Figure 2.

Molecular structure of retinoids divided per generation. The first generation is composed by natural retinoids, obtained modifying polar groups of vitamin A (retinol, tretinoin, isotretinoin, and alitretinoin). The second generation is constituted by monoaromatic compounds, in which a benzene ring replaces the cyclohexene ring (etretinate, and acitretin). The third generation is made up by polyaromatic molecules, resulting from the cyclization of a side chain (adapalene, bexarotene, and tazarotene). Trifarotene is a recently synthetized fourth-generation retinoid, highly specific for skin RAR-γ receptors (References in Supplementary materials).