A Trial of BXCL701 and Pembrolizumab in Patients With mCRPC Either Small Cell Neuroendocrine Prostate Cancer or Adenocarcinoma Phenotype.

An open-label, multicenter, Phase 1b/2 study to identify the recommended Phase 2 dose and assess the efficacy and safety of BXCL701 administered orally, as monotherapy and in combination with PEMBRO, in patients with mCRPC. Patients enrolled in the Phase 2a portion of the study will have either Small Cell Neuroendocrine Prostate Cancer(SCNC)(Cohort A) or adenocarcinoma phenotype (Cohort B), while the Phase 2b randomized portion of the study will enroll only the histologic subtype(s) showing preliminary evidence in Phase 2a. The study will also assess other efficacy parameters, such as rPFS, PSA PFS, OS, and DOR, as well as the safety of the combined treatment. The study will consist of three components.

1. Phase 1b: Safety and tolerability of the combination of BXCL701 administered once daily (QD) on Days 1 to 14 of a 21-day cycle plus PEMBRO 200 mg administered intravenously (IV) on Day 1 of every 21 days will be assessed and confirmed in patients with mCRPC. In the first cohort enrolled in the study, the initial dose level of BXCL701 will be 0.4 mg; if there are no safety concerns, this will be escalated to a total daily dose of 0.6 mg. 2. Phase 2a (Simon 2 Stage): Patients will be treated with BXCL701 in combination with PEMBRO. Patients will be grouped in 1 of 2 cohorts based on phenotype.

* Cohort A: Patients with any Small Cell/Neuroendocrine features, either de novo or treatment-emergent included mixed SCNC. * Cohort B: Cohort B: Patients with adenocarcinoma and no evidence of small cell or neuroendocrine features.

Phase 2b (for the histologic subtype\[s\] showing preliminary efficacy in Phase 2a): Patients within a given subtype will be randomized 2:1 to receive either BXCL701 combined with PEMBRO or BXCL701 monotherapy.

Phase 1b:

Patients will be observed for dose-limiting toxicity (DLT) during Cycle 1. Three patients will be treated initially with BXCL701 0.4 mg plus PEMBRO:

* If there are no DLTs in Cycle 1, the dose of BXCL701 will be escalated to a total daily dose of 0.6 mg in the next cohort of 3 patients. * If ≥1 of the 3 original patients has a DLT in Cycle 1, after a discussion between the sponsor and the investigator, either 3 patients (if 1 patient experiences a DLT) or 6 to 9 patients (if 2 or 3 patients experiences a DLT) will be added at the 0.4 mg BXCL701 dose level. For this expanded 0.4 mg cohort:

* If none of the patients experience a DLT, consideration will be given to dose escalation to BXCL701 0.6 mg plus PEMBRO * If 1/3 of the patients experience a DLT, the Phase 2 can commence * If \>1/3 of the patients experience a DLT, a discussion will be held between the investigators and sponsors as to how to proceed.