Investigation of the Interaction Mode of Phenothiazine Neuroleptics with α1-Acid Glycoprotein

Influence of Desialylation on the Drug Binding Affinity of Human Alpha-1-Acid Glycoprotein Assessed by Microscale Thermophoresis

Human serum alpha-1-acid glycoprotein (AAG) is an acute-phase plasma protein involved in the binding and transport of many drugs, especially basic and lipophilic substances. The sialic acid groups that terminate the N-glycan chains of AAG have been reported to change in response to numerous health conditions and may have an impact on the binding of drugs to AAG. In this study, we quantified the binding between native and desialylated AAG and seven drugs from different pharmacotherapeutic groups (carvedilol, diltiazem, dipyridamole, imipramine, lidocaine, propranolol, vinblastine) using microscale thermophoresis (MST). This method was chosen due to its robustness and high sensitivity, allowing precise quantification of molecular interactions based on the thermophoretic movement of fluorescent molecules. Detailed glycan analysis of native and desialylated AAG showed over 98% reduction in sialic acid content for the enzymatically desialylated AAG. The MST results indicate that desialylation generally alters the binding affinity between AAG and drugs, leading to either an increase or decrease in Kd values, probably due to conformational changes of AAG caused by the different sialic acid content. This effect is also reflected in an increased denaturation temperature of desialylated AAG. Our findings indicate that desialylation impacts free drug concentrations differently, depending on the binding affinity of the drug with AAG relative to human serum albumin (HSA). For drugs such as dipyridamole, lidocaine, and carvedilol, which have a higher affinity for AAG, desialylation significantly changes free drug concentrations. In contrast, drugs such as propranolol, imipramine, and vinblastine, which have a strong albumin binding, show only minimal changes. It is noteworthy that the free drug concentration of dipyridamole is particularly sensitive to changes in AAG concentration and glycosylation, with a decrease of up to 15% being observed, underscoring the need for dosage adjustments in personalized medicine.

Revisiting Chiral Recognition Mechanism on Chicken Alpha 1-Acid Glycoprotein: Location of Chiral Binding Sites and Insight into Chiral Binding Mechanism

Chiral stationary phases based on chicken alpha 1-acid glycoprotein (cAGP) have been used for enantioseparations of various compounds. However, the chiral binding sites and mechanism have not been clarified yet. Based on chromatographic properties of native and W26-modified cAGP columns and docking simulations of studied compounds into the generated model structure of cAGP, the chiral binding sites were located on cAGP and the chiral binding mechanism was discussed. On cAGP, there existed a binding cavity lined with H25, W26, Y47, R128, T129, D161 and E168, which contribute electrostatic or hydrogen bonding interactions. Benzoin and chlorpheniramine enantiomers interacted with cAGP at almost the same sites a little away from W26, while propranolol enantiomers docked, slightly shifting toward H25 and W26. Furthermore, in addition to hydrophobic interactions, ionic interactions between amino groups of chlorpheniramine enantiomers and a carboxy group of D161 or E168 played an important role in the chiral recognition, while hydrophobic interactions and hydrogen bonding interactions worked for the chiral recognition of benzoin and propranolol enantiomers.

Retarded photooxidation of cyamemazine in biomimetic microenvironments

Cyamemazine (CMZ) is a neuroleptic drug that mediates cutaneous phototoxicity in humans. Here, the photobehavior of CMZ has been examined within α1 -acid glycoproteins, β- and γ-cyclodextrins and SDS micelles. In all these microenvironments, CMZ emission was enhanced and blue-shifted, and its lifetime was longer. Irradiation of the entrapped drug at 355 nm, under air; led to the N,S-dioxide. Within glycoproteins or SDS micelles the reaction was clearly slower than in phosphate buffered solution (PBS); protection by cyclodextrins was less marked. Transient absorption spectroscopy in PBS revealed formation of the triplet state ((3)CMZ*) and the radical cation (CMZ(+•)). Upon addition of glycoprotein, the contribution of CMZ(+•) became negligible, whereas (3)CMZ* dominated the spectra; in addition, the triplet lifetime became considerably longer. In cyclodextrins, this occurred to a lower extent. In all microheterogeneous systems, quenching by oxygen was slower than in solution; this was most remarkable inside glycoproteins. The highest protection from photooxidation was achieved inside SDS micelles. The results are consistent with photooxidation of CMZ through photoionization and subsequent trapping of the resulting radical cation by oxygen. This reaction is extremely sensitive to the medium and constitutes an appropriate probe for localization of the drug within a variety of biological compartments.

Drug-binding energetics of human α-1-acid glycoprotein assessed by isothermal titration calorimetry and molecular docking simulations

This study utilizes sensitive, modern isothermal titration calorimetric methods to characterize the microscopic thermodynamic parameters that drive the binding of basic drugs to α-1-acid glycoprotein (AGP) and thereby rationalize the thermodynamic data in relation to docking models and crystallographic structures of the drug-AGP complexes. The binding of basic compounds from the tricyclic antidepressant series, together with miaserine, chlorpromazine, disopyramide and cimetidine, all displayed an exothermically driven binding interaction with AGP. The impact of protonation/deprotonation events, ionic strength, temperature and the individual selectivity of the A and F1*S AGP variants on drug-binding thermodynamics was characterized. A correlation plot of the thermodynamic parameters for all of the test compounds revealed that an enthalpy-entropy compensation is in effect. The exothermic binding energetics of the test compounds were driven by a combination of favorable (negative) enthalpic (∆Hº) and favorable (positive) entropic (∆Sº) contributions to the Gibbs free energy (∆Gº). Collectively, the data imply that the free energies that drive drug binding to AGP and its relationship to drug serum residency evolve from the complex interplay of enthalpic and entropic forces from interactions with explicit combinations of hydrophobic and polar side-chain sub-domains within the multi-lobed AGP ligand binding cavity.

Mechanism of interaction between human serum albumin and N-alkyl phenothiazines studied using spectroscopic methods

The binding characteristics of human serum albumin (HSA) with N-alkyl phenothiazines derivatives (NAP) viz., levomepromazine monomaleate (LMM) and propericiazine (PPC) have been studied by employing fluorescence, absorption, circular dichroism and FT-IR techniques. The Stern-Volmer quenching constant, K(SV) values were found to decrease with increase in temperature thereby indicating the presence of static quenching mechanism in the interactions of NAP with HSA. The number of binding sites, n and the binding constant, K were noticed to be, respectively, 1.11 and (5.188+/-0.034)x10(4) M(-1) for LMM and 1.06 and (4.436+/-0.066)x10(4) M(-1) for PPC at 298 K. The negative value of enthalpy change and positive value of entropy change in the present study indicated that the hydrophobic forces played a major role in the binding of NAP to HSA. The circular dichroism and FT-IR spectral data revealed the conformational changes in the structure of protein upon its interaction with NAP. The binding distances and the energy transfer efficiency between NAP and protein were determined based on Förster's theory of energy transfer. The decreased binding constants of HAS-LMM and HAS-PPC systems in presence of common ions indicated the availability of higher concentration of free drug in plasma.

The drug binding site of human α1-acid glycoprotein: Insight from induced circular dichroism and electronic absorption spectra

Human alpha(1)-acid glycoprotein (AGP) is an important drug binding plasma protein which affects pharmacokinetical properties of various therapeutic agents. For the first time, interpretation of the induced circular dichroism (ICD) spectra of drug-AGP complexes is presented yielding valuable information on the protein binding environment. ICD spectra were obtained by novel ligands of which AGP induced optical activity have never been reported (primaquine, mefloquine, propranolol, terazosin, carbamazepine, rhodamine B) and by re-investigation of ICD spectra of protein-bound drugs published earlier (chlorpromazine, dipyridamole, prazosin). Spectroscopic features of the ICD and absorption bands of drugs combined with native AGP indicated chiral non-degenerate exciton coupling between the guest chromophore and the indole ring of an adjacent tryptophan (Trp) residue. Results of additional CD experiments performed by using recombinant AGP mutants showed no changes in the ligand binding ability of W122A in sharp contrast with the W25A which was unable to induce extrinsic CD signal with either ligand. Thus, these findings unequivocally prove that, likely via pi-pi stacking mechanism, Trp25 is essentially involved in the AGP binding of drugs studied here as well as of related compounds. Survey of the AGP binding data published in the literature support this conclusion. Our results provide a fast and efficient spectroscopic tool to determine the inclusion of ligand molecules into the beta-barrel cavity of AGP where the conserved Trp25 is located and might be useful in ligand-binding studies of other lipocalin proteins.

Spectroscopic Studies and Life Time Measurements of Binding of a Bioactive Compound to Bovine Serum Albumin and the Effects of Common Ions and Other Drugs on Binding

The mechanism of binding of anti-inflammatory drug, nimesulide (NIM) with bovine serum albumin (BSA) was investigated by fluorescence, absorption, circular dichroism (CD) and lifetime measurements under simulative physiological conditions. The analysis of fluorescence data indicated the presence of both dynamic and static quenching mechanism in the binding. Various binding parameters have been evaluated. The CD spectral data revealed the decrease in alpha-helical content of BSA from 70.9% (in free BSA) to 42.03% (in bound form) thereby indicating the conformational change in BSA upon binding. The binding of NIM to BSA was also confirmed by absorption spectra. Based on the Förster's theory of non-radiation energy transfer, the binding average distance, r between the donor (BSA) and acceptor (NIM) was found to be 2.17 nm. The association constants of NIM-BSA decreased in presence of the common ions and other drugs thereby indicating the availability of higher concentration of free drug (NIM) in plasma.

Characterization of benzodiazepine binding site on human α1-acid glycoprotein using flunitrazepam as a photolabeling agent

The binding of flunitrazepam (FNZP) by human alpha1-acid glycoprotein (hAGP) and the relationships between the extent of drug binding and desialylation and the genetic variants of hAGP were examined. The photolabeling specificity of [3H]FNZP was confirmed by findings in which other hAGP-binding ligands inhibited the formation of covalent bonds between [3H]FNZP and hAGP. The photolabeling of asialo-hAGP suggested that sialic acid does not involve in the binding of [3H]FNZP. No difference in the labeling could be found between the F1*S variants and A variant. Similarly, FNZP did not show a difference in binding affinity to the two genetic variants of hAGP. Sequence analysis of the photolabeled peptide indicated a sequence corresponding to Tyr91-Arg105 of hAGP.

Characterization of ligand binding sites on the alpha1-acid glycoprotein in humans, bovines and dogs

The goal of this study was to classify and identify the ligand binding sites on alpha(1)-acid glycoprotein (AGP) from 3 species, in order to understand species differences with respect to both ligand binding properties and ligand interaction on protein binding. These characteristics of human, dog and bovine AGP were examined using the basic ligands chlorpromazine and auramine O, the acidic ligand acenocoumarin, and the steroid hormone progesterone. Ultrafiltration and fluorescence techniques were used to characterize the nature of the interactions, and the data were analyzed according to the method of Kragh-Hansen. Using a model analysis of the interaction, the ligand binding site on human AGP consists of at least 3 partially overlapping subsites: a basic ligand binding site, an acidic ligand binding site and a steroid hormone binding site. Moreover, dog and bovine AGP each have a basic ligand binding site and a steroid hormone binding site, which significantly overlap and affect each other. However, dog and bovine AGPs do not contain an acidic ligand binding region. The results of the fluorescence experiments indicate that the hydrophobic nature of the ligand binding pockets on the 3 AGPs are similar, but that their microviscosities are markedly different.

In vitro study on the interaction of mechanism of tricyclic compounds with bovine serum albumin

The mechanism of interaction of five phenothiazine drugs with bovine serum albumin has been investigated using fluorescence spectroscopy, circular dichroism and equilibrium dialysis methods. It was found that the phenothiazine ring common to all drugs makes major contribution to interaction. However, the nature of alkylamino group at position 10 influences the protein binding significantly. Binding affinities could be related to parachor values of drugs. Stern-Volmer plots indicated the presence of static component in the quenching mechanism. Results also showed that both tryptophan residues of protein are accessible to drug molecules. The high magnitude of rate constant of quenching indicated that the process of energy transfer occurs by intermolecular interaction forces and thus drug-binding site is in close proximity to tryptophan residues of BSA. Binding studies in presence of hydrophobic probe, 8-anilino-1-naphthalein-sulphonic acid showed that there is hydrophobic interaction between drugs and the probe and they do not share common sites in BSA. Fluorescence intensity data in the presence of additives showed that hydrophobic interactions play a significant role. Small decrease in critical micellar concentration of anionic surfactant, sodium dodecyl sulfate in the presence of drugs showed that the ionic character of drugs also contribute to binding. Thermodynamic parameters obtained from data at different temperatures showed that the binding of phenothiazine drugs to BSA involve hydrophobic bonds predominantly. The CD spectrum of BSA in presence of drugs shows that binding of drugs leads to change in the helicity of the protein. The binding of these drugs to BSA based on dialysis experiment has been characterized by association constant (K) and the number of binding sites (n).

Spectroscopic Studies on the Mode of Interaction of an Anticancer Drug with Bovine Serum Albumin

The mechanism of interaction of vinblastin sulphate (VBS) with bovine serum albumin (BSA) has been reported. Association constant for VBS-BSA binding was found to be 3.146+/-0.06 x 10(4) M(-1). Stern-Volmer analysis of fluorescence quenching data showed that the fraction of fluorophore (protein) accessible to the quencher (drug) was close to unity indicating thereby that both tryptophan residues of BSA are involved in drug-protein interaction. The rate constant for quenching, greater than 10(10) M(-1) S(-1), indicated that the drug-binding site is in close proximity to tryptophan residues of BSA. Binding studies in the presence of an hydrophobic probe, 8-anilino-1-naphthalein-sulphonic acid, sodium salt (ANS) indicated that there is hydrophobic interaction between VBS and probe and they do not share common sites in BSA. Thermodynamic parameters obtained from data at different temperatures showed that the binding of VBS to BSA involves predominant hydrophobic forces. The effects of some additives and paracetamol on binding of VBS-BSA have also been investigated. The CD spectrum of BSA in presence of VBS shows that the binding of VBS leads to change in the helicity of BSA.

Hydrophobic and ionic factors in the binding of local anesthetics to the major variant of human alpha1-acid glycoprotein

Understanding the interaction of local anesthetics (LAs) with plasma proteins is essential to understanding their systemic pharmacology and toxicology. The molecular determinants of LA binding to the major variant (F1*S) of human alpha1-acid glycoprotein (AGP) were therefore investigated spectrofluorometrically using whole AGP and a novel, F1*S variant-selective probe previously developed in our laboratory. Equilibrium- competitive displacement of this probe by LAs, observed by the recovery of AGP's fluorescence as the quenching probe was displaced from its high-affinity site, was characterized by inhibitory dissociation constants for the various LAs. The importance of electrostatic factors was assessed by examining the pH dependent binding of an ionizable LA, lidocaine, using the quaternary lidocaine derivative QX-314 [N-(2,6-dimethylphenylcarbamoylmethyl) triethylammonium chloride] to control for pH dependent ionization of AGP. Uncharged lidocaine bound with at least 8 times the affinity of protonated lidocaine (K(D) = 4.0 +/- 0.6 microM and >32 microM, respectively). This result is inconsistent with the current model of the AGP-binding site, which depicts a buried pocket having a negatively charged region that interacts with the amino termini of basic drugs. Consistent with the model, however, two sets of structurally homologous LAs (mepivacaine, ropivacaine, bupivacaine, and lidocaine, RAD-240, RAD-241, RAD-242, L-30, W-6603) demonstrated a strong positive correlation between hydrophobicity (measured as the octanol:buffer partition coefficient of the neutral species) and their free energies of dissociation. Given that the tertiary structure of AGP has proven refractory to resolution, these structure-activity studies should contribute to understanding the nature of the binding site on this important protein.

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.

Single-step isolation method for six glycoforms of human alpha1-acid glycoprotein by hydroxylapatite chromatography and study of their binding capacities for disopyramide

A single-step isolation method for the glycoforms of human serum alpha1-acid glycoprotein (AAG) using a hydroxylapatite column under a gradient elution program was developed. The concentrations of N-acetylneuraminic acid and monosaccharides (fucose, N-acetylglucosamine, galactose and mannose) of six AAG glycoforms were determined by the pulsedamperometric detection method. For each AAG glycoform, significant sex-related differences in carbohydrate content have been observed only for AAG glycoforms two and six, and not for each AAG glycoform. The relationship between the extent of the branch in the glycan chain and the binding capacity to disopyramide were examined. Female AAG contained highly sialylated AAG glycoforms compared to male glycoforms. Conversely, male AAG was rich in the lower sialylated AAG glycoform. Furthermore, it was found that the drug binding capacity decreases with increasing branching of the glycan chain. This suggests that the binding sites of AAG are hindered by a relatively large carbohydrate moiety, such as tetraantennary structures.

Enantioresolution of disopyramide by capillary affinity electrokinetic chromatography with human alpha1-acid glycoprotein (AGP) as chiral selector applying a partial filling technique

A method using alpha1-acid glycoprotein (AGP) as chiral selector for disopyramide by means of affinity electrokinetic chromatography has been developed. In order to avoid UV absorbance interferences, less than the effective length of the capillary was filled with the chiral selector. The electrophoretic conditions were chosen to give opposite migration directions for the chiral selector and the analyte; AGP migrated away from the detector. Enantiomers of disopyramide were separated on a methylcellulose-coated capillary with 20 cm length to the detector. The enantioresolution of the solute was affected by the concentration of the chiral selector, the plug length of the selector in the capillary, and the applied voltage. Resolution factors and migration times decreased with reduction of the plug length, while the efficiency of the separation system and peak performance were improved by decreasing the separation zone. A special feature of the technique is an enhanced selectivity due to increasing separation of the enantiomers when the fastest has migrated from the selector zone, while the second one still is retained. Equations relating selectivity and resolution with the difference in effective plug lengths between the two enantiomers are developed. Optimized conditions yielding complete resolution, requiring an 0.75 mM AGP plug of only 4.5 cm effective length, also gave high efficiencies (about 400,000 plates/m) for both enantiomer peaks.

Effect of sialic acid residues of human alpha 1-acid glycoprotein on stereoselectivity in basic drug-protein binding

The function of sialic acid groups at the terminal of sugar chains of human alpha 1-acid glycoprotein (AGP) was investigated with respect to chiral discrimination between optical isomers of basic drugs, using high-performance capillary electrophoresis/frontal analysis (HPCE/FA), a novel analytical method developed for the determination of unbound drug concentration with ultramicrosample volume (100-200 nl). Native human AGP and desialylated AGP were used as test proteins, and propranolol (PRO) and verapamil (VER) were used as model drugs. The unbound concentration of (S)-VER was 1.31 times higher than that of (R)-VER in native AGP solution. This selectivity was not affected by desialylation. Further, enzymatic elimination of galactose residues, which neighbored sialic acid groups, did not change the binding of either isomer of VER. On the other hand, the unbound concentration of (R)-PRO was 1.27 times higher than that of (S)-PRO in native AGP solution. Desialylation caused the unbound concentration of (S)-PRO to rise to the same level of (R)-PRO, resulting in loss of enantioselectivity. Thus, it follows that sialic acid groups of AGP, as a whole, are not responsible for chiral recognition between enantiomers of VER but are involved in enantioselectivity toward the isomers of PRO.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

Correlation between the short-term measurements of drug accumulation in living cells and the long-term growth inhibition

The basic distinguishing feature of all cells expressing functional P-glycoprotein-multidrug resistance (P-gp-MDR) is a decrease of steady state drug levels as compared to drug-sensitive controls. Recently it has been pointed out that there appears to be a discrepancy between the amount of drug accumulated at steady state by drug-sensitive and highly resistant cells and their degree of resistance. These observations could suggest two things: (1) that factors other than drug accumulation may be important in MDR, (2) that they reflect a discrepancy between the short-term measurements of drug accumulation at 60 min versus long-term (72 hr) growth inhibition. Due to the different experimental conditions and the different type of cells used it is very difficult to compare the literature data. For this reason we have investigated the effect of 12 compounds in overcoming resistance in relation to drug accumulation. We have used a spectrofluorometric method which allows the determination of the nuclear drug accumulation directly on living cells. Our data clearly establish that, at least for the compounds used in that study, there is a very good correlation between their ability to increase drug accumulation, measured at short-term, and their ability to reverse MDR accumulation, measured at short-term, and their ability to reverse MDR, but no correlation with their ability to inhibit protein kinase C activity. In addition, their efficiency to reverse MDR correlates with their pKa values, the efficiency being the highest when the pKa is the lowest.