Comprehensive Nonclinical Safety Assessment of Nirmatrelvir Supporting Timely Development of the SARS-COV-2 Antiviral Therapeutic, Paxlovid™

Abstract

COVID-19 is a potentially fatal infection caused by the SARS-CoV-2 virus. The SARS-CoV-2 3CL protease (Mpro) is a viral enzyme essential for replication and is the target for nirmatrelvir. Paxlovid (nirmatrelvir co-administered with the pharmacokinetic enhancer ritonavir) showed efficacy in COVID-19 patients at high risk of progressing to hospitalization and/or death. Nonclinical safety studies with nirmatrelvir are essential in informing benefit-risk of Paxlovid and were conducted to support clinical development. In vivo safety pharmacology assessments included a nervous system/pulmonary study in rats and a cardiovascular study in telemetered monkeys. Potential toxicities were assessed in repeat dose studies of up to 1 month in rats and monkeys. Nirmatrelvir administration (1,000 mg/kg, p.o.) to male rats produced transient increases in locomotor activity and respiratory rate but did not affect behavioral endpoints in the functional observational battery. Cardiovascular effects in monkeys were limited to transient increases in blood pressure and decreases in heart rate, observed only at the highest dose tested (75 mg/kg per dose b.i.d; p.o.). Nirmatrelvir did not prolong QTc-interval or induce arrhythmias. There were no adverse findings in repeat dose toxicity studies up to 1 month in rats (up to 1,000 mg/kg daily, p.o.) or monkeys (up to 600 mg/kg daily, p.o.). Nonadverse, reversible clinical pathology findings without clinical or microscopic correlates included prolonged coagulation times at ≥60 mg/kg in rats and increases in transaminases at 600 mg/kg in monkeys. The safety pharmacology and nonclinical toxicity profiles of nirmatrelvir support clinical development and use of Paxlovid for treatment of COVID-19.

Keywords: COVID-19; antiviral; drug safety; nirmatrelvir; paxlovid; safety pharmacology; toxicology.

Figure 1.

Changes in quantitative locomotor activity and pulmonary endpoints in rats following a single acute dose of nirmatrelvir. (A,B) Group means ± SEM (n = 6 males/group) are presented for quantitative locomotor activity recorded for 60 min at approximately 1-1.5 HPD (immediately following the FOB assessment at 1 h postdose). (C-E) Group means ± SEM (n = 6 males/group) are presented for pulmonary function parameters assessed continuously for 6 h postdose. The test article contained approximately 15% MTBE; therefore, the amount of MTBE administered to the MTBE control group (150 mg/kg) was approximately the amount of MTBE present in the high dose of 1,000 mg/kg nirmatrelvir. Statistically significant differences are highlighted between 1,000 mg/kg and vehicle (*P < 0.05), between 60 mg/kg and vehicle (#P < 0.05), and between MTBE Control and vehicle (^P < 0.05).

Figure 2

.Summary of effects of nirmatrelvir cardiovascular parameters in cynomolgus monkeys. Each individual animal (n = 8) was administered each treatment using a latin square cross-over design with sufficient washout period between dosing days to ensure clearance of nirmatrelvir. Following continuous collection, data were initially reduced to 1-minute means and further processed to provide 15 min mean values which were binned into four postdose periods. The data are represented as the change in each endpoint (mean ± 95% confidence interval) compared with vehicle control values for systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and QTc interval, respectively, which are available in Supplemental Table 1. Statistical differences due to treatment were determined using ANOVA analysis. *P < 0.05 indicates statistically significant difference compared to vehicle.

Figure 3.

Representative microscopic changes in the liver, thyroid gland, and pituitary gland of rats following administration of nirmatrelvir (1,000 mg/kg) in a 1-month toxicity study. (A) Liver: periportal hepatocellular hypertrophy characterized by slight enlargement of periportal hepatocytes with abundant finely granular eosinophilic cytoplasm and sinusoidal compression. Peri: periportal area. Ctr: centrilobular area. Two out of 10 males and 2/10 females in the 200 mg/kg group, and 9/10 males and 10/10 females in the 1,000 mg/kg group had periportal hepatocellular hypertrophy. (B) Liver: normal. (C) Thyroid gland: follicular cell hypertrophy characterized by increased size and height of follicular cells with more amphophilic to vacuolated cytoplasm and decreased colloid in the follicle lumens with variable staining intensity. One out of 10 males in the 60 mg/kg group, 4/10 males and 2/10 females in the 200 mg/kg group, and 10/10 males and 10/10 females in the 1000 mg/kg group had thyroid follicular cell hypertrophy. (D) Thyroid gland: normal. (E) Pituitary gland: vacuolated cells (basophils or chromophobes) within pars distalis are slightly enlarged and have a large single or several cytoplasmic vacuoles with displaced nuclei. Two out of 10 males in the 60 mg/kg group, 2/10 males in the 200 mg/kg group, and 5/10 males in the 1,000 mg/kg group exhibited vacuolated cells in the pituitary gland. (F) Pituitary gland: normal. Scale bar = 50 μm. H&E staining.