A Signal-finding Study of Abemaciclib in Heavily Pretreated Patients with Metastatic Castration-Resistant Prostate Cancer: Results from CYCLONE 1.

BACKGROUND:Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors radically changed the treatment paradigm for breast cancer. Similar to estrogen receptor in breast cancer, androgen receptor signaling activates cyclin D-CDK4/6, driving proliferation and resistance to hormonal manipulation in prostate cancer. This study was designed to detect signals of clinical activity for abemaciclib in treatment-refractory metastatic castration-resistant prostate cancer (mCRPC). METHODS:Eligible patients had progressive mCRPC, measurable disease, and previously received ≥1 novel hormonal agent(s) and 2 lines of taxane chemotherapy. Abemaciclib 200 mg BID was administered on a continuous dosing schedule. Primary endpoint was objective response rate (ORR) without concurrent bone progression. This study was designed to detect a minimum ORR of 12.5%. RESULTS:At trial entry, forty (90.9%) of 44 patients had objective radiographic disease progression, four (9.1%) had prostate-specific antigen (PSA)-only progression, and 20 (46.5%) had visceral metastases (of these, 60% had liver metastases). Efficacy analyses are as follows: ORR without concurrent bone progression: 6.8%; disease control rate: 45.5%; median time to PSA progression: 6.5 months (95% CI: 3.2, NA); median radiographic PFS; 2.7 months (95% CI: 1.9, 3.7); and median OS: 8.4 months (95% CI: 5.6, 12.7). Most frequent grade ≥3 TEAEs were neutropenia (25.0%), anemia, and fatigue (11.4% each). No grade 4 or 5 AEs were related to abemaciclib. CONCLUSION:Abemaciclib monotherapy was well tolerated and showed clinical activity in this heavily pretreated population, nearly half with visceral metastases. This study is considered preliminary proof-of-concept and designates CDK4/6 as a valid therapeutic target in prostate cancer.