Approach of Cocrystallization to Improve the Solubility and Photostability of Tranilast

The cocrystals and salts of an antiallergic drug, tranilast, were synthesized to improve its solubility and photostability. Two tranilast cocrystals with urea (1) and nicotinamide (2), as well as two salts with cytosine (3) and sodium ion (4), were obtained and characterized by infrared spectra, thermogravimetric analyses, differential scanning calorimetry, and powder and single crystal X-ray diffractions. The results of single crystal structure analyses of 1–3 indicate that tranilast combines amide groups in coformers via R₂²(8) synthon, resulting in a 1:1 stoichiometry. The complexes showed advantages in terms of solubility and photostability in comparison to pure tranilast. The maximum solubility values of 1–3 in phosphate buffer of pH 6.8 are approximately 1.6, 1.9, and 2.0 times as large as that of tranilast, and the residual of tranilast is 79.5, 92.9, 88.5, 86.2, and 87.4% for tranilast and 1–4, respectively, under the fluorescent lamp irradiation for 96 h.