C-13 AND P-31 NMR INVESTIGATION OF EFFECT OF 6-AMINONICOTINAMIDE ON METABOLISM OF RIF-1 TUMOR-CELLS IN-VITRO

The effect of 6-aminonicotinamide on the metabolism of RIF-1 tumor cel ls was investigated using C-13 and P-31 NMR spectroscopy, 6-Aminonicot inamide can be metabolized to 6-amino-NAD(P), a competitive inhibitor of NAD(P)-requiring processes. 40 mu M 6-aminonicotinamide led to an i nhibition of 6-phosphogluconate dehydrogenase and an accumulation of 6 -phosphogluconate. A subsequent accumulation of the 6-phosphogluconate precursor 6-phosphoglucono-delta-lactone was observed in the C-13 NMR spectrum, These metabolites were shown to be intracellular, although a small amount of leakage of 6-phosphoglucono-delta-lactone occurred. The intracellular concentrations of 6-phosphogluconate and 6-phosphogl ucono-delta-lactone were 1.9 +/- 0.8 mu mol/10(8) cells (+/- 1 standar d deviation) and 0.8 +/- 0.4 mu mol/10(8) cells, respectively, after 1 5 h. Glucose utilization and lactate production were significantly inh ibited by 6-aminonicotinamide (both p < 0.05), indicating inhibition o f glycolysis, P-31 NMR data showed that phosphocreatine was significan tly depleted in cells exposed to 6-aminonicotinamide (p < 0.05), Expos ure of RLF-1 cells to 6-aminonicotinamide prior to 3- or 6-Gy x-irradi ation induced a supra-additive cell kill, indicating that 6-aminonicot inamide is acting as a radiosensitizer. There was no effect of 6-amino nicotinamide alone or when the drug was given postradiation, suggestin g that its mechanism of action may be by inhibition of radiation-induc ed repair.