Poly (adenine diphosphate-ribosyl)ation of a variety of nuclear proteins is the immediate response in most eukaryotic cells to DNA strand breaks. This modification is catalysed' by the chromatin bound enzyme Poly (ADP-ribose) polymerase (PADPRP). The enzyme is thought to be intimately involved in several cellular processes including cell differentiation, gene expression, transformation of oncogenes and repair of DNA damage. As a consequence, inhibitors of PADPRP are able to potentiate the cytotoxicity of many anti-tumour agents whose actions include DNA damage, and as such these inhibitors are potential resistance-modifying agents for use in cancer therapy. In order to probe the active site of the enzyme a series of potential mimics (i) of NAD' were synthesised from readily available 3-hydroxybenzamide. The conformation of the amide bond is considered to be important and for increased potency the amide carbonyl should be anti with respect to the nicotinamide C2-C3 bond. Based on this reported observation a series of novel inhibitors were synthesised, which include a series of benzoxazole-4-carboxamide analogues( ii) and 8-hydroxy-2- (substituted)quinazolin-4-one analogues (iii). The structure of the benzoxazole analogues are such that the amide is anchored into the required conformation by an intramolecular hydrogen bond between the carboxamide N-H and the benzoxazole nitrogen. The benzyloxybenzamide analogues had comparable potency to 3-hydroxybenzamide against PADPRP. However, both the benzoxazole-4-carboxamide and 8-hydroxy quinazolin-4-one series of analogues exhibited outstanding inhibitory activity against PADPRP. The most potent of the benzoxazole-4-carboxamide analogues (ii, R= phenyl) had an IC50 value of 2.1 p. M. Exceptional PADPRP inhibitory activity was observed in the 8-hydroxyquinazolin-4-one (iii) series, where R= CH3 (IC50 = 0.4 μ M) and R= 4-nitrophenyl (IC50 = 0.2 μM). Further in vitro evaluation has shown that 8-hydroxy-2-methylquinazolin-4[3H]-one potentiates cytotoxicity in temozolomide treated cells.