Abemaciclib With or Without Atezolizumab for mCRPC

This is a multi-center, open label Phase II study of patients with metastatic castration resistant prostate cancer (mCRPC) who will be treated with abemaciclib and atezolizumab alone or in combination. The U.S. Food and Drug Administration (FDA) has not approved abemaciclib or atezolizumab alone or in combination for use in prostate cancer. Abemaciclib is an orally administered molecularly targeted chemotherapy drug called a cyclin-dependent kinase inhibitor, which acts to block the ability of cancer cells to divide and thus prevents tumors from growing. In the laboratory setting, this drug is effective in prostate cancer models that have become resistant to standard hormonal treatments, and this drug is currently being studied for its effectiveness in prostate cancer in other clinical trials. Atezolizumab is an intravenously administered drug called an immune checkpoint inhibitor, which acts to activate the immune system to kill cancer cells. Atezolizumab is ineffective on its own in most patients with prostate cancer, but is being tested in combination with other drugs for prostate cancer in other clinical trials. Multiple research groups have demonstrated in laboratory model systems that abemaciclib can may make immune checkpoint inhibitors more effective. The research study procedures include screening for eligibility and study treatment including evaluations and follow up visits. The study design divides study participants into two separate cohorts. The first cohort is a set of subjects whose tumors are not known to have mutations in the CDK12 gene (the "biomarker unselected cohort") - either because tumor tissue never underwent genetic profiling, or because genetic profiling was performed but did not demonstrate a mutation in the CDK12 gene. In this "biomarker unselected cohort," this study will be testing whether abemaciclib alone or in combination with atezolizumab is an effective treatment strategy. The second cohort of participants is a set of subjects whose tumors are known to have mutations in the CDK12 gene based on genetic profiling of the tumor that occurred prior to enrollment on this study. Prior studies suggest that cancers with mutations in the CDK12 gene can shrink in response to immune checkpoint inhibitors. This study will be testing in study participants whose tumors are known to have mutations in CDK12 whether atezolizumab alone or in combination with abemaciclib is an effective treatment strategy. In addition, the trial is testing the safety of the combination of the two drugs in both cohorts. Participants will receive study treatment for as long as they do not have serious side effects and their disease does not get worse. Participants will be followed after completion of study treatment for up to 24 months It is expected that about 75 people will take part in this research study. Eli Lilly and Company is supporting this research study by providing funding for research and the study drug abemaciclib. Genentech, Inc. is supporting the study by providing the study drug atezolizumab.