Evidence for the delivery of narcotic antagonists to the colon as their glucuronide conjugates.

Morphine-dependent rats were used to evaluate the effects of the narcotic antagonists, naloxone and nalmefene, and their glucuronide conjugates on the gastrointestinal tract and various parameters of brain-mediated withdrawal. When administered s.c. nalmefene HCl caused a dose-dependent tail skin temperature increase, whereas nalmefene glucuronide was ineffective. Nalmefene precipitated brain-mediated morphine withdrawal at doses as low as 10 micrograms/kg, whereas nalmefene glucuronide was ineffective at doses as high as 1 mg/kg. After p.o. administration of the drugs, naloxone HCl and nalmefene HCl caused diarrhea, withdrawal behavior and tail skin temperature responses by 15 min. In contrast, after p.o. administration of the glucuronide conjugate of either narcotic antagonist, diarrhea was delayed for 75 to 203 min. This latency probably reflects the required transit time to the lower gastrointestinal tract inasmuch as direct colonic administration of either nalmefene or nalmefene glucuronide caused diarrhea within 5 to 8 min. Additionally, the magnitude of brain-mediated withdrawal was smaller and its time of occurrence was delayed and compared to the diarrhea response after p.o. administration of the conjugated forms of the narcotic antagonists. Our calculations indicate that about 0.2 to 0.5% of the dose of the narcotic antagonist administered orally as the glucuronide was absorbed systemically. These results indicate that p.o. administration of the glucuronide conjugates of naloxone and nalmefene results in delivery of the narcotic antagonists to the colon. As such these conjugates may be useful in the prevention or relief of constipation caused by opiate use, without interfering with the central analgesic effects of the narcotics.