Clinical relevance of thymidylate synthase (TS) activity for S-1-based chemotherapy in squamous cell carcinoma of the oral cavity

S-1 is a newly developed oral fluoropyrimidine derivative that is now widely used as a chemotherapeutic agent in the treatment of oral squamous cell carcinoma (SCC). Thymidylate synthase (TS) is the rate-limiting enzyme in the de novo DNA biosynthetic pathway, and improves clinical response to chemotherapy with fluoropyrimidines. We have retrospectively evaluated the predictive value of thymidylate synthase activity in 75 patients with oral SCC with an enzyme-linked immunosorbent assay (ELISA). Mean (SD) activity (pmol/mg) in the specimens was 0.078 (0.080) (median 0.059). The median value was taken as the cut-off value based on which the patients were divided into high and low activity groups. Both the clinical and histopathological responses to chemotherapy and radiochemotherapy were higher in the group with low TS activity. The group with low TS activity also differed significantly in their clinical response to S-1-based chemotherapy (p < 0.05). However, there was no significant difference in cause-specific survival. Measurement of TS activity may aid in predicting the clinical response to chemotherapy including S-1 for oral SCC.