The invention provides a method of preparing a lyophilized pharmaceutical composition containing a dinucleotide compound or a pharmaceutically-acceptable salt thereof. The process comprises dissolving the dinucleotide 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.