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Theophylline is widely used in the treatment of patients with obstructive airway diseases. It is well known that theophylline has a narrow therapeutic range being a plasma concentration of 10-20 mg/ml 1,2) and is extensively metabolized in humans. 3,4) Many articles have been published on the drug interactions of theophylline with other coadministered drugs mediated by cytochrome P450 (CYP) 1A2, which is the major metabolizing enzyme for theophylline. [5][6][7][8][9][10][11][12] For example, it is well known that histamine H 2 -antagonist cimetidine and quinolone antimicrobial agent enoxacin cause the risk of the development of serious side effects by inhibiting CYP1A2-mediated theophylline metabolism in the liver. 7,9,13,14) The antivirus agent aciclovir, which has potent inhibitory activity against herpes simplex virus and varicella zoster virus, [15][16][17] is known to be mainly eliminated from the kidney. 18,19) Drug interaction between aciclovir and concomitantly administered drugs, such as cimetidine, probenecid and mycophenolate that are primarily excreted into the urine has been reported. [20][21][22] However, there is an interesting clinical report indicating that total body clearance of theophylline is decreased by multiple oral administrations of aciclovir (800 mg five times daily for two consecutive days) in healthy volunteers, 23) although theophylline is extensively metabolized in the liver. They proposed that the mechanism responsible for the decreases in theophylline clearance by coadministration of aciclovir might be due to inhibition of CYP1A2-mediated theophylline metabolism. However, the effect of aciclovir on the activity and expression of CYP1A2 remains unclear.We have previously reported that rats are a useful animal model for predicting drug interaction between theophylline and other drugs in humans. [24][25][26][27][28][29] The aim of the present study was to investigate the effect of aciclovir on the pharmacokinetics and metabolism of theophylline, and on the activity and expression of hepatic CYP1A2 in rats. To further clarify whether aciclovir has an inhibitory effect against hepatic CYP1A2, we also investigated the effect of aciclovir on the pharmacokinetics of the xanthine derivative 1-methyl-3-propylxanthine, which is almost completely metabolized by CYP1A2 in rats. 29,30) MATERIALS AND METHODS Chemicals Theophylline, aciclovir, resorufin and 7-ethoxyresorufin were purchased from Sigma Chemical Company (St. Louis, MO, U.S.A.). Aciclovir in the form of a commercial preparation for injection (Zovirax for IV infusion) was obtained from GlaxoSmithKline K.K. (Tokyo, Japan). Enprofylline, 1-methyl-3-propylxanthine and 1-methyl-3-buthylxanthine were synthesized in our laboratory and were identical to those previously described.29,31) Enprofylline and 1-methyl-3-buthylxanthine were used as internal standards for determination of concentrations of theophylline and 1-methyl-3-propylxanthine, respectively. Inulin was purchased from Nacalai Tesque (Kyoto, Japan). All other There is an interes...