Abstract PD2-03: Association between co-existing genomic alterations and abemaciclib benefit in patients with metastatic hormone receptor-positive breast cancer with ESR1 mutations following disease progression on prior endocrine therapy plus palbociclib or ribociclib

Abstract Background: For patients with hormone receptor-positive (HR+) metastatic breast cancer (MBC) and progression on combination endocrine therapy plus palbociclib/ribociclib, abemaciclib can be effective and well-tolerated, conferring durable clinical benefit in a subset of patients (Wander SA et al. JNCCN 2021). We previously reported that, even in patients with ESR1-mutant (ESR1-MUT) HR+ MBC, some - but not all - still achieve long-term disease control with abemaciclib (Wander SA et al. SABCS 2020). How to identify which patients are likely to benefit from abemaciclib after progression on combination endocrine therapy and CDK4/6 inhibition is not known and, more generally, predicting tumor response to CDK4/6 inhibition has been an area of ongoing research. Because resistance to CDK4/6 inhibition can occur through multiple mechanisms, we hypothesized that a comprehensive resistance panel, rather than single genetic markers or immunohistochemical readouts, would provide an effective predictive tool. Methods: To determine which patients with ESR1-MUT MBC and progression on endocrine therapy plus palbociclib/ribociclib benefit from abemaciclib, we examined a multicenter cohort of such patients who had genomic profiling by standard commercially available assays, the majority of which were via plasma-based cell-free DNA (cfDNA) genotyping assays. We generated a curated list based upon prior literature of CDK4/6i resistance drivers that had been validated in tumor biopsy specimens and in laboratory models: these genes were involved in cell cycle regulation (CCNE1/2, RB1, AURKA) and growth factor signaling pathways (ERBB2, FGFR1/2, AKT1, PTEN, KRAS, FAT1). Progression-free survival (PFS) was defined as time from abemaciclib initiation to time of discontinuation due to disease progression or death; patients who discontinued due to toxicity were right-censored. To examine the cellular effects of different mutations, we also studied the impact of ESR1-MUT, RB1 loss, and KRAS activation on the growth and survival (using an ATP abundance-based assay) of patient-derived circulating tumor cell (CTC) lines treated with palbociclib and abemaciclib in vitro. Results: Among patients with ESR1-MUT MBC with disease progression on endocrine therapy plus palbociclib/ribociclib (n=28), absence of co-existing genomic alterations in our curated panel was associated with greater clinical benefit with subsequent abemaciclib. Patients lacking a mutation in this resistance panel (n=17) had a median PFS of 7.0 months (95% CI: 4.1-13.2); patients with at least one mutation in this panel (n=11) had a median PFS of 3.5 months (95% CI: 2.1-5.4). The difference in PFS was statistically significant (p=0.02, log-rank test). On univariable Cox regression the hazard ratio for patients with a mutation in the resistance panel was 2.8 (95% CI: 1.1-7.1, p=0.03). In vitro, two out of three patient-derived cell lines with ESR1-MUT remained sensitive to abemaciclib, while those with mutation in RB1 or KRAS were less sensitive to abemaciclib. Conclusions: In our study, absence of co-existing genomic alterations in a curated panel was associated with greater clinical benefit with subsequent abemaciclib among patients with ESR1-MUT MBC with prior disease progression on endocrine therapy plus palbociclib/ribociclib. While a small dataset, this is the first demonstration of a genomic panel associated with continued CDK4/6 inhibitor sensitivity. Future directions include testing this panel outside of ESR1-MUT MBC and refining the panel in additional datasets with increased sample size, to guide therapy selection for patients with HR+ MBC. Citation Format: Jamie O Brett, Taronish D Dubash, Andrzej Niemierko, Veronica Mariotti, Leslie SL Kim, Jing Xi, Apurva Pandey, Siobhan Dunne, Azadeh Nasrazadani, Maxwell R Lloyd, Laura M Spring, Douglas Micalizzi, Maristela Onozato, Dante Che, Adam Brufsky, Kevin M Kalinsky, Cynthia X Ma, Joyce O’Shaughnessy, Hyo S Han, A. John Iafrate, Shyamala Maheswaran, Daniel A Haber, Aditya Bardia, Seth A Wander. Association between co-existing genomic alterations and abemaciclib benefit in patients with metastatic hormone receptor-positive breast cancer with ESR1 mutations following disease progression on prior endocrine therapy plus palbociclib or ribociclib [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD2-03.