Extrahepatic glucuronidation of morphine in the dog

Area of Science:

• Pharmacology
• Drug Metabolism
• Veterinary Medicine

Background:

• Morphine is a widely used analgesic.
• Understanding morphine's metabolic pathways is crucial for optimizing its clinical use.
• Hepatic and extrahepatic contributions to drug metabolism require further investigation.

Purpose of the Study:

• To investigate the pharmacokinetic disposition of morphine in dogs.
• To determine the role of the liver and bile excretion in morphine metabolism.
• To elucidate the involvement of extrahepatic glucuronidation in morphine processing.

Main Methods:

• Pharmacokinetic study in dogs using intravenous morphine administration (1 mg/kg).
• Comparison of morphine disposition in sham-operated dogs, dogs with hepatic devascularization, and dogs with bile duct and ureter ligation.
• Analysis of plasma concentrations of morphine and morphine glucuronide over time.

Main Results:

• In sham-operated dogs, morphine exhibited rapid distribution and elimination (half-life: 65 ± 30 min) with significant morphine glucuronide formation.
• Hepatic devascularization markedly delayed morphine elimination (47% reduction in clearance) and reduced morphine glucuronide AUC by 56%.
• Bile duct and ureter ligation increased morphine elimination and decreased morphine glucuronide elimination, indicating potential enterohepatic recirculation.

Conclusions:

• Morphine undergoes significant glucuronidation by both hepatic and extrahepatic glucuronyltransferases in dogs.
• Bile excretion plays a role in morphine's disposition, with evidence of hydrolysis and reabsorption of morphine glucuronide.
• The liver is a primary site for morphine metabolism, but extrahepatic pathways contribute substantially.