• Must have at least 1 measurable lesion per Response Evaluation Criteria in Solid Tumors (RECIST) guidance

• Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1

• Prior systemic radiation or whole brain radiation therapy must have been completed at least 4 weeks before investigational product (IP) administration. Other palliative radiotherapy must be completed 2 weeks before investigational product administration, if radiation therapy-related AEs have resolved to Grade ≤ 1.

• Monotherapy-specific criteria for dose escalation and PD cohorts:

‣ Dose Escalation: Participants may have any pathologically confirmed advanced or metastatic solid tumor for which standard therapy has proven ineffective, intolerable, or is considered inappropriate.

⁃ Pharmacodynamic Cohorts: Participants may have any pathologically confirmed advanced or metastatic solid tumors for which standard therapy has proven ineffective, intolerable, or is considered inappropriate. Participants must be able to undergo collection of a fresh frozen biopsy during screening, as well as provide an on-treatment fresh frozen biopsy.

• Dose Expansion cohort criteria:

‣ Histologically confirmed, documented diagnosis of HER2-negative locally advanced unresectable or metastatic gastric or GEJ adenocarcinoma.

⁃ No prior systemic treatment for locally advanced unresectable or metastatic disease.

⁃ Cannot have progressed within 6 months of prior platinum-based chemotherapy for earlier stage disease.