FOR46 in Combination With Enzalutamide in Patients With Metastatic Castration Resistant Prostate Cancer

PRIMARY OBJECTIVES: Phase 1b: I. To determine the maximally tolerated dose (MTD) and recommended phase 2 dose (RP2D) of FOR46 in combination with enzalutamide in patients with mCRPC. Phase 2: I. To determine the composite response rate of FOR46 plus enzalutamide, defined as: 1. Decline in serum PSA \>= 50% (PSA50) from baseline, confirmed by repeat measurement at least 4 weeks later, and/or 2. Objective response by Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 criteria. SECONDARY OBJECTIVES: Phase 1b and Phase 2: I. To determine the PSA50 response rate. II. To determine the objective response rate by RECIST 1.1 criteria among patients with measurable soft tissue by RECIST 1.1 criteria at baseline. III. To determine the median duration of objective response. IIII. To determine the median time to prostate-specific antigen (PSA) progression by Prostate Cancer Clinical Trials Working Group 3 (PCWG3) criteria. V. To determine the median radiographic progression-free survival by PCWG3 criteria. VI. To determine the median overall survival. VII. To determine the safety of the combination. OUTLINE: Participants may continue study treatment from the time of treatment initiation until confirmed radiographic disease progression per PCWG3 / RECIST 1.1 criteria, unequivocal clinical progression, unacceptable toxicity, or patient withdrawal, whichever occurs first. Patients will be followed for overall survival every 90 days (+/- 30 days) from last dose of study treatment, until death, withdrawal of consent, or the end of the study, whichever occurs first.