Distribution coefficients and in vitro human serum protein binding of spironolactone and its 7 alpha-carboxymethyl analog.

The distribution coefficients of spironolactone (I) and its 7 alpha-carboxymethyl analog (II) were determined at 22-25 degrees in systems of n-octanol or chloroform and 0.1 M phosphate buffer at pH 7.4. The respective values for I in the two systems were 153.9 and 15.1, and those for II were 15.9 and 3.1. Protein binding studies of I and II were conducted with human serum albumin and human gamma-globulin via equilibrium dialysis at 37 degrees. The I fractions bound to 4% (w/v) albumin and to 1.16% (w/v) gamma-globulin were 66 and 18%, respectively. The corresponding II fractions bound to the two proteins were 46 and 12%. The greater protein binding of I agrees with its superior lipophilicity to that of II. The binding of both I and II to albumin increased with increasing albumin concentration, whereas the binding of I and II to albumin did not change significantly as the concentrations of I or II were varied from 50 to 1300 ng/ml. Cooperativity and/or multiple classes of binding sites appear to be associated with the binding of I and II to albumin.