Safety and Efficacy of XmAb18087 ± Pembrolizumab in Advanced Merkel Cell Carcinoma or Extensive-stage Small Cell Lung Cancer

Inclusion Criteria: * Able to provide written informed consent * Adult participants ≥ 18 years * Disease measurable by RECIST 1.1 criteria using either computed tomography (CT) or magnetic resonance imaging (MRI) scan * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * All participants must have adequate archival tumor sample (slides or archival formalin-fixed paraffin-embedded \[FFPE\] block\[s\] containing tumor that has not been previously irradiated * Female participants of childbearing potential must agree to use a highly effective method of birth control during and for 4 weeks after completion of study. success), or sexual abstinence * Fertile male participants must be willing to practice a highly effective method of birth control for the duration of the study and continuing for 4 weeks after the last dose of XmAb18087 or pembrolizumab (when applicable * Able and willing to complete the entire study according to the study schedule Additional Inclusion Criteria for Part A and Part B Cohorts: • Histologically or cytologically confirmed metastatic MCC or locoregional MCC that has recurred following standard locoregional therapy with surgery and/or radiation therapy. Additional Inclusion Criteria for Part A Cohorts: • Participants must have progressed on or been ineligible for treatment with anti-PD1 or anti-PDL1 therapy. Additional Inclusion Criteria for Part B Cohorts: • Participants must be eligible to receive pembrolizumab as standard of care. Additional Inclusion Criteria for Part C Cohorts: • Histologically or cytologically confirmed extensive-stage SCLC that has progressed following standard therapies Exclusion Criteria: Additional Exclusion Criteria for Part B Cohorts: XmAb18087 in Combination with Pembrolizumab * Prior treatment with therapeutics directed at anti-programmed cell death 1 (anti-PD1) or anti-programmed cell death ligand 1 (anti-PDL1) * Have severe hypersensitivity (≥ Grade 3) to pembrolizumab and/or any of its excipients