Besides EGFR other tyrosine kinase

Besides EGFR, other tyrosine kinase receptors are currently gaining attention as potential therapeutic targets. Included among these is the fibroblast growth factor receptor (FGFR) family, which is involved in the progression of a variety of cancers.7, 8, 9, 10, 11 In lung cancer, fibroblast growth factor receptor 1 (FGFR1) has shown oncogenic properties in preclinical models of squamous cell carcinomas, in which amplification of the gene is frequently observed through the activation of relevant signaling pathways such as signal transducer and activator of transcription (STAT), AKT, and mitogen-activated protein kinase,12, 13, 14, 15 opening the door for potential therapeutic targeting in this histological type of lung cancer. However, even though the preclinical results of using FGFR inhibitors were promising, the efficacy of drugs targeting FGFR1 in clinical trials has so far been discrete in this tumor subtype. For this reason, the current main patient selection criterion for use of these inhibitors, namely, fibroblast growth factor receptor 1 gene (FGFR1) amplification, which is also seen in 1% to 3% of lung adenocarcinomas,17, 18 is being questioned, and there is in fact evidence showing that FGFR1 mRNA and FGFR1 protein recommended site may be better predictors of FGFR inhibition efficacy.19, 20 In light of these results, current research on FGFR inhibition is focused on the identification of optimal predictive biomarkers. Furthermore, there may be a cooperative interaction between EGFR and FGFR1 in lung adenocarcinomas, as FGFR1 upregulation has been reported as a mechanism of resistance to EGFR-targeted therapy.22, 23, 24 In this work, we have aimed to study the biological interactions between both signaling pathways and the potential therapeutic implications of their inhibition.
Materials and Methods
Results First, we measured protein expression levels of FGFR1 and EGFR in adenocarcinoma cell lines with different genetic backgrounds and in two immortalized epithelial lung cell lines (see Supplementary Table 1). Relevant EGFR activation was observed in the EGFR-mutated cell lines, as expected, and in two other additional cell lines, Calu3 and H1781, without known EGFR mutations (Fig. 1A). EGFR mutational profiling in these two cell lines revealed no EGFR activating mutations in either cell line (data not shown). FGFR1 protein expression was detected in a few of the cell lines tested, but no relevant pFGFR1 was detected by Western blot (data not shown).
Discussion EGFR was found to colocalize and coimmunoprecipitate with FGFR1, supporting a physical interaction between these molecules. Interestingly, pEGFR levels increased after stimulation with FGFR-specific ligands as phosphorylation of the FGFRs occurs after stimulation with EGF, thus providing biochemical evidence for EGFR-FGFR cooperation. These results are in accordance with previous data in the literature for another member of the FGFR family, showing that mouse embryonic fibroblasts transformed by EGFR overexpression display higher levels of FGFR4 expression than mouse embryonic fibroblasts transformed by other means. FGFR1 overexpression in this context may further activate EGFR, which could confer a selective growth and tumorigenic advantage. The EGFR-FGFR1 cooperation suggests a potential therapeutic opportunity for tumors with EGFR activation and high expression of these FGFRs. Indeed, we reported higher efficacy of dual EGFR/FGFR inhibition in EGFR-dependent models with FGFR1 overexpression in vitro and in vivo, showing dramatic results in EGFR-mutated cell line xenograft models overexpressing FGFR1 and in PDXs with high EGFR activation (one with an EGFR activating mutation and two with wild-type EGFR). Remarkably, two of these models, in which the combined EGFR/FGFR inhibition showed high efficacy, harbored the erlotinib resistance T790M mutation, which highlights the importance of the FGFR1-EGFR cooperation even in the presence of erlotinib resistance mutations. However, the high efficacy of this combined therapy in these two erlotinib-resistant models may be due to the relatively low duration of the treatments. In any case, our results support the therapeutic potential of combined inhibition of EGFR and FGFR in EGFR-activated FGFR1-overexpressing tumors regardless of the presence of EGFR mutations. It is important to note that therapeutic regimens using more than one cell-signaling inhibitor frequently present increased adverse effects, which hinder their clinical application.31, 32 Toxicity enhancement is typically reported when unselective inhibitors that are molecularly promiscuous with numerous off-target effects are combined.33, 34 In this regard, a recent phase I trial testing the safety of erlotinib combined with the FGFR unselective inhibitor dovitinib in patients with NSCLC was terminated because of unacceptable toxicity. In the present work, however, we have proposed combining EGFR inhibitors with a more selective FGFR1 inhibitor, hoping to result in a more favorable tolerability profile.