Effect of dose on phenytoin absorption

Area of Science:

• Pharmacokinetics
• Drug Absorption and Metabolism

Background:

• Phenytoin is a widely used antiepileptic drug.
• Understanding dose-dependent bioavailability is crucial for effective therapeutic drug monitoring.

Purpose of the Study:

• To investigate the impact of varying oral doses of phenytoin on its bioavailability.
• To compare oral versus intravenous phenytoin loading doses.

Main Methods:

• Six healthy male subjects received single intravenous (15 mg/kg) and single oral doses (400, 800, 1600 mg) of phenytoin.
• A divided oral dose (1600 mg) was also administered.
• Pharmacokinetic parameters, including Vmax and Km, were determined from the intravenous dose.

Main Results:

• No statistically significant difference in the extent of phenytoin absorption was observed across different oral doses.
• Time to reach maximum serum concentrations (Tmax) significantly increased with higher oral doses (400 mg: 8.4 hr, 800 mg: 13.2 hr, 1600 mg: 31.5 hr).
• Peak serum concentrations (Cmax) increased with dose, reaching 15.3 mg/l for the divided 1600 mg dose.

Conclusions:

• Large oral doses of phenytoin exhibit prolonged absorption, likely due to slow dissolution and colonic absorption.
• Higher oral loading doses may be necessary compared to intravenous loading doses to achieve equivalent peak serum phenytoin concentrations.
• These findings suggest adjustments in oral phenytoin loading dose strategies may be warranted.