A Study to Evaluate the Safety and Tolerability of AB680 in Participants With Gastrointestinal Malignancies

Key Inclusion Criteria: * Histologically or cytologically confirmed metastatic pancreatic adenocarcinoma * Naïve to any prior treatment, including chemotherapy, biological therapy, or targeted therapy for metastatic disease * Prior adjuvant therapy (including chemotherapy and/or radiotherapy) for pancreatic adenocarcinoma is permitted if neoadjuvant or adjuvant therapy was completed at least 6 months prior to study enrollment. Prior adjuvant therapy may include nab- paclitaxel or gemcitabine * Participants initially diagnosed with locally advanced pancreatic cancer who have undergone chemotherapy then resection and had no evidence of disease are eligible if relapse of metastatic disease has occurred and if the last dose of chemotherapy was received more than 6 months before study entry * Must have at least 1 measurable lesion per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. The measurable lesion must be outside of a radiation field if the participant received prior radiation * Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1 * Confirmation that an archival tissue sample is available; if not, a new biopsy of a tumor must be obtained * Prior radiation therapy for metastatic disease must have been completed * Immunosuppressive doses of systemic medications, such as corticosteroids or absorbed topical corticosteroids (doses \> 10 mg/day prednisone or equivalent) must be discontinued at least 2 weeks (14 days) before investigational…