COVID-19 therapeutics

SUMMARY

Since the emergence of COVID-19 in 2020, an unprecedented range of therapeutic options has been studied and deployed. Healthcare providers have multiple treatment approaches to choose from, but efficacy of those approaches often remains controversial or compromised by viral evolution. Uncertainties still persist regarding the best therapies for high-risk patients, and the drug pipeline is suffering fatigue and shortage of funding. In this article, we review the antiviral activity, mechanism of action, pharmacokinetics, and safety of COVID-19 antiviral therapies. Additionally, we summarize the evidence from randomized controlled trials on efficacy and safety of the various COVID-19 antivirals and discuss unmet needs which should be addressed.

INTRODUCTION

The SARS-CoV-2 panzootic has presented modern healthcare systems with unprecedented challenges and opportunities to test their capabilities to promptly develop and deploy antiviral therapies. The pharmaceutical industry in concert with large government agencies developed conventional small-molecule antivirals repurposed from other uses or specifically designed to inhibit vital steps of the SARS-CoV-2 replication cycle (1–3). Simultaneously, the pandemic induced the revival of the oldest antibody-based passive immunotherapy, namely, COVID-19 convalescent plasma (CCP), which was soon followed by the generation of anti-Spike protein monoclonal antibodies (mAb) (4–6).

In this narrative review, we summarize data regarding antiviral activity, mechanisms of action (shown in Fig. 1), efficacy, and safety of SARS-CoV-2 antiviral therapies that have been approved by the United States Food and Drug Administration (FDA) and/or the European Medicines Agency (EMA), focusing on small-molecule antivirals (Table 1) and anti-Spike mAbs (Table 2). We have generally limited our presentation of efficacy data to information gleaned from randomized controlled trials (RCT). Since most RCTs in COVID-19 have been specifically designed to address either outpatients (discussed in much detail in a recent metanalysis) (7) or inpatients, we will separately report usages and recommendations for these two populations.

Fig 1.

Graphical summary of the main mechanisms of action for the therapeutic classes discussed in this review.

TABLE 1.

Summary of RCTs investigating small-molecule antiviral treatments

Small-molecule antiviral	Setting	RCT	n	Main outcome(s)	Ref
Remdesivir	Inpatients	ACTT-1	1,062	Superior to placebo in reducing mortality (6.7% vs 11.9% at day 15, and 11.4% vs 15.2% at day 29) and shortening the time to recovery.	(8)
		NCT04257656	237	No statistically significant difference	(9)
		GS-US-540-5774	84	No statistically significant difference	(10)
		DisCoVeRy	857	No statistically significant difference	(11)
		SOLIDARITY	14,304	No statistically significant difference in ventilated	(12)
	Outpatients	PINETREE	562	Reduced the risk of hospitalization to 0.7% compared to 5.3% with placebo	(13)
Nirmatrelvir/ritonavir	Outpatients	EPIC-high risk (HR)	2,246	28-day hospitalization rate reduced by 5.8%	(14)
		EPIC-standard risk (SR)	1,153	No statistically significant reduction in hospitalization rates	(15)
	Inpatients	NNSF of China	264	No significant differences in mortality from any cause at 28 days or time to viral clearance vs placebo.	(16)
Molnupiravir	Outpatients	MOVe-OUT	1,433	Hospitalized or died reduced by 3%	(17)
		CTRI/2021/07/034588	1,220	No statistically significant clinical benefit	(18)
		AGILE-CST-2	180	No statistically significant reduction in hospitalizations	(19)
		PANORAMIC	25,783	No statistically significant reduction in hospitalizations	(20)
	Inpatients	MOVe-In	304	No reductions in 29-days all-cause mortality, improvement in sustained recovery, or SARS-CoV-2 viral load	(21, 22)

TABLE 2.

Summary of RCTs investigating anti-Spike monoclonal antibody treatments, including absolute (ARR) and relative risk reductions (RRR) in endpoints

mAb	Setting	RCT	n	Main endpoint(s)	Ref
Bamlanivimab	Treatment of outpatients	BLAZE-1	465	Statistically significant ARR in hospitalizations for the 700 mg group	(23)
		ACTIV-3	314	No significant reduction in hospitalizations	(24)
Bamlanivimab + etesevimab		BLAZE-1	268	Statistically significant 4.9% ARR in hospitalizations	(23)
Casirivimab + imdevimab		NCT04425629	275	3% ARR in medically attended visits	(25)
		NCT04519437	969	Reduction in symptomatic COVID-19	(26)
		NCT04452318	1,505	Reduction in symptomatic COVID-19	(27)
	Treatment of inpatients	RECOVERY	9785	reduction in mortality at day 28	(28)
Tixagevimab + cilgavimab	Preexposure prophylaxis	PROVENT	5,150	77% RRR in symptomatic COVID-19
	Postexposure prophylaxis	STORM CHASER	1,121	73% RRR in symptomatic COVID-19	(29)
	Outpatients	TACKLE	903	4.5% ARR in COVID-19 progression or death	(30)
Sotrovimab	Outpatients	COMET-ICE	583	Statistically significant −6% ARR in hospitalization or death	(31)
Regdanvimab	Outpatients	NCT04602000	1,315	72% RRR in hospitalizations	(32)
Bebtelovimab	Outpatients	BLAZE-4	253	Minimal changes in time to viral clearance and time to symptom resolution	(33)
Amubarvimab + romlusevimab	Inpatients	ACTIV-3/TICO	354	Improvement in WHO clinical scale	(34)

During the pandemic, multiple pharmaceutical agents, which had already been approved for other indications, were tested and deployed as antiviral agents (35). Discussion of these drugs, including the antimalarials such as hydroxychloroquine, and antiparasitic agents such as ivermectin, are not as exhaustively covered in this review, and shortly summarized in Table 3. Immunomodulatory therapies such as glucocorticoids, colchicine, metformin, fluvoxamine, interferon lambda, interleukin-1 and interleukin-6 inhibitors, Janus kinase inhibitors, and anti-complement therapies have also been tested and used for the treatment of COVID-19 (36–46). As the focus of this review is on antiviral therapies, a discussion of many of these drugs (shortly summarized in Table 4) is also not as exhaustive in this article. Multiple drugs used at various stages of COVID-19 have the potential for drug interactions. This is particularly complex for nirmatrelvir/ritonavir. The University of Liverpool drug interaction checker is a helpful tool for clinicians (https://www.covid19-druginteractions.org/checker) (47).

TABLE 3.

Summary of the main failed RCTs for COVID-19 therapeutics in outpatient settings, including absolute (ARR) and relative risk reduction (RRR) in hospitalization

Molecule	RCT	ARR percent (95% CI)	RRR percent (95% CI)	Ref
Interferon lambda	TOGETHER	1.7 (0.1, 3.2)	42.6 (3.4, 65.9)	(48)
	ILIAD	0 (−9.1, 9.1)	0 (−1426, 93.4)	(49)
	COVID-Lambda	0 (−6.4, 6.4)	0 (−586.9, 85.4)	(50)
Sofosbuvir and daclatasvir	SOVODAK	10.6 (−4.2, 25.4)	74.1 (−117, 96.9)	(51)
Favipiravir	Avi-Mild-19	−3.7 (−8.4, 1.1)	−219 (−1,447, 34.3)	(52)
	Iran	−5.4 (−17.4, 6.6)	−105.3 (−955.6, 60.1)	(53)
	FLARE	−1.7 (−5.0, 1.6)	NA	(54)
Favipiravir/lopinavir/ritonavir	FLARE	−1.6 (−4.8, 1.5)	NA	(54)
Lopinavir/ritonavir	FLARE	−1.7 (−5.0, 1.6)	NA	(54)
	TREAT NOW	−0.5 (−3.7, 2.6)	−19.8 (−251, 59.1)	(55)
	TOGETHER	−0.9 (−4.9, 3.1)	−18.4 (−155.4, 45.1)	(56)
Tenofovir disproxil fumarate plus emtricitabine	AR0-CORONA	−3.3 (−14.3, 7.7)	−100 (−1,989.8, 80.9)	(57)
Metformin	COVID-OUT	1.8 (0.1, 3.5)	57.5 (3.8, 81.3)	(58)
	TOGETHER	0.7 (−5.5, 6.8)	5.6 (−60.8, 44.6)	(59)
Fluvoxamine	TOGETHER	2.7 (−0.5, 5.9)	21.1 (−4.8, 40.6)	(60)
	STOP COVID	8.3 (1.9, 14.7)	1 (1, 1)	(61)
	COVID-OUT	−0.3 (−2.5, 1.9)	−17.6 (−281, 63.7)	(58)
	ACTIV-6	0.18 (−0.4, 0.7)	54.7 (−398.3, 95.9)	(62)
Fluvoxamine/budesonide	TOGETHER	0.1 (−0.9, 1.1)	12.5 (−140.1, 68.1)	(63)
Ivermectin	TOGETHER	2.4 (−1.2, 5.9)	16.8 (−9.9, 37.1)	(64)
	COVID-OUT	0.3 (−1.3, 1.9)	23.9 (−181, 79.4)	(58)
	Iran	−2.1 (−6.1, 1.9)	−42.3 (−178, 27.2)	(65)
	ACTIV-6	−0.1 (−1.1, 1.0)	−5.3 (−158, 57.0)	(66)
	High dose-ACTIV-6	−0.5 (−1.4, 0.4)	−150.1 (−1,188, 51.1)	(67)
Hydroxychloroquine	TOGETHER	1.1 (−2.7, 4.9)	22.9 (−88.1, 68.4)	(56)
	COVID-19 PEP	2.4 (−1.1, 5.9)	50.2 (−43.1, 82.7)	(68)
	AH COVID-19	−3.6 (−7.1, –0.1)	NA	(69)
	BCN PEP-CoV-2	1.1 (−4.5, 6.7)	16.0 (−103, 65.2)	(70)
	BMG	1.4 (−4.0, 6.9)	29.9 (−154, 80.6)	(71)
	Utah	−2.0 (−6.2, 2.2)	−73.8 (−481.6, 48.0)	(72)
Hydroxychloroquine/azithromycin	Brazil	0 (−6.5, 6.5)	0 (−1,447, 93.5)	(73)
Nitazoxanide	Romark	2.0 (−0.4, 4.5)	78.8 (−79.7, 97.5)	(74)
Colchicine	COLCORONA	1.2 (−0.1, 2.5)	20.0 (−2.8, 37.7)	(75)
Enzalutamide	COVIDENZA	−0.43, (−0.20–0.93)	NA	(76)

TABLE 4.

Summary of the main non-antiviral ant inflammatory treatments for inpatients with severe COVID-19^a

Small-molecule antiviral	Mechanism of action	RCT	n	Main outcome(s)	Ref
Baricitinib	JAK1/JAK2 inhibitor	TACTIC-R (short-course)	137	No significant benefit	(77)
		Bari-SolidAct	299	Halted and underpowered	(78)
		COV-BARRIER	101	19% ARR and 46% RRR in 28-day mortality (P = 0.03)	(79)
Baricitinib + remdesivir		ACTT-2	1,033	2.7% ARR in 28-day mortality over placebo	(80)
		ACTT-4	1,010	No benefit over dexamethasone + remdesivir	(81)
Tocilizumab	Anti-IL-6R mAb	RCT-TCZ-COVID-19	126	No benefit on disease progression over placebo	(82)
		CORIMUNO-19	131	No statistically significant benefit on disease progression or mortality	(83)
		BACC-Bay	243	No reduction in intubation or deaths	(84)
		EMPACTA		Reduced the likelihood of progression to the composite outcome of mechanical ventilation or death, but it did not improve survival.	(85)
		COVACTA	452	No significant improvement at day 28	(86)
		COVINTOC	180	4% ARR in 14-day progression to severe COVID-19	(87)
		NCT04403685	129	No clinical benefit	(88)
		REMAP-CAP	353	Improved 90-day survival (HR 1.61 vs placebo)	(89)
Sarilumab		REMAP-CAP	48
Siltuximab	Anti-IL-6 mAb	COV-AID	113	No clinical benefit	(90)
Anakinra	Anti-IL-1Ra		112	No clinical benefit
		ANA-COVID-GEAS	179	No clinical benefit	(91)
		SAVE-MORE	594	Reduced 28-day mortality (HR = 0.45, P = 0.045)	(38)
		CORIMUNO-ANA-1	153	No clinical benefit	(92)
Abatacept	Anti-CD88	ACTIV-1 IM	524	Reduced 28-day mortality (OR 0.62; 95% CI 0.41–0.94)	(93)
Infliximab	Anti-TNFa	ACTIV-1 IM	531	Reduced 28-day mortality (OR, 0.59; 95% CI 0.39–0.90)	(93)
Cenicriviroc	Anti-CCR2/CCR5	ACTIV-1 IM	360	No clinical benefit	(93)
Ravulizumab	Anti-C5 cleavage mAb	TACTIC-R	135	No significant benefit	(77)
Vilobelimab	Anti-C5a mAb	PANAMO	369	Reduced 28-day mortality (HR 0.67; 95% CI 0.48–0.96; P = 0.027)	(43)

^a ARR, absolute risk reduction; CCR, C-C chemokine receptor; CI, confidence interval; C5, complement component 5; HR, hazard ratio; IL, interleukin; ILR, interleukin receptor; JAK, janus kinase; OR, odds ratio; RCT, randomized controlled trial; RRR, relative risk reduction; TNFa, tumor necrosis factor-alpha.

The pandemic presented unique challenges and opportunities to harness the modern clinical trial infrastructure to address pressing issues. Scientifically valid answers were needed regarding efficacy and safety of numerous potentially active agents at a time when understanding of the natural history of the disease was incomplete and events unfolded in a rapidly changing clinical environment. A commonly used approach was the innovative adaptive platform clinical trials. In this context, an adaptive trial design is one that offers preplanned opportunities to use accumulating trial data to modify aspects of an ongoing trial while preserving the validity and integrity of that trial (94). The COVID-19 adaptive platform trials involve networks of clinical trial sites collaborating to conduct multistage trials that use interim evaluations to determine whether and when to halt study of an ineffective intervention and/or add a new intervention during the trial (95). Many of the studies listed in Tables 1 to 4 were conducted using adaptive platform clinical trial mechanisms.

SMALL-MOLECULE ANTIVIRALS

Small-molecule synthetic antivirals possess multiple advantages over passive immunotherapies, including easier scalability of manufacturing, easier storage (not requiring cold chains), and often easier routes of administration (e.g., via the oral route). The small-molecule agents authorized or approved by the FDA are remdesivir, nirmatrelvir/ritonavir, and molnupiravir.

Remdesivir

Remdesivir was the first clinically effective small-molecule antiviral to be introduced into clinical practice for COVID-19. This repurposed nucleoside analog antiviral was initially developed prior to the emergence of SARS-CoV-2 and arose from a collaboration between the U.S. Centers for Disease Control and Prevention (CDC), the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) and Gilead Sciences (2, 96).

Remdesivir inhibits viral replication by targeting the viral RNA-dependent RNA polymerase (RdRp) thereby leading to termination of new RNA chain formation. Remdesivir itself is an inactive prodrug that must first be metabolized within cells to yield remdesivir-triphosphate (also called GS443902). The active remdesivir-triphosphate molecule is then preferentially utilized by the RdRp over its naturally occurring nucleotide counterpart. This leads to the incorporation of remdesivir-monophosphate (RMP) into the growing RNA viral chain (97). Following the incorporation of the RMP, three more nucleotides are further incorporated into the viral RNA chain before further synthesis stalls (98). Once the fourth nucleotide is incorporated into the viral RNA chain, a portion of RMP containing a cyanogroup sterically clashes with a key viral RNA polymerase structure [non-structural-protein 12 (Nsp12) serine-861]. This, in turn, distorts the positioning of the RNA and hampers translocation of the RMP to the next position. The result is termination of further viral RNA chain synthesis. Moreover, remdesivir interferes with the Nsp12 polymerase even in the setting of intact viral exoribonuclease (ExoN) proofreading activity (99). This is important as the presence of such proofreading mechanisms significantly impairs the activity of other nucleotide antivirals when used against coronaviruses.

Remdesivir was found to be active against multiple pathogenic RNA viruses including Ebola virus (100) and a range of coronaviruses (CoV) that included SARS-CoV and MERS-CoV, bat CoVs, prepandemic bat CoVs, and circulating contemporary human CoV (101). Similarly, the drug was effective in murine models of a range of CoV infections with pandemic potential including SARS-CoV and MERS-CoV (102, 103). With the emergence of the COVID-19 pandemic, remdesivir’s activity against SARS-CoV-2 was demonstrated in vitro, and the drug was found to be most effective when applied to cells soon after they had been infected (104, 105). Remdesivir appears to have a high barrier to the development of resistance. Although mutations in the coronavirus RNA polymerase and helicase proteins can impair remdesivir activity, the drug has remained active in vitro against all the major SARS-CoV-2 sublineages (106–110).

The activity of remdesivir has also been demonstrated in non-human primates using a macaque model of SARS-CoV-2 infection (111). Macaques treated with remdesivir early in the course of infection did not show signs of respiratory disease and also had reduced pulmonary infiltrates on radiographs, virus titers in bronchoalveolar lavages, and lung damage on pathology. However, virus shedding from the upper respiratory tract was not reduced by remdesivir treatment.

As indicated earlier, remdesivir is a prodrug of its active triphosphate metabolite. Remdesivir that remains in the extracellular compartments is metabolized by extracellular kinases to GS-441524, which is then renally eliminated. GS-441524 itself can also be metabolized within cells into an active antiviral form. Remdesivir is highly protein bound, whereas its extracellular active metabolite (GS-441542) is not. Remdesivir and its metabolites have relatively linear pharmacokinetics, and high intracellular concentrations of the active triphosphate metabolite are achievable (112). The optimal serum level for efficacy is not known although one group identified a GS-441524 trough concentration ≥70 ng/mL as the level associated with improved clinical responses in COVID-19 pneumonia (113).

Based on pharmacokinetic bridging from animal data and available human data in healthy adult volunteers, the clinical dose regimen of a 200 mg loading dose on day 1 followed by 100 mg maintenance doses was selected for clinical use (114). This is the recommended dose for adults and for children weighing 40 kg or more. For children weighing less than 40 kg, a weight-based regimen is used (115). With such dosing, high intracellular concentrations of remdesivir-triphosphate can be attained. Remdesivir-triphosphate itself is not detectable in plasma. A population pharmacokinetic study involving 18 non-critically ill patients indicated that the estimated elimination half-life was about 30 minutes for remdesivir and 26.6 hours for its metabolite GS-441524 (116). However, there is extensive variability, with higher levels seen in older patients and in those with reduced renal function (117). Drug level variability also impacts patients receiving continuous renal replacement therapy. In patients receiving continuous renal replacement therapy, the mean trough serum concentrations of GS-441524 can be three- to sixfold higher than in those with preserved renal function (118). Nevertheless, in subjects with severe renal impairment, lung exposure of GS-443902, which is the active metabolite in the lung, is not expected to be impacted (119). Additionally, the elevated exposure to GS‐441524 when used in such patients did not impact toxicity (as measured by elevated liver enzymes in serum) (120).

As indicated above, remdesivir has the potential to cause hepatotoxicity (121). In vitro, remdesivir reduces hepatocyte viability and albumin synthesis, and increases the cleavage of caspase-8 and caspase-3, phosphorylation of histone H2AX, with release of alanine transaminase (ALT) and aspartate transaminase (AST) in a concentration-dependent manner (122). Current guidance is to consider stopping therapy with elevation of transaminases that exceed 10 times the upper limit of normal, especially if accompanied by signs or symptoms of liver inflammation. There was initial concern that the carrier molecule sulfobutylether-beta-cyclodextrin (SBECD), which is used to solubilize remdesivir, could cause renal or hepatic toxicity. However, the lack of a consistent safety signal in patients with reduced renal function has resulted in recommended dosing schedules that do not take renal function into account (123–125).

Cardiovascular events including hypotension, QT interval prolongation, T wave abnormalities, and profound bradycardia have been reported with remdesivir (126, 127). The mechanism seems to be related to remdesivir’s activity on G-protein-coupled receptors for adenosine A1 and urotensin-II (128, 129). While care should be taken to monitor for cardiac toxicity when using remdesivir, in a study including nearly 3,000 patients, remdesivir- associated bradycardia did not lead to an increased risk of death (130). Hypersensitivity reactions, including infusion-elated reactions and anaphylaxis, have been reported. Slowing infusion rates (e.g., to 120 minutes) may help reduce the risk of such reactions.

Remdesivir can only be given in IV formulation. However, the orally available prodrug GS-621763 is effective in vitro and in animal models of COVID-19 (131). This oral prodrug allows for delivery of remdesivir’s metabolite GS-441524 into the systemic circulation. GS-441524 is, in turn, converted by cellular kinases to the analog monophosphate metabolite before further metabolism to the active nucleoside triphosphate (132). Another related drug is VV116, also known as mindeudesivir. This deuterated remdesivir hydrobromide has improved oral bioavailability and has been shown efficacy in preclinical and clinical trials (133–135). Direct instillation of remdesivir into respiratory tract tissues via inhalation has also been studied and was found to be generally safe and well-tolerated with the most common adverse events being nausea, dizziness, and cough (136).

Data for the efficacy of remdesivir have been derived from several RCTs performed in the pre-vaccine era.

Remdesivir is widely used in immunocompromised patients based on extrapolations from results of RCTs in which such patients were a minority of subjects and on retrospective data (137). The ideal length of therapy is not known, and treatment durations are often individualized. Remdesivir is also used during pregnancy. Data from 86 pregnant and postpartum women with severe COVID-19 who received compassionate use remdesivir indicated that the drug was well tolerated and associated with low rates of serious adverse events in that population (138).

RCTs in inpatients

The ACTT-1 study was a double-blind, randomized, placebo-controlled trial of intravenous remdesivir involving 1,062 adults who were hospitalized with COVID-19 and had evidence of lower respiratory tract infection. Remdesivir was superior to placebo in reducing day-15 mortality (6.7% vs 11.9% at day 15, and 11.4% vs 15.2% at day 29) and shortening the time to recovery (10 vs 15 days, P < 0.001) (8). In a multicentre, double-blind, investigator initiated RCT in 237 inpatients in China, patients receiving remdesivir within 10 days of onset of symptoms had a faster (but not statistically significant) time to clinical improvement than those receiving placebo (9). In the GS-US-540–5774 open-label RCT, among 397 unvaccinated patients not requiring mechanical ventilation, no difference in day-14 improvement was found between the 5-day and the 10-day course (139). In a follow-up RCT from the same group, among 584 unvaccinated patients, neither the 10-days nor the 5-days course of 200 mg remdesivir led to changes in clinical status at day 11 compared to placebo (10). The importance of early treatment was confirmed in the DisCoVeRy open-label, multicentre RCT on 857 unvaccinated patients: no clinical benefit was observed from the use of remdesivir in patients who were admitted to hospital for COVID-19, were symptomatic for more than 7 days, and required oxygen support (11) although it led to a median reduction of 0.7 days in the time to viral clearance (140). In the RECOVERY RCT, remdesivir had no significant effect on patients with COVID-19 who were already being ventilated: among other hospitalized patients, it had a small effect against death or progression to ventilation (or both) (141). The WHO Solidarity RCT on 14,304 unvaccinated patients from 35 countries proved that remdesivir has no significant effect on patients with COVID-19 who are already being ventilated: among other hospitalized patients, it has a small effect against death (11.9% vs 13.5%) or progression to ventilation (14.1% vs 15.7%) (12).

In the inpatient setting, remdesivir is infused at the 200 mg loading dose on day 1 followed by an additional 4 days (5 days total) at the 100 mg daily dose. Although the impact of an antiviral alone is of unclear value in patients with COVID-19 severe enough to require mechanical ventilation, the combination of remdesivir and immunomodulators appears to be beneficial in the group of patients receiving non-invasive mechanical ventilation (36, 42, 142). The current FDA recommendation for people requiring mechanical ventilation and extra corporeal membrane oxygenation (ECMO) is to extend the antiviral course to 10 days. In some circumstances, such as in highly immunocompromised patients, longer than these recommended courses may be required.

Remdesivir treatment scenarios were cost-effective, ranging from ~8 to ~23% of the willingness-to-pay threshold for the respective country (143).

RCTs in outpatients

The PINETREE study was a randomized, double-blind, placebo-controlled trial, of remdesivir in the outpatient setting during the pre-vaccine era. This trial was sponsored by Gilead and involved 562 non-hospitalized patients with COVID-19 who had symptom onset within the previous 7 days and who had at least one risk factor for disease progression (e.g., age ≥60 years, obesity, diabetes mellitus, hypertension, chronic end organ disease, cancer, and immune compromise) (13). Among the 279 patients who received a 3-day course of remdesivir (200 mg on day 1 and 100 mg on days 2 and 3) the drug, with an acceptable safety profile, reduced the risk of hospitalization to 0.7% compared to 5.3% with placebo (hazard ratio, 0.13; 95% confidence interval [CI], 0.03 to 0.59; P = 0.008). This study had a major impact on COVID-19 clinical care. It provided an option for patients who could not safely use nirmatrelvir/ritonavir due to drug interactions and for whom other antivirals were ineffective (such as molnupiravir due to its weak activity, and the anti-Spike mAbs due to the development of resistance). The current dosage recommendation is for outpatient use in adults weighing at least 40 kg, the currently approved formulation is infused intravenously with a loading dose of 200 mg on day 1 followed by an additional 2 days at 100 mg daily (3 days in total). The requirement for daily infusions is logistically challenging for outpatients. Additionally, the current benefit of IV remdesivir in a highly immunized individual (due to past infection, vaccination, or both) is not known. Conversely, for some highly immunocompromised patients, outpatient courses longer than 3 days may be required.

Nirmatrelvir/ritonavir

Nirmatrelvir (PF-07321332) is an orally bioavailable inhibitor of the SARS-CoV-2 main polyprotein protease (M^pro) that was developed by Pfizer (3). It is co-administered with ritonavir, a potent cytochrome P-450 3A4 (CYP3A4) inhibitor that decreases the metabolism of nirmatrelvir. The combination of nirmatrelvir/ritonavir was authorized by the EMA and FDA at the end of 2021.

The SARS-CoV-2 M^pro, also referred to as 3C-like protease (3CL^pro) or nonstructural protein 5 (Nsp5), is a chymotrypsin-like cysteine protease, the catalytic site of which consists of H41 and C145 residues (144). Homologous enzymes are found in most positive-sense, single-stranded RNA viruses (145). A ketone-based covalent M^pro cysteine protease inhibitor, PF-00835231, originally developed by Pfizer for SARS-CoV, was effective against SARS-CoV-2 in vitro (146) but was not readily available via the oral route. The phosphate prodrug nirmatrelvir (PF-07321332), which could be taken orally was subsequently developed as a second-generation M^pro inhibitor. Nirmatrelvir occupies subsites S1, S2, and S4 of the M^pro molecule (147). It was shown to be effective in vitro and in a mouse-adapted SARS-CoV-2 model (146). Nirmatrelvir has also been found effective for treatment and for prevention of transmission of infection in hamster models of SARS-CoV-2, including those with infection due to Omicron variants (148, 149). The common M^pro mutations in Omicron (P132H) do not affect its catalytic site (150), and the drug has retained in vitro efficacy against the VOC Omicron BA.1 (150–157), BA.1.1 (151), BA.2 (151, 155, 158), BA.2.12.1 (151), BA.4 (151), BA.5 (151), BA.2.75 (159), BQ.1.1, and XBB (160) sublineages (<2-folds increases in IC₅₀), but resistant clades have been reported: nevertheless, those mutant have not been subject to convergent evolution so far and have shown reduced replication in vitro (161–166).

The pharmacokinetics of nirmatrelvir have been studied in animals and humans. The main mechanism responsible for the clearance of the drug is CYP3A4 (167). The phase 1 trial NCT04756531 showed that nirmatrelvir had a more prolonged half-life when combined with ritonavir and supported use of the standard dose of nirmatrelvir 300 mg + ritonavir 100 mg by mouth twice daily for a 5-day course (168). The dosing is intended so that >90% of those treated would achieve a minimum concentration of 292 ng/mL, which is the concentration at which 90% of inhibition of SARS-CoV-2 viral replication occurs (146, 169).

A major issue with nirmatrelvir/ritonavir is the potential for significant drug/drug interactions. The coadministration with ritonavir, a powerful inhibitor of CYP3A4, has led to increased blood levels of many commonly used medications catabolized by CYP3A4, such as anticoagulants, anticonvulsants, and the immunosuppressants tacrolimus, cyclosporine A, and sirolimus. Conversely, co-administration with potent CYP3A inducers may significantly reduce nirmatrelvir or ritonavir plasma concentrations, which, in turn, may impair the efficacy of nirmatrelvir/ritonavir. Prior to treatment with nirmatrelvir/ritonavir, all medications taken by the patient should be reviewed. Aside from drug interactions, the most common side effect with nirmatrelvir/ritonavir is abnormal taste. In one study, dysgeusia was reported in 17.5% of recipients (170). One proposed mechanism is activation by nirmatrelvir of the TAS2R1 receptor, which mediates bitter taste (171).

Nirmatrelvir is expected to accumulate with decreasing kidney function and, thus, requires adjustment in dosing for those with renal dysfunction. The FDA-recommended nirmatrelvir dose for patients with an estimated glomerular filtration rate of 30–to 60 mL/min is 150 mg nirmatrelvir/100 mg ritonavir twice daily × 5 days. For patients with severe renal dysfunction (estimated GFR <30 mL/min), doses of nirmatrelvir 300 mg/ritonavir 100 mg on day one, followed by 150 mg/100 mg daily for the next 4 days (dosed after dialysis for those on hemodialysis), have been recommended (172, 173). In a small study of patients on hemodialysis who were also receiving nirmatrelvir/ritonavir (150 mg/100 mg twice a day), nirmatrelvir peak concentrations ranged from 4,563 to 7,898  ng/mL and declined after hemodialysis. These drug concentrations were up to fourfold higher but still within the range observed in patients without end-stage renal disease and without accumulation of nirmatrelvir after the end of treatment (174).

Virological rebound, often with potentially transmissible virus and occasionally with symptoms, has emerged as an important phenomenon with nirmatrelvir/ritonavir (175–177). This has also been described with molnupiravir and with untreated patients (178). Nonetheless, viral rebound is more common with nirmatrelvir/ritonavir (approximately 20% in one study) and especially in people treated within 2 days of symptom onset (179, 180). A proposed mechanism for the relapses, which has now been demonstrated in mice, is blunting of the development of SARS-CoV-2-specific antibody and T cell responses by early administration of nirmatrelvir (181). Another potential mechanism is through the persistence of an intermediary form of infectious SARS-CoV-2 that has been identified in vitro with nirmatrelvir treatment but not with remdesivir (182).

Nirmatrelvir/ritonavir is mainly used in the outpatient setting. Data for the efficacy of nirmatrelvir/ritonavir have been derived from trials performed in the pre-vaccine era.

RCTs in outpatients

In the Evaluation of Protease Inhibition for COVID-19 (EPIC) series of placebo-controlled RCTs, nirmatrelvir/ritonavir 300/100 mg twice daily for 5 days administered within 3 days of onset of symptoms reduced 28-day hospitalization rates in high-risk (EPIC-HR, NCT04960202) (14), but not in standard-risk subjects (EPIC-SR, NCT05011513) (15). Both RCTs were run at the time of the Delta variant of concern (VOC) (partly the Omicron BA.1 VOC for EPIC-SR) and mostly in unvaccinated subjects. While further placebo-controlled RCTs would have been unethical, head-to-head RCTs comparing different antivirals for fully vaccinated subjects during the Omicron era have not been performed. This has limited the evidence to large retrospective controlled studies (183–188), a few of them being propensity-score matched (189, 190). Generally, such studies have confirmed a narrowing of the therapeutic benefit, limiting the cost-efficacy of the treatment (191). Accordingly, the current indication remains for high-risk patients older than 12 and weighing more than 40 kg (192). In order to circumvent pharmacokinetic contraindications, reduced-dose regimens have been used (193–195).

The efficacy of nirmatrelvir/ritonavir in a highly immunized population (as a result of prior infection, vaccination, or both) is not well understood. No RCTs have been run after the mass vaccine campaign. A study based on a large healthcare system medical record registry and designed to emulate a randomized trial found nirmatrelvir/ritonavir to be associated with a reduced risk of admission to hospital or death at 30 days in people who were not vaccinated (ARR 1.83%), vaccinated (ARR 1.27%), had received a booster vaccine (ARR 1.05%), and in those with a primary SARS-CoV-2 infection (ARR 1.36%) and reinfection (ARR 0.79%) (196). Given the high number needed to treat to prevent a single hospitalization in the vaccine era, the cost-effectiveness of nirmatrelvir/ritonavir remains unclear.

Pfizer has initiated a phase 2/3 pediatric RCT named EPIC-PEDS in 140 children aged 6–18 years comparing 300 mg vs 150 mg of nirmatrelvir within the nirmatrelvir/ritonavir formulation. Pfizer is also working to develop a body weight-adjusted formulation in three additional cohorts below the age of 6 (197). Pfizer has also launched a triple-blinded phase 2 study (NCT05438602) where immunocompromised patients will be randomized to treatment with nirmatrelvir/ritonavir for 5, 10, or 15 days and followed up for 24 weeks. The results of these studies are not available at the time of this writing.

Based on the above data, the FDA recommended dose for nirmatrelvir/ritonavir in outpatient adults with preserved renal function is 300 mg nirmatrelvir (two 150 mg tablets) with 100 mg ritonavir (one 100 mg tablet), with all three tablets taken together twice daily for 5 days. For those with moderate renal impairment (eGFR ≥30 to <60 mL/min): the recommended dosage is 150 mg nirmatrelvir (one 150 mg tablet) with 100 mg ritonavir (one 100 mg tablet), with both tablets taken together twice daily for 5 days. For patients with severe renal dysfunction (estimated GFR <30 mL/min), there is no specific FDA recommendations although others have suggested doses of nirmatrelvir 300 mg/ritonavir 100 mg on day 1, followed by 150 mg/100 mg daily for the next 4 days (dosed after dialysis for those on hemodialysis) have been recommended (172, 173). From a US perspective, nirmatrelvir/ritonavir administration to outpatients at high risk of severe COVID-19 resulted in an incremental cost-effectiveness ratio of $8931/quality-adjusted life years (198).

Nirmatrelvir/ritonavir has been successfully used in immunocompromised patients but more widespread use in that population has been limited by the potential for drug interactions between ritonavir and many immunosuppressive medications. When used judiciously, and with close attention to drug interactions, nirmatrelvir/ritonavir has been effective and could be safely used in immunocompromised patients (199, 200). Nirmatrelvir/ritonavir has also been used in pregnancy. In a retrospective comparative study including 211 pregnant women, nirmatrelvir/ritonavir treatment was associated with reduced 28-day maternal morbidity and mortality and preterm labor but not 28-days COVID-19-related hospitalization women with SARS-CoV-2 Omicron variant infection (201).

RCTs in inpatients

Inpatient use of nirmatrelvir/ritonavir was tested in an open-label, multicenter, RCT that included 264 hospitalized adult patients in China. Subjects were randomized to receive either 300 mg of nirmatrelvir plus 100 mg of ritonavir every 12 h for 5 days with standard treatment or only standard treatment. Standard treatment did not include remdesivir. There were no significant differences between the two groups in mortality from any cause at 28 days, duration of SARS-CoV-2 RNA clearance among the two groups (mean days, 10 in nirmatrelvir/ritonavir plus standard treatment group and 10.5 in the standard treatment group) or incidence of adverse events that occurred during the treatment period (16). Another study in the inpatient population was the company-sponsored EPIC-HOS (NCT05545319) placebo-controlled RCT of nirmatrelvir/ritonavir in hospitalized immunocompromised patients. It was terminated in March 2023 because of “operational feasibility” (202).

Despite the lack of evidence for use in hospitalized patients, nirmatrelvir/ritonavir is still used in that setting. Typical use scenarios include as continuation for patients who were started on it as an outpatient, as treatment for patients with COVID-19 who are hospitalized for an indication other than severe COVID-19, and as part of combination therapy (despite lack of clear-cut evidence for such use). As detailed earlier, drug interactions are a major problem with nirmatrelvir/ritonavir. The cumulative prevalence of pharmacokinetic contraindications among inpatients has been estimated at 14.6% (203).

Molnupiravir

Molnupiravir is an orally available tautomeric nucleoside analog prodrug (EIDD-28/MK4882) of β-d-N⁴-hydroxycytidine (NHC, or EIDD-1931). Molnupiravir was initially developed at Emory University as an antiviral therapy for alphaviruses (such as Venezuelan equine encephalitis virus) (1). It was found to be effective against a range of viruses including influenza and coronaviruses (204, 205). With the emergence of the SARS-CoV-2 pandemic and discovery that molnupiravir was effective against a range of coronaviruses, development was redirected toward SARS-CoV-2. Ridgeback Biotherapeutics LP licensed molnupiravir from Emory University, funded early-phase clinical trials, and then collaborated with Merck & Co. (known as MSD outside the US and Canada) to jointly develop the drug (1).

Molnupiravir, the orally available prodrug, is converted to NHC, which then acts as a pyrimidine analog. Inside cells, NHC is metabolized to NHC-triphosphate, which then targets the viral RNA polymerase (RdRp) and overcomes viral proofreading mechanisms (206, 207). RdRp uses NHC-triphosphate as a substrate instead of cytidine triphosphate or uridine triphosphate. NHC forms stable base pairs with either guanine (G) or adenine (A), thereby inducing synthesis of mutated RNA with resultant G→A and cytosine (C)→ uracil (U) transitions that escape viral proofreading mechanisms (208–211). This ultimately results in “lethal mutagenesis” (also termed “error catastrophe”) (212–214). This mechanism of action of molnupiravir has raised concerns regarding the generation of clinically significant SARS-CoV-2 variants (215) and mutated viruses with peculiar signatures have been observed in global repositories confirming spreading (216–221).

Molnupiravir is effective against a range of coronaviruses in animal models of infection. In mice infected with SARS-CoV or MERS-CoV, both prophylactic and therapeutic administration of molnupiravir improved pulmonary function and reduced virus titer and body weight loss (205). Both prophylactic and therapeutic administration of molnupiravir were also effective in coronavirus-infected immunodeficient mice implanted with human lung tissue (222). Molnupiravir was also shown effective against SARS-CoV-2 (including emerging variants) in a hamster model of infection and against SARS-CoV-2 in non-human primates (223–226). When used as prophylaxis and as treatment, molnupiravir is effective in reducing viral transmission in animal models of infection (227). Of note, treatment with molnupiravir was less effective in male hamsters infected with omicron (but unchanged in female hamsters) (228). The combination of molnupiravir with nirmatrelvir has been shown effective in mice and in non-human primates (229, 230). Additionally, a combination of molnupiravir with inhibitors of dihydroorotate dehydrogenase (DHODH) is synergistic in the inhibition of SARS-CoV-2 replication in vitro. The proposed mechanism for this synergism is that the lack of available pyrimidine nucleotides upon DHODH inhibition increases the incorporation of NHC into nascent viral RNA. This combination has proven effective in both hamsters and mice (231, 232).

Molnupiravir is dosed for outpatients at 800 mg by mouth twice daily for a 5-day course. The pharmacokinetics of molnupiravir are such that the prodrug (molnupiravir) is rapidly converted to NHC (EIDD-1931) (233). Drug concentration in the plasma follows linear dose-proportional pharmacokinetics when administered between doses of 50 and 1,600 mg (234, 235). NHC concentration in saliva, nasal secretions are directly correlated with plasma concentrations, but the levels attained in those fluids are lower (saliva ~3% that of plasma, nasal secretions, and tears ~20%). NHC can also reach significant levels in the central nervous system. In rats, it has been demonstrated that the drug crosses the blood-brain barrier with assistance of the equilibrative nucleoside transporter (ENT) (236).

Molnupiravir was granted emergency use authorization for outpatients in the United States at the end of 2021, a few weeks after nirmatrelvir-ritonavir. So far, it has preserved in vitro efficacy against all recent SARS-CoV-2 Omicron sublineages (237). It is not authorized for people needing hospitalization for COVID-19.

Molnupiravir is well tolerated during therapy. The most common adverse reactions reported are diarrhea, nausea, and dizziness. Hypersensitivity reactions including anaphylaxis have been reported. However, concerns have been raised regarding longer-term safety (238). Molnupiravir carries a risk for mutagenicity in vitro for human RNAs (103, 239–242). The mechanism of mutagenesis is suspected to be due to oxidative DNA damage via cytidine deaminase (CDA)-mediated metabolism of NHC (243). Molnupiravir is not recommended for use during pregnancy. Animal studies have demonstrated that NHC is readily transmitted across the blood/placenta barrier to reach the fetus where it can lead to embryonic and fetal developmental toxicity (244–246). Additionally, it may impact bone and cartilage growth and hence is not recommended for those under the age of 18.

RCTs in outpatients

Several RCTs have evaluated molnupiravir in the outpatient setting. The MOVe-OUT trial was a phase 3, double-blind, randomized, placebo-controlled trial to evaluate the efficacy and safety of treatment with molnupiravir started within 5 days after the onset of signs or symptoms in non-hospitalized, unvaccinated adults with mild-to-moderate COVID-19 and at least one risk factor for severe illness (17). A total of 1,433 participants underwent randomization. In the analysis of all participants who had undergone randomization, the percentage of participants who were hospitalized or died through day 29 was lower in the molnupiravir group than in the placebo group (6.8% [48 of 709] vs 9.7% [68 of 699]; difference, −3.0 percentage points; 95% CI, −5.9 to −0.1). One death was reported in the molnupiravir group and nine were reported in the placebo group through day 29. The trial was stopped early at the recommendation of the Data Safety and Monitoring Board because of presumed clear superiority after approximately 50% of the sample had been recruited (May-Sep 2021, with 18.3% being convalescents as defined by the occurrence of anti-N antibodies) (247). At the point of discontinuing the RCT, approximately 90% of the planned sample had been recruited and had available follow-up data accessible. Thorlund et al. noted that treatment effects reverse when examining only the post-interim analysis population (Sep-Nov 2021, with 21.7% being convalescents as defined by having anti-N antibodies), i.e., molnupiravir increasing hospitalizations by 35%, and are substantially attenuated when examining the full data set (248, 249). In a subgroup analysis of immunosuppressed patients, 55 of the participants were considered immunocompromised. Compared to placebo, fewer molnupiravir-treated immunocompromised participants were hospitalized or died through day 29 (22.6% vs 8.3%). However, conclusions regarding efficacy are difficult as the numbers were small (250).

The Indian Central Drugs Standard Control Organization approved 12 molnupiravir trials, 2 of which were stopped early: the ones conducted by Indian pharmaceutical companies, Aurobindo (CTRI/2021/07/034588) and MSN Laboratories Pvt Ltd, enrolled more than 2,000 patients with moderate COVID-19 but showed no significant clinical benefits, according to the Indian regulatory authorities, and were discontinued (251). The preprint from the Aurobindo RCT on 1,220 patients went out on 24 February 2022: despite no patient being hospitalized in any arm, molnupiravir treatment was associated with faster clinical improvement and viral clearance (18). The placebo-controlled, investigator-initiated AGILE-CST-2 RCT in UK in 2020–2022 (NCT04746183) found shorter viral clearance but a nonsignificant reduction (0 vs 4 out of 90 per arm) in hospitalizations (19).

Molnupiravir as early treatment for outpatients was reevaluated in the 2022 UK PANORAMIC RCT (ISRCTN30448031) coordinated by Oxford University, which recruited vaccinated patients during the Omicron wave. In PANORAMIC, the original sample size calculation of 10,600 patients assumed a 3% rate of hospital admission or death for standard of care, and 2% for patients taking molnupiravir. By 27 April 2022, PANORAMIC had enrolled 25,783 patients at 65 sites, over double the original estimate, without being halted by ethical concern. This suggested that rates of hospital admission or death could be even lower than originally predicted (252). Accordingly on 4 October 2022, a preprint of the PANORAMIC RCT was posted which found that molnupiravir did not reduce already low hospitalizations/deaths among higher risk, vaccinated adults with COVID-19 in the community (0.8% in both placebo and intervention arms) (20). The manuscript went out on 22 December 2022 (253), leading EMA to withdraw its authorization and Merck to withdraw the application (254). At the time of this writing, multiple organizations (2) and the FDA continue to recommend molnupiravir in certain circumstances. A study based on a large healthcare system medical record registry and designed to emulate a randomized trial suggested that molnupiravir may have prevented hospitalizations during the omicron predominant (255), but trust in effectiveness of this drug as monotherapy has declined among prescribers (256).

RCTs in inpatients

In the MOVe-IN study, which evaluated 304 unvaccinated hospitalized patients during the Alpha wave, no reductions in 29-days all-cause mortality, improvement in sustained recovery, or SARS-CoV-2 viral load were reported and the trial was stopped by the ethics committee after slightly higher death rates were observed in the molnupiravir arm (21, 22). In an emulated RCT performed during the Omicron waves, Wan et al. reported on the performance of molnupiravir and nirmatrelvir/ritonavir when started within 5 days of hospitalization. The oral agents were presumably effective at reducing 28-day all-cause mortality but not progression to ventilatory support or ICU admission in COVID-19 (257). Such studies, however, are prone to major statistical limitations (256).

ANTIBODY-BASED ANTIVIRALS

Passive immunotherapy containing neutralizing antibodies (nAb) has played an important role in the response to the COVID-19 pandemic. Products have ranged from CCP to hyperimmune serum globulin preparations and mAbs. In a recent systematic review and metanalysis of 58 RCT that investigated passive antibody therapies for the treatment or prevention of COVID-19, Stadler et al. found that earlier clinical stage at treatment initiation was highly predictive of the efficacy of both anti-Spike mAbs (P < 0·0001) and CCP therapy (P = 0·030) in preventing progression to subsequent stages, with either prophylaxis or treatment in outpatients showing the greatest effects. For the treatment of outpatients with COVID-19, a significant association between the dose administered and efficacy in preventing hospitalization was found (relative risk 0·77; P < 0·0001) (258).

COVID-19 convalescent plasma

Since the start of the pandemic, CCP has been used as treatment for COVID-19 and at time of this writing continues to serve as a valuable therapeutic for selected patients who cannot tolerate or relapse after treatment with small molecule antivirals (259). An advantage of CCP is that the product can be generated almost anywhere where there are modern blood banking capabilities. Additionally, the antibodies contained in recently acquired CCP can remain effective against circulating viral variants that have grown resistant to anti-Spike mAbs (260). A systematic review of RCTs has shown that the toxicities of CCP do not exceed those well known for plasma (261), a commonly used blood product. These include transfusion-associated circulatory overload (TACO), transfusion-related acute lung injury (TRALI), allergic reactions, and hypotensive reactions.

The mechanisms of action include direct and indirect antiviral activity and potentially immunomodulatory effects. Neutralizing Abs may be the most consequential plasma component. Such antibodies could block infection by impairing interactions between the virus and the host cells (262). While at the beginning of the pandemic, only a minority of convalescents had useful nAb titers (mostly from those who were hospitalized and with the most severe symptoms) (263, 264), by 2023, the vast majority of regular blood donors had high nAb titers as a consequence of multiple infections and boosts. In addition to quantity, another benefit of repeated exposure to different variants is hybrid immunity, which leads to high titers of heterologous antibodies against sublineages the donor has not yet encountered (265). Of the various antibodies in CCP, certain IgG subtypes and IgM may be particularly important for neutralization (266). Despite accounting for only approximately 12% of total immunoglobulin mass, collectively IgG₃ and IgM account for approximately 80% of the total neutralization (267).

However, nAbs are only part of the response. Additional mechanisms such as antibody-dependent cellular phagocytosis (ADCP) and cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) may be at play (268). CCP may also modulate immune responses via antibody-mediated neutralization of complement, pro-inflammatory cytokines, and autoantibodies generated by COVID-19 (269), and by Fc receptor binding and modulatory effects on T cells, B cells, and dendritic cells. In SARS-CoV-2 challenged mice, treatment with CCP that had low nAb titers but displayed moderate to high Fc-effector activity was more effective than treatment with CCP that had both low nAb titers and poor Fc function (270). The possibility that antibodies in plasma could enhance disease via facilitating entry of virus into cells has been raised; however, there is no clear evidence at this time for disease-enhancing activity with CCP (268).

CCP has been tested in animal models for SARS-CoV-2 infection in mice, hamsters, and non-human primates. In mice, pretreatment with CCP at titer 1:1,000 prior to infectious challenge prevented weight loss and lung tissue histological changes and accelerated the rate of virus clearance (271). In hamsters, passive transfer of convalescent serum to naïve hamsters efficiently suppressed the replication of the virus in the lungs and impaired progression of pulmonary infection (272). The impact of CCP on rhesus macaques with SARS-CoV-2 has been more modest. CCP with moderate levels of nAbs was ineffective (273). In another study, macaques treated with higher titer CCP had detectable but low levels of antiviral antibodies after infusion. In comparison to the control animals, CCP-treated animals had similar levels of viral RNA in upper and lower respiratory tract secretions, similar detection of viral RNA in lung tissues by in situ hybridization, but lower amounts of infectious virus in the lungs and a moderate, but statistically significant reduction in interstitial pneumonia, as measured by comprehensive lung histology (274).

RCTs in outpatients

RCTs of CCP have generally shown success when patients were transfused within 5 days of onset of symptoms, using units containing high titers of nAb (typically >1:160, although large heterogeneity in tittering methods) (275, 276). A single RCT of CCP in outpatients has failed to show benefit (277), but the CCP used in that trial had been treated using a potentially Fc-destroying pathogen reduction technology (278).

RCTs in inpatients

RCTs in inpatients have generally failed because the majority of patients recruited were at advanced disease stages and/or transfused with units containing low titers of nAb (275, 276). Sometimes, those titers were even lower than the titers detected in recipients at baseline (279). Signals of CCP efficacy were, however, present in many negative trials when subgroup analyses (i.e., patients treated early and with high-titer CCP) were performed (12). Of note, a recent RCT showed a beneficial effect of CCP on mortality in patients under invasive mechanical ventilation, suggesting a potential therapeutic role also in this category of critically ill COVID-19 patients (280). A systematic review and metanalysis evaluating RCTs and matched cohort studies have confirmed reduced mortality in inpatients (281). In addition to systematic reviews and metanalyses (282), CCP thus far represents the only SARS-CoV-2 antiviral for which a single successful RCT has been run specifically in immunocompromised patients (283).

CCP in immunocompromised patients

CCP has emerged as an important modality for the treatment of immunocompromised patients, particularly those with impaired B cell number or function and in those who have persistent infection (259). Multiple clinical trials and systemic reviews have examined the role of CCP in immunocompromised patients. In a retrospective analysis of 966 hematologic malignancy patients with COVID-19, CCP was associated with improved 30-day mortality (HR, 0.60; 95% CI, 0.37–0.97). This association remained significant after propensity score matching (HR, 0.52; 95% CI, 0.29–0.92) (284). CCP treatment was also measured in a cohort of 112 hematological malignancies patients with COVID-19. Its efficacy in that study was particularly marked in those with B-cell malignancies (285). An RCT including 56 patients with cancer demonstrated that the addition of convalescent/vaccinated anti-SARS-CoV-2 plasma to usual care was associated with a shortened median time to improvement (HR  =  2.50; P  =  0.003) and superior survival (HR  =  0.28; P  =  0.042) (286). In another trial that included 120 patients who were randomized 1:1 to CCP or usual care, mortality in immunosuppressed CCP recipients was lower than in those who received usual care alone (HR 0.37; 95% CI 0.14–0.97) (287).

Some immunocompromised patients with significant abnormalities in B lymphocyte function and number develop persistent COVID-19 infection with ongoing high-level SARS-CoV-2 viral loads, respiratory symptoms, and radiographic abnormalities on chest X-ray or computed tomography scans (288). Observational studies have demonstrated a role for CCP in such patients, usually as part of combination therapy with other antiviral agents (289–291).

Hyperimmune serum

In parallel with CCP collection and clinical use, several countries have investigated CCP fractionation to produce hyperimmune intravenous immunoglobulins (hIVIg) on a large scale. The aim has been to develop a plasma-derived product with high-titer nAb against SARS-CoV-2. In terms of titers, hIVIg is approximately 10-fold more concentrated than CCP and can, thus, be given in a smaller volume (292).

RCTs in outpatients

The use of hIVIg as pre-exposure or post-exposure prophylaxis against COVID-19 is potentially interesting and particularly valid for immunocompromised patients who represent a cohort at increased risk of COVID-19 morbidity and mortality, even with the less virulent Omicron variants. To date, however, there are no published RCTs on hIVIg use in outpatients.

RCTs in inpatients

The results of RCTs on the clinical use of hIVIg against COVID-19 for hospitalized patients are contradictory (293), overall documenting no beneficial effect in immunocompetent hospitalized COVID-19 patients (294), but a potential role in improving outcome in severe immunocompromised patients (295). In a recent head-to-head comparative RCT (hIVIg versus CCP), treatment outcomes were better in patients treated with hIVIg on day 28 but not on day 14 (296). Currently available standard IVIg, however, stemming mostly from plasma donations from COVID-19 vaccinated and recovered blood donors, have similar anti-SARS-CoV-2 nAb content to hIVIg thus rendering the creation of a dedicated plasma-derived product poorly cost-effective and obsolete (297, 298).

Anti-Spike monoclonal antibodies

Development and successful deployment of highly effective and safe anti-spike mAb in record time was a major advance in antimicrobial therapy (6). These agents were developed by pharmaceutical companies following selection of nAbs from convalescent patients or, in some cases, from humanized mice exposed to SARS-CoV-2 antigens (299). Within 2 years of the onset of the pandemic FDA granted emergency use authorization to multiple mAbs for the prevention or treatment of infection in the outpatient setting (5). mAbs authorized by FDA included bamlanivimab, etesevimab, casirivimab, imdevimab, sotrovimab, tixagevimab, and cilgavimab. None of the agents were effective as a treatment for people hospitalized with COVID-19 (300–302), and emergency use authorization has since been revoked in the USA for all such agents.

These mAbs bind to targets on SARS-CoV-2 spike (S) protein. As with other passive immunotherapy approaches, titer of effective antibody is important. A model has been developed that determined that if a treated population can maintain a mean in vivo mAb concentration of >96-fold of the in vitro IC₅₀ of the antibody to the circulating variant, they should maintain >50% efficacy against COVID-19 (303). Such levels are not readily attained by the previously authorized mAbs against the currently circulating variants.

When effective, mAbs work by disrupting viral binding to human receptors and by viral killing through ADCC and ADCP (304). Of note, manipulation of the antibody structure can facilitate prolonged half-lives allowing extended protection, as has been done with the pre-exposure prophylactic agent tixagevimab/cilgavimab, but this may alter antibody-induced effector function (305). Activity of the mAbs for treatment and for prevention has been established in animal models including mice, hamsters, and non-human primates (305–312).

The major problem with the SARS-CoV-2 mAbs is that ongoing alteration in the Spike glycoprotein has led to antibody evasion rendering the products ineffective (313). A combination of multiple mAbs (cocktails) was initially helpful in minimizing escape mutants but ultimately, that approach also failed in maintaining baseline mAb effectiveness (314, 315).

Thus, despite hitting the market with high hopes, we can now conclude that SARS-CoV-2 has also highlighted multiple limitations of antiviral mAb therapy: high cost, logistical hurdles, baseline resistance, and treatment-emergent resistance when used as monotherapies (316). For example, treatment emergent-emergent resistance developed with the mAb bamlanivimab. During a clinical trial with bamlanivimab, for example, resistance mutations in the spike protein were significantly more likely to be detected after bamlanivimab (700 mg) treatment than in recipients of placebo (7% of 111 vs 0% of 112 participants, P  =  0.003) (317). One after the other, the mAbs became ineffective with the advent of the Delta VOC, and have remained ineffective along the many waves of the Omicron VOC.

Nonetheless, misprescription of anti-Spike mAbs had continued in the USA after FDA deauthorization (318) and in Europe (319), where EMA has not yet withdrawn authorized mAbs and has only sporadically issued alerts about possible basal resistance (320). Some researchers have continued advocating for continued efficacy of anti-Spike mAbs based on marginal gains in surrogate endpoints in non-RCTs (321, 322) or no change in hospitalization rates of outpatients (323). While binding affinity seems a robust surrogate endpoint, advocates of residual in vivo activity argue that the concentrations achieved by mAbs in vivo can overcome several degrees of baseline resistance to neutralization or that alternative effector function still persists (324). However, with IC₅₀ above 1,000 (325) antigen binding by the mAb is compromised, and effector functions other than neutralization invariably require binding to the virus to activate Fc-receptors. Since the turnaround time of viral sequencing is incompatible with timely delivery of therapeutics, tools have been developed that can predict basal efficacy of anti-Spike mAbs based on regional genomic surveillance of the circulating variant milieu (326, 327).

Below, we will focus on the two authorized anti-Spike mAbs which still preserve some degree of in vitro efficacy at the time of this writing, i.e., sotrovimab and the cilgavimab + tixagevimab cocktail.

Sotrovimab

Sotrovimab targets a highly conserved epitope of the Spike protein and has so far granted the highest levels of in vitro activity against most SARS-CoV-2 sublineages, except for the recently emerged JN.1 (328).

RCTs in outpatients

In the interim analysis of the COMET-ICE double-blind RCT on 583 unvaccinated patients at risk for progression with symptoms since less than 5 days, treatment with intravenous sotrovimab 500 mg reduced hospitalization from 7% to 1% (329). The final results on 1,057 patients confirmed a reduction in hospitalization lasting longer than 24 hours or death from 6% to 1% at day 29 (330).

RCTs in inpatients

In the multicentre TICO double-blind RCT on 546 unvaccinated patients with symptoms of more than 12 days, treatment with sotrovimab did not result in fewer pulmonary complications on day 5, nor did it improve clinical recovery on day 90 as compared with placebo (34).

Tixagevimab + cilgavimab

Although the vast majority of tixagevimab + cilgavimab (Evusheld) use has thus far been for pre-exposure prophylaxis (PEP), it has also been authorized and used for the treatment of COVID-19 cases. Unfortunately, in vitro experiments found tixagevimab ineffective since the emergence of BA.1 (331, 332), and cilgavimab ineffective since the emergence of BA.4/5 (333, 334).

RCTs in outpatients

TACKLE was a therapeutic RCT ran at 95 sites across Europe, USA, Latin America, and Japan from January to July 2021, a time that coincided with the Alpha VOC wave, involving 1014 non-hospitalized unvaccinated adults with WHO scores 2–3 aged 18 years or older who received Evusheld 300 + 300 mg by the intramuscular route (IM) or placebo within 7 days of symptom onset. Severe COVID-19 or death occurred in 4% (18 of 407 patients) in the Evusheld group vs 9% (37 out of 415) in the placebo group, corresponding to a relative risk reduction 50.5% (30). AstraZeneca communicated the results in a press release on 11 October 2021 (335), and the study was published on 07 June 2022, a time when most people in many western countries had received three or more vaccine doses. In Italy, the national medicine’s agency (AIFA) expanded the indication for Evusheld to include early therapy in patients generically defined as intolerant of small-molecule antivirals or in accordance with the epidemiological landscape (336). The decision was made on the basis of the preliminary findings (including an interim analysis of the MANTICO-2 RCT; NCT05321394) regarding the noninferiority of Evusheld 300 + 300 mg and Paxlovid vs sotrovimab in outpatients older than 50 and despite the fact that EMA had not completed a final review planned for September 2022. This decision was unfortunate since it was announced at a time when the Evusheld-resistant BA.4/5 Omicron VOC was highly dominant in Italy (319).

RCTs in inpatients

In the ACTIV-3 RCT, the Therapeutics for Inpatients with COVID-19 (TICO) Study Group Investigated Evusheld as treatment for patients hospitalized with COVID-19 from 10 February 2021 to 30 September 2021. The study enrolled 1,455 adults with symptoms for a median of 8 days, of whom 47% were seronegative, 15% were vaccinated with two doses, and 12% were vaccinated with one dose. These patients were hospitalized with COVID-19 WHO stage 4–5 at 81 sites across Europe, USA, Uganda, and Singapore and were randomized 1:1 ratio to standard of care plus intravenous tixagevimab 300 mg–cilgavimab 300 mg (n = 710) or standard of care plus placebo (n = 707). There were no differences in sustained recovery at 3 months, regardless of serostatus, but Evusheld led to lower mortality (9% vs 12%) (337). The results of the Evusheld arm of ACTIV-2 (NCT04518410) have not been reported yet. The investigator-initiated DisCoVeRy RCT in France (NCT04315948), part of the WHO Solidarity Trial), could have investigated Evusheld vs other treatments or placebo but is also currently not recruiting.

COMBINED ANTIVIRAL THERAPIES

Immunocompromised patients who cannot mount endogenous immune response after both vaccination and/or natural infection tend to experience persistent SARS-CoV-2 infection with high viral loads. This circumstance is often unsuccessfully managed by the standard antiviral monotherapy regimens which were initially only validated for treatment of COVID-19 immunocompetent patients. The off-label prescription of monotherapy regimens in such persistently infected immunocompromised patients is likely to drive the emergence of treatment-related immune escape, relapses, excess morbidity and mortality from both COVID-19 and delayed treatment of the underlying disorders (338, 339). A possible treatment approach to mitigate such consequence is based on combined antiviral therapies. Combinations of either small molecule antivirals or small-molecule antivirals plus passive immunotherapies appear to be safe and effective in small cohorts (340). The challenge of antiviral resistance in this population, however, has been highlighted in a report of a patient with lymphoma who was treated with multiple antiviral and antibody therapies, including sotrovimab, remdesivir nirmatrelvir/ritonavir, and molnupiravir. This patient developed rebound infection with an omicron subvariant (BA.1.1), which evolved to have resistance mutations to nirmatrelvir, sotrovimab, and remdesivir (341). To date, no RCTs of combined therapies has been run in either outpatients or inpatients, leaving uncertainty about their optimal management.

REPURPOSED AND IMMUNE-MODULATING AGENTS

The COVID-19 pandemic spurred extensive testing to identify agents that might prove safe and effective at various stages of the infection. Beyond the small-molecule antiviral agents detailed above, which were repurposed for SARS-CoV-2, an astounding array of drugs from various classes was screened, and many progressed to RCTs. Tables 3 and 4 provide details regarding RCTs testing many of those drugs. Of particular importance were the anti-coagulant and immune-modulating agents. Anticoagulation in patients with COVID-19 has been extensively reviewed and strategies for reducing preventing and treating thrombotic complications have been published by multiple organizations including the American Society of Hematology (342–344). The identification of the efficacy of corticosteroids for patients with severe inflammatory manifestations of COVID-19 was a major therapeutic advance and opened the way for many other immunomodulating therapies (37, 345).

It has long been recognized that host damage as an outcome in host-microbe interactions stems from microbial traits or the host immune response or both (346). Early on in the COVID-19 pandemic, it was understood that the host response was a major driver of COVID-19 associated pathology (347). Viral replication and viral-induced injury in the first phase of infection are then followed by the recruitment of effector immune cells. Patients who are unable to mount an effective and timely response to the virus in the initial phase and are then unable to control the intensity of the immune response in the second phase may go on to develop severe disease with respiratory distress syndrome (ARDS), thrombosis, and multiorgan failure (348, 349). During earlier phases of the COVID-19 pandemic, it was common for clinicians to encounter patients who rapidly progressed from what appeared to be an uncomplicated and often improving upper respiratory viral infection, to a severe, life-threatening illness with respiratory failure, clotting complications, and deaths despite aggressive care. With widespread immunity induced by vaccinations and naturally occurring COVID-19, the use of antiviral therapy early in the course of infection, this devastating second phase of illness has become much less common.

Treatment of severe infection that is deemed to be due to a destructive immune response is treated with a combination of antiviral therapy (typically remdesivir) and immune modulation. Table 4 summarizes the various agents that have been tested in RCTs. These include systemic glucocorticoids, interleukin-1 (IL-1), IL-6, and tumor necrosis factor-alpha (TNF-a) inhibitors, Janus kinase inhibitors, cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) agonists, and inhibitors of the complement axis. Additional immunomodulatory targets are in testing including inhibition of IL-23 and 33.

THE IMPACT OF ANTIVIRALS ON LONG-COVID-19

Long-COVID is a term used to describe heterogenous post-acute-infection conditions affecting millions of people worldwide (350). Significant investment has been made in characterizing these conditions. Manifestations include fatigue, cognitive dysfunction, pain, gastrointestinal symptoms, dysautonomia, autonomic dysfunction, hypercoagulability, and mast cell activation syndrome. There is conflicting evidence regarding whether antiviral therapy during acute infection impacts the risk of developing long COVID (351). A systemic review of various therapies suggested that remdesivir, nirmatrelvir/ritonavir, dexamethasone, and metformin could have a potential protective effect for long-COVID (352). For patients with long-COVID symptoms, treatment paradigms focus on the management of clinical manifestations and are often extrapolated from therapies used for other conditions such as myalgic encephalomyelitis/chronic fatigue syndrome, and postural orthostatic tachycardia syndrome.

ANTIVIRAL PIPELINE

The mAb pipeline has suffered a shrinkage because of the high risk of baseline resistance in emerging Omicron sublineages and the investment risk faced by drug manufacturers. AstraZeneca is testing the AZD5156 cocktail (cilgavimab plus AZD3152) in the SUPERNOVA phase I study and AZD3152 monotherapy in the SUPERNOVA phase III RCT (353, 354, 355). The AZD7158 cocktail (AZD3152 plus AZD3959) is under evaluation at EMA (356). On 22 March 2024 FDA has granted EUA for pre-exposure prophylaxis to Invivyd’s VYD222/pemivibart (357) (a reengineered version of Adagio Therapeutics ADG20/adintrevimab). Regeneron has early clinical trials of REGN17092 (358), Aerium Therapeutics has advanced clinical trials of the AER-800 cocktail (359), Brii Biosciences is developing the amubarvimab plus romlusevimab cocktail (360), and several others also have advanced candidates [e.g., SA55 (361)].

The M^pro inhibitors pipeline includes molecules that are in advanced development stages such as ensitrelvir/S-217622 (Xocova, Shionogi), which do not require ritonavir boosting and are administered once daily (362). Such agents would be largely free of pharmacokinetics interactions. The clinical pipeline also includes EDP-235 (Enanta) and PBI-0451 (Pardes Bio), while the preclinical pipeline includes boceprevir, STI-1558 (Sorrento), SH-879 (Sosei Heptares), EDDC-2214 (Everest Medicine), ASC-11 (Ascletis), GC376 (Anivive Lifesciences), and NLC-V-01 (Tollovir, Todos Medical). Rupintrivir is selective for rhinoviruses, but its derivative M^pro-1 is also effective against SARS-CoV-2.

The RdRp inhibitors pipeline includes the previously discussed oral VV116 (135), and obeldesivir (GS-5245), an oral prodrug of the parent nucleoside GS-441524 (363). While Gilead halted the RCT investigating obeldesivir in inpatients (BIRCH, NCT05603143), the RCT investigating obeldesivir in outpatients (OAKTREE, NCT05715528) has been completed and results are awaited.

CONCLUSIONS

Since the onset of the COVID-19 pandemic, multiple antiviral agents have been developed at records breaking pace. However, much work remains to be done. As of this writing the armamentarium of authorized and highly effective antivirals options is limited to a single intravenous drug (remdesivir), a single oral drug (nirmatrelvir/ritonavir), and a single immunotherapy option (CCP). Molnupiravir has limited efficacy, especially in an immunized population and none of the previously authorized mAbs can be expected to be reliably effective. The mAb pipeline has suffered from failures, with manufacturers often reluctant to invest on novel cocktails, which invariably represent the most successful way to minimize baseline and treatment-emergent antibody resistance. Many key questions remain undefined. For example, it is unclear to what extent efficacy data developed in studies done prior to the vaccination era can be generalized to the current era. Another key question regards the timing in a patient’s trajectory of illness where antibody-based antivirals are no longer effective. Antibody-based therapies that have proven effective when delivered to COVID-19 outpatients have been proven ineffective in immunocompetent inpatients, likely marking the threshold between virus- and inflammation-driven pathology. With regard to immunocompromised patients, there is both a dearth of evidence-based antiviral treatment schedules and a lack of consensus about the role and makeup of combination therapies. Since immunocompromised patients represent one of the groups most vulnerable to severe complications of COVID-19 and are a key part of the persisting burden of the pandemic in the post-vaccination era, the design of combination RCTs is a public health urgency.

Biographies

Daniele Focosi is a hematologist employed as resident transfusion physician at the largest blood bank in Italy since 2009. He has been a transplant immunologist and immunogeneticist, quality assurance manager and production manager. He has received awards from the European Federation of Immunogenetics, the European Society of Organ Transplantation, and the Italian Society of Hematology. He has a Ph.D. degree in Clinical and Fundamental Virology, and a master’s degree in clinical Trials. He has authored 254 articles indexed in PubMed, for a global h-index of 42, on topics ranging from emerging viral infections to new markers of immune competence.

Massimo Franchini specialized in hematology (1995) at the University of Verona (Italy). He is currently Director of the Department of Hematology and Transfusion Medicine of the Hospital of Mantua (Italy). He is Associate Editor of Seminars in Thrombosis and Hemostasis and Member of the Committee of the Italian Association of Hemophilia Centers (AICE) for the revision of the Italian Guidelines on the Management of Hemophilia. He is a member of the Regional Hematology Network (REL, Lombardy Region), Subcommittee of Hemostasis, and served as a consultant to the Italian Ministry of Health–National Blood Center (2016 to 2020). His field of scientific research is predominantly dedicated to hemostasis and thrombosis. He is currently studying the COVID-19 convalescent plasma, from biological validation to its clinical use. He has authored more than 650 publications for a h-index of 83.

Fabrizio Maggi, M.D., graduated at the University of Pisa, Specialist in Microbiology and Virology, Ph.D. in “Clinical and Fundamental Virology” at the University of Pisa. He is Head of the Laboratory of Virology and High Containment Laboratories at the Spallanzani National Institute for Infectious Diseases, Rome. He is Associate Professor at the Department of Medicine and Surgery of the University of Insubria. He was Head of Division of Microbiology, ASST Settelaghi, and Director of Specialization School in Microbiology and Virology at the University of Insubria. He had awards for research on HIV from ANLAIDS Association (National Association against AIDS), Unesco, and International Foundation for AIDS Research and for research on HIV from the Istituto Superiore di Sanità. He holds teaching courses of Microbiology and Clinical Microbiology at the University of Insubria. His main interest is in the natural history and pathogenesis of anelloviruses infection and development of methods for anelloviruses diagnosis. Additional areas of research include emerging respiratory viruses (metapneumovirus, bocavirus), hepatitis C virus, human immunodeficiency virus, feline immunodeficiency virus, and diagnostic virology. He is member of various scientific societies such as Italian Society of Microbiology, Italian Society of Virology, Italian Society of Medical Virology, and Association of Italian Microbiology Clinicians, and serves as referee for a number of international scientific journals. He has published over 250 refereed papers and abstracts as well as several chapters in national and international scientific books. Since 2022 he is the Director of the Virology Laboratory at the National Institute for Infectious Diseases “Lazzaro Spallanzani” in Rome.

Shmuel Shoham is a professor of clinical medicine at Johns Hopkins University School of Medicine in Baltimore, Maryland. He has over 20 years of experience in management of patients with invasive infections and is the author or co-author of over 150 original articles, book chapters and topic reviews. His clinical work and research focused upon infections in transplant and cancer patients. Dr. Shoham serves as a reviewer and expert consultant to multiple journals, professional societies, hospitals and government agencies in the US and numerous countries abroad. He is a member of professional guideline committees for the Infectious Diseases Society of America (IDSA), the American Society of Transplantation (AST), the European Organization for Research and Treatment of Cancer/Invasive Fungal Infections Cooperative Group and Infectious Diseases Mycoses Study Group (EORTC/MSG) and the National Comprehensive Cancer Network (NCCN). Dr. Shoham received his medical degree from Thomas Jefferson University (Philadelphia, PA) and medical residency and infectious diseases fellowship training at Boston University School of Medicine (Boston, MA).