CHEMOPOTENTIATING EFFECTS OF A NOVEL NAD BIOSYNTHESIS INHIBITOR, FK866, IN COMBINATION WITH ANTINEOPLASTIC AGENTS

FK866 is a novel anticancer agent that was previously shown to interfere with NAD+ biosynthesis by inhibition of nicotinamide phosphoribosyltransferase and to initiate apoptosis in cancer cells. As NAD+ is involved in cellular DNA repair processes, the present in vitro study on THP-1 and K562 leukemia cells was conducted to investigate the cytotoxicity of FK866 combination treatment with various cytotoxic agents: the antimetabolite Ara-C, the DNA-intercalating agent daunorubicin and the alkylating compounds 1-methyl3-nitro-1-nitrosoguanidinium (MNNG) and melphalan. Cell viability after drug exposure was assessed by propidium iodide (PI) staining. Non-cytotoxic concentrations of FK866 (10-9M or less), applied simultaneously or 24 hours before adding cytotoxic agents, caused a depletion in the intracellular NAD+ and – to a lesser extent – NADH levels in THP-1 cells. After 48 and 72 hours treatment with daunorubicin and AraC, respectively, increased cell death was observed in THP-1 cells that were pretreated with FK866, as compared to cells exposed to antineoplastic drugs alone. However, this effect was transient, and there was no difference in cell survival after 72 hours incubation with daunorubicin or 96 hours with Ara-C. Non-toxic concentrations of FK866 added 8, 16, or 24 hours before starting treatment with the PARP-activating agent MNNG synergistically decreased intracellular NAD+ contents, and increased MNNG-induced cytotoxicity both in THP-1 and K562 cells for at least 72 hours. This effect was less pronounced when FK866 was used in combination with another alkylating agent, melphalan. The PARP inhibitor 3aminobenzamide delayed MNNG-induced cytotoxicity by 24 hours both in cells that were pretreated with FK866 and in non-pretreated cells. 48 hours later, the protective effect of 3-aminobenzamide could no longer be observed, but FK866-pretreated cells retained increased sensitivity to MNNG. In conclusion, the chemosensitizing effect of FK866 on cell death induced by antineoplastic drugs was particularly obvious in combination with substances like MNNG that cause NAD+ depletion per se. It was less pronounced and only transiently measurable in combination with daunorubicin, Ara-C, and melphalan, respectively. These results may indicate different levels of DNA damage implicated in the action of the cytotoxic agents used.