The present invention is directed to a compound represented by Structural Formula (I):or a pharmaceutically acceptable salt thereof. The variables for Structural Formula (I) are defined herein. Also described is a pharmaceutical composition comprising the compound of Structural Formula (I) and its therapeutic use.本發明係有關一種由結構式(I)表示之化合物:或其醫藥學上可接受之鹽。本文中定義結構式(I)之變數。亦描述一種包含結構式(I)之化合物的醫藥組成物及其治療用途。