This is an open label, Phase 1, dose escalation and dose confirmation study of ZEN003694 in patients with mCRPC.

Inclusion Criteria:

1. Males age ≥ 18 years
2. Metastatic, castrate resistant, histologically confirmed prostate cancer; surgically castrated or continuous medical castration for ≥ 8 weeks prior to screening
3. Serum testosterone < 50 ng/dL determined within 4 weeks of first administration of study drug
4. Prior progression on one or more androgen-receptor/androgen-synthesis inhibitor therapies (e.g. abiraterone, enzalutamide, apalutamide, TAK-700 and/or galeterone) by Prostate Cancer Working Group 2 (PCWG2) criteria. Prior progression on bicalutamide/nilutamide/flutamide/ketoconazole alone is not allowed.
5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
6. Adequate laboratory parameters [absolute neutrophil (ANC), platelets, aspartate aminotransferase (AST), alanine aminotransferase (ALT), total bilirubin, creatinine and coagulation parameters] at screening

Exclusion Criteria:

1. Any history of brain metastases or prior seizure or conditions predisposing to seizure activity
2. Have previously received an investigational BET inhibitor (including previous participation in this study or Study ZEN003694-002)
3. Have received prior systemic anti-cancer therapy or investigational therapy within 2 weeks or five half-lives, whichever is shorter, prior to the first administration of study drug
4. Failure to recover to Grade 1 or lower toxicity related to prior systemic therapy (excluding alopecia and neuropathy) prior to study entry
5. Radiation therapy within 2 weeks of first administration of study drug
6. Have received prior chemotherapy in the metastatic castration-resistant setting (prior chemotherapy in the hormone-sensitive setting is allowed provided last dose was at least 6 months prior to study entry)
7. Currently receiving medications known to be strong inducers or inhibitors of CYP3A4 with a narrow therapeutic window. Strong inducers and inhibitors of CYP3A4 with narrow therapeutic ranges must be discontinued at least 7 days prior to the first administration of study drug.