Disposition of phenytoin in analbuminemic rats

The disposition characteristics of phenytoin were investigated in analbuminemic rats to study the effect of plasma protein binding on phenytoin disposition. Blood and plasma phenytoin concentration determinations, measurement of plasma protein phenytoin binding, and whole-body autoradiography were performed after intravenous bolus injection of 10 mg kg-1 of phenytoin. When plasma phenytoin was assayed, total body phenytoin clearance (CL) was faster and its apparent volume of distribution (Vd) greater in the analbuminemic rats in comparison to the controls. The plasma protein binding of phenytoin was significantly lower in the analbuminemic rats, suggesting the disposition characteristics of phenytoin were altered in the presence of low plasma albumin concentrations due to reduced plasma phenytoin protein binding. On the other hand, when blood phenytoin levels were analyzed, no difference in CL and a less pronounced difference in Vd were noted between the two groups of rats. The red blood cell-to-plasma phenytoin concentration ratios were greater in analbuminemic rats, suggesting that the distribution of phenytoin into red blood cells was greater in the mutant rats.