This study is for adults with **advanced breast cancer** that is hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-), which means the cancer grows with hormones but not with a protein called HER2. It's a **First-in-Human (FIH)** study, testing a new drug, RGT-419B, taken by mouth alone or with **endocrine therapy**, which is hormone treatment. The study aims to see if this drug is safe, how it behaves in the body (**pharmacokinetics**), and if it works to stop cancer growth after other treatments have stopped working.

• Participants must be 18 or older and have previously received certain cancer treatments.
• The study involves taking a new medicine and attending regular check-ups.
• Participants must not have severe organ problems or be pregnant.