Disclosed is a compound of formula (I) in crystalline form (VSN16), wherein the compound is in the form of the free base or a pharmaceutically acceptable salt thereof, or a solvate of the free base or salt form thereof. Also disclosed is a pharmaceutical composition containing said crystalline form as an active ingredient, and use thereof in the prevention or treatment of a muscular disorder. Also disclosed is a process for preparing the crystalline form by crystallising from methyl acetate or ethyl acetate.
