Abemaciclib (Verzenio) is a commonly used treatment for hormone receptor-positive breast cancer, but it can cause gastrointestinal side effects such as diarrhea, which may affect quality of life and treatment tolerance. The gut microbiome, which is the collection of bacteria living in the digestive tract, may play a role in these symptoms.

The purpose of this study is to examine how the gut microbiome changes during the early phase of abemaciclib treatment and how these changes relate to gastrointestinal symptoms experienced by patients. Participants will provide stool samples at multiple time points using mailed collection kits and will complete questionnaires about gastrointestinal symptoms while receiving standard-of-care abemaciclib therapy. All study procedures will be conducted remotely.

Information gained from this study may help inform future supportive care strategies for breast cancer patients receiving abemaciclib.

Inclusion Criteria:

• Age ≥ 18 years.
• Histologically confirmed hormone receptor-positive, HER2-negative breast cancer.
• Planned initiation of adjuvant abemaciclib (Verzenio) as part of standard clinical care, with enrollment occurring prior to or within 7 days of starting abemaciclib.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.
• Women of child-bearing potential must agree to use contraception prior to study entry, for the duration of study participation, and for 90 days following completion of study participation. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately.

A female of child-bearing potential is any woman (regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria:

• Has not undergone a hysterectomy or bilateral oophorectomy; or

• Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months).

  • Ability to understand and the willingness to sign a written informed consent.

Exclusion Criteria:

• Concurrent participation in another interventional clinical trial that, in the opinion of the investigator, would interfere with study participation or interpretation of results.
• Known gastrointestinal conditions that would significantly interfere with study procedures or interpretation of results, including but not limited to inflammatory bowel disease, short bowel syndrome, or chronic diarrheal disorders.
• Known allergy or intolerance to resistant starch or any component of the study supplement.
• Pregnant or breastfeeding individuals.
• Any medical, psychiatric, or social condition that, in the judgment of the investigator, would limit the participant's ability to comply with study procedures.