Use of real-world data (RWD) to assess the utility of cell-free circulating tumor DNA (cfDNA) in identifying resistance to early treatment in advanced breast cancer (aBC).

Abstract

1011 Background: The approvals of CDK4/6 inhibitors (CDK4/6i) and alpelisib (a PI3Ka inhibitor, PI3Ki) have overhauled early treatment of hormone positive aBC. While some clinical trials have investigated mechanisms of resistance to these drugs, their impact on tumor evolution requires further exploration. Here we use cfDNA to examine molecular changes pre- and post-CDK4/6i or PI3Ki treatment and use RWD to assess the impact of putative resistance alterations on response to treatment. Methods: Patients (pts) with aBC were identified via the Guardant INFORM database and included if they had a cfDNA test within 90 days prior to therapy initiation and/or 90 days after therapy discontinuation with CDK4/6i or PI3Ki. Pts with RB1 loss of function (LOF) alterations (alts) who received CDK4/6i and pts with PTEN LOF alts who received PI3Ki were separately identified, and these cohorts were matched 1:3 with a RB1/PTEN negative population respectively, by age (+/- 5 years), sex, year of cfDNA test, and line of therapy. Log-rank tests were used to assess differences in time to discontinuation (TTD) and time to next treatment (TTNT). Results: Differences in the frequencies of certain alts detected in pts pre- and post-CDK4/6i or PI3Ki treatment are shown (Table). Pts with RB1 LOF alts prior to the start of CDK4/6i had significantly worse TTD and numerically worse TTNT versus controls (TTD = 3 mos vs 4.7 mos, p=0.018; TTNT = 7.3 mos vs 8 mos, p=0.082). Pts with PTEN LOF alts prior to start of PI3Ki had no significant difference in TTD or TTNT versus controls (TTD = 4.1 mos vs 4.1 mos, p=0.92; TTNT = 7.4 mos vs 7 mos, p=0.32). Notably, 54% of pts receiving CDK4/6i and 84% of pts receiving PI3Ki were on their third or later line of therapy. Conclusions: Using cfDNA, we were able to further characterize the resistance landscape of both CDK4/6i and PI3Ki, and identified specific ESR1, RB1 and PTEN alterations that appear likely to occur under the pressure of therapy. Our real-world analysis examining RB1 LOF alts added further evidence to suggest it may be both a primary and acquired resistance mechanism to CDK4/6i. As a non-invasive alternative to tissue biopsies, this data further illustrates that cfDNA can provide unique insight into tumor evolution and disease progression in the aBC setting. [Table: see text]