title: Drug interaction between phenytoin and allopurinol.
abstract: The combination of phenytoin (DPH) and allopurinol is used for the treatment of a neurological disease. However, interactions between DHP and allopurinol and the mechanism are little known. The repeated dosing of allopurinol at higher doses (20 and 50 mg/kg) significantly retarded the elimination of DPH from the circulation and dramatically decreased the urinary excretion of p-hydroxyphenytoin (HPPH), a major metabolite of DPH. However, a single administration of allopurinol (10 or 50 mg/kg) did not give rise to these effects. Allopurinol did not affect the hepatic extraction of DPH and renal plasma flow rate. Allopurinol (50 mg/kg/d) dosed repeatedly could not inhibit the hepatic drug metabolizing enzyme activities. The in vitro hydroxylation of DPH was inhibited only slightly and the kinetic plots gave apparently non-competitive inhibition. The less inhibitory effect of allopurinol on the in vitro hydroxylation did not agree with the in vivo data. These results indicate that the inhibitory effect of allopurinol is not mediated by cytochrome P-450 dependent monooxygenase reactions.
authors.affiliation: Faculty of Pharmaceutical Sciences, Kinki University, Osaka, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Ogiso
authors.forename: Y
authors.initials: Y
authors.lastname: Ito
authors.forename: M
authors.initials: M
authors.lastname: Iwaki
authors.forename: K
authors.initials: K
authors.lastname: Tsunekawa
mesh_terms: D000284:Administration, Oral; D000493:Allopurinol / Q000494:pharmacology*; D000818:Animals; D004334:Drug Administration Schedule; D004347:Drug Interactions; D004359:Drug Therapy, Combination; D007668:Kidney / Q000098:blood supply; D008099:Liver / Q000201:enzymology / Q000378:metabolism; D008297:Male; D010672:Phenytoin / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology / Q000652:urine; D051381:Rats; D011919:Rats, Inbred Strains; D012039:Regional Blood Flow / Q000187:drug effects
chemical_list: C001419:hydroxyphenytoin; D010672:Phenytoin; D000493:Allopurinol