Study of the Kinesin Oral Molecular Degrader BBI-940 in Subjects With Advanced or Metastatic Breast Cancer

Key Inclusion Criteria * Adults with locally advanced or metastatic breast cancer, including estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER+/HER2-) disease or triple-negative breast cancer with luminal androgen receptor subtype (TNBC-LAR; androgen receptor expression ≥10% by immunohistochemistry), as applicable by study part. * Prior treatment with standard therapies known to provide clinical benefit, appropriate for disease subtype and study part, including endocrine therapy with CDK4/6 inhibition for ER+/HER2- disease. * Measurable disease per RECIST v1.1, except for participants enrolled in Part 1A. * Molecular eligibility as applicable by study part, including absence of an ESR1 mutation (Part 2A) or presence of FGFR1 amplification (Part 2B), based on prior local testing. * Availability of archival or newly obtained formalin-fixed, paraffin-embedded (FFPE) tumor tissue suitable for protocol-specified biomarker analyses. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Adequate hematologic, hepatic, renal, and coagulation function per protocol-defined laboratory criteria. * Estimated life expectancy of at least 12 weeks. * Ability to swallow oral medication and provide written informed consent. Key Exclusion Criteria * Prior exposure to an inhibitor or degrader of Kinesin. * Known hypersensitivity to study intervention(s) or excipients. * Receipt of recent anticancer therapy within protocol-defined washout periods. * Other active malignancy likely to interfere with study assessment. * Baseline QTcF \>470 msec or congenital long QT syndrome. * Clinically significant pulmonary embolism within 6 weeks prior to first dose. * Major surgery within 4 weeks or minor surgery within 2 weeks prior to first dose. * Active infection requiring systemic therapy within 2 weeks prior to first dose. * Pregnant or breastfeeding, or planning conception or gamete donation during the study or required post-treatment period. * Prior solid organ transplant or allogeneic stem cell transplant with protocol-defined exceptions. * Failure to recover to CTCAE Grade ≤1 (or baseline) from prior anticancer therapy, with protocol-specified exceptions. * Any serious or uncontrolled medical, laboratory, or psychiatric condition that could compromise safety or study integrity. * Other exclusion criteria as specified in the study protocol.