In vitro transesterification of cocaethylene (ethylcocaine) in the presence of ethanol. esterase-mediated ethyl ester exchange esterase-mediated ethyl ester exchange.

This study reports that cocaethylene undergoes an esterase-mediated ethyl ester exchange with ethanol, resulting in an increase in the apparent in vitro t1/2, compared with control conditions. Homogenized liver from male Sprague Dawley rats in pH 7.4 phosphate buffer was centrifuged at 9000g, and the resulting supernatant (S9) fraction was collected. Tubes containing the rat S9 fraction and 50 microM cocaethylene plus aqueous buffer (control), 50 mM ethanol, or 51. 3 mM 2H6-ethanol were incubated at 37 degrees C for 4 hr. Samples were collected from the incubation tubes at various times, extracted with a solid-phase extraction system, and assayed for cocaethylene and 2H5-cocaethylene by GC/MS. Concentration-time profiles were constructed and kinetic parameters were determined. The experiment was repeated in the presence of specific and nonspecific esterase inhibitors. Enzyme kinetic parameters were also determined. Cocaethylene underwent ethyl ester exchange, being converted to 2H5-cocaethylene in the presence of 2H6-ethanol. The average apparent in vitro t1/2 value for cocaethylene (13.0 +/- 1.4 min) incubated with the S9 fraction and buffer only was increased approximately 5-fold (67.8 +/- 0.3 min) in the presence of ethanol. Formation of 2H5-cocaethylene was totally blocked with the addition of bis-(p-nitrophenyl)phosphate but was unaffected by physostigmine. The intrinsic metabolite formation clearance of 2H5-cocaethylene from cocaethylene and 2H6-ethanol (1.92 +/- 0.03 microl/min.mg protein) was several times greater than the corresponding value for cocaethylene formation from cocaine and ethanol (0.94 +/- 0.01 microl/min.mg protein) or 2H6-ethanol (0.87 +/- 0.04 microl/min.mg protein).