PO-524 MT4-MMP, EGFR and Rb expressions are predictive biomarkers of response to erlotinib-palbociclib combination in TNBC

Introduction Triple negative breast cancer (TNBC) comprises heterogeneous agressive tumours. The standard treatment relies entirely on chemotherapy. While EGFR is expressed in 50%–75% of TNBC, clinical trials with erlotinib or cetuximab fail to show clinical benefit. Recently, we demonstrated that EGFR interacts with MT4-MMP, the membrane-type 4 matrix metalloprotease in TNBC. Here, we investigated the clinical significance of the MT4-MMP/EGFR axis and inactivation of protein of retinoblastoma (Rb) in human TNBC samples and patient-derived xenografts (PDX). We tested the efficacy of targeting this pathway in vivo in xenografts and PDX with a distinct expression profile of MT4-MMP, EGFR and Rb. Material and methods Immunohistochemistry (IHC) analysis for MT4-MMP, EGFR, Rb and Ki67 expression were performed on human TNBC samples (n=78) and Patient-Derived Xenograft (PDX-TNBC) (n=38). Single and combined treatments with erlotinib and palbociclib were applied in vitro and in vivo. In vitro 2D and 3D proliferation assays were performed on TNBC cells naturally producing EGFR and transfected or not with MT4-MMP cDNA. For the in vivo study, xenografts and PDX were engrafted with Rag1-/- mice and nude mice, respectively. Mice bearing tumours producing or not MT4-MMP, EGFR and Rb were treated with vehicle, erlotinib (50 mg/kg/day), palbociclib (75 mg/kg/day) or the combination. Results and discussions By IHC analysis we reveal that Rb is maintained functional in 68% of human samples and 59% of PDX-TNBC. Co-expression of MT4-MMP, EGFR and Rb is obtained in 49% of human samples and 43% in PDX-TNBC. In a 2D culture model, response to erlotinib and palbociclib is enhanced in cells expressing both MT4-MMP and EGFR. In 3D culture, their combination is synergistic for the inhibition of proliferation of MT4-MMP and EGFR expressing cells. In vivo, tumours expressing MT4-MMP and EGFR show a better response to erlotinib and palbociclib than tumours negative for MT4-MMP with a striking effect of the combination on MT4-MMP tumours. Moreover, PDX-TNBC expressing only EGFR and Rb show no response to erlotinib, a partial response to palbociclib and a strong response to the combination. Importantly, co-expression of EGFR and MT4-MMP strongly sensitises Rb positive PDX-TNBC to both erlotinib and palbociclib with a striking effect of the combination, whereas PDX-TNBC with Rb loss didn’t respond to any single or combination therapy. Conclusion Our data highlight MT4-MMP, EGFR and Rb as predictive biomarkers for response to erlotinib-palbociclib combination.