Characterization of the properties of cocaine in blood: Blood clearance, blood to plasma ratio, and plasma protein binding

As part of a study to examine cocaine disposition and interaction with ethanol, it was necessary to characterize various properties of cocaine in the blood of the experimental animal. All studies were conducted using blood from healthy adult male Sprague-Dawley rats. Cocaine was incubated in whole blood at 37 °C at concentrations of 500-4000 ng/mL. The apparent first-order rate constant for cocaine loss was independent of concentration. Blood clearance, calculated assuming blood volume to be 64 mL/kg, was estimated to be 0.056 ± 0.003 mL/(min·kg); a value considerably smaller than estimates of systemic clearance. The addition of NaF increased the rate of loss to form benzoylecgonine, as a result of increased chemical degradation and as a consequence of increased pH (to pH 8.0 over 30 h). This NaF-enhanced degradation was abolished when NaF was added to blood buffered to pH 7.4. Ethanol had no influence on cocaine degradation, and there was no evidence of cocaethylene (ethylcocaine) formation. Blood to plasma ratios determined in spiked and authentic samples were constant (0.94-1.05 and 0.99-1.03, respectively) and independent of concentration (100-1500 ng/mL) and pH (7.2-7.6). This ratio was not influenced by NaF or ethanol. The unbound fraction (f(u)) of cocaine determined in spiked plasma varied from 0.62 to 0.63 over the concentration range (75-2025 ng/mL). Ethanol had no effect on binding. The values for f(u) determined from authentic blood samples taken from rats dosed intravenously with cocaine (10 mg/kg) ranged from 0.67 to 0.69 (over the concentration range 300- 1500 ng/mL). Cocaine plasma protein binding was independent of concentration but depended upon plasma pH (f(u), 0.765 and 0.486, at pHs 7.0 and 7.8, respectively).