The invention relates to prodrug compounds of formula I:wherein R2, R3, R5, R7and X are as defined herein. The invention also provides pharmaceutically acceptable compositions comprising the compounds of the invention and methods of using the compositions in the treatment of various disorders, including pain. The compounds of formula I possess advantageous solubility and physicochemical properties.本發明係關於式I之前藥化合物:其中R2、R3、R5、R7及X如本文所定義。本發明亦提供包含本發明化合物之醫藥學上可接受之組合物及使用該等組合物於治療包括疼痛之各種病症之方法。式I化合物具有有利溶解性及物理化學特性。