TORONTO (Reuters) - Psoriasis and smoking don't have a lot in common - except a drug that might treat both the skin condition and people who want to quit the tobacco habit.

Dr. Edward Sellers, a pharmacology professor at the University of Toronto, has found that the psoriasis medication Methoxsalen relieves heavy smokers from their longing for nicotine. Normally Methoxsalen is given in combination with intense ultraviolet light to patients with psoriasis, which is a chronic skin disorder characterised by reddish plaques covered with dry silvery scales and often severe itching. Sellers and his team identified a positive side effect of Methoxsalen. ``We found out that this drug is a very potent inhibitor of the enzyme CYP2A6, that metabolises nicotine,'' he told Reuters. The enzyme breaks down the nicotine in the human body, so that half the amount contained in a cigarette is removed within an hour. ``Therefore nicotine-addicted smokers have to 'fill up the tank' with more nicotine. Typically a smoker will have a cigarette every hour,'' Sellers said. Methoxsalen blocks the enzyme so nicotine will stay in the body longer. ``If you can slow the breakdown of nicotine, people will smoke less and get into a position where they can actually stop,'' Sellers explained. The University of Toronto's recent study was on a small scale - 12 people got only a single dose of the drug. Nevertheless the study gave strong evidence the medication can help smokers wean themselves from the habit. Smokers in the study who got Methoxsalen smoked 25 per cent less. In combination with a nicotine replacement treatment, such as the patch, the figure rose to as much as 50 per cent less. Long term studies will need to evaluate the treatment to ensure the drug is both effective and safe in high doses. For psoriasis, only single acute doses are given. ``For it to be effective in smoking, we would have to give it chronically every day,'' Sellers said. He is already working on a bigger study with about 30 smokers, which should be completed within a few months.

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