Disclosed herein is a salt, a crystalline anhydrous form, a hydrate, a solvate, or a co-crystal of a free base compound 2-(6-azaspiro[2.5]octan-6-yl)-N-[2-(4,4-difluoropiperidin-1-yl)-6-methylpyrimidin-4-yl]-4-[(2-hydroxyethanesulfonyl)amino]benzamide (Compound A); method of preparation, pharmaceutical compositions, and method of treating a disease mediated by a motor protein kinesin family member 18A (KIF18A) inhibition, wherein said disease is a neoplastic disease, including a cancer or a tumor.