COVID-19 redux: clinical, virologic, and immunologic evaluation of clinical rebound after nirmatrelvir/ritonavir

Viral Rebound After Antiviral Treatment: A Mathematical Modeling Study of the Role of Antiviral Mechanism of Action

The development of antiviral treatments for SARS-CoV-2 was an important turning point for the pandemic. Availability of safe and effective antivirals has allowed people to return back to normal life. While SARS-CoV-2 antivirals are highly effective at preventing severe disease, there have been concerning reports of viral rebound in some patients after cessation of antiviral treatment. In this study, we use a mathematical model of viral infection to study the potential of different antivirals to prevent viral rebound. We find that antivirals that block production are most likely to result in viral rebound if the treatment time course is not sufficiently long. Since these antivirals do not prevent infection of cells, cells continue to be infected during treatment. When treatment is stopped, the infected cells will begin producing virus at the usual rate. Antivirals that prevent infection of cells are less likely to result in viral rebound since cells are not being infected during treatment. This study highlights the role of antiviral mechanism of action in increasing or reducing the probability of viral rebound.

The Design, Synthesis and Mechanism of Action of Paxlovid, a Protease Inhibitor Drug Combination for the Treatment of COVID-19

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has presented an enormous challenge to health care systems and medicine. As a result of global research efforts aimed at preventing and effectively treating SARS-CoV-2 infection, vaccines with fundamentally new mechanisms of action and some small-molecule antiviral drugs targeting key proteins in the viral cycle have been developed. The most effective small-molecule drug approved to date for the treatment of COVID-19 is PaxlovidTM, which is a combination of two protease inhibitors, nirmatrelvir and ritonavir. Nirmatrelvir is a reversible covalent peptidomimetic inhibitor of the main protease (Mpro) of SARS-CoV-2, which enzyme plays a crucial role in viral reproduction. In this combination, ritonavir serves as a pharmacokinetic enhancer, it irreversibly inhibits the cytochrome CYP3A4 enzyme responsible for the rapid metabolism of nirmatrelvir, thereby increasing the half-life and bioavailability of nirmatrelvir. In this tutorial review, we summarize the development and pharmaceutical chemistry aspects of Paxlovid, covering the evolution of protease inhibitors, the warhead design, synthesis and the mechanism of action of nirmatrelvir, as well as the synthesis of ritonavir and its CYP3A4 inhibition mechanism. The efficacy of Paxlovid to novel virus mutants is also overviewed.

Safety Profile of Paxlovid in the Treatment of COVID-19

BACKGROUND

With the urgent and widespread application of Paxlovid, a novel antiviral drug for Coronavirus Disease 2019 (COVID-19) in clinical practice, concerns regarding its actual efficacy and safety have emerged. In order to provide more evidence to support its clinical application, we sought to perform a descriptive analysis of cases who experienced at least one Paxlovid-related adverse event (AEs) and reported to the FDA Adverse Event Reporting System (FAERS) in the post-marketing period.

METHODS

Individual adverse event reports between January 1, 2022 and September 30, 2022, were downloaded from the FAERS website. We completed a descriptive study about the safety of Paxlovid in the treatment of COVID-19. Further, we also analyzed the onset time of Paxlovid-related AEs.

RESULTS

As of 30 September 2022, 16,529 de-duplicated cases were submitted to the FDA, and 5,860 (35.45%) were female. The average age was 58.38 years (S.D. 15.50). Most reports (12,390, 74.96%) were submitted by consumers and 1,436 (8.68%) concerned serious outcomes. The most frequently reported AEs were disease recurrence (7,724, 16.23%), dysgeusia (2,877, 6.05%), and diarrhoea (1,448, 3.04%). The median onset time of Paxlovid-related AEs was 8 days (interquartile range,1-10 days), and most of the cases (2,629, 19.12%) occurred on the day after Paxlovid initiation.

CONCLUSION

This study indicates that the most common AEs reported with Paxlovid in post-marketing experience are consistent with the safety assessment of antiviral drugs. Even without emerging apparent safety concerns, the incidence of serious outcomes was unexpectedly high, and a few cases of potential new AEs occurred.

Nirmatrelvir plus ritonavir in COVID-19: a profile of its use

Oral nirmatrelvir plus ritonavir (Paxlovid™) is an effective treatment option for coronavirus disease 2019 (COVID-19), the illness caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Nirmatrelvir inhibits the main protease of SARS-CoV-2, with ritonavir acting as a pharmacokinetic booster. In the phase II/III EPIC-HR trial, nirmatrelvir plus ritonavir reduced the risk of progression to severe COVID-19 in symptomatic, unvaccinated, non-hospitalized adults with mild-to-moderate COVID-19 at high risk for progression to severe disease. The incidence of COVID-19-related hospitalization or death through day 28 was significantly lower with nirmatrelvir plus ritonavir than with placebo. The efficacy of nirmatrelvir plus ritonavir has also been demonstrated in the real-world setting. Nirmatrelvir plus ritonavir is generally well tolerated, with most adverse events being of mild or moderate severity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40267-022-00971-1.

Incidence of Viral Rebound After Treatment With Nirmatrelvir-Ritonavir and Molnupiravir

Importance

Some patients treated with nirmatrelvir-ritonavir have experienced rebound of COVID-19 infections and symptoms; however, data are scarce on whether viral rebound also occurs in patients with COVID-19 receiving or not receiving molnupiravir.

Objective

To examine the incidence of viral rebound in patients with COVID-19 who were treated with the oral antiviral agents nirmatrelvir-ritonavir and molnupiravir.

Design, Setting, and Participants

This cohort study identified 41 255 patients with COVID-19 who were hospitalized from January 1, 2022, to March 31, 2022, in Hong Kong and assessed 12 629 patients with serial cycle threshold (Ct) values measured. Patients were followed up until the occurrence of the clinical end point of interest, death, date of data retrieval (July 31, 2022), or up to 30 days of follow-up, whichever came first.

Exposures

Molnupiravir or nirmatrelvir-ritonavir treatment.

Main Outcomes and Measures

Viral rebound, defined as a Ct value greater than 40 that decreased to 40 or less.

Results

Of 12 629 patients (mean [SD] age, 65.4 [20.9] years; 6624 [52.5%] male), 11 688 (92.5%) were oral antiviral nonusers, 746 (5.9%) were molnupiravir users, and 195 (1.5%) were nirmatrelvir-ritonavir users. Compared with nonusers, oral antiviral users were older, had more comorbidities, and had lower complete vaccination rates. The mean (SD) baseline Ct value was slightly higher in nirmatrelvir-ritonavir users (22.2 [6.0]) than nonusers (21.0 [5.4]) and molnupiravir users (20.9 [5.4]) (P = .04). Viral rebound occurred in 68 nonusers (0.6%), 2 nirmatrelvir-ritonavir users (1.0%), and 6 molnupiravir users (0.8%). Among 76 patients with viral rebound, 12 of 68 nonusers, 1 of 6 molnupiravir users, and neither of the nirmatrelvir-ritonavir users died of COVID-19.

Conclusions and Relevance

In this cohort study, viral rebound was uncommon in patients taking molnupiravir or nirmatrelvir-ritonavir and was not associated with increased risk of mortality. Given these findings, novel oral antivirals should be considered as a treatment for more patients with COVID-19 in the early phase of the infection.

Viral Kinetics of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Omicron Infection in mRNA-Vaccinated Individuals Treated and Not Treated with Nirmatrelvir-Ritonavir

We measured viral kinetics of SARS-CoV-2 Omicron infection in 36 mRNA-vaccinated individuals, 11 of whom were treated with nirmatrelvir-ritonavir (NMV-r). We found that NMV-r was associated with greater incidence of viral rebound compared to no treatment. For those that did not rebound, NMV-r significantly reduced time to PCR conversion.

Viral and Symptom Rebound in Untreated COVID-19 Infection

Background

There are reports of viral RNA and symptom rebound in people with COVID-19 treated with nirmatrelvir/ritonavir. Since the natural course of viral and symptom trajectories of COVID-19 has not been well described, we evaluated the incidence of viral and symptom rebound in untreated outpatients with mild-moderate COVID-19.

Methods

The study population included 568 participants enrolled in the ACTIV-2/A5401 platform trial who received placebo. Anterior nasal swabs were collected for SARS-CoV-2 RNA testing on days 0-14, 21 and 28. Participants recorded the severity of 13 targeted symptoms daily from day 0 to 28. Viral rebound was defined as ≥0.5 log10 viral RNA copies/mL increase and symptom rebound was defined as a 4-point total symptom score increase from baseline. Baseline was defined as study day 4 (primary analysis) or 8 days from symptom onset (secondary analysis).

Findings

In both the primary and secondary analyses, 12% of participants had viral rebound. Viral rebounders were older than non-rebounders (median 54 vs 47 years, P=0.04). Symptom rebound occurred in 27% of participants after initial symptom improvement and in 10% of participants after initial symptom resolution. The combination of high-level viral rebound to ≥5.0 log10 RNA copies/mL and symptom rebound after initial improvement was observed in 1-2% of participants.

Interpretation

Viral RNA rebound or symptom relapse in the absence of antiviral treatment is common, but the combination of high-level viral and symptom rebound is rare.