p-Hydroxymethamphetamine enantiomer pharmacokinetics and metabolism in rats: absence of N-demethylation.

p-hydroxymethamphetamine (OHMAP) is one of the major metabolites of the widely abused drug methamphetamine (MAP). The demethylation of OHMAP to p-hydroxyamphetamine (OHAP) has been shown in vitro but has never been reported in vivo. The disposition kinetics as well as the metabolism of OHMAP was investigated employing a sensitive HPLC method which can separate the enantiomers of OHMAP and OHAP. Both conjugated and unconjugated forms of these compounds can be quantitated. Male Sprague-Dawley rats were given an iv bolus of racemic OHMAP (20 mg kg-1) and serum and urine samples were collected at selected times. The serum concentration-time data for OHMAP enantiomers could be described by a biexponential equation. The clearance of D-OHMAP (93.5 mL min-1 kg-1) was slightly, but statistically significantly, greater than that of the L-enantiomer (83.9 mL min-1 kg-1). The steady-state volumes of distribution of L- and D-OHMAP were (mean +/- SD) 3.15 +/- 0.84 and 4.23 +/- 1.76 L kg-1, respectively. No significant concentrations or amounts of OHAP enantiomers could be detected in any serum or urine sample. Rats excreted more unchanged L-OHMAP (34%) than D-OHMAP (29%). In contrast, more conjugated D-OHMAP (57%) was recovered compared to the conjugated L-OHMAP (52%). The results suggest that there is slight stereoselectivity in the disposition of OHMAP enantiomers. The N-demethylation product (OHAP) was not produced in vivo.