This is a Phase I dose escalation and dose expansion and Phase II monotherapy open--label, first--in--human study to determine the dose limiting toxicity (DLT), maximum tolerated dose (MTD) and/or recommended Phase II dose (RP2D), to characterize the safety and pharmacokinetic (PK) profile, and to estimate the preliminary anti-tumor activity of OP-1250 as a single agent in adult subjects with hormone receptor (HR)-positive/human epidermal growth factor receptor 2 (HER2)-negative metastatic or locally advanced breast cancer. This study comprises 2 Phases: Phase I (Part A \[Dose Escalation\] and Part B \[Dose Expansion\]) and Phase II. Patients 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. Patients will be evaluated for treatment emergent adverse events (AEs) during study participation, and toxicity will be assessed according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events, Version 5.0.
