Cyclazocine: comparison to hydromorphone and interaction with cocaine

Kappa-opioid agonists produce neurobiological and behavioral effects opposite to those of cocaine and may be useful for the treatment of cocaine dependence. To evaluate the kappa- and mu-agonist effects of cyclazocine and to test whether cyclazocine pretreatment would attenuate the effects of cocaine, healthy, male and female, experienced opiate and cocaine users (n = 13) were enrolled in a two-phase study. In Phase 1, placebo, cyclazocine (0.2, 0.4 and 0.8 mg) and the mu-agonist hydromorphone (5 and 15 mg) were administered orally in six 4.5-hour sessions separated by at least 72 h. In Phase 2, cocaine (100 mg intranasal) was given 2 h after oral pretreatment with cyclazocine (0, 0.1, 0.2, 0.4, 0.8 and 0 mg, in that order) in each of six sessions conducted daily Monday to Friday and the following Monday. Physiological, subjective and behavioral measures were collected in each session. Nine participants completed Phase 1; eight completed Phase 2. Hydromorphone (15 mg) produced prototypic mu-agonist effects. Cyclazocine exhibited only modest kappa-like effects. Cyclazocine also had only modest, non-dose-related effects on response to cocaine. However, cocaine effects were consistently lower on the last administration (cyclazocine 0 mg pretreatment) following 4 days of cyclazocine pretreatment, compared to the first administration (0 mg pretreatment). This finding is unlikely to be fully attributable to cocaine tolerance and is not accounted for by pharmacokinetic changes; plasma concentrations of cocaine were not altered by cyclazocine. This study is suggestive but not strongly supportive for the use of kappa-opiate drugs to diminish acute effects of cocaine administration or for the use of these kappa agonists in drug abuse treatment applications.