['The effect of combined administration of the multi-targeted receptor tyrosine kinase (RTK) inhibitor (Sorafenib) and chemotherapy (Pemetrexed) is still unknown. The cytotoxicity, the optimal combined modality, and the mechanisms underlying the cytotoxic synergism between sorafenib and pemetrexed for EGFR TKI-resistant NSCLC cell lines were then investigated.', 'A549 (EGFR wild-type and KRAS mutation) and H1975 (EGFR mutation and KRAS wild-type) cell lines were treated with pemetrexed and/or sorafenib in vitro. IC50 values, CI (combination index), cell cycle distribution, and phospho-p44/42MAPK were assessed for both cell lines.', 'The cytotoxic interactions between sorafenib and pemetrexed were dose dependent in EGFR TKI-resistant NSCLC cell lines. The administration of the pemetrexed-sorafenib sequence had a synergistic effect and an advantage over the sorafenib-pemetrexed sequence and concomitant administration in both cell lines. Cell cycle analysis showed that sorafenib arrested cells mainly in G1 phase while pemetrexed arrested cell mainly in S phase. Exposure to sorafenib first induced G1 arrest and subsequently prevented the cytotoxicity of S phase specific drug pemetrexed. Exposure to pemetrexed resulted in an increased phospho-p44/42MAPK level which was inhibited by subsequent exposure to sorafenib. U0126, an inhibitor of the MAPK kinase, also enhanced cytotoxicity of pemetrexed in a sequence dependent manner in TKI-resistant cell lines. Likewise, the pemetrexed-activated MAPK signaling pathway was subsequently inhibited by U0126.'] Is the inhibition of the pemetrexed-activated MAPK pathway via sorafenib involved in the synergistic mechanism of sorafenib subsequent potentiation of pemetrexed cytotoxicity in EGFR TKI-resistant cell lines?