Antiviral efficacy of oral ensitrelvir versus oral ritonavir-boosted nirmatrelvir in COVID-19

Background: Ensitrelvir is an oral antiviral treatment for COVID-19 with the same molecular target as ritonavir-boosted nirmatrelvir - the current oral first-line treatment. There have been no direct comparisons between the two drugs. Methods: In an open label controlled adaptive pharmacometric platform trial, low-risk adult patients aged 18-60 years with early symptomatic COVID-19 (<4 days of symptoms) were randomised concurrently to one of eight treatment arms including ensitrelvir, ritonavir-boosted nirmatrelvir, and no study drug. The primary endpoint was the rate of oropharyngeal viral clearance assessed in a modified intention-to-treat population (mITT), defined as patients with >3 days of follow-up. Viral clearance rate was derived under a Bayesian hierarchical linear model fitted to the log10 viral densities in standardised duplicate oropharyngeal swab eluates taken daily over five days (14 measurements). This trial is registered at ClinicalTrials.gov ( NCT05041907 ). Findings: Between March 2023 and April 2024 the three study arms randomised 604 patients concurrently in Thailand and Lao PDR (ensitrelvir 202; ritonavir-boosted nirmatrelvir 207; no study drug 195) among 903 patients enrolled. All patients recovered uneventfully. Ensitrelvir was very well tolerated and did not cause dysgeusia. Median (interquartile range) estimated SARS-CoV-2 clearance half-lives were 5.9 hours (4.0 to 8.6) with ensitrelvir; 5.2 hours (3.8 to 6.6) with nirmatrelvir; and 11.6 hours (8.1 to 14.5) with no study drug. Viral clearance following ensitrelvir was 82% (95% credible interval, CrI: 61 to 104%) faster than no study drug and 16% (95% CrI: 5 to 25%) slower than ritonavir-boosted nirmatrelvir. Viral rebound occurred in 15 (7%) of the nirmatrelvir group and 10 (5%) of the ensitrelvir group (p=0.4). Conclusions: Both ensitrelvir and nirmatrelvir markedly accelerate oropharyngeal SARS-CoV-2 viral clearance. Ensitrelvir is an efficacious and well tolerated alternative to currently available antivirals in treating COVID-19. Funding: Finding treatments for COVID-19: A phase 2 multi-centre adaptive platform trial to assess antiviral pharmacodynamics in early symptomatic COVID-19 (PLAT-COV) is supported by the Wellcome Trust Grant ref: 223195/Z/21/Z through the COVID-19 Therapeutics Accelerator.