A Randomized Phase 2/3 Study of Ensitrelvir, a Novel Oral SARS-CoV-2 3C-Like Protease Inhibitor, in Japanese Patients with Mild-to-Moderate COVID-19 or Asymptomatic SARS-CoV-2 Infection: Results of the Phase 2a Part

Antimicrob Agents Chemother. 2022 Oct 18;66(10):e0069722. doi: 10.1128/aac.00697-22. Epub 2022 Sep 13.


This multicenter, double-blind, phase 2a part of a phase 2/3 study assessed the efficacy and safety of ensitrelvir, a novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) 3C-like protease inhibitor, in Japanese patients with mild-to-moderate coronavirus disease 2019 (COVID-19) or asymptomatic SARS-CoV-2 infection. Sixty-nine patients were randomized (1:1:1) to orally receive 5-day ensitrelvir fumaric acid (375 mg on day 1 followed by 125 mg daily, or 750 mg on day 1 followed by 250 mg daily) or placebo and followed up until day 28. The primary outcome was the change from baseline in the SARS-CoV-2 viral titer. A total of 16, 14, and 17 patients in the ensitrelvir 125 mg, ensitrelvir 250 mg, and placebo groups, respectively, were included in the intention-to-treat population (mean age: 38.0 to 40.4 years). On day 4, the change from baseline in SARS-CoV-2 viral titer (log10 50% tissue culture infectious dose/mL) in patients with positive viral titer and viral RNA at baseline was greater with ensitrelvir 125 mg (mean [standard deviation], -2.42 [1.42]; P = 0.0712) and 250 mg (-2.81 [1.21]; P = 0.0083) versus placebo (-1.54 [0.74]); ensitrelvir treatment reduced SARS-CoV-2 RNA by -1.4 to -1.5 log10 copies/mL versus placebo. The viral titer and viral RNA were similar across groups on and after day 6. The median time to infectious viral clearance decreased by approximately 50 h with ensitrelvir treatment. All adverse events were mild to moderate. Ensitrelvir treatment demonstrated rapid SARS-CoV-2 clearance and was well tolerated (Japan Registry of Clinical Trials identifier: jRCT2031210350).

Keywords: COVID-19; Japanese; S-217622; SARS-CoV-2 3C-like protease inhibitor; ensitrelvir; proof-of-concept study; viral titer.

Publication types

  • Randomized Controlled Trial
  • Multicenter Study
  • Clinical Trial, Phase II
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Anti-Infective Agents*
  • Antiviral Agents
  • COVID-19 Drug Treatment*
  • Double-Blind Method
  • Enzyme Inhibitors
  • Humans
  • Japan
  • Protease Inhibitors
  • RNA, Viral
  • SARS-CoV-2


  • RNA, Viral
  • Protease Inhibitors
  • Antiviral Agents
  • Enzyme Inhibitors
  • Anti-Infective Agents

Associated data

  • JPRN/jRCT2031210350