Oral naltrexone, a nonselective opioid antagonist, is approved for the treatment of alcohol and opioid dependence. However, the efficacy of oral naltrexone is limited by poor patient compliance. To overcome this limitation, attempts have been made to develop an injectable extended-release formulation of naltrexone, including encapsulation into biodegradable polymer microspheres (e.g. Medisorb® Naltrexone, Vivitrex® (naltrexone long acting injection)). In 1980, NIDA established development goals that they considered optimal for an extended-release formulation. At Alkermes, different formulations were tested with in vitro assays and in vivo models to select a lead formulation. Pharmacokinetic studies in rats confirmed that the principle formulation produced stable, pharmacologically relevant plasma levels of naltrexone for approximately one month following a single injection. The pharmacodynamic effects (antagonism of morphine analgesia) of extended-release naltrexone corresponded well with the pharmacokinetic profile from the same animals. While brain mu-opioid receptor density was found to increase over time in these rats, it did not appear to affect the ability of naltrexone to suppress morphine analgesia. Finally the pharmacokinetic profile of extended-release naltrexone in monkeys confirmed long duration of elevated plasma concentrations of naltrexone. Both naltrexone and the PLG polymer matrix in which it is encapsulated are well tolerated. Clinical trials of Vivitrex are currently ongoing in alcohol dependent patients.