The effects of cystine and theanine mixture on the chronic survival rate and tumor incidence of rats after total body X-ray irradiation†.

Cystine and theanine (CT), an amino acid mixture, provides the substrates cysteine and glutamic acid that promote glutathione synthesis. We previously reported that CT pre-treatment significantly improved the acute survival rate and reduced acute radiation injury of the small intestine and bone marrow of rats after 5 Gy of total body X-ray irradiation. To examine the long-term effects of CT administration after irradiation, we investigated the effects of CT pre-treatment and pre- and post-treatment on the chronic survival rate and solid tumor (spleen, skin and subcutis, and thyroid) incidence after irradiation using 7-week-old male Wistar rats. CT pre-treatment of 280 mg/kg was administered orally for 5 days before 5 Gy irradiation, and CT pre- and post-treatment was administered 5 days before and 5 days after irradiation. A 0.5% carboxymethyl cellulose solution was administered as a control. The chronic survival rate of the pre-treated rats was higher than that of the control rats at 441 days after irradiation (40 vs 8.1%, P = 0.011). However, the survival rate did not significantly differ between the pre- and post-treatment and control rats at 467 days after irradiation (33.8 vs 30.2%, P = 0.792). In addition, more solid tumors, especially subcutis sarcomas, were observed in the pre-treatment rats (26.1%, 6/23) than in the control rats (4.5%, 1/22) after irradiation. Therefore, pre-administration of CT improves the chronic survival rate after irradiation; however, the occurrence of solid tumors was not suppressed.