This is a Phase 1/2 dose escalation and cohort expansion study and will assess the safety, tolerability and preliminary efficacy of HP568 alone and in combination with palbociclib in patients with ER+/HER2- locally advanced or metastatic breast cancer.

Inclusion Criteria:

1. Women aged 18-75 years old (inclusive of both ends) at the time of signing the informed consent form.
2. Patients with locally advanced inoperable or recurrent or metastatic breast cancer ER+/HER2- advanced breast cancer is confirmed by histopathology have confirmed that the primary and/or metastatic lesion.
3. Previously received at least 1-line endocrine therapy (endocrine therapy duration ≥ 6 months) and ≤ 2-line chemotherapy (≤ 2-line chemotherapy limited to dose escalation stage) for the recurrence or metastasis stage of the disease. The third stage : Inclusion of patients who have not received prior treatment but are suitable for CDK4/6i therapy.
4. Disease progression confirmed by imaging occurs during or after the last systemic anti-tumor treatment before the first medication.

Exclusion Criteria:

1. Known or suspected allergy to any ingredient of HP568 formulation, and allergy to any ingredient of palbociclib (only applicable to stage III).
2. Within 42 days prior to the first administration, Fluvistran was used; Other endocrine therapies such as tamoxifen, toremifene, letrozole, anastrozole, and exemestane were used within 14 days prior to the first administration.
3. Previously received other ER-ROTAC drugs such as ARV-471.
4. Within 6 weeks before the first administration of HP568 in this study, nitrosoureas or mitomycin were used; Received any anti-tumor treatment, including immunotherapy, chemotherapy, radiotherapy, or targeted therapy, within 28 days prior to the first administration (or of the drug's 5 half lives,take the shorter one).