Abemaciclib With or Without Atezolizumab for mCRPC

Inclusion Criteria: * Diagnosis of metastatic castration resistant prostate cancer (mCRPC), with histologic confirmation of adenocarcinoma of the prostate (without evidence of small cell carcinoma), with progressive disease at the time of study entry by either * Sequence of at least 2 rising PSA values at a minimum of 1-week intervals * Radiographic progression per RECIST1.1 for soft tissue and/or per PCWG3 for bone, with or without PSA progression * Adult males 18 years of age or older. * ECOG performance status of 0 or 1 * Patients must have metastatic disease by bone scintigraphy or other nodal or visceral lesions on CT or MRI with a bone or soft tissue lesion amenable to image-guided percutaneous biopsy, and the patient must be evaluable for disease response by either * Baseline PSA ≥ 2.0 ng/mL OR * Measurable disease per RECIST 1.1 * Past progression or intolerance to at least one novel antiandrogen therapy (abiraterone, enzalutamide, galeterone, apalutamide, darolutamide, orteronel, seviteronel or equivalent) in either the hormone-sensitive or castration-resistant disease setting. * Not a candidate for docetaxel or cabazitaxel chemotherapy due to: * progression within 12 months of completion or intolerance to prior taxane OR * refusal of taxane OR * contraindication to, or lack of fitness for taxane OR * Investigator assessment that taxane is not clinically indicated or preferred. * Maintenance of castration status, defined as serum testosterone leve…