1. A compound of formula (I):wherein:A is selected from the group consisting of:
O, S, O(C?S)S, ONH, ONR?, NR?O, NHO,
Y1(CH2)nY2, where n is an integer from 1 to 8, and
Y1(CH2CH2O)aCH2CH2Y2 where a is an integer from 1 to 5,
wherein Y1 and Y2 are independently selected from O, NH, S, OC(O), C(O)NH, NHC(O), OC(S)S, SC(S)ONR?, ONH, NHO, ONR?, NR?O, OC(O)O, NR?C(S)S, SC(S)NR?, NR?C(O), C(O)NR? and C(O)O, and R? is a C1-C6 alkyl,
and wherein R1, R2 and R3 are independently selected from the group consisting of H, CH(CH3)CH2X and (CH2)2X, X being a halogen atom,
or a pharmaceutically acceptable salt or solvate thereof.