The invention provides a method of preparing a lyophilized pharmaceutical composition containing a compound described herein or a pharmaceutically acceptable salt thereof. The process comprises dissolving the compound in a solvent comprising dimethylsulfoxide and optionally one or more co solvents to form a solution and then removing the solvent and any co solvents by a freeze drying process. Also provided by the invention are lyophilized pharmaceutical compositions and their use in medicine and in particular in the treatment of cancer.