Determination of drug binding to plasma proteins using competitive equilibrium binding to dextran-coated charcoal

A method for determination of drug binding to plasma proteins, which is based on the comparison of drug affinities to plasma proteins and dextran-coated charcoal, is described. The method is free of nonspecific binding feature. The fractions unbound obtained by the charcoal method are in good agreement with values from a traditional ultrafiltration method for various low and highly bound compounds. The method presently requires drug concentrations much less than that of plasma proteins. A possibility of using the method to determine protein binding at an arbitrary drug concentration is discussed. A mechanism of nonspecific binding of drug to ultrafiltration membranes, which yields a good agreement with experimental observations, is suggested.