The primary purpose of this study is to determine whether the investigational drug XmAb541 is safe and well tolerated, and to determine an optimal and safe dose(s) for further study. The study will also evaluate the effect of XmAb541 on tumor outcomes.

Key Inclusion Criteria:

• Age ≥ 18 years. For US only: subjects with GCTs, age ≥15 years
• CLDN6+ tumor
• Histological or cytological documentation of locally advanced, recurrent, or metastatic ovarian, fallopian tube, or peritoneal cancer, adenocarcinoma of the endometrium (endometrial cancer, uterine cancer, or carcinoma of the uterine corpus), GCT resistant to previous treatment
• Adequate Eastern Cooperative Oncology Group performance status
• Life expectancy ≥ 3 months
• Adequate liver, kidney, and bone marrow function

Key Exclusion Criteria:

• Patients with treated brain metastases may participate, provided they are radiologically stable.
• Active known or suspected autoimmune disease
• Have any condition requiring systemic treatment with corticosteroids, prednisone equivalents, or other immunosuppressive medications within 14 days prior to first dose of study drug
• Clinically significant cardiovascular, pulmonary or gastrointestinal disease
• Active hepatitis B or hepatitis C