Study of AMG 199 in Subjects With MUC17-Positive Solid Tumors Including Gastric, Gastroesophageal Junction, Colorectal, and Pancreatic Cancers

Inclusion Criteria: Key Inclusion Criteria: • Subjects with histologically or cytologically confirmed metastatic or locally advanced unresectable gastric adenocarcinoma or gastroesophageal junction (GEJ) adenocarcinoma positive for MUC17. Subjects should have been refractory to or have relapsed after two or more prior lines of standard systemic therapy that included a platinum, a fluoropyrimidine, nivolumab (in combination with a platinum and a fluoropyrimidine), either a taxane or irinotecan, and an approved vascular endothelial growth factor receptor (VEGFR) antibody/tyrosine kinase inhibitor (TKI). OR • Subjects with histologically or cytologically confirmed metastatic or locally advanced unresectable colorectal cancer positive for MUC17. Subjects should have been refractory to or have relapsed after at least two and up to five prior lines of standard systemic therapy. Therapy should have included an approved vascular endothelial growth factor (VEGF) antibody (if clinically appropriate) and epidermal growth factor receptor (EGFR) antibody (if kirsten rat sarcoma \[KRAS\]/ neuroblastoma RAS viral oncogene homolog \[NRAS\]/ v-Raf murine sarcoma viral oncogene homolog B1 \[BRAF\] wild type tumor). OR * Subjects with histologically or cytologically confirmed unresectable or metastatic pancreatic ductal adenocarcinoma positive for MUC17. Subjects should have been refractory to or have relapsed after at least one and up to three prior lines of standard systemic therapy. * Gastric adenocarcinoma and GEJ adenocarcinoma: Subjects eligible for human epidermal growth factor receptor 2 (HER2) directed therapy, prior systemic therapy should have an approved HER2 targeting antibody approved for treatment of gastric cancer. For subjects with microsatellite instability high (MSI H) or mismatch repair deficient (dMMR) tumors a prior line of treatment should have included an approved programmed cell death protein-1 (PD-1) blocking antibody. OR * Colorectal cancer: For subjects with MSI H or dMMR tumors a prior line of treatment should have included an approved PD-1-blocking antibody. For subjects with BRAF V600E mutation positive tumors a prior line of treatment should have included a BRAF inhibitor. * Subjects may also be included if the aforementioned therapeutic options were medically not appropriate for them. In these cases, the reason(s) why required prior therapies for solid tumors were medically not appropriate should be documented in the subject's electronic case report form (eCRF). Subjects may also be included if the aforementioned therapeutic options have not been available or accessible for them. * For dose expansion only: Subjects with at least one measurable lesion ≥ 10mm which has not undergone biopsy within 3 months of screening scan. This lesion cannot be biopsied at any time during the study. Exclusion Criteria: Key Exclusion Criteria: * Any anticancer therapy or immunotherapy within 4 weeks of start of first dose. * Central nervous system (CNS) metastases, leptomeningeal, or spinal cord compression. * Autoimmune disorders requiring chronic systemic steroid therapy or any other form of immunosuppressive therapy. Subjects may be included if the treatment is discontinued more than 3 months prior to the first dose of AMG 199, there is a low likelihood of relapse from the autoimmune disorder, AND there is agreement between the investigator and the Amgen Medical Monitor.