ASPEN-09-03: A Study of Evorpacept in Combination With Trastuzumab and Chemotherapy in Metastatic HER2-Positive Breast Cancer

* Histologically confirmed invasive HER2+ breast cancer.* Received at least one prior line of therapy including T-DXd (ENHERTU) for locally advanced/metastatic HER2+ breast cancer. Prior neoadjuvant therapy which resulted in relapse within 6 months of completion of T-DXd will be considered a line of treatment for metastatic disease. Participants who discontinue T-DXd due to intolerance are considered eligible. * Progressed on or following the most recent line of therapy. * Eligible to receive one of the following chemotherapy options (capecitabine, eribulin, gemcitabine, paclitaxel or vinorelbine). * Measurable disease as defined by RECIST v1.1. * LVEF ≥50%. * Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) must be 0 to 1. * Life expectancy of at least 3 months. * Adequate renal function (estimated creatinine clearance ≥30 mL/min as calculated using the Cockcroft-Gault equation or Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. * Adequate liver function:

* Total bilirubin ≤1.5 x upper limit of normal (ULN) (≤3.0 x ULN if the participant has documented Gilbert syndrome); * Aspartate and alanine transaminase (AST and ALT) ≤3 x ULN (≤5.0 x ULN if liver involved by metastatic disease). * Participants must have recovered from all AEs due to previous therapies, procedures, and surgeries to baseline severity or ≤Grade 1 per NCI CTCAE v5.0 except for AEs not deemed reversible and which do not constitute a safety risk by Investigator judgment.