A Phase 1 study of abemaciclib plus abiraterone in Japanese patients with metastatic castration-resistant prostate cancer.

The aim of this Phase 1, multicentre, open-label study was to evaluate the safety, tolerability and pharmacokinetics (PK) of abemaciclib administered at global recommended Phase 2 dose (RP2D) of 200 mg twice daily, combined with standard doses of abiraterone and prednisolone, in Japanese patients with metastatic castration-resistant prostate cancer (mCRPC). Dose-limiting toxicities (DLTs) were assessed for 28 days post-first dose. Six patients were treated, and all experienced at least one treatment-emergent adverse event (TEAE), mostly low grade; no Grade 4 or 5 TEAEs occurred. Diarrhoea was the most common TEAE (all events were Grade 1 except for one Grade 2). Three patients experienced serious adverse events (SAEs), leading to treatment discontinuation in two cases. The PK profile was consistent with non-Japanese patients, with no PK drug-drug interactions detected. The study confirms that the global RP2D of abemaciclib is suitable for Japanese patients with mCRPC treated with abiraterone and prednisolone.