Nonetheless, certain compounds of the present invention are novel and accordingly the present invention provides a compound of Formula I or an acid addition salt thereof wherein R1 to R8 are as hereindefined, with the proviso that at least one of R4 to R8 is other than hydrogen, and further with the proviso that when R1 and R2 are both amino, or when R1 is hydroxy and R2 is amino, and R3 is alkyl