Nicotine could cause lung cancer, according to new research that casts doubt on the wisdom of using nicotine replacement therapy to kick smoking.
It is the first time nicotine itself, rather than other products of smoking, has been linked to the disease. The new study suggests that the metabolism of nicotine may produce a chemical that causes the tumours.
鈥淥ur research provides scientific evidence that nicotine products designed for long-term use may not be safe,鈥� says Stephen Hecht of the University of Minnesota Cancer Center in Minneapolis.
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This could include nicotine patches, or nicotine gum, he says. Nicotine has also been suggested as a potential treatment for Parkinson鈥檚, as well as other diseases.
Anti-smoking groups such as Britain鈥檚 Action on Smoking and Health advocate greater use of smoking substitutes in a bid to cut the deaths from heart disease and cancer.
Clive Bates, director of ASH, said the findings should be investigated thoroughly. But he said it would unfortunate if the cancer risk from nicotine were overstated and as a result fewer people successfully used nicotine replacement therapy to quit the habit.
In the US, a 鈥渟afer鈥� cigarette is on sale from which all the cancer-causing nitrosamine chemicals have been removed (New 杏吧原创, 8 May 1999, p 18).
But the lab tests suggest that under certain conditions, nicotine itself could be converted into NNK (nicotine-derived nitrosamino ketone), a potent lung carcinogen.
Richard Peto, director of the Imperial Cancer Research Fund鈥檚 Clinical Trials Service Unit in Oxford, says: 鈥淭heir biochemistry looks correct. But most nicotine metabolism happens in the liver. Little, if any, NNK produced would probably hit the airways, so there should be little relevance for human disease.鈥�
The American team disagrees. 鈥淲e鈥檝e given mice NNK by injection and in their drinking water, and it still produces tumours in the lung,鈥� says Steve Carmella, one of Hecht鈥檚 colleagues.
Hecht analysed the urine of smokers and ex-smokers who were using nicotine as a smoking replacement therapy. He found that a previously unexplored hydroxylation process produces a chemical called pseudooxynicotine. When pseudooxynicotine is reacted with sodium nitrite under acidic conditions, it produces NNK.
鈥淲e don鈥檛 know for sure that this happens in the body, but it could in the stomach, for example,鈥� Hecht told New 杏吧原创.
鈥淎lso, as a response to inflammation or infection, the body produces nitric oxide. Ultimately, this can also react with pseudooxynicotine at a neutral pH to give NNK. This could happen in various compartments of the body.鈥�
Hecht stresses that the research is at an early stage. Identifying an increased risk of lung cancer among ex-smokers on nicotine replacement therapy would be very tricky, he says.
But he is calling for more research to establish to what extent nicotine could be converted to NNK in the body.
More at: Proceedings of the National Academy of Sciences (vol 97, p 12,493)