Abstract
L-[methyl-11C]Methionine (11C-Met) is useful for estimating the therapeutic efficacy of particle radiotherapy at early stages of the treatment. Given the short half-life of 11C, the development of longer-lived 18F- and 123I-labeled probes that afford diagnostic information similar to 11C-Met, are being sought. Tumor uptake of 11C-Met is involved in many cellular functions such as amino acid transport System-L, protein synthesis, and transmethylation. Among these processes, since the energy-dependent intracellular functions involved with 11C-Met are more reflective of the radiotherapeutic effects, we evaluated the activity of the amino acid transport System-A as an another energy-dependent cellular function in order to estimate radiotherapeutic effects. In this study, using a carbon-ion beam as the radiation source, the activity of System-A was evaluated by a specific System-A substrate, alpha-[1-14C]-methyl-aminoisobutyric acid (14C-MeAIB). Cellular growth and the accumulation of 14C-MeAIB or 14C-Met were evaluated over time in vitro in cultured human salivary gland (HSG) tumor cells (3-Gy) or in vivo in murine xenografts of HSG tumors (6- or 25-Gy) before and after irradiation with the carbon-ion beam. Post 3-Gy irradiation, in vitro accumulation of 14C-Met and 14C-MeAIB decreased over a 5-day period. In xenografts of HSG tumors in mice, tumor re-growth was observed in vivo on day-10 after a 6-Gy irradiation dose, but no re-growth was detected after the 25-Gy irradiation dose. Consistent with the growth results, the in vivo tumor accumulation of 14C-MeAIB did not decrease after the 6-Gy irradiation dose, whereas a significant decrease was observed after the 25-Gy irradiation dose. These results indicate that the activity of energy dependent System-A transporter may reflect the therapeutic efficacy of carbon-ion radiotherapy and suggests that longer half-life radionuclide-labeled probes for System-A may also provide widely available probes to evaluate the effects of particle radiotherapy on tumors at early stage of the treatment.
Highlights
The use of particle beams such as carbon-ions and protons have several advantages over conventional X- or γ-ray beam radiotherapies, the most prominent of which is a lower radiation dose delivered to normal tissues surrounding the tumor [1]
Among the cellular functions involved in 11C-Met such as transport (System-L), protein synthesis, and transmethylation [8, 17,18,19,20], energy-dependent intracellular functions are more reflective of the therapeutic effects of carbon-ion radiotherapy [21] and conventional radiotherapy [40, 41]
It is with these considerations, we evaluated whether the activity of the amino acid transport System-A may have a potential utility in estimating the radiotherapeutic effect at early stage of the treatment, analogues to those observed with 11C-Met
Summary
The use of particle beams such as carbon-ions and protons have several advantages over conventional X- or γ-ray beam radiotherapies, the most prominent of which is a lower radiation dose delivered to normal tissues surrounding the tumor [1]. Other potential advantages over the use of X- or γ-ray radiotherapies include a higher relative biological effectiveness (RBE) and a lower oxygen enhancement ratio [1, 2]. Particle beams have a relatively higher RBE and display less difference in sensitivity between the cell lines, compared with X- or γ-ray beams, the radiotherapeutic response of tumors post beamirradiation differed from the cell-lines or the tumor site [1, 6]. An effective non-invasive means to evaluate the therapeutic efficacy of particle beam radiotherapies, as well as X- or γ-ray radiotherapy, early in the treatment protocol would be valuable
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