Inhibition of lung tumourigenesis by sulindac: comparison of two experimental protocols

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent lung carcinogen in mice and is most likely involved in the aetiology of tobacco-induced lung cancer. Two protocols using NNK and A/J mice have been developed. In the single-dose protocol, each mouse was injected once with 2 mg of NNK. In the 7-week protocol, each mouse received 9.1 mg of NNK in drinking water during 7 weeks. Mice were killed 16 weeks after NNK treatment. We observed a near-Gaussian distribution in the number of tumours per mouse in the single protocol, but not in the 7-week protocol. In the 7-week protocol, a significant number (8.6%) of mice had more than 20 tumours/mouse. In the single-dose protocol, no mouse had more than 20 tumours. Sulindac at a dose of 123 mg/kg of diet inhibits lung tumourigenesis in the 7-week protocol, but not in the single-dose protocol. We observed that the inhibition of tumourigenesis in the 7-week protocol was proportional to the logarithm of the dose of sulindac between 15 and 123 mg/kg of diet. Treatment of mice for 7 weeks inhibits the primary humoral response to sheep red blood cells by 70%. This observation is particularly significant considering that NNK is present in tobacco smoke and that tobacco smoking suppresses both the specific and non-specific humoral and cellular immunity. Single injections of 2.0, 3.5 or 5.0 mg of NNK had no effect on this response. Our results suggest that the immunosuppressive effects of NNK contribute to its high carcinogenic potency particularly in sustained or life-time exposure models. We hypothesize that sulindac promotes the recovery of immune system from the NNK-mediated suppression observed in the 7-week protocol. This study illustrates the importance of selecting the most appropriate protocol of carcinogen treatment in investigating the efficacies of cancer chemopreventive agents.