There are provided according to the invention novel compounds of formula (I) and pharmaceutically acceptable salts and solvates thereof: wherein R1 is selected from the group consisting of C1-4alkyl, C4-6cycloalkyl(CH2)m—, methoxyC2-4alkyl, HOCH2CH2—, R3(O)2S(CH2)2—, R5R4NCO(CH2)n—, and heterocyclyl(CH2)m— wherein any nitrogen heteroatom of the heterocyclyl radical may be unsubstituted or substituted by methyl; R2 is hydrogen or methyl; R3 is methyl or NH2; R4-5 independently represent methyl, m is 0, 1 or 2; and n is 1 or 2; or R1 and R2 together with the nitrogen atom to which they are attached may form a heterocyclyl ring, which may be unsubstituted or substituted by one or two substituents selected from the group consisting of; methyl, ═O and (CH3)2N—.