Abstract: The present invention relates to compounds of formula (I), and pharmaceutically acceptable salts, prodrugs, solvates and crystalline forms thereof, in which R1 and R2 independently represent: a C1-6alkyl group; an optionally substituted (amino)C1-4alkyl-group; an optionally substituted non-aromatic C3-15carbocyclic group; a (C3-12cycloalkyl)C1-3alkyl-group; a group —(CH2)r(phenyl)s in which r is 0, 1, 2, 3 or 4, s is 1 when r is 0 otherwise s is 1 or 2 and the phenyl groups are optionally independently substituted by Z; naphthyl; anthracenyl; an optionally substituted saturated 5 to 8 membered heterocyclic group containing one nitrogen and optionally one of the following: oxygen, sulphur or an additional nitrogen; 1-adamantylmethyl; a group —(CH2)t Het in which t is 0, 1, 2, 3 or 4, and the alkylene chain is optionally substituted and Het represents an optionally substituted aromatic heterocycle; or R1 represents H and R2 is as defined above; or R1 and R2 together with the nitrogen atom to which they are atta