23. Liver Damage and Oxidative Stress Caused by Irradiation of Heavy Ion Beams

Keizo Takeshita, Hideyuki Majima and Toshihiko Ozawa

Keywords: in vivo ESR, radical, heavy ion beam, radiation, liver, reactive oxygen species

Heavy ion beam irradiation is a promising therapeutic technique for inveterate cancers because of its excellent dose distribution and high biological effects. It is very important to know the damage mechanisms by heavy ion beams to prevent the damage of normal tissues and to obtain the best therapeutic effects. However, the damage mechanisms for normal tissues are not fully clear. To clarify them, we examined the relation of oxidative stress to liver damage caused by whole body irradiation of heavy ion beams to mice.

Heavy ion beams (290 MeV/u carbon beams, 6 cm spread-out Bragg Peak, 60 KeV/mm LET) were generated with the Heavy Ion Medical Accelerator at NIRS. Body weight and liver wet weight of mice decreased after more than 16 h with a 7.5 Gy irradiation. A remarkable increase of serum GOT was also observed 16 h after irradiation. Thiobarbituric acid-reactive substances (TBARS) in liver homogenates significantly increased more than 2 days after irradiation, indicating occurrence of lipid peroxidation in liver. The mortality was about 30 % on the 4th day after a 15 Gy irradiation, and liver TBARS of surviving mice were similar to those for the 7.5 Gy irradiation.

To evaluate enhancement of in vivo radical reaction, the in vivo ESR technique was used with a nitroxyl redox probe, 3-carbamoyl-2,2,5,5-tetramethylpyrrodine-1-yloxy (carbamoyl-PROXYL), which is known to be converted to an ESR-silent form by reaction with oxygen radicals and other reducing compounds. In vivo ESR measurements were made at the upper abdomen of mice immediately after intravenous injection of an aqueous solution of probe. Signal decay rate of probe (spin clearance) increased at 1-2 h after the 7.5 Gy irradiation and then decreased after more than 16 h. Spin clearance measured at 1-2 h after the 15 Gy irradiation was similar to that after the 7.5 Gy irradiation. This dose response was different from that for X-rays which we have reported previously. This may reflect difference in biological effects of heavy ion beams and X-rays.

Increases of liver TBARS and spin clearance at the upper abdomen suggest the possibility of oxidative stress in liver. The early increase of spin clearance may reflect enhancement of the radical reaction in the initial stage of heavy ion damage. Increases of spin clearance prior to damage have been reported with animals which received various oxidative stresses, including liver damage by carbon tetrachloride and ammonia-induced gastric ulcer. In these cases, the increase of spin clearance was reduced by administration of radical scavengers, indicating that the increase of spin clearance resulted from radical reactions in animal bodies. To clarify the meaning of the increase of spin clearance observed in this study, further experiments are necessary.