A potentially simpler approach to measure aqueous pKa of insoluble basic drugs containing amino groups

Determination of the aqueous pKa of very insoluble drugs by capillary electrophoresis: Internal standards for methanol-water extrapolation

A fast determination of acidity constants (pK_a) of very insoluble drugs has become a necessity in drug discovery process because it often produces molecules that are highly lipophilic and sparingly soluble in water. In this work the high throughput internal standard capillary electrophoresis (IS-CE) method has been adapted to the determination of pK_a of water insoluble compounds by measurement in methanol/aqueous buffer mixtures. For this purpose, the reference pK_a values for a set of 46 acid-base compounds of varied structure (internal standards) have been established in methanol-water mixtures at several solvent composition levels (with a maximum of 40% methanol). The IS-CE method has been successfully applied to seven test drugs of different chemical nature with intrinsic solubilities lower than 10^-6 M. pK_a values have been determined at different methanol/aqueous buffer compositions and afterwards Yasuda-Shedlovsky extrapolation method has been applied to obtain the aqueous pK_a. The obtained results have successfully been compared to literature ones obtained by other methods. It is concluded that the IS-CE method allows the determination of aqueous pK_a values using low proportions of methanol, becoming then more accurate in the extrapolation procedure than other reference methods.

Chromatographic and Surfactant Based Potentiometric Determination of Aqueous Dissociation Constant of Mupirocin

Background:

The available data concerning aqueous dissociation constant of mupirocin (sparingly soluble drug) are scarce. In this study, chromatographic, surfactant based potentiometric concept and spectrophotometric method were used for determination of the aqueous pKa value of mupirocin.

Methods:

Different approaches were used for estimation of the aqueous pKa value from the apparent pKa values obtained at four ACN concentrations, ranging from 22% to 30%. The potentiometric determination of the pKa value of mupirocin was performed using different concentration of Tween 80 as a surfactant.

Results:

The aqueous pKa value of mupirocin, determined for the first time by reverse-phase liquid chromatography method, was found to be 4.76. The obtained value was confirmed by potentiometric method (4.85). It was found that Tween 80 increases the pKa values. The linear relationship between the apparent pKa values and the surfactant concentrations was used as an approach for estimation of the aqueous pKa value. Both methods gave similar values for aqueous pKa which correspond with the theoretically obtained pKa value (4.88) using Pallas computer program. It was found that mupirocin gives pH-indipendent spectra, thus spectrophotometric method is not applicable for determination of pKa of this compound.

Conclusion:

This comprehensive approach used for the pKa determination enable us to obtained reliable results for the aqueous pKa value of mupirocin. The linear relationship between the pKa values and the nonionic surfactant concentrations could be used as a reliable and simple approach for determination of aqueous pKa value of sparingly soluble drugs.

Structural elements of a pH-sensitive inhibitor binding site in NMDA receptors

Context-dependent inhibition of N-methyl-D-aspartate (NMDA) receptors has important therapeutic implications for the treatment of neurological diseases that are associated with altered neuronal firing and signaling. This is especially true in stroke, where the proton concentration in the afflicted area can increase by an order of magnitude. A class of allosteric inhibitors, the 93-series, shows greater potency against GluN1-GluN2B NMDA receptors in such low pH environments, allowing targeted therapy only within the ischemic region. Here we map the 93-series compound binding site in the GluN1-GluN2B NMDA receptor amino terminal domain and show that the interaction of the N-alkyl group with a hydrophobic cage of the binding site is critical for pH-dependent inhibition. Mutation of residues in the hydrophobic cage alters pH-dependent potency, and remarkably, can convert inhibitors into potentiators. Our study provides a foundation for the development of highly specific neuroprotective compounds for the treatment of neurological diseases.

Context-dependent inhibition of NMDA receptors has important therapeutic implications for treatment of neurological diseases. Here, the authors use structural biology and biophysics to describe the basis for pH-dependent inhibition for a class of allosteric NMDAR inhibitors, called the 93-series.

Radical scavenging propensity of Cu2+, Fe3+ complexes of flavonoids and in-vivo radical scavenging by Fe3+-primuletin

Cu²⁺ and Fe³⁺ complexes of three flavonoids (morin or mo, quercetin or quer and primuletin or prim) were synthesized with the objective of improving antioxidant capacities of flavonoids. The radical scavenging activities of pure flavonoids and their metal complexes were assayed to monitor their tendencies towards sequestering of radicals at physiological conditions. The scavenger potencies of metal-flavonoid complexes were significantly higher than those of the parent flavonoids. Further, influence of the solvent polarity on the radical capturing by flavonoids and their metal complexes was in favor for the polar solvent. Fe³⁺-prim displayed its radical scavenging ability via up gradation of CAT and SOD activities in in-vivo antioxidant assays.

Effect of solute–solvent interactions on DPPH radical scavenging efficiency of some flavonoid antioxidants in various binary water–methanol mixtures

Scavenging ability of three kinds of natural flavonoid antioxidants including chrysin, naringenin, and quercetin against the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical was evaluated by the UV-vis spectrophotometric technique in different aqueous mixtures of methanol (50%–90% v/v) at physiological pH. The studied flavonoids showed their activity to be comparable with ascorbic acid (vitamin C), which is used as a standard reference material in most testing methods. Our findings indicated that an increase in the organic solvent percentage (v/v) has different effects on the radical scavenging efficiency of flavonoids presumably due to solute–solvent interactions. These variations were analyzed in light of various simple and multiple regression equations using the normalized polarity parameter ([Formula: see text]) and Kamlet, Abboud, and Taft solvatochromic parameters. Moreover, The IC50 values of the samples were then obtained by the Yasuda–Shedlovsky extrapolation procedure in pure water.

Equilibrium studies of triphenyltin(IV) complexes with glycine, glycyl-glycine, and glycyl-glycyl-glycine in different aqueous solutions of ethanol

The protonation equilibria of glycine (gly), glycyl-glycine (gly-gly), and glycyl-glycyl-glycine (gly-gly-gly) and their formation constants with triphenyltin(IV) chloride were studied over a wide pH range (pH 1–11), using a combination of spectrophotometric and potentiometric methods at constant temperature (25 °C), different ethanol–water mixtures (50%–80%, v/v), and constant ionic strength (0.1 mol dm–3 NaClO4). Least-squares regression calculations are consistent with the formation of ph3SnHL+, ph3SnL, and ph3SnH–1L– complex species, where L– represents the fully dissociated form of each ligand. The stability constant of the formed complexes in different media were analyzed in terms of Kamlet, Abboud, and Taft (KAT) parameters. Single-parameter correlations of the stability constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and for π* (dipolarity/polarizability) are relatively poor in all solutions, but multi-parameter correlations represent significant improvements with regard to the single- and dual-parameter models. Linear correlation is observed when the experimental logβxyz values are plotted versus the calculated ones, while all the KAT parameters are considered. Also, the stability constant values of the formed complexes are determined in zero percent of organic solvent using the Yasuda–Shedlovsky extrapolation approach. Finally, the results are discussed in terms of the effect of solvent on complexation.

Predicting pKa

One of the most important physicochemical properties of small molecules and macromolecules are the dissociation constants for any weakly acidic or basic groups, generally expressed as the pK(a) of each group. This is a major factor in the pharmacokinetics of drugs and in the interactions of proteins with other molecules. For both the protein and small molecule cases, we survey the sources of experimental pK(a) values and then focus on current methods for predicting them. Of particular concern is an analysis of the scope, statistical validity, and predictive power of methods as well as their accuracy.

In silico prediction of ionization constants of drugs

Most pharmacologically active molecules contain one or more ionizing groups, and it is well-known that knowledge of the ionization state of a drug, indicated by the pKa value, is critical for understanding many properties important to the drug discovery and development process. The ionization state of a compound directly influences such important pharmaceutical characteristics as aqueous solubility, permeability, crystal structure, etc. Tremendous advances have been made in the field of experimental determination of pKa, in terms of both quantity/speed and quality/accuracy. However, there still remains a need for accurate in silico predictions of pKa both to estimate this parameter for virtual compounds and to focus screening efforts of real compounds. The computer program SPARC (SPARC Performs Automated Reasoning in Chemistry) was used to predict the ionization state of a drug. This program has been developed based on the solid physical chemistry of reactivity models and applied to successfully predict numerous physical properties as well as chemical reactivity parameters. SPARC predicts both macroscopic and microscopic pKa values strictly from molecular structure. In this paper, we describe the details of the SPARC reactivity computational methods and its performance on predicting the pKa values of known drugs as well as Pfizer internal discovery/development compounds. A high correlation (r2=0.92) between experimental and the SPARC calculated pKa values was obtained with root-mean-square error (RMSE) of 0.78 log unit for a set of 123 compounds including many known drugs. For a set of 537 compounds from the Pfizer internal dataset, correlation coefficient r2=0.80 and RMSE=1.05 were obtained.

Novel 5-substituted 1-pyrazolol analogues of ibotenic acid: synthesis and pharmacology at glutamate receptors

5-Substituted 1-pyrazolol analogues of ibotenic acid have been synthesized and pharmacologically characterized on ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs). The syntheses involved introduction of bromide, alkyls, phenyl and arylalkyls in the 5-position of 1-benzyloxypyrazole leading to 5-substituted (RS)-2-amino-(1-hydroxy-4-pyrazolyl)acetic acids (5a-l). The pharmacological activities of the synthesized analogues ranged from the 5-cyclopropylmethyl analogue (5f) with weak but selective affinity for NMDA receptors (IC(50)=35 microM), over the 5-n-propyl analogue (5c), which was a selective mGluR2 agonist (EC(50)=72 microM), to the 5-cyclohexylmethyl analogue (5g), which was a selective mGluR2 antagonist (K(i)=32 microM), and the 5-phenylethyl analogue (5j), which was a weak but apparently selective mGluR1 antagonist (K(i)=230 microM). This series of compounds afforded GluR ligands with a broad spectrum of pharmacological profiles, and showing potential for development of new compounds with subtype-selective activities at various GluRs.

Background electrolytes in 50% methanol/water for the determination of acidity constants of basic drugs by capillary zone electrophoresis

The acidic dissociation constants of several hydrophobic drugs, amiodarone and a series of antidepressants that show a secondary or tertiary amino group, were determined in a 50% methanol/water mixture by capillary zone electrophoresis. The electrophoretic behavior of buffers prepared from sodium acetate, tris(hydroxymethyl) aminomethane hydrochloride, sodium hydrogenphosphate, ammonium chloride, ethanolamine, butilammonium chloride, and sodium borate in the hydroalcoholic solution was tested. Thus, all of them follow the Ohm's law until about 25 kV and, therefore, they can be used without significant Joule heat dissipation at 20 kV. For the studied drugs, buffers prepared with phosphate or borate give effective mobility measurements lower than those from other buffers. The wide pKa range of the studied drugs provides a wide pH range where the protonated forms of the amino compounds coexist with hydrogenphosphate ions and where the neutral amines coexist with boric acid. The decrease of the experimental effective mobilities in these instances can be explained through the interactions between coexisting species. Therefore, phosphate and borate buffers should be avoided to determine the mobility of amines with aqueous pKa higher than 8, at least in solutions with high methanol content. Independent measurements of acidic dissociation constants of drugs validate this statement.

Modeling Retention and Selectivity as a Function of pH and Column Temperature in Liquid Chromatography

In reversed-phase liquid chromatography (RPLC), the retention of weak acids and bases is a sigmoidal function of the mobile-phase pH. Therefore, pH is a key chromatographic variable to optimize retention and selectivity. Furthermore, at an eluent pH close to the pKa of the solute, the dependence of ionization of the buffer and solute on temperature can be used to improve chromatographic separations involving ionizable solutes by an adequate handling of column temperature. In this paper, we derive a general equation for the prediction of the retentive behavior of ionizable compounds upon simultaneous changes in mobile-phase pH and column temperature. Four experiments, two limiting pH values and two temperatures, provide the input data that allow predictions in the whole range of these two variables, based on the thermodynamic fundamentals of the involved equilibria. Also, the study demonstrates the significant role that the choice of the buffer compound would have on selectivity factors in RPLC at temperatures higher than 25 degrees C.

Retention of ionisable compounds on high-performance liquid chromatography

In agreement with our previous studies and those of other authors, it is shown that much better fits of retention time as a function of pH are obtained for acid-base analytes when pH is measured in the mobile phase, than when pH is measured in the aqueous buffer when buffers of different nature are used. However, in some instances it may be more practical to measure the pH in the aqueous buffer before addition of the organic modifier. Thus, an open methodology is presented that allows prediction of chromatographic retention of acid-base analytes from the pH measured in the aqueous buffer. The model presented estimates the pH of the buffer and the pKa of the analyte in a particular acetonitrile/water mobile phase from the pH and pKa values in water. The retention of the analyte can be easily estimated, at a buffer pH close to the solute pKa, from these values and from the retentions of the pure acidic and basic forms of the analyte. Since in many instances, the analyte pKa values in water are not known, the methodology has been also tested by using Internet software, at reach of many chemists, which calculates analyte pKa values from chemical structure. The approach is successfully tested for some pharmaceutical drugs.

Acidity constants in methanol/water mixtures of polycarboxylic acids used in drug salt preparations

The acidic dissociation constants in a number of methanol/water mixtures of mono and polycarboxylic acids commonly used in the preparation of drug salts were determined. These solvent mixtures are usually used to determine the pKa of drugs of low aqueous solubility. However, when these drugs are prepared in salt form, the acid-base equilibria of both the basic drug and the counter-anion are involved in the potentiometric titration curves. In these instances, the inclusion of the pKa of acids as constant values in the curve fitting provides easy computation of the drug pKa without the need of any previous step to get the free base. As an application example, the aqueous pKa values of the quetiapine formulated as hemifumarate (Seroquel) were estimated by extrapolation from the experimental pKa in several methanol/water mixtures, which were then calculated according to the suitable constants of fumaric acid. The estimated aqueous pKa values of quetiapine are compared with those directly obtained in aqueous solution by potentiometry and by capillary electrophoresis.

BIOACTIVATION OF THE NONTRICYCLIC ANTIDEPRESSANT NEFAZODONE TO A REACTIVE QUINONE-IMINE SPECIES IN HUMAN LIVER MICROSOMES AND RECOMBINANT CYTOCHROME P450 3A4

The therapeutic benefits of the antidepressant nefazodone have been hampered by several cases of acute hepatotoxicity/liver failure. Although the mechanism of hepatotoxicity remains unknown, it is possible that reactive metabolites of nefazodone play a causative role. Studies were initiated to determine whether nefazodone undergoes bioactivation in human liver microsomes to electrophilic intermediates. Following incubation of nefazodone with microsomes or recombinant P4503A4 in the presence of sulfydryl nucleophiles, conjugates derived from the addition of thiol to a monohydroxylated nefazodone metabolite were observed. Product ion spectra suggested that hydroxylation and sulfydryl conjugation occurred on the 3-chlorophenylpiperazine-ring, consistent with a bioactivation pathway involving initial formation of p-hydroxynefazodone, followed by its two-electron oxidation to the reactive quinone-imine intermediate. The formation of novel N-dearylated nefazodone metabolites was also discernible in these incubations, and 2-chloro-1,4-benzoquinone, a by-product of N-dearylation, was trapped with glutathione to afford the corresponding hydroquinone-sulfydryl adduct. Nefazodone also displayed NADPH-, time-, and concentration-dependent inactivation of P4503A4 activity, suggesting that reactive metabolites derived from nefazodone bioactivation are capable of covalently modifying P4503A4. A causative role for 2-chloro-1,4-benzoquinone and/or the quinone-imine intermediate(s) in nefazodone hepatotoxicity is speculated. Although the antianxiety agent buspirone, which contains a pyrimidine ring in place of the 3-chlorophenyl-ring, also generated p-hydroxybuspirone in liver microsomes, no sulfydryl conjugates of this metabolite were observed. This finding is consistent with the proposal that two-electron oxidation of p-hydroxybuspirone to the corresponding quinone-imine is less favorable due to differences in the protonation state at physiological pH and due to weaker resonance stabilization of the oxidation products as predicted from ab initio measurements.

Effect of temperature on the chromatographic retention of ionizable compounds

The retention mechanism of acids and bases in reversed-phase liquid chromatography (RPLC) has been experimentally studied by examining the temperature dependence of retention, with emphasis on the role of the buffer ionization equilibria in the retention and selectivity. Retention factors of several ionizable compounds in a typical octadecylsilica column and using buffers dissolved in 50% (w/w) methanol as eluents at three temperatures in the range of 25-50 degrees C were measured. Two pairs of buffer solutions were prepared by a close adjusting of their pH at 25 degrees C; differences in their ionization enthalpies determined a different degree of ionization when temperature was raised and, as a consequence, a different shift in the eluent pH. Predictive equations of retention that take into account the temperature effect on both the transfer and the ionization processes are proposed. This study demonstrates the significant role that the selected buffer would have in retention and selectivity in RPLC at temperatures higher than 25 degrees C, particularly for co-eluted solutes.

Determination of acid dissociation constants by capillary electrophoresis

Capillary electrophoresis affords a simple, automated approach for the measurement of pKa values in the range 2-11 at a throughput of less than 1 h per sample per instrument. Agreement with literature values is usually within 0.20 log units with a precision better than 0.07 log units. The attractive features of capillary electrophoresis for pKa measurements are: (1) conventional instrumentation with a high level of automation are suitable for all measurements; (2) because it is a separation method samples need not be of high purity; (3) samples of low water solubility with suitable chromophores are easily handled (detection limits in the microM range); (4) sample consumption per measurement is in the microgram range; and (5) since only mobilities are measured, exact knowledge of concentrations is not needed. The general approach can be extended to pKa measurements in aqueous-organic solvent mixtures and non-aqueous solvents with suitable calibration. The widespread use of absorbance detection in capillary electrophoresis means that the sample must have a suitable chromophore for detection. The main source of controllable error is the accuracy of buffer standardization and their stability in use, and uncontrollable error, the retentive interactions of the sample with the column wall. The latter seems to be a rare problem in practice for typical operating conditions.

Hydrophobic and cation exchange mechanisms in the retention of basic compounds in a polymeric column

A cation exchange retention mechanism concomitant with the well-known hydrophobic partition mechanism in a polymeric column has been observed and investigated. This exchange process is attributed to ionization of some acidic sites present in the polymer column at basic mobile phase pH values. Several drugs of different basicity have been chromatographed on a polymeric PLRP-S column with methanol-water and acetonitrile-water mobile phases. The cation exchange between the protonated basic drug and the buffer cations (Na+, K+ and BuNH4+) is observed at the pH range where the protonated drug and the ionized sites of the column coexist. This process produces a shift of the retention versus pH plot of the base to pH values lower than those expected from the pKa of the base as well as a maximum in the plot at basic pH values. These effects are more pronounced for acetonitrile-water mobile phases.