A Study of 9MW2821 Versus Chemotherapy in Participants With Previously Treated Locally Advanced or Metastatic Triple-Negative Breast Cancer

• 9MW2821 (Drug): Subjects will receive intravenous (IV) infusion of 9MW2821 as per protocol
• Chemotherapy (Drug): Participants will receive investigator's choice of chemotherapy determined prior to randomization from 1 of the 4 allowed regimens as per protocol: eribulin, or vinorelbine or capecitabine or gemcitabine.

Primary outcomes

• Overall Survival [Up to approximately 2 years]: Time from the date of randomization until the date of death from any cause

Secondary outcomes

• Objective Response Rate per investigator [Up to approximately 2 years]: The percentage of subjects who experience a best response of either CR or PR.
• Duration of Response per investigator [Up to approximately 2 years]: Time from the date of the first complete response (CR) or partial response (PR) to the earliest date of disease progression or death from any cause.
• Time to response per investigator [Up to approximately 2 years]: Time from the date of randomization to the date of CR or PR.
• Disease Control Rate per investigator [Up to approximately 2 years]: The percentage of subjects who experience a best response of CR, PR or stable disease (SD).
• Progression Free Survival per investigator [Up to approximately 2 years]: Time from the date of first randomization to the earliest date of documented disease progression per radiological evidence or death from any cause.
• Incidence of adverse events [Up to approximately 2 years]
• Incidence of Anti-Drug Antibody (ADA) [Up to approximately 2 years]

Sex: All, ages: 18 Years to 75 Years

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.