| abstract |
A compound of formula I: ** Formula ** or a pharmaceutically acceptable salt thereof, wherein: R is C2-4 alkyl; W is chosen from NRA, O and S; Z is chosen from a simple link, -C (O) -, NRA, O and S; X1 and X2 are independently selected from -OH, -NHRA and -C (O) OH; RA is H or C1-3 alkyl; m is 1, 2, 3 or 4; and n is 1, 2, 3 or 4. |