The location of drug binding sites in human serum albumin

Drug-carrying albumin film for blood-contacting biomaterials

Surface-induced thrombosis is a major complication in the development of blood-contacting medical devices. Serum albumin has the ability to bind to a wide variety of compounds, including drugs, and neither cells nor proteins adsorb to an albumin-coated surface. These properties of albumin are useful for improving the blood compatibility of biomaterial surfaces. In the present study, we prepared a water-insoluble film by cross-linking pharmaceutical grade recombinant human serum albumin aiming to the clinical applications, and loaded the film with a synthetic antiplatelet drug, cilostazol. The resultant film possessed native albumin characteristics such as drug binding ability and resistance to cell adhesion. Mouse fibroblast L929 cells did not adhere on the albumin film, just as they did not adhere on native albumin-coated surfaces. Furthermore, when the albumin film carrying cilostazol was placed in PBS containing Tween-80, the release of cilostazol was sustained over 144 h. The results indicate that the surface coating with thus prepared albumin film can confer the biomaterials with antithrombogenic surface by virtue of its non-adhesiveness to cells and its release of cilostazol.

Albumin-based nanoparticles as magnetic resonance contrast agents: I. Concept, first syntheses and characterisation

To develop a platform for molecular magnetic resonance imaging, we prepared gadolinium-bearing albumin-polylactic acid nanoparticles in the size range 20-40 nm diameter. Iterative cycles of design and testing upscaled the synthesis procedures to gram amounts for physicochemical characterisation and for pharmacokinetic testing. Morphological analyses showed that the nanoparticles were spheroidal with rough surfaces. Particle sizes were measured by direct transmission electron microscopical measurements from negatively contrasted preparations, and by use of photon correlation spectroscopy; the two methods each documented nanoparticle sizes less than 100 nm and generally 10-40 nm diameter, though with significant intrabatch and interbatch variability. The particles' charge sufficed to hold them in suspension. HSA retained its tertiary structure in the particles. The nanoparticles were stable against turbulent flow conditions and against heat, though not against detergents. MRI imaging of liquid columns was possible at nanoparticle concentrations below 10 mg/ml. The particles were non-cytotoxic, non-thrombogenic and non-immunogenic in a range of assay systems developed for toxicity testing of nanoparticles. They were micellar prior to lyophilisation, but loosely structured aggregated masses after lyophilisation and subsequent resuspension. These nanoparticles provide a platform for further development, based on non-toxic materials of low immunogenicity already in clinical use, not expensive, and synthesized using methods which can be upscaled for industrial production.

In-vitro study on the competitive binding of diflunisal and uraemic toxins to serum albumin and human plasma using a potentiometric ion-probe technique

The competitive binding of diflunisal and three well-known uraemic toxins (3-indoxyl sulfate, indole-3-acetic acid and hippuric acid) to bovine serum albumin (BSA), human serum albumin (HSA) and human plasma was studied by direct potentiometry. The method used the potentiometric drug ion-probe technique with a home-made ion sensor (electrode) selective to the drug anion. The site-oriented Scatchard model was used to describe the binding of diflunisal to BSA, HSA and human plasma, while the general competitive binding model was used to calculate the binding parameters of the three uraemic toxins to BSA. Diflunisal binding parameters, number of binding sites, ni and association constants for each class of binding site, Ki, were calculated in the absence and presence of uraemic toxins. Although diflunisal exhibits high binding affinity for site I of HSA and the three uraemic toxins bind primarily to site II, strong interaction was observed between the drug and the three toxins, which were found to affect the binding of diflunisal on its primary class of binding sites on both BSA and HSA molecules and on human plasma. These results are strong evidence that the decreased binding of diflunisal that occurs in uraemic plasma may not be solely attributed to the lower albumin concentration observed in many patients with renal failure. The uraemic toxins that accumulate in uraemic plasma may displace the drug from its specific binding sites on plasma proteins, resulting in increased free drug plasma concentration in uraemic patients.

Interaction of polyphenols with proteins: binding of (-)-epigallocatechin gallate to serum albumin, estimated by induced circular dichroism

The binding of (-)-epigallocatechin gallate (EGCG), a representative natural polyphenol, to human serum albumin (HSA) and bovine serum albumin (BSA) was investigated using induced circular dichroism (CD). The site of the binding EGCG-HSA was analyzed based on the competition with drugs with known binding sites on HSA, such as phenylbutazone (PB) and diazepam (DP). Double-reciprocal plot analyses showed the competitive relations with the site-I- (PB and tolbutamide, TB) and site-II-binding drugs (DP and ibuprofen, IP) indicating the binding of EGCG to sites I and II on HSA, while digitoxin (DG), a site-III-binding drug, did not affect the binding of EGCG. In an analogous way, the competitive relations were observed between EGCG and the site-I- (PB and TB) and site-II-binding (ethacrynic acid, EA) drugs for the binding of EGCG and BSA. The site-III drug DG also showed competitive binding with EGCG to BSA. The binding of EGCG to the albumins indicated its affinity to sites I and II on HSA, while competitive binding for all three sites was observed on BSA.

Spectrometric and voltammetric studies of the interaction between quercetin and bovine serum albumin using warfarin as site marker with the aid of chemometrics

The interaction of quercetin, which is a bioflavonoid, with bovine serum albumin (BSA) was investigated under pseudo-physiological conditions by the application of UV-vis spectrometry, spectrofluorimetry and cyclic voltammetry (CV). These studies indicated a cooperative interaction between the quercetin-BSA complex and warfarin, which produced a ternary complex, quercetin-BSA-warfarin. It was found that both quercetin and warfarin were located in site I. However, the spectra of these three components overlapped and the chemometrics method - multivariate curve resolution-alternating least squares (MCR-ALS) was applied to resolve the spectra. The resolved spectra of quercetin-BSA and warfarin agreed well with their measured spectra, and importantly, the spectrum of the quercetin-BSA-warfarin complex was extracted. These results allowed the rationalization of the behaviour of the overlapping spectra. At lower concentrations ([warfarin]<1x10(-5) mol L(-1)), most of the site marker reacted with the quercetin-BSA, but free warfarin was present at higher concentrations. Interestingly, the ratio between quercetin-BSA and warfarin was found to be 1:2, suggesting a quercetin-BSA-(warfarin)(2) complex, and the estimated equilibrium constant was 1.4x10(11)M(-2). The results suggest that at low concentrations, warfarin binds at the high-affinity sites (HAS), while low-affinity binding sites (LAS) are occupied at higher concentrations.

Preparation of water-insoluble albumin film possessing nonadherent surface for cells and ligand binding ability

Serum albumin is the most abundant protein in blood plasma. Albumin has the ability to bind to a wide variety of compounds including drugs, and cells as well as proteins do not attach to an albumin-coated surface. These properties of albumin are attractive for biomaterials utilized in biomedical fields. In the present study, we aimed to prepare a water-insoluble albumin film possessing suitable flexibility and native albumin characteristics, such as drug binding ability and resistance to cell adhesion. To confer the film with both water insolubility and flexibility without losing albumin characteristics, we searched a suitable condition for the cross-linking of albumin. As a result, we found that a film having aimed properties could be obtained by conducting the cross-linking reaction at room temperature for 24 h using 215 mM ethylene glycol diglycidyl ether. Mouse fibroblast L929 cells did not adhere on thus obtained film in a similar manner to a native albumin-coated surface. In addition, the film could bind 2-(4'-hydroxyphenylazo)-benzoic acid, a representative albumin binding dye, and gradually release it in a pH-dependent manner.

In vitro drug-drug interaction studies with febuxostat, a novel non-purine selective inhibitor of xanthine oxidase: plasma protein binding, identification of metabolic enzymes and cytochrome P450 inhibition

1. The potential for drug-drug interactions with febuxostat was examined in the following three in vitro systems: the characteristics of the binding of febuxostat to human plasma proteins; identification of the cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) enzymes participating in the metabolism of febuxostat; and the potential inhibitory effects of febuxostat on typical CYP reactions. 2. The results have shown that the presence of ibuprofen or warfarin did not change the plasma protein binding of febuxostat, and that febuxostat did not influence the plasma protein binding of ibuprofen or warfarin. These results indicate that there is little possibility that febuxostat causes a drug-drug interaction by binding to albumin. 3. The UGT 1 and 2 families were involved in the glucuronidation, and several CYPs participated in the metabolism of febuxostat, suggesting that there is little possibility that the blood concentration of febuxostat varies widely even if febuxostat is concomitantly administered with drugs that inhibit CYP or UGT enzyme. Examination of the inhibitory effect of febuxostat on CYP enzymes suggests that febuxostat minimally inhibits the activities of any CYP. 4. The results demonstrate that febuxostat is a novel anti-hyperuricaemia drug with low drug-drug interaction potential in clinical use.

The Bcl-2 antagonist HA14-1 forms a fluorescent albumin complex that can be mistaken for several oxidized ROS probes

The proapoptotic effects of the Bcl-2 antagonist HA14-1 are believed to derive from its affinity for the hydrophobic groove on Bcl-2 and Bcl-x(L), thereby displacing proapoptotic factors, e.g. Bax and Bak. We have reported that HA14-1 promotes the efficacy of low-dose photodynamic therapy (PDT). A recent report proposed that the proapoptotic activity of HA14-1 reflects its ability to generate reactive oxygen species (ROS) when incubated in an aqueous environment. This later study, like several other HA14-1 investigations, relied on the use of fluorescent probes for ROS detection. We found that HA14-1 reacts with the albumin in serum to yield a fluorescent product. After correcting for this effect, the putative formation of ROS by HA14-1 could not be demonstrated with the fluorescent probes H(2)DCFDA, dihydroethidium or dihydrorhodamine. Indeed, the fluorescence excitation/emission spectra of HA14-1 encompassed the excitation/emission wavelengths used to detect these ROS probes. Cells cultured in a medium supplemented with ovalbumin, instead of serum, underwent apoptosis following HA14-1 addition, but did not exhibit fluorescence. Hence, HA14-1 fluorescence was unrelated to its proapoptotic activity. We conclude that the enhancement of PDT by HA14-1 reflects a pharmacologic effect, rather than its direct contribution of ROS.

Bioreversible derivatives of phenol. 1. The role of human serum albumin as related to the stability and binding properties of carbonate esters with fatty acid-like structures in aqueous solution and biological media

With the overall objective of assessing the potential of utilizing plasma protein binding interactions in combination with the prodrug approach for improving the pharmacokinetics of drug substances, a series of model carbonate ester prodrugs of phenol, encompassing derivatives with fatty acid-like structures, were characterized in vitro. Stability of the derivatives was studied in aqueous solution, human serum albumin solution, human plasma, and rat liver homogenate at 37 degrees C. Stability of the derivatives in aqueous solution varied widely, with half-lives ranging from 31 to 1.7 x 10(4) min at pH 7.4 and 37 degrees C. The carbonate esters were subject to catalysis by plasma esterases except for the t-butyl and acetic acid derivatives, which were stabilized in both human plasma and human serum albumin solutions relative to buffer. In most cases, however, hydrolysis was accelerated in the presence of human serum albumin indicating that the derivatives interacted with the protein, a finding which was confirmed using the p-nitrophenyl acetate kinetic assay. Different human serum albumin binding properties of the phenol model prodrugs with fatty acid-like structure and neutral carbonate esters were observed. In the context of utilizing plasma protein binding in combination with the prodrug approach for optimizing drug pharmacokinetics, the esterase-like properties of human serum albumin towards the carbonate esters potentially allowing the protein to act as a catalyst of parent compound regenerations is interesting.

Albumin-bound paclitaxel: a next-generation taxane

Taxanes are standard treatment for metastatic breast cancer; however, the solvents used as vehicles in these formulations cause severe toxicities. The FDA recently approved a solvent-free formulation of paclitaxel for the treatment of metastatic breast cancer that utilises 130-nanometer albumin-bound (nab) technology (Abraxane; nab-paclitaxel) to circumvent the requirement for solvents. nab-Paclitaxel utilises the natural properties of albumin to reversibly bind paclitaxel, transport it across the endothelial cell and concentrate it in areas of tumour. The proposed mechanism of drug delivery involves, in part, glycoprotein 60-mediated endothelial cell transcytosis of paclitaxel-bound albumin and accumulation in the area of tumour by albumin binding to SPARC (secreted protein, acidic and rich in cysteine). Clinical studies have shown that nab-paclitaxel is significantly more effective than paclitaxel formulated as Cremophor EL (CrEL, Taxol, CrEL-paclitaxel), with almost double the response rate, increased time to disease progression and increased survival in second-line patients. The absence of CrEL from the formulation is associated with decreased neutropenia and rapid improvement of peripheral neuropathy with nab-paclitaxel, compared with CrEL-paclitaxel. For these reasons, nab-paclitaxel can be administered using higher doses of paclitaxel than that achievable with CrEL-paclitaxel, with shorter infusion duration and without the requirement for corticosteroid and antihistamine premedication to reduce the risk of solvent-mediated hypersensitivity reactions. Taken together, these studies have demonstrated that nab technology has increased the therapeutic index of paclitaxel compared with the conventional, solvent-based formulation.

Drug binding to human serum albumin: abridged review of results obtained with high-performance liquid chromatography and circular dichroism

The drug binding to plasma and tissue proteins are fundamental factors in determining the overall pharmacological activity of a drug. Human serum albumin (HSA), together with alpha1-acid glycoprotein (AGP), are the most important plasma proteins, which act as drug carriers, with drug pharmacokinetic implications, resulting in important clinical impacts for drugs that have a relatively narrow therapeutic index. This review focuses on the combination of biochromatography and circular dichroism as an effective approach for the characterization of albumin binding sites and their enantioselectivity. Furthermore, their applications to the study of changes in the binding properties of the protein arising by the reversible or covalent binding of drugs are discussed, and examples of physiological relevance reported. Perspectives of these studies reside in supporting the development of new drugs, which require miniaturization to facilitate the screening of classes of compounds for their binding to the target protein, and a deeper characterization of the mechanisms involved in the molecular recognition processes.

[Interpretations of laboratory test data on serum protein binding]

Serum proteins that are important in the serum protein binding of drugs are human serum albumin (HSA) and alpha1-acid glycoprotein (AGP). Several binding sites exist on HSA and AGP molecules. HSA, AGP, free fatty acid (FFA), blood urea nitrogen (BUN) and bilirubin, which can all be determined by laboratory test, affect the binding capacities of binding sites on these proteins. The increase and decrease of HSA and AGP influence the binding capacities of all binding sites. As an additional influence on the binding sites on protein molecules, the increment of FFA decrease the binding capacity of site II, while binding capacity of site I is enhanced by FFA. Increase in bilirubin remarkably decreases the binding capacity of site I. BUN data are associated with the amounts of several uremic toxins, 3-carboxy-4-methyl-5-propyl-2-furanpropanate (CMPF), indole-3-acetate (IA), indoxyl sulfate (IS) and hippurate (HA). With CMPF, the binding capacity of site I is decreased, while IA, IS, HA contribute to the binding inhibition of site II of HSA. If we can monitor binding capacities of binding sites of HSA and AGP, laboratory test data can be interpreted from a pharmaceutical perspective regarding protein binding, because changes in laboratory test data that are endogenous substance concentrations have an influence on the binding capacities of those binding sites.

A molecular dynamics study of human serum albumin binding sites

A 2.0 ns unrestrained Molecular Dynamics was used to elucidate the geometric and dynamic properties of the HSA binding sites. The structure is not stress affected and the rmsds calculated from the published crystallographic data are almost constant for all the simulation time, with an averaged value of 2.4A. The major variability is in the C-terminus region. The trajectory analysis of the IIA binding site put in evidence fast oscillations for the Cgamma@Leu203...Cgamma@Leu275 and Cgamma@Leu219...Cgamma@Leu260 distances, with fluctuations around 250 ps, 1000 ps and over for the first, while the second is smoothly increasing with the simulation time from 7 to 10A. These variations are consistent with a volume increase up to 20% confirmed by the inter-domain contacts analysis, in particular for the pair O@Pro148...Ogamma@Ser283, representing the change of distance between IB-h9 and IIA-h6, O@Glu149...Ogamma@Ser189 for sub-domains IB-h9/IIA-h1 and N@Val339...Odelta2@Asp447 sub-domains IIB-h9/IIIA-h1. These inter-domain motions confirm the flexibility of the unfatted HSA with possible binding site pre-formation.

Chromatographic analysis of carbamazepine binding to human serum albumin

Recent studies with carbamazepine on human serum albumin (HSA) columns have noted an appreciable degree of non-specific binding on supports prepared by the Schiff base immobilization method. This work examines an alternative immobilization method for HSA based on N-hydroxysuccinimide (NHS)-activated silica. This support was prepared by reacting HPLC-grade silica directly with disuccinimidyl carbonate. The resulting material was compared to an HSA support prepared by the Schiff base method in terms of its activity for carbamazepine and non-specific interactions with this drug. When examined by frontal analysis, both supports gave comparable association equilibrium constants for carbamazepine interactions with HSA ((0.53-0.55) x 10(4)M(-1) at 37 degrees C). However, columns prepared by the Schiff base method gave greater non-specific binding. These columns, as well as control columns prepared using the carbonyldiimidazole (CDI) immobilization method, were also evaluated for their non-specific binding to a variety of other solutes known to interact with HSA. From these results it was concluded that the NHS method was an attractive alternative to the Schiff base technique in the preparation of immobilized HSA for HPLC-based binding studies for carbamazepine. However, it was also noted that non-specific binding varies from one drug to the next in these immobilization methods, indicating that such properties should be evaluated on a case-by-case basis in the use and development of HSA columns for binding studies.

Quantitative characterization of binding of small molecules to extracellular matrix

Extracellular matrix (ECM) is a major tissue component that, besides its cell support function, is implicated in cell-cell signaling, wound repair, cell adhesion, and other cell and tissue functions. For small molecules acting in tissues, including chemicals, signaling peptides, effectors, inhibitors, and other man-made and physiological compounds, non-specific binding to ECM is a critical phenomenon affecting their disposition. We describe here a method for a quantitative characterization of the ECM binding, using a solidified ECM layer incubated with medium containing studied small molecules. Working conditions of Matrigel, a commercial basement membrane preparation, were optimized in terms of the protein concentration, surface area, gel layer thickness, solidification time, and mixing speed. The release of proteins from the solidified layer into the buffer was monitored and taken into account. Two major proteins, laminin and collagen IV, dissolve at different rates. The Matrigel stability data, obtained under varying incubation conditions and gentle mixing, can also be useful in other ECM-related research. The experimental binding data, averaged over all binding sites, were analyzed assuming a fast linear binding. The binding constants were determined for 10 small organic molecules for both dissolved proteins and the solidified layer. The binding constants tend to increase with lipophilicity of the compounds, as characterized by the 1-octanol/water partition coefficients.

Drug release from hydrogel containing albumin as crosslinker

Albumin is the major plasma protein and acts as a physiological carrier for various compounds including drugs. To take advantage of the drug-binding ability of albumin for a drug delivery system, we have prepared hydrogels consisting of acrylamide (AAm) and bovine serum albumin (BSA) by introducing three to four vinyl groups into one BSA molecule and subsequently copolymerizing it with AAm. The resultant hydrogel was solubilized by trypsin treatment, since BSA served as a crosslinker in the hydrogel. The BSA-crosslinked hydrogel (BSA-AAm hydrogel) was loaded with salicylic acid or sodium benzoate and their release was investigated. The BSA-AAm hydrogel released much more salicylic acid than sodium benzoate. In addition, the amount of released salicylic acid increased with the BSA content of the hydrogel, despite a decrease in the swelling ratio of the hydrogel. On the other hand, the amount of released sodium benzoate increased with the swelling ratio. When a hydrogel crosslinked with N,N'-methylenebis (acrylamide) was used as a control, both drugs showed release tendencies similar to that of sodium benzoate from the BSA-AAm hydrogel. Furthermore, the salicylic acid release was sustained longer on the BSA-AAm hydrogel than the sodium benzoate release. Taken together, it is thought that albumin in the BSA-AAm hydrogel preferentially adsorbs salicylic acid and contributes to the high drug loading and the sustained release of salicylic acid.

Equilibrium studies of a fluorescent tacrolimus binding to surfactant protein A

Tacrolimus (FK506) is a hydrophobic immunosuppressive agent used in kidney, liver, and lung transplantation. The objective of this study was to characterize the binding of FK506 to surfactant protein A (SP-A), an abundant lipoprotein found in the alveolar fluid that functions as part of the innate immune system in the lung. We have synthesized a novel derivative of FK506 in which a dansyl moiety was covalently bound via cadaverine to the C22 position of the FK506 molecule (DNS-FK). Using the fluorescence and anisotropy properties of DNS-FK, we demonstrated that tacrolimus avidly binds to SP-A with an apparent equilibrium association constant (K(app)) of 10(7)M(-1) and a Gibbs binding free energy of -40 kJ mol(-1)K(-1). Derivatization of FK506 at the C22 position did not block FK506 binding to the cytosolic immunophilin FK506-binding protein (FK-BP) or human serum albumin (HSA), both used as controls of tacrolimus-binding proteins. K(app) for FK-BP/DNS-FK and HSA/DNS-FK complexes were 1.5 x 10(7) and 10(7)M(-1), respectively. The high sensitivity of this analytical technique makes it suitable for binding analysis of FK506 to proteins.

Electrohydrodynamic atomization of protein (bovine serum albumin)

Bovine serum albumin (BSA) was chosen as a model protein. Three solutions of different concentrations of 5, 20 and 50 mg/ml were prepared, characterised and subjected to electrohydrodynamic atomization (EHDA). The 5 and 20 mg/ml solutions were atomized successfully and mode selection (M-S) maps were drawn for both concentrations to find out regions of stable cone-jet mode atomizaton. Droplet relics of these two solutions were investigated by electron microscopy. Samples were investigated by UV spectroscopy and circular dichroism (CD) spectroscopy before and after electrohydrodynamic atomization. We conclude that, particularly at the higher concentration of protein, EHDA does not result in significant structural change of BSA, and therefore is a processing route that can be considered for encapsulating drugs in proteins.

Characterization of bovine serum albumin-tryptophan interaction by capillary isoelectric focusing with whole column imaging detection

Capillary isoelectric focusing (cIEF) with whole column imaging detection (WCID) was explored for the characterization of bovine serum albumin (BSA)-tryptophan interaction, to further understand protein-drug interactions. The BSA-tryptophan interaction was dynamically monitored by cIEF-WCID, to provide the cIEF profiles of the BSA-tryptophan interaction system at different focusing times. Our study demonstrated that the cIEF behavior of BSA can serve as a probe into the study of BSA-tryptophan interaction, through monitoring the change in its cIEF profile when the interaction occurred. The study illustrated that the BSA peak split due to the BSA-tryptophan interaction, and the peak of BSA-tryptophan complex was clearly identified in the cIEF electropherograms. By comparing the cIEF profiles of BSA/L-tryptophan and BSA/D-tryptophan, respectively, our study demonstrated that BSA interacted with the enantiomers of tryptophan with a chiral recognition. L-Tryptophan demonstrated a very strong interaction with BSA, while D-tryptophan exhibited a much weaker interaction with BSA. The effects of the BSA concentration, the tryptophan concentration, the focusing time and the incubation time on the BSA-tryptophan interaction were investigated. This study offers a novel approach for the study of protein-drug interactions.

Drug-human serum albumin binding studied by capillary electrophoresis with electrochemiluminescence detection

A new technique for investigating drug-protein binding was developed employing capillary electrophoresis (CE) coupled with tris(2,2'-bipyridyl) ruthenium(II) [Ru(bpy)(3) (2+)] electrochemiluminescence (ECL) (CE-ECL) detection after equilibrium dialysis. Three basic drugs, namely pridinol, procyclidine and its analogue trihexyphenidyl, were successfully separated by capillary zone electrophoresis with end-column Ru(bpy)(3) (2+) ECL detection. The relative drug binding to human serum albumin (HSA) for each single drug as well as for the three drugs binding simultaneously was calculated. It was found that the three antiparkinsonian drugs compete for the same binding site on HSA. This work demonstrated that Ru(bpy)(3) (2+) CE-ECL can be a suitable technique for studying drug-protein binding.

Chromatographic analysis of carbamazepine binding to human serum albumin

In this study, high-performance affinity chromatography was used to characterize the binding of carbamazepine to an immobilized human serum albumin (HSA) column. Frontal analysis was first used to determine the association equilibrium constant and binding capacity for carbamazepine on this column at various temperatures. The non-specific binding of carbamazepine within the column was also considered. The results indicated that carbamazepine had a single binding site on HSA with an association equilibrium constant of 5.3 x 10(3)M(-1) at pH 7.4 and 37 degrees C. This was confirmed through zonal elution self-competition studies. The value of DeltaG for this reaction was -5.35 kcal/mol at 37 degrees C, with an associated change in enthalpy (DeltaH) of -6.45 kcal/mol and a change in entropy (DeltaS) of -3.56 cal/molK. The location of this binding region was examined by competitive zonal elution experiments using probe compounds with known sites on HSA. It was found that carbamazepine had direct competition with l-tryptophan, a probe for the indole-benzodiazepine site of HSA, but allosteric interactions with probes for the warfarin, tamoxifen and digitoxin sites. Changes in the pH, ionic strength, and organic modifier content of the mobile phase were used to identify the predominant forces in the carbamazepine-HSA interaction.

The interaction of human serum albumin with a novel antidiabetic agent—SU-118

SU-118 is a newly synthesized antidiabetic agent and shows the best hypoglycemic effect among a series of analogs. Its binding properties and binding sites located on human serum albumin (HSA) have been studied using UV absorption and fluorescence spectroscopy. The results of spectroscopic study and the thermodynamic parameters obtained suggest that SU-118 binds to the hydrophobic cavity of human serum albumin and the hydrophobic interaction is the predominant intermolecular force stabilizing the complex. Fluorescent probe displacement studies show that SU-118 can displace competitively both dansylamide and dansylsarcosine from HSA. It is suggested that SU-118 can bind to both site I and site II, but the primary interaction may take place at site I. A binding constant of 1.4 x 10(4) M(-1) and a binding site of 2.0 are obtained from absorbance titration data. The value of binding constant is of the same order of magnitude as that from fluorescence titration. This study provides a molecular basis for elucidating the mechanism of drug acting and predicting unfavorable drug interaction.

Anti-coagulant rodenticide binding properties of human serum albumin: a biochromatographic approach

In this paper, the anti-coagulant rodenticide-human serum albumin (HSA) binding was investigated using a perturbation method to calculate the solute distribution isotherms. It was shown that rodenticide can bound either on the benzodiazepine HSA site with low affinity (site I) or on the warfarin HSA site with high affinity (site II). The thermodynamic parameters of this association were calculated for the two HSA binding sites. For the site II, the rodenticide-HSA association was governed enthalpically whereas for the site I, this one was driven entropically. Moreover, the role of the magnesium (Mg(2+)) and calcium (Ca(2+)) on this association was carried out. It was clearly demonstrated that the rodenticide affinity for the site I was not affected by modifying the bulk solvent surface tension whereas for the site II the association constant increased strongly with the Mg(2+) or the Ca(2+) concentration in the bulk solvent. These results showed that the rodenticide-HSA affinity and thus the rodenticide toxicological effect depends on the Mg(2+) or Ca(2+) concentration.

19F NMR studies of tryptophan/serum albumin binding

19F NMR provides direct measures of the Trp binding avidity of 'fatty acid free' bovine serum albumin when D- and L-6-fluorotryptophan are used as the probes. Both a high and low affinity binding site are present. The addition of octanoate either displaces the ligand from both sites or greatly decreases the affinity such that little binding occurs at 2 mM levels. In the case of L-6-fluorotryptophan separate signals are observed for the high and low affinity binding sites and titrations with competing ligands can be used to establish the relative affinities of ligands at the high affinity site. Binding at this site appears to be hydrophobic and shape specific with L-Phe being a very poor ligand (K(D)[L-Phe]/K(D)[L-Trp]=800) while both GHKalphaNal and GHKW displace L-6-fluorotryptophan from this site. In tripeptides of the general formula GHK[ epsilon NH(CH(2))(n)(CO)W], affinity increases with tether length and binding at the low affinity site is restored. This NMR assay appears well-suited for the discovery of selective binding agents in this and other biorecognition phenomena.

Studies on the metabolic fate of M17055, a novel diuretic (6). Assessment for drug-drug interactions of M17055 in metabolism, distribution and excretion

1. The potential of M17055, a novel diuretic candidate, to affect the activities of human CYP enzymes, alter the plasma unbound fraction and compete with concomitant drugs in renal secretion as part of an assessment for drug-drug interactions in metabolism, distribution and excretion was investigated. 2. The effects of M17055 on the activities of human CYP1A2, CYP2E1, CYP2C9, CYP2C19, CYP2D6 and CYP3A4 were considered negligible at clinically relevant concentrations. 3. The majority of M17055 (99%) was bound to human plasma proteins, but it is unlikely to alter the binding of other clinically relevant drugs. 4. The renal clearance of M17055 (corrected for the plasma unbound fraction in male rats) substantially exceeded the glomerular filtration rate and was markedly reduced by treatment with probenecid, suggesting that the renal excretion of M17055 is controlled predominantly by an active secretion mechanism. 5. The results show that M17055 is unlikely to cause or undergo significant pharmacokinetic interactions with concomitant drugs in metabolism and distribution. However, when it is administered simultaneously with certain organic anions, drug-drug interactions during kidney excretion may be possible.

Acoustic coupling of transverse waves as a mechanism for the label-free detection of protein-small molecule interactions

An on-line acoustic transverse wave device has been used to study the binding interactions of human serum albumin with the small molecule drug, warfarin. Four linking systems for the covalent attachment of the protein to the surface of the gold electrode of the sensor were employed, namely thioctic acid, cysteamine, an N-hydroxysuccinimide ester and 11-mercaptoundecanoic acid. All the attachment protocols involve the ability of thiols to form gold-sulfur bonds at the metal surface. The functional group present at the distal end of each thiol was chemically activated in order to facilitate covalent attachment of the protein. On-line sensor measurements of acoustic parameters show that the binding of warfarin to the protein can be detected, and depending on the linking monolayer used three of four possible combinations of changes in series resonance frequency and motional resistance are observed. Calculations of possible mass and thickness viscoelastic effects demonstrate that these conventional notions are invalid in terms of an explanation of the acoustic signals observed for the warfarin-protein interaction. The responses are ascribed to acoustic coupling phenomena.

Predicting plasma protein binding of drugs: a new approach

In spite of the large amount of plasma protein binding data for drugs, it is not obvious and there is no clear consensus among different disciplines how to deal with this parameter in multidimensional lead optimization strategies. In this work, we have made a comprehensive study on the importance of plasma protein binding and the influencing factors in order to get new insights for this molecular property. Our analysis of the distribution of percentage plasma protein binding among therapeutic drugs showed that no general rules for protein binding can be derived, except for the class of chemotherapeutics, where a clear trend towards lower binding could be observed. For the majority of indication areas, however, empirical rules are missing. We present here an extensive list of multiply determined primary association constants for binding to human serum albumin (HSA) for 138 compounds from the literature. Correlating these binding constants with the percentage fraction of protein bound showed that the percentage data above 90%, corresponding to a binding constant below 6 microM, are of insufficient accuracy. Furthermore, it could be demonstrated that the lipophilicity of drugs, traditionally felt to dominate binding to HSA, is not the only relevant descriptor. Here, we report a generic model for the prediction of drug association constants to HSA, which uses a pharmacophoric similarity concept and partial least square analysis (PLS) to construct a quantitative structure-activity relationship. It is able to single out the submicromolar to nanomolar binders, i.e. to differentiate between 99.0 and 99.99% plasma protein binding. Depending on the system, this can be important in medicinal chemistry programs and may together with other computed physicochemical and ADME properties assist in the prioritization of synthetic strategies.

Differential binding of tetracyclines with serum albumin and induced structural alterations in drug-bound protein

Interaction of tetracycline (TC) derivatives viz. oxytetracycline, doxycycline, demeclocycline and chlorotetracycline with bovine serum albumin (BSA) and concomitant changes in protein conformation were studied using fluorescence quenching and circular dichroism measurements. Fluorescence data revealed the presence of one to three binding sites on BSA for different TC derivatives. Binding studies with the marker ligands, warfarin and bilirubin, elucidated site-I as a primary binding site for TCs on albumin. Scatchard analysis revealed the binding affinity (K(a)) and capacity (n) for these derivatives vary in the range from 0.8 to 3.2 x 10(6) l/mole and 1.3-3.4, respectively. Significant reduction (60-45%) in secondary structure (alpha-helical content) of BSA was noticed upon interaction with different TC derivatives in presence of Cu (II) ions. High affinity binding of TCs with BSA signifies drug stability. However, excessive binding at higher TC concentrations in combination with Cu (II) induces conformational change in protein structure, which may exert detrimental effect on cellular protein.

Evalution of capillary electrophoresis-frontal analysis for the study of low molecular weight drug-human serum albumin interactions

Capillary electrophoresis frontal analysis was applied to 12 low molecular weight compounds including 8 drug substances displaying a range of different properties with respect to binding affinity, binding location, structure, lipophilicity, charge at physiological pH, and electrophoretic mobility. It was found that capillary electrophoresis frontal analysis can be used as a general method to study and quantify drug-human serum albumin interactions. The binding parameters obtained were consistent with literature values. Dextran was in some cases added to the run buffer to improve separation of the drug and human serum albumin plateau peaks. Results indicate that mobility differences between free and complexed human serum albumin give rise to only minor errors. Capillary electrophoresis frontal analysis was also found applicable to the study of human serum albumin drug displacement reactions. Low sensitivity of the UV-detection system was found to be the major limitation of capillary electrophoresis frontal analysis. The method is simple, and minimal effort has to be put into method development, which makes it well suited for screening in early drug development.

Photosensitized paraquat-induced structural alterations and free radical mediated fragmentation of serum albumin

Photosensitization of paraquat with photosynthetically active radiations (PAR) induced substantial production of both the hydroxyl radicals (*OH) and superoxide anions (O(2)(*-)) under in vitro conditions. Addition of transition metal ion, Cu (II) enhanced the paraquat-induced *OH radical production by 1.8-fold. Treatment of bovine serum albumin (BSA) with photosensitized paraquat resulted in a dose dependent fragmentation of protein. The quantitative analysis revealed the release of 73 microM acid soluble amino groups and 450 microM carbonyl groups from treated albumin at the highest albumin-paraquat molar ratio of 1:8 in presence of 200 microM Cu (II). The results with the free radical quenchers such as mannitol and superoxide dismutase (SOD) clearly reflected the involvement of *OH radicals in protein fragmentation process. The fluorescence data revealed significantly higher binding of paraquat with serum albumin. The binding constants (K(a)) and binding capacity (n) of albumin for paraquat were determined to be 3.4 x 10(5) l/mole and 12.9, respectively. Fluorescence emission spectra exhibited significant quenching of the intrinsic fluorescence of albumin upon addition of paraquat at increasing molar ratios. This is attributed to induced conformational changes in protein structure upon paraquat interaction at specific sites on albumin molecule. Most likely, the alkyl group transfers occur from N1 and/or N1' positions of paraquat to the electron rich sites at critical amino acid residues on treated protein. At higher paraquat concentrations, the albumin-paraquat interaction resulted in adduct formation with concurrent protein alkylation and free radical mediated fragmentation. Thus, on the basis of these results, the paraquat-protein interaction leading to alkylation, structural alterations and/or fragmentation of biological macromolecules has been suggested as an important factor for agrochemical-induced toxicity.

Practical aspects of the ligand-binding and enzymatic properties of human serum albumin

Recent work with approaches like recombinant mutants and X-ray crystallography has given much new information about the ligand-binding properties of human serum albumin (HSA). The information increases the understanding of this unique transport and depot protein and could give a structural basis for the possible construction of therapeutic agents with altered HSA-binding properties. A tabulation of high-affinity binding sites for both endogenous and exogenous compounds has been made; it could be useful for the above-mentioned purpose, but it could also be of value when trying to predict potential drug interactions at the protein-binding level. Drug displacement is not always a complication to therapy; it can be used to increase the biological effect of a drug. However, due to rebinding at other sites, the increase in the free concentration of a displaced ligand can be less than expected. Drugs and drug metabolites can also interact covalently with HSA; thiol-containing drugs often bind to the single free cysteine residue of HSA, and glucuronidated drugs react irreversibly with other residues of the protein. Reversible binding of ligands is often stereospecific, and therefore immobilized HSA can be used to separate drug isomers. Albumin-containing dialysates are useful for extracorporeal removal of endogenous toxins and in the treatment of drug overdoses. HSA has different types of hydrolytic activities, which also can be stereospecific. The esterase-like property seems especially useful in converting prodrugs to active drugs in plasma.