Theophylline Cocrystallized with Salicylic Acid and Picolinic Acid: Preparation and Physical Properties

Abstract

Theophylline (TP) is an oral bronchodilator medicine, used to treat respiratory diseases. The problem of TP is low aqueous solubility, slow release tablets, and solid state interconversion between anhydrous and monohydrate forms as a function of relative humidity (RH). Physicochemical properties of this medicine, including solubility, humidity sensitivity, and chemical and physical stability can be improved by cocrystallizing with another compound. Herein we present the cocrystallization of TP with salicylic acid (SA) and picolinic acid (PI). Mixtures of 2:1 TP-SA and TP-PI were screened by solvent-drop grinding, crystal growth by slow evaporation and pH control at 2.0 and 7.0, and compared to the 1:1 crystalline products of TP-SA and TP-PI. The products were characterized by XRD, FT-IR/FT-Raman, and DSC/TG. Crystals of the 1:1 TP-SA, 1:1 TP-PI, and pure TP were stored at room temperature (342 °C) with 582% RH for one month and their stability examined by FT-IR.