Entinostat is a histone deacetylase inhibitor undergoing clinical investigation in multiple types of solid tumors, such as breast cancer, and hematologic cancers. Crystalline form D and E of Entinostat and crystal form A in high crystal purity are provided. Crystalline form D can be obtained in high chemical purity, exhibits improved water solubility and allows efficient purification of Entinostat with removal of coloured impurities. Processes for the preparation of such crystalline forms and of form A with improved chemical and crystal purity are also provided.
