abstract |
A compound of Formula I: ** Formula ** or a pharmaceutically acceptable salt thereof, wherein: each of Y1, Y2, Y3, Y4, Z1, Z2, Z3, Z4, W1 and W2 is independently selected from between hydrogen and deuterium; with the condition that: a. Y1, Y2, Y3, Y4, Z1, Z2, Z3 and Z4, each be deuterium; or b. W1 and W2 are each deuterium. |