ADXS31-142 Alone and in Combination With Pembrolizumab (MK-3475) in Participants With Previously Treated Metastatic Castration-Resistant Prostate Cancer (mCRPC)

Inclusion Criteria: 1. Have progressive mCRPC, on androgen deprivation therapy, based on at least one of the following criteria: 1. Prostate-specific antigen (PSA) progression, defined as 25% increase over baseline value with an increase in the absolute value of at least 2 ng/mL that is confirmed by another PSA level with a minimum of a 1 week interval with a minimum PSA of 2 ng/mL. 2. Progression of bi-dimensionally measurable soft tissue (nodal metastasis) assessed within 1 month prior to registration by computed tomography (CT) scan or magnetic resonance imaging (MRI) of the abdomen and pelvis. 3. Progression of bone disease (evaluable disease) (new bone lesion\[s\]) by bone scan. 2. Has discontinued antiandrogens (bicalutamide, nilutamide) \>6 weeks and enzalutamide \>4 weeks prior to Day 1 of trial treatment 3. Have a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) performance scale. Exclusion Criteria: 1. Received more than 3 prior systemic treatment regimens with chemotherapy, hormonal, or immunotherapy in the metastatic setting or received more than 1 prior chemotherapeutic regimen in the metastatic setting 2. Has a diagnosis of immunodeficiency or is receiving any systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to Day 1 of trial treatment. The use of physiologic doses of corticosteroids may be approved after consultation with the Sponsor. 3. Has had a prior monoclonal antibody within 4 weeks prior to study Day 1 or who has not recovered (i.e., Grade ≤1 or at baseline) from adverse events due to agents administered more than 4 weeks earlier. 4. Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study Day 1 or who has not recovered (i.e., Grade ≤1 or at baseline) from adverse events due to a previously administered agent. 5. Has received prior therapy with an anti-programmed cell death protein-1 (PD-1), anti-programmed death-ligand-1 (PD-L1), or anti-Programmed death-ligand-2 (PD-L2) agent or if the participant has previously participated in a Merck MK-3475 clinical trial. 6. Has a contraindication to administration of ampicillin or trimethoprim/ sulfamethoxazole. 7. Has implanted medical device(s) that pose a high risk for colonization and/or cannot be easily removed (e.g., prosthetic joints, artificial heart valves, pacemakers, orthopedic screw(s), metal plate(s), bone graft(s), or other exogenous implant(s)). NOTE: More common devices and prosthetics which include arterial and venous stents, dental and breast implants, and venous access devices (e.g., Port-a-Cath or Mediport) are permitted. Sponsor must be contacted prior to consenting any subject who has any other device and/or implant.