Protein and mRNA expression of folic acid-associated enzymes as biomarkers for the cytotoxicity of the thymidylate synthase-targeted drugs, pemetrexed and S-1, in non-small cell lung cancer

The thymidylate synthase (TS)-targeted drugs, pemetrexed and S-1, exert an important role in advanced non-small cell lung cancer (NSCLC) treatment; folic acid-associated enzymes are expected to behave as biomarkers, although their role has yet to be fully elucidated. In the present study, a single-institutional prospective analysis, in which the mRNA and protein expression levels of five folic acid-associated enzymes were evaluated with surgical specimens of NSCLC, was performed. Drug sensitivity was evaluated using a collagen gel droplet-embedded culture drug sensitivity test (CD-DST) in vitro. A total of 50 patients with NSCLC were enrolled, and the mRNA and protein expression levels of five enzymes were assessed in 47 and 46 patients, respectively. A significant association was identified between mRNA and protein expression in TS (r=0.6266), but the correlation between mRNA and protein expression levels for the other four enzymes was poor. TS mRNA expression was significantly higher in poorly differentiated tumors compared with moderately differentiated tumors (P=0.0399). TS protein expression was significantly higher in patients with pleural invasion or lymphatic invasion compared with those lacking them (P=0.027 and 0.030, respectively). CD-DST revealed that none of the tumors that were sensitive to pemetrexed, but not to S-1, were well differentiated, whereas none of the tumors that were sensitive to S-1, but not to pemetrexed, were poorly differentiated. More prominent vascular invasion was observed in the tumors that were sensitive to S-1. The only factors that exhibited the potential to discriminate the cytotoxicity of pemetrexed from S-1 were tumor differentiation grade and vascular invasion.