Influence of infused catecholamines on the pharmacokinetics of cocaine and benzoylecgonine formation after bolus dose or continuous cocaine administration in the rat

The purpose of this study was to determine whether a catecholamine infusion administered to simulate a stress state could alter the pharmacokinetics of administered cocaine and effect the formation of benzoylecgonine, its major metabolite, in the rat. In a previous investigation we determined that catecholamine infusion enhanced the toxicity of continuous cocaine infusion by reducing the time before the onset of convulsions and respiratory arrest. We postulated that this enhanced toxicity was an effect of catecholamines on the pharmacokinetics of cocaine. To test this hypothesis we studied plasma cocaine and benzoylecgonine disposition after intravenous bolus administration of cocaine (5 mg kg^-1) to 19 male Sprague-Dawley rats and to 10 rats which received an initial loading-dose cocaine infusion of 1 mg kg^-1 min^-1 (for 5 min) followed by continuous infusion of 100 μg kg^-1 min^-1. Rats in both studies randomly received either continuous catecholamine infusion comprising adrenaline (7.25 μg mL^-1), noradrenaline (4.4 μg mL^-1) and dopamine (8.0 μg mL^-1) or saline, administered at a similar rate. Bolus dose cocaine administration, simultaneously with catecholamine infusion, resulted in significantly higher C(max) levels for cocaine (3.8 compared with 2.5 μg mL^-1) and lower distribution half-lives (3.3 compared with 5.9 min) and central compartment volumes of distribution (1.5 compared with 2.1 L kg^-1) compared with saline infusion. Benzoylecgonine formation was significantly reduced in rats receiving catecholamines whereas the elimination half-lives (26.3 compared with 25.0 min) and systemic clearances (146 compared with 146 mL kg^-1 min^-1) were not different. Continuous cocaine infusion (after an initial loading infusion) resulted in the doubling of plasma cocaine levels in rats receiving catecholamines compared with the control group. These data indicate that elevated plasma catecholamines have significant effects on cocaine pharmacokinetics. This might serve to explain the enhanced toxicity from concomitant cocaine and catecholamine infusion demonstrated in previous experiments.