Pyrimidine derivatives and guanine derivatives, and their use in treating tumor cells

The present invention provides certain 6-hetarylalkyloxy pyrimidine derivatives of formula II wherein R is (i) a cyclic group having at least one 5- or 6-membered heterocyclic ring, optionally with a carbocyclic or heterocyclic ring fused thereto, the or each heterocyclic ring having at least one hetero atom chosen from O, N, or S, or a substituted derivative thereof; or (ii) phenyl or a substituted derivative thereof, R2 is selected from H, C1-C5 alkyl, halogen or NH2, R4 and R5 which are the same or different are selected from H, NH2 or NOn where n=1 or 2, or R4 and R5 together with the pyrimidine ring form a 5- or 6-membered ring structure containing one or more heterocyclic atoms, and pharmaceutically acceptable salts thereof, exhibit the ability to deplete O6-alkylguanine-DNA alkyltransferase (ATase) activity.