The invention provides a compound of formula (I):or a pharmaceutically acceptable salt thereof, wherein the variablesY1-Y6, X, Z, Z1, Z2, R2-R3, and R6have the meaning as described herein, and compositions containing such compounds and methods for using such compounds and compositions.本發明提供式(I)化合物,或其醫藥學上可接受之鹽,其中變數Y1至Y6、X、Z、Z1、Z2、R2至R3及R6具有如本文所述之含義;及含有此等化合物之組合物;及使用此等化合物及組合物之方法。