A compound of formula I: ##STR00001## and isomers, salts, solvates, chemically protected forms, and prodrugs thereof, wherein: R.sup.1 and R.sup.2 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 R.sup.N1 is selected from hydrogen, an optionally substituted C.sub.1-7 alkyl group, an optionally substituted C.sub.3-20 heterocyclyl group, an optionally substituted C.sub.5-20 aryl group, an acyl group, an ester group and an amido group, and its use as a pharmaceutical.