This study is a multicenter, Phase 1 study to evaluate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics of AGEN1777 as a single agent and when used in combination with a PD-1 inhibitor in participants with advanced, metastatic solid tumors.

Key Inclusion Criteria:

1. Histologically or cytologically confirmed diagnosis of metastatic or locally advanced solid tumor for which no acceptable standard therapy available or progressed on or after standard therapies.
2. Measurable disease on baseline imaging based on Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1).
3. Life expectancy of at least 3 months and an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

Key Exclusion Criteria:

1. Active infection requiring treatment.
2. Lack of recovery for participants who had major surgical procedure within 4 weeks prior to first dose of protocol therapy.
3. Clinically significant cardiovascular disease: cerebral vascular accident/stroke or myocardial infarction within 6 months of enrollment, unstable angina, congestive heart failure (New York Heart Association class ≥ II), or serious uncontrolled cardiac arrhythmia requiring medication.
4. Corrected QT interval (QTc) (corrected for heart rate using Fridericia's formula prolongation) >480 msec at screening except for right bundle branch block.