-C6)alkyl (C3-C10)cycloalkyl, (C1-C6)alkyl-N,N???C1-C6 dialkylamine, (C1-C6)alkyl-pyrrolidine, (C1-C6)alkyl-piperidine, or (C1-C6)alkyl-morpholine, with the proviso that at least one of R5, R6, or R8 is other than hydrogen; with the further proviso that where R2 and R4 each independently represent fluorine, then the compound is of the formula or a pharmaceutically acceptable salt thereof.