Affiliation: Preston Robert Tisch Brain Tumor Center, Department of Surgery, Duke University Medical Center, Box 3624 DUMC, Durham, North Carolina 27710 USA.
Despite intensive efforts to improve multimodal treatment of brain tumors, survival remains limited. Current therapy consists of a combination of surgery, irradiation and chemotherapy with predisposition to long-term complications. Identifying novel targeted therapies is therefore at the forefront of brain tumor research. This study explores the utility of a manganese porphyrin in a brain tumor model. The compound used is ortho isomer, manganese(III) meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin, MnTnHex-2-PyP5+. It is a powerful SOD mimic and peroxynitrite scavenger and a potent modulator of redox-based cellular transcriptional activity, able to suppress excessive immune and inflammatory responses and in turn proliferative pathways. It is further one of the most lipophilic compounds among cationic Mn(III) N-alkylpyridylporphyrins, and thus accumulates predominantly in mitochondria relative to cytosol. In mitochondria, MnTnHex-2-PyP5+ could mimic our key antioxidant system, mitochondrial superoxide dismutase, MnSOD, whose overexpression has been widely shown to suppress tumor growth. Importantly, MnTnHex-2-PyP5+ crosses blood brain barrier in sufficient amounts to demonstrate efficacy in treating CNS injuries. For those reasons we elected to test its effects in inhibiting brain tumor growth. This study is the first report of the antitumor properties of MnTnHex-2-PyP5+ as a single agent in adult and pediatric glioblastoma multiforme (D-54 MG, D-245 MG, D-256 MG, D-456 MG) and pediatric medulloblastoma (D-341 MED), and is the first case where a redox-able metal complex has been used in glioma therapy. When given subcutaneously to mice bearing subcutaneous and intracranial xenografts, MnTnHex-2-PyP5+ caused a significant (P ≤ 0.001) growth delay in D-245 MG, D-256 MG, D-341 MED, and D-456 MG tumors. Growth delay for mice bearing subcutaneous xenografts ranged from 3 days in D-54 MG to 34 days in D-341 MED. With mice bearing intracranial xenografts, MnTnHex-2-PyP5+ increases median survival by 33% in adult glioblastoma multiforme (D-256 MG; P ≤ 0.001) and 173% in pediatric medulloblastoma (D-341 MED, ≤ 0.001). The beneficial effects of MnTnHex-2-PyP5+ are presumably achieved either (1) indirectly via elimination of signaling reactive oxygen and nitrogen species (in particular superoxide and peroxynitrite) which in turn would prevent activation of transcription factors; or (2) directly by coupling with cellular reductants and redox-sensitive signaling proteins. The former action is antioxidative while the latter action is presumably pro-oxidative in nature. Our findings suggest that the use of Mn porphyrin-based SOD mimics, and in particular lipophilic analogues such as MnTnHex-2-PyP5+, is a promising approach for brain tumor therapy.