This is a single-arm, Phase II clinical study to explore the efficacy and safety of dalpiciclib combined with fluvestrant and compound gossypol acetate tablets in advanced HR-positive and HER2-negative breast cancer after CDK4/6 treatment failed.

Dalpiciclib: According to the standard dose, 150mg orally for three weeks and stop for one week.

Fluvestrant: At the standard dose, 500mg once every four weeks. A further 500mg dose two weeks after the first dose.

Compound gossypol acetate tablets: 20mg once a day.

Inclusion Criteria:

1.Written informed consent. 2.Women aged 18 years or older. 3.Eastern Cooperative Oncology Group(ECOG) has a physical fitness score of 0 or 1. 4.Life expectancy is more than six months. 5.Diagnosed as HR-positive HER2-negative metastatic breast cancer. 6.There was at least one measurable lesion according to RECIST 1.1. 7.Failed in treating with CDK4/6 inhibitors combined with endocrine therapy at the metastatic stage. 8.Good organ function. 9.Fertile female patients must have a negative serum pregnancy test within seven days prior to study treatment and consent to effective contraceptive use for 180 days from screening to the last dose of study treatment. 10.Female patients must agree not to breastfeed during the study period or for 180 days after the last dose of study therapy.

Exclusion Criteria:

1.Patients enrolled in any interventional clinical trial at the same time and received the investigational therapy ≤ four weeks prior to initiation of the regimen or at least five half-lives of the investigational drug. 2.Patients who had received radiation therapy with bone marrow coverage >20% within two weeks before the start of treatment, except for minor palliative radiation therapy more than one week before the first day of the study. 3.Patients with a visceral crisis, and requiring chemotherapy. 4.Patients allergic to dalpiciclib or compound gossyrol acetate tablets. 5.A history of platelet transfusion for chemotherapy-induced thrombocytopenia or prior cancer treatment (lasting > 4 weeks and associated with recent treatment) is known to result in ≥ grade 3 hematological toxicity. 6.The patient had any known history of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). 7.Patients have a severe, uncontrolled medical condition, a non-malignant systemic disease, or an active, uncontrolled infection. 8.Patients diagnosed, detected, or treated for another type of cancer within ≤2 years prior to beginning regimen therapy.

Patients with brain metastases or pial metastases uncontrolled. 9.Patients have received an allogeneic bone marrow transplant or double umbilical cord blood transplant. 10.Patients cannot swallow oral medications. 11.Patients with gastrointestinal disorders that may interfere with the absorption of investigational drugs. 12.Patients have had systemic active autoimmune disease (i.e., disease modulators, corticosteroids, or immunosuppressants) within the past two years. 13.Patients with a history of human immunodeficiency virus, active-hepatitis -B or C. 14.Pregnant or nursing women. Fertile adults without effective contraceptive methods.