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.

Co-solvent solubilization of some poorly-soluble antidiabetic drugs.

Co-solvent solubilization approach has been used to enhance the solubility of seven antidiabetic drugs: gliclazide, glyburide, glipizide, glimepiride, repaglinide, pioglitazone, and roziglitazone. Solubility in water, phosphate buffer (pH 7.4), six co-solvent solutions prepared in water as well as phosphate buffer (pH 7.4) and pH-solubility profile of various drugs have been determined at 25 degrees C. Aqueous solubility of various drugs was found to be less than 0.04 mg/mL. Solubility of gliclazide, glipizide and repaglinide increased by 3-6 times by using phosphate buffer (pH 7.4) as solvent. Solubility enhancement by pH modification was not sufficient. Significant enhancement in solubility could be achieved by the use of co-solvents. The combined effect of co-solvent and buffer was synergistic and enormous increase in solubility of sulfonylureas and repaglinide could be achieved. In the case of glitazones, however, co-solvent alone caused significant enhancement; the presence of buffer had negative effect on the solubilization potential of the co-solvents. Up to 763, 316, 153, 524, 297, 792 and 513 times increase in solubility could be achieved in the case of gliclazide, glyburide, glimepiride, glipizide, repaglinide, pioglitazone and rosiglitazone, respectively.

Reversible binding of antidiabetic drugs, repaglinide and gliclazide, with human serum albumin.

Mechanism of interaction of antidiabetic drugs, repaglinide and gliclazide, to human serum albumin has been studied using fluorescence spectroscopic technique. Repaglinide had much higher affinity for human serum albumin when compared with gliclazide. The order of association constants was 10(5) for both the drugs. The size, hydrophobicity and flexibility of the drug molecules play a major role in explaining the binding behaviour of these drugs. Hydrophobic interactions are predominantly involved in the binding. However, drugs do not share common sites with 1-anilinonaphthalene-8-sulphonate on the human serum albumin molecule. Both tyrosine and tryptophan residues participate in the interaction. Repaglinide and gliclazide are bound to site II on the human serum albumin molecule, and the aromatic ring of 411Tyr appears to be involved in binding within site II. Although they do not bind at site I, their binding at site II may cause conformational changes thereby affecting the binding of other ligands to site I. Site-specificity can be useful in predicting the competitive displacement of these drugs by other co-administered drugs, resulting in fluctuations of the blood glucose levels in diabetic patients. Stern-Volmer analysis of quenching data indicated that the tryptophan residues are not fully accessible to the drugs and predominantly dynamic quenching mechanism is involved in the binding.