Phase 1, First-in-human, Dose-finding and Expansion Study to Evaluate XmAb®808 in Combination With Pembrolizumab in Advanced Solid Tumors

Key Inclusion Criteria: * Part A: Histologically confirmed advanced/metastatic castration-resistant prostate adenocarcinoma, epithelial ovarian cancer, head and neck squamous cell carcinoma, non-small cell lung cancer, urothelial carcinoma, melanoma, renal cell carcinoma, triple-negative breast cancer, or colorectal cancer that has progressed on standard therapies * Part B: Histologically confirmed advanced/metastatic castration-resistant prostate cancer that is PD1-naïve; head and neck squamous cell carcinoma that is PD1-naïve or has progressed on prior PD1 therapy; or melanoma that is PD1-naïve or has progressed on prior PD1 therapy * Measurable disease by RECIST 1.1; subjects with prostate cancer who have evaluable disease according to PCWG3 criteria may enroll * Life expectancy \> 3 months * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 Key Exclusion Criteria: * Subjects currently receiving other anticancer therapies * Any prior treatment with an investigational agent targeting CD28 * History of a life-threatening adverse event related to prior immunotherapy * Known active central nervous system involvement by malignant disease. Subjects with previously treated brain metastases may participate provided they are radiologically and clinically stable