EVALUATION OF CAFFEINE AS A TEST DRUG FOR CYPIA2, NAT2 AND CYP2E1 PHENOTYPING IN MAN BY IN-VIVO VERSUS IN-VITRO CORRELATIONS

Caffeine is used to phenotype subjects in vivo for the cytochrome P450 isoforms CYP1A2 and CYP2E1, and for N-acetyltransferase type 2 (NAT2) , However, how much of the variation in phenotyping parameters may be attributed to variations in CYP1A2 and CYP2E1 activities has not been determined, Therefore, this study intraindividually compared enzyme ac tivities and/or content in liver samples with pharmacokinetic paramete rs of caffeine in vivo after administration of a test dose in 25 patie nts undergoing hepatectomy, Parameters measured in vitro were the high affinity components of caffeine 3-demethylation and phenacetin O-deet hylation, microsomal CYP1A2 and CYP2E1 immunoreactivity, and cytosolic sulfamethazine N-acetylation. Caffeine parameters in vivo included ca ffeine clearance from plasma and/or saliva, paraxanthine/caffeine rati os in plasma and saliva, plasma theophylline/caffeine ratio, and sever al metabolite ratios from spot urine sampled 6 h postdose, Correlation s between parameters were determined using weighted linear regression analyses, Caffeine clearance and paraxanthine/caffeine ratios correlat ed most highly to intrinsic clearance of caffeine 3-demethylation and to CYP1A2 immunoreactivity (r = 0.58-0.82), whereas urinary CYP1A2 rat ios correlated less strongly with CYP1A2 parameters in vitro, Assignme nt of acetylator phenotype by urinary NAT2 ratios was concordant with sulfamethazine N-acetylation in vitro, In contrast to CYP1A2 paramters in vitro, CYP2E1 immunoreactivity was not related to the theophylline /caffeine plasma ratio, CYP1A2 activity, thus, is the major determinan t of caffeine clearance and the paraxanthine/caffeine ratios in vivo, of which the saliva ratio 6 h postdose appears as the most advantageou s parameter, The results confirm that phenotyping using caffeine provi des valid estimates of CYP1A2 and NAT2, activity.