A Phase II Clinical Trial to Evaluate Safety and Efficacy of XmAb20717 in Advanced Rare Cancers

Primary Objective: Efficacy of XmAb20717 as defined by objective response (defined as a complete response \[CR\] or partial response \[PR\] on two consecutive occasions ≥4 weeks apart) as determined by an independent radiologist according to RECIST v1.1 (modified RECIST for pleural mesothelioma) Secondary Objectives: 1. Objective response as determined by an independent radiologist according to immune-modified RECIST 2. Progression-free survival (PFS) (defined as the time from enrollment to the first occurrence of disease progression or death from any cause, whichever occurs first) as determined by an independent radiologist according to RECIST v1.1 (modified RECIST for pleural mesothelioma). 3. Duration of response (DoR) (defined as the time from the first occurrence of a documented objective response to disease progression or death from any cause, whichever occurs first) as determined by an independent radiologist according to RECIST v1.1 (modified RECIST for pleural mesothelioma) 4. Disease control as determined by an independent radiologist according to RECIST v1.1 (modified RECIST for pleural mesothelioma) 5. Overall survival (OS) (defined as the time from enrollment to death from any cause) 6. PFS as determined by an independent radiologist according to immune-modified RECIST 7. DoR as determined by an independent radiologist according to immune-modified RECIST 8. Disease control as determined by an independent radiologist according to immune-modified RECIST 9. Occurrence and severity of AEs, with severity determined according to National Cancer Institute Common Terminology Criteria for Adverse Events, version 5.0 Exploratory Objective To identify biomarkers that are predictive of response and/or are associated with progression to a more severe disease state (i.e., prognostic biomarkers).