Phase I Study of BEBT-209 in Women With Advanced Breast Cancer

* Dose escalation phase: To conduct approximately 6 dose levels, with a starting dose of 25 mg/day of BEBT-209, and subsequent dose groups were first dose escalated at 100%, with dose escalation at 50% for subsequent dose groups if one drug-related Grade 2 nonhematologic or Grade 3 hematologic toxicity was identified and dose-limiting toxicity was not reached. If one dose limiting toxicity was identified and the maximum tolerated dose was not reached, dose escalation was performed at 33% for the subsequent dose groups. * Phase Ib of BEBT-209 as single therapy, in Combination with Letrozole, in Combination with Fulvestrant: According to the pharmacokinetics, safety and preliminary efficacy of BEBT-209 in the dose escalation phase, one dose was selected for the BEBT-209 monotherapy group, two doses were selected for the combination with letrozole group, and two doses were selected for the combination with fulvestrant group, and all five groups were continuously administered until disease progression or unacceptable toxicity or patient withdrawal or death. Late phase clinical trials were conducted as appropriate based on preliminary safety tolerability, pharmacokinetics, and preliminary efficacy results from Phase 1b. Participants will need to understand the requirements and risks of the trial, sign an informed consent form, accept the dosing regimen required by the trial protocol, and follow the investigator's guidance.