Thymidylate synthase (TS) is an important target for cancer chemotherapy. However,several mechanisms of resistance to TS inhibitors have been described. One mechanism that may be relevant to short-term exposure to TS inhibitors occurs as a result of disruption of the autoregulatory loop,which allows TS to control its own translation. This disruption leads to up-regulation of TS protein and is generally thought to decrease efficacy. This study has investigated TS protein up-regulation using a range of TS inhibitors in both tumor and nonmalignant cell lines in vitro and in vivo.

Up-regulation of TS protein showed a time-, dose-, and cell-type-specific response to treatment with ZD9331. This response was observed in W1L2 cells treated for 24 h at equitoxic doses of raltitrexed (6-fold), ZD9331 (10-fold), fluorouracil (5-fold), LY231514(7-fold), AG337 (7-fold), and BW1843U89 (3-fold). Up-regulation was observed over a range of doses. Elevation of TS protein only persisted up to 12 h after removal of drug. The extent of induction does not depend on basal TS levels. Nontransformed human fibroblasts showed significantly greater up-regulation of TS protein than tumor cells exposed to an equitoxic dose of ZD9331. In vivoexperiments using the L5178Y thymidine kinase −/− mouse lymphoma implanted into DBA2 mice also showed greater up-regulation of TS protein in normal intestinal epithelial cells compared with tumor cells.

These results confirm that TS up-regulation is a common feature of TS inhibition in tumor cells and that it may occur to a greater extent in normal tissues, although the clinical implications of these findings remain to be determined.