Abemaciclib With or Without Atezolizumab in Metastatic Castration Resistant Prostate Cancer

This research is studying two experimental drugs, abemaciclib and atezolizumab, alone and in combination with each other, to learn about the safety and effectiveness of these treatments and their side effects. This is an investigational study treatment for adult men with metastatic castrate resistant prostate cancer (mCRPC) who have progressive disease despite previous treatment with androgen deprivation therapy (ADT). One group of men (men without a genetic mutation called "CDK12 loss") will receive abemaciclib therapy alone. Two other groups of men (men with CDK12 loss in one group and men without CDK12 loss in the other) will receive the combination of abemaciclib and atezolizumab. Another group of men with CDK12 loss will receive atezolizumab therapy alone.