INTERACTION BETWEEN 2 PROBES USED FOR PHENOTYPING CYTOCHROMES P4501A2(CAFFEINE) AND P4502E1 (CHLORZOXAZONE) IN HUMANS

The first steps in the metabolism of caffeine and chlorzoxazone are pr imarily catalysed by CYP1A2 and CYP2E1, respectively, Accordingly, the se compounds have been developed as metabolic probes for non-invasive phenotyping of these two P450s. Their specificities, however, have bee n shown to overlap. In this study, 140 mg of caffeine and 500 mg of ch lorzoxazone were administered alone or together in 16 healthy subjects under standardized conditions, The metabolites of these two probes we re measured in the blood and also in the urine for caffeine, CYP1A2, a ctivity was determined either by the paraxanthine/caffeine ratio in th e blood or by the usual caffeine metabolic ratio in the urine, The CYP 2E1 activity was determined by the 6-OH-chlorzoxazone/chlorzoxazone ra tio in blood. CYP1A2 activities measured in blood and urine were highl y significantly correlated CYP2E1 activity was not modified when chlor zoxazone was given together with caffeine, In contrast, an inhibition of CYP1A2 by chlorzoxazone was demonstrated by a 16% decrease in the c affeine metabolic ratio in urine when both caffeine and chlorzoxazone were given together, Under the same conditions, the paraxanthine/caffe ine ratio in plasma also decreased by about 20%, These results were co nfirmed in vitro by the incubation of 1 mM caffeine with human hepatic liver microsomes in the presence of 0.4 mM chlorzoxazone. The overall metabolism of caffeine decreased by 38% compared to controls incubate d without chlorzoxazone. As all three N-demethylations of caffeine wer e inhibited by chlorzoxazone with an apparent K-i of 0.18, 0.3 and 0.5 mM for N-3-, N-1- and, N-7-demethylations respectively, it is suggest ed that chlorzoxazone is metabolized by CYP1A2 and, as a result, is a competitive inhibitor of caffeine.