This is an open-label, single-center, exploratory, Phase Ib/II clinical trial evaluating the safety and efficacy of entinostat combined with pyrotinib and endocrine therapy in patients with advanced hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-positive (HER2+) breast cancer who have failed prior trastuzumab-based therapy.

The study consists of two parts: Part I (Phase Ib) employs a 3+3 dose-escalation design to determine the dose-limiting toxicity (DLT), maximum tolerated dose (MTD), and recommended Phase II dose (RP2D) of entinostat when given in combination with pyrotinib (400 mg daily) and endocrine therapy. Part II (Phase II) uses a Simon two-stage design to further evaluate the efficacy and safety of the combination at the RP2D. The primary efficacy endpoint is progression-free survival (PFS) based on RECIST v1.1. A total of approximately 79-94 participants will be enrolled.

Background: Triple-positive breast cancer (TPBC), characterized by co-expression of HR and HER2, accounts for approximately 10% of all breast cancer cases. Despite the availability of anti-HER2 targeted therapies, patients with TPBC often have suboptimal responses to chemotherapy-containing regimens, and endocrine therapy resistance remains a clinical challenge. Histone deacetylase (HDAC) inhibitors have shown synergistic effects with HER2-targeted agents and endocrine therapy through epigenetic modulation, potentially reversing drug resistance and enhancing anti-tumor immunity.

Objective: To explore the DLT, MTD, and RP2D of entinostat in combination with pyrotinib and endocrine therapy, and to preliminarily evaluate the safety and efficacy of this regimen in HR+/HER2+ advanced breast cancer patients who have failed trastuzumab therapy.

Study Design: This is a two-part study. Part I (Phase Ib) is a single-arm, open-label, dose-escalation trial following the traditional 3+3 design. Entinostat is administered at escalating doses (3 mg, 4 mg, and 5 mg) once weekly, in combination with a fixed dose of pyrotinib (400 mg daily) and physician-selected endocrine therapy, in 28-day cycles. The DLT observation period is the first cycle.

Part II (Phase II) is a single-arm, two-stage (Simon's optimal design) trial. Participants will receive entinostat at the MTD/RP2D established in Part I, combined with pyrotinib (400 mg daily) and endocrine therapy. A total of 76 evaluable participants are planned for Part II, with an interim analysis after the first 24 participants. The study is expected to enroll approximately 79-94 participants over 24 months, with an additional 12 months of follow-up.

Key Eligibility: Female patients aged 18-75 years with histologically confirmed HR+/HER2+ locally advanced or metastatic breast cancer, who have received prior trastuzumab and taxane therapy, with 1-2 prior lines of systemic therapy in the advanced setting, ECOG PS 0-2, and measurable disease per RECIST v1.1.

Statistical Methods: PFS will be estimated using the Kaplan-Meier method. ORR, DCR, CBR, DoR, and PRO will be summarized with descriptive statistics and 95% confidence intervals. Safety data will be tabulated by type, severity, and relationship to study drugs.

Inclusion Criteria:

1. Age ≥ 18 and ≤ 75 years, female, premenopausal or postmenopausal.
2. Histologically confirmed HR+/HER2+ breast cancer (HER2 positive defined as IHC 3+ or FISH confirmed by the pathology department of the study center).
3. Histologically confirmed locally advanced breast cancer (not amenable to curative local therapy) or recurrent/metastatic breast cancer.
4. Prior treatment with trastuzumab and taxane-based anticancer therapy.
5. Received 1-2 prior lines of systemic anticancer therapy in the advanced setting.
6. Life expectancy ≥ 3 months.
7. At least one measurable lesion per RECIST v1.1.
8. ECOG performance status 0-2.
9. All acute toxicities from prior anticancer therapy resolved to Grade 0-1 (per NCI CTCAE v5.0), except alopecia and toxicities deemed by the investigator to pose no safety risk.
10. Adequate bone marrow function: ANC ≥ 1.5×10^9/L, platelet ≥ 100×10^9/L, hemoglobin ≥ 90 g/L.
11. Adequate hepatic and renal function: TBIL ≤ 1.5×ULN; ALT and AST ≤ 2.5×ULN (or ≤ 5×ULN in the presence of liver metastases); BUN and Cr ≤ 1.5×ULN with creatinine clearance ≥ 50 mL/min.
12. Voluntarily participate and sign written informed consent. -

Exclusion Criteria:

1. Factors significantly affecting oral drug absorption, such as inability to swallow, chronic diarrhea, or intestinal obstruction.

2. Prior treatment with any HDAC inhibitor for antitumor therapy.

3. Prior or current use of any HER2-targeted tyrosine kinase inhibitor (including lapatinib, neratinib, pyrotinib, etc.).

4. Known allergy to any component of the study drugs.

5. Other malignancy within 5 years prior to enrollment, except cured papillary thyroid carcinoma, cervical carcinoma in situ, basal cell carcinoma, or squamous cell carcinoma of the skin.

6. Participation in another drug clinical trial within 4 weeks prior to enrollment.

7. History of immunodeficiency, including HIV positivity, other acquired or congenital immunodeficiency diseases, or history of organ transplantation.

8. Uncontrolled significant cardiovascular disease: clinically significant QTc interval prolongation or QTc > 450 ms at screening; severe cardiac impairment (NYHA > Class II); unstable angina or myocardial infarction within 6 months; or severe arrhythmia.

9. Pregnant or lactating women, or positive pregnancy test at baseline; or women of childbearing potential who are unwilling to use effective contraception during the study and for at least 8 weeks after the last dose.

10. Concomitant diseases that, in the investigator's judgment, could seriously endanger patient safety or affect study completion (e.g., severe hypertension, diabetes, thyroid disease, active infection).

11. Known history of neurological or psychiatric disorders, including epilepsy or dementia.

12. Active hepatitis (HBV: HBsAg positive with HBV DNA ≥ 500 IU/mL; HCV: HCV antibody positive with HCV RNA > ULN).

13. Any condition that, in the investigator's judgment, makes the patient unsuitable for study participation.

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