Safety, pharmacokinetics, and antiviral activity of the capsid inhibitor AB‐506 from Phase 1 studies in healthy subjects and those with hepatitis B

AB‐506 is a potent, pan‐genotypic small molecule capsid inhibitor that inhibits hepatitis B virus (HBV) pregenomic RNA encapsidation. We assessed the safety, pharmacokinetics, and antiviral activity of AB‐506 in two randomized, double‐blinded Phase 1 studies in healthy subjects (HS) and subjects with chronic HBV infection (CHB). Single ascending and multiple doses of AB‐506 or placebo (30–1000 mg or 400 mg daily for 10 days) were assessed in HS. AB‐506 or placebo was assessed at either 160 mg or 400 mg daily for 28 days in subjects with CHB. A second follow‐up study examined AB‐506 or placebo at 400 mg daily for 28 days in 14 Caucasian and 14 East‐Asian HS. Twenty‐eight days of AB‐506 at 160 mg and 400 mg produced mean HBV‐DNA declines from baseline of 2.1 log(10) IU/ml and 2.8 log(10) IU/ml, respectively. Four subjects with CHB (all Asian) had Grade 4 alanine aminotransferase (ALT) elevations (2 at each dose) as HBV DNA was declining; three events led to treatment discontinuation. In the second follow‐up study, 2 Asian HS had serious transaminitis events leading to treatment and study termination. No subjects had bilirubin elevations or signs of hepatic decompensation. Conclusion: AB‐506 demonstrated mean HBV‐DNA declines of >2 log(10); however, transient but severe ALT flares were observed in 4 Asian subjects with CHB. In the follow‐up study in HS, 2 additional Asian HS had Grade 4 flares, suggesting that AB‐506 hepatotoxicity contributed to the ALT elevations. The AB‐506 development program was terminated because of these findings.