Mechanism of action and clinical potential of the selective tumor radiosensitizer nicotinamide

Abstract

We have shown recently that the vitamin nicotinamide (niacinamide) can enhance the radiation response of several mouse tumors with little or no increase in the radiosensitivity of normal tissues (Horsman et al., Radiat. Res. in press). Using an in vivolin vitro survival assay, performed 24 hours after -Frxiation with 250 kV X-rays, we found tGt?-Targe single dose of nicotinamide (1000 mg/Kg) injected 1-2 hours prior to irradiation produces enhancement ratios (ER's) of 1.4-1.7 in 3 tumor models (EMT6, Lewis Lung and RIF-1). For the EMT6 tumor these ER's were dependent on the administered nicotinamide dose, but even at doses as low as 25% of the LD50 value (LD50 = 2000 mg/Kg) an ER greater than 1.5 could be observed. In 2 normal tissue assays (jejunal crypt cell survival and mean skin reaction) ER's of less than 1.2 were obtained using 1000 mg/Kg nicotinamide. In the present study, we have attempted to understand the mechanism for this preferential tumor radiosensitization. Our data suggest it is not a direct effect of nicotinamide or its metabolites on the tumor cells. First, we have found no radiosensitization of hypoxic or aerobic EMT6 cell in vitro with concentrations of nicotinamide up to 10 mM (greater than measured tumor concentrations foTo_a dose of 1000 mg/Kg). Second, if a clamp is placed around the tumors 90 minutes after injecting nicotinamide and immediately prior to irradiation, no radiosensitization of the hypoxic tumors is observed. Tumor clamping does not affect the extent of radiosensitization with the electron-affinic radiosensitizer SR 2508. Third, no tumor radiosensitization is seen with the major in vivo metabolite nicotinic acid. These findings suggest an indirect effect of nicotinamide, and 0% studies suggest that radiosensitization is a consequence of improved tumor oxygenation and blood flow. The oxygenation status of tumors after nicotinamide treatment was measured using the binding of I4C-misonidazole (I4C-MISO) to tumor cells, since a good correlation exists between tumor radioresistance produced by hypoxia and 14C-MISO binding. Nicotinamide (1000 mg/Kg) reduces the binding of I4C-MISO in the EMT6 tumor by as much as 8056, suggesting that nicotinamide increases tumor oxygenation. We have also measured tumor blood flow following nicotinamide administration. With one technique we observed a 40-60% increase in mean tumor cell fluorescence from the fluorescent dye Hoechst 33342 following nicotinamide injection indicating increased tumor perfusion by this i.v. injected drug. We are also in the process of measuring the effect of nicotinamide on tumor blood flow using clearance of I33Xenon injected directly into the tumor. Nicotinamide has been administered to patients with a variety of disorders. A reasonably safe dose associated with few side effects is 6 g daily. This is approximately double the daily dose achievable with SR 2508, and at these concentrations, SR 2508 and nicotinamide produce equivalent tumor radiosensitization in mice. Thus, it would appear that if similar increases in tumor oxygenation and blood flow can be produced in human tumors by nicotinamide, this drug may prove a simple and useful sensitizer in radiotherapy. Supported by Grant CA 15201 from the National Cancer Institute, DHHS.