Adenocarcinomas 5-Fluorouracil-sensitive and -resistant Murine Colon Kinetics of Thymidylate Synthetase Inhibition in In Vivo

The predictive utility of several biochemical parameters of 5fluorouracil (5-FUra) action was evaluated in four murine coIonic adenocarcinomas: 5-FUra-sensitive Tumor 38 and 5FUra-resistant Tumors 07/A, 51, and 06/A. Thymidylate synthetase (TS) was determined by a tritiated 5-fluoro-2'-deoxyuridylate (FdUMP)-binding assay. Bolus 5-FUra (80 mg/kg, i.p.) administration caused in all tumors a rapid decrease in free TS levels. Only Tumor 38, however, showed inhibition of TS to undetectable (<0.05 pmol/g) levels, which lasted up to 6 hr after treatment; correction for dissociation of endogenous TS: FdUMP:folate ternary complex during the TS assay was re quired. Total TS (free enzyme plus ternary complex) was de termined with experimental conditions that achieved quantita tive recovery of free TS from ternary complex. By 48 hr after 5-FUra, Tumor 38 showed a decrease in total TS proportional to the estimated log kill/dose of 5-FUra; in contrast, the resist ant tumors showed no such decrease from pretreatment levels. Assay of FdUMP showed that the free nucleotide was formed rapidly in all tumors in excess over available TS-binding sites. However, tumor sensitivity did not correlate with peak or resid ual FdUMP levels or with deoxyuridylate levels, which were low and remained so in all tumors. Tumor sensitivity to 5-FUra also could not be explained by the small differences among the tumors in total perchloric acid-soluble metabolites of 5-FUra or drug incorporation into RNA. We conclude from these data that levels of free TS in the tumor after 5-FUra treatment are predictive of chemotherapeutic response in these murine models of human colonie adenocarcinoma.