The disclosure provides a novel polymorph of Compound (I): , 2-((1-(2-(4-Fluorophenyl)-2-oxoethyl)piperidin-4-yl)methyl)isoindolin-1-one monohydrochloride dihydrate, i.e., Form (A) of Compound (I)‧HCl‧2H2O. Pharmaceutical compositions comprising Form (A) of Compound (I)‧HCl‧2H2O and related methods of treatment are also disclosed.本揭示內容係提供一種化合物(I):之新穎的多形體,2-((1-(2-(4-氟苯基)-2-側氧基乙基)哌啶-4-基)甲基)異吲哚啉-1-酮單鹽酸鹽二水合物,即,化合物(I)‧HCl‧2H2O的晶型(A)。亦揭示包含化合物(I)‧HCl‧2H2O的晶型(A)之醫藥組成物以及相關的治療方法。