Clinical Research Directory

Inclusion Criteria: * Subjects voluntarily join the study, sign the informed consent form, and have good compliance * Aged 18 to 75 years, with an Eastern Cooperative Oncology Group Performance Status (ECOG PS) score of 0\~1; expected survival time of more than 3 months. * Postmenopausal or premenopausal/perimenopausal female patients * Progressed after prior treatment with CDK4/6 inhibitors * At least one measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria * Good major organ function * Women of childbearing potential must agree to use contraception during the study and for 6 months after its completion. Exclusion Criteria: * Subjects with a previous pathological diagnosis of HER2-positive breast cancer. * Subjects with inflammatory breast cancer or occult breast cancer. * Subjects who have had or currently have other malignant tumors within 5 years prior to randomization. * Subjects with unresolved toxicity (greater than Common Terminology Criteria for Adverse Events (CTCAE) Grade 1) from prior treatment, excluding alopecia. * Subjects who have undergone major surgical procedures, incisional biopsies, or significant traumatic injuries within 28 days prior to the first dose. * Subjects with non-tumor-related unresolved wounds, ulcers, or fractures. * Subjects with multiple factors affecting oral medication intake and absorption. * Subjects with arterial or deep vein thrombotic events within 6 months prior to the first dose. * Subjects with ≥ Grade 2 myocardial ischemia or myocardial infarction, arrhythmia, angina requiring anti-anginal medication, clinically significant valvular heart disease, or uncontrolled hypertension. * Subjects with a history of interstitial lung disease/pneumonitis (non-infectious) requiring steroid intervention or currently having interstitial lung disease/pneumonitis, or subjects with suspected interstitial lung disease/pneumonitis on screening imaging that cannot be excluded. * Subjects with active or uncontrolled serious infections (≥CTCAE Grade 2 infection) or unexplained fever \>38.5°C within 28 days prior to the first dose. * Subjects with a history of abuse of psychotropic drugs that cannot be abstained from or those with psychiatric disorders. * Subjects with (pseudo) cirrhosis, active hepatitis. * Subjects with renal failure requiring hemodialysis or peritoneal dialysis. * Subjects with a history of immunodeficiency diseases, organ transplants, or hematopoietic stem cell transplants. * Subjects who have previously received fulvestrant or other oral Selective Estrogen Receptor Degrader (SERD) class drugs. * Subjects who have previously received anti-HER2 therapy. * Subjects who have previously received antibody-drug conjugate therapy. * Subjects who have participated in other anti-tumor clinical trials and taken investigational drugs within 4 weeks prior to the first dose. * Subjects judged by the investigator to have serious accompanying diseases that severely endanger the safety of the subject or affect the completion of the study, or subjects who are deemed unsuitable for enrollment for other reasons