We conducted an exploratory study to assess the safety tolerability, and anti-fibrotic effects of PRI-724, a CBP/β-catenin inhibitor, in patients with hepatitis C virus (HCV)- and hepatitis B virus (HBV)-induced cirrhosis.This multicentre, open-label, non-randomised, non-placebo-controlled phase 1/2a trial was conducted at three hospitals in Japan. Between July 27, 2018, and July 13, 2021, we enrolled patients with HCV- and HBV-induced cirrhosis classified as Child-Pugh (CP) class A or B. In phase 1, 15 patients received intravenous infusions of PRI-724 at escalating doses of 140, 280, and 380 mg/m2/4 h twice weekly for 12 weeks. In phase 2a, 12 patients received the recommended PRI-724 dose. The primary endpoints of phases 1 and 2a were the frequency and severity of adverse events and efficacy in treating cirrhosis based on liver biopsy. This study was registered at ClinicalTrials.gov (no. NCT03620474).Three patients from phase 1 who received the recommended PRI-724 dose were evaluated to obtain efficacy and safety data in phase 2a. Serious adverse events occurred in three patients, one of which was possibly related to PRI-724. The most common adverse events were diarrhoea and nausea. PRI-724 did not decrease hepatic fibrosis with any statistical significance, either by ordinal scoring or measurement of collagen proportionate area at 12 weeks; however, we observed statistically significant improvements in liver stiffness, Model for End-stage Liver Disease score, and serum albumin level.Intravenous administration of 280 mg/m2/4 h PRI-724 over 12 weeks was preliminarily assessed to be well tolerated; however, further evaluation of anti-fibrotic effects in patients with cirrhosis is warranted.AMED, Ohara Pharmaceutical.