Novel Synthetic Retinoids and Separation of the Pleiotropic Retinoidal Activities

Retinoids, all-trans-retinoic acid (1a) and its analogs, act as specific modulators of cellular differentiation and proliferation, through binding to and activating specific nuclear receptors, retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Retinoids have chemotherapeutic roles in dermatology and oncology, but their usefulness is restricted by the high toxicity of retinoic acid and its hydrophobic analogs. We have developed various retinoidal benzoic acid derivatives, and named them retinobenzoic acids. Among them, aromatic amides such as Am80 (7) and Am580 (8) have superior pharmacological characteristics, including RAR subtype selectivity. Structural modification based on the ligand superfamily concept afforded several types of RAR antagonists, benzimidazole derivatives, BIPh (41) and BIBn (42), and dibenzodiazepine derivatives, LE135 (46) and LE540 (47). LE135 (46) is a unique antagonist with RARβ-selectivity. During investigations on the structureactivity relationships of retinobenzoic acids, several retinoid synergists (RXRs agonists), such as HX600 (49), DA113 (55h) and TZ335 (57), have been found. These compounds are expected to modulate other nuclear receptors which form heterodimers with RXRs, besides retinoids. Further, we found some RXRs antagonists, HX531 (60) and HX603 (61), which inhibit the activation of both RXR homodimers and RXR·RAR heterodimers. In this review, we describe our investigations on these structurally and biologically unique retinoids and retinoid-regulatory compounds.