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

A Phase 1/2 Study of REGN5668 (MUC16xCD28, a Costimulatory Bispecific Antibody) Administered in Combination With Other Agents in MUC16 + Malignancies

This study is researching an investigational drug called REGN5668 :

• alone or,
• combined with cemiplimab (also known as REGN2810) or,
• combined with both cemiplimab and fianlimab (also known as REGN3767), or
• combined with ubamatamab (also known as REGN4018), with or without sarilumab.

The main purposes of this study are to:

• Learn about the safety and profile of any side effects from the study drugs and to determine the highest, safe dose that can be given to participants with ovarian cancer or cancer of the uterus
• Look for signs that the study drugs can treat ovarian cancer or cancer of the uterus

This study has 2 parts. The purpose of Part 1 (Escalation) is to find the highest, safe dose of the study drug(s). The purpose of Part 2 (Expansion) is to use the doses chosen in Part 1. Participants with cancer of the uterus will only participate in Part 2.

The study is looking at several other research questions, including:

• Side effects that may be experienced by participants taking REGN5668 alone and/or in combination with cemiplimab, cemiplimab and fianlimab, or ubamatamab
• How REGN5668 works in the body either alone and/or in combination with cemiplimab, cemiplimab and fianlimab, or ubamatamab
• How much of the study drugs (REGN5668, cemiplimab, fianlimab, ubamatamab) are in the blood
• To see if REGN5668 in combination with cemiplimab, cemiplimab and fianlimab, or ubamatamab works to treat cancer

Key Inclusion Criteria:

1. Ovarian Cancer Cohorts Only: Has histologically or cytologically confirmed diagnosis of advanced epithelial ovarian cancer (except carcinosarcoma), primary peritoneal, or fallopian tube cancer that has received at least 1 line of platinum-based systemic therapy as defined in the protocol
2. Expansion cohorts only: Has at least 1 lesion that is measurable by RECIST 1.1 as described in the protocol.
3. Has a serum CA-125 level ≥2x ULN (in screening, not applicable to endometrial cohorts)
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 that has progressed or recurrent after prior anti-PD-1 therapy and platinum-based chemotherapy as described in the protocol

Key Exclusion Criteria:

1. Current or recent (as defined in the protocol) treatment with an investigational agent, systemic biologic therapy, or anti-cancer immunotherapy
2. Has had another malignancy within the last 5 years that is progressing, requires active 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 defined in 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 disease that required treatment with systemic immunosuppressive treatments as defined in the protocol
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 the protocol
9. Has known allergy or hypersensitivity to cemiplimab and/or components of study drug(s).

Note: Other protocol-defined Inclusion/Exclusion criteria apply