Aromatase Inhibitor Therapy With or Without Fulvestrant for the Treatment of HR Positive Metastatic Breast Cancer With an ERS1 Activating Mutation, the INTERACT Study

PRIMARY OBJECTIVE: I. To assess progression free survival (PFS) with transition to fulvestrant compared with continuing aromatase inhibitor (AI) therapy in patients with emergence of estrogen receptor 1 (ESR1) mutations in plasma. SECONDARY OBJECTIVES: I. To assess circulating tumor deoxyribonucleic acid (ctDNA) ESR1 mutant allele fraction (MAF) and kinetics with fulvestrant compared with AI. II. To assess the prevalence of ESR1 mutations in patients with secondary resistance to endocrine therapy. III. To correlate ctDNA with cancer antigens (CA) 15-3 tumor marker changes. IV. To assess overall survival (OS) with transition to fulvestrant compared with continuing AI therapy in patients with emergence of ESR1 mutations. V. To assess PFS and time to next treatment (TTNT) on next line of therapy after progression on fulvestrant versus (vs.) AI in combination with CDKI. EXPLORATORY OBJECTIVES: I. Characterize other co-existing actionable genomic alterations of interest in relation to ESR1 and clinical outcomes. II. To determine frequency of other actionable genomic alterations and frequency of enrollment on genotype-matched therapy. OUTLINE: Patients are randomized to 1 of 2 arms. ARM I: Patients receive ribociclib orally (PO) once daily (QD), palbociclib PO QD on days 1-21, and/or abemaciclib PO twice daily (BID) on days 1-28. Patients also receive fulvestrant intramuscularly (IM) for 2 injections over 1-2 minutes each on days 1 and 15 of cycle 1 and day 2 of subsequent cycles. Cycles repeat every 28 days in the absence of disease progression or unacceptable toxicity. ARM II: Patients receive ribociclib orally PO QD, palbociclib PO QD on days 1-21, and/or abemaciclib PO BID on days 1-28. Patients also receive letrozole PO QD or anastrozole PO QD on days 1-28. Cycles repeat every 28 days in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up for 30 days.