A Study to Find Out How Safe REGN5668 is and How Well it Works In Adult Women When Given With Either Cemiplimab, or Cemiplimab + Fianlimab, or Ubamatamab

Key Inclusion Criteria:

1. Ovarian Cancer Cohorts Only: Has histologically or cytologically confirmed diagnosisof advanced epithelial ovarian cancer (except carcinosarcoma), primary peritoneal,or fallopian tube cancer that has received at least 1 line of platinum-basedsystemic therapy as defined in the protocol

2. Expansion cohorts only: Has at least 1 lesion that is measurable by RECIST 1.1 asdescribed in the protocol.

3. Has a serum CA-125 level ≥2x ULN (in screening, not applicable to endometrialcohorts)

4. Has adequate organ and bone marrow function as defined in the protocol

5. Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

6. Has a life expectancy of at least 3 months

7. Endometrial Cancer Cohorts Only: histologically confirmed endometrial cancer thathas progressed or recurrent after prior anti-PD-1 therapy and platinum-basedchemotherapy as described in the protocol

Key Exclusion Criteria:

1. Current or recent (as defined in the protocol) treatment with an investigationalagent, systemic biologic therapy, or anti-cancer immunotherapy

2. Has had another malignancy within the last 5 years that is progressing, requiresactive treatment, or has a high likelihood of recurrence as defined in the protocol

3. Prior treatment with a Mucin 16 (MUC16)-targeted therapy

4. Ovarian Expansion cohorts only: More than 5 prior lines of systemic therapy

5. Has any condition that requires ongoing/continuous corticosteroid therapy as definedin the protocol within 1 week prior to the first dose of study drug

6. Has ongoing or recent (within 5 years) evidence of significant autoimmune diseasethat required treatment with systemic immunosuppressive treatments as defined in theprotocol

7. Has untreated or active primary brain tumor, CNS metastases, leptomeningeal disease,or spinal cord compression as defined in the protocol

8. Has history of clinically significant cardiovascular disease as defined in theprotocol

9. Has known allergy or hypersensitivity to cemiplimab and/or components of studydrug(s).

Note: Other protocol-defined Inclusion/Exclusion criteria apply