The Substudy Protocol ASPEN-09-03 is a Phase 2, single-arm, multicenter study evaluating the efficacy, safety, and tolerability of evorpacept in combination with trastuzumab and chemotherapy in participants with HER2-positive metastatic breast cancer who have previously received trastuzumab-deruxtecan. This substudy is actively recruiting. ASPEN-09-03 is a substudy under Master Protocol ASPEN-09, and additional substudies are as follows: * Metastatic colorectal cancer (CRC) - dose escalation phase to evaluate evorpacept in combination with other drugs. This substudy is not open. * Recurrent/metastatic head and neck cancer (HNSCC) - dose escalation phase to evaluate evorpacept in combination with other drugs. This substudy is not open.