Phase II Trial of Ubamatamab Alone or in Combination With Cemiplimab in MUC16-Expressing SMARCB1-Deficient Malignancies

Stage 1 of study treatment involves being given ubamatamab by itself by vein. The first dose will be given over about 4 hours. Depending on how participants respond, later doses may be given over shorter periods of time (possibly down to 30 minutes per infusion). Participants dose of ubamatamab will be increased from a starting dose on Day 1 to a full dose on Day 15. Participants will receive ubamatamab 1 time every week for the first 4 weeks, then every 3 weeks after that, unless the disease gets worse or intolerable side effects occur. • If the disease gets worse after completing 6 weeks of treatment, participants will move to Stage 2 and receive combination therapy.

Participants in Stage 2 will begin receiving ubamatamab and cemiplimab. * If participants are enrolled directly into Stage 2, but did not have therapy with ubamatamab alone, participants will receive ubamatamab 1 time each week for 4 weeks, before beginning combination therapy. * If participants have already completed Stage 1 and are moving on to Stage 2, participants will begin with combination therapy. During Stage 2, ubamatamab is given by vein over 30 minutes to 4 hours, as described above. Cemiplimab is given by vein over 30 minutes.