Abemaciclib Before 177Lu-PSMA-617 for the Treatment of Metastatic Castrate Resistant Prostate Cancer

PRIMARY OBJECTIVES: I. To determine the recommended phase II dose (RP2D) for abemaciclib given as lead-in treatment prior to lutetium Lu 177 vipivotide tetraxetan (177Lu-PSMA-617) for each treatment cycle, as well as dose limiting toxicities (DLTs) of this combination regimen. (Part A) II. To determine the change in prostate-specific membrane antigen (PSMA) uptake on gallium Ga 68 gozetotide (68Ga-PSMA-11) positron emission tomography (PET) scan following fourteen days of priming with abemaciclib treatment, relative to the pre-treatment baseline scan. (Part B (Expanded Cohort)). SECONDARY OBJECTIVES: I. To determine the (proportion of patients who experience \>= 50% decline from baseline in serum prostate specific antigen (PSA) (PSA50) response of this combination treatment. (Part B \[Expanded Cohort\]) II. To describe the safety of this combination treatment regimen using National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version (v.)5.0. (Part B \[Expanded Cohort\]) III. To determine the radiographic progression free survival (rPFS) according to Prostate Cancer Working Group 3 (PCWG3) guidelines among patients treated with this combination regimen. (Part B \[Expanded Cohort\]) IV. To determine the objective response rate (ORR) (complete response \[CR\] + partial response \[PR\]) as measured by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 response in soft tissue, lymph node and visceral lesions. (Part B \[Expanded Cohort\]) V. To determine the disease control rate (DCR) (CR + PR + stable disease (SD)) as measured by RECIST v1.1 response in soft tissue, lymph node and visceral lesions. (Part B \[Expanded Cohort\]) VI. To determine the overall survival (OS) of patients treated with this combination regimen. (Part B \[Expanded Cohort\]) VII. To determine the median duration of response (DOR) in patients treated with this combination regimen who achieve CR or PR as measured by RECIST v1.1. (Part B \[Expanded Cohort\]) EXPLORATORY OBJECTIVES: I. To assess changes in tumor microenvironment using ribonucleic acid (RNA) and whole exome sequencing by comparing biopsies obtained pre and post-combination treatment using the established institutional biopsy protocol (PSMA biopsy study). II. To assess changes in PSMA expression in biopsies at the time of progression relative to pre-treatment biopsies using immunohistochemistry (IHC). III. To describe PSMA upregulation on imaging following 7 days of treatment with abemaciclib, and in particular compare to PSMA upregulation on imaging seen following 14 days of abemaciclib treatment. IV. To describe PSMA expression and upregulation on imaging following combined treatment with abemaciclib and 177Lu-PSMA-617 at the time of subsequent therapy cycles (pre and post abemaciclib treatment with cycle 3) in some patients treated with RP2D in Part B (expansion cohort). V. To describe patterns of progression following completion of treatment, including in patients with available 68Ga-PSMA-11 PET scans. VI. To compare response to treatment and clinical outcomes with different dose levels of abemaciclib used in this study. OUTLINE: This is a dose-escalation study of abemaciclib. Patients receive abemaciclib orally (PO) twice daily (BID) on days 1-14 and lutetium Lu 177 vipivotide tetraxetan intravenously (IV) over 30 minutes on day 15. Treatment repeats every 6 weeks for up to 4 cycles in the absence of disease progression or unacceptable toxicity. After completion of study intervention, patients are followed up at 30 days, and then will subsequently be followed for overall survival in the long-term follow-up period and contacted approximately every 3 months until consent is withdrawn, death occurs, patient is lost to follow-up or the study is concluded.