Abstract

TPS1134 Background: Aromatase inhibitor (AI) + CDK4/6 inhibitor (CDK4/6i) has been established as a standard of care of 1 st line treatment for hormone receptor (HR)-positive/HER2-negative advanced metastatic breast cancer (AMBC). ESR1 mutations are considered to be one of the causes of acquiring resistance against the AI +CDK4/6i. PADA-1 trial showed that replacing AI with fulvestrant (FUL) as a partner of palbociclib (PAL) at occurrence of some ESR1 mutations improved progression-free survival (PFS) and PFS2, suggesting PAL should be used based on the strategy of selecting endocrine agents according to ESR1 mutation status by ctDNA analysis. On the other hand, we don’t know if this strategy is effective in the case of abemaciclib (ABM), another CDK4/6i established as the standard of care for AMBC. We are conducting a study to make evidence on this clinical question. Methods: This multicenter, single-arm phase II study is designed to evaluate the efficacy and safety of the strategy to replace AI with FUL for detectable ESR1 mutations in combination with ABM. Key eligibility criteria include patients receiving AI+ABM for at least 6 months as first-line treatment for HR-positive/ HER2-negative AMBC without progressive disease (PD). While continuing AI+ABM, ctDNA analysis will be performed at approximately 3, 9, 15 months from enrollment. If ESR1 mutation is identified at any point and non-PD is confirmed by CT, FUL+ABM will be initiated instead of AI+ABM. In patients with ESR1 mutations completely cleared by ctDNA analysis 6 months after the start of FUL+ABM, AI+ABM will be resumed (Re-AI+ABM arm); the Re-AI+ABM arm is intended to investigate whether the discontinuation of FUL will allow clones with ESR1 mutations to proliferate again. Primary endpoint is 2-year PFS rate in full analysis set. Key secondary endpoints include 2-year PFS rate by treatment sequences, and rate of disappearance of ESR1mutations in ctDNA with FUL+ABM. Plasma-SeqSensei Breast Cancer IVD Kit, which can simultaneously detect mutations in a wide range of six genes including ESR1, as well as PIK3CA, AKT1, ERBB2, KRAS, and TP53, is used for ctDNA analysis in this study. Based on PFS data in MONARCH3 study, the threshold and expected value of the 2-year PFS rate were estimated to be 45% and 55%, respectively, and target sample size was calculated as 160 patients. Patient enrollment started in December 2022 and run through December 2024; as of December 2023, 54 patients have been enrolled. Clinical trial information: 051220133.