Biological effects of novel poly (adenosine diphosphate ribose) polymerase inhibitors
Poly(ADP-ribose) polymerase (PADPRP) is a nuclear enzyme with a well documented role in DNA repair. Inhibitors of PADPRP, (e.g. 3' substituted benzamides) potentiate the cytotoxicity of a wide range of antitumour drugs. The results presented in this thesis represent, to the best of my knowledge, the first comprehensive and quantitative assessment of the ability of a range of P ADPRP inhibitors to modulate the cellular responses to damaging agents. Two novel PADPRP inhibitors, 8-hydroxy-2-methyl quinazolin-4(3H)-one (NU1025) and 3,4 dihydro-5-methoxyisoquinolin-1-(2H)-one (PD 128763) were compared with two "classical" PADPRP inhibitors, 3-aminobenzamide (3AB) and benzamide (BZ). The relative potencies for 3AB, BZ, NU1025 and PD 128763 as PADPRP inhibitors in vitro were 1.0, ~1.0, ~43 and ~53 respectively. All compounds potentiated the growth inhibition and cytotoxicity of the monofunctional alkylating agent temozolomide (TM) in L1210 cells. For example, 10/-lM NUI025 and PD 128763 gave dose enhancement factors (DEF) of ~2 at 100/0 survival, whereas ImM 3AB and 0.5mM BZ where required to give similar DEF values. Cellular NADl- levels were depleted up to 50% by 1-2mM TM and this depletion was completely prevented by coincubation with 50-100µM PD 128763 and 1-3mM 3AB. TM induced DNA single strand break levels were increased in a concentration dependent manner by the P ADPRP inhibitors. Overall, the relative potencies for ability of the compounds to potentiate TM induced growth inhibition, cytotoxicity and DNA single strand breaks showed good correlation with those determined in an in vitro inhibition study, with both NU1025 and PD 128763 exhibiting ~60 fold increased inhibitory activity as compared to 3AB. The PADPRP inhibitors per se did not effect the growth or survival of the L 121 0 cells, nor increase DNA strand breakage. NAD+ is the substrate for PADPRP. A L1210 cell line made resistant to tiazofurin (TZ) utilising a step wise selection protocol was shown to be deficient in nicotinamide mononucleotide adenyl transferase (NMNAT) , the final enzyme required for NAD+ biosynthesis. The consequences of a reduced NMNAT activity (<3% of the parental line ) and an ~40% reduction in intracellular NAD+ levels were determined. The resistant cells showed an ~3 fold increased sensitivity to TM as compared to the parental cells. Upon coincubation with increasing concentrations of NU1025 in the presence of a fixed concentration of TM, growth inhibition was potentiated ~70 fold in the resistant cells but only ~10 fold in the parental cell line, demonstrating the reduced level of competition between NAD+ and NUI025 for PADPRP. However, DNA single strand breaks were increased in the resistant compared to the parental cell line only when NU1025 was coincubated with TM. In contrast, in the presence of the PADPRP inhibitors alone, equivalent growth inhibitory effects were observed in each of the cell lines, suggesting inhibition of PADPRP was not the cytotoxic effector. The ~40% NAD+ depletion observed could therefore suggest, that NAD+ levels in the resistant cells were reduced to, or near to the KmNAD+ for PADPRP.