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