Advances in the development of retinoids as chemopreventive agents.

With the inclusion of brief discussions of retinoid drug development in animal carcinogenesis models (e.g., skin, breast, oral cavity, lung, prostate or bladder) and clinical trials (e.g., head and neck or cervix), this review will focus on recent advances in retinoid molecular targeting studies designed primarily to develop retinoids with reduced toxicity, while maintaining or enhancing activity in the context of chemoprevention. Major current retinoid molecular targets include the six known nuclear retinoid receptors (RAR and RXR). Receptor numbers, distinct functions, tissue-expression patterns, ligand specificities, functional redundancy and regulation of multiple pathways make retinoid signaling highly complex. Development of receptor-selective synthetic retinoids is a major focus of molecular retinoid development. RAR heterodimerize with RXR and mediate classic retinoid activity/toxicity. RXR are more promiscuous, heterodimerizing with several other members of the steroid receptor superfamily [e.g., peroxisome proliferator-activated receptors (PPAR) or vitamin D receptors]. RXR-selective ligands are less toxic and more active in animal breast cancer prevention studies and less toxic than RAR ligands in clinical trials. Other new avenues of retinoid molecular drug development include newly identified retinoid-regulated genes, orphan-receptor ligands/functions, novel retinoid mechanisms involving potent receptor-independent apoptosis-inducing activity (e.g., 4-HPR or anhydroretinol), synergistic combinations [e.g., RXR agonists plus selective estrogen receptor modulators (SERM)], activity in other diseases and novel delivery systems.