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.