PHASE 1/2 STUDY OF REGN5668 (MUC16 X CD28, A COSTIMULATORY BISPECIFIC) ADMINISTERED IN COMBINATION WITH CEMIPLIMAB OR REGN4018 (MUC16 X CD3)

This study is researching an investigational drug called REGN5668. Participants will receive
additional investigational drugs in combination with REGN5668. These additional drugs include
cemiplimab or 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) 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 or REGN4018

- How REGN5668 works in the body either alone and/or in combination with cemiplimab or
REGN4018

- How much of the study drugs (REGN5668, cemiplimab, REGN4018) are in the blood

- To see if REGN5668 in combination with cemiplimab or REGN4018 works to treat cancer

- To find out how safe, tolerable, and effective in mitigating Cytokine Release Syndrome
(CRS) sarilumab pretreatment is when given before REGN4018

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. Prior anti-cancer immunotherapy as defined in the protocol

2. Recent treatment with anti-Programmed Cell Death (PD-1)/PDL-1 therapy

3. 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

4. Prior treatment with a MUC16-targeted therapy

5. Expansion cohorts only: More than 4 prior lines of cytotoxic chemotherapy (including
antibody drug conjugates)

6. 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

7. Has ongoing or recent (within 5 years) evidence of significant autoimmune disease that
required treatment with systemic immunosuppressive treatments as defined in the
protocol

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

9. Has history of clinically significant cardiovascular disease as defined in the
protocol

10. Has known allergy or hypersensitivity to cemiplimab and/or components of study
drug(s).

Note: Other protocol-defined Inclusion/Exclusion criteria apply