Abstract PD2-11: Breast cancer index is a molecular signature of endocrine responsiveness that determines extended endocrine benefit independent of prognostic risk
Authors: Liefers Gerrit-Jan , Noordhoek Iris , Treuner Kai , Putter Hein , Zhang Yi , Wong Jenna , Kranenbarg Elma Meershoek-Klein , Duijm-de Carpentier Marjolijn , Van De Velde Cornelis JH , Schnabel Catherine A 
Published: February 2021

Background: For patients with early stage, hormone receptor positive (HR+) breast cancer, extension of endocrine therapy beyond 5 years reduces the ongoing risk of late distant recurrence and new primary breast cancer but is associated with potentially serious toxicities and side effects that can impair quality of life. While risk stratification by clinicopathological factors and prognostic biomarkers is recommended to guide patient selection, predictive biomarkers that are directly informative of therapeutic response are critical for clinical utility. Breast Cancer Index (BCI) is a gene expression-based signature that reports: 1) BCI Prognostic (BCI Score), which combines the Molecular Grade Index (MGI) with the HOXB13/IL17BR (H/I) ratio to provide individualized risk of late (post-5y) distant recurrence, and 2) BCI Predictive (BCI H/I), which predicts benefit from extended endocrine therapy. In this study, BCI and endocrine benefit from 5 vs 2.5y of extended letrozole were examined by comparing the deterministic effects of prognostic risk vs endocrine response in patients treated in the IDEAL trial.Methods: 908 patients with available tumor tissue were included in this pre-specified and blinded analysis, with recurrence-free interval (RFI) as the primary endpoint. Kaplan-Meier and Cox proportional hazards regression were performed using pre-defined assay cut-points. Clinically high prognostic risk was defined as pN+ and ≥pT2; low prognostic risk was defined as pN0 or pN1 and pT1 or G1. Genomic risk was defined by BCI prognostic categories (High vs Low risk). Results: Patients classified with endocrine responsive disease by BCI (H/I)-High status showed a statistically significant benefit from 5 vs 2.5y of letrozole irrespective of clinical risk, whereas those classified as BCI (H/I)-Low did not show benefit in either clinical risk category. Both clinically High and Low risk patients derived a similar magnitude of benefit when stratified as BCI (H/I)-High (12.5% and 15.1%, respectively).

Evaluation of BCI (H/I) predictive performance in the context of BCI prognostic risk categories showed similar results. In particular, patients with High Risk/High Likelihood to benefit showed significantly improved outcomes with extended therapy (HR 0.42 [0.20-0.89]; absolute benefit 10.7%; P=0.020), whereas those with High Risk but Low Likelihood to benefit did not show response to extended endocrine therapy (HR 1.05 [0.58-1.88]; absolute benefit -0.7%; P=0.880), despite considerable risk of late distant recurrence (~20%). Conclusion: Results from this study highlight distinct tumor biology underlying recurrence risk vs endocrine responsiveness in HR+ early stage breast cancer. BCI (H/I) as a molecular signature to identify patients with endocrine responsive disease is a key determinant of benefit and improved outcome with extended endocrine therapy. Prediction of endocrine benefit with BCI (H/I) is a more actionable approach vs risk assessment to individualize discontinuation of or continued treatment with prolonged durations of endocrine therapy.

Prognostic RiskBCI Predictive NHR (95% CI)P valueClinical RiskHigh Risk: N+/ T2+ (N=353)High BCI(H/I)1620.32 (0.10-0.98)0.035Low BCI(H/I)1911.13 (0.55-2.31)0.742Low Risk: N0 or N1 & T1 or grade 1 (N=404)High BCI(H/I)1910.15 (0.03-0.66)0.004Low BCI(H/I)2130.75 (0.33-1.70)0.484Genomic RiskHigh BCI Score(N=621)High BCI(H/I)3410.42 (0.20-0.89)0.020Low BCI(H/I)2801.05 (0.58-1.88)0.880Low BCI Score(N=227)High BCI(H/I)600.35 (0.04-3.38)0.344Low BCI(H/I)1670.64 (0.22-1.92)0.425

Citation Format: Gerrit-Jan Liefers, Iris Noordhoek, Kai Treuner, Hein Putter, Yi Zhang, Jenna Wong, Elma Meershoek-Klein Kranenbarg, Marjolijn Duijm-de Carpentier, Cornelis JH Van De Velde, Catherine A Schnabel. Breast cancer index is a molecular signature of endocrine responsiveness that determines extended endocrine benefit independent of prognostic risk [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 PD2-11.