Determinants of Acquired Endocrine Resistance in Metastatic Breast Cancer: A Pilot Study (ENDO-RESIST)

Inclusion Criteria:

• Sign written and voluntary informed consent
• Histological confirmation of advanced ER positive and HER2 negative breast cancer.
• Adult patients at least 18 years of age
• ECOG performance status equal to 0 or 1
• Able to provide written informed consent
• Must be willing to provide blood for ctDNA analysis on study enrollment and at specified study time points
• Must be willing and able to perform stool sample collection
• Patients must be suitable, as per their treating physician, for initiation of first line endocrine therapy with an aromatase inhibitor and a CDK 4/6 inhibitor for metastatic disease

Exclusion Criteria:

• Prior treatment with CDK 4/6 inhibitors in the neoadjuvant or adjuvant setting
• Use of targeted therapies other than endocrine therapies alone in the neoadjuvant or adjuvant setting
• Relapse on prior endocrine therapy or within 6 months of discontinuation of prior adjuvant endocrine therapy
• History of inflammatory bowel disease, chronic diarrhea or malabsorption syndromes and significant prior bowel resection as judged by the study investigator
• Use of immunosuppressants including steroids within the previous 4 weeks of planned Cycle 1 Day 1 treatment