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

The study consists of two parts: Part 1 (dose escalation) and Part 2 (dose expansion). Part 1 is a dose-escalation phase designed to evaluate the safety and tolerability of BBI-940 and to determine the recommended dose for expansion (RDE). Participants may have estrogen receptor-positive, HER2-negative (ER+/HER2-) breast cancer or triple-negative breast cancer of the luminal androgen receptor subtype (TNBC-LAR). Part 2 is a dose-expansion phase designed to further evaluate BBI-940 at the selected RDE in defined participant populations. Part 2A evaluates BBI-940 in combination with fulvestrant, including multiple dose cohorts to evaluate the safety of the combination regimen and to determine the combination RDE in participants with ER+/HER2- breast cancer without an ESR1 mutation. Part 2B evaluates BBI-940 monotherapy at the RDE in participants with ER+/HER2- breast cancer with FGFR1 amplification. Part 2C evaluates BBI-940 monotherapy at the RDE in participants with TNBC-LAR. Across all parts of the study, treatment is administered in repeated 28-day cycles, and participants undergo protocol-specified safety assessments.