Molecular Features and Pathways in Predicting Drug Resistance in Patients With Metastatic Castration-Resistant Prostate Cancer Receiving Enzalutamide

Inclusion Criteria: * Histologically or cytologically confirmed adenocarcinoma of the prostate without neuroendocrine differentiation or small cell features * Ongoing androgen deprivation therapy with a gonadotropin-releasing hormone (GnRH) analogue or orchiectomy (i.e., surgical or medical castration); for patients who have not had an orchiectomy, there must be a plan to maintain effective GnRH-analogue therapy for the duration of the trial * Radiographic evidence of regional or distant metastases with suspected tumor in an area that is safe to biopsy * Willingness to undergo a tumor biopsy at baseline and at disease progression * Serum testosterone level \< 50 ng/dL at screening * Progressive disease by PSA or imaging in the setting of medical or surgical castration; disease progression for study entry is defined as one or more of the following three criteria: * PSA progression defined by a minimum of three rising PSA levels with an interval of \>= 1 week between each determination; the PSA value at screening should be \>= 2 ug/L (2 ng/ml) * Soft tissue disease progression defined by Response Evaluation Criteria in Solid Tumors (RECIST 1.1) * Bone disease progression defined by two or more new lesions on bone scan * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 * Clinically able, in the opinion of the investigator, to receive MDV3100 (enzalutamide) * Willing and able to give informed consent * A minimum of 4 weeks elapsed off of anti-androgen t…