Lung cancer: chemoprevention and intermediate effect markers.
Abstract
Even after smoking cessation, genetic damage in the airways epithelium may lead to focal progression of lung carcinogenesis. Some centres now report as many new lung cancer cases among former smokers as among current smokers. Chemoprevention is a potential approach to diminish the progression of pre-clinical genetic damage. The most intensively studied lung cancer chemoprevention agents are the retinoids, including vitamin A and its synthetic analogues and precursors. While effective in suppressing lung carcinogenesis in animal models, retinoids have failed to inhibit carcinogenesis in human chemoprevention trials with premalignant end-points (sputum atypia, bronchial metaplasia). In trials with lung cancer end-points, administration of retinoids either was ineffective or, in the case of beta-carotene, led to greater lung cancer incidence and mortality. In view of these findings, markers of specific retinoid effect (i.e., levels of RAR-beta) become less relevant. Other markers of genetic instability and proliferation may be useful for both early detection and potentially as intermediate-effect markers for new chemoprevention trials. Cytological atypia, bronchial metaplasia, protein (hnRNP A2/B1) overexpression, ras oncogene activation and tumour-suppressor gene deletion, genomic instability (loss of heterozygosity, microsatellite alterations), abnormal methylation, helical CT detection of atypical adenomatous hyperplasia and fluorescent bronchoscopic detection of angiogenic squamous dysplasia offer great promise for molecular diagnosis of lung cancer far in advance of clinical presentation. These end-points can now be evaluated as monitors of response to chemoprevention as potential intermediate-effect markers.