A compound of formula I: ##STR00001## and isomers, salts, solvates, chemically protected forms, and prodrugs thereof, wherein: R.sup.1 and R.sup.2 are independently selected from hydrogen, an optionally substituted C.sub.1-7 alkyl group, C.sub.3-20 heterocyclyl group, or C.sub.5-20 aryl group, or may together form, along with the nitrogen atom to which they are attached, an optionally substituted heterocyclic ring having from 4 to 8 ring atoms; Q is --NH--C(.dbd.O)-- or --O--; Y is an optionally substituted C.sub.1-5 alkylene group; X is selected from SR.sup.3 or NR.sup.4R.sup.5, wherein, R.sup.3, or R.sup.4 and R.sup.5 are independently selected from hydrogen, optionally substituted C.sub.1-7 alkyl, C.sub.5-20 aryl, or C.sub.3-20 heterocyclyl groups, or R.sup.4 and R.sup.5 may together form, along with the nitrogen atom to which they are attached, an optionally substituted heterocyclic ring having from 4 to 8 ring atoms; if Q is --O--, X is additionally selected from --C(.dbd.O)--NR.sup.6R.sup.7, wherein R.sup.6 and R.sup.7 are independently selected from hydrogen, optionally substituted C.sub.1-7 alkyl, C.sub.5-20 aryl, or C.sub.3-20 heterocyclyl groups, or R.sup.6 and R.sup.7 may together form, along with the nitrogen atom to which they are attached, an optionally substituted heterocyclic ring having from 4 to 8 ring atoms; and if Q is --NH--C(.dbd.O)--, --Y--X may additionally be selected from C.sub.1-7 alkyl.