Probiotics for Prevention of Neratinib-Induced Diarrhea in Breast Cancer Patients

Inclusion Criteria Participants must meet all of the following criteria: 1. Female patients aged ≥18 years, diagnosed with HER2-positive breast cancer. 2. Scheduled to receive Neratinib therapy (monotherapy or in combination), based on clinical guidelines. 3. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2, with an expected survival of at least 3 months. 4. Left ventricular ejection fraction (LVEF) ≥ 50%. 5. Resolution of any prior treatment-related toxicity to Grade ≤1 (per CTCAE v5.0), with AST and ALT ≤ 2.5 × the upper limit of normal (ULN), and total bilirubin ≤ 1.5 × ULN. 6. Adequate bone marrow function, defined as: White blood cell count ≥ 3.0 × 10⁹/L Neutrophil count ≥ 1.5 × 10⁹/L Platelet count ≥ 100 × 10⁹/L Hemoglobin ≥ 90 g/L Serum creatinine ≤ 1.5 × ULN 7. No persistent gastrointestinal symptoms, such as hematochezia, chronic constipation, or abdominal pain. 8. No evidence of structural gastrointestinal abnormalities confirmed by gastroscopy or other relevant examinations. Exclusion Criteria Participants will be excluded if they meet any of the following criteria: 1. Conditions that significantly impair swallowing, digestion, or gastrointestinal drug absorption. 2. History of chronic gastrointestinal diseases, including but not limited to inflammatory bowel disease (IBD), gastrointestinal tumors, or malabsorption syndromes. 3. Severe cardiovascular diseases that may interfere with study treatment, including but not limited to: Life-threatening arrhythmias Advanced atrioventricular block Unstable angina Clinically significant pericardial disease Myocardial fibrosis Uncontrolled hypertension 4. Known hypersensitivity to any component of Neratinib, probiotics, placebo, or loperamide. 5. Prior participation in any clinical trial involving investigational drugs within 4 weeks prior to enrollment, or chronic use of medications that may induce constipation within 6 months. 6. Pregnant or lactating women, or those unwilling to use effective contraception during the study period. 7. Any medical, psychiatric, or social condition that, in the investigator's judgment, could compromise the safety of the participant, interfere with study participation, or confound the study results.