Arginine deiminase: A novel radiosensitizer in pancreatic cancer in vitro and in vivo.

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221 Background: The benefits of chemoradiation therapy in patients with locally advanced pancreatic cancer (LAPC) are limited due to the inherent radioresistance of pancreatic cancer (PC) and high systemic toxicity of current radiosensitizers (e.g., gemcitabine). Hence, the search for newer radiosensitizers with unique anticancer properties continues. Single amino acid arginine starvation is a new promising therapeutic approach for solid tumors (e.g., PC), that are auxotrophic for non-essential amino acids. Arginine degrading enzyme, arginine deiminase (ADI), deprives cells of arginine and thereby exerts its anti-proliferative effects, especially in cancer cells deficient in enzyme argininosuccinate synthase (ASS1). Here we evaluate the effects of ADI-polyethylene glycol formulation (ADI-PEG20) as a radiosensitizer in PC. Methods: The toxicity of ADI-PEG20 in vitro was evaluated using XTT. Effect of ADI-PEG20 as radiosensitizer was determined by clonogenic cell survival. For in vivo, mice with PC tumor xenografts (Panc1), randomized into four groups, were treated with vehicle (PBS), ADI-PEG20 (5 IU/mouse; twice weekly), radiation (IR; 2 Gy × 5 times), and ADI-PEG20 with IR. Tumors were measured following treatment and the tumor re-growth delay time for each group was calculated. Immunohistochemical analysis of Ki-67 and VEGF was done on tumor tissues (paraffin sections) by routine immunofluorescence. Results: ADI-PEG20 selectively sensitized ASS1 deficient PC cells to IR at low, non-toxic concentrations (0.04 and 0.08 μg/mL for 72 h; DER at 10% SF for Panc1 was 1.39 and 1.52; for Miapaca-2, 1.09 and 1.25 respectively), but not ASS1 positive cells (L3.6pl). In vivo, ADI-PEG20 profoundly sensitized PC cells to IR. IR treatment alone delayed the tumor doubling time (7.6 ± 1.7 days compared to the non-treated controls); however, combining ADI-PEG20 with IR delayed the tumor growth by an additional 10 ± 1.3 days (p<0.05). Immunohistochemical analysis of tumor tissues suggested that ADI-PEG20 with IR down-regulates the expression of Ki-67 and VEGF. Conclusions: ADI-PEG20 potently radiosensitizes PC cells in vitro and in vivo. The detailed molecular mechanism of this radiosensitization warrants further investigations.