Kinetics of biotransformation of clonazepam to its 7-amino metabolite in the monkey

Human total clearance values and volumes of distribution of typical human cytochrome P450 2C9/19 substrates predicted by single-species allometric scaling using pharmacokinetic data sets from common marmosets genotyped for P450 2C19

Common marmosets (Callithrix jacchus) are small non-human primates that genetically lack cytochrome P450 2C9 (CYP2C9). Polymorphic marmoset CYP2C19 compensates by mediating oxidations of typical human CYP2C9/19 substrates.Twenty-four probe substrates were intravenously administered in combinations to marmosets assigned to extensive or poor metaboliser (PM) groups by CYP2C19 genotyping. Eliminations from plasma of cilomilast, phenytoin, repaglinide, tolbutamide, and S-warfarin in the CYP2C19 PM group were significantly slow; these drugs are known substrates of human CYP2C8/9/19.Human total clearance values and volumes of distribution of the 24 test compounds were extrapolated using single-species allometric scaling with experimental data from marmosets and found to be mostly comparable with the reported values.Human total clearance values and volumes of distribution of 15 of the 24 test compounds similarly extrapolated using reported data sets from cynomolgus or rhesus monkeys were comparable to the present predicted results, especially to those based on data from PM marmosets.These results suggest that single-species allometric scaling using marmosets, being small, has advantages over multiple-species-based allometry and could be applicable for pharmacokinetic predictions at the discovery stage of drug development.

Metabolite to parent drug concentration ratio as a function of parent drug extraction ratio: cases of nonportal route of administration

The ratio of metabolite to parent drug concentration (Cm/Cp) of a medium or high extraction ratio (E greater than 0.1) drug administered intravenously has been shown to depend on intrinsic clearance of drug by other metabolic routes (CLr,int) as well as on organ blood flow (Q). In contrast, for a low extraction ratio drug given intravenously or for any drug given by a portal route, this ratio is equal to the ratio of formation clearance (CLf,int) and metabolite clearance (CLm,int). The sensitivity of Cm/Cp to changes in CLr,int and CLf,int has been analyzed quantitatively. It was shown to be dependent on the fraction metabolized to that particular metabolite (fm).