This is an open-label, Phase 1b dose escalation and expansion study to determine the maximum tolerated dose (MTD) of OP-1250 in combination with palbociclib (Ibrance®️, Pfizer Inc.). Purpose of study is to evaluate the safety and pharmacokinetic (PK) profile, and estimate the preliminary anti-tumor activity of the combination in adult subjects with hormone receptor-positive (ER+ / HER2-) advanced or metastatic breast cancer (MBC).

Inclusion Criteria:

• Confirmed and evaluable locally advanced or metastatic breast cancer
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Must not have received prior oral endocrine or targeted therapy ≤ 2 weeks prior to first dose
• Must not have received prior chemotherapy, antibody therapy, or investigational therapy ≤ 4 weeks prior to the first dose
• Prior radiotherapy must have been completed 2 weeks prior to first dose
• Adequate safety laboratory tests
• Willingness to use effective contraception

Exclusion Criteria:

• Gastrointestinal disease
• Significant hepatic disease
• Significant cardiovascular disease
• Significant ECG abnormalities
• History of pulmonary embolism or high risk of thrombosis
• Known HIV infection
• Active infection (requiring antimicrobial therapy)
• Pregnant