east one of X, Y, and Z is a nitrogen atom; and b) where n=0 and X and Y are CH, R1 and R2 are not the same and neither of R1 and R2 is a ##STR44## group wherein R4 is a substituted or unsubstituted vinyl group or a group of formula ##STR45## in which Ra, Rb, R5 and R6 are as defined above, or a pharmaceutically acceptable salt thereof. 2.