A Trial to Find Out if REGN5678 (Nezastomig) is Safe and How Well it Works Alone or in Combination With Cemiplimab for Adult Participants With Metastatic Castration-Resistant Prostate Cancer and Other Tumors

Prior treatment

• 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, as described in the protocol
• Has received any previous systemic biologic therapy within 5 half-lives of first dose of study therapy, as described in the protocol
• Has received prior PSMA-targeting therapy with the exception of a PSMA targeting radioligand (eg. 177Lu-PSMA-617) in mCRPC
• Dose Escalation: Has had prior anti-cancer immunotherapy (other than sipuleucel-T) within 5 half-lives prior to study therapy.
• Dose Expansion (mCRPC only): Has had prior anti-cancer immunotherapy, as described in the protocol
• 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, as described in the protocol
• 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

Other illness

• 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

• mCRPC cohorts (men):
• Men with histologically or cytologically confirmed adenocarcinoma of the prostate without pure small cell carcinoma.
• PSA value at screening ≥4 ng/mL that 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)
  • 177Lu-PSMA-617 radiotherapy, or another lutetium-based PSMA targeted radioligand, as described in the protocol
  • ccRCC cohorts (men and women):
• Histologically or cytologically confirmed RCC with a clear-cell component.
• Diagnosis of metastatic ccRCC with at least one measurable lesion via RECIST 1.1 criteria
• Has progressed on or after ≥1 line prior systemic therapy approved in the metastatic setting. Prior treatment must include an anti-Programmed Death-1 (receptor) \[PD-1\]/Programmed Death-Ligand 1 (PD-L1) therapy and either ipilimumab and/or a tyrosine kinase inhibitor