The chemical stability of suplatast tosilate (ST) was evaluated under dry conditions at 60 °C, with a focus on the polymorphic forms and crystal uniformity. The intact α-form, namely α-C type crystals, was crystallized rapidly by adding isopropyl ether into an ethanolic solution of ST. The α-C type crystals exhibited low stability at 60 °C. In addition to the α-C type crystals, two types of α-form crystals with different uniformities, namely α-A and α-B type crystals, were prepared. The α-A type crystals were prepared by conversion from the η-form in aqueous acetone, while the α-B type crystals were prepared by recrystallization from 2-propanol at a low supersaturation level. The stability of the α-form crystals was as follows: α-A > α-B > α-C. The uniformity of ST crystals was determined according to the newly developed high performance liquid chromatography (HPLC) method. The α-C type crystals exhibited inferior chemical stability due to unstable crystalline phases. They showed preferential enrichment and exhibited a unique optical resolution phenomenon. The observed unstable crystalline phases were produced during the crystallization process. α-A type crystals, which had high regularity, exhibited excellent stability. Overall, we prepared and evaluated the structures of α-form crystals with different uniformities.
