Quantitative assessment of silicone oil release with siliconized and silicone oil-free syringes by microflow imaging microscopy / Avaliação quantitativa da liberação de óleo de silicone por seringas siliconizadas e sem óleo de silicone usando microflow imaging microscopy

ABSTRACT Purpose: Since particles are released in syringes during intravitreal injections, we assessed them quantitatively after agitating syringes commonly used for intravitreal injections. Methods: With and without agitation, the SR 1-ml insulin, Becton-Dickinson Ultra-Fine 0.3-ml Short Needle with a half-unit scale, HSW Norm-Ject Tuberculin, and Becton-Di­ckinson 1-ml Luer Lok Tip were examined with buffer and bevacizumab, aflibercept, and ziv-aflibercept. Flow imaging microscopy was performed to assess the particle numbers, concentrations, morphology, and size distribution. Results: Using the Becton-Dickinson Ultra-Fine syringe, the average particle count after agitation was higher than in the no-agitation group. For particles greater than 10 and 25 µm, differences were observed using the SR syringe between the two studied conditions. There were no significant differences in the means for the other syringes. Without agitation, the SR syringe had the highest number of particles (2,417,361.7 ± 3,421,575.5) followed by the Becton-Dickinson Ultra-Fine with 812.530,9 ± 996.187,2. The Becton-Dickinson Luer Lok Tip and HSW Norm-Ject performed equally with 398,396.8 ± 484,239.2 and 416,016.4 ± 242,650.1 particles, respectively. Conclusions: Flicking syringes to eliminate air bubbles results in increased numbers of particles released during intravitreal injections into the human vitreous.

RESUMO Objetivo: Visto que partículas são liberadas nas seringas durante as injeções intravítreas (IVIs), estas foram avaliadas quantitativamente após a agitação das seringas mais comumente usadas para injeções intravítreas. Métodos: A seringa SR de 1 ml de insulina, a agulha curta Becton-Dickinson Ultra-Fine 0,3 ml com escala de meia unidade, HSW Norm-Ject Tuberculin e a Becton-Dickinson Luer Lok Tip de 1 ml foram estudadas com placedo e com bevacizumabe, aflibercept e ziv-aflibercept, com e sem agitação. MicroFlow Imaging Mi­croscopy foi realizada para avaliar o número de partículas, con­centração, morfologia e distribuição das mesmas por tamanho. Resultados: A contagem média de partículas após agitação foi maior do que no grupo sem agitação usando a seringa Becton-Di­ckinson Ultra-Fine. Diferenças foram observadas usando a seringa SR entre as duas condições estudadas para partículas maiores que 10 e 25 µm. Para as demais seringas, não foram observadas diferenças significativas nas médias. A seringa SR apresentou o maior número de partículas sem agitação (2.417.361,7 ± 3.421.575,5) seguida da Becton-Dickinson Ultra-Fine com 812.530,9 ± 996.187,2. A BD Luer Lok Tip e a HSW Norm-Ject se comportaram de forma semelhante com 398.396,8 ± 484.239,2 e 416.016,4 ± 242.650,1 partículas, respectivamente. Conclusões: Agitar seringas para remover bolhas de ar resulta em um maior número de partículas liberadas durante Becton-Dickinson no vítreo humano.

Intrinsic affinity of protein - ligand binding by differential scanning calorimetry.

Differential scanning calorimetry (DSC) determines the enthalpy change upon protein unfolding and the melting temperature of the protein. Performing DSC of a protein in the presence of increasing concentrations of specifically-binding ligand yields a series of curves that can be fit to obtain the protein-ligand dissociation constant as done in the fluorescence-based thermal shift assay (FTSA, ThermoFluor, DSF). The enthalpy of unfolding, as directly determined by DSC, helps improving the precision of the fit. If the ligand binding is linked to protonation reactions, the intrinsic binding constant can be determined by performing the affinity determination at a series of pH values. Here, the intrinsic, pH-independent, affinity of acetazolamide binding to carbonic anhydrase (CA) II was determined. A series of high-affinity ligands binding to CAIX, an anticancer drug target, and CAII showed recognition and selectivity for the anticancer isozyme. Performing the DSC experiment in buffers of highly different enthalpies of protonation enabled to observe the ligand unbinding-linked protonation reactions and estimate the intrinsic enthalpy of binding. The heat capacity of combined unfolding and unbinding was determined by varying the ligand concentrations. Taken together, these parameters provided a detailed thermodynamic picture of the linked ligand binding and protein unfolding process.

Quantitative assessment of silicone oil release with siliconized and silicone oil-free syringes by microflow imaging microscopy.

PURPOSE: Since particles are released in syringes during intravitreal injections, we assessed them quantitatively after agitating syringes commonly used for intravitreal injections. METHODS: With and without agitation, the SR 1-ml insulin, Becton-Dickinson Ultra-Fine 0.3-ml Short Needle with a half-unit scale, HSW Norm-Ject Tuberculin, and Becton-Di-ckinson 1-ml Luer Lok Tip were examined with buffer and bevacizumab, aflibercept, and ziv-aflibercept. Flow imaging microscopy was performed to assess the particle numbers, concentrations, morphology, and size distribution. RESULTS: Using the Becton-Dickinson Ultra-Fine syringe, the average particle count after agitation was higher than in the no-agitation group. For particles greater than 10 and 25 µm, differences were observed using the SR syringe between the two studied conditions. There were no significant differences in the means for the other syringes. Without agitation, the SR syringe had the highest number of particles (2,417,361.7 ± 3,421,575.5) followed by the Becton-Dickinson Ultra-Fine with 812.530,9 ± 996.187,2. The Becton-Dickinson Luer Lok Tip and HSW Norm-Ject performed equally with 398,396.8 ± 484,239.2 and 416,016.4 ± 242,650.1 particles, respectively. CONCLUSIONS: Flicking syringes to eliminate air bubbles results in increased numbers of particles released during intravitreal injections into the human vitreous.

Effects of Transportation of IV Bags Containing Protein Formulations Via Hospital Pneumatic Tube System: Particle Characterization by Multiple Methods.

In hospitals, often drug products in intravenous (IV) bags are transported via pneumatic tube systems (PTS). The goal of this study was to evaluate the effects of such transportation of protein products on particle formation in polyvinyl chloride (PVC) and polyolefin (PO) IV bags, containing either IV saline or dextrose. We studied intravenous immunoglobulin (IVIG) and a monoclonal antibody (mAb). Particles were quantified with flow imaging, light obscuration and nanoparticle tracking analysis. PTS transportation of IVIG caused large increases in protein particle concentrations, with much greater increases observed in saline than in dextrose. The increases were greater in IV solutions in PO than those in PVC bags. With the mAb, PTS transportation in saline caused increases in protein particle levels in PO bags, but not in PVC bags. Transportation in dextrose did not result in significant increases in mAb particle concentrations in IV bags made of either material. Overall, the results document that the PTS transportation can result in large increases in protein particles and that magnitude of these increases depends the protein itself, the bag material and the IV solution. The main conclusion is that protein products in IV solutions should not be transported in hospital PTS.

Uncertainty in protein-ligand binding constants: asymmetric confidence intervals versus standard errors.

Equilibrium binding constants (Kb) between chemical compounds and target proteins or between interacting proteins provide a quantitative understanding of biological interaction mechanisms. Reported uncertainties of measured experimental parameters are critical for decision-making in many scientific areas, e.g., in lead compound discovery processes and in comparing computational predictions with experimental results. Uncertainties in measured Kb values are commonly represented by a symmetric normal distribution, often quoted in terms of the experimental value plus-minus the standard deviation. However, in general, the distributions of measured Kb (and equivalent Kd) values and the corresponding free energy change ΔGb are all asymmetric to varying degree. Here, using a simulation approach, we illustrate the effect of asymmetric Kb distributions within the realm of isothermal titration calorimetry (ITC) experiments. Further we illustrate the known, but perhaps not widely appreciated, fact that when distributions of any of Kb, Kd and ΔGb are transformed into each other, their degree of asymmetry is changed. Consequently, we recommend that a more accurate way of expressing the uncertainties of Kb, Kd, and ΔGb values is to consistently report 95% confidence intervals, in line with other authors' suggestions. The ways to obtain such error ranges are discussed in detail and exemplified for a binding reaction obtained by ITC.

Silicone oil-free syringes, siliconized syringes and needles: quantitative assessment of silicone oil release with drugs used for intravitreal injection.

PURPOSE: This study aimed to quantify the amount of silicone oil (SO) released across a variety of syringe and needle models routinely used for intravitreal injection. METHODS: The release of SO was assessed in eight models of syringes, two of which were reported to be 'SO-free', and eleven models of needles with unknown SO content. To evaluate SO release within the context of anti-VEGF therapeutics, syringes were evaluated using aflibercept, bevacizumab, buffer, ziv-aflibercept and formulation buffer. All syringe tests were performed with or without agitation by flicking for syringes. Needles were evaluated without agitation only. Samples were fluorescently labelled to identify SO, and triplicate measurements were collected using imaging flow cytometry. RESULTS: Seven out of 8 syringe models showed a statistically significant increase in the SO particle count after agitation. The two SO-free syringe models (HSW Norm-Ject, Daikyo Crystal Zenith) released the least SO particles, with or without agitation, whereas the BD Ultra-Fine and Saldanha-Rodrigues syringes released the most. More SO was released when the syringes were prefilled with formulation buffer than with ziv-aflibercept. Syringes filled with aflibercept and bevacizumab had intermediate levels. Agitation increased the release of SO into each of the drug solutions. Silicone oil (SO) was detected in all needles. CONCLUSIONS: Agitation of the syringe by flicking leads to a substantial increase in the number of SO particles. Silicone oil (SO)-free syringes had the best performance, but physicians must also be aware that needles are siliconized and also contribute to the injection of SO into the vitreous.

High particle variability across siliconized and oil-free syringes and needles from the same lots.

Previous studies have reported silicone oil (SO) applied to needles and syringes in the vitreous of patients after intravitreal injections. We evaluated four syringes (SR 1-mL insulin, Saldanha-Rodrigues; BD 1-mL Tuberculin Slip Tip, Becton-Dickinson; BD Ultra-Fine 0.3 mL, HSW Norm-Ject Tuberculin, Henke Sass Wolf) and 10 needles (BD PrecisionGlide 27- and 30-gauge (G); BD Eclipse and JBP Nanoneedle 27-, 30-, 33- and 34-G; TSK Invisible Needle and 27 and 30-G Steriject Control Hub). The protein-free buffer samples injected into the syringes and needles under study were collected in an Eppendorf tube and taken to Flow imaging microscopy, that characterized the concentration and morphology of the microsized particles. The number of particles was analyzed. The coefficients of variation (CV) were the primary outcome. The Feltz and Miller test compared the CVs. The significance level was 5%. Numerous particles and high CVs were associated with both devices, needles and syringes; the comparisons among them did not reach significance. The BD Ultrafine 0.3 mL syringe (149.7%) had the highest CV and the SO-free HSW Norm-Ject (66.4%) syringe the lowest, and the TSK Invisible needle (149.5%) had the highest and the BD Precision Glide 30G needle (35.9%) needle the lowest. In conclusion, particle release, including those with SO morphology, varied greatly among instruments, even from the same lots, which is relevant considering that fewer particles are injected into some eyes compared with others.

Methyl 2-Halo-4-Substituted-5-Sulfamoyl-Benzoates as High Affinity and Selective Inhibitors of Carbonic Anhydrase IX.

Among the twelve catalytically active carbonic anhydrase isozymes present in the human body, the CAIX is highly overexpressed in various solid tumors. The enzyme acidifies the tumor microenvironment enabling invasion and metastatic processes. Therefore, many attempts have been made to design chemical compounds that would exhibit high affinity and selective binding to CAIX over the remaining eleven catalytically active CA isozymes to limit undesired side effects. It has been postulated that such drugs may have anticancer properties and could be used in tumor treatment. Here we have designed a series of compounds, methyl 5-sulfamoyl-benzoates, which bear a primary sulfonamide group, a well-known marker of CA inhibitors, and determined their affinities for all twelve CA isozymes. Variations of substituents on the benzenesulfonamide ring led to compound 4b, which exhibited an extremely high observed binding affinity to CAIX; the Kd was 0.12 nM. The intrinsic dissociation constant, where the binding-linked protonation reactions have been subtracted, reached 0.08 pM. The compound also exhibited more than 100-fold selectivity over the remaining CA isozymes. The X-ray crystallographic structure of compound 3b bound to CAIX showed the structural position, while several structures of compounds bound to other CA isozymes showed structural reasons for compound selectivity towards CAIX. Since this series of compounds possess physicochemical properties suitable for drugs, they may be developed for anticancer therapeutic purposes.

Halogenated and di-substituted benzenesulfonamides as selective inhibitors of carbonic anhydrase isoforms.

By applying an approach of a "ring with two tails", a series of novel inhibitors possessing high-affinity and significant selectivity towards selected carbonic anhydrase (CA) isoforms has been designed. The "ring" consists of 2-chloro/bromo-benzenesulfonamide, where the sulfonamide group is as an anchor coordinating the Zn(II) in the active site of CAs, and halogen atom orients the ring affecting the affinity and selectivity. First "tail" is a substituent containing carbonyl, carboxyl, hydroxyl, ether groups or hydrophilic amide linkage. The second "tail" contains aryl- or alkyl-substituents attached through a sulfanyl or sulfonyl group. Both "tails" are connected to the benzene ring and play a crucial role in selectivity. Varying the substituents, we designed compounds selective for CA VII, CA IX, CA XII, or CA XIV. Since due to binding-linked protonation reactions the binding-ready fractions of the compound and protein are much lower than one, the "intrinsic" affinities were calculated that should be used to study correlations between crystal structures and the thermodynamics of binding for rational drug design. The "intrinsic" affinities together with the intrinsic enthalpies and entropies of binding together with co-crystal structures were used demonstrate structural factors determining major contributions for compound affinity and selectivity.

Cofactor Analogues as Active Site Probes in Lysine Acetyltransferases.

Lysine acetyltransferases (KATs, also termed histone acetyltransferases, HATs) catalyze the acetylation of substrate lysine residues by employing the cofactor acetyl-coenzyme A (AcCoA), thereby providing a dynamic control mechanism of protein function. Because of their major involvement in cell development and homeostasis, small-molecule modulators of KAT activity are urgently needed to assess their therapeutic potential and for probing their underlying biology. Recent advances in the field suggest that targeting the cofactor binding site represents a promising strategy for identifying potent and selective ligands. Here, we present the synthesis of two functional cofactor-based chemical probes and their usage as mechanistic tools in a broadly applicable assay platform. A fluorescence polarization (FP)-based binding assay was combined with biolayer interferometry competition analysis and a FP competition activity immunoassay to enable easy, reliable, and profound evaluation of ligands that target the KAT cofactor binding site.

Repeatability, precision, and accuracy of the enthalpies and Gibbs energies of a protein-ligand binding reaction measured by isothermal titration calorimetry.

In rational drug design, it is important to determine accurately and with high precision the binding constant (the affinity or the change in Gibbs energy, ∆G), the change in enthalpy (ΔH), and the entropy change upon small molecule drug binding to a disease-related target protein. These thermodynamic parameters of the protein-ligand association reaction are usually determined by isothermal titration calorimetry (ITC). Here, the repeatability, precision, and accuracy of the measurement of the affinity and the change in enthalpy upon acetazolamide (AZM) interaction with human carbonic anhydrase II (CA II) are discussed based on the measurements using several ITC instruments. The AZM-CA II reaction was performed at decreasing protein-ligand concentrations until the determination of ∆G and ΔH was not possible, indicating a lower limit for accuracy. To obtain the confidence intervals (CI) of the ∆G and ΔH of AZM binding to CA II, the binding reaction was repeated numerous times at the optimal concentration of 10 µM and 25 °C temperature. The CI (at a confidence level α = 0.95) for ΔH = - 51.2 ± 1.0 kJ/mol and ∆G = - 45.4 ± 0.5 kJ/mol was determined by averaging the results of multiple repeats.

Design of two-tail compounds with rotationally fixed benzenesulfonamide ring as inhibitors of carbonic anhydrases.

Rational design of compounds that would bind specific pockets of the target proteins is a difficult task in drug design. The 12 isoforms of catalytically active human carbonic anhydrases (CAs) have highly similar active sites that make it difficult to design inhibitors selective for one or several CA isoforms. A series of CA inhibitors based on 2-chloro/bromo-benzenesulfonamide that is largely fixed in the CA active site together with one or two tails yielded compounds that were synthesized and evaluated as inhibitors of CA isoforms. Introduction of a second tail had significant influence on the binding affinity and two-tailed compounds in most cases provided high affinity and selectivity for CA IX and CA XIV. The contacts between several compounds and CA amino acids were determined by X-ray crystallography. Together with the intrinsic enthalpy and entropy of binding they provided the structure-thermodynamics correlations for this series of compounds with the insight how to rationally build compounds with desired CA isoform as a target.

Introduction of Intrinsic Kinetics of Protein-Ligand Interactions and Their Implications for Drug Design.

Structure-kinetic relationship analyses and identification of dominating interactions for optimization of lead compounds should ideally be based on intrinsic rate constants instead of the more easily accessible observed kinetic constants, which also account for binding-linked reactions. The intrinsic rate constants for sulfonamide inhibitors and pharmacologically relevant isoforms of carbonic anhydrase were determined by a novel surface plasmon resonance (SPR) biosensor-based approach, using chemodynamic analysis of binding-linked pH-dependent effects. The observed association rates ( kaobs) were pH-dependent and correlated with the fraction of deprotonated inhibitor and protonated zinc-bound water molecule. The intrinsic association rate constants ( kaintr) were pH independent and higher than kaobs. By contrast, the observed and intrinsic dissociation rate constants were identical and pH-independent, demonstrating that the observed association and dissociation mechanisms are inherently different. A model accounting for the differences between intrinsic and observed rate constants was developed, useful also for other interactions with binding-linked protonation reactions.

Benzimidazole design, synthesis, and docking to build selective carbonic anhydrase VA inhibitors.

The similarity of human carbonic anhydrase (CA) active sites makes it difficult to design selective inhibitors for one or several CA isoforms that are drug targets. Here we synthesize a series of compounds that are based on 5-[2-(benzimidazol-1-yl)acetyl]-2-chloro-benzenesulfonamide (1a) which demonstrated picomolar binding affinity and significant selectivity for CA isoform five A (VA), and explain the structural influence of inhibitor functional groups to the binding affinity and selectivity. A series of chloro-substituted benzenesulfonamides bearing a heterocyclic tail, together with molecular docking, was used to build inhibitors that explore substituent influence on the binding affinity to the CA VA isoform.

Thermodynamic, kinetic, and structural parameterization of human carbonic anhydrase interactions toward enhanced inhibitor design.

The aim of rational drug design is to develop small molecules using a quantitative approach to optimize affinity. This should enhance the development of chemical compounds that would specifically, selectively, reversibly, and with high affinity interact with a target protein. It is not yet possible to develop such compounds using computational (i.e., in silico) approach and instead the lead molecules are discovered in high-throughput screening searches of large compound libraries. The main reason why in silico methods are not capable to deliver is our poor understanding of the compound structure-thermodynamics and structure-kinetics correlations. There is a need for databases of intrinsic binding parameters (e.g., the change upon binding in standard Gibbs energy (ΔGint), enthalpy (ΔHint), entropy (ΔSint), volume (ΔVintr), heat capacity (ΔCp,int), association rate (ka,int), and dissociation rate (kd,int)) between a series of closely related proteins and a chemically diverse, but pharmacophoric group-guided library of compounds together with the co-crystal structures that could help explain the structure-energetics correlations and rationally design novel compounds. Assembly of these data will facilitate attempts to provide correlations and train data for modeling of compound binding. Here, we report large datasets of the intrinsic thermodynamic and kinetic data including over 400 primary sulfonamide compound binding to a family of 12 catalytically active human carbonic anhydrases (CA). Thermodynamic parameters have been determined by the fluorescent thermal shift assay, isothermal titration calorimetry, and by the stopped-flow assay of the inhibition of enzymatic activity. Kinetic measurements were performed using surface plasmon resonance. Intrinsic thermodynamic and kinetic parameters of binding were determined by dissecting the binding-linked protonation reactions of the protein and sulfonamide. The compound structure-thermodynamics and kinetics correlations reported here helped to discover compounds that exhibited picomolar affinities, hour-long residence times, and million-fold selectivities over non-target CA isoforms. Drug-lead compounds are suggested for anticancer target CA IX and CA XII, antiglaucoma CA IV, antiobesity CA VA and CA VB, and other isoforms. Together with 85 X-ray crystallographic structures of 60 compounds bound to six CA isoforms, the database should be of help to continue developing the principles of rational target-based drug design.

Intrinsic thermodynamics of high affinity inhibitor binding to recombinant human carbonic anhydrase IV.

Membrane-associated carbonic anhydrase (CA) isoform IV participates in carbon metabolism and pH homeostasis and is implicated in the development of eye diseases such as retinitis pigmentosa and glaucoma. A series of substituted benzenesulfonamides were designed and their binding affinity to CA IV was determined by fluorescent thermal shift assay and isothermal titration calorimetry (ITC). Compound [(4-chloro-2-phenylsulfanyl-5-sulfamoyl-benzoyl)amino]propyl acetate (19) bound CA IV with the K d of 1.0 nM and exhibited significant selectivity over the remaining 11 human CA isoforms. The compound could be developed as a drug targeting CA IV. Various forms of recombinant CA IV were produced in Escherichia coli and mammalian cell cultures. Comparison of their temperature stability in various buffers and salt solutions demonstrated that CA IV is most stable at slightly alkaline conditions and at elevated sodium sulfate concentrations. High-resolution X-ray crystallographic structures of ortho-Cl and meta-thiazole-substituted benzene sulfonamide in complex with CA IV revealed the position of and interactions between the ligand and the protein. Sulfonamide inhibitor binding to CA IV is linked to several reactions-the deprotonation of the sulfonamide amino group, the protonation of CA-Zn(II)-bound hydroxide at the active site of CA IV, and the compensating reactions of the buffer. The dissection of binding-linked reactions yielded the intrinsic thermodynamic parameters, characterizing the interaction between CA IV and the sulfonamides in the binding-able protonation forms, including Gibbs energy, enthalpy, and entropy, that could be used for the characterization of binding to any CA in the process of drug design.

Isothermal titration calorimetry for drug design: Precision of the enthalpy and binding constant measurements and comparison of the instruments.

Isothermal titration calorimetry (ITC) is one of the most robust label- and immobilization-free techniques used to measure protein - small molecule interactions in drug design for the simultaneous determination of the binding affinity (ΔG) and the enthalpy (ΔH), both of which are important parameters for structure-thermodynamics correlations. It is important to evaluate the precision of the method and of various ITC instrument models by performing a single well-characterized reaction. The binding between carbonic anhydrase II and acetazolamide was measured by four ITC instruments - PEAQ-ITC, iTC200, VP-ITC, and MCS-ITC and the standard deviation of ΔG and ΔH was determined. Furthermore, the limit of an approach to reduce the protein concentration was studied for a high-affinity reaction (Kd = 0.3 nM), too tight to be measured by direct (non-displacement) ITC. Chemical validation of the enthalpy measurements is discussed.

Kinetically Selective Inhibitors of Human Carbonic Anhydrase Isozymes I, II, VII, IX, XII, and XIII.

To get a better understanding of the possibility of developing selective carbonic anhydrase (CA) inhibitors, interactions between 17 benzenesulphonamide ligands and 6 human CAs (full-length CA I, II, VII, and XIII and catalytic domains of CA IX and XII) were characterized using surface plasmon resonance and fluorescent-based thermal shift assays. Kinetics revealed that the strongest binders had subnanomolar affinities with low dissociation rates (i.e., kd values around 1 × 10(-3) s(-1)) or were essentially irreversible. Chemodynamic analysis of the interactions highlighted an intrinsic mechanism of the CA-sulphonamide interaction kinetics and showed that slow dissociation rates were mediated by large hydrophobic contacts. The studied inhibitors demonstrated a high cross-reactivity within the protein family. However, according to chemical phylogenetic analysis developed for kinetic data, several ligands were found to be selective against certain CA isozymes, indicating that it should be possible to develop selective CA inhibitors suitable for clinical use.

Intrinsic thermodynamics of inhibitor binding to human carbonic anhydrase IX.

BACKGROUND: Human carbonic anhydrase 9th isoform (CA IX) is an important marker of numerous cancers and is increasingly interesting as a potential anticancer drug target. Various synthetic aromatic sulfonamide-bearing compounds are being designed as potent inhibitors of CA IX. However, sulfonamide compound binding to CA IX is linked to several reactions, the deprotonation of the sulfonamide amino group and the protonation of the CA active site Zn(II)-bound hydroxide. These linked reactions significantly affect the affinities and other thermodynamic parameters such as enthalpies and entropies of binding. METHODS: The observed and intrinsic affinities of compound binding to CA IX were determined by the fluorescent thermal shift assay. The enthalpies and entropies of binding were determined by the isothermal titration calorimetry. RESULTS: The pKa of CA IX was determined to be 6.8 and the enthalpy of CA IX-Zn(II)-bound hydroxide protonation was -24 kJ/mol. These values enabled the analysis of intrinsic thermodynamics of a library of compounds binding to CA IX. The most strongly binding compounds exhibited the intrinsic affinity of 0.01 nM and the observed affinity of 2 nM. CONCLUSIONS: The intrinsic thermodynamic parameters of compound binding to CA IX helped to draw the compound structure to thermodynamics relationship. GENERAL SIGNIFICANCE: It is important to distinguish the intrinsic from observed parameters of any disease target protein interaction with its inhibitors as drug candidates when drawing detailed compound structure to thermodynamics correlations.