An Open-Label, Multicenter, Phase Ib Dose Randomization Study of Bulumtatug Furvedotin (BFv; 9MW2821) in Subjects With Recurrent or Metastatic Triple-Negative Breast Cancer Previously Treated With Antibody-Drug Conjugates

Inclusion Criteria * Patient has measurable disease by RECIST v1.1 * Recurrent or metastatic triple-negative breast cancer patients as per current ASCO/CAP guidelines * Patient has received prior treatment with a taxane and an antibody-drug conjugate with a topoisomerase inhibitor payload. * Patient has received no more than 3 prior lines of cytotoxic therapy in the locally advanced or metastatic setting. * Provision of archival tumor tissue or fresh tumor biopsy. * Capable of giving informed consent * Male or female subjects aged ≥ 18 years. * Subjects must be willing to receive blood transfusions if medically indicated. * ECOG 0-1 * Adequate hematologic and organ function * Life expectancy of at least 3 months as assessed by the investigator * Compliance with contraceptive requirement Exclusion Criteria: * Have received any prior treatment with enfortumab vedotin, tisotumab vedotin or other MMAE based or nectin-4 targeted antibody-drug conjugates. * Unstable CNS metastasis requiring treatment in the last 28 days. * Acute infection requiring IV treatment in the last 14 days. * Grade ≥2 peripheral neuropathy. * Pregnant or breastfeeding women. * Life-threatening illness or uncontrolled medical conditions that could compromise the subject's safety or put the study outcomes at risk * Any systemic anticancer therapy in the last 28 days prior to first administration of study drug. * Active HCV, HBV or HIV infection unless well controlled with anti-viral therapy. * Active or chronic corneal disorder, keratitis, corneal ulcerations or Sjogren's syndrome. * Have any ongoing acute inflammatory skin disease or chronic skin disease not well controlled. * Have been diagnosed with another primary malignancy except for adequately treated non-melanoma skin cancer or cervical cancer in situ; definitively treated non-metastatic prostate cancer; or subjects with another primary malignancy who are definitively relapse-free with at least 3 years elapsed since the diagnosis of the other primary malignancy. * Have significant, uncontrolled or active cardiovascular disease * Have active or a history of pneumonitis or interstitial lung disease that requires corticosteroid treatment. Patients with radiation pneumonitis that does not require treatment is allowed. * Have uncontrolled diabetes. * Have received any strong CYP3A4 inhibitors within 14 days prior to the first dose of study drug. * Subjects known to be hypersensitive to bulumtatug fuvedotin or to any components of the formulation. * History of drug abuse including narcotic and psychiatric drugs within 12 months prior to screening. * Have received a live vaccine within 30 days of planned start of study therapy.