This is a multicenter, open-label, single-arm, prospective, phase II study. conducted to evaluate the efficacy, safety and tolerability of nab-paclitaxel plus pyrotinib in patients with lymph node-negative and tumor size ≤3 cm, HER2 positive breast cancer.

Inclusion Criteria:

• Enrollment subjects must have a pathological diagnosis of HER2-positive primary invasive breast cancer with an immunohistochemistry (IHC) score of 3 +, or 2 + and HER2 gene amplification by in situ hybridization (ISH) (ratio of HER2/CEP17 ≥ 2.0).
• The invasive tumor had to measure no more than 3cm and with histologically confirmed lymph node-negative or one lymph node micrometastasis (T ≤ 3 cm, N0/N1mi, M0).
• Tumor should has known ER/PR hormone receptor status.
• All patients must be women above18 years old with Eastern Cooperative Oncology Group score 0 to 1.
• Adequate hematopoietic function and organ function as defined as follows: neutrophil count ≥ 1.5 x 109/L; Platelet count ≥ 90 × 109/L; Hemoglobin ≥ 90 g/L; total bilirubin ≤ 1.5 × ULN; alanine transaminase (ALT) and aspartate transaminase (AST) ≤ 3 × ULN; serum creatinine Cr ≤ 1.5 × ULN and creatinine clearance ≥ 50 mL/min (Cockcroft-Gault formula).
• left ventricular ejection fraction (LVEF) ≥ 50% and Electrocardiogram Fridericia-corrected QT interval (QTcF) ≤ 480 ms.
• Provide written informed consent.

Exclusion Criteria:

• Clinical or radiologic evidence of local or regional recurrence of disease or metastatic disease prior to or at the time of study entry.
• Previous treatment with chemotherapeutic drugs, or tyrosine kinase inhibitors targeting HER2 (lapatinib, neratinib or pyrotinib, etc.).
• Other malignancies within the past 5 years, excluding cured cervical carcinoma in situ, cutaneous basal cell carcinoma, or cutaneous squamous cell carcinoma.
• Inability to swallow, chronic diarrhea, or intestinal obstruction.
• Known to be allergic to the drug components.
• Have a history of immunodeficiency, including HIV positive, HCV positive, active viral hepatitis B or other immunodeficiency diseases.
• Have a history of organ transplantation.
• Pregnant, lactating female patients, or female patients who are unwilling to take effective contraceptive.
• Any heart disease, including: (1) arrhythmia; (2) myocardial infarction; (3) heart failure.
• Any other concomitant diseases assessed by investigator as unsuitable for study.
• Previous history of definite neurological or psychiatric disorders.
• Concomitant use of CYP3A4 inhibitors or inducers or drugs that prolong the QT interval.