Background: Despite recent advances, nearly all patients (pts) with metastatic castration-resistant prostate cancer (mCRPC) experience disease progression and cancer-specific mortality. Persistent or reactivated androgen receptor (AR) signaling and/or activation of pathways in cross-talk with AR signaling are key drivers of mCRPC progression. Evidence suggests that AR signaling promotes translation of D-type cyclins resulting in cyclin-dependent kinase 4 and 6 (CDK4&6) activation and cell cycle progression. Abemaciclib is an oral selective inhibitor of CDK4&6 dosed on a continuous schedule, that is FDA-approved in combination with endocrine therapy or as monotherapy to treat HR+, HER2- metastatic breast cancer pts. Preclinical studies with prostate cancer cell lines and xenograft models showed that abemaciclib induces cell cycle arrest and tumor growth inhibition. The hypothesis is that addition of abemaciclib to AR targeted therapy may be an effective treatment for mCRPC pts. Methods: CYCLONE 2 (NCT03706365) is a phase II, randomized, double-blind, multicenter, placebo-controlled study to assess the safety and efficacy of abemaciclib in combination with abiraterone acetate plus prednisone (AA+P) as first-line treatment of pts with mCRPC. The study is designed in two parts. Part 1 is a 30-patient safety lead-in to determine the recommended phase II dose (RP2D; 150 mg or 200 mg, twice daily) of abemaciclib in combination with AA (1000 mg, once daily) + P (5 mg, twice daily). In part 2, 150 pts are randomized 1:1 to abemaciclib at the RP2D with AA+P or placebo with AA+P. Pts who received prior AA+P, enzalutamide, apalutamide, darolutamide, radiopharmaceuticals, or sipuleucel-T are excluded. Prior docetaxel for metastatic hormone-sensitive prostate cancer, but not for mCRPC, is allowed. Pts must have progressive mCRPC (by PSA and/or imaging) and an accessible metastatic lesion for tumor biopsy. The co-primary objectives are radiographic PFS (per RECIST1.1 for soft tissue and PCWG3 for bone) and time to PSA progression. Secondary objectives include safety, objective response rate, duration of response, OS, time to symptomatic progression, and pharmacokinetics. Assuming hazard ratios of 0.64 (rPFS) and 0.6 (PSA progression), the study is powered to 80% and 85%, respectively, to test the superiority of abemaciclib plus AA+P vs. placebo plus AA+P at one-sided α=0.1 using stratified log-rank tests. Part 1 is completed and part 2 is enrolling in 70 sites worldwide. Clinical trial information: NCT03706365.