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