We examined HPA axis response to 50 mg oral naltrexone compared with placebo in 17 healthy male and female nonalcoholic subjects, approximately half of whom had a positive family history of alcoholism (FH ϩ ) and half of whom who did not (FH Ϫ)The endogenous opioid system plays a role in the modulation of the hypothalamic-pituitary-adrenal (HPA) axis (Cushman and Kreek 1974;Johnson et al. 1992;Kreek 1972Kreek , 1973Kreek , 1978. Opioid antagonist administration blocks the tonic opioid inhibition of HPA axis activity, thereby resulting in release of POMC-derived hormones in the pituitary and cortisol from the adrenal gland. Indeed, numerous studies have demonstrated acute increases in adrenocorticotropin (ACTH) and cortisol levels in man after intravenous infusion of the opioid antagonists naloxone (Cohen et al. 1983;Conaglen et al. 1985;Delitala et al. 1994;Naber et al. 1981;Martin del Campo et al. 1994;Morley et al. 1980;Schluger et al. 1998;Volavka et al. 1979a) and nalmefene . However, few studies have examined acute neuroendocrine response to the opioid antagonist naltrexone, which, in contrast to naloxone and nalmefene, is not available for intravenous administration.The two published studies on the acute HPA axis effects of orally administered naltrexone have both been conducted with relatively small sample sizes (n р 10). NO . 6 Naltrexone, HPA Axis, and Biotransformation 779The first study (Volavka et al. 1979b), conducted in ten normal male subjects, showed that oral naltrexone significantly increased cortisol levels two hours after administration and directionally (but nonsignificantly) increased ACTH levels one hour after administration. However, due to the small sample studied, the power may not have been sufficient to detect differences. The results may also have been confounded by the co-occurrence of pain sensitivity testing (i.e., 2-min cold pressor task) during the protocol, which may have produced independent HPA axis responses. A more recent study in six abstinent alcoholic patients (Farren et al. 1999) showed significant increases in cortisol and ACTH to three doses of oral naltrexone (25, 50, and 100mg) with no dose-dependent effects (Farren et al. 1999). It has been hypothesized that alterations in HPA axis function may play a role in various stages of addiction, including initiation, maintenance, and relapse (Kreek 1992). Naltrexone (50 mg oral) has shown efficacy in the treatment of alcohol dependence (Volpicelli et al. 1992(Volpicelli et al. , 1997O'Malley et al. 1992;Anton et al. 1999), which led to its FDA-approval for adjunctive treatment of alcoholism. Studies examining HPA axis response after oral naltrexone administration might be of potential clinical relevance as neuroendocrine changes and sensitivity to naltrexone could relate to individual differences and treatment response. Hypothetically, the effects of an opioid antagonist, such as naltrexone, in modulating tonic inhibition of the opioid system at various receptors, including primarily , but also ␦ and , may relate to tr...

Among other actions, opioid antagonists modulate the control endogenous opioids exert on the hypothalamic-pituitary-adrenal (HPA) axis. Naloxone, nalmefene, and naltrexone are the opioid antagonists approved for use in man and are primarily mu-opioid selective. Naltrexone and nalmefene have been demonstrated to be useful in the treatment of alcoholism. Compared with naloxone, nalmefene has a longer half-life, is more potent at the mu-receptor, and has a higher affinity for kappa- and delta-opioid receptors. We conducted an inpatient study comparing the effects of 10 and 30 mg doses of intravenous naloxone and nalmefene in normal, nonsubstance nor alcohol-abusing, volunteers. Significant increases in ACTH and cortisol were observed after both antagonists, without an apparent dose-response relationship; however, both doses of nalmefene resulted in greater HPA axis activation than either dose of naloxone (ACTH: p <0.005). These results indicate that kappa- and delta-opioids may play important roles in the regulation of the HPA axis; nalmefene may be useful as both a probe to explore the HPA axis physiology and as a pharmacotherapeutic agent.

A comprehensive analysis on the prediction of human clearance based on intravenous pharmacokinetic data from rat, dog, and monkey for approximately 400 compounds was undertaken. This data set has been carefully compiled from literature reports and expanded with some in-house determinations for plasma protein binding and rat clearance. To the authors- knowledge, this is the largest publicly available data set. The present examination offers a comparison of 37 different methods for prediction of human clearance across compounds of diverse physicochemical properties. Furthermore, this work demonstrates the application of each prediction method to each charge class of the compounds, thus presenting an additional dimension to prediction of human pharmacokinetics. In general, the observations suggest that methods employing monkey clearance values and a method incorporating differences in plasma protein binding between rat and human yield the best overall predictions as suggested by approximately 60% compounds within 2-fold geometric mean-fold error. Other single-species scaling or proportionality methods incorporating the fraction unbound in the corresponding preclinical species for prediction of free clearance in human were generally unsuccessful.

The authors present a comprehensive analysis on the estimation of volume of distribution at steady state (VD(ss) ) in human based on rat, dog, and monkey data on nearly 400 compounds for which there are also associated human data. This data set, to the authors- knowledge, is the largest publicly available, has been carefully compiled from literature reports, and was expanded with some in-house determinations such as plasma protein binding data. This work offers a good statistical basis for the evaluation of applicable prediction methods, their accuracy, and some methods-dependent diagnostic tools. The authors also grouped the compounds according to their charge classes and show the applicability of each method considered to each class, offering further insight into the probability of a successful prediction. Furthermore, they found that the use of fraction unbound in plasma, to obtain unbound volume of distribution, is generally detrimental to accuracy of several methods, and they discuss possible reasons. Overall, the approach using dog and monkey data in the íie-Tozer equation offers the highest probability of success, with an intrinsic diagnostic tool based on aberrant values (<0 or >1) for the calculated fraction unbound in tissue. Alternatively, methods based on dog data (single-species scaling) and rat and dog data (íie-Tozer equation with 2 species or multiple regression methods) may be considered reasonable approaches while not requiring data in nonhuman primates.

When administered orally, naltrexone undergoes extensive biotransformation and is metabolized to 6 beta-naltrexol and other minor metabolites. Naltrexone has been recently approved by the Food and Drug Administration for the treatment of alcohol dependence. An important clinical issue with naltrexone treatment is predicting patient compliance, which may be influenced by adverse side effects experienced during the medication. We investigated whether subjective side effects were related to urinary concentrations of naltrexone and its metabolite 6 beta-naltrexol 3 hr after administration of 50 mg po naltrexone in 24 male moderate-to-heavy social drinkers. The results showed significantly higher levels of urinary 6 beta-naltrexol (p < 0.05) in those subjects who experienced one or more side effect (i.e., headache, nausea, anxiety, or erection). Urinary naltrexone levels did not differ between the groups. Results also showed an approximate 10:1 ratio of 6 beta-naltrexol to naltrexone levels and a significant positive correlation between the parent compound and metabolite, suggesting parallel renal clearance. The results of this study suggest a possible mechanism for the side effects observed after acute administration of naltrexone.