A Dose Escalation/Expansion Study of Oral OP-1250 in Subjects With Advanced and/or Metastatic HR+, HER2- Breast Cancer

Key Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Must have received at least 1 prior hormonal regimen and at least 6 months of a prior continuous endocrine therapy for locally advanced or metastatic disease * Must not have received prior oral endocrine therapy \< 2 weeks prior to first dose * Must not have received prior, chemotherapy in 2 weeks or within 5 half-lives whichever is earlier, antibody therapy within 4 weeks or investigational therapy within 4 weeks or 5 half-lives whichever is earlier, prior to the first dose * Adequate hepatic function * Adequate renal function * Normal coagulation panel * Willingness to use effective contraception Key Exclusion Criteria: * Gastrointestinal disease * Significant renal disease * Significant cardiovascular disease * Significant ECG abnormalities * Ongoing systemic bacterial, fungal, or viral infection (requiring antimicrobial therapy) * Pregnancy or breastfeeding