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In human clinical trials it was found that the putative chemopreventive agent beta-carotene not only failed to protect active smokers against the carcinogenic action of tobacco smoke, but actually increased their risk of developing lung cancer. In preclinical animal studies, beta-carotene had been effective against some chemically induced cancers, but not against tumors in the respiratory tract. We exposed male strain A/J mice to tobacco smoke at a concentration of 140 mg/m(3) of total suspended particulate matter, 6 h a day, 5 days a week, for either 4 or 5 months, followed by a recovery period in air for 4 or 5 months, or for 9 months without recovery period. beta-carotene was added in the form of gelatin beadlets to the AIN-93G diet either during or following tobacco smoke exposure at concentrations of 0.005, 0.05 and 0.5%. In the supplement-fed animals, plasma and lung levels of beta-carotene were higher than they were in animals fed control diets. Exposure to tobacco smoke increased rather than decreased plasma beta-carotene levels, but had no significant effect on lung levels. After 9 months, lung tumor multiplicities and incidence were determined. Tobacco smoke increased both lung tumor multiplicities and incidences, but beta-carotene failed to modulate tumor development under all exposure conditions. Animal studies in a model of tobacco smoke carcinogenesis would thus have predicted the absence of any beneficial effects of beta-carotene supplementation in current or former smokers, but would have failed to anticipate the increase in lung cancer risk.

Lung Tumor Development in Mice Exposed to Tobacco Smoke and Fed beta-Carotene Diets