The invention provides a compound having the formula (1): and salts thereof； wherein: R1 is hydrogen or C1-2 alkyl； and R2, R3 and R4 are the same or different and each is selected from hydrogen, C1-2 alkyl, fluorine, chlorine, C1-2 alkoxy and trifluoromethyl, provided that no more than two of R2, R3 and R4 are other than hydrogen. Also provided are pharmaceutical compositions containing the compounds and their use in medicine, and in particular in the treatment of cancer.
