Relationship between Dihydropyrimidine Dehydrogenase Activity and Plasma 5-FluorouraciI Levels with Evidence for Orcadian Variation of Enzyme Activity and Plasma Drug Levels in Cancer Patients Receiving 5-Fluorouracil by Protracted Continuous Infusion1

The activity of dihydropyrimidine dehydrogenase (DPD) in peripheral blood mononudear cells and plasma concentration of 5-fluorouracil (FUra) were simultaneously determined in cancer patients receiving FUra by protracted continuous infusion (300 mg/m!/day). Blood samples were drawn every 3 h over 24-h period and the resulting DPD and FUra values analyzed for circadian periodicity. In the seven patients studied, a circadian rhythm of DPD activity was observed (P < 0.00001, Cosinor analysis) with the peak of activity at 1 a.m. (0.197 ±0.007 nmol/min/ mg) and the trough at a 1 p.m. (0.113 ±0.007 nmol/min/mg). In addition, a circadian rhythm was observed for the plasma concentrations of FUra obtained over a 24-h period (/' < 0.00001, Cosinor analysis) with peak values (27.4 ±1.3 ng/ml) occurring at 11 a.m. and trough values (5.6 ± 1.3 ng/ml) occurring at 11 p.m. The ratio of the maximum concentration of Ilia to the minimum concentration observed was almost 5-fold. This study demonstrates a circadian variation of DPD activity in human peripheral blood mononuclear cells and a circadian variation of FUra plasma levels in patients receiving FUra by protracted continuous infu sion. An inverse relationship between the circadian patterns of DPD activity and FUra plasma levels was also noted, suggesting that an association may exist between DPD activity and FUra plasma concentra tion. Further evidence of an association between DPD activity in periph eral blood mononuclear cells and plasma FUra concentration was dem onstrated by a linear relationship between the two parameters in all patients (r = —¿ 0.627) and within individual patients (—0.978< r < —¿ 0.742). With the recent advent of programmable pumps, information on the circadian pattern of FUra and/or DPD may be useful in planning continuous infusion schedules in order that optimal plasma drug concen tration may be maintained over a 24-h cycle, thereby enhancing the therapeutic efficacy of FUra administered by continuous infusion.