vitamin-k-semiquinone-radical and penciclovir

Although 7-hydroxymethotrexate is a major metabolite of methotrexate during high-dose therapy, negligible methotrexate-oxidizing activity has been found in-vitro in the liver in man. The goals of this study were to determine the role of aldehyde oxidase in the metabolism of methotrexate to 7-hydroxymethotrexate in the liver and to study the effects of inhibitors and other substrates on the metabolism of methotrexate. Methotrexate, (+/-)-methotrexate and (-)-methotrexate were incubated with partially purified aldehyde oxidase from the liver of rabbit, guinea-pig and man and the products analysed by HPLC. Rabbit liver aldehyde oxidase was used for purposes of comparison. In-vitro aldehyde oxidase from the liver of man catalyses the oxidation of methotrexate to 7-hydroxymethotrexate, but the turnover is low. However, formation of 7-hydroxy-methotrexate from all forms of methotrexate by the liver in guinea-pig and man was significantly inhibited in the presence of 100 microM menadione and chlorpromazine, potent inhibitors of aldehyde oxidase. Allopurinol (100 microM) had a negligible inhibitory effect on liver aldehyde oxidase from guinea-pig and man. Allopurinol is a xanthine oxidase inhibitor. The production of 7-hydroxymethotrexate was enhanced in the presence of allopurinol. Although aldehyde oxidase is also responsible for some of this conversion, it is also possible that the closely related xanthine oxidase is responsible for the formation of 7-hydroxymethotrexate. By employing potent selective inhibitors of aldehyde oxidase, menadione and chlorpromazine, we have demonstrated for the first time that liver aldehyde oxidase from man is minimally involved in methotrexate oxidation.

Topics: Acyclovir; Aldehyde Oxidase; Aldehyde Oxidoreductases; Allopurinol; Animals; Chlorpromazine; Enzyme Inhibitors; Guanine; Guinea Pigs; Humans; Kinetics; Liver; Methotrexate; Oxidation-Reduction; Rabbits; Species Specificity; Stereoisomerism; Substrate Specificity; Vitamin K

Famciclovir is the diacetyl 6-deoxy derivative of the active antiviral penciclovir that is for use in the treatment of infections caused by the herpes family of viruses. The major pathway of conversion is via di-deacetylation to BRL 42359, followed by oxidation to penciclovir. On oral dosing of famciclovir to humans, only penciclovir and BRL 42359 can be detected consistently in the plasma; thus, attention was focused on the oxidation reaction. This 6-oxidation occurred rapidly in human liver cytosol, had no requirement for cofactors, and followed simple Michaelis-Menten kinetics with a KM of 115 microM +/- 23 (N = 3). Using inhibitors of xanthine oxidase (allopurinol) and aldehyde oxidase (menadione and isovanillin), the relative roles of these enzymes in this process were determined. At a concentration of BRL 42359 that reflected plasma concentrations observed in humans (4 microM), both menadione (IC50 7 microM) and isovanillin (IC50 15 microM) caused extensive inhibition of the 6-oxidation reaction. In contrast, allopurinol caused no significant inhibition, confirming earlier in vivo work. At higher substrate concentrations (50 and 200 microM), the results with these inhibitors were broadly similar. These results provide strong evidence that aldehyde oxidase and not xanthine oxidase is responsible for the 6-oxidation of BRL 42359 to penciclovir in human liver cytosol, and this is likely to reflect the in vivo situation.

Topics: 2-Aminopurine; Acyclovir; Aldehyde Oxidase; Aldehyde Oxidoreductases; Allopurinol; Antiviral Agents; Benzaldehydes; Cytosol; Famciclovir; Guanine; Humans; Kinetics; Liver; Oxidation-Reduction; Prodrugs; Vitamin K; Xanthine Oxidase