COMPARATIVE PHARMACOKINETICS OF COUMARIN ANTICOAGULANTS .I. UNUSUAL INTERACTION OF BISHYDROXYCOUMARIN WITH PLASMA PROTEINS - DEVELOPMENT OF A NEW ASSAY

A widely used method for the determination of bishydroxycoumarin (BHC) in plasma, which involves extraction of BHC from acidified plasma (pH <1) into an organic solvent, fails at high concentrations of BHC. Evidence is presented to show that this is not due to a limited solubility of the drug in the organic phase but rather to a concentration‐ and pH‐dependent interaction of plasma protein with BHC. Complete extraction of BHC from the blood plasma of rats, guinea pigs, dogs, monkeys, and man is obtained readily only in the narrow pH range of 3.0 to 3.5. A new method of analysis for BHC in plasma has been developed on the basis of these findings and its specificity is demonstrated. It is shown that the development of analytical methods for drugs in plasma or serum, which involve extraction of the drug into an organic phase, cannot be based solely upon a consideration of the effect of pH on the distribution of the drug between the organic and aqueous phases. It is necessary also to consider the effect of pH (and possibly of other factors, such as the type of organic solvent, the buffer system, and ionic strength) on the physical‐chemical properties of plasma proteins as they affect the type and magnitude of interaction of these proteins with the drug. Copyright © 1968 Wiley‐Liss, Inc., A Wiley Company