Pyrotinib Combined With Trastuzumab and Pertuzumab for Maintenance Therapy in HER2-Positive Advanced Breast Cancer

Inclusion Criteria: 1. Age ≥ 18 years, male or female. 2. Histologically or cytologically confirmed HER2-positive advanced breast cancer (IHC 3+, or IHC 2+ with ISH amplification), with known HR status. 3. Have unresectable locally advanced or metastatic disease. If recurrent (after \[neo\]adjuvant therapy), must be at least 6 month treatment free from any trastuzumab and pertuzumab received in the early breast cancer setting for advanced HER2+ disease. 4. Have received 4-8 cycles of pre-study induction therapy including only trastuzumab, pertuzumab, and taxane as first-line of therapy for the treatment of advanced breast cancer prior to study enrollment. Participants are eligible provided they are without evidence of disease progression following completion of induction therapy. 5. CNS Inclusion - Based on screening contrast-enhanced brain magnetic resonance imaging (MRI), participants may have any of the following: No evidence of brain metastases. Untreated brain metastases which are asymptomatic not needing immediate local treatment and, if identified on prior brain imaging, without evidence of progression since starting first-line induction therapy with trastuzumab, pertuzumab, and taxane 6. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2 7. Have planned to receive maintenance anti-tumor therapy with pyrotinib in combination with HP 8. Negative serum pregnancy test; women of childbearing potential must use a highly effective contraceptive method from study initiation until at least 6 months after the last dose of study medication. 9. Voluntary participation with written informed consent obtained prior to any study-related procedures. Exclusion Criteria: 1. Other malignancy diagnosed within 5 years prior to enrollment. 2. Prior treatment with any tyrosine kinase inhibitor targeting HER2 and/or epidermal growth factor receptor (EGFR) including pyrotinib, lapatinib, tucatinib, neratinib, and afatinib. 3. Patients who are difficult or unable to be followed-up. 4. Other reasons that, in the investigator's judgment, make the patient unsuitable for participation in this study.