Saturable first-pass kinetics of propranolol

Area of Science:

• Pharmacokinetics
• Drug Metabolism
• Biopharmaceutics

Background:

• Reduced oral bioavailability is often caused by hepatic first-pass extraction.
• Michaelis-Menten kinetics describe drug metabolism, but integrated solutions for first-pass effects are complex.

Purpose of the Study:

• To derive an integrated solution for Michaelis-Menten kinetics of the first-pass effect for oral drug doses.
• To extend single-dose equations to steady-state kinetics after multiple dosing.
• To calculate key pharmacokinetic parameters for propranolol.

Main Methods:

• Derived an integrated equation for first-pass effect using maximal metabolic rate (Vm), volume of distribution (Vd), absorption rate constant (ka), Michaelis constant (Km), and liver blood flow (Q).
• Extended single-dose equations to steady-state kinetics, considering drug amount in hepatic circulation.
• Calculated Vm and Km for propranolol based on literature data for single and multiple doses.

Main Results:

• The derived equation successfully models the first-pass effect for single and multiple oral doses.
• For propranolol, Vm was calculated as 0.043 mg/L/h (310 mg/day) and Km as 0.10 mg/L.
• Bioavailability of propranolol increases with multiple dosing (0.22 to 0.36).

Conclusions:

• Factors like multiple dosing, increased plasma protein binding, high absorption rate, and increased portal venous blood flow enhance bioavailability.
• Slow-release formulations, fractional dosing, and saturable absorption kinetics decrease bioavailability of first-pass drugs.
• Nonlinear protein binding and zero-order absorption may explain observed phenomena in propranolol dosing.