Abstract LB-98: Promiscuous transactivation of multiple receptor tyrosine kinases by EGFR mutations: mechanism and therapeutic significance.
Authors: Greenall Sameer A. , Donoghue Jacqueline F. , Adams Timothy E. , Johns Terrance G. 
Published: August 2014

De2-7EGFR (or EGFRvIII) is an extra-cellular domain mutation of the EGFR expressed in 30% of high grade gliomas. Previously we reported that the de2-7EGFR mediates its tumorigenic activity in part through the promiscuous transactivation of other receptor tyrosine kinases (RTKs), including c-Met. Using proximity ligation assay technology we now show that de2-7EGFR directly associates with c-Met in a heterodimeric complex through a scaffold that involves focal adhesion kinase (FAK). Chemical and siRNA ablation of FAK prevented the de2-7EGFR-mediated transactivation of c-Met and inhibited downstream signalling. U87MG glioma cells have a c-Met/HGF autocrine loop that is critical for in vivo growth as evidenced by their complete inhibition by Rilotumumab; a clinical antibody that inhibits HGF the c-Met ligand. Transfection with the de2-7EGFR (U87MG.Δ2-7) switches the primary driver of in vivo growth to this receptor. U87MG.Δ2-7 xenografts are responsive to Panitumumab (anti-EGFR) but resistant to Rilotumumab due to the transactivation of c-Met by de2-7EGFR. To further elucidate the clinical significance of this transactivation we treated U87MG.Δ2-7 cells with the EGFR tyrosine kinase inhibitor (TKI) erlotinib in the presence of Rilotumumab. Surprisingly, when using concentrations of erlotinib that partially inhibited the de2-7EGFR, we observed increased activation of c-Met. Mechanistically, Rilotumumab increased the amount of cell surface c-Met by decreasing internalization, leaving it free to interact with the de2-7EGFR. Since EGFR TKIs and HGF neutralizing antibodies are in trials in lung cancer patients with EGFR kinase mutations, this suggests that HGF blockade may increase c-Met signalling in patients. We are exploring this possibility in lung cancer cell lines. These studies suggest that the direct inhibition of the de2-7EGFR combined with inhibition of de2-7EGFR transactivation maybe an effective anti-tumor strategy. The combination of a FAK TKI and Panitumumab was effective in preventing the in vitro growth of U87MG.▵2-7 cells. We also examined the anti-tumor activity of Panitumumab in combination with Rilotumumab or AMG 706 in the U87MG.▵2-7 xenograft model. The TKI AMG 706 was selected as it inhibits several RTKs transactivated by de2-7EGFR. Rilotumumab had no single agent activity, while AMG 706 displayed modest anti-tumor activity. Unexpectedly, the combination of Rilotumumab and AMG 706 had significant anti-tumor activity despite neither therapeutic directly targeting the de2-7EGFR; suggesting that targeting de2-7EGFR transactivation is a viable therapeutic approach. Greater than additive anti-tumor responses were seen with Rilotumumab and AMG 706 when used in combination with Panitumumab. These studies indicate that targeting de2-7EGFR transactivation is a valid therapeutic strategy and enhances the efficacy of Panitumumab.

Citation Format: Sameer A. Greenall, Jacqueline F. Donoghue, Timothy E. Adams, Terrance G. Johns. Promiscuous transactivation of multiple receptor tyrosine kinases by EGFR mutations: mechanism and therapeutic significance. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-98. doi:10.1158/1538-7445.AM2013-LB-98