1-HYDROXYLATION AND 3-HYDROXYLATION, IN ADDITION TO 4-HYDROXYLATION, OF DEBRISOQUINE ARE CATALYZED BY CYTOCHROME-P450 2D6 IN HUMANS

Twenty-one healthy Swedish Caucasian volunteers, representing differen t groups with 0-13 functional cytochrome P450 (CYP) 206 genes, were gi ven a single oral dose of 20 mg of debrisoquine. The hypothesis of fur ther oxidation of the main metabolite, (S)-4-hydroxydebrisoquine, in s ubjects with multiple CYP2D6 genes was tested by screening the 0-8-hr urine samples for dihydroxylated metabolites of debrisoquine with prot onated molecular ions at m/z 208, using LC/ MS. Three peaks were detec ted in a subject with 13 functional CYP2D6 genes. One compound was ide ntified as dihydroxylated debrisoquine (presumably with hydroxylation at position 4 plus one of the positions in the aromatic ring). This me tabolite had not been previously demonstrated in humans and was detect ed only in this subject. The other two compounds, which were measurabl e in various amounts in all subjects investigated, were identified as 2-(guanidinomethyl)phenylacetic acid and 2-(guanidinoethyl)benzoic aci d. They had been previously detected in the urine of humans, dogs, and rats. They were distinguished by acid-catalyzed deuterium exchange of the hydrogens at the alpha-position, with respect to the carboxylic a cid group, of the former but not the latter acid. The acids are formed by 3- and 1-hydroxylation of debrisoquine, respectively, followed by ring opening to aldehydes, which are further oxidized to acids. Strong Spearman rank correlations between debrisoquine products of 1- or 3-h ydroxydebrisoquine and debrisoquine/4-hydroxydebrisoquine ratios (r(S) = 0.97 and r(S) = 0.96, respectively), using the intensity of the pea ks of the reconstructed ion-current chromatograms, clearly showed that both hydroxylation steps are catalyzed by CYP2D6. Because reference c ompounds for the two acids were not available, the absolute quantities could not be determined.