This study aims to evaluate the efficacy and safety of probiotics for the prevention of diarrhea in patients with breast cancer receiving the tyrosine kinase inhibitor (TKI) Neratinib.

Study Design: This is a prospective, randomized controlled clinical trial. Participants will be randomly assigned to either a probiotics intervention group or a placebo-controlled group. Both groups will receive prophylactic loperamide according to the FDA-recommended dosing schedule for neratinib-associated diarrhea.

Primary Objective: To evaluate the efficacy of probiotics in reducing the incidence and severity of diarrhea in patients receiving Neratinib.

Secondary Objectives: This study will also investigate the effects of probiotics on gut microbiota composition and their potential impact on drug efficacy.

Study Duration: Enrollment is planned from August 2025 to June 2027 at Sun Yat-Sen Memorial Hospital, Sun Yat-Sen University. Both the intervention and control groups will receive treatment for a total of six weeks (two cycles of three weeks each). No post-treatment observation period is included.

Eligibility Criteria: Participants must be diagnosed with HER2-positive breast cancer and scheduled to receive Neratinib. Exclusion criteria include patients with severe gastrointestinal disorders or recent probiotic consumption.

Breast cancer patients treated with the tyrosine kinase inhibitor (TKI) Neratinib often experience severe diarrhea, leading to treatment interruptions, dose reductions, or discontinuation. Emerging evidence highlights the role of gut microbiota in drug metabolism and gastrointestinal toxicity. Probiotics have been proposed as a potential intervention to modulate gut microbiota and mitigate diarrhea; however, their efficacy in preventing Neratinib-induced diarrhea remains inconclusive.

This randomized, double-blind, placebo-controlled clinical trial aims to evaluate the efficacy and safety of probiotics in preventing diarrhea in HER2-positive breast cancer patients receiving Neratinib. The study will also explore the impact of probiotics on gut microbiota composition and potential microbiome-mediated mechanisms that may enhance treatment adherence and therapeutic outcomes.

Participants will be randomly assigned in a 1:1 ratio to either a probiotics group or a placebo group. Both groups will receive prophylactic loperamide in accordance with the dosing schedule recommended by the U.S. Food and Drug Administration (FDA) for the prevention of Neratinib-associated diarrhea, ensuring consistency with the approved prescribing information.

Participants will receive treatment for a total of six weeks, corresponding to two treatment cycles.

Metagenomic sequencing and targeted metabolomics will be performed on stool samples to investigate gut microbial composition and metabolic changes. Additional analyses will assess the association between gut-derived metabolites and immune cell activation, aiming to elucidate potential interactions among gut microbiota, metabolites, and immune responses.

Sun Yat-sen Memorial Hospital will serve as the coordinating center for this planned multicenter trial. Currently, it is the only active site, and additional participating centers will be added progressively as they are initiated.

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.