Why is this study being done? Many patients with a type of breast cancer (called HR-positive) take a medicine called Abemaciclib. While this medicine is effective, a very common side effect is diarrhea, which can be severe enough to disrupt treatment and reduce quality of life. The reason why some patients get diarrhea and others do not is not well understood. This study aims to investigate whether the natural bacteria living in the gut (known as the gut microbiome) play a role in this side effect. Researchers will compare the gut bacteria of patients who develop diarrhea with those who do not.

What will happen in the study? This is an observational study, which means that patients will receive their normal cancer treatment and will not be given any new or experimental drugs as part of this initial phase.

• Patients who are already being treated with Abemaciclib will be invited to join.
• They will be placed into one of two groups: those who experience diarrhea and those who do not.
• Participants will be asked to provide stool (feces) samples and may also provide optional blood samples at specific times during their treatment.
• Researchers will analyze these samples in the lab to study the types and functions of the gut bacteria.

Who can participate?

• Adult women (aged 18-75) diagnosed with HR-positive breast cancer.
• Currently receiving treatment with Abemaciclib for at least 2 weeks.
• Must be willing to provide informed consent and follow the study procedures.

What are the potential benefits? Participants will not receive any direct medical benefit from taking part in this study. However, the information learned may help researchers better understand why diarrhea occurs and, in the future, could lead to new ways to prevent or treat this side effect for other cancer patients.

How is privacy protected? All personal information and samples collected will be de-identified using a unique code. This means that the data used for analysis cannot be directly linked back to the participant's identity. All data is stored securely according to strict ethical guidelines.

Inclusion Criteria:

1. Aged 18 to 75 years.
2. Diagnosed with hormone receptor-positive (HR⁺) breast cancer.
3. Currently receiving treatment with Abemaciclib (either as monotherapy or in combination with endocrine therapy) for a duration of at least 2 weeks.
4. Willing and able to provide written informed consent for participation in the study.

Exclusion Criteria:

1. History of major gastrointestinal diseases, such as inflammatory bowel disease, Crohn's disease, ulcerative colitis, or intestinal obstruction, or having undergone major gastrointestinal surgery.
2. Recent use (within 1 month) of antibiotics, probiotics, or traditional Chinese medicine that may alter gut function.
3. Pregnant or lactating women.
4. Unwilling to provide informed consent or considered by the investigator to be unsuitable for the study for any other reason.

This study will enroll a prospective cohort of female patients with hormone receptor-positive (HR⁺) breast cancer who are receiving treatment with Abemaciclib (as monotherapy or in combination) as part of their standard clinical care at Hubei Cancer Hospital. Participants will be prospectively assigned into two groups based on the presence or absence of drug-induced diarrhea, as defined by CTCAE v5.0 criteria: the Diarrhea Group and the Non-Diarrhea (Control) Group. The study involves the collection of clinical data, fecal samples, and optional blood samples for microbiome and biomarker analysis. No investigational intervention is administered in this phase of the study.

Individual participant data (IPD) that underlie the results reported in this study will not be shared publicly. The decision is based on the following considerations: to protect patient privacy and confidentiality, as the data contain sensitive clinical and genomic information; the informed consent obtained from participants does not include provisions for public data sharing; and the samples and data are collected under a specific ethical approval that limits their use to this research protocol. Summary results and aggregated data will be made available through publication.