Fluorescence spectroscopic study for competitive binding of antidiabetic drugs and endogenous substances on serum albumin.

Background: The hypoglycemic effect of antidiabetic drugs varies with change in the level of endogenous substances in the body in diseased states largely due to alteration in drug-serum albumin binding affinity. The aim of the present study was to understand and quantify this effect.

Methods: Quenching of intrinsic fluorescence of human serum albumin was used to monitor the competitive binding of antidiabetic drugs (gliclazide, glimepiride, glipizide and repaglinide) and bilirubin/hemin/chloride ions.

Results and conclusions: Bilirubin and hemin were bound at site I and site II in human serum albumin with association constants of the order of 105. The presence of bilirubin decreased the binding affinity of all the antidiabetic drugs. In the presence of hemin, the binding of gliclazide and glimepiride increased significantly, whereas that of glipizide and repaglinide decreased. The presence of chloride ions decreased the association constants of all drugs except glimepiride. More than 20% increase in the percentage of free drug was observed for gliclazide in the presence of bilirubin and for repaglinide in the presence of bilirubin and hemin. A large decrease was also observed in the percentage of free gliclazide in the presence of hemin, and free glimepiride in the presence of hemin and chloride ions. Competitive binding mechanism has also been proposed. Significant changes (increase/decrease) in the availability of free pharmacologically active antidiabetic drugs, observed in some cases, can result in fluctuations in the blood glucose level of diabetic patients. The effect, which varied with the nature of the drug and the competing substance, was relatively large for gliclazide and repaglinide compared to other drugs.