AMG 208 Tumor Microenvironment in Metastatic Castration Resistant Prostate Cancer (mCRPC)

Study Drug Administration: If participant is found to be eligible to take part in this study, they will take capsules of AMG208 by mouth 1 time every day while they are on study. Participant should fast (not have anything except water) for 2 hours before and 1 hour after they take the AMG208 capsules. Participant will be asked to keep a diary to help them keep track of when they take each dose of the study drug. Participant should record any missed or vomited doses in the diary. If participant vomits their dose, they should not re-take their dose. Participant should wait until their next scheduled dose. Participant should bring the diary with them to each visit. Every 6 weeks is considered a cycle. Participant should return all unused study drug and/or empty pill bottles at the end of each cycle. Study Visits: On Day 1 of Cycle 1 (+/ - 3 days): * Blood (about 3-4 tablespoons) will be drawn for biomarker testing. * Participant will have a bone marrow aspiration and biopsy to check the status of the disease and for biomarker testing. To collect a bone marrow aspirate/biopsy, an area of the hip is numbed with anesthetic, and a small amount of bone marrow and bone is withdrawn through a large needle. Biomarkers are found in the blood/tissue and may be related to participant's reaction to the study drug. On Day 22 of Cycle 1: * Participant will have a physical exam. * Participant will have an EKG. Participant will have 2 more EKGs if the study doctor thinks it is needed. * Blood (about 3-4 tablespoons) and urine will be collected for routine tests. This blood will also be used to check participant's PSA level, to measure CTCs, and for biomarker testing. * Participant will fill out the questionnaire about pain. On Day 1 of Cycles 2 and every 6 weeks after that: * Participant will have a physical exam. * Blood (about 3-4 tablespoons) and urine will be collected for routine tests. This blood will also be used to check participant's PSA and testosterone levels. * Participant will fill out the questionnaire about pain. On Day 1 of Cycles 2 and 3, and every 12 weeks after that: * Participant will have a bone scan and a CT or MRI scan of the chest, abdomen, pelvis, and any other area the study doctor thinks is needed to check the status of the disease. * Participant will have an EKG. Participant will have 2 more EKGs if the study doctor thinks it is needed. * Blood (about 2-3 tablespoons) will be drawn to check participant's thyroid function, and for CTC and biomarker testing. * Participant will have either an ECHO or a MUGA scan. (Cycles 2 and 5 only and every 12 weeks after that) On Day 1 of Cycle 2 participant will have a bone marrow aspiration and biopsy to check the status of the disease. On Day 22 of Cycle 2: * Participant will have a physical exam. * Blood (about 3-4 tablespoons) and urine will be collected for routine tests. This blood will also be used to check participant's PSA level. * Participant will fill out the questionnaire about pain. On Day 1 of Cycle 5, participant will have a bone marrow aspiration to check the status of the disease. Length of Study: Participant may continue taking the study drug for as long as the doctor thinks it is in their best interest. Participant will no longer be able to take the study drug if the disease gets worse, if intolerable side effects occur, or if they are unable to follow study directions. Patient's participation on the study will be over after the follow-up visit. End-of-Study Visit: If they have not been done within the last week, within 5 days after participant's last dose of study drug, the following tests and procedures will be performed: * Participant will have a physical exam. * Blood (about 3-4 tablespoons) and urine will be collected for routine tests. This blood will also be used to check participant's PSA levels, thyroid function, for biomarker testing, and to measure CTCs. * Participant will have a bone scan and a CT or MRI scan of the chest, abdomen, pelvis, and any other area the study doctor thinks is needed to check the status of the disease. * Participant will have a bone marrow aspiration and biopsy to check the status of the disease. * Participant will fill out the questionnaire about pain. Follow-Up Visit: About 30 days after participant stops taking the study drug, the following procedures will be performed: * Participant will have a physical exam. * Blood (about 3-4 tablespoons) and urine will be collected for routine tests. This blood will also be used to check participant's PSA levels. This is an investigational study. AMG208 is not FDA approved or commercially available. It is being used for research purposes only. The study doctor can explain how the study drug is designed to work. Up to 30 participants will be enrolled in this study. All will take part at MD Anderson.