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W Loscher
The serum protein binding of valproate (di-n-propylacetate) has been determined at therapeutic concentrations by equilibrium dialysis in man (94.8%), dog (78.5%), rat (63.4%) and mouse (11.9%). In dog serum, the binding was found to be independent of the valproate concentration in the range of 5 to about 70 microgram/ml, but fell with higher concentrations. In addition, the kinetics of valproate has been determined in dogs and rats. After intravenous administration, serum concentrations declined biexponentially in both species, the half-life of elimination (T 0.5 (beta)) being 1.7 hours in dogs and 4.6 hours in rats. In comparison with the pharmacokinetics of valproate in man and mouse, it can be assumed that the protein binding of valproate is rate- limiting for its clearance by the liver and may be responsible for the striking differences in the half-lives of the drug in different species. Increased drug binding was associated with a decrease in the total clearance and in all species examined, the calculated hepatic extraction ratios (0.009-0,17) were smaller that the free fraction, indicating that valproate fits into the group of drugs with restrictive and liver blood flow independent elimination, i.e., only the unbound drug can be cleared.
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