A Phase II Clinical Study of the Efficacy and Safety of Culmerciclib Rechallenge in HR-positive, HER2-negative Breast Cancer Patients With Resistance to First-line Endocrine Therapy.

Inclusion Criteria: * 1\) Female, ≥18 years old; ≤ 75years old 2)ECOG score 0-2; 3) Predicted survival ≥3 months;Patients with locally advanced and/or metastatic breast cancer confirmed by histopathology with positive ER expression and negative HER2 expression; 5) Enrolled subjects must meet one of the following criteria regarding prior endocrine therapy: i) Received CDK4/6 inhibitor combined with endocrine therapy as adjuvant endocrine therapy, experienced recurrence or progression during or within 1 year after completion of adjuvant CDK4/6 inhibitor therapy, and did not receive subsequent endocrine therapy; ii) Recurrence or progression more than 1 year after completion of adjuvant endocrine monotherapy, followed by progression after receiving CDK4/6 inhibitor combined with endocrine therapy as first-line salvage endocrine therapy; iii) Newly diagnosed locally advanced or metastatic disease, with disease progression after receiving CDK4/6 inhibitor combined with endocrine therapy as first-line salvage endocrine therapy; 6)Participants with recurrent or metastatic disease may receive rescue chemotherapy, ADC, or rescue endocrine therapy not exceeding first-line treatment; 7) The time interval between non-endocrine therapy should be ≥2 weeks; 8) At least one extracranial measurable lesion as defined by RECIST V1.1 criteria; 9) The functions of vital organs meet the requirements; 10) Fertile subjects must have a negative pregnancy test 7 days before starting treatment and mus…