Plasmolysis, red blood cell partitioning, and plasma protein binding of etofibrate, clofibrate, and their degradation products.

Etofibrate (I), the ethylene glycol diester of clofibric and nicotinic acids, degrades almost equally through both half-esters with half-lives of approximately 10 and 1 min in fresh dog and human plasma, respectively. The nicotinate V degrades with half-lives of approximately 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 half-lives of 12 and 26 min. Tetraethyl pyrophosphate at 100 micrograms/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 micrograms/ml of I. Clofibrate (VI) showed a saturable erythrocyte partitioning that decreased from 7.8 (10 micrograms/ml) to 1 (50 micrograms/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 micrograms/ml) and 98.2% (13.6-108.4 micrograms/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.