Retinoids are naturally occurring compounds and synthetic derivatives of retinol (vitamin-A alcohol) that show vitamin A activity. There are three generations of synthetic retinoids today. Manipulation of the polar group and the polyene side chain of vitamin A forms the first generation of retinoids, which includes tretinoin (all-trans-retinoic acid), isotretinoin (13-cis-retinoic acid), and alitretinoin (9-cis-retinoic acid). The aromatic retinoids, etretinate and acitretin, are produced by replacing the cyclic end group of vitamin A with different substituted and nonsubstituted ring systems and are synthetic retinoids of the second generation. The third-generation retinoids, tazarotene and adapalene, known as polyaromatic compounds, are topical agents for the treatment of psoriasis and acne (Fig. 27.3). Bexarotene is also a third-generation retinoid and is approved for the systemic treatment of cutaneous T-cell lymphoma (Brecher and Orlow 2003, Orfanos et al. 1987).
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