Antiviral Drugs for Influenza and Other Respiratory Virus Infections

In this chapter, antiviral agents against influenza viruses and certain other respiratory viruses such as parainfluenza and respiratory syn­cytial virus are reviewed (Table 44-1 ). The antiviral agents are presented in alphabetical order and include licensed (approved) as well as investigational agents. Agents that have been investigated in rhinovirus infections but have been utilized primarily in non–respiratory tract infections, such as interferons and pleconaril, are discussed in Chapter 47.

TABLE 44-1.

Antiviral Agents of Established Therapeutic Effectiveness for Respiratory Virus Infection

VIRAL INFECTION	DRUG	ROUTE	USUAL ADULT DOSAGE
Influenza A and B viruses	Oseltamivir	Oral	75 mg bid for 5 daysa
	Peramivir	Intravenous	300 or 600 mg once
	Zanamivir	Inhalation	10 mg bid by inhaler for 5 daysb
	Laninamivir octanoate	Inhalation	40 mg oncec
Influenza A virus	Amantadine	Oral	100 mg bid for 5 days for treatmentd
	Rimantadine	Oral	100 mg bid for 5 days for treatmente
Respiratory syncytial virus	Ribavirin	Aerosol	Aerosol treatment 18 hr/day for 3-7 daysf

Note: Please consult text and manufacturer's product prescribing information for dosage adjustments in renal or hepatic insufficiency and in other circumstances.

^aPediatric dosages: For infants 2 wk to <1 yr of age dose is 3 mg/kg twice daily. For children ≥1 yr of age, doses are weight adjusted: 30 mg bid for <15 kg, 45 mg bid for 16-23 kg, 60 mg bid for 24-40 kg, and 75 mg bid for >40 kg. Prophylactic dosage is given once daily (one half of total daily treatment dosage). Not FDA approved currently for prophylaxis in children <1 yr old or treatment in children <2 wk old.

^bFDA approved at same dosage for treatment of children ≥7 yr of age. Prophylactic dosage is 10 mg inhaled once daily for adults and children ≥5 yr of age.

^cAdult dose and for children ≥10 yr of age. Pediatric dose: 20 mg once for children <10 yr of age.

^dMaximum recommended dosage for older adults (≥65 yr) is 100 mg/day. Recommended pediatric dosage is 5 mg/kg/day up to a maximum of 150 mg/day in divided doses. For prophylaxis, the same daily dosage should be given for period at risk.

^ePediatric dosage is 5 mg/kg up to a maximum of 150 mg/day in divided doses. Not approved by FDA for treatment in children <13 yr of age. For prophylaxis, same daily dosage should be given for period at risk.

^fReservoir concentration of 20 mg/mL. Special aerosol-generating device (available from manufacturer) and expert respiratory therapy monitoring for administration are required. Higher reservoir concentration (60 mg/mL) given for 2 hr tid is an alternative.

Amantadine and Rimantadine

Spectrum

Amantadine (1-adamantanamine hydrochloride; Symmetrel) and rimantadine (α-methyl-1-adamantane methylamine hydrochloride; Flumadine) are symmetrical tricyclic amines (Fig. 44-1A and B ) that specifically inhibit the replication of influenza A viruses at low con­centrations (<1 µg/mL). Influenza B and C viruses are resistant.¹ In the past, epidemic human and avian strains of influenza viruses have generally been susceptible to amantadine.² However, since 2008-2009, isolates of influenza A/H1N1 and H3N2, highly pathogenic avian H5N1, and A (H1N1)pdm09 are resistant to amantadine and rimantadine (see later discussion).³ By plaque assay, inhibitory con­centrations of the drugs range from 0.1 to 0.4 µg/mL or less for sensitive human influenza A viruses. Rimantadine is 4 to 10 times more active than amantadine in some assay systems. Both drugs are inhibitory for virus containing the M protein from the 1918 pandemic strain.⁴

FIGURE 44-1.

Chemical structures of amantadine hydrochloride (A) and rimantadine hydrochloride (B).

Higher concentrations (10 to 50 µg/mL) inhibit other enveloped viruses in vitro, including parainfluenza, influenza B, rubella, dengue, several arenaviruses (Junin, Lassa, Pichinde), rabies, and African swine fever virus, but these concentrations are not achievable clinically and can be cytotoxic in vitro.⁵ Rimantadine has pH-dependent trypanocidal activity at concentrations of approximately 1 µg/mL⁶; amantadine at the same concentration in combination with doxycycline inhibits Coxiella burnetii. ⁷ Amantadine may transiently inhibit hepatitis C virus (HCV) replication in humans.⁸

These agents have prophylactic and therapeutic activity in experimental influenza A virus infection of animals after oral or parenteral dosing. Combinations of M2 inhibitors and neuraminidase inhibitors and ribavirin show enhanced antiviral and therapeutic effects in vitro or in animal models of influenza.9, 10, 11, 12

Mechanism of Action

Amantadine and rimantadine share two concentration-dependent mechanisms of anti-influenza action. Low concentrations inhibit the ion channel function of the M2 protein of influenza A viruses, which affects two different stages in virus replication.13, 14, 15 The primary effect involves inhibition of viral uncoating or disassembly of the virion during endocytosis. For subtype H5 and H7 viruses, a late effect on hemagglutinin maturation and viral assembly is presumably mediated through altered pH regulation of the trans-Golgi network. Amantadine and rimantadine block proton permeation and prevent M2-mediated changes in pH. This action probably accounts for inhibition of the acid-mediated dissociation of the matrix protein from the ribonucleoprotein complex within endosomes early in replication and potentiation of acidic pH–induced alterations in the hemagglutinin during its transport late in infection.

Amantadine and rimantadine are also concentrated in the lysosomal fraction of mammalian cells. Drug-mediated increases in lysosomal pH may inhibit virus-induced membrane fusion events and account for the broader antiviral spectrum at higher concentrations. In contrast, the selective anti–influenza A virus effects are quickly lost after removal of the drug from the surrounding medium, which suggests that drug must be present in extracellular fluid early in the replicative cycle.

Amantadine inhibits the ion channel activity of expressed HCV p7 protein at low concentrations,¹⁶ an effect that might account for its reported anti-HCV effects in vivo. Neither agent inhibits HCV enzyme functions or internal ribosome entry in biochemical assays.¹⁷

Resistance

Amantadine-resistant virus is readily selected by virus passage in the presence of drug. Resistance with more than 100-fold increases in inhibitory concentrations has been associated with single amino-acid substitutions at critical sites (positions 26, 27, 30, 31, 34) in the trans-membrane region of the M2 protein.¹³ Amantadine and rimantadine share cross-resistance. In avian models, resistant viruses are virulent, genetically stable, and able to compete with wild-type virus so that transmission of drug-resistant virus may occur after cessation of drug use.

Before 2003, a small percentage of untreated patients (<1%) had infection with resistant influenza A virus.¹⁸ Approximately 30% of drug-treated ambulatory children and adults and 80% of hospitalized children or immunocompromised patients shed resistant virus.19, 20, 21 Immunocompetent individuals shedding resistant virus resolve their illness promptly,²² whereas immunocompromised hosts may experience prolonged illness associated with persistent virus shedding.²⁰ Transmission of M2 inhibitor–resistant virus, associated with failure of drug prophylaxis, occurs in household contacts of treated index cases²³ and in nursing home residents.²⁴ Resistant variants can cause typical influenza illness. It is prudent to avoid contact between treated patients and susceptible high-risk contacts and to avoid use of treatment (specifically of young children) and postexposure prophylaxis in the same household.

Globally, up to 2003, epidemic influenza A H1N1 and H3N2 strains were M2 inhibitor sensitive. Since 2003, the prevalence of amantadine resistance has increased progressively, although rates vary by virus type and geography.25, 26 Among H3N2 isolates, amantadine resistance increased from 12% worldwide in 2003²⁵ to 91% by 2005 and greater than 95% in 2008-2009.²⁶ In the United States prior to March 2009, nearly all of the A/H1N1 isolates tested were sensitive to the adamantanes and, subsequently, virtually all A/H1N1 isolates have been resistant up to the present, including the A (H1N1)pdm09 virus.²⁷ Among nonpandemic H1N1 isolates, the prevalence of amantadine resistance was 4% in 2004-2005 worldwide and 16% in isolates from 2005-2006, with rates ranging from 2% in South Korea to 72% in China.26, 28, 29 The reason for the emergence and global spread of amantadine-resistant strains is unclear. Widespread inappropriate use of amantadine³⁰ and acquisition of undefined advantageous mutations combined with lack of fitness impairment may have been contributing factors. Ribavirin and the neuraminidase inhibitors zanamivir and oseltamivir carboxylate are active in vitro against M2 inhibitor–resistant strains.

The triple combination of amantadine, oseltamivir, and ribavirin impedes the selection of drug-resistant influenza A virus in vitro at clinically achievable concentrations³¹ compared with double combinations and the agents used singly in vitro.³² The same combination of drugs was also synergistic in vitro in inhibiting the growth of both amantadine- and oseltamivir-resistant influenza A virus strains at concentrations that had no activity as single agents.³²

Pharmacokinetics

The clinical pharmacokinetic characteristics of amantadine and rimantadine are shown in Table 44-2 .

TABLE 44-2.

Clinical Pharmacokinetic Characteristics of Amantadine and Rimantadine in Healthy Adults

CHARACTERISTIC	AMANTADINE		RIMANTADINE
	Young	Elderly	Young	Elderly
Relative oral bioavailability (%)	62-93	53-100	75-93	NA
Vd (L/kg) at 200 mg/day	6.1 ± 2.1	3.6 ± 1.1	18.4 ± 9.6	11.5 ± 2.9
Plasma protein binding (%)	67	NA	40	NA
Clearance (mL/min/kg)
Plasma or total	5 ± 2.1	2 ± 0.9	6.1 ± 1.9	4.7 ± 2
Renal	6.4 ± 3.7	2 ± 1.1	1.2 ± 0.4	NA
Nonrenal	0	0	6.4 ± 1.4	NA
Urinary excretion of unchanged drug (%)	62-93	53-100	8.3-43	NA
Plasma half-life (hr)	14.8 ± 6.2	26.1 ± 9.7	29.1 ± 9.7	36.5 ± 14.5
Therapeutic range (ng/mL)
Cmax
200 mg/day	475 ± 110	—	416 ± 108	447 ± 108
100 mg/day	—	362 ± 158	—	—
Ctrough
200 mg/day	302 ± 80	—	300 ± 75	310 ± 87
100 mg/day	—	301 ± 75	—	—

NA, not available.

Adapted from Hayden FG, Aoki FY. Amantadine, rimantadine, and related agents. In: Yu VL, Edwards D, McKinnon S, et al, eds. Antimicrobial Therapy and Vaccines. 2nd ed. Pittsburgh: E Sun Technologies; 2002:714.

Amantadine

Amantadine is well absorbed after oral administration of capsule, tablet, or syrup forms.⁵ Steady-state peak plasma concentrations average 0.5 to 0.8 µg/mL with a 100-mg twice-daily regimen in healthy young adults. Older adults require only one half of the weight-adjusted dosage needed for young adults to achieve equivalent trough plasma levels of 0.3 µg/mL. Plasma protein binding of amantadine is about 67%, and amantadine's volume of distribution (V_d) is large (4 to 5 L/kg). Nasal secretion and salivary levels of amantadine approximate those found in the serum. Cerebrospinal fluid levels are 52% to 96% of those in plasma, and amantadine is excreted in breast milk.

Amantadine is eliminated largely unchanged in the urine by glomerular filtration and probably by tubular secretion by a bicarbonate-dependent organic cation transporter.³³ The plasma elimination half-life () is 12 to 18 hours, ranges widely, and correlates with the creatinine clearance (CrCl). Because of age-related declines in renal function, increases twofold in older adults and even more in patients with impaired renal function. Dosage reductions are required in renal insufficiency (Table 44-3 ). Amantadine is inefficiently cleared in patients receiving hemodialysis or continuous ambulatory peritoneal dialysis, and additional doses are not required. Monitoring of plasma concentrations in such patients is desirable but impractical.

TABLE 44-3.

Amantadine Dosage Regimens for Prophylaxis and Alterations in Renal Failure

CONDITION	SUGGESTED DOSAGE
No Renal Insufficiency
Children 1-9 yr	5 mg/kg/day in two divided doses, ≤150 mg/day
Ages 10-64 yr	100 mg twice daily
Ages ≥65 yr	100 mg once daily*
Creatinine Clearance (mL/min/1.73 m2)†
≥80	100 mg (1.4 mg/kg) twice daily
79-35	100 mg once daily
34-25	100 mg every 2 days
24-15	100 mg every 3 days
<15	100 mg every 7 days
Older Adults and Creatinine Clearance (mL/min/1.73 m2)‡
≥80	100 mg daily
60-79	100 mg and 50 mg on alternate days
40-59	100 mg every 2 days
30-39	100 mg twice weekly
20-29	50 mg twice weekly
10-19	100 mg and 50 mg on alternate weeks

^*Use weight-adjusted dosing for smaller patients (<50 kg). Dosages of 1.4 mg/kg/day have been suggested.⁵

^†Based on adult dosage of 200 mg/day. Proportionate reductions should be made for older adults receiving lower dosages and for children.

^‡This dosing schedule for older adults with renal insufficiency is taken from the Canadian guidelines and has been found to be reasonably well tolerated.⁴⁴

Modified from Wu MJ, Ing TS, Soung LS, et al. Amantadine hydrochloride pharmacokinetics in patients with impaired renal function. Clin Nephrol. 1982;17:19-23.

Amantadine pharmacokinetics remained unaffected by concurrent administration of oseltamivir and ribavirin in healthy adult volunteers or stable immunocompromised patients.³⁴

Rimantadine

Rimantadine is well but slowly absorbed, with the time to peak plasma concentration averaging 2 to 6 hours. Absorption does not seem to be decreased by food. With multiple doses of 100 mg twice daily, the steady-state peak and trough plasma concentrations in healthy adults are 0.4 to 0.5 µg/mL and 0.2 to 0.4 µg/mL. In infants receiving dosages of 3 mg/kg each day, peak serum levels range from 0.1 to 0.6 µg/mL. No important age-related changes in pharmacokinetics have been found in healthy older adults or in children. However, steady-state plasma concentrations in older nursing home residents receiving 100 mg twice daily average more than twofold higher (mean, 1.2 µg/mL) than concentrations observed in healthy adults, which indicates the need for lower dosages in these patients. Plasma protein binding is about 40%. Rimantadine has a very large V_d (~12 L/kg), and concentrations in nasal mucus average 50% higher than those in plasma.

In contrast to amantadine, rimantadine undergoes extensive metabolism by hydroxylation, conjugation, and glucuronidation before renal excretion.⁵ The plasma of rimantadine averages 24 to 36 hours. No clinically important differences in pharmacokinetics are found in patients with chronic liver disease without significant hepatocellular dysfunction. In hemodialysis patients with severe renal failure, the clearance of rimantadine is decreased by 40% and the is about 55% longer. Reducing dosages by one half (e.g., to 100 mg/day) is recommended for marked hepatic or renal insufficiency (CrCl <10 mL/min). Hemodialysis removes only a small amount of rimantadine, so supplemental doses are not required.

Interactions

The risks for central nervous system (CNS) adverse effects with amantadine and possibly with rimantadine are increased by concomitant ingestion of antihistamines, antidepressants, anticholinergic drugs, and other drugs affecting CNS function. Concurrent use of trimethoprim-sulfamethoxazole or triamterene-hydrochlorothiazide has been associated with CNS toxicity resulting from decreased renal clearance of amantadine. Cimetidine is associated with 15% to 20% increases, and aspirin or acetaminophen is associated with 10% decreases in plasma rimantadine concentrations, but such changes are unlikely to be significant. Neither adverse clinical nor adverse pharmacokinetic effects are observed when amantadine and oseltamivir are co-administered.³⁵

Concurrent administration of recommended doses of amantadine, oseltamivir, and ribavirin for 10 days was well tolerated.³⁴

Toxicity

Amantadine or rimantadine given in treatment courses of 5 days is generally well tolerated in young healthy adults.³⁶ Longer periods of administration, such as 6 weeks for seasonal prophylaxis in young adults,³⁶ and administration to fragile, elderly nursing home residents, such as octogenarians, for 10 days for outbreak control are associated with a significant frequency of adverse reactions and drug withdrawals.³⁷

A case-control study demonstrated that in children younger than 12 months of age, amantadine and rimantadine were well tolerated, as was oseltamivir.³⁸ No evidence of adverse maternal or neonatal outcomes were observed after antepartum influenza treatment with adamantane antiviral agents.³⁹

The most common side effects related to amantadine ingestion are minor, dose-related gastrointestinal and CNS complaints, including nervousness, lightheadedness, difficulty concentrating, confusion, insomnia, and loss of appetite or nausea.⁴⁰ Complaints typically develop within the first week of administration, often resolve despite continued ingestion, and are reversible on drug discontinuation. CNS side effects occur in 5% to 33% of amantadine recipients at dosages of 200 mg/day but are significantly less frequent with rimantadine. When used for influenza prophylaxis in ambulatory adults, dosages of 200 mg/day are associated with excess withdrawals in 6% to 11% of recipients because of drug side effects. Dosages of 100 mg/day are better tolerated and may be protective against influenza illness. Amantadine dosage reductions are required in older adults (100 mg/day), but 20% to 40% of nursing home residents experience significant adverse effects on this lower dosage despite some adjustment for renal insufficiency.41, 42, 43 Consequently, further dosage reductions based on CrCl are warranted in this population.⁴⁴

In the setting of renal insufficiency or high dosages, serious neurotoxic reactions, including delirium, hostility, hallucinations, tremor, myoclonus, seizures, or coma; cardiac arrhythmias; and death can occur in association with elevated amantadine plasma concentrations (1 to 5 µg/mL).⁴⁵ Neurotoxic reactions may be transiently reversed by physostigmine administration, and lidocaine has been used to treat ventricular arrhythmias. Long-term amantadine ingestion has been associated with livedo reticularis, peripheral edema, orthostatic hypotension, and, rarely, congestive heart failure, vision loss, or urinary retention. Peripheral edema and livedo reticularis may improve if treatment is switched from amantadine to rimantadine.⁴⁶ Patients with preexisting seizure disorders have an increased frequency of major motor seizures during amantadine use, and dosage reductions are advised. Psychiatric side effects in patients with Parkinson's disease and psychotic exacerbations in patients with schizophrenia may occur with addition of amantadine. Rash and leukopenia have been described rarely.

Rimantadine administration is associated with dose-related side effects similar to side effects observed with amantadine, although the risk for CNS side effects is lower with rimantadine at dosages of 200 mg/day or 300 mg/day in ambulatory adults.⁵ During prophylaxis, excess withdrawal rates are usually less than 5%. In older nursing home residents, dosages of 200 mg/day are associated with higher side effect rates, whereas dosages of 100 mg/day seem to be better tolerated.41, 47 Rimantadine may uncommonly cause exacerbations of seizures in patients not receiving anticonvulsants and was associated with an unexplained excess mortality in one nursing home study.⁴⁷

The clinical observations of dry mouth, pupillary dilation, toxic psychosis, and urinary retention in acute amantadine overdose suggest that anticholinergic activity is present in humans. Amantadine shows activity on the adrenergic nervous system by affecting accumulation, release, and reuptake of catecholamines in the CNS and in the peripheral nervous system. Malignant ventricular arrhythmia after amantadine overdose has been described in humans.

Amantadine and rimantadine lack mutagenicity in vitro; carcinogenicity studies have not been reported for either. Amantadine is teratogenic and embryotoxic in rats, and rimantadine may cause teratogenic effects in rabbits and maternal toxicity and embryotoxicity at high dosages in rodents. Both drugs are classified in pregnancy category C. Birth defects have been reported after amantadine exposure during pregnancy.⁴⁸ The safety of neither amantadine nor rimantadine has been established in pregnancy. Because of excretion in breast milk, use is not recommended in nursing mothers.

Clinical Studies

Influenza A

Amantadine and rimantadine have been efficacious for the prevention and treatment of influenza A virus infections in young healthy adults.5, 40, 49 A systematic review of published studies in children and the elderly concluded that available data only demonstrate that amantadine has prophylactic efficacy and a modest therapeutic effect in children.⁵⁰ In the elderly, no data were available to support a conclusion of prophylactic or therapeutic efficacy of either adamantane. The emergence of widespread and nearly complete amantadine resistance among influenza A/H3N2 isolates,²⁶ as well as the amantadine resistance of the pandemic A (H1N1)pdm09 strains, precludes the empirical use of adamantanes for management of an untyped influenza A outbreak. Amantadine and rimantadine, both at a dosage of 200 mg/day in adults, are about 70% to 90% protective against clinical illness caused by various susceptible influenza A subtypes, including susceptible pandemic strains.⁵¹ Prophylaxis is effective in preventing nosocomial influenza and possibly in curtailing nosocomial outbreaks caused by such strains. Protection seems to be additive to that provided by vaccine.⁵²

Rimantadine was less effective than zanamivir in reducing cases of influenza A illness in adults in a long-term care facility.⁵³ The difference in protective efficacy was largely due to the emergence of rimantadine-resistant viruses that caused rimantadine prophylactic failure; no zanamivir-resistant viruses were isolated. Rimantadine administration to school-aged children (5 mg/kg/day) decreased the risk for influenza A illness in recipients and possibly in their family contacts. Postexposure prophylaxis with these drugs provided inconsistent protection to family contacts, however, in part, depending on whether ill index children were treated.¹⁹ Dosages of 100 mg/day seem to be protective against influenza A illness and are well tolerated in adults.⁵⁴

Amantadine and rimantadine are also effective therapies for uncomplicated adamantane-susceptible influenza A illness in healthy adults,5, 22 but it is uncertain whether treatment reduces the risk for complications in high-risk patients or is useful in patients with established pulmonary complications. Early treatment in ambulatory adults (200 mg/day for 5 days) reduces the duration of fever and systemic complaints by 1 to 2 days, decreases virus shedding, and shortens time to resumption of usual activities.²² In illness caused by H3N2-subtype influenza viruses, certain abnormalities of peripheral airways function, but not of airway hyperreactivity, resolve more quickly in amantadine-treated patients. Amantadine or rimantadine treatment in adults with leukemia or stem cell transplantation may reduce the risk for pneumonia,⁵⁵ but more recent data suggest that in stem cell transplant recipients, early neuraminidase inhibitor therapy may be preferred to adamantanes, because it may prevent progression to pneumonia and decrease viral shedding, thereby possibly preventing both influenza-related death in index patients and nosocomial transmission to others.⁵⁶ In children, rimantadine treatment is associated with lower symptom burden, fever, and viral titers during the first 2 days of treatment compared with acetaminophen administration, but rimantadine-treated children have more prolonged shedding of virus. Treatment generally does not seem to affect humoral immune responses to infection, but may blunt secretory antibody levels.⁵⁷

Intermittent aerosol administration of amantadine or rimantadine seems to be therapeutically useful in uncomplicated influenza. No injectable formulation of either drug is available in the United States.

Other Viruses

Amantadine has been used in multiple trials for treatment of chronic hepatitis C with inconsistent evidence for increases in sustained viral response (SVR). In treatment-naïve patients, the addition of amantadine (200 mg daily in single or divided doses) to interferon58, 59 or to interferon plus ribavirin⁶⁰ may modestly increase biochemical responses and the likelihood of SVR. In re-treatment of interferon nonresponders, the combination of interferon plus amantadine is ineffective⁶¹ but the addition of amantadine to the combination of interferon plus ribavirin may be associated with SVR in 10% to 25%.⁶² Amantadine plus combined pegylated interferon and ribavirin may increase SVR modestly in treatment-experienced patients compared with pegylated interferon plus ribavirin.⁶³ Reports of possible activity in bornavirus infections and associated neuropsychiatric symptoms require confirmation.

DAS181 (Fludase)

DAS181 is an investigational antiviral agent with activity against influenza A and B viruses and parainfluenza viruses types 1-3.64, 65, 66, 67, 68, 69 It has a novel mechanism of action in that it is a sialidase from Actinomyces viscosus (Fig. 44-2 ) linked to a respiratory epithelium-anchoring domain.⁷⁰ It cleaves the terminal sialic acid residues on the surface of human respiratory cells, thus reducing the binding of respiratory viruses, which use those as receptors. Desialylation is rapid and results in an antiviral effect, which lasts for at least 2 days.⁶⁴ The effective concentration (EC) for 50% of all isolates (EC₅₀) against influenza A and B viruses ranges from 0.04 to 0.9 µM.⁶⁵ DAS181 is active against influenza viruses that are resistant to neuraminidase inhibitors.⁷¹ Low-level resistance to DAS181 can be induced, but resistant variants appear to be reduced in fitness.⁷²

FIGURE 44-2.

Molecular model of DAS181.

The catalytic domain of the sialidase (AvCD) is shown in green and the protruding anchoring domain (AR) on the carboxyl terminus in blue.

(From Malakhov M, Aschenbrenner L, Smee D, et al. Sialidase fusion protein as a novel broad-spectrum inhibitor of influenza virus infection. Antimicrob Agents Chemother. 2006;50:1470.)

DAS181 is administered by oral inhalation and appears to be generally well tolerated. A phase II placebo-controlled study was recently conducted in 177 subjects with influenza A and B virus infections.⁷³ DAS181 was administered either as a single 10-mg dose or as a daily 10-mg dose for 3 days. Compared with placebo recipients, DAS181 recipients had a statistically significant decrease in virus load determined by polymerase chain reaction (PCR) assay between days 1 and 3 and days 1 and 5. However, there were no differences in resolution of clinical illness among the groups. Administration of DAS181 appeared to be generally well tolerated, although transient elevations in alkaline phosphatase level were reported.⁷³

DAS181 has also been utilized to treat parainfluenza virus type 3 infections in lung transplant and stem cell transplant patients.74, 75 These case reports described clinical improvement, increased pulmonary function, and decreased virus loads. Additional clinical studies of DAS181 are being planned.

Laninamivir Octanoate

Laninamivir octanoate (Inavir) is an investigational drug except for its approval in Japan. It is the prodrug of laninamivir, an inhibitor of influenza A and B neuraminidases.⁷⁶ Laninamivir is (2R,3R,4S)-3-acetamido-2-[CIR,2R-2,3-dihydroxy-1-methoxypropyl]-4-quanidino-3,4-dihydro-2-H-pyram-6-carboxylic acid (Fig. 44-3D ). Laninamivir octanoate consists of an octanoic acid ester side chain attached at the C₃ position of laninamivir. Laninamivir octanoate, like polymeric zanamivir conjugates, shares the pharmacokinetic characteristic of persisting for a prolonged period in the respiratory tract after administration intranasally or intratracheally in animals or by oral inhalation in humans. These observations have presaged therapeutic effects of a single dose in animals with experimentally induced influenza in patients as well.

FIGURE 44-3.

Chemical structures of oseltamivir carboxylate (A), zanamivir (B), peramivir (C) and laninamivir (D).

Spectrum

Laninamivir octanoate exhibits little or no influenza virus neuraminidase inhibitory activity in vitro.⁷⁷ However, its hydrolysis product is a potent inhibitor of neuraminidases of N1 to N9 influenza A viruses plus influenza B and their replication in cell culture at nanomolar concentrations.⁷⁸ These include seasonal and pandemic influenza A/H1N1, highly pathogenic avian influenza (HPAI) H5N1 viruses, and clinical isolates of oseltamivir-resistant H1N1, H3N2, H5N1, and A (H1N1)pdm09. Median inhibitory concentrations in cell culture vary over a wide range and in general appear to be intermediate between those of oseltamivir carboxylate (lower) and zanamivir (higher), but the clinical importance of these differences is not yet known.

In preclinical studies, laninamivir octanoate reduced fever in ferrets, mortality in mice, and virus concentrations in lung in ferrets and mice and brain in mice after induced influenza with a variety of viruses: A/PR/8/34, HPAI H5N1, A (H1N1)pdm09, B/Malaysia/2506/2004, as well as oseltamivir-resistant A H1N1 and HPAI H5N1 clinical isolates possessing the H274Y mutation, as reviewed by Yamashita and associates.⁷⁸ In these studies, laninamivir octanoate was administered as a single intranasal dose after intranasal inoculation of virus and was either as or more efficacious than multiple doses of oral oseltamivir or intranasal zanamivir. The results of these studies in animals with experimental influenza have been replicated in part in therapeutic trials of a single laninamivir octanoate dose in the clinic (see later).

Single doses of laninamivir octanoate are also efficacious prophylactically in mice. One dose prevents mortality and reduces virus concentration in lungs and brain when administered as much as 7 days before virus challenge.⁷⁹

Mechanism of Action

See subsequent discussion of mechanism of action under “Oseltamivir.”

The basis for the prolonged persistence of laninamivir in the respiratory tract after intranasal or intratracheal administration of laninamivir octanoate in animals or oral inhalation in humans is not completely understood. In human volunteers, bronchoalveolar lavage samples obtained serially over 24 hours after oral inhalation of a single 40-mg dose of laninamivir octanoate reveal concentrations that exceed influenza virus neuraminidase inhibitory concentrations at all test times.⁸⁰ In mice, intranasal administration of carbon-14 (¹⁴C)–labeled laninamivir octanoate demonstrates prolonged retention of laninamivir in lung tissues. Microautoradiography indicates that laninamivir octanoate is taken into airway epithelial cells, seemingly hydrolyzed to the antiviral molecule laninamivir by intracellular esterases, and then released slowly extracellularly, perhaps as a result of its hydrophobic poor membrane permeability.⁸¹ The cellular and molecular processes underlying these observations are not yet determined.

Resistance

No extensive studies have been reported on the emergence of laninamivir-resistant strains after laninamivir exposure in vitro or laninamivir octanoate treatment in animals or patients. However, in one study in mice infected with an A H1N1 virus, no viruses with reduced susceptibility to laninamivir were recovered.⁸²

Pharmacokinetics

Epithelial lining fluid concentrations of laninamivir octanoate and laninamivir calculated from analysis of bronchoalveolar lavage washings after a single oral inhalation of 40 mg laninamivir octanoate were 102.4 and 8.6 µg/mL, respectively, at 4 hours in healthy adult volunteers.⁷⁶ The disappearance half-times in bronchoalveolar lavage fluid were 41 and 141 to 241 hours, respectively. The plasma values were 2.6 and 45.7 hours, respectively. Laninamivir concentrations in epithelial lining fluid exceeded the median inhibitory concentrations for influenza neuraminidases at all time points for 240 hours after dose inhalation. In other healthy adult volunteers, evaluation of the pharmacokinetics of laninamivir octanoate and laninamivir was done after oral inhalation of single doses from 5 to 120 mg.⁸³ Laninamivir octanoate appeared rapidly in plasma with a C_max at 0.5 to 1.0 hour compared with 4.0 hours for laninamivir. Plasma values were 1.8 and 71.6 to 80.8 hours, respectively. The plasma area under the concentration-time curve (AUC) of laninamivir octanoate was linearly related to dose, while that of laninamivir increased disproproportionately. The mean cumulative excretion in urine over 144 hours was 2.3% to 3.6% and 10.7% to 14.6%, respectively.

After intravenous administration of ¹⁴C-laninamivir in rats, almost 90% of the radioactivity was recovered in urine.⁸⁴ In human volunteers, the clearance of both laninamivir octanoate and laninamivir is linearly related to CrCl.⁸⁵ In subjects with none, mild, moderate, or severe renal impairment given a single orally inhaled dose of 20 mg laninamivir octanoate, the renal clearance of laninamivir octanoate and laninamivir is directly related to CrCl, whereas values are not. Geometric mean laninamivir octanoate clearance values declined from 26.0 mL/min in normal control subjects to 6.5 mL/min in patients with severe renal impairment. However, values were 2.3 to 3.5 hours and not different among the four groups. Laninamivir renal clearance declined from 65.0 to 12.7 mL/min across the four groups, whereas was not different among the groups, ranging from 53.2 to 57.0 hours. The likely explanation is that the elimination of both laninamivir octanoate and laninamivir reflect slow release of these compounds from tissues into plasma, rather than renal elimination, a pharmacokinetic concept called “flip-flop.”⁸⁶ These pharmacokinetic data indicate that reduction of laninamivir octanoate doses may be appropriate for patients with renal impairment for pharmacokinetic reasons, but the lack of clear dose-related toxicity (see later) and the minimal absorption of orally inhaled drugs suggest that no dose adjustment will be needed.

Toxicity

Like orally inhaled zanamivir, orally inhaled laninamivir octanoate powder is well tolerated. In a double-blind study in healthy adult volunteers, single doses from 5 to 120 mg or multiple doses of 20 or 40 mg twice daily for 5 days were as well tolerated as placebo.⁸⁵

In clinical trials, patients with influenza were randomized to single laninamivir octanoate doses of 20 or 40 mg in adults or children 10 years old or older, 20 mg in children younger than 10 years old, or inhaled zanamivir as the control neuraminidase inhibitor treatments. Laninamivir octanoate inhaled once was as well tolerated as inhaled zanamivir 20 mg twice daily for 5 days.⁸⁷ In a double-blind trial in children 9 years of age or younger with influenza, a single dose of inhaled laninamivir octanoate of 20 or 40 mg was as well tolerated as oseltamivir at 2 mg/kg body weight twice daily for 5 days.⁸⁸ In a phase III double-blind trial in adults 20 years of age or older with influenza, a single dose of inhaled laninamivir octanoate of 20 or 40 mg was as well tolerated as oral oseltamivir at 75 mg twice daily for 5 days.⁸⁹ Notwithstanding the lack of data from large, randomized, placebo-controlled, double-blind trials to establish the tolerability of laninamivir octanoate across the range of persons in healthy and high-risk groups, these published data on laninamivir octanoate tolerance plus those from studies of orally inhaled zanamivir collectively suggest that orally inhaled laninamivir octanoate will likely prove to be well tolerated and safe in the clinic.

Postmarketing studies of laninamivir octanoate in Japan concluded that the safety profile of laninamivir octanoate for abnormal behavior/delirium and syncope is similar to that of other neuraminidase inhibitors.⁹⁰ In Japan, it is recommended in the product labeling that teenage patients inhaling laninamivir octanoate should remain under constant parental supervision for at least 2 days to monitor for behavioral changes to prevent associated self-injury. To avoid syncope, patients should inhale laninamivir octanoate in a relaxed sitting position. In another postmarketing survey for laninamivir octanoate tolerance, 50 patients of 3542 (1.4%) reported an adverse event.⁹¹ Commonly reported adverse events included psychiatric disorders (abnormal behavior), gastrointestinal symptoms, and nervous system disorders such as dizziness, with frequencies of 0.48%, 0.45%, and 0.17%, respectively. These usually appeared on the day of laninamivir octanoate treatment and resolved in 3 days. These adverse reactions and their frequency were considered comparable to those previously observed during clinical trials, and thus were believed to confirm no noticeable problem with safety.

Clinical Studies

Limited data from controlled trials are available on the efficacy of orally inhaled laninamivir octanoate for influenza treatment, although three randomized, controlled trials on the efficacy and tolerance of laninamivir octanoate and one observational study comparing it with other neuraminidase inhibitors have been reported. In these trials, laninamivir octanoate has been administered as an orally inhaled powder with a proprietary device that has two containers of 10-mg dry laninamivir octanoate powder. The manufacturer's instructions recommend two inhalations from each 10-mg changer. For children, four inhalations are necessary, whereas eight inhalations from two devices are required for adults. Occasionally, young children do not inhale the medication completely owing to technical difficulty with the device.⁸⁷

Of 87 pediatric patients with influenza of less than 48 hours in duration, 44 were randomized to treatment with a single inhaled dose of laninamivir octanoate (N = 55), 20 or 40 mg, according to age, or inhaled zanamivir, 10 mg twice daily for 5 days (N = 41).⁸⁷ Median times to fever resolution were 36 hours in the laninamivir octanoate groups and 37 hours in the zanamivir-treated group. This relatively small study suggested that a single dose of inhaled laninamivir octanoate was as efficacious as the recommended 5-day treatment with zanamivir. In another study, 180 children 9 years or younger with influenza of less than 36 hours in duration were randomized to a single oral inhalation of 40 (N = 61) or 20 mg (N = 61) laninamivir octanoate or oseltamivir 2 mg/kg (N = 62) ingested twice daily for 5 days.⁸⁸ Of the 180 children, 62% (112) were infected with influenza A H1N1 virus, of which all but 4 possessed the H274Y mutation, mediating oseltamivir resistance. Oseltamivir therapy was likely not to have been different from placebo. The median times to alleviation of influenza illness in children were significantly less (49.6 and 44.3 hours) in the 40- and 20-mg laninamivir groups, respectively, than in the oseltamivir-treated group (110.5 hours). Treatment effects on virus concentration and persistence in upper airway secretions were inconsistent, although on day 3, 10%, none, and 25% of subjects in the three groups, respectively, were still excreting virus. There were no clinical therapeutic or virologic differences among children infected with influenza A H3N2 or B viruses, but the numbers of cases were small.

In a double-blind, randomized noninferiority trial, 1003 young healthy adults with febrile influenza for no more than 36 hours were randomized to receive either 40 mg or 20 mg of laninamivir octanoate by oral inhalation once or oseltamivir, 75 mg twice daily orally, for 5 days.⁸⁹ The primary end point was time to influenza illness alleviation. Unfortunately, as in the pediatric study of Sugaya and Ohashi,⁸⁸ 66% of the subjects were infected with oseltamivir-resistant influenza A H1N1 virus. The median times to resolution of illness in patients infected with this virus were 74.0, 85.8, and 77.8 hours, respectively, which were not different. Virus was detected by culture significantly less often at day 3 in the laninamivir octanoate 40-mg (28%) and 20-mg (32%) groups than in the oseltamivir group, which might be con­sidered analogous to a placebo-treated cohort. Among individuals infected with influenza A H3N2 virus, median times to illness alleviation were not different between those treated with laninamivir octanoate 40 mg (73.5 hours) and oseltamivir (67.5 hours) but significantly longer in the group treated with laninamivir octanoate 20 mg (91.2 hours). There were no differences among the groups in H3N2 virus concentration in upper airway secretions or persistence. The 95% confidence intervals of the pooled analysis of all data were less than the prescribed noninferiority margin. It was concluded that a single inhalation of laninamivir octanoate is effective for treatment of seasonal influenza including that caused by oseltamivir-resistant virus in adults.

In an observational study, 211 children with febrile influenza of less than 48 hours due to influenza A H3N2 infection and 45 with A (H1N1)pdm09 infection were treated according to the recommendations of clinicians and the preference of patients or their guardians.⁹² Of the 256 children, 119 were treated with oseltamivir in weight-appropriate doses, zanamivir (124 cases), one dose of intravenous peramivir (4 children),⁷⁹ or a single dose of orally inhaled laninamivir octanoate of 40 mg for children 10 years or older or 20 mg for those younger than 10 years (9 children). The primary end point was duration of fever from the first dose of neuraminidase inhibitor. There were no differences in the duration of fever among the oseltamivir, zanamivir, or laninamivir octanoate groups. The median time to resolution of fever in the peramivir group (17.0 hours) was significantly less than in the other three groups.

Available data suggest that a single inhaled dose of laninamivir octanoate is efficacious in children with influenza of less than 48 hours, but efficacy in other populations, especially those with high-risk conditions, remains to be evaluated, as does the impact on complications of influenza.

Oseltamivir

Spectrum

Oseltamivir phosphate (Tamiflu) is the ethyl ester prodrug of oseltamivir carboxylate, a sialic acid analogue (see Fig. 44-3A) that is a potent, specific inhibitor of the neuraminidases of influenza A and B viruses.93, 94 The metabolite, oseltamivir carboxylate, is approximately 50-fold more potent than the phosphate prodrug.⁹⁵ Oseltamivir carboxylate competitively and reversibly interacts with the active enzyme site to inhibit neuraminidase activity at low nanomolar concentrations.⁹⁶ Inhibitory concentrations for neuraminidase inhibitors in cell culture have a broad range (≥1000-fold), depending on the assay method, and may not correlate with in vivo activity.97, 98 Oseltamivir carboxylate is active against viruses containing all nine influenza A neuraminidase subtypes recognized in nature, including more recent pathogenic avian viruses (H5N1, H7N7, H9N2), reassortant virus containing neuraminidase from the 1918 pandemic strain, M2 inhibitor–resistant strains,4, 99 and the recently circulating (2009) pandemic A/H1N1 viruses (S-OIV).²⁷ Resistance to oseltamivir has been recently reported in an H7N9 isolate.¹⁰⁰

Influenza B viruses are 10-fold to 20-fold less susceptible to oseltamivir carboxylate than influenza A viruses, and influenza B virus illness responds less well clinically and virologically to oseltamivir than influenza A illness.101, 102, 103 The carboxylate is not cytotoxic and inhibits neuraminidases from mammalian sources or other pathogens only at 10⁶-fold higher concentrations. Oral oseltamivir is active in murine and ferret models of influenza.94, 97 A prophylactic regimen given orally twice daily for 10 days completely protected ferrets against morbidity and mortality caused by H5N1 infection and did not interfere with development of a protective immunity against subsequent H5N1 infection.¹⁰⁴ Neuraminidase inhibitors combined with M2 inhibitors or ribavirin show enhanced antiviral activity in vitro and in animal models of influenza A virus infection,¹⁰⁵ including H5N1 virus.106, 107 Amantadine combined with oseltamivir prevented the emergence of amantadine resistance in cell culture.¹⁰⁸

Mechanism of Action

The neuraminidase inhibitor drugs oseltamivir, zanamivir, peramivir, and laninamivir share a common mechanism of action. Influenza neuraminidase cleaves terminal sialic acid residues on glycoconjugates and destroys the receptors recognized by viral hemagglutinin on cells, on newly released virions, and on respiratory tract mucins. This action is essential for release of virus from infected cells and for spread within the respiratory tract.¹⁰⁹ Inhibition of neuraminidase action causes newly formed virions to adhere to the cell surface and to form viral aggregates. Inhibitors limit spread of virus within the respiratory tract and may prevent virus penetration of respiratory secretions to initiate replication.

Resistance

Resistant variants selected by in vitro passage with oseltamivir carboxylate or zanamivir have point mutations in the viral hemagglutinin or neuraminidase genes.98, 110 Hemagglutinin variants generally have mutations in or near the receptor binding site that make them less dependent on neuraminidase action for release from cells in vitro and that confer cross-resistance among neuraminidase inhibitors. Most of these variants retain full susceptibility in vivo.⁹⁸ Neuraminidase variants contain single amino-acid substitutions in the framework or catalytic residues of the active enzyme site that alter drug binding and cause approximately 30-fold to more than 1000-fold reduced susceptibility in enzyme inhibition assays.⁹⁶ Influenza A variants selected by oseltamivir carboxylate are subtype specific, most commonly Arg292Lys in N2 and H275Y in N1, without cross-resistance to zanamivir. The altered neuraminidases have reduced activity or stability in vitro, and early studies of these variants usually demonstrated decreased infectivity and transmissibility in animals.¹¹¹

Oseltamivir therapy has been associated with recovery of viruses with reduced susceptibility in about 1% of immunocompetent adult and 18% of pediatric recipients.112, 113 Generally, emergence of resistant variants has not been associated with clinical worsening, although prolonged recovery of resistant variants, sometimes in combination with M2 inhibitor resistance, has been observed in highly immunocompromised hosts.¹¹⁴ Transmission of oseltamivir-resistant virus has been documented.115, 116

Although isolation of oseltamivir-resistant strains from treated immunocompetent patients was uncommon, in 2007-2008, oseltamivir-resistant seasonal H1N1 virus appeared widely in immunocompetent individuals in Norway in the absence of antiviral pressure.¹¹⁷ This mutant virus became the transmissible, pathogenic prevalent global H1N1 virus strain. Similarly, during the 2009 A (H1N1)pdm09 pandemic, there was no linkage between prevalent use of oseltamivir in immunocompetent patients and the appearance of oseltamivir-resistant A (H1N1)pdm09 strains, which was uncommon. The prevalence of oseltamivir-resistance ranged from 0.6% (5/804 strains) tested in Ontario, Canada,¹¹⁸ to 1.0% in the United States¹¹⁹ and 1.1% (16/1488 isolates) in Southeast Asia.¹²⁰ The prevalence was 8.11% in children whose immunocompetence was not specified.¹²¹

On the other hand, oseltamivir-resistant isolates are not uncommonly recovered from immunocompromised patients being treated with the drug. Reports indicated that some of the A (H1N1)pdm09 oseltamivir-resistant strains retained replicative fitness,¹¹⁶ transmissibility,¹²² and pathogenicity comparable with wild-type oseltamivir strains in murine and ferret models of influenza infection.¹²³ Clinical illness caused by oseltamivir-resistant H1N1 strains in immunocompetent children responded less well to oseltamivir,¹²⁴ as evidenced by higher fever at day 4 or 5 of treatment, although some found no evidence of prolonged illness in children infected with drug-resistant virus.¹²⁵ Others reported a significantly longer time to achieve nondetectable virus load in patients with oseltamivir-resistant H1N1 compared with oseltamivir-sensitive strains.¹²⁶

Pharmacokinetics

Oral oseltamivir is rapidly absorbed and metabolized by esterases in the gastrointestinal tract, liver, and blood to the active carboxylate. The estimated bioavailability of the carboxylate is approximately 80%,¹²⁷ and its time to maximal plasma concentrations averages 2 to 4 hours. Dose proportionality of oseltamivir has been reported over the dose range from 75 to 675 mg. Only low blood levels of the prodrug are detectable. Rarely, possession of a constitutive variant of carboxylesterase 1, the enzyme that normally catalyzes the conversion of oseltamivir phosphate to carboxylate, can markedly impair the hydrolysis of the parent compound, resulting in the potential for a compromised antiviral effect after oseltamivir administration.128, 129 Ingestion with food delays absorption slightly but does not decrease overall bioavailability. Oseltamivir administered via a nasogastric tube to patients with respiratory failure requiring mechanical ventilation was well absorbed and converted to oseltamivir carboxylate.130, 131 In healthy adults, peak and trough plasma concentrations average 0.35 µg/mL and 0.14 µg/mL after 75-mg doses.¹³² In infants up to 1 year of age, systemic exposure (AUC_0-12 hr) to the carboxylate exhibits decreasing variability while clearance increases.¹²¹ Recommended doses of oseltamivir are 3.0 mg/kg twice daily for infants from birth to 8 months of age and 3.5 mg/kg twice daily for those 9 to 11 months of age. In children older than 1 year, carboxylate exposure increases gradually with increasing age¹³² so that weight-based dosing is recommended.¹³³ In healthy elderly adults, overall drug exposure is about 25% greater than in younger adults, most likely owing to differences in renal elimination. Morbid obesity (body mass index ≥40 kg/m²) does not alter oseltamivir pharmacokinetics.¹³⁴ The effects of pregnancy on the pharmacokinetics of oseltamivir are unclear. One study reported no differences among women in the third trimester of pregnancy and historical controls,¹³⁵ whereas another reported a 25% to 30% reduction in systemic (AUC_0-12 hr) oseltamivir-carboxylate exposure in pregnant women compared with concurrent nonpregnant controls, perhaps suggesting a need for 75 mg three times a day of oseltamivir for treatment.¹³⁶

Plasma protein binding of the prodrug (42%) and the carboxylate (<3%) is low.¹²⁷ The V_d is moderate (23 to 26 L). In animals, lower respiratory tract levels are similar to or exceed the levels in blood¹³⁷; and in humans, the carboxylate is detectable in middle ear and maxillary sinus fluid at concentrations similar to those in plasma.¹³⁸

Oseltamivir concentrations occur in breast milk.¹³⁹ In the ex vivo human placenta model, oseltamivir was extensively metabolized to the carboxylate moiety, but transplacental passage of oseltamivir carboxylate occurred at a low rate, inferring that fetal exposure during maternal treatment with oseltamivir may be minimal.¹⁴⁰ No carboxylate was detected in cerebrospinal fluid in one child,¹⁴¹ whereas C_max values in cerebrospinal fluid were 2.1% and 3.5% for corresponding plasma concentrations for oseltamivir and oseltamivir carboxylate in eight healthy adults after ingestion of 150 mg of oseltamivir.¹⁴² After oral oseltamivir, the plasma of the carboxylate averages 6 to 10 hours in healthy adults. The prodrug and carboxylate are excreted primarily unchanged through the kidney; the carboxylate is eliminated by glomerular filtration and tubular secretion via a probenecid-sensitive anionic transporter. Clearance varies linearly with CrCl, such that increases to 22 hours in patients with CrCl less than 30 mL/min, and dosage reductions are needed.¹²⁷ Oseltamivir carboxylate is removed with different degrees of efficiency by different renal replacement therapies (peritoneal, hemodialysis, and continuous renal replacement therapies). Doses of oseltamivir for patients with renal impairment receiving renal replacement therapy have been published.¹⁴³

Uncomplicated influenza illness does not seem to alter the pharmacokinetics of oseltamivir.¹²⁷ Cystic fibrosis patients appear to clear oseltamivir carboxylate more rapidly than patients who do not have the disease.¹⁴⁴

Interactions

Probenecid reduces renal clearance of oseltamivir by about 50%.¹⁴⁵ Few other clinically important drug interactions have been recognized. Sotalol appeared to induce a torsades de pointes cardiac arrhythmia during oseltamivir therapy for influenza.¹⁴⁶ Specific studies have found no interactions with antacids, acetaminophen, or aspirin or known inhibitors of selected renal tubular secretion pathways, amoxicillin, cimetidine, cyclosporine, mycophenolate, tacrolimus,127, 147 warfarin,¹⁴⁸ or rimantadine.¹⁴⁹

Toxicity

Preclinical studies have found no evidence of mutagenic, teratogenic, or oncogenic effects. High-dose oseltamivir causes renal tubular mineralization in mice and maternal toxicity in rabbits. It is classified as pregnancy category C.

Oseltamivir is generally well tolerated in patients of all ages, including pregnant women and fetuses,150, 151 and no serious end-organ toxicity has been recognized.97, 152, 153, 154 Oral administration is associated with nausea, epigastric distress, or emesis in 10% to 15% of adults receiving 75 to 150 mg twice daily. These gastrointestinal complaints are usually mild to moderate in intensity, resolve despite continued dosing, and are ameliorated by administration with food. Nausea and vomiting (and possibly, dizziness) are dose related in adults.¹⁵⁵ Discontinuation rates of 1% to 2% were observed in controlled treatment studies. The mechanism of nausea and vomiting is uncertain, but the risk seems to be lower in older adults. Long-term prophylaxis has not been associated with an increased risk for adverse events,97, 156 although headache may occur in older recipients. Self-injury, delirium, and psychiatric illness have been reported in patients, primarily pediatric or adolescent, with influenza treated with oseltamivir, mostly in Japan.¹⁵⁷ Analyses of neuropsychiatric reactions among patients with influenza treated with oseltamivir in three large U.S. administrative databases did not demonstrate such an association.158, 159, 160 The decline in cases in Japan after a regulatory recommendation to restrict oseltamivir use in children 10 to 19 years of age has been associated with a decline in oseltamivir-related cases but a corresponding rise in cases associated with zanamivir, the inhaled, minimally systemically bioavailable neuraminidase inhibitor. The latter fact raises further doubts about a causal association between oseltamivir therapy and neuropsychiatric and behavioral adverse reactions in patients with influenza.¹⁶¹ Erythematous rashes and rare instances of severe eruptions or Stevens-Johnson syndrome, hepatic inflammation, hemorrhagic colitis, anaphylaxis, and thrombocytopenia have been reported, but their relationship to oseltamivir is uncertain.

Clinical Studies

Oseltamivir is efficacious for the prevention and treatment of influenza A and B virus infection. In the United States, it is approved for the prevention of influenza in patients 1 year and older and the treatment of acute uncomplicated influenza in patients 2 weeks of age and older who have been symptomatic for no more than 2 days.¹³³

In early clinical experiments in volunteers with induced influenza it was demonstrated that oral oseltamivir is highly protective against experimental human influenza, and early treatment is associated with reductions in viral titers, symptoms, nasal cytokines, and middle ear pressure abnormalities.⁹⁷ Subsequent controlled trials in patients—mostly healthy adults and children with naturally acquired seasonal influenza A infection—demonstrated that early oseltamivir treatment of acute influenza reduces the time to illness alleviation by 1 to days, fever duration, and viral titers in the upper respiratory tract.112, 162, 163, 164 Earlier treatment maximizes the speed of resolution of illness.¹⁶⁵ Treatment of children reduces the risk for otitis media and decreases overall antibiotic use.¹¹² In healthy and high-risk adults, early treatment has been reported to decrease the risk for lower respiratory tract complications leading to antibiotic administration and to hospitalization,¹⁶⁶ but this has been questioned.¹⁶⁷ A meta-analysis of observational studies of high-risk patients with seasonal influenza concluded that oseltamivir treatment may reduce hospitalization, whereas treatment of hospitalized patients reduces respiratory failure, intensive care unit admission, and mortality.168, 169 A recent meta-analysis based on a large number of observational data from individual cases suggested that oseltamivir treatment may be associated with a reduction in mortality risk.^169a However, a Cochrane analysis did not conclude that the evidence indicated that oseltamivir treatment reduced complications or hospitaliza­tions.^169b In hospitalized patients with infection with influenza A (H1N1)pdm09, oseltamivir provides similar benefits even if treatment is started more than 48 hours after clinical illness has begun.170, 171, 172

It is uncertain to what extent oseltamivir treatment may reduce transmission, although a review of four trials of prophylaxis suggests that oseltamivir may have reduced transmission.173, 174

Oseltamivir is less efficacious for the treatment of influenza B than for influenza A virus infection in children175, 176 and adults.¹⁷⁶ An analysis of 284 cumulated cases of influenza A (H5N1) infections in a global registry demonstrated that crude mortality was significantly less in those treated with oseltamivir (40%) than in those not treated (76%) when started up to 6 to 8 days after symptoms onset.¹⁷⁷

Oseltamivir treatment of hematopoietic stem cell transplant recipients with influenza may prevent the development of pneumonia and virus shedding, thereby both preventing influenza-related death in index patients and nosocomial transmission to others.¹⁷⁸ Of 21 patients with leukemia who developed influenza and were treated with oseltamivir, none died, compared with 3 of 8 who were not treated.¹⁷⁹

Prophylactic administration of once-daily oral oseltamivir (75 mg) is highly effective in reducing the risk for developing febrile illness during influenza season in unimmunized adults (efficacy 84%),¹⁸⁰ immunized nursing home residents (efficacy 92%),¹⁸¹ and transplant recipients (efficacy 80%).¹⁵⁶ Prevention of influenza may reduce secondary complications in institutionalized older adults.¹⁸¹ Once-daily oseltamivir for 7 to 10 days is also effective for postexposure prophylaxis in household contacts, including children, and when ill index cases receive concurrent treatment.182, 183 Oseltamivir chemoprophylaxis has been used to control institutional outbreaks of influenza A continuing despite M2 inhibitor use and of influenza B.¹⁸⁴

Peramivir

Spectrum

Peramivir ([1S,2S,3S,4R]-3-[(1S)-1-(acetylamino)-2-ethylbutyl]-4-[(aminoiminomethyl)amino]-2-hydroxy-cyclopentanecarboxylic acid; Rapiacta) (see Fig. 44-3C) is an investigational agent in the United States but is approved in Japan, China, and South Korea. It is a potent, selective inhibitor of influenza A and B virus neuraminidases, including that of all nine avian NA subtypes¹⁸⁵ and influenza A (H1N1)pdm09.¹⁸⁶ It is a sialic acid analogue designed to be structurally distinct from oseltamivir and zanamivir such that cross-resistance to it among oseltamivir-resistant and zanamivir-resistant strains is not consistently observed.187, 188 Like oseltamivir and zanamivir, peramivir inhibits influenza neuraminidase in enzyme assays at nanomolar concentrations¹⁸⁹ and requires micromolar concentrations to inhibit influenza replication in cell culture.¹⁹⁰ It is a more potent inhibitor of influenza A than B viruses in vitro than is oseltamivir or zanamivir.¹⁹⁰ The clinical relevance of this difference has not yet been evaluated. Combination treatment of influenza A virus infection in cell culture and in mice with peramivir and ribavirin yields additive or synergistic interactions with no increase in toxicity.¹⁹¹ The antiviral effect of combinations of peramivir plus rimantadine in vitro is variable, ranging from additive to synergistic.¹⁹² In mice with experimental influenza infections, the combination of peramivir and rimantadine is synergistic.¹⁹³ In murine and ferret models of influenza infection, peramivir is effective when administered intranasally,¹⁹⁴ orally,¹⁹⁵ and intramuscularly.¹⁹⁶

Mechanism of Action

See previous discussion of mechanism of action under “Oseltamivir.”

Resistance

Peramivir-resistant influenza virus has been selected in vitro,187, 188, 197, 198 but not from peramivir-treated mice with experimental influenza infection,¹⁹⁹ healthy volunteers given peramivir for prevention or treatment of experimentally induced influenza A or B infection, or healthy treated patients.²⁰⁰ A peramivir-resistant virus possessing the H275Y mutation emerged during intravenous therapy for pandemic 2009 influenza A/H1N1 in an immunocompromised patient.²⁰¹ Peramivir-resistant mutants generated in vitro may possess unaltered or diminished virulence and replicative capacity in mice and ferrets.²⁰² Peramivir resistance associated solely with an alteration in the hemagglutinin gene conferred cross-resistance to oseltamivir and zanamivir and could cause lethal disease in mice. Infection with the resistant virus in mice was still amenable to peramivir therapy, however.¹⁹⁸

Naturally occurring oseltamivir-resistant influenza viruses possessing the H275Y mutation have a 100-¹⁸⁶ to 661-fold²⁰² reduced susceptibility to peramivir, less than that of oseltamivir (982-fold). Studies suggest that infection due to viruses possessing the H275Y mutation may be successfully treated with higher dose regimens of injected peramivir in mice²⁰³ and high-risk patients.²⁰⁴ However, intravenous peramivir was not more effective than oseltamivir in a case report²⁰¹ and in an observational study of influenza caused by H275Y mutant strains.²⁰⁴ In 2009, the World Health Organization recommended that for treatment of infection due to influenza A (H1N1)pdm-09 strains possessing the H275Y mutation, intravenous peramivir is likely to be suboptimal and intravenous zanamivir is preferred.²⁰⁵

Pharmacokinetics

The absolute oral bioavailability of peramivir is 2%.²⁰⁶ As a result, clinical development has focused on its efficacy and safety after intramuscular and intravenous injection. Fortunately, its long elimination half-life supports single-dose intravenous treatment regimens. At doses up to 2 mg/kg in adults, plasma and AUC_0-α increase in proportion to dose. At higher doses of greater than 2 mg/kg being used in clinical trials in adults, the plasma in healthy adults is approximately 20 hours, which supports single-dose treatment; apparent V_d is approximately 2 L/kg, and systemic clearance is 85 mL/hr/kg. The corresponding values for children with mean age of 9 years are 7.7 hours, 0.3 L/kg, and 173 mL/hr/kg.²⁰⁷ The physiologic counterpart of this large V_d is unknown, because no locus of drug sequestration has been identified. Plasma protein binding is less than 30%. Peramivir concentrations in plasma are 10-fold to 50-fold higher than concurrent levels in nasal wash or pharyngeal gargle solutions.²⁰⁶ Peramivir is detectable at these sites 24 hours after dosing, at concentrations greater than levels that inhibit neuraminidases of most strains of influenza virus. The clinical relevance of these data is unknown.

A 300-mg dose injected intravenously once in young healthy adults with influenza illness of less than 48 hours' duration is efficacious and well tolerated.²⁰⁸ Infusion of this dose over a median of 38 minutes produced median plasma concentrations of 18,100 ng/mL at the end of the infusion and 14.8 ng/mL 18 to 24 hours later. The 600-mg dose yielded corresponding values of 36,300 and 32.8 ng/mL. The median inhibitory concentration for 50% of isolates (IC₅₀) for the neuraminidase of the patient viruses ranged from 1.15 nmol/L for influenza A/H1N1, 1.36 nmol/L for influenza A/H3N2, and 2.81 nmol/L for influenza B isolates.²⁰⁴ In pediatric patients 1 month to 15 years of age infected with influenza A (H1N1)pdm09, an intravenous infusion of 10 mg/kg once daily produced comparable plasma concentrations to those seen in young healthy adults (see earlier): median peramivir plasma concentrations were 33,150 ng/mL at the end of the infusion and 20.7 ng/mL 18 to 24 hours later.²⁰⁷ The relationship of these plasma concentration data to efficacy is unclear. In mice, plasma AUC of peramivir is the pharmacokinetic characteristic related to efficacy.²⁰⁹

Data on peramivir distribution into breast milk in humans are unavailable.²⁰⁶ Less than 5% of ¹⁴C-labeled peramivir administered to rats is recovered in breast milk. Peramivir is eliminated unchanged into urine by glomerular filtration, and probenecid does not affect its excretion. In patients with renal insufficiency, mean ranges from 24 to 30 hours in subjects with mean CrCl of 21 to 68 mL/min. In individuals with dialysis-dependent renal failure, averages 79 hours.

In October 2009, the U.S. Food and Drug Administration issued an Emergency Use Authorization (EUA) for the administration of intravenous peramivir for the treatment of hospitalized patients with suspected or confirmed cases of influenza A (H1N1)pdm09 infection, because no other intravenous neuraminidase inhibitor drug was available. In adults with normal renal function, the recommended intravenous dose was 600 mg/day; and in children 6 to 17 years of age it was 10 mg/kg intravenously once daily. Doses for other age groups and patients with renal impairment including end-stage renal disease requiring different renal-replacement therapies have been suggested.²¹⁰ The EUA was terminated in 2010, and peramivir was to be available only through clinical trials (see “Clinical Studies,” later).

Interactions

Adverse drug-drug interactions have not been reported in subjects given peramivir, but the number of individuals exposed is still modest. No pharmacokinetic interaction of intravenous peramivir and oral oseltamivir or rimantadine was observed in healthy volunteers.²¹¹ Drug-drug interactions in individuals receiving peramivir are unlikely because it neither induces nor inhibits important drug-metabolizing cytochrome P-450 enzymes.

Toxicity

Peramivir is generally nontoxic and well tolerated. Preclinical studies revealed no genotoxicity, reproductive toxicity, or developmental toxicity.²⁰⁶ In multiple species of animals, the only apparent adverse effect is reversible nephrotoxicity, which is species (rabbit only) and gender (female) specific. The nephrotoxic dose is greater than 200 mg/kg/day intravenously for 9 days.

The largest doses administered to humans, 800 mg orally²⁰⁰ and 600 mg intravenously,²⁰⁸ have not been associated with consistent adverse symptoms or laboratory abnormalities compared with placebo.²⁰⁸ In placebo-controlled clinical trials of peramivir orally up to 800 mg/day for 4 to 5 days,²⁰⁰ 300 mg/day intramuscularly once,²¹² and 600 mg intravenously once,²⁰⁸ adverse symptoms were not reported more frequently in peramivir recipients than in placebo recipients.

In controlled, blinded trials as well as uncontrolled studies of intravenous peramivir, it has been generally well tolerated and safe. In a randomized, double-blind study comparing a single dose of peramivir of 300 or 600 mg and a matching placebo given intravenously to 300 young healthy adults in an outpatient setting,²⁰⁸ nausea may have been reported more frequently in drug recipients (3.0%, 6.1%, and 1.0%, respectively, in the three groups). Extensive blood and urine laboratory tests revealed no differences among groups. In a randomized, double-blind, double-dummy trial in young healthy adults with influenza treated with peramivir, 300 mg and 600 mg intravenously once, or oseltamivir, 75 mg orally twice daily for 5 days, the overall incidence of adverse effects was lowest in the 300-mg group: 14.0% compared with 18.1% and 20.0% in the other groups, respectively. Diarrhea (3.8%, 5.5%, and 5.2%), nausea (0.5%, 1.9%, and 4.4%), and a decreased neutrophil count (2.5%, 3.8%, and 3.6%) all tended to be lowest in the 300-mg peramivir group.²¹³ In a randomized, unblinded study in hospitalized patients treated for influenza with intravenous peramivir at 200 or 400 mg once daily, or oseltamivir at 75 mg orally twice daily, all for 5 days, the “incidence of adverse events was low and generally similar among treatment groups.”²¹⁴

Assessment of side effects of intravenous peramivir in uncontrolled studies in hospitalized adults with high-risk comorbid conditions²⁰⁴ also suggested that the drug was generally well tolerated. A single case of dilated cardiomyopathy or myocarditis in a volunteer infected with an influenza B challenge virus and treated with peramivir has been reported.²⁰⁰ The relationship of the cardiac disorder to the drug is unknown.

Clinical Studies

In a study in serosusceptible volunteers, peramivir prophylaxis with 50 to 800 mg orally daily or placebo, initiated 24 hours before influenza A or B virus challenge and continued for 5 days, tended to prevent illness at doses of 200 mg or greater and to reduce viral shedding and titer in nasal washings in subjects inoculated with influenza A virus. No effect on preventing illness caused by influenza B virus was observed, although the duration of virus shedding tended to be less in individuals receiving 400 mg and 800 mg of peramivir.²⁰⁰

In early studies in patients with influenza, oral peramivir therapy with doses of 400 to 800 mg daily for 5 days²⁰⁰ and single intramuscular doses of 150 or 300 mg²⁰⁶ reduced median times to relief of symptoms, but the differences were not statistically significant from controls. Subsequently, controlled trials with an intravenous formulation demonstrated peramivir therapeutic efficacy and tolerance in patients with influenza due to susceptible virus strains. Peramivir treatment of naturally acquired influenza in young adults with illness of 48 hours' duration or less with 300 or 600 mg injected once intravenously versus placebo, reduced median time to relief of symptoms significantly from 82 hours in the placebo group to 59 hours and 60 hours in the peramivir 300-mg and 600-mg groups in the outpatient setting.²⁰⁸ Peramivir treatments also significantly reduced the proportion of subjects still excreting virus in nasal and throat secretions at day 3 from 51% in placebo recipients to 26% to 37% in those treated with peramivir, 600 or 300 mg, respectively.²⁰⁸

In 137 hospitalized patients randomized to 5 days' treatment with intravenous peramivir, 200 or 400 g/day, compared with historical reports with oral oseltamivir at 75 mg twice daily, the reduction in virus concentration in nasopharyngeal secretion was similar across the three treatments.²¹¹ An additional study utilizing a higher dose of peramivir (300 mg twice daily or 600 mg four times a day) in 234 hospitalized patients also showed no differences in virologic or clinical end points among the peramivir-treated regimens or compared with reported patients treated with oseltamivir.²¹⁵ A randomized controlled study of intravenously administered peramivir plus standard of care versus standard of care alone in patients hospitalized with influenza was recently terminated because of futility to show a difference between the peramivir and control groups.215, 216

As noted earlier, intravenous peramivir is probably not effective for treatment of patients with oseltamivir resistance due to possession of the H275Y mutation; intravenous zanamivir has been recommended.

Ribavirin

Spectrum

Ribavirin (1-β-d-ribofuranosyl-1,2,4-thiazole-3-carboxamide; Virazole, Rebetol, Copegus) is a guanosine analogue (Fig. 44-4A ) in which the base and the d-ribose sugar are necessary for antiviral activity. Ribavirin inhibits the in vitro replication of a wide range of RNA and DNA viruses, including myxoviruses, paramyxoviruses, arenaviruses, flaviviruses, bunyaviruses, coronaviruses, togaviruses, reoviruses, herpesviruses, adenoviruses, poxviruses, and retroviruses. By plaque assay, inhibitory concentrations range from 3 to 10 µg/mL for influenza, parainfluenza, and respiratory syncytial virus (RSV). High concentrations inhibit group C adenoviruses²¹⁴ and pathogenic flaviviruses,²¹⁷ including West Nile virus in neural cells. Ribavirin does not inhibit severe acute respiratory virus (SARS) coronavirus in vitro.²¹⁸

FIGURE 44-4.

Chemical structure of ribavirin (A) and the nucleoside guanosine (B).

Low concentrations of ribavirin (1 to 10 µg/mL) reversibly inhibit macromolecular synthesis and the proliferation of rapidly dividing cells.²¹⁹ Ribavirin decreases nucleic acid and protein synthesis, inhibits interferon-γ release, and increases apoptosis in human peripheral blood mononuclear cells in vitro,218, 220 but it does not adversely affect polymorphonuclear leukocyte functions.²²¹ Ribavirin has been postulated to enhance cell-mediated immune responses by increasing type 1 and suppressing type 2 cytokine responses in T cells²²¹ and to decrease proinflammatory cytokine elaboration and inflammatory cell numbers. Inhibition of mast cell secretory responses occurs in vitro.

Aerosol administration is more effective than parenteral dosing in animal models of influenza and RSV infection. Parenteral ribavirin has antiviral and therapeutic activity in animal models of infection with Lassa virus, other arenaviruses, and bunyavirus (see Chapters 47, 168, and 169Chapter 47Chapter 168Chapter 169). Combinations of ribavirin with immunoglobulin in RSV infection and with M2 or neuraminidase inhibitors in influenza A infection or with neuraminidase inhibitors in influenza B infection show enhanced antiviral activity.¹² The use of ribavirin in treatment of hepatitis B and C is discussed in Chapter 46.

Mechanism of Action

The antiviral mechanisms of action of ribavirin are complex and most likely vary for different viruses. Ribavirin causes alterations of cellular nucleotide pools, inhibits viral RNA synthesis, and may cause lethal mutagenesis of certain RNA virus genomes.221, 222, 223 Intracellular phosphorylation to the monophosphate, diphosphate, and triphosphate derivatives is mediated by host cell enzymes. In uninfected and RSV-infected cells, the predominant derivative (>80%) is the triphosphate, which is rapidly lost, with an intracellular of less than 2 hours.

Ribavirin monophosphate competitively inhibits inosine monophosphate dehydrogenase and interferes with the synthesis of guanosine triphosphate (GTP) and with nucleic acid synthesis. Decreased concentrations of competing GTP likely potentiate ribavirin's other antiviral effects. Ribavirin triphosphate inhibits influenza virus RNA polymerase activity and the GTP-dependent 5′-capping of viral mRNA. The monophosphate is incorporated inefficiently into viral RNA genomes, and this may lead to lethal mutagenesis and contribute to antiviral activity.²²² HCV RNA polymerase incorporates ribavirin monophosphate into viral RNA, which causes mutations and inhibits viral RNA synthesis.²²⁴ Ribavirin diphosphates and triphosphates also inhibit human immunodeficiency virus (HIV) reverse transcriptase activity.²²⁵

Ribavirin has immunosuppressive effects in experimental animals and shows therapeutic activity against transplantable virus-induced tumors and certain autoimmune diseases. Ribavirin increases type 1 cytokine–mediated immune responses in vivo, an effect that may contribute to its therapeutic activities,²²¹ and seems to augment type-1 cytokine responses ex vivo in peripheral blood mononuclear cells from patients with chronic hepatitis C.²²³

Resistance

Antiviral resistance to ribavirin has been documented only in Sindbis virus and HCV to date. One HCV RNA polymerase variant (F415Y) selected in genotype 1a–infected, ribavirin-treated patients has been associated with ribavirin resistance in vitro.²²⁶ No ribavirin-resistant RSVs have been detected during aerosol therapy of children.

Pharmacokinetics

Oral ribavirin is well absorbed, but bioavailability averages 45% to 65% in adults because of first-pass metabolism.227, 228, 229, 230 Administration with food increases absorption and peak plasma concentrations by 70%.²²⁷ After single oral doses of 600 mg, 1200 mg, or 2400 mg, peak plasma concentrations occur at 1 to 2 hours and average 1.3 µg/mL, 2.5 µg/mL, and 3.2 µg/mL. Plasma concentrations average approximately 24 µg/mL and 17 µg/mL after intravenous doses of 1000 mg and 500 mg in patients with Lassa fever. During long-term administration, overall exposure and increase substantially.²²⁷ Steady-state plasma levels of about 1 to 4 µg/mL occur by about 4 weeks with weight-adjusted dosing in chronic hepatitis C, and higher concentrations at 4 weeks correlate with decline in hemoglobin and likelihood of sustained viral responses.²³¹ Plasma protein binding is negligible, and ribavirin has a large V_d (>2000 L). At steady state, cerebrospinal fluid levels are about 70% of those in plasma.²²⁹

The disposition of ribavirin is complex, involving renal elimination and metabolism. After rapid initial distribution, there is a prolonged terminal of 37 to 79 hours.227, 228, 229 Ribavirin triphosphate concentrates in erythrocytes with an erythrocyte-to-plasma ratio of 40 : 1 or greater, and erythrocyte levels gradually decrease, with an apparent of 40 days. Renal excretion accounts for 30% to 60% of ribavirin's overall clearance, but hepatic metabolism is contributory. About 5% to 10% is recovered unchanged in the urine, and a much greater fraction is excreted as triazole carboxamide and carboxylic acid metabolites.²²⁷ Plasma clearance is reduced threefold in patients with advanced renal impairment (CrCl ≤30 mL/min). Dosage adjustments are needed for renal insufficiency, and ribavirin should be used with caution in patients with CrCl less than 50 mL/min. Hemodialysis and hemofiltration remove small amounts of drug. Higher initial blood levels occur in severe hepatic dysfunction.²³⁰

With aerosol administration, systemic absorption is low (<1% of deposited dose). Peak plasma levels range from 0.5 to 2.2 µg/mL after 8 hours' exposure and from 0.8 to 3.3 µg/mL after 20 hours in pediatric patients. Respiratory secretion levels often exceed 1000 µg/mL and persist with a of 1.4 to 2.5 hours. A special aerosol generator (SPAG-2, ICN Pharmaceuticals) is needed to produce particles of proper aerodynamic size to reach the lower respiratory tract. The delivered dose is twice as high in infants (1.8 mg/kg/hr) than in adults.

Toxicity

Systemic ribavirin causes dose-related anemia because of extravascular hemolysis and, at higher dosages, suppression of bone marrow release of erythroid elements.²³² Reversible increases of serum bilirubin (in one fourth of recipients), serum iron, and uric acid concentrations occur during short-term oral administration. Long-term use of oral ribavirin at dosages greater than 800 mg daily causes hemoglobin decreases of 2 to 4 g/dL in most recipients, usually within 4 weeks. When used in combination with interferon, hemoglobin levels less than 11 g/dL develop in 25% to 30% of patients.²³³ Renal impairment increases the risk for hemolysis. Severe anemia requires dosage reduction or cessation, although erythropoietin has been used effectively.²³³ Other reported side effects include pruritus, myalgia, rash, nausea, depression, nervousness, and cough or respiratory symptoms.²³⁴ High-dose intravenous ribavirin is associated with headache, hypomagnesemia, and hypocalcemia.²³⁵ Bolus intravenous dosing may cause rigors, and infusion over 10 to 15 minutes is advised.

Aerosolized ribavirin may cause conjunctival irritation, rash, bronchospasm, reversible deterioration in pulmonary function, and, rarely, acute water intoxication. No adverse hematologic effects have been associated with aerosolized ribavirin. The drug may precipitate on contact lenses, so they should not be worn during aerosol exposure. Ribavirin exposure may occur in health care workers working in the environment of aerosol-treated infants.235, 236 Health care worker exposure is higher during delivery by oxygen hood than by ventilator or vacuum-exhausted hood systems.²³⁵ Use of aerosol containment and scavenging systems, turning off the aerosol generator before providing routine care, and use of personal protective equipment have been recommended.²³⁶

When ribavirin is used in conjunction with mechanical ventilation, in-line filters, modified circuitry, and frequent monitoring are required to prevent plugging of ventilator valves and tubing with precipitates of ribavirin. The possible effects of such modifications on drug delivery to the lower respiratory tract are undefined.

In preclinical studies, ribavirin is mutagenic, gonadotoxic, and teratogenic.²³² Low oral dosages have been teratogenic or embryotoxic in multiple species. Use of ribavirin is relatively contraindicated during pregnancy, and pregnant women should not directly care for patients receiving ribavirin aerosol. Ribavirin is categorized as pregnancy category X, and effective means of contraception for men and women are recommended for at least 6 months after discontinuation of treatment or exposure.

Interactions

Antacids slightly decrease the oral bioavailability of ribavirin. During co-administration clinically, ribavirin, amantadine, and oseltamivir do not interact pharmacokinetically.³⁴ Ribavirin antagonizes the anti-HIV-1 effects of zidovudine but enhances the activity of purine dideoxynucleosides. Ribavirin use in patients who are coinfected with HIV and HCV and on antiretroviral drugs, particularly combined with didanosine, seems to increase the risk for mitochondrial toxicity and lactic acidosis. Ribavirin may inhibit the effect of warfarin.

Clinical Studies

Ribavirin aerosol is approved in the United States for treatment of RSV bronchiolitis and pneumonia in hospitalized children. Oral ribavirin in combination with various interferons is approved for treatment of chronic hepatitis C. The following describes only clinical studies on the prevention and treatment of respiratory virus infection with ribavirin. Treatment for infection with HCV is discussed elsewhere (see Chapters 46, 119, and 156Chapter 46Chapter 119Chapter 156).

Respiratory Syncytial Virus

Aerosolized ribavirin (18-hour exposure daily for 3 to 6 days) variably shortens the duration of virus shedding and may improve certain clinical measures in infants hospitalized with RSV illness.²³⁸ No consistent reductions in need for ventilatory support or duration of hospitalization have been documented, however. In infants receiving mechanical ventilation for RSV-related respiratory failure, no significant reductions in duration of ventilatory support, hospitalization, or mortality have been found.238, 239 Intermittent, high-dose therapy (2-hour exposures three times daily for 5 days) is well tolerated and may be as effective as prolonged exposure.²⁴⁰

Use of aerosolized ribavirin is limited by concerns regarding its efficacy, ease of administration, risk of occupational exposure, and cost. The American Academy of Pediatrics states that aerosol treatment for RSV infection “is not recommended for routine use but may be considered for use in selected patients with documented, potentially life-threatening RSV infection.”²⁴¹ Decreased RSV-specific serum neutralizing antibody titers and diminished nasopharyngeal secretion RSV-specific IgE and IgA responses may occur in ribavirin-treated children. No long-term adverse or beneficial effects of ribavirin therapy have been documented in children.²⁴²

Combinations of aerosolized ribavirin and intravenous immunoglobulin or palivizumab may be beneficial in treating RSV pneumonia in hematopoietic stem cell transplant recipients,243, 244, 245, 246 whereas intravenous ribavirin alone is ineffective.²⁴⁷ Therapy with either aerosolized248, 249, 250 or oral251, 252 ribavirin appears to prevent progression from upper to lower respiratory tract illness in such patients. A similar benefit of preemptive treatment of RSV upper respiratory tract infection in lung transplant recipients with oral and inhaled ribavirin has been reported.253, 254

Other Respiratory Viruses

Intravenous and aerosolized forms of ribavirin have been used to treat severe influenza virus infections.255, 256 Aerosolized ribavirin inconsistently reduces viral titers and illness measures in adults with uncomplicated influenza A or B and has modest efficacy in children hospitalized with influenza.²⁵⁷ However, oral ribavirin 300 mg three times per day combined with amantadine and oseltamivir may possibly be effective for treatment of influenza A (H1N1)pdm09 disease and more so than oseltamivir alone.²⁵⁸ Oral, intravenous, and aerosolized ribavirin have been used in immunosuppressed patients with severe parainfluenza virus and adenovirus infections with inconsistent clinical benefits.255, 259, 260 Intravenous ribavirin has been used to treat adenovirus-associated hemorrhagic cystitis, pneumonia, and invasive infections in immunocompromised patients, and it may be effective even in severe disease.261, 262 Treatment with intravenous263, 264 and oral²⁶⁵ ribavirin of human metapneumovirus pneumonia in immunocompromised patients has been associated with resolution. Aerosolized ribavirin has been used in treating parainfluenza virus infections in solid-organ transplant recipients, but seems ineffective in parainfluenza virus pneumonia in hematopoietic stem cell transplant recipients.²⁵⁹ Oral ribavirin was effective in accelerating functional graft recovery and reducing late bronchiolitis obliterans in 38 lung transplant recipients²⁶⁶ and in a bone marrow transplant recipient²⁶⁷ with paramyxovirus respiratory infection. Intravenous ribavirin therapy was associated with successful treatment of paramyxovirus type 3 respiratory infection in cardiac transplant recipients.268, 269 Ribavirin has been used extensively in treating SARS coronavirus infections without proven antiviral effects in vitro²¹⁸ or in patients²⁷⁰ and has been associated with frequent adverse effects.²³⁵ Intravenous ribavirin seems to be ineffective in treatment of hantavirus cardiopulmonary syndrome.²⁷¹ However, it inhibits Andes virus in vitro, an important cause of this syndrome, and is effective in a hamster model of hantavirus cardiopulmonary syndrome caused by this virus (see Chapter 168).²⁷²

RSV604

RSV604 is an oral benzodiazepine compound (C₂₂H₁₇FN₄O₂) under development for treatment of RSV infections (Fig. 44-5 ).273, 274 It inhibits both RSV A and B subtypes at submicromolar concentrations. Its antiviral activity is expressed through interaction with the RSV nucleocapsid (N) protein, which is highly conserved.²⁷⁵ The drug is well absorbed orally, and a single dose per day is sufficient to achieve antiviral EC₉₀ levels. In vitro resistance can be elicited, at an apparent low rate, and resistant virus appears similarly fit to wild-type RSV in terms of replication.²⁷⁵ Phase II studies of RSV604 are underway in transplant patients with RSV infections.

FIGURE 44-5.

Chemical structure of RSV604.

Zanamivir

Spectrum

Zanamivir (4-guanidino-2,4-dideoxy-N-acetylneuraminic acid; Relen­za) is a sialic acid analogue (see Fig. 44-3B) that is a potent and specific inhibitor of the neuraminidases of influenza A and B viruses.²⁷⁶ It inhibits influenza neuraminidase activity at nanomolar concentrations but has a higher and broader range of inhibitory concentrations in cell culture.277, 278 Compared with oseltamivir carboxylate, zanamivir is more active against influenza B, and data from a comparative trial in children indicate that this difference is clinically important.²⁷⁹ Zanamivir is less active against neuraminidases of influenza A/N2 clinical isolates,²⁸⁰ but the clinical importance of this difference is uncertain. Zanamivir inhibits certain influenza A neuraminidase variants that are resistant to oseltamivir carboxylate.⁹⁶ Combinations of zanamivir plus rimantadine inhibit strains of influenza A/H1N1 and H3N2 viruses synergistically, but some concentrations seem antagonistic when assessed by reductions in cell-associated virus yield.²⁸¹ Zanamivir is not cytotoxic and is highly selective for influenza neuraminidase, inhibiting neuraminidases from human²⁸² and other mammalian sources or other pathogens only at 10⁶-fold higher concentrations. Millimolar concentrations inhibit parainfluenza virus type 3 in cell culture, most likely by blocking attachment.²⁸³ Topical zanamivir in the respiratory tract is active in murine and ferret models of influenza.²⁷⁷

Resistance

Resistance to neuraminidase inhibitor drugs has developed less frequently than to the adamantane compounds and less frequently to zanamivir than to oseltamivir.

A recent systematic review of the prevalence of neuraminidase inhibitor resistance among influenza viruses cultured from immunocompetent ambulatory adults enrolled in prophylactic and therapeutic trials of zanamivir found no reports of zanamivir resistance.²⁸⁴ In surveys of other collections of influenza isolates, a similar absence or dearth of zanamivir resistance was reported: influenza A H1N1 viruses circulating in the 2008-2009 influenza season in the United States prior to emergence of the 2009 pandemic were resistant to oseltamivir but susceptible to zanamivir.²⁸⁵ Among 391 nonpandemic A/H1N1 isolates from Australia and Southeast Asia patients from 2006 to 2008, 2.3% were resistant to zanamivir²⁸⁶ but susceptible to oseltamivir. Zanamivir resistance was not demonstrated among 3359 influenza A (H1N1)-pdm09 global isolates²⁸⁷ nor among 304 oseltamivir-resistant isolates reported by the World Health Organization to August 2010.²⁸⁸ Avian influenza A/H5N1 isolates from 2003 to 2005 were susceptible to zanamivir.²⁸⁹ Of 680 influenza B viruses isolated in China from 2010 and 2011, one with D197N amino-acid substitution was resistant to zanamivir.²⁹⁰

Several neuraminidase mutations mediate diminished susceptibility to zanamivir: Q136K in an A (H1N1) seasonal virus (300-fold reduction in zanamivir susceptibility)²⁸⁶ and S274N²⁹¹ in nonpandemic A/H1N1 virus and I223R (5-fold reduction)²⁹² in an A (H1N1)pdm09 isolate. The relationship of these virus resistance mutations and prior zanamivir therapy and immune competence was not consistently apparent. An influenza B virus with an Arg152Lys mutation resistant to both zanamivir and oseltamivir was recovered from an immunocompromised child with prolonged virus excretion despite receipt of nebulized zanamivir.²⁹³ The effect of these neuraminidase mutations on infectivity and transmissibility compared with the wild-type parental strains is variable, but only some mutants have been characterized in this regard.293, 294, 295

An observational study in pediatric patients with influenza treated with oseltamivir or zanamivir suggested that the lower prevalence of zanamivir than oseltamivir resistance is more related to the intrinsic properties of the drugs than to differences in the prevalence of use of the drugs.²⁹⁶

Pharmacokinetics

The oral bioavailability of zanamivir is low (<5%). The approved formulation is a dry powder containing a lactose carrier delivered by oral inhalation with a proprietary Diskhaler device. The proprietary inhaler device for delivering zanamivir is breath activated and requires a cooperative, trained patient. The use of the Diskhaler device is unreliable in young children, very infirm or elderly patients, or cognitively impaired patients. Although the inhaler has been used effectively in many older adults,²⁹⁷ more than half of hospitalized older adults could not correctly use the device after instruction.²⁹⁸

After inhalation of the dry powder using the Diskhaler, approximately 15% is deposited in the lower respiratory tract while the remainder is deposited in the oropharynx.²⁷⁷ Zanamivir concentrations in epithelial lining fluid obtained by bronchoalveolar lavage may approximate concentrations in alveoli. Median epithelial lining fluid concentrations of zanamivir 12 hours after oral inhalation of the recommended 10-mg dose by Diskhaler in healthy volunteers ranged from 0.3 to 0.9 µg/mL.²⁹⁹ In other uninfected individuals, median zanamivir levels in induced sputum were 1.34 µg/mL, 0.30 µg/mL, and 0.05 µg/mL at 6 hours, 12 hours, and 24 hours after dosing, with the pulmonary estimated to be 2.8 hours.³⁰⁰ Approximately 4% to 17% of an inhaled dose is absorbed systemically, and peak plasma levels are low, averaging 0.04 to 0.05 µg/mL.²⁷⁷ Because of the low bioavailability of zanamivir inhaled orally, dosage adjustments are not indicated in renal insufficiency.

After intravenous dosing, the plasma of zanamivir ranges from 1.6 to 2.9 hours,277, 299 with about 90% eliminated unchanged in the urine.²⁷⁷ After intravenous administration of 600 mg zanamivir to healthy adults, the median serum C_max is 39.4 µg/mL, AUC_0-12 hr is 86.6 µg/mL, and C_trough is 0.6 µg/mL. The median epithelial lining fluid concentration 12 hours after dosing is 0.4 µg/mL, very similar to the value after inhalation of 10 mg (see earlier). This is 552 to 1653 times the in vitro IC₅₀ for influenza A and B neuraminidases, respectively.²⁸⁰

The pharmacokinetics of zanamivir in 103 adults with influenza receiving 600 mg intravenously twice daily with dose adjustments for renal impairment were similar to those in previously described studies.³⁰¹ Zanamivir renal clearance declines linearly with increasing renal impairment.³⁰² The suggested dose for adults with normal renal function is 600 mg intravenously given twice daily. Doses for children and for patients with renal impairment who are or are not receiving replacement therapy have been published.³⁰³

Interactions

No clinically significant drug interactions have been recognized for inhaled zanamivir. No clinically relevant pharmacokinetic interaction was demonstrated between oseltamivir, 150 mg taken orally twice daily, and zanamivir, 600 mg administered intravenously every 12 hours, in healthy volunteers.³⁰⁴ Zanamivir does not affect the immune response to injected inactivated influenza vaccine, but, similar to all antiviral medications, it has the potential to impair the immunogenicity of attenuated live influenza vaccine administered concurrently. Zanamivir should not be administered from 48 hours before to 2 weeks after intranasal administration of an attenuated influenza vaccine.³⁰⁵

Toxicity

Preclinical studies of zanamivir found no evidence of mutagenic, teratogenic, or oncogenic effects. In cell culture, the inhibitory effect of zanamivir on influenza virus replication was not impaired by analgesics, antihistamines, decongestants, or antibacterial drugs.³⁰⁶ Zanamivir is classified as a pregnancy category C agent.

Orally inhaled zanamivir is generally well tolerated, and the frequencies of complaints are not significantly different from those in placebo recipients among adults and children 5 years old or older.277, 306, 307 This includes once-daily oral inhalation for prophylaxis by adults for 16 weeks.³⁰⁸ Most reported symptoms in treatment studies are likely the result of the underlying illness. Similarly, in high-risk patients receiving zanamivir or placebo, no differences in adverse reactions have been seen in controlled trials.³⁰⁹ In patients with mild to moderate asthma or chronic obstructive pulmonary disease, orally inhaled zanamivir is associated with fewer bronchitis episodes, similar measurements of forced expired volume in 1 second, and more rapid improvement in peak expiratory flow rate than with inhaled placebo.³¹⁰ However, postmarketing reports indicate a potential risk for acute bronchospasm, respiratory arrest, or worsening of chronic obstructive pulmonary disease accompanied by pulmonary edema after zanamivir inhalation, particularly in patients with underlying airway disease.³¹¹ Apparent declines in respiratory function have also been rarely reported in patients without recognized airway disease. Consequently, use in patients with underlying airway disease is not generally recommended in the United States, although treatment in at-risk patients is used in other countries.³¹² If used in patients with obstructive airway disease, zanamivir should be administered cautiously under close observation and with availability of fast-acting bronchodilators.

Zanamivir inhaled as an experimental nebulized solution containing 16 mg/mL for 10 minutes four times a day for 5 days for treatment of serious influenza with lower respiratory tract signs in hospitalized patients 10 years or older was well tolerated.³¹³ However, when the oral formulation containing lactose has been reformulated as a solution and administered into the airway during mechanical ventilation, lactose precipitation in the airway filters has caused obstruction,³¹⁴ precluding the reformulation of the powder in the orally inhaled formulation into a solution for nebulization and inhalation.

Zanamivir injected intravenously to healthy volunteers in doses from 50 to 600 mg twice daily for 5 days was also well tolerated.³¹⁵ In 130 hospitalized adults with influenza treated with zanamivir, 600 mg intravenously twice daily for 5 days, or reduced doses in those with renal impairment, no safety signals or clinically significant trends in laboratory values, vital signs, or electrocardiograms were identified that were considered attributable to the drug.³¹⁶

Clinical Studies

Zanamivir has been administered to patients intranasally as a spray, by oral inhalation as a dry powder, by nasal inhalation as an aerosol from a nebulized solution, and by intravenous injections.

Intranasal and intravenous zanamivir are highly protective against experimental human influenza, and early treatment is associated with reductions in viral titers, symptoms, and middle ear pressure abnormalities.163, 277, 317 Orally inhaled zanamivir powder is approved in the United States for prevention of influenza in individuals 5 years old and older, and for treatment of influenza in individuals 7 years old and older. Zanamivir (10 mg twice daily for 5 days) inhaled early in the course of illness for treatment of uncomplicated influenza in previously healthy adults and children 5 to 12 years old shortens the times to illness resolution and return to usual activities by 1 to 3 days.307, 318, 319 Treatment benefits seem to be greater in patients with severe symptoms at entry, in patients older than 50 years, and in higher-risk patients.³²⁰ Inhaled zanamivir treatment in adults is associated with a 40% reduction in lower respiratory tract events leading to antibiotic use and a 28% overall reduction in antibiotic prescriptions.³²¹

Zanamivir inhaled orally is equally efficacious for treatment of influenza A and B infection.319, 322 In individuals with influenza B illness, zanamivir reduces the median duration of fever by 32%, from 53 hours to 36 hours, compared with oseltamivir.²⁷⁹ In high-risk patients with primarily mild to moderate asthma or other chronic cardiopulmonary conditions, orally inhaled zanamivir treatment reduces illness duration and the incidence of complications leading to antibiotic use.310, 323 It has been used to treat immunocompromised hosts with influenza A and B infections,³²⁴ including a child to whom an aqueous zanamivir solution (16 mg/mL) was administered by aerosol and nebulizer via an endotracheal tube.³²⁵ More recently, in an observational study, orally inhaled zanamivir was more efficacious for treatment of oseltamivir-resistant influenza A/H1N1 than oseltamivir.³²⁶

Prophylactic administration of once-daily inhaled zanamivir (10 mg) prevents febrile influenza illness during influenza season (84% efficacy),³²⁷ or when used for postexposure prophylaxis in households with or without treatment of the ill index case (82% efficacy).328, 329 In an observational study with limited numbers of patients, orally inhaled zanamivir and oral oseltamivir were not different for prevention of secondary cases during nosocomial outbreaks on pediatric wards.³³⁰ In nursing home residents, 2 weeks of inhaled zanamivir was superior to oral rimantadine in preventing influenza A infection, in part because of a high frequency of rimantadine resistance,³³¹ and inhaled zanamivir has been used to curtail transmission of amantadine-resistant influenza A in nursing homes.²⁹⁷

Orally inhaled zanamivir has been administered in combination with oral oseltamivir. For postexposure prophylaxis in families, such combined zanamivir-oseltamivir administration was not more efficacious than either agent alone.³³² However, a subgroup analysis suggests greater efficacy of the combination treatment among contacts whose prophylaxis was begun within 24 hours of exposure to the index case compared with oseltamivir or zanamivir alone. For treatment of adults with mainly A/H3N2 influenza, zanamivir-oseltamivir combination treatment was not more efficacious than zanamivir alone and was less efficacious than oseltamivir monotherapy.³³³

Zanamivir has been administered intravenously to treat patients seriously ill with influenza who could not receive or who had failed oral oseltamivir therapy. Immunocompetent³³⁴ and immunocompromised335, 336 patients who were infected with oseltamivir-resistant³³⁷ and oseltamivir-susceptible336, 338 influenza A/H1N1 nonpandemic viruses or oseltamivir-resistant pandemic virus³³⁹ or oseltamivir-sensitive influenza A (H1N1)pdm09 virus335, 340 have been successfully treated with intravenous zanamivir. There is a sense that intravenous zanamivir may be lifesaving.³⁴¹ However, an apparent lack of a relationship between intravenous zanamivir treatment–associated reductions in pandemic virus load in upper and lower respiratory tract secretions and mortality have prompted questions about its effectiveness in seriously ill patients.³⁴² A phase III study comparing intravenous zanamivir and oseltamivir in hospitalized patients is underway.

Polymeric Zanamivir Conjugates

Polymeric zanamivir conjugates are experimental, high-molecular-weight anti-influenza compounds comprising multiple zanamivir monomers connected at the 7-0 position to backbone or linker molecules of various types and lengths.343, 344, 345, 346, 347, 348, 349 These compounds are potential second-generation inhaled neuraminidase inhibitors for influenza chemoprophylaxis and therapy with enhanced potency and prolonged lung retention time compared with zanamivir. In mice, one of these compounds has been associated with prophylactic efficacy for 7 days after a single intranasal administration.

Spectrum

Polymeric zanamivir conjugates exhibit broad-spectrum anti-influenza activity, inhibiting human influenza A N1, N2, and B viruses and an avian influenza A/H5N1 virus.³⁴³ Inhibitory potency varies according to the length³⁴⁵ and type of linker molecule³⁴⁴ and the number of zanamivir derivatives, whether dimeric,³⁴³ trimeric, or tetrameric.³⁴⁶ The most potent polymeric zanamivir conjugate is a dimer with a 14-carbon linker, which is 10-fold less potent in a neuraminidase assay enzyme inhibition test (IC₅₀, 7.86 nM vs. 0.76 nM for zanamivir) but is 500,000-fold more potent in inhibiting influenza A/WSN/33 (H1N1) in a cytopathic reduction assay (IC₅₀, 0.0001 nM vs. 56 nM for zanamivir).³⁴³ In mice, this dimeric conjugate is 100 times more potent than zanamivir in preventing influenza virus replication in the lung for 7 days after a single intranasal dose of drug and 1 day after intranasal virus challenge (drug doses to reduce lung virus titer by 90% were 0.03 mg/kg and 2.92 mg/kg for the dimeric conjugate and zanamivir). The prophylactic effect is associated with prolonged persistence of dimer conjugate in lung tissue after intranasal administration, as discussed in the section on pharmacokinetics. The specificity of polymeric zanamivir conjugates for influenza A and B neuraminidase is presumed but not yet reported.

A zanamivir polymer can overcome zanamivir resistance. A zanamivir polymer bound to the neuraminidase of zanamivir-resistant avian influenza A viruses possessing a resistance mutation at position 119 bound as much as 2000 times more strongly than did monomeric zanamivir.³⁵⁰

Mechanism of Action

The synthesis of polymeric conjugates of zanamivir that retain neuraminidase inhibitory activity is possible because of the unique position of the molecule when it is docked in the enzymatic pocket, with the 7-OH group pointing out and away from the target site, making it accessible to linkage to different backbone molecules. Electron micrographs show influenza virus clumping in the presence of dimeric zanamivir conjugates. The marked potency of some conjugates is postulated to reflect clumping caused by three types of bivalent binding: between two neuraminidase molecules in the tetrameric transmembrane spike protein (intratetramer), binding between sites on different tetramers on the same virion (intravirionic), and head-to-head binding between different neuraminidase sites on separate virions (intervirionic binding).³⁴³

An additional mechanism for the marked enhancement of potency observed by synthesis of polymer-attached zanamivir is postulated to be the result of interference with intracellular trafficking of endocytosed virus and subsequent virus-endosome fusion.³⁵¹

Resistance

Studies describing attempts to induce resistance in vitro by repeated passage in the presence of drug have not been reported.

Pharmacokinetics

Prolonged retention of polymeric zanamivir compared with monomeric zanamivir in lung tissue accounts for the enhanced antiviral effect of polymeric conjugates. After intratracheal instillation of the same single dose of a polymeric zanamivir conjugate or monomeric zanamivir solution to rats, lung homogenate drug concentrations of the polymeric compound after 48 hours and 168 hours are 35 times and 160 times greater than zanamivir concentrations.³⁴³ Generally, lung retention time is directly related to molecular weight because small polar molecules leave the lung by passing through tight junctions between cells. Prolonged retention of high-molecular-weight polymeric conjugates compared with monomeric zanamivir is expected. However, the prolonged lung retention time of some smaller conjugates indicates that aqueous insolubility and aggregate formation plus partitioning into cell membrane phospholipids may also play a role in the prolonged retention of zanamivir polymeric conjugates in the lung after inhalation.³⁴³

Interactions and Toxicity

Toxicity studies have been limited to assessments of in vitro cyto­toxicity. For a series of dimeric conjugates, concentrations of 100 to 1000 ng/mL caused no cytotoxicity.³⁴⁵

Clinical Studies

No clinical studies have been reported.