[A case of lung cancer resistant to pemetrexed responding to S-1 monotherapy].

Pemetrexed(PEM)and S-1 are both cytotoxic anti-tumor drugs that target thymidylate synthase(TS). Here, we report a case of lung cancer resistant to PEM, but responsive to S-1 monotherapy. A 50-year-old male with lung cancer(adenocarcinoma, EGFR mutation negative)at Stage III B(T4N2M0)received PEM as third-line chemotherapy. However, metastatic mediastinal lymph nodes were increased in size. A switch to gemcitabine(GEM)resulted in further enlargement of the nodes and elevation of the serum CEA level. Introduction of S-1 monotherapy(120mg/body/day divided into two doses, 4-week administration every 6 weeks, 6 courses)reduced the size of the nodes and serum CEA was normalized. Although S-1 as well as PEM, target TS, the mechanisms in these two drugs that lead to resistance are not the same. Furthermore, S-1 induces dysfunction of RNA through the generation of fluoro-uridine monophosphate(FUMP). We conclude that S-1 can be a drug of choice, even in cases that show resistance to PEM.