The concentrations of amitriptyline (AMT) and its demethylated metabolite nortriptyline (NRT) in the serum and in specific brain regions were determined periodically after acute or chronic administration of 20 mg/kg of AMT in rats. Both AMT and NRT declined from the serum in a biexponential manner and were eliminated monoexponentially from the brain regions, with no significant difference in elimination among the eight brain regions examined. In the brain, both AMT and NRT were unevenly distributed after chronic administration, whereas an even distribution was observed after acute administration. The AUCbrain:AUCserum ratio of AMT was higher than that of NRT, indicating greater transport of AMT into the brain regions. The AUCAMT value in the serum increased 1.6 times after chronic administration, whereas no significant changes were observed in the brain regions. The AUCNRT values increased 9.0 times in the serum and 6.8 times in the brain, with the increase in the serum being greater. These results suggest inhibited distribution of the drugs into the tissues, including the brain regions, and enhanced metabolism of AMT.
Aldehyde oxidase (AO) plays an important role in metabolizing many drugs, so AO activity in individual patients may be a useful parameter for dose adjustment to avoid severe toxicity. In this study, we investigated the developmental changes of AO activity in 101 children. Urine was collected in the morning, and AO activity was assessed in terms of the ratio of pyridone formation from N(1)-methylnicotinamide, an AO substrate. Significant correlations were found between AO activity and various growth indices (age, body weight, body surface area, and liver volume). Age showed the moderate correlation (r(2)=0.506). AO activity rapidly increased with increase of the subjects' age up to about 1 year. These findings suggest that the AO activity begins to increase soon after birth. Because AO activity is immature in children below 1 year of age, dose adjustment based on individual AO activity should be made for such patients.
Aldehyde oxidase (AO) plays a role in metabolizing many drugs, such as methotrexate and 6-mercaptopurine. We previously showed that AO activity in rat liver rapidly increases from birth, reaching a plateau within 4 weeks, and is regulated at the protein expression level. However, developmental changes of AO activity and protein expression in human liver have not been reported. Here, we investigated the developmental changes and variability of AO in 16 human livers (13 children ranging from 13 days to 12 years old and 3 adults, 17, 34 and 45 years old). Young children (13 days to 4 months after birth) showed little liver AO activity, evaluated in terms of the activities for oxidation of N-1-methylnicotinamide to N-1-methyl-2-pyridone-5-carboxamide and N-1-methyl-4-pyridone-3-carboxamide in liver cytosol. However, these oxidase activities were markedly increased after 4 months, reaching the adult level by about 2 years of age. The AO band density in immunoblotting analysis was well correlated with the AO activity among all subjects (p < 0.01, r(2) = 0.771). Therefore, AO activity in the liver of young children is regulated at the AO protein expression level. Thus, as in rats, the AO activity in humans rapidly increases soon after birth, and is regulated at the protein expression level.
Aldehyde oxidase (AO) plays an important role in metabolizing antitumor and antiviral drugs, including methotrexate, cyclophosphamide and acyclovir. Green tea and its catechins have been shown to modulate the activities of various xenobiotic-metabolizing cytochrome P450 species, both in vivo and in vitro, but their effect on AO has not been studied. Therefore, we evaluated the effect of tea beverages on AO activity in rat and human liver cytosol. We also investigated the influence of several catechins on AO activity in rat liver cytosol. AO activity was evaluated in terms of oxidation of N-1-methylnicotinamide to N-1-methyl-2-pyridone-5-carboxamide and N-1-methyl-4-pyridone-3-carboxamide. Bottled green tea beverages at 10% (vol/vol) inhibited AO activity by 90.0-93.5%, while at 1.0% (vol/vol), they reduced AO activity by 73.9-90.0%. At 0.1% (vol/vol), green tea II and III, which have high contents of catechins and their derivatives, inhibited AO activity by 24.3% and 38.8%, respectively. Bottled mineral water had no effect. AO activity was inhibited potently by epicatechin and epicatechin gallate. These results indicate that the AO-inhibitory activity of tea beverages is predominantly due to catechins and their derivatives. Thus, consumption of tea beverages may cause a decrease of AO activity, which may result in reduced clearance of drugs that are AO substrates.
The effects of zonisamide (ZNS) on the pharmacokinetics of phenobarbital (PB), valproic acid (VPA), carbamazepine (CBZ) and phenytoin (PHT) were investigated in rats. Additionally, the influence of ZNS on the serum protein binding, erythrocyte distribution and metabolism of these antiepileptics was studied in vitro. The t 1/2 and AUC values of PB were significantly increased by ZNS coadministration, and a significant decrease in the Vd/F value of PHT was observed after multiple dosing of ZNS. By contrast, ZNS showed no significant effect on VPA and CBZ kinetics. No significant effect of ZNS was observed in the serum protein binding, erythrocyte distribution or metabolism of other antiepileptics. These results suggest that ZNS has little effect on the pharmacokinetic behaviors of other antiepileptic drugs.
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