A compound of the following formula: ** Formula ** in which R1 is selected from: -CH3 and -CF3; R2 is selected from: H and -CH3; R3 is selected from: H and -CH3; R4 is selected from: H, -OC1-3 alkyl, and halogen; R5 is selected from: H, -CF3, -OCH3, and halogen; R6 is selected from: H and halogen; provided that at least one of R4, R5, and R6 is H; or a pharmaceutically acceptable salt thereof.