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.

Competitive binding of fluoroquinolone antibiotics and some other drugs to human serum albumin: a luminescence spectroscopic study.

Co-administration of several drugs in multidrug therapy may alter the binding of each to human serum albumin (HSA) and hence their pharmacological activity. Thirty-two frequently prescribed drug combinations, consisting of four fluoroquinolone antibiotics and eight competing drugs, have been studied using fluorescence and circular dichroism spectroscopic techniques. Competitive binding studies on the drug combinations are not available in the literature. In most cases, the presence of competing drug decreased the binding affinity of fluoroquinolone, resulting in an increase in the concentration of free pharmacologically active drug. The competitive binding mechanism involved could be interpreted in terms of the site specificity of the binding and competing drugs. For levofloxacin, the change in the binding affinity was small because in the presence of site II-specific competing drugs, levofloxacin mainly occupied site I. A competitive interference mechanism was operative for sparfloxacin, whereas competitive interference as well as site-to-site displacement of competing drugs was observed in the case of ciprofloxacin hydrochloride. For enrofloxacin, a different behavior was observed for different combinations; site-to-site displacement and conformational changes as well as independent binding has been observed for various drug combinations. Circular dichroism spectral studies showed that competitive binding did not cause any major structural changes in the HSA molecule.

Effect of metal ions on some pharmacologically relevant interactions involving fluoroquinolone antibiotics.

BACKGROUND: Complexation of five metal cations, Fe(3+), Al(3+), Zn(2+), Cu(2+) and Mg(2+) with four fluoroquinolones, levofloxacin, sparfloxacin, ciprofloxacin hydrochloride and enrofloxacin and human serum albumin (HSA) has been studied for better understanding of bioavailability of drugs interacting with metals and proteins. METHODS: The binding parameters have been determined using fluorescence and ultraviolet absorption spectroscopic techniques. The effect of metal cations on the interaction of fluoroquinolones with HSA has also been investigated. RESULTS: The association constants were of the order of 10(2)-10(4) for the fluoroquinolone-metal ion interaction. For a given drug, the chelation potential of Al(3+) was highest, whereas that of Mg(2+) was lowest. At a metal ion/drug ratio of 1:1, approximately 50%-73% of metal ion was bound per mole drug in most cases. In the case of HSA-metal ion interaction, for Fe(3+) and Zn(2+) ions, there was only one class of binding site, whereas for Al(3+) and Cu(2+) ions, two types of binding sites were found. The relative affinity of various metal ions was found to vary as Al(3+)>Cu(2+)>Zn(2+)>Fe(3+). The extent of binding was found to be independent of the charge on the ion. Owing to very weak quenching of fluorescence, the association constant for the interaction of Mg(2+) ion could not be determined by this technique. The binding affinity of all the fluoroquinolones to HSA was found to increase in the presence of Cu(2+) ions, whereas all other metal ions decreased the binding -affinity with the exception of levofloxacin in the presence of Zn(2+) and Al(3+) ions. Increase in the binding affinity indicated that the metal ions facilitate HSA-fluoroquinolone interaction and fluoroquinolones probably interact with HSA via a metal ion bridge. Decrease in the binding affinity, by contrast, can either be due to the fact that fluoroquinolone-metal ion complex inhibits fluoroquinolone-HSA interaction or metal ions produce conformational changes in the HSA molecule. CONCLUSIONS: Results indicate that metal chelate formation can cause significant reduction in the antimicrobial activity of fluoroquinolone antibiotics. Alteration in the HSA-fluoroquinolone binding affinity in the presence of metal ions could have significant pharmacological effects. Quantitative estimate of the magnitude of interaction of different metal ions could also be obtained from the data.

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.

Competition between COX-2 inhibitors and some other drugs for binding sites on human serum albumin.

Competitive binding of six COX-2 inhibitors (celecoxib, valdecoxib, etoricoxib, parecoxib sodium, meloxicam and nimesulide) in the presence of three categories of drugs: an antidiabetic (gliclazide), antipsychotic (chlorpromazine) and antibiotic (ceftriaxone sodium) were studied by fluorescence spectroscopy. Data are expressed in terms of the percentage of drug bound in the absence and presence of competing drug and change in the percentage of free drug due to competitive binding. The results are discussed in terms of three cases: decrease in the binding of the parent drug; increase in binding; and no effect by the presence of the competing drug. The relative binding affinity of the parent drug and the displacing compound for human serum albumin (HSA) is not the only factor involved in the competitive mechanism. Binding behaviour of individual drugs was analysed, and explanations for different cases based on the competitive displacement, non-competitive interference, conformational changes in the HSA molecule, and independent binding are presented.

Complexation of cox-2 inhibitors with bovine serum albumin: interaction mechanism.

Mechanism of interaction of six cox-2 inhibitors--celecoxib, valdecoxib, etoricoxib, parecoxib sodium, meloxicam and nimesulide--with bovine serum albumin (BSA) was studied using fluorescence spectroscopic technique. Results were discussed in terms of the binding parameters, thermodynamics of the binding process, the nature of forces involved in the interaction and the fluorescence quenching mechanism involved. Association constants were of the order of 10(4)-10(5) for various drugs. Binding affinity varied with the nature of drug. Nature of forces involved in the interaction could be predicted from the thermodynamic parameters for the binding. Meloxicam and nimesulide shared common sites with hydrophobic probe, 1-anilinonaphthalene-8-sulfonate (ANS) on BSA molecule. Stern-Volmer analysis of the quenching data indicated that both tryptophan residues of BSA are fully accessible to the drugs and predominantly static quenching mechanism is involved in the interaction.

Various solvent systems for solubility enhancement of enrofloxacin.

Solubility enhancement of antimicrobial drug enrofloxacin has been studied using a series of co-solvents and surfactants. Aqueous solubility of enrofloxacin could be increased up to 26 times. Co-solvents alone produced only small increase in solubility. However, the combined effect of co-solvents and buffer was synergistic and a large increase in solubility could be attained. Ionic surfactants were found to be much better solubilizing agents than non-ionic surfactant. Amongst ionic surfactants, solubility was found to be very high in anionic surfactant, sodium dodecylsulphate as compared to the cationic surfactant, cetyltrimethylammonium bromide. Up to 3.8 mg/ml of enrofloxacin could be dissolved in sodium dodecylsulphate. Mechanism of solubilization has been proposed and surfactant solubilization parameters have been calculated.

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.

Quantitative correlation between theoretical molecular descriptors and drug-HSA binding affinities for various cox-2 inhibitors.

In this study, quantitative structure-protein binding relationships have been derived to predict the binding affinities of cox-2 inhibitor drugs to HSA from the structure of drug molecules. Drug-HSA binding affinities were determined using fluorescence spectroscopic technique. For the calculation of theoretical molecular descriptors and statistical treatment of data dragon, codessa and spss software were used. In the quantitative structure-protein binding model, one-parameter correlations were statistically not very sound. Quadratic equations gave better fit of the data in some cases. Two-parameter correlations could explain 94-98% of the variance in the data. Steric, geometric and topological descriptors outweighed all other parameters in the developed quantitative structure-protein binding relationships. There was some contribution from electrostatic descriptors as well, but the role of hydrophobic binding was completely ruled out in the protein binding of coxibs. The quantitative correlations derived in this paper are useful in predicting the protein binding of coxibs and may help in the design of cox-2 inhibitors with appropriate HSA binding properties.

Interaction of non-steroidal anti-inflammatory drugs, etoricoxib and parecoxib sodium, with human serum albumin studied by fluorescence spectroscopy.

The mechanism of interaction of the non-steroidal anti-inflammatory drugs, etoricoxib and parecoxib sodium, with human serum albumin (HSA) was studied using fluorescence spectroscopy. There was only one class of binding site with association constants of the order of 10(4). Thermodynamic parameters suggest that van der Waals and hydrogen bonding interactions in the case of etoricoxib, and electrostatic and hydrogen bonding interactions in the case of parecoxib sodium, are predominantly involved in the binding. Studies in the presence of the hydrophobic probe, 1-anilinonaphthalene-8-sulfonate (ANS), showed that hydrophobic interactions are not involved in the binding of these drugs to HSA. Displacement studies using the site-specific probe, dansylsarcosine piperidinium salt (DSS), showed that the drugs are bound at site II on the HSA molecule. However, etoricoxib and parecoxib sodium are bound at different regions within site II. Increase of pH and the presence of salt caused significant changes in the association constants and the concentration of free pharmacologically active drug. Stern-Volmer analysis of the binding data indicated that the tryptophan residues of albumin are not fully accessible to anionic parecoxib sodium and a predominantly static quenching mechanism is operative in each case.

Reversible binding of celecoxib and valdecoxib with human serum albumin using fluorescence spectroscopic technique.

Mechanism of interaction of non-steroidal anti-inflammatory drugs, celecoxib and valdecoxib with human serum albumin has been studied using fluorescence spectroscopic technique. There was only one high affinity site on serum albumin for both the drugs with association constants of the order of 10(4) in the case of celecoxib and 10(5) in the case of valdecoxib. Thermodynamic parameters for the binding indicated that hydrogen bonding interactions are predominantly involved in the binding of these drugs to human serum albumin. Binding studies in the presence of hydrophobic probe, 1-anilinonaphthalene-8-sulfonate (ANS) suggested that the mode of interaction of drugs and ANS with HSA is different and hydrophobic interactions are not primarily involved in the binding. Studies carried out in the presence of site-specific probe showed that drugs are bound at site II and phenolic oxygen of (411)Tyr is involved in binding. Stern-Volmer analysis of the quenching data indicated that predominantly static quenching mechanism is operative and the tryptophan residues of albumin are fully accessible to celecoxib and only partially accessible to valdecoxib. The presence of salt caused a decrease in the association constant and significant increase in the concentration of free drug.

Mechanism of interaction of the non-steroidal antiinflammatory drugs meloxicam and nimesulide with serum albumin.

The mechanism of interaction of the non-steroidal antiinflammatory drugs meloxicam and nimesulide with human and bovine serum albumin has been studied using fluorescence spectroscopy. There was only one high affinity site on serum albumin for both the drugs with association constants of the order of 10(5). Negative enthalpy (DeltaH(0)) and positive entropy (DeltaS(0)) values in the case of both meloxicam and nimesulide showed that both hydrogen bonding and hydrophobic interactions play a role in the binding of these drugs. Binding studies in the presence of the hydrophobic probe 1-anilinonaphthalene-8-sulfonate (ANS) showed that the binding of meloxicam and nimesulide to serum albumin involves predominantly hydrophobic interactions. Stern-Volmer analysis of the quenching data showed that quenching is highly efficient and that the tryptophan residues in hydrophobic regions of the proteins are fully exposed to the drugs. Thus these drugs are bound to albumin by hydrophobic interactions as well as hydrogen bonding at a site, which is close to the tryptophan residues. An increase of the pH and ionic strength caused an increase in the concentration of free drug, although the effect was not very significant.