Effect of Pyrimidine Nucleosides on Body Temperatures of Man and Rabbit in Relation to Pharmacokinetic Data

The effect of high-dose uridine on body temperatures of rabbits and man has been studied in relation to plasma concentrations of uridine and its catabolite uracil. Uridine induced fever in both rabbits and man. High-dose cytidine had no influence on body temperature in rabbits. Plasma concentrations of uridine were between 1 and 1.5 m M at 30 min after an iv bolus injection of 400 mg uridine/kg in rabbits and reached peak levels of 2 m M after a 1-hr infusion of 12 g uridine/m² in man. The plasma concentration of cytidine in rabbits was about 0.5 m M and that of uridine was 0.30 m M at 30 min after an iv bolus injection of 400 mg cytidine/kg. The mean residence time for uridine in patients and rabbits varied between 80 and 195 min. The area under the plasma concentration–time curve (AUC) for uridine in rabbits was 2.0 mmol · hr/liter, and that for cytidine was 0.6 mmol · hr/liter. A large AUC for uridine indicates a prolonged exposure of tissues to uridine, which might lead to extensive formation of degradation products. The administration of some of these catabolites, dihydrouracil (at 20–40 mg/kg), carbamyl-β-alanine (at 60 mg/kg), and β-alanine (at 300–400 mg/kg), resulted in a significant increase in body temperature. It is concluded that the change in body temperature associated with uridine administration was not due to bacterial pyrogens but that one of the degradation products might be involved in thermoregulation.