Purpose: The aim of the present study was to analyze the role of thymidylate synthase (TS; main cellular target of fluorouracil [FU]) and dihydropyrimidine dehydrogenase (DPD; rate-limiting enzyme of FU catabolism) in tumoral biopsies with respect to FU responsiveness.
Patients And Methods: This prospective study was conducted on 62 head and neck cancer patients (six stage II, 16 stage III, and 40 stage IV). All received first-line chemotherapy with biomodulated FU (5-day continuous infusion). Before treatment, a tumor biopsy and control biopsy (symmetrical nontumoral area) were obtained. Cytosolic TS and DPD activities were measured using radioenzymatic assays.
Results: DPD activity was detectable in all samples, without a significant difference between tumoral (median, 60 pmol/min/mg protein; range, 13 to 193) and nontumoral samples (median, 68 pmol/min/mg protein; range, 12 to 150). Tumoral TS and tumoral DPD were not significantly influenced by tumor localization or tumor staging. Among 52 tumors assessable for clinical response, we observed 46% complete responses (CRs), 33% partial responses (PRs), and 21% no responses (NRs). No relationship was demonstrated between TS activity and response to FU therapy. The comparison of tumoral DPD between complete responders and partial or nonresponders showed a trend toward significance (P = .06). In an attempt to reduce variability, we analyzed the tumoral/nontumoral DPD activity ratio; complete responders exhibited a significantly lower normalized DPD than partial or nonresponding patients (median, 0.86, 1.18, and 1.42 for CR, PR, and NR, respectively; CR v PR plus NR, P = .03).
Conclusion: Although resistance to FU is multifactorial, the present clinical study suggests that FU catabolism in target cells is probably a determinant factor for FU responsiveness in cancer patients and justifies the clinical use of specific DPD inhibitors as FU biomodulators.
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