The stereoselectivity of the 'single drug binding site' of human alpha 1-acid glycoprotein (orosomucoid)

High-throughput 1,536-well fluorescence polarization assays for α(1)-acid glycoprotein and human serum albumin binding

Two major plasma proteins in humans are primarily responsible for drug binding, the α(1)-acid-glycoprotein (AGP) and human serum albumin (HSA). The availability of at least a semiquantitative high-throughput assay for assessment of protein binding is expected to aid in bridging the current gap between high-throughput screening and early lead discovery, where cell-based and biochemical assays are deployed routinely to test up to several million compounds rapidly, as opposed to the late-stage candidate drug profiling methods which test at most dozens of compounds at a time. Here, we describe the miniaturization of a pair of assays based on the binding- and displacement-induced changes in fluorescence polarization (FP) of fluorescent small molecule probes known to specifically target the drug-binding sites of these two proteins. A robust and reproducible assay performance was achieved in ≤4 µL assay volume in 1,536-well format. The assays were tested against a validation set of 10 known protein binders, and the results compared favorably with data obtained using protein-coated beads with high-performance liquid chromatography analysis. The miniaturized assays were taken to a high-throughput level in a screen of the LOPAC(1280) collection of 1,280 pharmacologically active compounds. The adaptation of the AGP and HSA FP assays to a 1,536-well format should allow their use in early-stage profiling of large-size compound sets.

Use of plasma proteins as solubilizing agents in in vitro permeability experiments: Correction for unbound drug concentration using the reciprocal permeability approach

The purpose of the present study was to explore the applicability of the reciprocal permeability approach to correct for changes in thermodynamic activity when in vitro permeability data are generated in the presence of plasma proteins. Diazepam (DIA), digoxin (DIG), and propranolol (PRO) permeability was assessed in the presence of bovine serum albumin (BSA) and bovine alpha-1-acid glycoprotein (AAG). The reciprocal permeability approach was subsequently employed to calculate the true permeability coefficient (Papp(corr)) and the operational protein association constant (nK(a)). For BSA binding, good agreement was observed between the Papp(corr) values and Papp values obtained in the absence of protein. For PRO and AAG, where binding affinity was high, deviation in the reciprocal permeability plots was evident suggesting ligand depletion at low drug/high protein concentrations. Bidirectional DIG permeability data in the presence of either BSA or AAG indicated that neither protein had an effect on the efflux transporters involved in DIG permeability. The data suggest that plasma proteins can be utilized in permeability experiments with no adverse effects on transporter function and that the reciprocal permeability approach can be used to correct permeability data for changes in unbound drug concentration.

Determination of drug–serum protein interactions via fluorescence polarization measurements

New fast methods for the determination of pharmacokinetic behaviour of potential drug candidates are receiving increasing interest. We present a new homogeneous method for the determination of drug binding and drug competition for human serum albumin and alpha(1)-acid glycoprotein that is amenable to high-throughput-screening. It is based on selective fluorescent probes and the measurement of fluorescence polarization. This leads to decreased interference with fluorescent drugs as compared with previously published methods based on similar probes and the measurement of fluorescence intensity. The binding of highly fluorescent drugs that still interfere with the probes can be measured by simply titrating the drugs in a two-component system with the serum protein. The assay may also be used to discover strongly binding protein ligands that are interesting for drug-targeting strategies. Additionally, binding data could be obtained from larger libraries of compounds for in silico predictive pharmacokinetics. Figure Fluorescence polarization displacement titration of dansylsarcosine (3D-structure as insert) bound to human serum albumin (HSA) by naproxene.

Determinants of histamine recognition: implications for the design of antihistamines

Towards understanding how histamine, a vital neurotransmitter, can perform multiple physiological tasks, an analysis of the different proteins that bind histamine is reported here. Their structural comparison reveals conformational rigidity of histamine. Yet, flexibility in the modes of histamine binding has been observed, which appears to suit specific biological roles of the proteins. These results will be helpful in developing specific antihistamines and also in understanding the pharmacological and toxicological profiles of existing antihistamines.

Stereoselective protein binding of alprenolol in the renal diseased state

The investigation was undertaken to study the stereoselective protein binding of alprenolol in renal disease patient sera, compared to that in the sera of healthy volunteers. The in vitro stereoselective protein binding of beta-blockers was determined in undiluted serum and in isolated alpha(1)-acid glycoprotein (AGP) solutions by ultrafiltration. The stereoselctive serum protein binding of alprenolol, a beta-adrenergic blocking agent, in healthy volunteers was significantly altered in renal disease patients. We investigated the effects of AGP concentration and endogenous substances, including uremic toxins, on the stereoselective protein binding of alprenolol in renal disease patients. A good correlation between the unbound (R)/(S) ratio (F(R)/F(S) ratio), an apparent index of stereoselectivity in alprenolol serum binding and AGP concentration in serum, was found. However, stereoselective protein binding was not influenced by endogenous substances. This result can be explained by the difference in binding affinities of (R) and (S)-isomers of alprenolol to AGP. We conclude that the stereoselective protein binding of alprenolol in healthy volunteers and renal disease patients varies as a result of changes in AGP concentration. Accordingly, these findings might be useful in alprenolol therapy in renal disease patients.

Human alpha-1-glycoprotein and its interactions with drugs

For about half a century, the binding of drugs to plasma albumin, the "silent receptor," has been recognized as one of the major determinants of drug action, distribution, and disposition. In the last decade, the binding of drugs, especially but not exclusively basic entities, to another plasma protein, alpha 1-acid glycoprotein (AAG), has increasingly become important in this regard. The present review points out that hundreds of drugs with diverse structures bind to this glycoprotein. Although plasma concentration of AAG is much lower than that of albumin, AAG can become the major drug binding macromolecule in plasma with significant clinical implications. Also, briefly reviewed are the physiological, pathological, and genetic factors that influence binding, the role of AAG in drug-drug interactions, especially the displacement of drugs and endogenous substances from AAG binding sites, and pharmacokinetic and clinical consequences of such interactions. It can be predicted that in the future, rapid automatic methods to measure binding to albumin and/or AAG will routinely be used in drug development and in clinical practice to predict and/or guide therapy.

Tick histamine-binding proteins: lipocalins with a second binding cavity

Tick histamine-binding proteins (HBPs) are lipocalins with two binding pockets. One of these binds histamine with a high affinity and is found at the position expected from other lipocalins, adjacent to the omega-loop at the open-end of the beta-barrel. A second binding cavity, which is a low-affinity site for histamine in one of the HBPs, is located at the end of the barrel that is closed off in other lipocalins. In order to create the second site, the 'closed-end' region has undergone a major reconstruction. Typical lipocalin characteristics, such as the 3(10) helix and a structural cluster of highly conserved residues, have been lost, while an alpha-helix now shields the cavity from the exterior. The prominence of acidic residues in the binding pockets is another distinctive characteristic of HBPs. Whereas most lipocalins have highly hydrophobic binding cavities designed to bind lipophilic compounds, HBPs have evolved to trap cationic, hydrophilic molecules.

Mechanisms of retention of pyrrolidinyl norephedrine on immobilized alpha(1)-acid glycoprotein

The HPLC separation of the R,S and S,R enantiomers of pyrrolidinyl norephedrine on immobilized alpha-1 glycoprotein (AGP) was investigated. Conditions for the separation were varied using a premixed mobile phase containing an ammonium phosphate buffer and an organic modifier. The influence of mobile phase pH, ionic strength, organic modifier composition, modifier type, and temperature on the chiral selectivity and retention were investigated. The presented data demonstrate that independent phenomena govern the enantioselectivity and retention. Retention is a function of both ion exchange equilibria and hydrophobic adsorption. Thermodynamic data derived from van't Hoff plots illustrates that while enantioselectivity is also enthalpically driven, the magnitude of the enthalpy term is governed by pH. Enantioselectivity has little dependence on ionic strength. Hydrophobic interactions appear to foster hydrogen bonding interactions; the two appear to be mutually responsible for chiral selectivity. The chiral selectivity decreases as the pH is decreased and increases with mobile phase buffer strength.

Ligand specificity of the genetic variants of human alpha1-acid glycoprotein: generation of a three-dimensional quantitative structure-activity relationship model for drug binding to the A variant

Human alpha1-acid glycoprotein (AAG) is a mixture of at least two genetic variants: the A variant and the F1 and/or S variant or variants, which are encoded by two different genes. In a continuation of previous studies indicating specific drug transport roles for each AAG variant according to its separate genetic origin, this work was designed to (1) determine the affinities of the two main gene products of AAG (i.e., the A variant and a mixture of the F1 and S variants) for 35 chemically diverse drugs and (2) to obtain meaningful 3D-QSARs for each binding site. Affinities were obtained by displacement experiments, leading to qualitative indications about binding site characteristics. In particular, drugs binding selectively to the A variant displayed some common structural features, but this was not seen for the F1*S variants. Three-dimensional QSAR analyses using the CoMFA method yielded a steric model for binding to the A variant, from which a simplified haptophoric model was derived. In contrast, no statistically sound model was found for the F1*S variants, possibly due (among other reasons) to an insufficient number of high affinity ligands in the set.

Quantitative structure-retention relationships in the examination of the topography of the binding site of antihistamine drugs on alpha 1-acid glycoprotein

Quantitative relationships between the structure of antihistamine drugs (AHD) and their retention on an alpha 1-acid glycoprotein (AGP) HPLC column (QSRR) were studied in order to identify characteristic structural features of the binding site for AHD on AGP. The hydrophobicity of AHD was determined by HPLC on an immobilized artificial membrane (IAM) column. A highly significant QSRR equation was obtained which describes the retention of AHD on AGP in terms of the chromatographically determined hydrophobicity parameter, electron excess charge on the aliphatic nitrogen and a molecular size descriptor. The topography of the AHD-binding site on AGP was suggested to be a conical pocket with lipophilic regions at the mouth of the receptor and an anionic region close to the spike of the cone. Protonated aliphatic nitrogen is supposed to guide a drug molecule towards the anionic region of the binding site. Hydrophobic aryl moieties provide anchoring of the molecule in the lipophilic regions of the binding site. Steric hindrance prevents the molecule from plunging into the binding site.

Binding site for basic drugs on alpha 1-acid glycoprotein as revealed by chemometric analysis of biochromatographic data

Interactions between alpha 1-acid glycoprotein (AGP) and 52 basic drugs were quantified by means of high-performance liquid chromatography (HPLC). The HPLC retention parameters were related quantitatively to the hydrophobicity and molecular modelling parameters, giving rise to the prediction of relative drug-AGP binding from the chemical structure of a drug. A structural model of one binding site on AGP, common for various classes of drugs, was defined which accounted for the observed and reported differences in binding to AGP. A combination of biochromatography and chemometrics has been presented as a promising new approach in biochemical/pharmacological studies.

An investigation of the use of nile red as a long-wavelength fluorescent probe for the study of alpha 1-acid glycoprotein-drug interactions

Spectrofluorimetry in the long-wavelength region of the electromagnetic spectrum (600-1000 nm) is a fairly recent development in photoluminescence spectroscopy, which has numerous advantages over measurements in the more conventional ultraviolet and visible spectral region. 9-Diethylamino-5H-benzophenoxazine-5-one (Nile Red) is an unchanged, hydrophobic molecule, and long-wavelength fluorescence of which is strongly influenced by the polarity of its environment. When Nile Red was added to solutions of alpha 1-acid glycoprotein (Orosomucoid. OMD), it showed an enhancement in fluorescence intensity and a shift to blue in emission wavelength, suggesting it was binding hydrophobically to a non-polar site on the protein. The association constant (12,261,000 +/- 900,000 M-1) and number of binding sites (0.746 +/- 0.044) were calculated for the probe. Upon addition of both acidic and basic drugs, the Nile Red fluorescence reverted to its unbound form, indicating that OMD probably has one high-affinity, wide and flexible binding area for such drugs. Possible enantiomeric selectivity was shown with ephedrine, and the association constant determined for a racemic mixture of propranolol was found to be comparable to other values obtained with alternative, more conventional techniques.

Propafenone binding interaction with human alpha 1-acid glycoprotein: assessing experimental design and data evaluation

Binding data on racemic RS-propafenone as well as individual R- and S-drug enantiomers interacting reversibly with human alpha 1-acid glycoprotein, as obtained by a high-performance liquid chromatographic method, are evaluated according to three different approaches introduced, respectively, by Scatchard, Bjerrum, and by Tobler and Engel. A non-linear curve-fitting procedure was applied to compute the binding parameters exclusively for the binary system comprising the examined protein and R- and S-propafenone, individually. The exactness of the study design rather than the numerical values were the focus of attention in the evaluation of the data found.

Effects of turpentine oil pretreatment on beta-blocker pharmacokinetic parameters in rats

Turpentine oil treatment (0.2 mL kg-1, s.c.) was used to increase the plasma concentration of alpha 1-acid glycoprotein (0.13 mg mL-1 in control rats) to 1.72 mg mL-1 after 2 days, and allow assessment of its effects on the pharmacokinetics and stereoselective binding of three beta-blockers. Racemates (5 mg kg-1) were administered intravenously to control and turpentine oil-pretreated rats and the plasma concentrations were determined up to 90 min. Stereoselective analysis showed the apparent distribution volume and the area under plasma concentration-time curves (AUC) of R-(+)-propranolol to be, respectively, one-quarter and twice those of the S-(-)-enantiomer and differences in pharmacokinetic parameters between the two were magnified by turpentine oil pretreatment. Pharmacokinetic parameters of oxprenolol enantiomers were essentially similar for the controls but after turpentine oil pretreatment, a higher affinity of the R-(+)-enantiomer for plasma was observed. Acebutolol enantiomers behaved non-stereospecifically throughout. These results were consistent with predictions from the in-vitro stereospecific binding properties of these agents to purified rat alpha 1-acid glycoprotein.

Chiral high-performance liquid chromatographic separations of vinca alkaloid analogues on alpha 1-acid glycoprotein and human serum albumin columns

Separations of the stereoisomers of a series of tetracyclic and pentacyclic vinca alkaloid analogues having two or three chiral centres were performed on Chiral-AGP and Chiral-HSA high-performance liquid chromatographic columns. Phosphate buffers with pH 5-7 containing 5-35% acetonitrile or 2-propanol were used as mobile phases. The results were in accordance with previous binding data obtained with native AGP and on an HSA-Sepharose column. Whereas on Chiral-AGP the retention of the trans isomers having 1(R),12b(S)-indolo[2,3-a]quinolizidine or the corresponding 3(S),16(R)-eburnane absolute configurations was exceedingly high, on Chiral-HSA the trans isomers, independently of their absolute configurations, were more retained. Eburnane-type compounds could also be separated according to the configuration of the chiral centre at position 14. A comparison of the chromatographic properties of the vinca alkaloids on the Chiral-AGP and Chiral-HSA columns demonstrates that these compounds are bound with higher affinity to the AGP phase. The AGP column resolves a very broad range of vinca alkaloids compared with the HSA column. Higher stereoselectivity and a much better chromatographic performance were also obtained on the Chiral-AGP column.

Stereoselective binding of tertatolol and of 4-hydroxytertatolol to human plasma proteins

The binding of racemic tertatolol and 4-hydroxytertatolol and of their enantiomers was compared in alpha 1-acid glycoprotein and albumin solutions. The binding rate of S(-)tertatolol to alpha 1-acid glycoprotein was much greater than that of R(+)tertatolol, the binding of the racemate being intermediate. It was the reverse for the binding to albumin, although the differences were slight. The binding of 4-hydroxytertatolol racemate and enantiomers was very low as compared to the binding of tertatolol, and there were no statistically significant differences in the binding of the 4-hydroxytertatolol enantiomers to either alpha 1-acid glycoprotein or albumin.

Binding of vinca alkaloid analogues to human serum albumin and to alpha 1-acid glycoprotein

The binding of a series of vinca alkaloid analogues having eburnane or indolo[2,3-a]quinolizidine skeletons was studied with human serum albumin (HSA) by affinity chromatography and with alpha 1-acid glycoprotein by means of competition experiments. On HSA the binding occurs at the benzodiazepine-indole binding site via hydrophobic interaction and shows slight stereoselectivity preferring the trans isomers. The binding to alpha 1-AGP proved to be highly stereoselective in favour of the trans isomers having 3(S),16(R)eburnane or 1(R),12b(S)indolo[2,3-a]quinolizidine absolute configurations.

Interaction of propafenone enantiomers with human alpha 1-acid glycoprotein

The interaction of propafenone enantiomers with human alpha 1-acid glycoprotein was studied using high-performance liquid chromatography. Each of the two optical antipodes interacted with one class of high-affinity binding sites characterized by Ka(R) = (6.18 +/- 0.93) x 10(5) M-1, n(R) = 1.34 +/- 0.09 for the (R)-isomer and Ka(S) = (8.93 +/- 1.82) x 10(5) M-1, n(S) = 0.99 +/- 0.08 for the (S)-isomer. Nonspecific binding to secondary low-affinity high-capacity binding site(s) was only slightly greater in the case of the (S)-enantiomer (n'k'(S) = (1.06 +/- 0.09) x 10(4) M-1) compared to the (R)-enantiomer (n'k'(R) = (6.87 +/- 0.72) x 10(3) M-1). It was concluded that both enantiomers interact with common single class of high-affinity binding sites on AAG (along with nonspecific binding) exhibiting only slight stereoselectivity for propafenone.

Separation of the enantiomers of beta-receptor blocking agents and other cationic drugs using a CHIRAL-AGP column. Binding properties and characterization of immobilized alpha 1-acid glycoprotein

The effect of the immobilization procedure on the conformation of alpha 1-acid glycoprotein (AGP) was investigated by recording the fluorescence spectra of native and immobilized AGP. A 20-nm red shift was obtained for the immobilized form of AGP compared with the emission maximum of 338 nm obtained for native AGP. This demonstrates that the tryptophan residues are exposed on the protein surface after immobilization, indicating that the immobilized form of AGP has a more unfolded structure than the native AGP. The effect of N,N-dimethyloctylamine on the enantioselectivity for some fentiazine derivatives, observed with immobilized AGP, was equal to that obtained with AGP as a chiral complexing agent in the mobile phase. This demonstrates that even though the immobilization procedure affects the conformation of the protein there still exist large similarities between native and immobilized AGP concerning chiral recognition. The adsorption isotherm of (-)-terodiline was studied by use of the breakthrough technique. The adsorption isotherm indicates that (-)-terodiline is adsorbed to one site with high affinity and at least one more site with lower affinity. It was also observed that the enantiomers of amines, acids and non-protolytic compounds compete with the cationic compound, (-)-terodiline, for binding to the same sites. The beta-receptor blocking agents atenolol, metoprolol, pindolol, alprenolol, oxprenolol and propranolol were resolved on a CHIRAL-AGP column. The retention and enantioselectivity are highly influenced by the structure of the solute and the nature of the uncharged mobile phase additives. Separation factors of 1.2-1.8 were obtained for the beta-blockers under the studied conditions.

Rapid high-performance liquid chromatographic method for the measurement of the enantiomers of metoprolol in serum using a chiral stationary phase

Metoprolol, a beta-adrenergic blocker, is only available as a racemic mixture for clinical use. A high-performance liquid chromatographic method for the separation of the optical isomers (enantiomers) in human serum is described utilizing a commercially available column with a cellulose tris (3,5-dimethylcarbamate) chiral stationary phase. Separation of the enantiomers is accomplished without precolumn derivatization using a mobile phase of hexane-2-propanol-diethylamine (91:8:1, v/v). The method can be successfully applied to pharmacokinetic studies in man.

Different binding of propranolol enantiomers to human alpha 1-acid glycoprotein

The binding of propranolol enantiomers to human alpha 1-acid glycoprotein was studied using high performance liquid chromatography in order to provide insight into binding models and to describe individual binding parameters of both enantiomers. The binding of (-)-propranolol was shown to be saturable with one major binding site (n = 0.81, k = 2.73 x 10(5)/M). The saturation process achieved its upper asymptotic value at drug/protein molar ratio of approximately 1. In the case of the opposite (+)-enantiomer the binding isotherm did not show evidence of saturation even at higher drug/protein molar ratios (up to 50). The individual binding parameters for (+)-enantiomer were n = 0.38, k = 3.4 x 10(6)/M and n'k' = 1.39 x 10(4)/M for the saturable and nonsaturable binding component, respectively. At drug/protein molar ratio 2 the circular dichroism measurements confirmed the existence of different binding models for individual propranolol enantiomers.

Inverse stereoselectivity in the binding of acenocoumarol to human serum albumin and to alpha 1-acid glycoprotein

Stereoselective binding of rac-acenocoumarol to human serum albumin (HSA) and alpha 1-acid glycoprotein (alpha 1-AGP) was investigated by affinity chromatography and by combined ultrafiltration (UF) and circular dichroism (CD) methods. For HSA, the ratio of the enantiomeric constants was KR/KS = 2, while for alpha 1-AGP, KS/KR = 3.

Alpha 1-acid glycoprotein high-performance liquid chromatography column (EnantioPAC) as a screening tool for protein binding

An attempt was made to correlate retention behavior on a high-performance liquid chromatographic (HPLC) system employing an immobilized alpha 1-acid glycoprotein (AAG) column with AAG binding behavior for various compounds. Protein binding was assessed by propranolol displacement studies in an equilibrium dialysis system using isolated AAG. HPLC retention behavior poorly correlated with propranolol displacement from AAG. This system is not suitable for use as a screening tool for AAG affinity.