This is a study which consists of phase studydose escalation stage) and phase 2a study (expansion stage)

Phase 1 study will adopt the classical 3+3 dose escalation design, exploring the safety and tolerance of 5 dose cohorts (25mg, 75mg, 150mg, 250mg and 400mg) in subjects with advanced solid tumor and determining the maximum tolerated dose of BPI-1178 for phase 2a study.

Phase 2a is an expansion study in subjects of HR+/HER2- breast cancer using 3+3 design, to evaluate the efficacy and safety of BPI-1178 in combination with endocrine therapy. Cohort A is BPI-1178 in combination with fulvestrant for advanced or recurrent HR+/HER2- breast cancer after failure or intolerance of first-line standard therapy. Cohort B is BPI-1178 in combination with letrozole for advanced or recurrent HR+/HER2- breast cancer as first-line treatment.

Phase 1 and 2a consist of screening period (28 days before enrollment), treatment period and follow up period (every 3 months until death or the end of study). Subjects will receive BPI-1178 daily for 3 weeks, followed by 1 week-off treatment. Once the maximum tolerated dose (MTD) is reached in phase 1, phase 2a study will explore the dose of BPI-1178 from MTD to MTD-1 and MTD-2 level in combination with fulvestrant or letrozole.

Dose limiting toxicity (DLT) will be recorded for the single dose period of 7 days and the multiple dose period up to 28 days in phase 1 study, as well as 28 days after the first dose of BPI-1178 in phase 2a. Efficacy will be evaluated by RECIST v1.1 and the Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) every 2 months. Adverse events will be monitored throughout the trial. Other exploration of pharmacokinetic information will be assessed throughout the trial.