Additional Study Information: In Phase 1b, the purpose is to learn how well patients with Metastatic Castration-Resistant Prostate Cancer (mCRPC) tolerate the treatment when PLX2853 is added to ongoing treatment with abiraterone acetate and prednisone. We are looking for the highest dose of PLX2853 that can be given safely to patients with your type of disease who are already on abiraterone acetate. We will also look at how your body absorbs and breaks down PLX2853 and abiraterone and how effective the combination of PLX2853 and abiraterone acetate is in treating your disease. PLX2853 works by targeting and slowing down certain activities within cells that promote tumor growth. By inhibiting these activities, PLX2853 may help to stabilize or reduce the growth of tumor cells. PLX2853 has shown anti-cancer activity in laboratory studies; however, it is not yet known whether it will work in humans, or if taking PLX2853 and abiraterone acetate together is better than taking abiraterone acetate alone. In the other Phase 2a part of the study we are trying to determine how effective PLX2853 in combination with abiraterone acetate or PLX2853 in combination with olaparib is at treating subjects with mCRPC. We will also look at how PLX2853 and abiraterone or olaparib are broken down and absorbed in the body.