The present invention relates to new crystalline forms of 2-hydroxy-6-((2-(1-isopropyl-1H-pyrazol-5-yl)pyridin-3-yl)methoxy)benzaldehyde (also referred to as "voxelotor"), in particular to crystalline salts of voxelotor with 1,5-naphthalenedisulfonic acid (also referred to as "voxelotor napadisylate" salts) and methods for preparing same. The invention also refers to a pharmaceutical composition comprising the said voxelotor napadisylate salts and one or more pharmaceutically acceptable excipients. The pharmaceutical composition of the present invention can be used as a medicament for increasing oxygen affinity of hemoglobin S, in particular for the treatment of oxygen deficiency associated with stickle cell anemia.
