Long acting delivery systems for narcotic antagonists II: release rates of naltrexone from poly(lactic acid) composites.

Parallel in vitro and in vivo release rates of tritiated naltrexone from poly(lactic acid) composites were studied. The in vitro release of naltrexone was 67% of the dose over a 35-day test period, while the in vivo release was only 24% within 70 days. Apparently, an exchange of the tritium for the hydrogen of the body water takes place, indicating that urinary excretion radioactivity is not a reliable measure for estimating the naltrexone released. Naltrexone-poly(lactic acid) composites showed effective blocking action to morphine in rats (24 days), dogs (29 days), monkeys (20 days), and mice (21 days).