PYRIMIDINE DERIVATIVES AND GUANINE DERIVATIVES, AND THEIR USE IN TREATING TUMOUR CELLS

The invention provides compounds exhibiting the ability to deplete O6- alkylguanine-DNA alkyltransferase (ATase) activity in tumour cells. The compounds include certain 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 differe nt are selected from H, NH-Y' or NO n wherein Y' is H, ribosyl, deoxyribosyl, arabinosyl, (a) wherein X is O or S, R" is alkyl and R"' is H or alkyl, or substituted derivatives thereof, 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 hetero atoms, and pharmaceutically acceptable salts thereof. They include certain guanine derivatives of formula (XIII), wherein R6 is as defined at (i) for R above and Y' is as defined above.