Acid-base equilibria and solubility of loratadine and desloratadine in water and micellar media

Abstract

Acid-base equilibria in homogeneous and heterogeneous systems of two antihistaminics, loratadine and desloratadine were studied spectrophotometrically in Britton-Robinson's buffer at 25 °C. Acidity constant of loratadine was found to be pKa 5.25 and those of desloratadine pKa1 4.41 and pKa2 9.97. The values of intrinsic solubilities of loratadine and desloratadine were 8.65 × 10-6 M and 3.82 × 10-4 M, respectively. Based on the pKa values and intrinsic solubilities, solubility curves of these two drugs as a function of pH were calculated. The effects of anionic, cationic and non-ionic surfactants applied in the concentration exceeding critical micelle concentration (cmc) on acid-base properties of loratadine and desloratadine, as well as on intrinsic solubility of loratadine were also examined. The results revealed a shift of pKa values in micellar media comparing to the values obtained in water. These shifts (ΔpKa) ranged from -2.24 to +1.24. © 2008 Elsevier B.V. All rights reserved.