Purpose: Radiation causes damage to irradiated tissues and also tissues that do not receive
direct irradiation through a phenomenon called bystander effects. Melatonin as a potent antioxidant
and anti-inflammatory agent is known for protection of normal tissues against ionizing irradiation. In
addition, some studies have suggested that Melatonin may have some anti-cancer properties. Although
the complete mechanisms remain unknown, it can act via immunomodulatory effects. The aim
of this study was to evaluate the effect of pretreatment with melatonin on oxidative damage caused by
direct irradiation and bystander effects on the lung and heart tissue after xenograft mice colon cancer
irradiation in Balb/c mice.
Materials and Methods: Forty nine (49) Balb/c mice were evenly divided into 7 groups including
control, irradiation of 5 Gy directly to tumor, melatonin treatment (20 mg/kg) and irradiation (5 Gy
single fraction) directly to tumor, irradiation of 5 Gy directly to chest area, melatonin treatment (20
mg/kg) and radiation directly to chest, only melatonin treatment and whole-body scatter group (which
gave radiation dose equal to the amount of radiation that the lung had received from the localized pelvic
irradiation) to evaluate the effect of melatonin on the MDA level as well as SOD and GPx activity
after 24 and 72 h of irradiation of 5 Gy single fraction directly to the tumor tissue and chest area, in
the tumor tissue, the lung tissue, and the heart tissue.
Results: The results revealed that exposure to irradiation resulted in an increase in MDA level and
suppressed SOD and GPx activity in the targeted and non-targeted lung and heart tissues, and the tumor
tissue. Melatonin decreased MDA level in the lung and heart tissues. Also, melatonin improved
SOD and GPx activity in non-targeted tissues, while it was able to reduce these two enzymes and decreased
MDA level in the tumor tissue.
Conclusion: Melatonin exhibited its ability to ameliorate oxidative stress in both targeted and nontargeted
tissues. Administration of melatonin boosted SOD and GPx activity in the normal tissues, but
not in the tumor cells. Through stimulation and suppression of the antioxidant system, Melatonin may
cause sensitization of the tumor cells while protecting the normal tissues.