Propranolol: pooled Michaelis-Menten parameters and the effect of input rate on bioavailability

Area of Science:

• Pharmacokinetics and Drug Metabolism
• Clinical Pharmacology

Background:

• Propranolol exhibits variable oral bioavailability, influenced by first-pass metabolism.
• Understanding propranolol's elimination kinetics is crucial for optimizing dosing strategies.

Purpose of the Study:

• To investigate the relationship between propranolol input rate and its steady-state plasma concentration (Css).
• To elucidate the pharmacokinetic basis for altered propranolol bioavailability under different dosing conditions.

Main Methods:

• Calculated average steady-state propranolol plasma concentration (Css) from published clearance data for various daily dose rates (Ro).
• Fitted Css-Ro data using a Michaelis-Menten equation (Css = KmRo / (Vm-Ro)) for four subjects.
• Assessed the contribution of four parallel Michaelis-Menten metabolic pathways to propranolol clearance.

Main Results:

• The Michaelis-Menten equation accurately described the Css-Ro data, indicating dose-dependent elimination.
• Similar Vm and Km values were derived from individual and parallel pathway analyses.
• Propranolol input rate significantly impacts bioavailability due to Michaelis-Menten elimination kinetics.

Conclusions:

• The study explains the reduced bioavailability of sustained-release propranolol formulations.
• Dosing frequency influences propranolol bioavailability, with increased subdivisions potentially decreasing it.
• Michaelis-Menten kinetics are key to understanding propranolol's oral bioavailability and formulation effects.