Binding of erythromycin base to human plasma proteins.

The binding of erythromycin base (EB) to human plasma (HP) proteins was measured by equilibrium dialysis using EB-14C at approximately 0.5 microgram/ml in the plasma phase, expected therapeutic concentration. EB was 64.5 +/- 0.4% (mean +/- S.D.) bound to HP. The percentage of bound EB was linearly related to the logarithm of HP dilution. Physiological concentrations linearly related to the logarithm of HP dilution. Physiological concentrations of human serum albumin (HSA) and a1 acid glycoprotein (a1AGP) bound 8.7 +/- 1.6 and 54.5 +/- 0.6%, respectively, of the drug. No binding of EB to HDL or LDL or human gamma globulins was found. For HSA separately, EB binding followed a non-saturable phenomenon with n'K' = 100 +/- 5, while for a1AGP the binding was following a saturable phenomenon with n" = 1 +/- 0.045, K" = 35,000 +/- 5000 M-1. It can then be shown that, in human plasma, EB is only bound to those two proteins: firstly by computing the binding of EB to HSA and a1AGP with the previously indicated parameters, and secondly by measuring the binding of EB to HP. The computed curve fairly fits to the measured one. Interaction of EB with HP involved hydrophobic, ionic and hydrogen forces. Lower than 37 degrees C temperatures decreased EB binding. Significant competitive binding was demonstrated between EB and acetylsalicylic acid, furosemide, phenylbutazone, warfarin and bilirubin.