Inclusion Criteria:

          -  Male ≥ 18 years of age

          -  Has histologically confirmed prostate cancer that is metastatic and has progressed as
             defined by PCWG3 criteria during or after treatment with at least 1 ASI (eg,
             abiraterone, enzalutamide, apalutamide), or another second-generation anti-androgen or
             cytochrome P450 (CYP)17A1 inhibitor, with the most recent ASI administered in the
             castration-resistant setting

          -  Has serum testosterone levels < 50 ng/dL during screening. Patients without a history
             of bilateral orchiectomy are required to remain on luteinizing hormone-releasing
             hormone (LHRH) analog during the course of protocol therapy

          -  ECOG performance status of 0 or 1

          -  Adequate hematologic, renal and hepatic function

          -  Males with female partners of childbearing potential must agree to use 2 effective
             methods of contraception

          -  Patients must provide signed informed consent

          -  Patients enrolled into the dose expansion phase must have prostate carcinoma without
             histologic evidence of small-cell/neuroendocrine carcinoma features on prior biopsy or
             must have unequivocal histologic evidence of small-cell/neuroendocrine prostate
             carcinoma (pure or mixed). Patients with treatment-emergent small-cell neuroendocrine
             cancer (pure or mixed) may have received no more than on prior chemotherapy regimen
             for mCRPC

          -  Patients enrolled into the dose expansion phase must be willing to undergo a
             metastatic tumor biopsy or has tissue available from a prior post-castration resistant
             tumor biopsy

        Exclusion Criteria:

          -  Persistent clinically significant toxicities from previous anticancer therapy

          -  Has NCI CTCAE Grade ≥ 2 peripheral neuropathy from any etiology or has a genetic
             disorder that is associated with peripheral neuropathy even without current
             neuropathic manifestations

          -  Prior treatment with cytotoxic chemotherapy for mCRPC (chemotherapy in the
             hormone-sensitive setting is allowed if > 6 months before study entry)

          -  Has received external-beam radiation or systemic anticancer therapy within 14 days
             before first dose of FOR46

          -  Has received treatment with an investigational drug within 28 days before first dose
             of FOR46

          -  Has had a major surgical procedure within 28 days before administration of FOR46 dose

          -  Clinically significant cardiovascular disease

          -  Uncontrolled, clinically significant pulmonary disease

          -  Has a history of brain or leptomeningeal metastases.

          -  Uncontrolled intercurrent illness

          -  Has a known positive status for HIV or either active/chronic hepatitis B/C

          -  Requires medications that are strong inhibitors or strong inducers of CYP3A4

          -  [Dose escalation only] Has a history of episodic atrial fibrillation or flutter
             (patients with chronic atrial fibrillation are not excluded)