Determination of the Physicochemical Properties of Piroxicam

Objectives

The aim of this study was to determine the acid dissociation constant (pKa) of piroxicam using high performance liquid chromatography (HPLC) and ultraviolet-visible (UV-Vis) spectrophotometry, and to determine the partition coefficient (Log P), distribution coefficient (Log D), and "Log kw" values of piroxicam using HPLC.

Materials and Methods

The HPLC studies were performed on a reversed-phase ACE C18 (150x4.6 mm ID, 5 μm) column at a flow rate of 1.0 mL min^-1. The detector was set to 360 nm. Log D at different pH values (3.0-6.5) was examined with a phosphate buffer (20 mM) and acetonitrile (30:70 v/v) mixture as the mobile phase. For pKa determination, HPLC studies were performed with a mixture of phosphate buffer (20 mM) and methanol within the pH range of 3.50-6.00. Log kw measurements were performed with phosphate buffer (20 mM) and MeOH (from 20:80 v/v to 10:90 v/v) mixtures within the pH range of 3.50-6.00. UV-Vis spectrophotometric pKa measurements were performed at 285 nm wavelength.

Results

The pKa value of piroxicam was found to be 5.3 by HPLC and 5.7 by UV-Vis spectrophotometry. Log P of piroxicam was determined as 1.58 in our experimental conditions. Log D values were 1.57, 1.57, 1.44, 1.13, and 0.46 for pH values of 3.17, 3.79, 4.44, 5.42, and 6.56, respectively.

Conclusion

In the literature, different Log P (3.1, 2.2, and 0.6) and pKa (6.3 and 4.8) values were reported for piroxicam. The Log P (1.58) and pKa (5.3 and 5.7) values obtained for piroxicam in our study were within the range of the literature values. All these results indicate that different experimental approaches used for the determination of physicochemical properties could provide different values. Although UV spectrophotometry is easy to apply, HPLC is a unique technique for simultaneous determination of pKa, Log D, and Log P values of compounds.

Determination of pKa values of alendronate sodium in aqueous solution by piecewise linear regression based on acid-base potentiometric titration

As a mono-sodium salt form of alendronic acid, alendronate sodium presents multi-level ionization for the dissociation of its four hydroxyl groups. The dissociation constants of alendronate sodium were determined in this work by studying the piecewise linear relationship between volume of titrant and pH value based on acid-base potentiometric titration reaction. The distribution curves of alendronate sodium were drawn according to the determined pKa values. There were 4 dissociation constants (pKa₁=2.43, pKa₂=7.55, pKa₃=10.80, pKa₄=11.99, respectively) of alendronate sodium, and 12 existing forms, of which 4 could be ignored, existing in different pH environments.

Ligand-controlled stereodivergent 1,3-dipolar cycloaddition of azomethine ylides with 3-methyl-4-nitro-5-styrylisoxazoles

An unprecedented Ag(i)-catalyzed ligand-controlled stereodivergent 1,3-dipolar cycloaddition of azomethine ylides with 3-methyl-4-nitro-5-styrylisoxazoles has been developed to afford heterocycles bearing both methylisoxazole and pyrrolidine moieties.

Application of capillary electrophoresis in determination of acid dissociation constant values

The chemical groups undergoing protonation or deprotonation in solution are described by the acid dissociation constant value, the key parameter for physicochemical characterization of biologically- and pharmacologically-important compounds. Capillary electrophoresis (CE) proved to be suitable technique for its determination: it enables automated and accurate measurements even for minute amount of sample, does not require the information about concentration, and handle both the impure and complex samples. In this review, a number of contributions reporting on the application of CE in pKa prediction has been summarized and critically discussed. The reader will find herein the brief introduction of theory, summary of all works published in the last decade, considerations on the most important innovations and achievements, and the discussion of pKa-related issues as e.g. the role of pKa-shifts in the chiral separation mechanism or the elucidation of migration order reversals observed during CE-mediated separations.

Determination of Acid Dissociation Constants (pK a ) of Bicyclic Thiohydantoin-Pyrrolidine Compounds in 20% Ethanol-Water Hydroorganic Solvent

The acid dissociation constants of potential bioactive fused ring thiohydantoin-pyrrolidine compounds were determined by potentiometric titration in 20% (v/v) ethanol-water mixed at 25 ± 0.1°C, at an ionic background of 0.1 mol/L of NaCl using the HYPERQUAD computer program. Proton affinities of potential donor atoms of the ligands were calculated by AM1 and PM3 semiempiric methods. We found, potentiometrically, three different acid dissociation constants for 1a–f. We suggest that these acid dissociation constants are related to the carboxyl, enol, and amino groups.