Binding of coumarins to site I of human serum albumin. Effect of the fatty acids

Human Serum Albumin Immobilized On Magnetizable Beads: A Rapid Method for Compound HSA Binding Study

Immobilized human serum albumin (HSA) was developed by coupling His-tagged HSA onto Ni2+-coupled magnetizable beads (HSA-beads), allowing the HSA to be easily removed from incubation components. The HSA-beads system provides a rapid and convenient method to study HSA compound binding. In this study, the HSA-beads system was characterized and evaluated as a tool for assessing compound HSA binding properties. The free fraction (fu) values of test compounds measured using HSA-beads were comparable to those determined by equilibrium dialysis (ED), which is commonly used to evaluate albumin binding in vitro. The equilibrium dissociation constant (Kd) values determined for a series of compounds using the HSA-beads method demonstrated good correlation with literature data. This good correlation also suggests that the binding of His-HSA to the beads does not impact the conformations of the two compound binding sites of HSA, as the range of compounds tested encompassed binding to both sites. Furthermore, the Kd values of representative compounds itraconazole and BIRT2584 that were difficult to assess using ED, due to significant cellulose membrane adsorption, were successfully determined. The HSA-beads provide several advantages over ED, such as simple preparation, short assay incubation duration, and the ability to quantify both free and HSA-bound species of the test compound, facilitated by the simple separation of HSA-beads from the solution phase using a magnetic field. These properties render the HSA-beads method suitable for high-throughput studies on compound HSA binding.

Using Human Serum Albumin Binding Affinities as a Proactive Strategy to Affect the Pharmacodynamics and Pharmacokinetics of Preclinical Drug Candidates

We report on a new preclinical drug optimization strategy that measures drug candidates' binding affinity with human serum albumin (HSA) as an assessment of increasing or decreasing serum T1/2. Three common scaffolds were used as drug prototypes. Common polar and nonpolar substituents attached to the scaffolds have been identified as opportunities for increasing or decreasing the HSA binding affinity. This approach of adjusting HSA binding could be proactively established for preclinical drug candidates by appending functionality to sites on the drug scaffold not involved in biological target interactions. This strategy complements other medicinal chemistry efforts to identify longer or shorter human dosing regimens.

Synthesis and biological evaluation of 3-(4-aminophenyl)-coumarin derivatives as potential anti-Alzheimer's disease agents

The work is focused on the design of drugs that prevent and treat Alzheimer’s disease (AD) and its complications. A series of 3–(4-aminophenyl)-coumarin derivatives designed, synthesised, fully characterised and evaluated in vitro/vivo. The biological assay experiments showed that some compounds displayed a clearly selective inhibition for acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). Among all compounds, compound 4m exhibited the highest AChE inhibition with an IC₅₀ value of 0.091 ± 0.011 µM and compound 4k exhibited the highest BuChE inhibition with an IC₅₀ value of 0.559 ± 0.017 µM. A zebrafish behaviour analyser (Zebrobox) was used to determine the behavioural effects of the active compound on the movement distance of the aluminium chloride-induced zebrafish. Compound 4m offered a potential drug design concept for the development of therapeutic or preventive agents for AD and its complications.

Evaluation of blood-brain barrier penetration and examination of binding to human serum albumin of 7-O-arylpiperazinylcoumarins as potential antipsychotic agents

The delivery of drugs to the brain is complicated by the multiple factors including low blood-brain barrier (BBB) passive permeability, active BBB efflux systems, and plasma protein binding. Thus, a detailed understanding of the transport of the new potent substances through the membranes is vitally important and their physico-chemical characteristics should be analyzed at first. This work presents an evaluation of drug likeness of eight 7-O-arylpiperazinylcoumarin derivatives with high affinity towards serotoninergic receptors 5-HT_1A and 5-HT_2A with particular analysis of the requirements for the CNS chemotherapeutics. The binding constants to human serum albumin (HSA) were determined at physiological pH using fluorescence spectroscopy, and then their mode of action was explained by analysis of theoretical HSA complexes. Dynamic simulation of systems allowed for reliable evaluation of the interaction strength. The analyzed coumarins were able to pass BBB, and they present good drug likeness properties. They showed high affinities to HSA (log K_Q = 5.3-6.0 which corresponds to -8.12 to -7.15 kcalmol^-1 of Gibbs free energy). The changes of the emission intensity upon binding to HSA were scrutinized showing the different mode of action for 4-phenylpiperazinylcoumarins. The values of computed Gibbs free energy and determined on the basis of experimentally obtained binding constants log K_Q coincide suggesting a good quality of the theoretical model. Overall the 8-acetyl-7-O-arylpiperazinyl-4-methylcoumarin derivatives represent valuable lead compounds to be further tested in various preclinical assays as a possible chemotherapeutics against CNS diseases. Studied coumarins can be metabolized by cytochrome P450 to aldehydes and hydroxy derivatives. The existence of other binding sites inside HSA than Sudlow's site 1 was postulated. The longer aliphatic linker between coumarin and piperazine moieties favored binding to HSA in other than Sudlow site 1 pocket.

In Vivo Gold Complex Catalysis within Live Mice

Metal complex catalysis within biological systems is largely limited to cell and bacterial systems. In this work, a glycoalbumin-Au^III complex was designed and developed that enables organ-specific, localized propargyl ester amidation with nearby proteins within live mice. The targeted reactivity can be imaged through the use of Cy7.5- and TAMRA-linked propargyl ester based fluorescent probes. This targeting system could enable the exploitation of other metal catalysis strategies for biomedical and clinical applications.

Location of high and low affinity fatty acid binding sites on human serum albumin revealed by NMR drug-competition analysis

Human serum albumin (HSA) is an abundant plasma protein that transports a wide variety of drugs and endogenous compounds. The complex binding capacity of HSA has made it a challenging system to study in detail but in order to develop our understanding of the interactions between ligands for HSA, the locations and relative affinities of different ligand binding sites must be determined. Albumin possesses multiple binding sites for its primary physiological ligand, non-esterified fatty acids (FA). Previously, titration of BSA with (13)C-labeled FA revealed multiple chemical shifts and allowed identification of a subset of three chemical shifts that were associated with high affinity FA binding. Recent crystallographic studies of HSA have mapped at least seven FA binding sites for long-chain FA and delineated the overlap with binding sites for drugs and other endogenous compounds. We aim to correlate NMR and structural data for FA to provide a more complete description of the binding capacity of HSA. Our recent mutagenesis studies allowed us to identify two high affinity binding sites in domain III of HSA. Here, we use NMR to study the binding of (13)C-carboxyl labeled palmitate to HSA in the presence and absence of competitor ligands to complete the correlation of NMR chemical shifts with specific structural binding sites. We carefully selected ligands with specific binding sites identified by crystallography and used them, either singly or in combination, to compete with [(13)C]palmitate for binding to HSA. We show that FA sites 2, 4 and 5 bind FA with high affinity, while sites 1, 3, 6 and 7 exhibit low affinity for FA, thus providing the first complete determination of relative affinities of all seven long-chain FA sites on HSA. Our results also yield direct insights into the interactions between FA and other ligands.

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.

Interaction of ochratoxin A with human serum albumin. Binding sites localized by competitive interactions with the native protein and its recombinant fragments

Competitive interactions of ochratoxin A (OTA) and several other acidic compounds were utilized to gain insight into the localization of binding sites and the nature of binding interactions between anionic species and human serum albumin (HSA). Depolarization of OTA fluorescence in the presence of a competing anion was used to quantify ligand-protein interactions. The results obtained were rationalized in terms of OTA displacement from its major binding site. Based on their ability to displace OTA, two distinct groups of the anionic ligands were revealed. The first group contained structurally diverse compounds that shared a common binding site in subdomain IIA (Sudlow Site I). The second group consisted of three non-steroidal anti-inflammatory drugs, which showed much lower affinity to Site I than the OTA dianion. The major site for these drugs was located in domain III. Fluorescence spectroscopy measurements of OTA, warfarin (WAR) and naproxen (NAP) complexes with recombinant proteins corresponding to the domains of HSA (D1-D3) revealed binding to all domains but with different affinities. The binding constants for OTA and WAR decreased in the series D2z.Gt;D3>D1. In contrast, NAP showed the most favorable interaction with D3 and comparable affinities to the two remaining domains. The OTA binding constant for D2, 7.9 x 10(5) M(-1), was smaller than the largest constant for HSA by a factor of approximately 7. The binding constant for OTA with D3, 1.1 x 10(5) M(-1), was very close to that of the secondary binding site for HSA.

High-resolution and high-throughput protocols for measuring drug/human serum albumin interactions using BIACORE

Characterizing how chemical compounds bind to human serum albumin (HSA) is essential in evaluating drug candidates. Using warfarin as a test system, we validate the application of BIACORE SPR biosensors to reliably determine binding constants for drug/HSA interactions. The binding responses for warfarin over HSA surfaces were extremely reproducible even though warfarin is small compared to the size of the immobilized protein. At high concentrations, warfarin bound at more than one site on HSA, which is consistent with its known binding properties. The affinity we determined for the high-affinity site (K(25 degrees C)(d) = 3.7 +/- 1.2 microM), as well as the dissociation rate constant (k(25 degrees C)(d) = 1.2 s(-1)), are also consistent with binding constants determined previously. These results validate the biosensor technology and illustrate how BIACORE can be used to study drug/HSA interactions in a high-resolution mode. Using a set of 10 test compounds, we present a protocol for determining equilibrium dissociation constants for HSA in a high-throughput mode. Our method involves working at low compound concentrations and fitting the equilibrium data for all compounds simultaneously. We show that the % bound values determined by SPR correlate with the values determined by solution-based methods. The ability to examine directly the binding of small molecules (130-800 Da), coupled with minimal sample requirements and automated instrumentation, makes BIACORE technology applicable for evaluating drug/HSA interactions.

Data plotting of warfarin binding to human serum albumin

The binding of warfarin to human serum albumin was studied by equilibrium dialysis at pH 7.4 in a 67 mM sodium phosphate buffer at 37 degrees C. The equilibrium data were analysed using a computer program for curve fitting. The analysis was made fitting the data to equations for one, two and three classes of binding sites with one, two and three sites at the primary binding site (n(1)=1, 2 or 3). The data fitting was acceptable for two and three classes of binding sites but the best fit was obtained with the equation for two classes of binding sites, allowing us to define the binding by a model with two independent classes of binding sites on the serum albumin molecule.

Biosensor analysis of the interaction between immobilized human serum albumin and drug compounds for prediction of human serum albumin binding levels

The interactions between a set of drugs, selected on the basis of reported human serum albumin (HSA) binding levels, and immobilized HSA were investigated using surface plasmon resonance technology. Major HSA binding sites were available after immobilization. The intensity of the signal obtained from the interaction of the drug with the HSA surface was correlated with the reported HSA binding level. Drugs were classified into groups corresponding to high, medium, or low HSA binding based on the injection of the drug at 80 microM concentration. A set of 10 drugs binding to alpha(1)-acid glycoprotein (AGP) was also investigated and correlated with reported AGP binding data. The throughput of the presented assay is 100 compounds/24 h, and the sample consumption is less than 100 microL (8 nmol).

Study of heterocycle rings binding to human serum albumin

The binding of drugs to human serum albumin determines the drug distribution through the systemic circulation and its pharmacological effects on the organism. Then, with the aim of obtaining information concerning the drug structural features which favour their binding on seroalbumin we have studied the seroalbumin binding to the heterocyclic drugs such as warfarin, propylthiouracil and cromoglycate and to similar compounds such as 4-hydroxy-coumarin, 3-acetylcoumarin, coumarin, benzylthiouracil, propyluracil, thiouracil, chromone and chromanol. These compounds were competitively displaced by warfarin at their primary binding sites on seroalbumin. The comparative analysis of the binding data showed that heterocyclic compounds such as benzopyranes (coumarins and chromanol) and benzyl pyrimidines with 4-hydroxyl groups bind specifically in the warfarin binding site. Then, 4-hydroxyl-bencene heterocycles will displace other ligands from the subdomain IIA of the seroalbumin molecule. Therefore, we can predict that the administration concomitant of warfarin, cromoglycate, propylthiouracil and analogous heterocyclic drugs involves the displacement of the drug without 4-hydroxyl and benzyl groups, increasing their free fraction in serum and the amount of active drug.