Stereochemical features of 1,4-benzodiazepin-2-ones bound to human serum albumin: difference CD and UV studies

A protein-coated magnetic beads as a tool for the rapid drug-protein binding study

A simple and fast method for the determination of the association constant (K(a)) of ligand to human serum albumin (HSA) has been developed by using human serum albumin-coated magnetic beads (HSA-MB). To date, magnetic beads (MB) have been increasingly used as a bioseparation tool, especially for DNA, RNA, protein, enzyme and cell isolation or purification. In this study, HSA-MB were used as a new tool to determine the affinity of known ligands to HSA. The K(a) for l-tryptofan, fenoprofen, ketoprofen, tolbutamide and warfarin obtained from Schathard analysis are consistent with previously reported values. The different K(a) values for ketoprofen after the acetylation of HSA-MB by preincubation with acetylosalicylic acid indicate that these beads can be successfully adapted in combined experiment. In addition, the HSA-MB experiment with phenytoin and valproic acid proved to be a simple method to examine drug displacement effect.

Conformation selectivity in the binding of diazepam and analogues to α1-acid glycoprotein

Diazepam, a 1,4-benzodiazepine lacking chiral centre, exists in an equimolar mixture of two chiral conformers. Induced circular dichroism spectra for the binding of diazepam and its 3,3-dimethyl substituted analogues to alpha1-acid glycoprotein (AGP) revealed that opposite to human serum albumin, AGP preferably binds the P-conformers. Accordingly, slightly favoured binding of (R)-enantiomers of 3-alkyl derivatives having P-conformation was found. In case of 3-acyloxy derivatives, however, AGP preferably binds the (S)-enantiomers. Studies with the separated genetic variants of AGP proved similar binding affinities, but markedly different conformation selectivities. For diazepam bound by the F1-S variant, a P/M selectivity of about 2 could be estimated.

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.

Drug affinity to immobilized target bio-polymers by high-performance liquid chromatography and capillary electrophoresis

This review addresses the use of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) as affinity separation methods to characterise drugs or potential drugs-bio-polymer interactions. Targets for the development of new drugs such as enzymes (IMERs), receptors, and membrane proteins were immobilized on solid supports. After the insertion in the HPLC system, these immobilized bio-polymers were used for the determination of binding constants of specific ligands, substrates and inhibitors of pharmaceutical interest, by frontal analyses and zonal elution methods. The most used bio-polymer immobilization techniques and methods for assessing the amount of active immobilized protein are reported. Examples of increased stability of immobilized enzymes with reduced amount of used protein were shown and the advantages in terms of recovery for reuse, reproducibility and on-line high-throughput screening for potential ligands are evidenced. Dealing with the acquisition of relevant pharmacokinetic data, examples concerning human serum albumin binding studies are reviewed. In particular, papers are reported in which the serum carrier has been studied to monitor the enantioselective binding of chiral drugs and the mutual interaction between co-administered drugs by CE and HPLC. Finally CE, as merging techniques with very promising and interesting application of microscale analysis of drugs' binding parameters to immobilized bio-polymers is examined.

Inhibition of drug binding to human serum albumin by cholecystographic agents

The binding of two cholecystographic agents to human serum albumin (HSA) was evaluated by means of two different complementary methodologies. In particular, the inhibition of drug HSA binding caused by iopanoic- and iophenoxic-acid was investigated by circular dichroism (CD) and resonant mirror (RM) optical biosensor techniques. The CD study allowed to obtain information both on the cholecystographic agent binding site and on the effect of the binding on the protein conformation. Iopanoic acid (IOP), a drug potentially useful for thyrotoxic disorders, resulted a direct competitor for ligands that selectively bind to site II, in agreement to literature data. No definite evidence was obtained for the highest affinity binding site of iophenoxic acid (IOPH), however, this diagnostic tool markedly affected the binding of ligands to the most characterized high affinity sites on HSA, namely sites I, II and III. Binding parameters were obtained by optical biosensor analysis: K(D) values were 3.6 x 10(-7) and 2.8 x 10(-8) M for IOP and IOPH, respectively.

Rapid screening of small ligand affinity to human serum albumin by an optical biosensor

Here we report the use of IAsys biosensor technology for determining the binding parameters of low molecular weight compounds, such as warfarin and bilirubin, to surface immobilized human serum albumin. The protein was covalently immobilized on the surface of the biosensor cuvette, bearing a carboxymethyl dextran layer, through a condensing reaction between the carboxyl groups of the biosensor surface and epsilon-amine groups of protein lysine residues. This system detects and quantifies the changes in refractive index in the vicinity of the surface of the sensor chip to which the protein is immobilized. The changes in the refractive index are proportional to the change in the absorbed mass, thus the analysis allows the monitoring of the interaction process and the determination of the binding parameters. Optical biosensor analysis, most suited for studying protein/protein or protein/nucleic acid interactions, was sensitive enough to monitor the binding of low molecular weight compounds to human serum albumin and then suitable for a rapid screening of libraries of potential drugs when bioavailability is the research target.

Use of an immobilised human serum albumin HPLC column as a probe of drug-protein interactions: the reversible binding of valproate

The reversible binding of valproate to human serum albumin determines a decrease of the binding of ligands that selectively bind to site I, site II, and bilirubin binding site. The binding inhibition was followed by displacement chromatography methodology using increasing concentrations of the competitor, i.e. valproate, in the mobile phase. Significant binding inhibition was observed for drugs binding at site I and site II. The greater displacement was observed for the more retained enantiomer of benzodiazepines and profens. A reduction of the affinity was observed also in the case of phenol red, this compound being selected as representative of bilirubin binding site. Difference circular dichroism spectroscopy was also used to characterise the binding of valproate to human serum albumin. This antiepilectic drug was proved to affect the binding at site I, II, and bilirubin binding site. The data have physiological relevance because significant inhibition of the binding resulted at clinic concentrations of valproate.

Direct resolution, characterization, and stereospecific binding properties of an atropisomeric 1,4-benzodiazepine

The chromatographic resolution of 7-chloro-1,3-dihydro-1-(1,1-dimethylethyl)-5- (2-fluorophenyl)-2H-1,4-benzodiazepin-2-on (7), the 2'-fluoro, N1-tertbutyl analogue of diazepam, was attained on both analytical and preparative (mgs) scales, by using several chiral stationary phases (CSPs). The stereochemistry of this compound was characterized by means of 1H-NMR Nuclear Overhauser Effect (NOE) analysis. The single enantiomers of 7 were tested for their configuration and stereochemical stability by circular dichroism (CD), and their interaction with the central nervous system (CNS) benzodiazepine receptor was assayed, showing a significant difference in their binding affinities. Protein binding studies with human serum albumin (HSA, the main benzodiazepine carrier in human plasma) immobilized on a silica stationary phase revealed that HSA also preferentially binds one stereoisomer of 7. However, both on line CD detection and stereospecific interaction with other common drugs clearly demonstrated that the stereoselectivity of immobilized HSA for 7 is opposite to that for all the other studied benzodiazepines. In addition, HSA stereoselectivity for 7 is opposite to CNS receptor binding stereoselectivity for the same compound. Such HSA anomalous stereoselectivity for 7 was also confirmed in aqueous buffer solution by competitive displacement studies. Compared to other chiral 1,4-benzodiazepines, compound 7 thus shows several anomalous binding properties: HSA and the CNS receptor demonstrated opposite enantioselective discrimination; HSA has reversed enantioselectivity for compound 7; and HSA stereospecifically binds the low-affinity enantiomer.

Allosteric modulation by single enantiomers of a C3-chiral 1,4-benzodiazepine of the gamma aminobutyric acid type A receptor channel expressed in Xenopus oocytes

Xenopus laevis oocytes injected with Poly(A)(+)-RNA isolated from neuronal tissue express membrane proteins peculiar to the origin of mRNA. The translation of gamma aminobutyric acid type A (GABAA) receptors has been shown by dose/ response behavior of GABA and the reversible blockade of the GABA-induced current by picrotoxin. This current was analyzed quantitatively under two electrode voltage-clamp conditions. This methodology has been applied for the first time to study the functional properties of the receptor as a function of the stereochemistry of the ligands. The (+)-S and (-)-R enantiomers of a water-soluble benzodiazepine derivative, 7-chloro-1,3-dihydro-3-hemisuccinyloxy-5-phenyl-1,4-benzodiazep in-2-one (OXHEM), obtained by preparative high performance liquid chromatographic (HPLC) resolution on chiral stationary phase, act as agonists in the in vitro modulation of the chloride channel. The (+)-S-OXHEM enantiomer was the more active.

The binding of 5-fluorouracil to native and modified human serum albumin: UV, CD, and 1H and 19F NMR investigation

5-Fluorouracil (FU) is an important and widely used antineoplastic drug that is carried in the serum by plasma proteins. Protein binding studies of this drug to human serum albumin (HSA) have been carried out by several spectroscopic techniques. Difference circular dichroism and UV studies provided information on the class of binding sites involved in the interaction. In particular, displacement experiments showed that FU has at least one secondary binding site in the coumarin binding area, but does not interact with the benzodiazepine binding area. Binding was also investigated by difference 1H NMR and by measuring the increase in the 19F NMR signal of FU when bound to HSA. Finally, evidence was obtained that chemical acetylation of Lys199 results in a decreased apparent binding affinity constant (nK) for FU. Such a modification is induced under physiological conditions by aspirin.

Stereospecific and competitive binding of drugs to human serum albumin: a difference circular dichroism approach

Simultaneous binding of two drugs to human serum albumin (HSA) was investigated by difference circular dichroism (delta CD) spectroscopy. Phenylbutazone and diazepam were chosen as specific markers for binding areas I (cumarines) and II (indoles), respectively, and their stereospecific interactions with protein were selectively characterized. Displacers were drugs known to specifically bind to areas I (salicylate) and II (racemic ibuprofen). The results indicate two different interaction mechanisms: a direct competition one (diazepam-ibuprofen and phenylbutazone-salicylate) and an indirect competition one (diazepam-salicylate and phenylbutazone-ibuprofen). The two major binding areas on HSA are distinct, but not independent, entities. Finally, the dissociation constants of marker ligands and competitors complexed to HSA were determined by quantitative analysis of CD data.

Protein binding investigation by difference circular dichroism: native and acetylated human serum albumins

A modified albumin was prepared by selective reaction of Lys199 with acetyl salicylic acid. Protein binding investigation was carried out on native and modified proteins by difference circular dichroism (delta CD). Acetylation of Lys199 reduces significantly the effects of the interaction between drugs in the stereoselective HSA binding at specific binding areas.