A Phase 1 Study of FOR46 in Patients With Metastatic Castration-Resistant Prostate Cancer (mCRPC)

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)