[Melatonin suppresses benz(a)pyrene-induced carcinogenesis in mice]

Skin tumors were induced in 3 groups of out-bred mice SHR by painting with 0.05% solution of benz(a)pyrene and 0.2ml acetone beginning from the age of 3 months. Each group included 40 mice; another 10 intact animals were in control. From day 2 on, several experimental animals received melatonin 2 and 20 mg/l with drinking water daily at nighttime. Mice were decapitated 24 weeks later. Among the parameters under study were frequency, multiplicity, size, morphological pattern, latent period of tumors and survival. Lipid peroxidation was evaluated on the basis of levels of malonic dialdehyde (MDA) and catalase in blood serum and tumor tissue. Tumor frequency in controls was 69.4%. That index in melatonin-treated mice fell as follows: 2 mg/l- 1.9 times (p<0.01); 20 mg/l - 2.2 times (p<0.05). Following melatonin 2 mg/l and 20 mg/l, the number of tumors per animal fell by 30.6% (p<0.05) and 27.4% (p<0.05), respectively; medium and maximum size of tumor decreased significantly too. There was no correlation between melatonin treatment and latent period duration. Melatonin 20 mg/l was followed by shorter survival after tumor development whereas 2 mg/l produced the opposite effect. Benz(a)pyrene boosted blood serum MDA by 190%, catalase - by 267% and 116% in tumor tissue as compared with untreated controls. Melatonin treatment supressed MDA and catalase levels in blood serum but not in tumor tissue. Relatively smaller doses exerted a more marked effect.