Background: Metalloporphyrin antioxidants can protect tissues against radiation-induced damage. However, for effective use in radiotherapy as normal tissue radioprotectants, they must not protect the cancer. The major objectives were to evaluate the effects of Mn (III) tetrakis (N-ethylpyridinium-2-yl) porphyrin (MnTE-2-PyP) on tumor response to radiation and to explore mechanisms responsible for the observed effects.
Materials and methods: C57BL/6 mice were subcutaneously (s.c.) injected with RM-9 prostate tumor cells on day 0 and grouped according to treatment with MnTE-2-PyP (s.c. 6 mg/kg/day beginning on day 1 for 16 maximum days), 10 Gray (Gy) single fraction radiation on day 7, a combination of both or neither. Subsets per group and non-tumor bearing controls were evaluated for leukocyte populations, red blood cell (RBC) and platelet characteristics and cytokines on day 12; the remaining mice were followed for tumor growth.
Results: Although radiation alone significantly slowed tumor growth and the addition of MnTE-2-PyP resulted in slightly slower tumor progression, the difference between radiation and radiation plus drug was not statistically significant. However, the treatment with drug alone significantly elevated T (helper, Th and cyotoxic, Tc) and natural killer (NK) cells in the spleen, B-cells in the blood and spleen, and the capacity to produce interleukin-2. The addition of the drug to radiation did not ameliorate the depression seen in all the major leukocyte types, but did protect against radiation-induced decreases in RBC counts, hemoglobin and hematocrit. Vascular endothelial growth factor (VEGF) increased in the plasma from both the irradiated groups and a trend for increased transforming growth factor-beta1 (TGF-beta1) was noted with radiation alone.
Conclusion: MnTE-2-PyP did not protect RM-9 prostate tumors against radiation damage and was not toxic under the conditions used. The drug-induced enhancement of certain immune parameters suggests that MnTE-2-PyP may be beneficial not only as a normal tissue radioprotectant, but also as a facilitator of antitumor immunity.