In vivo and in vitro studies on cocaine metabolism: ecgonine methyl ester as a major metabolite of cocaine.

The levels of cocaine, benzoylecgonine and ecgonine methyl ester were measured periodically in the urine of dogs and rabbits by gas chromatography-chemical ionization selected ion monitoring mass spectrometry after subcutaneous injection of 2.9 mumol/kg (1.0 mg/kg) cocaine--HCl. Ecgonine methyl ester persisted much longer than benzoylecgonine. Urinary ecgonine methyl ester could be identified for 72 h in all the experimental dogs. The ester accounted for 6.6-27.1% of a dose of cocaine in dogs and 8.8-31.9% in rabbits 24 h after administration. Excretion of benzoylecgonine in urine was 6.3-19.2% in dogs and 7.5-32.9% in rabbits. This confirms that ecgonine methyl ester is one of the major metabolites of cocaine as well as benzoylecgonine. Excretion of the parent drug and its two hydrolyzed metabolites in faeces was very small, less than 1% of administered dose. Tri-o-tolylphosphate and eserine significantly inhibited cocaine hydrolysis to benzoylecgonine and ecgonine methyl ester, respectively. Parathion and EDTA, however, had no effects on cocaine hydrolysis in vivo. In vitro demethylation of cocaine to benzoylecgonine was demonstrated in the plasma from dogs and this production of benzoylecgonine was much more than that of non-enzymatic formation in buffer at physiological pH. It was concluded that a large part of the benzoylecgonine excreted in urine is an in vivo enzymatic product of cocaine. On the other hand, plasma cholinesterase and liver esterase mediated ecgonine methyl ester formation. This liver esterase was abundant in the soluble fraction and could be found in both of microsomal and mitochondrial fractions.