Plasmolysis, Red Blood Cell Partitioning, and Plasma Protein Binding of Etofibrate, Clofibrate, and Their Degradation Products

doi:10.1002/jps.2600721119

Journal of Pharmaceutical Sciences

Volume 72, Issue 11, November 1983, Pages 1309-1318

https://doi.org/10.1002/jps.2600721119 Get rights and content

Abstract

Etofibrate (I), the ethylene glycol diester of clofibric and nicotinic acids, degrades almost equally through both half-esters with half-lives of ~ 10 and 1 min in fresh dog and human plasma, respectively. The nicotinate V degrades with half-lives of ~ 12 hr and 50 min in fresh dog and human plasma, respectively. Ester III and clofibrate VI degrade by saturable Michaelis–Menten kinetics in fresh human plasma, with similar maximum initial rates and respective terminal first-order halflives of 12 and 26 min. Tetraethyl pyrophosphate at 100 μg/ml inhibited human plasma and red blood cell esterases permitting plasma protein binding and red blood cell partitioning studies. The red blood cell-plasma water partition coefficient was 5.4 for 0.2-80 μg/ml of I. Clofibrate (VI) showed a saturable erythrocyte partitioning that decreased from 7.8 (10 μg/ml) to 1 (50 μg/ml). The strong binding of I and VI to ultrafiltration membranes necessitated the determination of their plasma protein binding by the method of variable plasma concentrations of erythrocyte suspensions to give 96.6% (0.2-80 μg/ml) and 98.2% (13.6-108.4 μg/ml) binding, respectively. Methods for the determination of the parameters of saturable and nonsaturable plasma protein binding for unstable and membrane-binding drugs by the method of variable plasma concentrations in partitioning erythrocyte suspensions are presented.

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Citation Excerpt :
Assuming binding to plasma proteins is an equilibrium (as opposed to a covalent and/or coordinate binding), and drugs encounter blood, not plasma alone, a drug is simultaneously partitioned into erythrocytes while binding and equilibrating with plasma protein. Schuhmacher et al. [24] have modified and improved a method [25] to exploit this situation. The technique is a little more labor-intensive than the standard techniques.
The binding of a drug to plasma protein reduces free drug in the blood circulation that would otherwise be available for penetration into tissues to reach the therapeutic target or the kidney for elimination. Therefore, the binding event affects drug elimination from the body, efficacy, duration of action and toxicity. Co-administration of other drugs, food and pathological conditions of patients can significantly change percentage binding of the drug and result in serious consequences. Here, we present the largest and newest information on plasma protein binding for 222 drugs, of which 50% show 90–100% binding, a range that could be considered as a favorable element for future lead selection. We also provide critical and comprehensive evaluations on the methods and techniques established to determine plasma protein binding, pinpoint advantages and pitfalls of individual approaches, and offer detailed guidance for experimental designs, including ultrafiltration, equilibrium dialysis, ultracentrifugation, charcoal adsorption, high-performance affinity chromatography, high-performance frontal analysis, solid-phase microextraction and in vivo microdialysis.
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Plasmolysis, Red Blood Cell Partitioning, and Plasma Protein Binding of Etofibrate, Clofibrate, and Their Degradation Products