Addition of S-1 to the Epidermal Growth Factor Receptor Inhibitor Gefitinib Overcomes Gefitinib Resistance in Non–small cell Lung Cancer Cell Lines withMETAmplification

PURPOSE: Most non-small cell lung cancer (NSCLC) tumors with activating mutations in the epidermal growth factor receptor (EGFR) are initially responsive to EGFR tyrosine kinase inhibitors (EGFR-TKI) such as gefitinib and erlotinib, but they almost invariably develop resistance to these drugs. A secondary mutation in EGFR (T790M) and amplification of the MET proto-oncogene have been identified as mechanisms of such acquired resistance to EGFR-TKIs. We have now investigated whether addition of the oral fluoropyrimidine derivative S-1 to gefitinib might overcome gefitinib resistance in NSCLC cell lines. EXPERIMENTAL DESIGN: The effects of gefitinib on EGFR signaling and on the expression both of thymidylate synthase and of the transcription factor E2F-1 in gefitinib-resistant NSCLC cells were examined by immunoblot analysis. The effects of S-1 (or 5-fluorouracil) and gefitinib on the growth of NSCLC cells were examined in vitro as well as in nude mice. RESULTS: Gefitinib induced down-regulation of thymidylate synthase and E2F-1 in gefitinib-resistant NSCLC cells with MET amplification but not in those harboring the T790M mutation of EGFR. The combination of 5-fluorouracil and gefitinib synergistically inhibited the proliferation of cells with MET amplification, but not that of those with the T790M mutation of EGFR, in vitro. Similarly, the combination of S-1 and gefitinib synergistically inhibited the growth only of NSCLC xenografts with MET amplification. CONCLUSIONS: Our results suggest that the addition of S-1 to EGFR-TKIs is a promising strategy to overcome EGFR-TKI resistance in NSCLC with MET amplification.