The invention relates to a compound of Formula (I):(see formula I)or a pharmaceutically acceptable or a cosmetically acceptable salt thereof,wherein: R1 isCOOR10; Z is a bond or O-C1-4alkyl; R2 is hydrogen; R3 is hydroxyl; X is -CH=CH- or-N=N-; R4 is hydroxy or -NR15R16; R5 is hydrogen or -NR15R16; R6 is hydrogen;R7 ishydrogen; R8 is hydrogen or halo; R9 is hydroxy; R10 is hydrogen; R15 and R16are eachindependently hydrogen.