A Study of the Gut Microbiome in Hormone Receptor-positive HER2-negative Breast Cancer Treated With CDK4/6 Inhibitors

The combination of endocrine therapy and a CDK4/6 inhibitor is the preferred treatment option in advanced hormone-receptor positive (HR+) and HER2-negative (HER2-) breast cancer (BC). This combination (with abemaciclib or ribociclib) is now also considered standard of care for early HR+ HER2- BC deemed at high risk of relapse. Biomarkers predicting response to CDK4/6 inhibition remain largely unknown. Of note, research primarily focused on acquired genomic and transcriptomic tumor cell alterations, requiring invasive biopsies of tumor content. Thus, enlarging the scope of biomarkers of response to CDK4/6 inhibition to more easily accessible biological samples and to areas other than tumoral gene pathway alterations is urgently required. CDK4/6 inhibitors have been demonstrated to enhance the activity of cytotoxic T cells in BC, but without deep knowledge of mechanisms and implications. Studies of multiple cancer types have demonstrated the impact of the gut microbiome on the adaptive anti-cancer immunity. Ciclibiome is a prospective study of BC patients starting treatment with a CDK4/6 inhibitor (both in the advanced and adjuvant setting), aiming to study the interplay between the gut microbiome (its composition and evolution during treatment), circulating immune, metabolic and cytokine biomarkers (before and during treatment), and response outcomes to the CDK4/6 inhibitor. This study will explore the existence of microbial, immune and metabolic biomarkers correlated with the clinical outcomes to CDK4/6 inhibition in BC. The aim is to find an easily available biomarker, assessable by a stool or blood sample examination. This study will generate new hypotheses, that ultimately could give rise to potential combination strategies to test in a population considered of poor prognosis.