Intracellular reactions affecting 2-amino-4-([(11)C]methylthio)butyric acid ([(11)C]methionine) response to carbon ion radiotherapy in C10 glioma cells.

PURPOSE: The response of 2-amino-4-([(14)C]methylthio)butyric acid ([(14)C]Met) uptake and [(125)I]3-iodo-alpha-methyl-l-tyrosine ([(125)I]IMT) uptake to radiotherapy of C10 glioma cells was compared to elucidate the intracellular reactions that affect the response of 2-amino-4-([(11)C]methylthio)butyric acid ([(11)C]Met) uptake to radiotherapy. METHODS: After irradiation of cultured (3 Gy) or xenografted C10 glioma cells (25 Gy) using a carbon ion beam, the accumulation of [(14)C]Met and [(125)I]IMT in the tumors was investigated. The radiometabolites in xenografted tumors after radiotherapy were analyzed by size-exclusion HPLC. RESULTS: [(14)C]Met provided earlier responses to the carbon ion beam irradiation than [(125)I]IMT in both cultured and xenografted tumors. While [(125)I]IMT remained intact in xenografted tumor before and after irradiation, the radioactivity derived from [(14)C]Met was observed both in high molecular fractions and intact fractions, and the former decreased after irradiation. CONCLUSION: The earlier response of [(11)C]Met uptake to tumor radiotherapy could be attributable to the decline in the intracellular energy-dependent reactions of tumors due to radiotherapy.