Blood-brain distribution of morphine-6-glucuronide in sheep

Area of Science:

• Pharmacology
• Neuroscience
• Drug Metabolism

Background:

• Limited data exists on the brain-blood partitioning of morphine-6-glucuronide (M6G), an active metabolite of morphine.
• Understanding M6G's cerebral kinetics is crucial for its clinical use and potential central nervous system effects.

Purpose of the Study:

• To determine the rate and extent of M6G partitioning into the brain.
• To investigate the cerebral kinetics of M6G following intravenous infusion in conscious sheep.

Main Methods:

• Intravenous infusion of M6G (2.2 mg kg(-1) over 4 minutes) in five instrumented sheep.
• Non-linear mixed-effects analysis with hybrid physiologically based kinetic models.
• Estimation of cerebral kinetics using arterio-sagittal sinus concentration gradients and cerebral blood flow.

Main Results:

• A membrane-limited model best described M6G blood-brain equilibration.
• Slow equilibration observed: 5.8 minutes to reach 50% equilibration in the deep compartment.
• Low membrane permeability (PS: 2.5 ml min(-1)) and a small deep distribution volume (V2: 18.4 ml) were found.

Conclusions:

• Cerebral pharmacokinetic modeling of M6G indicates delayed brain-blood equilibration.
• Blood-brain barrier permeability is the primary limitation for M6G brain entry, consistent with its physicochemical properties.