Rate-limiting diffusion processes following intrathecal administration of morphine

Area of Science:

• Pharmacology
• Neuroscience
• Clinical Pharmacy

Background:

• Intrathecal (i.t.) administration of analgesics bypasses first-pass metabolism.
• Understanding morphine pharmacokinetics after i.t. administration is crucial for optimizing pain management.

Purpose of the Study:

• To compare morphine pharmacokinetics following intrathecal versus intravenous administration.
• To elucidate the bioavailability and elimination pathways of i.t. morphine.

Main Methods:

• Plasma morphine concentrations were measured using radioimmunoassay (RIA) in patients receiving i.t. or i.v. morphine.
• Pharmacokinetic analysis was performed to determine clearance, bioavailability, and half-life.

Main Results:

• Intrathecal morphine demonstrated complete bioavailability to the systemic circulation.
• Similar apparent total body clearance was observed for both i.t. and i.v. morphine.
• A flip-flop phenomenon contributed to a prolonged terminal half-life for i.t. morphine.

Conclusions:

• Morphine likely does not undergo metabolism within the central nervous system (CNS).
• Slow diffusion from the cerebrospinal fluid (CSF) to plasma explains the extended duration of analgesia after i.t. administration.
• Intrathecal morphine is a viable route for sustained pain relief.