This study was designed in accordance with the FDA draft guidance on pharmacokinetics in participants with impaired renal function [10]. The renal function was classified based on the FDA draft guidance [10], and renal impairment participants were assigned according to their estimated glomerular filtration rate (eGFR), which was calculated by modification of diet using the renal disease (MDRD) formula [11] and the participant’s body surface area at screening (day − 28 to day − 2 relative to day 1): normal renal function (control participants eGFR ≥ 90 ml/min, n = 8), mild impairment (eGFR 60–89 ml/min, n = 8), moderate impairment (eGFR 30–59 ml/min, n = 8), and severe impairment (eGFR < 30 ml/min, n = 8). Participants with severe renal impairment were not included if they required hemodialysis. Each of the eight participants with normal renal function (control participants) was matched to a participant with moderate renal impairment with respect to sex, age (± 5 years), and BMI (± 10%).

A Phase 1, open-label, nonrandomized, parallel-group study was conducted to access the pharmacokinetics, safety, and tolerability of ensitrelvir in participants with mild, moderate, and severe renal impairment and participants with normal renal function. The participants received a single oral administration of ensitrelvir 375 mg (three 125-mg tablets). This clinical study was conducted from July 2022 to May 2023.

This study (NCT05363215) was conducted in accordance with the study protocol approved by the Declaration of Helsinki and Council for International Organizations of Medical Sciences International Ethical Guidelines, the Institutional Review Board, the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use Good Clinical Practice Guidelines, and other regulations and applicable laws. The Institutional Review Board also approved the study. All participants in this study signed a written informed consent form before participation [12,13,14].

Plasma ensitrelvir concentrations were determined by liquid chromatography with tandem mass spectrometry (LC/MS/MS). Acetonitrile was used for the protein precipitation. API4000 (Sciex, Framingham, MA) was used for the LC/MS/MS analysis. The determination method was validated over a range of 200–200,000 ng/ml, and the lower limit of quantification (LLOQ) of the method was 200 ng/ml.

Pharmacokinetic blood samples were collected at 0, 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 12, 16, 24, 48, 72, 96, 120, 144, 192, 240, 288, and 336 h postdose. Blood samples for protein binding were collected at 3 and 24 h postdose. Pharmacokinetic urine samples were collected at 0–24, 24–48, 48–72, 72–96, 96–120, 120–144, 144–168, 168–192, 192–216, 216–240, 240–264, 264–288, 288–312, and 312–336 h postdose.

The mean and standard deviations (SD) for plasma ensitrelvir concentrations were calculated by group and sampling time. In this study, the plasma ensitrelvir concentrations below the LLOQ value (200 ng/ml) were treated as zero for calculating the mean and SD for plasma ensitrelvir concentrations. Non-compartmental analysis was used to calculate the following pharmacokinetic parameters based on the plasma ensitrelvir concentrations: Cmax, time to maximum plasma concentration (Tmax), area under the plasma concentration-time curve from time zero to the time of last quantifiable concentration after dosing (AUC0–last), AUC0–inf, terminal elimination half-life (t1/2,z), terminal elimination rate constant (λz), and mean residence time (MRT). In addition, the unbound fraction in plasma (fu) was also assessed at 3 and 24 h following a single-dose administration of ensitrelvir. The cumulative amount of drug excreted unchanged in the urine (Ae), fraction of dose excreted unchanged into urine (Feu), and renal clearance of ensitrelvir (CLR) were estimated for each participant or patient with urinary excretion data. The pharmacokinetic parameters were calculated using Phoenix WinNonlin version 8.3 (Certara L.P., Princeton, NJ, USA).

Analysis of variance (ANOVA) was conducted using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA) using the Proc Mixed procedure, which included terms for renal status as a fixed effect for the ln-transformed values of the pharmacokinetic parameters. Point estimates and 90% confidence intervals (CIs) were calculated for the ratios of parameters for participants with mild, moderate, and severe renal impairment compared with participants with normal renal function.

Safety was assessed including the analysis of all treatment-emergent adverse events (TEAEs), which were categorized according to the System Organ Class and Preferred Term of MedDRA version 25.0. The nature, frequency, and severity of TEAEs were evaluated and recorded.