Sonia Espinosa

Retention of ionizable compounds in high-performance liquid chromatography: 14. Acid–base pK values in acetonitrile–water mobile phases

Linear relationships between sspKa values in acetonitrile-water mixtures and wwpKa values in pure water have been established for five families of compounds: aliphatic carboxylic acids, aromatic carboxylic acids, phenols, amines, and pyridines. The parameters (slope and intercept) of the linear correlations have been related with acetonitrile-water composition. The proposed equations allow accurate estimation of the pKa values of any member of the studied families at any acetonitrile-water composition up to 60% of acetonitrile in volume (100% for pyridines). Conversely, the same equations can be used to estimate aqueous pKa values from chromatographic pKa values obtained from any acetonitrile-water mobile phase between the composition range studied. Estimation of pKa values have been tested with chromatographic literature data.

Retention of ionizable compounds on high-performance liquid chromatography III. Variation of pK values of acids and pH values of buffers in acetonitrile-water mobile phases

Abstract A critical compilation of the literature pK data for acetonitrile–water is presented and equations and parameters that allow estimation of the pK of these compounds for any acetonitrile–water mixture up to 60% of acetonitrile by volume are proposed. The data and equations have been used to calculate the pH of different buffers, which have been prepared and used to calibrate a potentiometric system for several acetonitrile–water mixtures. The measured potential vs. pH plots follow the Nernst equation for all solvent mixtures after elimination of some outliers. This procedure allows us to identify the most reliable pK data and from them to propose buffered solutions of accurate pH. These solutions have been used for every-day calibration of the pH-meter in solute retention–mobile phase pH relationships. A model developed in earlier works has been used to relate the retention of benzoic acid in a C18 column with the measured pH of the mobile phase (60% acetonitrile). The results obtained demonstrate that the best relationships are obtained when the retention is related with the true pH of the buffer in the mobile phase, instead of the pH value of the buffer in water as it is usually done.

Retention of ionizable compounds in high-performance liquid chromatography. IX. Modelling retention in reversed-phase liquid chromatography as a function of pH and solvent composition with acetonitrile-water mobile phases.

The influence of pH and solvent composition of acetonitrile-water mobile phases on the retention of acids and bases on a polymeric stationary phase is studied. Very good relationships between retention and mobile phase pH are obtained if the pH is measured in the proper pH scale. The fit of retention to pH for a particular solvent composition provides the pKa values of the equilibria between the different acid-base species and the retention parameters of these species at this solvent composition. Several models are tested that relate these parameters to solvent composition and properties in order to propose a general model to predict retention for any mobile phase pH and composition.

Unique selectivity of perfluorinated stationary phases with 2,2,2-trifluoroethanol as organic mobile phase modifier

The selectivity of Luna C18 Xterra C18 and Fluophase (perfluorinated C6) stationary phases has been investigated with aqueous acetonitrile, methanol and 2,2,2-trifluoroethanol mobile phases using linear solvation equations. The gradient retention times of a set of 60 compounds with known molecular descriptors have been determined. Linear solvation equations have been set up to describe the relationship between the gradient retention times and the molecular properties. The selectivity of the stationary phase/mobile phase systems was characterised by the regression coefficients of the molecular descriptors. The perfluorinated stationary phase showed very different selectivity using 2,2,2-trifluoroethanol (TFE) as co-solvent. Compounds with H-bond donor functionality were retained much less than in the other investigated high-performance liquid chromatography (HPLC) systems. This unique selectivity can be explained by the stronger adsorption of trifluoroethanol on the perfluorinated stationary phase surface, than on the hydrocarbon surface. It suggests the importance of the adsorbed organic modifiers in the separation mechanism during reversed-phase HPLC.

Retention of ionizable compounds on high-performance liquid chromatography: XI. Global linear solvation energy relationships for neutral and ionizable compounds

A global linear solvation energy relationship (LSER) that simultaneously models retention in reversed-phase liquid chromatography as a function of solute LSER descriptors and mobile phase pH and composition has been derived from both the local LSER model and the linear solvent strength theory. At most only 13 mobile phase parameters and seven solute parameters are required to establish the global LSER model for neutral and ionizable solutes. This model implies only one mobile phase and two solute parameters more than the model previously set for neutral solutes. The additional mobile phase and solute parameters account for the ionization of the solute. The model has been successfully tested for 30 solutes of different type (acids, bases and non ionizable compounds) at 10 different pH values in three different acetonitrile-water mobile phases.

Acid–base constants of neutral bases in acetonitrile–water mixtures

Abstract The acid–base constants of several bases (aniline, pyridine, ammonia, ethanolamine, and triethylamine) in acetonitrile–water mixtures up to 60% of acetonitrile by volume have been potentiometrically determined. Two different pH scales have been used: the pH scale relative to each acetonitrile–water mixture as standard state solvent ( s s pH ) and the absolute pH scale in reference to water as standard state solvent ( w s pH ). The two sets of pK values are in a reasonable good agreement. In contrast with the variation of pKa values of neutral and anionic acids, the pKa values of the bases decrease with the increase of the acetonitrile contents in the solvent mixture. In the studied range of solvent composition, linear relationships have been obtained between the w s p K a values of the bases and the volume fraction of acetonitrile in the mixture.

Setup and validation of shake-flask procedures for the determination of partition coefficients (logD) from low drug amounts.

Several procedures based on the shake-flask method and designed to require a minimum amount of drug for octanol-water partition coefficient determination have been established and developed. The procedures have been validated by a 28 substance set with a lipophilicity range from -2.0 to 4.5 (logD7.4). The experimental partition is carried out using aqueous phases buffered with phosphate (pH 7.4) and n-octanol saturated with buffered water and the analysis is performed by liquid chromatography. In order to have accurate results, four procedures and eight different ratios between phase volumes are proposed. Each procedure has been designed and optimized (for partition ratios) for a specific range of drug lipophilicity (low, regular and high lipophilicity) and solubility (high and low aqueous solubility). The procedures have been developed to minimize the measurement in the octanolic phase. Experimental logD7.4 values obtained from different procedures and partition ratios show a standard deviation lower than 0.3 and there is a nice agreement when these values are compared with the reference literature ones.

Change of mobile phase pH during gradient reversed-phase chromatography with 2,2,2-trifluoroethanol–water as mobile phase and its effect on the chromatographic hydrophobicity index determination

We have shown previously that using a trifluoroethanol containing mobile phase provides a unique chromatographic selectivity. This is essential to derive molecular descriptors by HPLC which requires retention data from several systems. It also requires that the ionisation is suppressed so that retention times reflect the properties of the neutral molecules. Therefore the pH change of the mobile phase during gradient elution and its effect on the solute ionisation have been studied. During gradient elution of mixtures of ammonium acetate and butylammonium formate with trifluoroethanol as an organic modifier it was found that the pH was almost constant when the gradient started with a low pH. However, when the starting mobile phase pH was above 8 the pH dropped very quickly as the trifluoroethanol concentration increased in the mobile phase. The CHI descriptor (a retention index derived directly from gradient retention times) of several basic compounds as a function of starting mobile phase pH has been measured using trifluoroethanol gradient. The effect of the trifluoroethanol on the pKa change of the compounds has been investigated. The experimental data fit closely to a previously derived equation that describes gradient retention times as a function of mobile phase pH and analyte ionisation constant (pKa). This equation makes it possible to predict the CHI descriptor for ionisable compounds at various pH values. We have used butylamine for high pH mobile phase preparation as is more basic than ammonia and for many basic drugs the retention of the neutral form could be obtained directly (without extrapolation).

Abediterol (LAS100977), an inhaled long-acting β 2 -adrenoceptor agonist, has a fast association rate and long residence time at receptor

ABSTRACT This study describes the association rate and residence time of abediterol, a novel long‐acting &bgr;2‐adrenoceptor agonist (LABA) in Phase II development for treatment of asthma and COPD, in comparison with indacaterol, olodaterol, vilanterol and salmeterol, for both human &bgr;1‐ and &bgr;2‐adrenoceptors. Abediterol association and dissociation rates were monitored directly by using its tritiated form. Moreover, association was determined indirectly using experimental Ki and koff obtained from assays performed with unlabelled compound. Dissociation was also studied indirectly by measuring the association rate of 3H‐CGP12177 to beta adrenoceptors previously occupied by unlabelled compounds. Abediterol shows a fast association for the &bgr;2‐adrenoceptor (kon 1.4 × 107 ± 1.8 × 106 M−1 min−1) while its dissociation rate is between 30 and 64 times slower than that of the reference LABA compounds tested, with a residence time of 91.3 ± 13.3 min (measured directly) and 185.5 ± 7.5 min (measured indirectly). Abediterol shows kinetic selectivity for the &bgr;2‐ over the &bgr;1‐adrenoceptor, with a dissociation rate from the &bgr;1‐adrenoceptor similar to the other LABA compounds tested. In conclusion, abediterol is a potent LABA with a fast association rate and a long residence time at &bgr;2‐adrenoceptors. These data are in agreement with the onset and duration of action of abediterol shown in humans.

Discovery of a novel inhaled PI3Kδ inhibitor for the treatment of respiratory diseases

Oral PI3Kδ inhibitors such as Idelalisib and Duvelisib have shown efficacy as anticancer agents and Idelalisib has been approved for the treatment of three B-cell cancers. However, Idelalisib has a black box warning on its product label regarding the risks of fatal and serious toxicities including hepatic toxicity, severe diarrhea, colitis, pneumonitis, infections, and intestinal perforation. Some of these side effects are mechanism-related and could hinder the development of Idelalisib for less severe conditions. For respiratory diseases, compounds administered by inhalation are delivered directly to the site of action and may improve the therapeutic index of a drug, minimizing undesired side effects. This work describes the discovery and optimization of inhaled PI3Kδ inhibitors intended for the treatment of severe asthma and COPD. Once the potency was in the desired range, efforts were focused on identifying the particular physicochemical properties that could translate into better lung retention. This medicinal chemistry exercise led to the identification of LAS195319 as a candidate for clinical development.