Background: CDK4/6 inhibitors have emerged as the standard of care for HR+ MBC. However, there is limited insight into the potential benefit of abemaciclib following prior progression on palbociclib or ribociclib. Based on a multi-center cohort of patients with HR+ MBC who had received abemaciclib after prior palbociclib progression (Wander SA et al ASCO 2019), we have previously reported that abemaciclib after prior CDK4/6i progression was well tolerated and that a subset of patients derived durable clinical benefit. Identifying molecular predictors of sensitivity to abemaciclib after prior CDK4/6i progression constitutes an important area of research. Given the high frequency of ESR1 mutations in HR+ MBC with antiestrogen resistance, we evaluated the translational impact of ESR1 mutations in mediating response to abemaciclib in this setting.

Methods: To evaluate abemaciclib sensitivity in ESR1 mutant cell lines, T47D HR+ breast cancer cells were modified to over-express multiple mutant ESR1 isoforms via lentiviral infection and antibiotic selection. These isoforms included ESR1 Y537S, Y537N, and D538G. In an additional T47D cell line, RB1 expression was knocked down via CRISPR. The resulting derivative cell lines were grown in the absence of estrogen (via charcoal-stripped serum, CSS) or in escalating doses of abemaciclib. Cell viability was measured via cell-titer-glo assay. For clinical validation, we identified patients with MBC who had ESR1 mutations detected by targeted sequencing of cell-free DNA (cfDNA), via CLIA certified Guardant assay, and had abemaciclib exposure following prior progression on palbociclib or ribociclib in the existing multi-center cohort from six US institutions.

Results: All ESR1 mutant derivative cells demonstrated enhanced growth in estrogen deprivation compared to GFP controls, as expected, and were similarly sensitive to escalating doses of abemaciclib monotherapy in vitro, suggesting that ESR1 mutations do not confer resistance to abemaciclib. Interestingly, two patients with ESR1 mutations (in the absence of concurrent driver alterations in RB1, FGFR, CCNE2, and ERBB2) demonstrated progression on palbociclib and sensitivity to abemaciclib. In one patient, cfDNA obtained prior to palbociclib and fulvestrant exposure failed to reveal any ESR1 alteration. Following progression on palbociclib, and prior to sequential exposure to abemaciclib, an ESR1 Y537N alteration was identified. The patient went on to receive 16 months of abemaciclib monotherapy. In a second patient, an ESR1 D538G alteration was identified following progression on palbociclib and fulvestrant. The patient had several intervening regimens, and subsequently went on to receive abemaciclib and fulvestrant for 16 months. RB1-null T47D cells were resistant to abemaciclib monotherapy in vitro, as expected and, in the clinical dataset, the presence of alterations in previously identified genomic mediators of CDK4/6i resistance, such as RB1, were associated with progression on both palbociclib and abemaciclib.

Conclusions: HR+ breast cancer cells expressing mutant ESR1 isoforms were resistant to estrogen deprivation but retained sensitivity to abemaciclib in vitro. Furthermore, patients harboring ESR1 mutations via targeted sequencing of cfDNA, in the absence of other known mediators of CDK4/6i resistance, were shown to derive clinical benefit from abemaciclib following prior progression on palbociclib. These results suggest that patients with HR+ MBC, ESR1 mutation, and clinical resistance to anti-estrogen treatment and palbociclib may be candidates for abemaciclib treatment. Further research is warranted to confirm these novel translational observations.

Citation Format: Seth A. Wander, Hyo S. Han, Gabriela N. Johnson, Maxwell R. Lloyd, Pingping Mao, Utthara Nayar, Kailey Kowalski, Casey R. Stein, Veronica Mariotti, Leslie SL Kim, Maren Levin, Jing Xi, Apurva Pandey, Siobhan Dunne, Azadeh Nasrazadani, Adam Brufsky, Kevin Kalinsky, Cynthia X Ma, Joyce O’Shaughnessy, Nikhil Wagle, Aditya Bardia. Esr1 mutation as a potential predictor of abemaciclib benefit following prior cdk4/6 inhibitor (cdk4/6i) progression in hormone receptor-positive (hr+) metastatic breast cancer (mbc): A translational investigation [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-10.