Relation between chloroguanide bioactivation to cycloguanil and the genetically determined metabolism of mephenytoin in humans.

It has been suggested recently that the bioactivation of chloroguanide hydrochloride (proguanil) to its active antimalarial metabolite cycloguanil cosegregates with the genetically determined polymorphism of mephenytoin hydroxylation. We determined the chloroguanide to cycloguanil ratio in urine after oral administration of a single dose of 200 mg proguanil either alone or together with 100 mg racemic mephenytoin or 40 mg dextromethorphan in a randomized crossover study performed in 24 healthy subjects. The mephenytoin hydroxylation index was also determined after administration of 100 mg racemic mephenytoin either alone or together with 200 mg proguanil. Two subjects were poor metabolizers and one subject was an intermediate metabolizer of mephenytoin. These three subjects had chloroguanide to cycloguanil ratios of more than 50. The 21 subjects with the extensive metabolizer phenotype for mephenytoin hydroxylation had chloroguanide to cycloguanil ratios of less than 10. The chloroguanide to cycloguanil ratio was not significantly altered by mephenytoin or dextromethorphan coadministration. The trend toward a correlation between chloroguanide/cycloguanil ratio and log mephenytoin hydroxylation index did not reach statistical significance. Inclusion of the dextromethorphan metabolic ratio into the model did not improve the relationship. These findings confirm that the bioactivation of chloroguanide to cycloguanil cosegregates with the genetically determined activity of the CYP2C family. However, the chloroguanide to cycloguanil ratio and the mephenytoin hydroxylation index do not similarly reflect the variable activity of CYP2C.