Kinetics of drug--drug interactions

The Øie-Tozer model of drug distribution and its suitability for drugs with different pharmacokinetic behavior

INTRODUCTION

Drug distribution is a major pharmacokinetic process that affects the time course of drug concentrations in tissues, biological fluids and the resulting pharmacological activities. Drug distribution may follow different pathways and patterns, and is governed by the drug's physicochemical properties and the body's physiology. The classical Øie-Tozer model is frequently used for predicting volume of drug distribution and for pharmacokinetic calculations.

AREAS COVERED

In this review, the suitability of the Øie-Tozer model for drugs that exhibit different distribution patterns is critically analyzed and illustrated. The method used is a pharmacokinetic modeling and simulation approach. It is demonstrated that the major limitation of the Øie-Tozer model stems from its focus on the total drug concentrations and not on the active (unbound) concentrations. Moreover, the Øie-Tozer model may be inappropriate for drugs with nonlinear or complex pharmacokinetic behavior, such as biopharmaceuticals, drug conjugates or for drugs incorporated into drug delivery systems. Distribution mechanisms and alternative distribution models for these drugs are discussed.

EXPERT OPINION

The Øie-Tozer model can serve for predicting unbound volume of drug distribution for 'classical' small molecular mass drugs with linear pharmacokinetics. However, more detailed mechanism-based distribution models should be used in preclinical and clinical settings for drugs that exhibit more complex pharmacokinetic behavior.

Effect of a protein binding change on unbound and total plasma concentrations for drugs of intermediate hepatic extraction

For substances eliminated from blood by the liver, the effect of a change in unbound fraction of drug (fu(b)) on steady state total (Cb) and unbound (Cu(b)) blood concentrations has hitherto only been considered for the two limiting cases, i.e., at the upper and lower extremes of hepatic intrinsic clearance (CL(int)). For a substance of very low CL(int), if fu(b) changes, Cb will change and Cub will remain constant, whereas if CL(int) is very high, Cub will change and Cb will remain constant. The present study defines the effects of a change in fu(b) on Cb and Cub over the whole CL(int) range. Computer simulations were undertaken which predicted that, for a given change in fu(b), absolute and relative changes in Cb would decrease nonlinearly with increasing CL(int), while the relative change in Cub would increase with CL(int). The absolute change in Cub would be independent of CL(int). Significant changes in Cb and Cub would be observed at intermediate values of CL(int) not just at the high and low extremes. These theoretical predictions were investigated experimentally in the isolated perfused rat liver by examining the effects of a change in fu(b) of sodium taurocholate a substance with intermediate CL(int) (such that at fu(b) = 0.27, hepatic extraction ratio = 0.71) induced by concurrent administration of sodium oleate. Sodium 24-14 C-taurocholate (specific activity 52 microCi/mmol) was infused into the reservoir in a recycling system at 30 mumol/hr for 105 min (n = 6). At 45 min a bolus dose of sodium oleate (50 mmol) was administered to the reservoir, followed by a constant infusion of 143 mmol/hr for 1 hr. Following the administration of oleate, taurocholate fu(b) fell promptly by 55% (0.27-0.12). There was a relative increase of taurocholate Cb of 22.7% and a relative decrease in Cub of 45.4%, in accordance with the simulations (p less than 0.05). We conclude that important changes in unbound steady-state concentration, the pharmacologically active moiety, can occur upon changes in unbound fraction with compounds of intermediate hepatic intrinsic clearance.

Kinetics of drug metabolism inhibition: use of metabolite concentration-time profiles

A simple model simulating the kinetics of drug metabolism inhibition interaction is investigated. The relative sensitivity of drug and metabolite concentration-time profiles as indices of inhibition is assessed. Under steady-state conditions where inhibitor concentrations are maintained constant, the determination of metabolite in addition to drug kinetics provides little additional information when inhibition is nonselective in nature. However metabolite profiles do offer increased sensitivity when parallel routes of metabolism exist and there is selectivity of inhibitory action. Non-steady-state conditions are also investigated as they often apply in inhibition studies; the inhibitor is often administered as a single dose or as a multiple dosing regimen rather than by a constant rate intravenous infusion. Under the former conditions, when inhibitor concentrations in the body fluctuate during the study, metabolite kinetics can be more useful than drug kinetics. The changes evident in the metabolite concentration-time profiles are substantial due to both the inhibition per se and the nonlinearity in the system arising from inhibitor elimination. It is concluded that metabolite kinetics provide a useful aid in the detection of drug metabolism inhibition interactions.

Phenytoin and phenobarbital protein binding alterations in a uremic burn patient

A case is reported in which dramatic decreases in the protein binding of phenytoin and phenobarbital were noted. A 57-year-old burn patient with a history of a seizure disorder had increased free fractions of phenytoin and phenobarbital. The patient later developed renal failure and the free fractions of both drugs were further increased. Decreases in the protein binding of phenytoin in patients with hypoalbuminemia and/or uremia have been well documented, but the effects of these conditions on phenobarbital protein binding have not been well studied. In the case described here, the presence of both hypoalbuminemia and uremia were necessary before a doubling of the free fraction of phenobarbital was noted. Although it is not a highly protein-bound drug, in some circumstances protein binding alterations of phenobarbital may be of clinical significance.