A Phase 1/2 Study of REGN5678 (Anti-PSMAxCD28) With Cemiplimab (Anti-PD-1) in Patients With Metastatic Castration-resistant Prostate Cancer

Primary Outcome Measures  :

1. Incidence and severity of treatment-emergent adverse events (TEAEs) [ Time Frame: Through study completion, Up to 5 years ]Dose Escalation Phase
   
2. Incidence and severity of adverse event of special interests (AESIs) [ Time Frame: Through study completion, Up to 5 years ]Dose Escalation Phase
   
3. Incidence and severity of serious adverse events (SAEs) [ Time Frame: Through study completion, Up to 5 years ]Dose Escalation Phase
   
4. Number of patients with grade ≥3 laboratory abnormalities [ Time Frame: Through study completion, Up to 5 years ]Dose Escalation Phase
   
5. Incidence of dose-limiting toxicities (DLTs) [ Time Frame: First dose through day 42 of last patient in each dose level ]Dose Escalation Phase
   
6. Concentration of REGN5678 in serum over time [ Time Frame: Through study completion, Up to 5 years ]Dose Escalation Phase
   
7. Concentration of REGN5678 in combination with cemiplimab in serum over time [ Time Frame: Through study completion, Up to 5 years ]Dose Escalation Phase
   
8. Objective response rate (ORR) per modified Prostate Cancer Working Group 3 (PCWG3) criteria [ Time Frame: Through study completion, Up to 5 years ]Dose Expansion Phase

Key Inclusion Criteria:

• Histologically or cytologically confirmed adenocarcinoma of the prostate without pure small cell carcinoma
• Metastatic, castration-resistant prostate cancer (mCRPC) with PSA value at screening

  ≥4 ng/mLthat has progressed within 6 months prior to screening as defined in the protocol

• Has received ≥2 lines prior systemic therapy approved in the metastatic and/or castration-resistant setting (in addition to androgen deprivation therapy [ADT]) including at least one second-generation anti-androgen therapy (eg, abiraterone, enzalutamide, apalutamide, or darolutamide)

Key Exclusion Criteria:

• Has received treatment with an approved systemic therapy within 3 weeks of dosing or has not yet recovered (ie, grade ≤1 or baseline) from any acute toxicities
• Has received any previous systemic biologic therapy within 5 half-lives of first dose of study therapy
• Has received prior PSMA-targeting therapy
• Dose Expansion Only: Has had prior anti-cancer immunotherapy
• Any condition that requires ongoing/continuous corticosteroid therapy (>10 mg prednisone/day or anti-inflammatory equivalent) within 1 week prior to the first dose of study therapy
• Ongoing or recent (within 5 years) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments
• Encephalitis, meningitis, neurodegenerative disease (with the exception of mild dementia that does not interfere with activities of daily living [ADLs]) or uncontrolled seizures in the year prior to first dose of study therapy
• Uncontrolled infection with human immunodeficiency virus (HIV), hepatitis B or hepatitis C infection; or diagnosis of immunodeficiency

NOTE: Other protocol defined Inclusion/Exclusion Criteria apply