Enadoline and butorphanol: Evaluation of κ-agonists on cocaine pharmacodynamics and cocaine self-administration in humans

Preclinical studies have demonstrated that κ-opioid agonists can attenuate the neurochemical and behavioral effects of cocaine that are related to its reinforcing efficacy, suggesting that κ-agonists may serve as pharmacotherapies for cocaine dependence. This 8-week inpatient study examined the ability of enadoline, a selective and high-efficacy κ-agonist, and butorphanol, a mixed agonist with intermediate efficacy at both μ-and κ-receptors, to reduce the direct pharmacodynamic effects and self-administration of intravenous cocaine in humans (n = 8). Acute doses of intramuscular enadoline (20, 40, and 80 μg/kg), butorphanol (1.5, 3, and 6 mg/70 kg) and placebo were examined separately as pretreatments during each of three test sessions with cocaine in a constrained random order. A cocaine dose-effect session (0, 20, and 40 mg cocaine i.v., 1 h apart) examined direct pharmacodynamic interactions on subjective and physiological indices; self-administration sessions examined choice behavior for cocaine (40 mg i.v. for six trials) versus money 1) in the presence of a sample cocaine dose with money choices presented in ascending value, and 2) in the absence of a sample dose with money choices presented in descending values. Enadoline (80 μg/70 kg) significantly (p < 0.05) reduced some of the positive subjective effects of cocaine (e.g., ratings of "high"), while butorphanol failed to modify subjective responses. Both agents were safely tolerated in combination with cocaine without adverse physiological responses. Cocaine self-administration was significantly greater across all pretreatment conditions when the sample dose was given and ascending money choices were used. Enadoline and butorphanol failed to modify cocaine self-administration. These data suggest that these κ-agonists may be safely administered in the presence of cocaine but do not produce significant attenuation of cocaine's direct effects or self-administration under these acute dosing conditions.