Testing for drug-human serum albumin binding using fluorescent probes and other methods

Noncovalent Association Thermodynamics of Turn-On Fluorescent Probes with Human Serum Albumin: Dual-Concentration Ratio Method

Efficient quantification of the affinity of a drug and the targeted protein is critical for strategic drug design. Among the various molecules, turn-on fluorescent probes are the most promising signal transducers to reveal the binding strength and site-specificity of designed drugs. However, the conventional method of measuring the binding ability of turn-on fluorescent probes by using the fractional occupancy under the law of mass action is time-consuming and a massive sample is required. Here, we report a new method, called dual-concentration ratio method, for quantifying the binding affinity of fluorescent probes and human serum albumin (HSA). Temperature-dependent fluorescence intensity ratios of a one-to-one complex (L ⋅ HSA) for a turn-on fluorescent probe (L), e. g., ThT (thioflavin T) or DG (dansylglycine), with HSA at two different values of [L]0 /[HSA]0 under the constraint [HSA]0 >[L]0 were collected. The van't Hoff analysis on these association constants further resulted in the thermodynamic properties. Since only two samples at different [L]0 /[HSA]0 are required without the need of [L]0 /[HSA]0 at a wide range, the dual-concentration ratio method is an easy way to greatly reduce the amounts of fluorescent probes and proteins, as well as the acquisition time.

Chiral 8-aminoBODIPY-based fluorescent probes with site selectivity for the quantitative detection of HSA in biological samples

Human serum albumin (HSA) is one of the vital proteins in blood serum, and its optimum level is a reflection of the physiological well-being of an individual. Any abnormalities in serum HSA levels could often be a sign of disguised physiological disorders. The importance of fast and accurate determination of serum HSA levels has led to the development of various quantification methods. Among these, fluorescence-based methods employ molecular probes capable of producing selective responses on interaction with HSA. Herein, we report chiral 8-aminoBODIPY-based probes having blue emission for the quantitative detection of HSA in buffer and human blood serum. A pair of 8-aminoBODIPY enantiomers, namely R-PEB and S-PEB, were synthesized. They exhibited a fast 'turn-on' fluorescence response towards HSA, allowing its detection and quantification. In PBS buffer, R-PEB and S-PEB showed very good sensitivity with a limit of detection (LoD) of 25 nM (K_D = 9.84 ± 0.14 μM) and 39 nM (K_D = 18.67 ± 0.21 μM), respectively. The linear relationship observed between the fluorescence intensity of R-PEB/S-PEB and the HSA concentration in serum samples allowed us to generate a reference curve for HSA estimation for practical applications. Examination of unknown serum samples showed a good correlation with the results obtained by the benchmark BCG method. Interestingly, the difference in these probes' dissociation constants and LoD indicated their differential binding to HSA. Considering the availability of multiple ligand binding sites in HSA, their binding preferences were investigated in detail by displacement assays using site-specific drugs. These studies showed the preferential affinity of R-PEB towards site II, which was further substantiated using molecular docking studies. However, these displacement assays could not identify the preferred binding site of S-PEB. Blind docking studies indicated that S-PEB occupied a site closer to FA5. Selective binding of R-PEB to site II and its characteristic photophysical response can be utilized to quickly screen potential site II binding drugs.

Evaluating the Merit of a Syringol Derived Fluorophore as a Charge Transfer Probe for Detection of Serum Albumins

In this article a syringol-π-benz[e]indolium based donor-acceptor fluorophore has been reported. The fluorophore shows a solvent polarity dependent change in the absorption and emission spectra in solution. A combined spectroscopic and time dependent density functional theory (TDDFT) studies reveal higher dipole moment of the fluorophore in the excited state, resulting positive solvatochromism. In physiological pH, the phenol group in the fluorophore is easily deprotonated owing to the electron pulling effect of the substituents. Consequently, the phenolate (PhO-) becomes a strong active donor in the new donor-acceptor pair. In aqueous solution, the new phenolate fluorochrome shows negligible fluorescence due to energy loss via non-radiative pathways from the low-lying polar excited states. The fluorochrome can detect human and bovine serum albumins in physiological buffer solution with high selectivity. The underlying mechanism of human serum albumin (HSA) detection was estimated to be strong (1.46 × 105 M-1, ΔG = -7.05 kcal/mol) supramolecular complexation between the fluorophore and albumin in hydrophobic binding site III-B. The linear relationship between fluorescence intensity and HSA concentration extends from 40 mg/L to an impressive upper limit (540 mg/L), thereby opening an opportunity for albumin detection in a broad range of health conditions. The practical applicability of the fluorophore was tested in spiked urine samples and a good correlation was observed between fluorescence intensity and the concentration of human serum albumin in neutral aqueous samples.

Ligand binding constants for human serum albumin evaluated by ratiometric analysis of DSC thermograms

This paper describes an expanded application of our recently reported method (Eskew et al., Analytical Biochemistry 621,1 2021) utilizing thermogram signals for thermal denaturation measured by differential scanning calorimetry. Characteristic signals were used to quantitatively evaluate ligand binding constants for human serum albumin. In our approach the ensemble of temperature dependent calorimetric responses for various protein-ligand mixtures and native HSA were compared, in a ratiometric manner, to extract binding constants and stoichiometries. Protein/ligand mixtures were prepared at various ligand concentrations and subjected to thermal denaturation analysis by calorimetry. Measurements provided the melting temperature, T_m, and free-energy ΔG_cal(37°C) for melting ligand-bound Albumin as a function of ligand concentration. Concentration dependent behaviors of these parameters derived from protein/ligand mixtures were used to construct dose-response curves. Fitting of dose-response curves yielded quantitative evaluation of the ligand binding constant and semi-quantitative estimates of the binding stoichiometry. Many of the ligands had known binding affinity for Albumin with binding constants reported in the literature. Evaluated binding parameters for the ligands impressively agreed with reported literature values determined using other standard experimental methods. Results are reported for 29 drug ligands binding to Albumin. These validate our calorimetry-based process for applications in pre-clinical drug screening.

Probing the interactions of novel europium coordination complexes with serum albumin

Molecular interactions between novel europium coordination complexes (EC) possessing superior cytotoxic activity and bovine serum albumin (BSA), the most prominent representative of plasma proteins, were assessed using fluorescence spectroscopy and molecular docking techniques. Cumulative results from fluorescent probe binding, fluorescence quenching and Förster resonance energy transfer studies revealed that the europium complexes V4 and V8 do not perturb the BSA structure, while V3, V5, and V7 induce partial unfolding of the polypeptide chain. Molecular docking studies coupled with analysis of the three-dimensional structure of the BSA-EC complexes showed that V4 and V8 reside in the vicinity of the protein IIA subdomain (Sudlow's site I), while V3, V5 and V5 were localized predominantly in the BSA IIIA subdomain (Sudlow's site II). Due to the intactness of the protein structure upon association with V4 and V8, these compounds may be recommended for further evaluation as potential antineoplastic agents.

Testing for Ketoprofen Binding to HSA Coated Magnetic Nanoparticles under Normal Conditions and after Oxidative Stress

Binding and transport of ligands is one of the most important functions of human blood serum proteins. Human serum albumin is found in plasma at the highest concentration. Because of this, it is important to study protein–drug interactions for this albumin. Since there is no single model describing this interaction, it is necessary to measure it for each active substance. Drug binding should also be studied in conditions that simulate pathological conditions of the body, i.e., after oxidative stress. Due to this, it is expected that the methods for testing these interactions need to be easy and fast. In this study, albumin immobilized on magnetic nanoparticles was successfully applied in the study of protein–drug binding. Ketoprofen was selected as a model drug and interactions were tested under normal conditions and artificially induced oxidative stress. The quality of obtained results for immobilized protein was confirmed with those for free albumin and literature data. It was shown that the type of magnetic core coverage does not affect the quality of the obtained results. In summary, a new, fast, effective, and universal method for testing protein–drug interactions was proposed, which can be performed in most laboratories.

Human Serum Albumin Binding in a Vial: A Novel UV-pH Titration Method To Assist Drug Design

The knowledge on human serum albumin (HSA) binding is of utmost importance as it affects pharmacokinetic behavior and bioavailability of drugs. In this article, we report a novel method to screen for ionizable molecules with high HSA binding affinity based on pK_a shifts using UV-pH titration. We investigated the HSA binding of 27 drugs and compared the results to experimental data from conventional methods. In most cases, significant shifts (ΔpK_a > 0.1) were observed for drugs with high HSA binding, while no change could be detected for low-affinity binders. We showed the pivotal role of ionization centers in the formation of strong interactions between drug and HSA using molecular docking studies. We also verified our findings by testing five modified analogues designed by structural considerations. Significant decreases in their HSA binding proved that the UV-pH titration method combined with an in silico support can be used as a medicinal chemistry tool to assist rational molecular design.