A compound of formula (I-a): wherein R1 is hydrogen, hydroxy, halo, C1-6 alkoxy, or C1-6 alkyl the drawing "---" and R2 together represent a double bond or the drawing "---" represents a single bond and R2 is hydrogen or hydroxy R3, R4 and R5 are independently hydrogen, hydroxy, NR10R11, C 1-6 alkoxy or C1-6 alkyl R6 is hydrogen, hydroxy, halo, C1-6 alkoxy or C1-6 alkyl R7 is hydroxy R8 and R9 are independently hydrogen, hydroxy, halo, C1-6 alkoxy or C1-6 alkyl and R10 and R11 are independently hydrogen, C1-6 alkyl or trialkyl silyl or a pharmaceutically acceptable salt thereof, with the proviso that when R1 represents hydrogen and "---" is a single bond then R2 does not represent hydrogen.
