New Perspectives on Antimicrobial Agents: Tecovirimat for Treatment of Human Monkeypox Virus

ABSTRACT

Tecovirimat is an antiviral drug initially developed against variola virus (VARV) to treat smallpox infection. Due to its mechanism of action, it has activity against the family of orthopoxviruses, including vaccinia and the human monkeypox virus (HMPXV). Efficacy studies have thus far been limited to animal models, with human safety trials showing no serious adverse events. Currently approved by the FDA only for the treatment of smallpox, tecovirimat shows promise for the treatment of HMPXV. Tecovirimat has been prescribed via an expanded access for an investigational new drug protocol during the 2022 outbreak. This review will examine the literature surrounding tecovirimat’s mechanism of action, pharmacokinetics, safety, efficacy, and potential for resistance.

INTRODUCTION

Orthopoxviruses comprise several viruses that cause human disease, including variola virus (VARV), vaccinia virus, and human monkeypox virus (HMPXV). Research on orthopoxviruses has centered on VARV, the causative agent of human smallpox disease. Smallpox infection is highly virulent and transmissible, with an overall mortality rate of 30% (1). Through worldwide vaccination efforts, smallpox was eradicated in 1980 but remained a global concern due to the potential for use of VARV as an agent of bioterrorism (2, 3).

Tecovirimat (TPOXX, ST-246) was created through efforts to develop an orally available antiviral against smallpox for biodefense. Tecovirimat was approved in 2018 via the Food and Drug Administration (FDA) Animal Efficacy Rule or Animal Rule, which allows a pathway for approval of drugs for severe or life-threatening conditions when it is not ethical or feasible to conduct efficacy trials in humans (4, 5). In this model, efficacy is established based on well-controlled animal studies of the human disease with separate safety studies in humans. Efficacy studies against VARV in humans were not feasible nor ethical since the global eradication of smallpox.

Interestingly, one particular feature of the virus that helped eradication efforts (that VARV infection is limited to humans) hindered the ability to conduct studies of VARV in animal models. Given that VARV infection in animals does not mimic human disease and the restriction of VARV to two containment laboratories (one within the United States and one within Russia), related orthopoxviruses, primarily rabbitpox and monkeypox, were used in the studies that led to tecovirimat’s approval (5).

While smallpox has been eradicated, other orthopoxviruses remain endemic worldwide and pose ongoing human infection, most notably HMPXV. HMPXV was first discovered and named after being identified in research monkeys in a Danish laboratory in 1958 (6). The name is likely a misnomer, as rodents rather than monkeys are thought to be the definitive host (7). The first human case was diagnosed in the Democratic Republic of Congo (DRC) in a 9-month-old child in 1970 following the eradication of smallpox in the area (8). Active surveillance in the 1980s revealed over 300 cases of HMPXV within the DRC (8). More recent outbreaks were confirmed within Nigeria from 2017 to 2018 (9). In 2003, an outbreak within the United States occurred in 71 individuals who had contact with infected pet prairie dogs that had been housed with imported African rodents (7). The classic disease involves a prodrome of fever, malaise, lymphadenopathy, and headache followed by a skin eruption 2 to 4 days later (10). Two distinct clades have been discovered, including clade I (Congo Basin) and clade II (West African Clade), with worsened mortality in clade I (10.6% case fatality versus 3.6%) (11).

The year 2022 represented an unprecedented outbreak of HMPXV in areas of nonendemicity, including the United States, Europe, and around the world, disproportionately affecting men who have sex with men (MSM) and persons living with HIV (PLWH). Recent data from three observational studies from the spring and early summer of 2022 have revealed the changing epidemiology and clinical features of the current outbreak, with greater than 95% of infected individuals identifying as MSM, up to 41% of individuals living with HIV, and one-third of individuals having a concurrent sexually transmitted infection (12). While most individuals experience a prodrome, including fever and malaise, one in five lacks a prodrome, and the timing of these symptoms varies, with some individuals experiencing a prodrome after the eruption of rash (10, 12, 13). More severe manifestations, including ocular involvement, painful anogenital lesions, and bacterial superinfection, are also described (12). Given the morbidity of the disease, tecovirimat is available for treatment under an expanded-access investigational new drug protocol, leading to increased prescribing of this medication (14). This review examines the available literature regarding tecovirimat’s mechanism of action, pharmacokinetics, safety, efficacy, and resistance.

MECHANISM OF ACTION

Tecovirimat is an antiviral drug that inhibits viral protein p37, encoded by the F13 gene of the variola virus (15). This protein is highly conserved in orthopoxviruses, allowing tecovirimat to have in vitro activity against several orthopoxviruses, including vaccinia, variola, cowpox, and monkeypox viruses (15, 16). The p37 protein is involved in the final steps of maturation of the virus and is required for the intracellular mature virus (IMV) to further envelope without a double membrane layer to form intracellular enveloped virus (IEV). IEV then fuses with the cytoplasmic membrane to release virions to disseminate from the site of infection (15). The p37 protein is specific to the orthopoxvirus family, making tecovirimat highly selective with the inability to inhibit replication of other classes of viruses, including herpesviruses (16).

DOSING/DRUG ADMINISTRATION

Tecovirimat is currently available in two formulations: oral capsules and an intravenous (IV) formulation. Each oral capsule contains 200 mg of tecovirimat, and existing recommended dosing regimens are illustrated in Table 1. The capsules can be swallowed whole or opened with the contents mixed with liquid for those who cannot swallow capsules. It is recommended to take within 30 min of a moderate- to high-fat meal containing at least 600 cal and 25 g of fat to optimize absorption (17).

TABLE 1.

Tecovirimat formulations

Oral dosage			Intravenous dosage
Wt	Dosage	No. of capsules	Wt	Dosage	Vol of TPOXX/diluent
13 kg to <25 kg	200 mg every 12 h	1 capsule every 12 h	3 kg to <35 kg	6 mg/kg every 12 h	0.6 mL/kg/1.2 mL/kg
25 kg to <40 kg	400 mg every 12 h	2 capsules every 12 h	35 kg to <120 kg	200 mg every 12 h	20 mL/40 mL
40 kg to <120 kg	600 mg every 12 h	3 capsules every 12 h	120 kg and above	300 mg every 12 h	30 mL/60 mL
120 kg and above	600 mg every 8 h	3 capsules every 8 h

The intravenous formulation is supplied in vials containing 200 mg/20 mL (10 mg/mL) and requires dilution with two equal parts of either 0.9% sodium chloride injection or 5% dextrose injection. Each dose is infused over 6 h (17).

PHARMACOLOGY

Tecovirimat is a tetracyclic acyl hydrazide compound with a molecular weight of 376.33 g/mol. Tecovirimat exhibits high permeability and low solubility in solutions of gastric pH range (18). The oral bioavailability of tecovirimat (i.e., the portion of the dose that enters the body) is 48%. Food enhances absorption, and tecovirimat exposures are 39% higher with food. Thus, the drug is approved for oral administration within 30 min of a moderate- or high-fat meal. At steady state, the maximum concentration occurs at 4 h postdose on average with the oral formulation. With intravenous administration, the maximum concentration occurs at the end of the 6-h infusion. Seventy-five to 82% of the drug is bound to plasma proteins. Tecovirimat is metabolized by amide hydrolysis and glucuronidation by uridine glucuronosyl transferase enzyme 1A1 (UGT1A1) and UGT1A4 to ~10 metabolites. The most abundant metabolite is 4-trifluoromethyl benzoic acid (TFMBA), constituting 70.4% of total exposure. The other metabolites are N-{3,5-dioxo-4-azatetracyclo[5.3.2.0(2,6)0.0(8,10)]dodec-11-en-4 yl}amine and 3,5 dioxo-4-aminotetracyclo[5.3.2.0(2,6)0.0(8,10)]dodec-11-ene. Metabolites are not pharmacologically active. Tecovirimat concentrations decline biphasically, and the terminal elimination half-life is ~19 h (coefficient of variation [CV] of 29%). Table 2 shows the pharmacokinetic properties for tecovirimat at the approved recommended oral dosage.

TABLE 2.

Tecovirimat pharmacokinetic properties

Pharmacokinetic parametera	Data
AUC (ng*h/mL), mean (CV%)	28,791 (35%)
Cmax (ng/mL), mean (CV%)	2,106 (33%)
Cmin (ng/mL), mean (CV%)	587 (38%)
Apparent oral clearance, CL/F (L/h)	31
Apparent vol of distribution, V/F (L)	1,030

^aC_max, maximum concentration of drug in serum; C_min, minimum concentration of drug in serum.

Tecovirimat dose selection for the treatment of orthopoxviruses is based on the extrapolation of pharmacokinetics in animal models of infection and healthy human volunteers. Tecovirimat doses of 3 mg/kg and above demonstrated protection against lethal orthopoxvirus infection in several animal models (described below). The lowest exposures at the fifth percentile with 400 mg of oral tecovirimat exceeds the mean and median exposure in nonhuman primates at the 3 mg/kg dose (19). A 600-mg oral dose provided higher exposures with equivalent safety.

DRUG-DRUG INTERACTIONS

In terms of its potential to act as a victim of drug interactions, potent inducers, such as rifampin, rifapentine, rifabutin, St. John’s Wort, carbamazepine, phenytoin, phenobarbital, oxcarbazepine, or tipranavir/ritonavir, may lower exposures of tecovirimat but have not been studied. Tecovirimat is not a substrate for cytochrome P450 (CYP) enzymes CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, or CYP3A4. Additionally, tecovirimat is not a substrate for the drug transporters p-glycoprotein (P-gp), breast cancer resistance protein (BCRP), or organic anion transporting polypeptide 1B1 (OATP1B1) or OATP1B3.

Regarding its potential to act as a perpetrator of drug interactions, tecovirimat is not an inhibitor of CYP1A2, CYP2D6, CYP2E1, or CYP3A4 and is not an inducer of CYP1A2. Tecovirimat inhibits BCRP in vitro but is not an inhibitor of P-gp, OATP1B1, OATP1B3, organic anion transporter 1 (OAT1), OAT3, or organic cation transporter 2 (OCT2). There are limited in vivo data on the drug interaction potential of tecovirimat. The potential for tecovirimat to act as a perpetrator in drug interactions has been evaluated in healthy volunteers using five CYP probe substrates: flurbiprofen (CYP2C9), bupropion (CYP2B6), repaglinide (CYP2C8), omeprazole (CYP2C19), and midazolam (CYP3A4). The area under the concentration-time curve (AUC) for flurbiprofen was not altered, indicating no impact on CYP2C9. Bupropion was minimally altered (16% reduction), indicating no significant impact of tecovirimat on CYP2B6 substrates. However, the AUCs of the other three drugs were altered. The repaglinide AUC was increased by 29%, and the combination may lead to mild or moderate hypoglycemia. The omeprazole AUC was increased by 73%, and while this is not a clinically significant impact for omeprazole, other CYP2C19 substrates with narrow therapeutic indices may have increased exposures. The midazolam AUC was reduced by 32%, indicating that tecovirimat has CYP3A induction potential; this is relevant for drugs that are substrates for CYP3A4, which may have reduced exposures and compromised therapeutic effects.

SPECIAL POPULATIONS

Children.

Early surveillance studies of HMPXV from the DRC showed a high prevalence of cases in children, with 90% of patients aged less than 15 years old and over half of the patients less than 5 years old (20). Increased mortality rates occurred in children, with 14.9% mortality in children 0 to 4 years and 6.5% mortality in those 5 to 9 (20). More recent studies have confirmed the severity of disease in children, with a 1997 surveillance study from the DRC showing 3.7% mortality, with all deaths occurring in children less than 3 years old (21). During the 2003 US outbreak, the most critically ill patients were school-aged children, with pediatric patients having higher hospitalization rates in intensive care units and severe infections, including encephalitis and a retropharyngeal abscess (22). Despite the prevalence and mortality of orthopoxviruses in children, pharmacokinetics and safety trials of tecovirimat were not performed (17). Therefore, all dosing recommendations for children are based on pharmacokinetic simulations, and no safety data exist. Recommended dosing is based on total body weight with a minimum weight of 13 kg for the oral formulation, limiting pediatric access. In pediatric patients under 2 years of age, the potential exists for drug accumulation, particularly with the IV formulation and the ingredient hydroxypropyl-β-cyclodextrin due to renal immaturity. Close monitoring of tecovirimat use in patients less than 2 years is recommended (17).

Pregnancy/lactation.

Pregnant women represent another at-risk population from orthopoxvirus infection. Women infected with VARV had higher rates of spontaneous abortion, stillbirth, and premature birth than others (23). Similarly, a case report of HMPXV in 4 pregnant women described how 3 out of 4 women suffered a miscarriage or fetal demise (24). Nonetheless, studies of tecovirimat were not performed in human pregnant or lactating individuals. In animal reproduction studies, no developmental toxicities were noted in mouse embryos or fetuses during the period of organogenesis at tecovirimat levels up to 23 times higher than the human exposure at the recommended human dose (17). In contrast, animal reproduction studies for brincidofovir, the second FDA-approved smallpox medication, showed decreased embryo/fetal survival and structural malformations at doses lower than the human recommended dose, suggesting that tecovirimat is likely the safer option for smallpox in pregnancy (25). Breastfeeding is not recommended in patients with smallpox infection, given the potential for variola virus transmission through direct contact. No data on tecovirimat in human milk or drug effects on infants exist, although tecovirimat was present in animal milk (17).

Dose adjustments.

No dose adjustments are required for mild, moderate, or severe hepatic impairment.

No dose adjustment is required for renal dysfunction with the oral formulation of tecovirimat. The IV formulation includes an ingredient, hydroxypropyl-β-cyclodextrin, which is eliminated through glomerular filtration. Dose adjustment is not required for mild or moderate renal impairment (creatinine clearance [CrCl] > 30 mL/min); however, IV tecovirimat is contraindicated in those with severe renal impairment (CrCl < 30 mL/min) (17).

SAFETY/ADVERSE EVENTS

Two phase I trials, one phase II trial, and one phase III trial have demonstrated tecovirimat’s safety and limited adverse events. Phase I trials examined the safety of single-dose tecovirimat daily for up to 21 days. The initial trial contained 38 healthy volunteers who received 500 mg, 1,000 mg, or 2,000 mg daily for 21 days. No serious adverse events occurred, with the most common adverse events being headache and constipation (26). The subsequent phase I trial included 30 nonfasted healthy volunteers randomized to receive a single dose of 250 mg, 400 mg, or 800 mg of tecovirimat or placebo for 21 days. Eight subjects in the active treatment group (33.3%) and two placebo subjects (33.3%) reported at least one adverse event considered to be at least possibly drug related. The most common adverse event reported across all interventions was a headache, while gastrointestinal disorders (dry mouth, flatulence, and nausea) were noted in those receiving the higher dosage of 800 mg (27).

Phase II safety trials examined a single daily oral dose of either 400 mg or 600 mg of tecovirimat for 14 days. One hundred and seven participants were randomized to placebo (N = 16), 400 mg daily (N = 45), or 600 mg daily (N = 46). Forty-eight subjects (44.9%) reported at least one treatment adverse event. The most common adverse events were headaches and nausea. No deaths or serious adverse events were reported (28).

The most extensive safety study was a randomized, double-blind, multicenter phase III trial of 449 healthy volunteers. Three hundred and fifty-nine participants received 600 mg of tecovirimat twice a day for 14 days, while 90 participants received a placebo. There were 208 nonserious adverse events identified that were thought to be related to tecovirimat or placebo; 16.7% nonserious adverse events were related to placebo compared to 19.8% nonserious adverse events related to tecovirimat. The most common adverse events included headache (12% of those receiving tecovirimat and 8% of those receiving placebo) and nausea (5% of participants receiving tecovirimat and 4% of participants receiving placebo). One fatal adverse event occurred in a participant who suffered a pulmonary embolism 1 week after completion of tecovirimat that was not considered to be associated with tecovirimat (1).

EFFICACY

Tecovirimat was approved for use against variola virus based on the Animal Rule. Four studies in nonhuman primates and two studies in rabbits were conducted that demonstrated efficacy of tecovirimat. In nonhuman primate studies, crab-eating macaques were challenged on day 0 with a lethal dose of HMPXV (Zaire 1979 strain). In rabbit studies, New Zealand white rabbits were challenged on day 0 with a lethal dose of rabbitpox virus Utrecht. Tecovirimat was initiated a minimum of 4 to 6 days after the exposure once clinical signs of infection were present. The studies looked at a primary endpoint of survival and secondary endpoints of lesion formation and viral load (1).

Nonhuman primate studies showed a decreased incidence of mortality in nonhuman primates treated with tecovirimat at dose levels of ≥3mg/kg/day. One study of crab-eating macaques (N = 24) showed significantly higher survival rates with tecovirimat at 3, 10, and 20 mg/kg once daily for 14 days than observed in placebo-treated macaques (100% survival with drug versus 0% with placebo). A subsequent study showed improved survival with tecovirimat at 3 mg/kg (80%) and 10 mg/kg (80%) compared to lower dosing and placebo (0%). Treatment with an increased dose at greater than 10 mg/kg/day for 14 days did not further increase survival but resulted in lower circulating MPXV DNA levels and fewer clinical signs of infection. A delayed treatment study in 21 macaques suggested improved mortality with earlier drug administration on days 4 and 5 (83% survival) compared to initiation of tecovirimat on day 6 after exposure (50%). No macaques survived without treatment. The final study examined survival based on treatment duration (N = 25) of tecovirimat 10 mg/kg once daily for 3, 5, 7, or 10 days. Survival rates for tecovirimat were 50% for 3 days, 100% for 5 days, 100% for 7 days, and 80% for 10 days compared to 25% for placebo. Pooled data from four studies suggested that a dose of tecovirimat at greater than 3 mg/kg led to 95% survival in the treatment arm compared to less than 5% survival in the placebo group (1).

In rabbit studies, tecovirimat treatment at greater than 20 mg/kg/day for 14 days decreased the incidence of mortality, blood levels of circulating virus, and clinical signs of infection. Greater than 90% of rabbits treated with any dose of tecovirimat survived, while all untreated rabbits died (1).

No randomized control trials or prospective trials of tecovirimat in humans exist. A recent observational study was released regarding HMPXV infection in 7 patients in the United Kingdom from 2018 to 2021. One of seven patients received tecovirimat for treatment. After initiating TPOXX, blood and upper respiratory tract samples became PCR negative for HMPXV 48 h after commencing treatment. No new lesions developed after 24 h of therapy (29). Tecovirimat has been used in the United States in isolated individuals for primary vaccinia associated with smallpox vaccination and a middle-aged man with HMPXV infection associated with recent travel to Nigeria. In the case of imported HMPXV, the patient received 14 days of tecovirimat (initially 600 mg orally twice daily [PO BID] for the first 19 doses and 200 mg intravenously twice daily [IV BID] for the remaining 9 doses) without adverse events and with the resolution of infection (30–32). Case reports and series from the current outbreak are emerging. Two patients from Washington, DC, with severe proctitis started on tecovirimat therapy in the second week of symptoms. Both patients showed improvement in rectal pain within 48 h of initiating therapy (33). A case series from Sacramento, CA, in which 24 patients received 14 days of tecovirimat and 1 patient received 21 days of tecovirimat described the resolution of lesions in 40% of patients by day 7 of therapy and 92% of patients by follow-up visit at day 21 (34).

RESISTANCE

Reports of tecovirimat resistance are limited. Early in vitro studies developed to help elucidate the target of tecovirimat used a resistant cowpox virus variant. Analysis of the resistant variant showed a single base change within the V061 gene of the virus (homologous to the F13L gene in the variola virus), which encodes the p37 protein. This base change leads to an amino acid change at position 277 in the protein from a glycine (G) to a cysteine (C). The resulting effective concentration of the drug to protect half the cells from virus-induced destruction (EC₅₀) of the resistant variant (EC₅₀ > 40μM) was more than 800-fold higher than the wild-type cowpox virus (EC₅₀ = 0.050 μM) (16).

In vivo, there is one documented case report of the development of tecovirimat resistance. A US Marine Corps member received the ACAM2000 smallpox vaccine predeployment and soon after was diagnosed with acute myelogenous leukemia (AML), necessitating chemotherapy. He developed progressive vaccinia infection at the injection site, prompting combination treatment with vaccinia immune globulin and tecovirimat with the addition of brincidofovir several weeks into therapy. Following 3 weeks of oral tecovirimat therapy, in vitro studies demonstrated resistance, with a 13.5-fold increase in EC₅₀ compared to virus isolated before initiation of treatment. Importantly, initial doses of tecovirimat were low with subtherapeutic target plasma drug concentrations. The starting dose of 400 mg daily was increased to 800 mg daily following low plasma drug levels, and, finally, 1,200 mg daily after resistance was discovered. Sequencing of the F13 gene of the resistant viruses was performed. The G277C amino acid change associated with resistance in prior in vitro studies was not detected. Variation was seen throughout several single nucleotide polymorphisms (SNP) in the sequenced region in the pre- and post-treatment virus but without a clear pattern to explain the developed resistance (31).

ALTERNATIVE TREATMENT OPTIONS

There are currently no FDA-approved treatments for HMPXV. Potential alternative treatments for orthopoxviruses include brincidofovir and cidofovir. Cidofovir, an FDA-approved antiviral against cytomegalovirus, has activity against orthopoxviruses. However, its utility is limited given the need for IV formulation and nephrotoxicity due to drug concentration in renal tubules (35). In 2021, the FDA approved brincidofovir for the treatment of smallpox infection. It is an orally bioavailable lipid conjugate of cidofovir, a viral DNA polymerase inhibitor. Animal studies in rabbits with rabbitpox virus and mice with ectromelia virus, a member of the orthopoxvirus family causing mousepox, showed improved survival with brincidofovir compared to placebo (36). Unlike tecovirimat, brincidofovir has activity against an array of double-stranded DNA viruses, including herpesviruses, and has been studied against cytomegalovirus in hematopoietic stem cell (HSCT) patients. A safety trial of 392 HSCT patients showed increased adverse events compared to placebo, most frequently diarrhea, nausea, and hepatoxicity (35, 37). A randomized controlled trial in 23 pediatric patients showed similar adverse events (38). Three patients in the United Kingdom who received brincidofovir for the treatment of human monkeypox virus developed elevated liver enzymes, prompting early cessation of the drug (29).

FUTURE DIRECTIONS

Currently, tecovirimat is approved only for the treatment of smallpox infection via the FDA Animal Rule. Case reports of success in treating human vaccinia virus infections and HMPXV infection suggest that it may be efficacious for treating the spectrum of human orthopoxviruses. The suggested efficacy of tecovirimat is further supported by its viral target, which is conserved throughout the virus family. Given that these infections are endemic in the human population, conducting randomized human clinical trials is critical to test for efficacy, safety, and the development of resistance. Randomized controlled trials are planned in the Democratic Republic of Congo, the United Kingdom, and the United States by the AIDS Clinical Trials Group (39, 40).