Thymidylate synthase and dihydrofolate reductase expression in non-small cell lung carcinoma: The association with treatment efficacy of pemetrexed

Thymidylate synthase (TS) and dihydrofolate reductase (DHFR) are target enzymes of inhibition by pemetrexed, an antifolate for treatment of advanced non-small-cell lung cancer (NSCLC). This study is to evaluate the association of TS and DHFR expressions and the treatment efficacy of pemetrexed in NSCLC patients. From January 2006 to October 2008, patients with advanced NSCLC treated with pemetrexed after prior chemotherapy were included. The TS and DHFR expressions in tumor tissues were examined by immunohistochemistry and evaluated by a semiquantitative histologic score (H-score). The H-score was derived from the degrees of intensity of tumor cells multiplied by the percentage of positive neoplastic cells. The medical records were reviewed and analyzed with respect to patients' characteristics, histology types, treatment responses and survivals. Among 268 NSCLC patients treated with pemetrexed, 49 had tumor specimens available for TS and DHFR evaluation. The TS expression was positively correlated with DHFR expression (r2 = 0.11, p = 0.02). Patients with low TS (≤150) expression had a longer median progression-free survival (PFS) than those with high TS (>150) expression (4.8 vs. 3.4 months; p = 0.01). Patients with low DHFR expression (≤120) also had a longer median PFS than patients with high DHFR expression (>120), which was not statistically significant (5.8 vs. 3.6 months; p = 0.33). In patients with adenocarcinoma, the low TS patient group also had a longer median PFS and a longer median overall survival (OS) as compared with patients with high TS expression (PFS, 4.8 vs. 3.8 months, p = 0.03; OS, 21.4 vs. 10.0 months, p = 0.03). Nevertheless, the association of DHFR expression level and median PFS as well as OS were not statistically significant. TS expression, rather than DHFR, may be an important predictive factor for treatment efficacy of pemetrexed in NSCLC patients.