The purpose of this study is to compare the efficacy and safety of 9MW2821 versus investigator's choice of chemotherapy in participants with locally advanced or metastatic triple-negative breast cancer who have previously received taxane-based chemotherapy with or without immunotherapy and an antibody-drug conjugate with a topoisomerase inhibitor payload

Inclusion Criteria:

• Competent to comprehend, sign, and date an independent ethics committee/institutional review board/research ethics board (IEC/IRB/REB) approved informed consent form.
• Male or female subjects aged 18 to 75 years (including 18 and 75 years).
• ECOG 0-1.
• Histopathological diagnosed triple-negative breast cancer.
• Patient has previously received taxane-based chemotherapy with or without immunotherapy and an antibody-drug conjugate with a topoisomerase inhibitor payload.
• Suitable for one of active comparator chemotherapy assessed by investigator.
• An archival tumor tissue sample or a fresh tissue sample should be provided.
• Life expectancy of ≥ 12 weeks.
• Subjects must have measurable disease according to RECIST (version 1.1).
• Adequate organ functions.
• Sexually active fertile participants, and their partners, must agree to use methods of contraception during the study and at least 6 months after termination of study therapy.

Exclusion Criteria:

• Preexisting treatment related toxicity Grade ≥ 2 and Grade ≥ 3 immune related adverse reactions.
• Preexisting peripheral neuropathy Grade ≥ 2.
• Hemoglobin A1C (HbA1c) ≥ 8%.
• Has ocular conditions or symptoms that may increase the risk of the study.
• History of ILD or pneumotitis, other severe or uncontrolled disease or central nervous system metastases and/or meningeal metastasis.
• Previous medication does not meet the requirements.
• Known sensitivity to any of the ingredients of the investigational product; History of drug abuse or mental illness.
• Documented history of pulmonary embolism or clinically significant cardiac or cerebrovascular diseases .
• Active autoimmune disease.
• History of another malignancy .
• Pleural, abdominal, or pericardial effusion with clinical symptoms or that require drainage treatment.
• Not suitable to receive study treatment for other conditions as per investigator.