First-in-Human, Escalating Oral Dose Study of RGT-419B Given Alone and With Endocrine Therapy in Subjects With Hormone Receptor Positive, Human Epidermal Growth Factor Receptor 2 Negative Advanced/Metastatic Breast Cancer

This is a phase I, First-in-Human (FIH), open-label study to evaluate the safety, tolerability, pharmacokinetic (PK) profile, and preliminary efficacy of RGT-419B administered orally as monotherapy OR in combination with Hormonal Therapy in subjects with HR+, HER2- locally advanced and unresectable (Stage III) or metastatic (Stage IV) breast cancer whose disease has progressed during prior therapy with an approved CDK4/6i plus hormonal therapy.

Drug: RGT-419B in combination with hormonal therapy