CANCER CHEMOPREVENTION

Purpose: To review the most important recent advances in clinical trials and biologic studies within the growing field of chemoprevention. Methods: The most critical methods issue concerns the definitive end point of phase III trials, which is now cancer incidence. This end point usually needs thousands of subjects monitored for 5 to 10 or more years to determine efficacy. Biologic markers of potential intermediate end points are under intensive study and may one day replace cancer incidence. Validated intermediate end point biomarkers could greatly reduce phase III trial populations, durations, and costs. Results: Randomized clinical trials over the last 5 years have produced significant activity in reversing oral, skin, colon, and cervical premalignancy; in preventing primary skin and stomach cancer; and in preventing second primary tumors associated with head and neck and lung cancer. These clinical advances have been paralleled at the basic science level by elegant molecular studies of premalignant carcinogenesis and of chemopreventive agents' mechanisms of action. One major laboratory advance is the discovery of nuclear retinoic acid receptors and strong evidence of their roles both in carcinogenic progression and in its response to retinoids. Conclusion: Chemoprevention has matured greatly in recent years with the significant reversal or suppression of premalignancy by chemopreventive agents in several sites. The future of chemoprevention will be determined largely by several ongoing phase III trials, including trials of retinoids, β-carotene, and α-tocopherol in the aerodigestive tract, of tamoxifen and fenretinide in the breast, and of finasteride in the prostate.