Use of the fluorescence probe 1-anilino-8-naphthalenesulfonate in predicting interindividual differences in the plasma protein binding of acidic drugs in rats.

The possibility that the fluorescence probe, 1-anilino-8-naphthalenesulfonate (I), might be used for predicting the interindividual differences in the plasma protein binding of acidic drugs was examined. The interindividual differences in the free fraction of I (fI) were found not to be due to corresponding differences in plasma albumin concentration, but to those differences in binding constant. The binding constant of I to the plasma of 18 individual rats ranged from 1.75 X 10(6) M-1 to 2.3 X 10(7) M-1. The free fraction of I had a highly significant statistical correlation with plasma concentration of free fatty acids, but had no significant correlation with the infinite fluorescence of I or the degree of the polarization of I. Each free fraction of five acidic drugs (warfarin, phenylbutazone, salicylic acid, indomethacin, and sulfaphenazole ) was correlated with fI with high statistical significance. Therefore, the simple and convenient method using I may predict the interindividual differences in the plasma protein binding of acidic drugs in rats.