The effects of inhibition of plasma cholinesterase and hepatic microsomal enzyme activity on cocaine, benzoylecgonine, ecgonine methyl ester, and norcocaine blood levels in pigs

We measured the blood levels of cocaine and its three major metabolites, benzoylecgonine, ecgonine methyl ester, and norcocaine, in three groups of male pigs weighing about 26 kg (25.75 ± 0.25 kg) to determine the effects of inhibition of plasma cholinesterase and hepatic microsomal enzyme activity on cocaine metabolism. In addition, systemic elimination half-life, volume of distribution, and clearance of cocaine were calculated for the three groups. Group 1 pigs (n = 4) were pretreated with normal saline solution, group 2 pigs (n = 4) were pretreated with tetraisopropyl pyrophosphoramide, a specific plasma cholinesterase inhibitor, and group 3 pigs (n = 4) were pretreated with cimetidine, a hepatic microsomal enzyme inhibitor, all administered intramuscularly. Pigs were anesthetized with intravenous sodium thiopental; a carotid arterial cannula and an external jugular catheter were then inserted for the administration of cocaine and for blood sampling. Forty-five minutes later, when pigs were again completely awake, cocaine 3 mg/kg was given intravenously. Arterial blood samples were collected for the analysis of cocaine and cocaine metabolite levels just before and at 5, 10, 15, 30, 45, 60, 120, 180, and 1440 minutes after the administration of cocaine. Cocaine and cocaine metabolite blood levels were analyzed with high-pressure liquid chromatography methods and plasma cholinesterase activity was measured with a colorimetric method. The blood levels of cocaine and cocaine metabolites were significantly different among the three groups (p < 0.05, analysis of variance). Statistically significant differences in half-life, volume of distribution and clearance were also seen among the three groups. There was a significant decrease in ecgonine methyl ester and a significant increase in benzoylecgonine blood levels in group 2 compared with those in groups 1 and 3 (p < 0.05). We conclude from these experiments that inhibition of plasma cholinesterase activity accelerated the metabolism of cocaine by an alternative route, resulting in significant alterations in the blood levels of benzoylecgonine and ecgonine methyl ester.