Background: Mucin 16 (MUC16) is a cell surface glycoprotein that is overexpressed in ovarian cancer (OC) and endometrial cancer (EC). Ubamatamab is a MUC16 × cluster of differentiation 3 (MUC16×CD3) bispecific antibody that bridges MUC16 on tumor cells and CD3 on T cells to promote T-cell–mediated cytotoxicity. Cemiplimab, a monoclonal antibody targeting the programmed cell death-1 (PD-1) receptor, enhanced MUC16×CD3 activity in preclinical models. In Phase 1 of this first-in-human study (NCT03564340), ubamatamab alone and in combination with cemiplimab demonstrated an acceptable safety profile, durable clinical activity across a range of doses, and linear pharmacokinetics (PK) in patients (pts) with recurrent OC (Liu et al. IGCS 2023. Plenary 03). Phase 2 will assess the efficacy of ubamatamab alone and in combination with cemiplimab in pts with recurrent OC. Ubamatamab will also be evaluated for recurrent advanced or metastatic EC that is MUC16+. Methods: For individuals with recurrent OC, elevated serum cancer antigen (CA)-125 ≥2× the upper limit of normal, and up to four prior lines of cytotoxic therapy, weekly dosing of ubamatamab alone or in combination with cemiplimab will be evaluated in monotherapy and combination therapy expansion cohorts. A Phase 2 cohort will also be randomized 1:1:1 to receive ubamatamab 250 mg IV Q3W or 800 mg IV Q3W or ubamatamab 250 mg IV Q3W + cemiplimab 350 mg IV Q3W. For the EC cohort (≥25% of tumor cells MUC16+, as assessed by immunohistochemistry [IHC]) pts with advanced or metastatic EC that has progressed or is recurrent after prior anti–PD-1 therapy and prior platinum-based chemotherapy will be enrolled. The initial EC cohort will receive ubamatamab 250 mg IV Q3W. Based upon observed safety and activity, additional cohorts may be enrolled to receive ubamatamab 800 mg IV Q3W and/or ubamatamab 250 mg IV Q3W + cemiplimab 350 mg IV Q3W. To limit the risk of cytokine release syndrome, all pts will receive ubamatamab once-weekly step-up dosing prior to addition of cemiplimab and/or Q3W dosing. Expansion cohorts will use a Simon 2-stage study design, with an interim analysis after the first 20 pts. Any treatment arm with ≥3 objective responses may be expanded to 50 pts. In Phase 2, the primary endpoint will be the objective response rate for each arm (RECIST v1.1). Secondary endpoints include duration of response and progression-free survival, safety, PK, and change from baseline in quality of life. Exploratory objectives include clinical correlation with serum CA-125, MUC16 IHC, and immune markers. The study is currently enrolling to these OC and EC expansion cohorts. Clinical trial information: NCT03564340.