Abstract PO2-16-02: RBsig gene-expression signature in patients with endocrine resistant metastatic breast cancer treated with palbociclib and fulvestrant in the PYTHIA trial

Abstract Background: We have previously identified RBsig, a gene-expression signature of resistance to CDK4/6 inhibitors composed of E2F1/E2F2 dependent genes, which is associated with genetic loss of RB1. Although RBsig has been previously tested in vitro and in clinical trials with CDK4/6 inhibitors, further clinical validation is needed. Additionally, data on the correlation of RBsig with other prognostic/predictive biomarkers for CDK4/6 inhibitors such as serum Thymidine Kinase activity (sTKa), is lacking. Here, we aimed to investigate RBsig in patients (pts) treated with palbociclib (P) + fulvestrant (F) within the PYTHIA trial (IBCSG 53- 14/BIG 14-04; NCT02536742), a downstream trial of the AURORA molecular program (BIG 14-01; NCT02102165). Methods: PYTHIA is a biomarker discovery phase II trial which enrolled 122 pts with endocrine-resistant ER+ and HER2 - metastatic breast cancer (MBC) who received P+F at standard schedule and dose. We analyzed a subset of 38 pts enrolled in PYTHIA and successfully included in AURORA who had available RNAseq from tumor tissue samples (24 primary, 24 metastatic, 10 both). Ki67 by IHC was also available. The PYTHIA biomarker assessment included sTKa at baseline and during treatment (D15 and D28) measured using DiviTumÒ TKa (Biovica International). Evaluation of the prognostic role of RBsig was a pre-specified primary objective of PYTHIA. RBsig is composed of 87 genes and was calculated as previously described. Wilcoxon rank sum test was used to compare continuous RBsig values between groups defined by sTKa. Median progression free survival (mPFS) was estimated using Kaplan-Meier method for groups defined by dichotomizing RBsig at zero. Results: Median RBsig was higher in metastatic than in primary samples; in paired samples, RBsig increased in metastatic for 7/10 pts vs primary. When evaluated as continuous variables, RBsig correlated with tissue Ki67 both in primary (Spearman R 0.8) and in metastatic (R 0.68) samples. Additionally, RBsig was higher in Basal and Luminal B intrinsic subtypes in metastatic samples. RBsig was significantly higher in pts with high baseline sTKa (median cut-off) (Wilcoxon test, p=0.04 and p=0.03 in primary and metastatic samples respectively), with a similar direction, albeit not statistically significant, observed in the group of pts with no sTKa clearance at D15 (below limit of detection cut-off). When evaluated in the combined cohort, where RBSig values were taken from metastatic samples when available and from primary samples otherwise, higher RBsig values were associated with poorer PFS as compared to lower RBsig values mPFS 5.5 months (95% CI 3.7, 14) vs 13 months (95% CI 8.6, 28); Log-rank p-value=0.004. Conclusions: Higher RBsig was associated with higher tumor proliferation (measured by both tissue ki67 and sTKa), luminal B status, and shorter PFS on treatment with P+F. This analysis reinforces prior data on RBsig prognostic significance in pts treated with CDK4/6 inhibitors. Further investigation on the added value of RBsig as a novel biomarker for patients’ stratification, including in the early setting, is warranted. Citation Format: Matteo Benelli, Svitlana Tyekucheva, Thayane Crestani, Michail Ignatiadis, Patrick Neven, Marco Colleoni, Stéphanie Henry, Konstantinos Papadimitriou, Antonio Bernardo, Elena Seles, François Duhoux, Iain Macpherson, Alastair Thomson, David Mark Davies, Ilenia Migliaccio, Gabriele Zoppoli, Roswitha Kammler, Heidi De Swert, Barbara Ruepp, Philippe Aftimos, Jose Seoane, Dario Romagnoli, David Venet, Theodora Goulioti, David Cameron, Sherene Loi, Martine Piccart, Meredith Regan, Luca Malorni. RBsig gene-expression signature in patients with endocrine resistant metastatic breast cancer treated with palbociclib and fulvestrant in the PYTHIA trial abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-16-02.