Effects of PR-350, a newly developed radiosensitizer, on dihydropyrimidine dehydrogenase activity and 5-fluorouracil pharmacokinetics.

This study was designed to investigate the effects of PR-350, a newly developed radiosensitizer, on dihydropyrimidine dehydrogenase (DPD) activity and 5-fluorouracil (5-FU) pharmacokinetics in 8-week-old male Sprague-Dawley rats. In an in vitro study with hepatic cytosol, DPD activity was dose-dependently reduced by PR-350 at 0.5, 1.0, and 2.0 mmol/l to 75.5%, 64.9%, and 61.5%, respectively, of the control values. In an ex vivo study, DPD activities in hepatic cytosols obtained from animals which had received PR-350 over 4 days (200 mg/kg per day) were not significantly different from those in animals which had not. In an in vivo study, none of the pharmacokinetic parameters obtained from the plasma concentration-time profile of 5-FU were significantly altered by single i.v. injections of PR-350 (50, 100, or 200 mg/kg). However, (E)-5-(2)-(bromovinyl)uracil (BVU), a DPD inhibitor, significantly increased the half-life and area under the curve of 5-FU to 238.1% and 323.2%, respectively, of the control values. Administration of PR-350 over 4 days (200 mg/kg per day) did not affect either of these parameters. The administration of PR-350 significantly reduced the clearance (73.5% of control) and volume of distribution (71.0% of control) of 5-FU, but the alterations were much less than those caused by BVU. These results suggest that the effect of PR-350 on 5-FU pharmacokinetics is much less than that of BVU and that the enhancement of 5-FU toxicity by PR-350 is less than we initially anticipated.