The First Potent Subtype‐Selective Retinoid X Receptor (RXR) Agonist Possessing a 3‐Isopropoxy‐4‐isopropylphenylamino Moiety, NEt‐3IP (RXRα/β‐dual agonist)

Retinoid X receptor (RXR) agonists (rexinoids) are attracting much attention for their use in treatment of cancers, including tamoxifen‐resistant breast cancer and taxol‐resistant lung cancer, and metabolic disease. However, known RXR agonists have a highly lipophilic character. In addition, no subtype‐selective RXR agonists have been found. We previously reported an RXRα‐preferential agonist 4‐[ N ‐methanesulfonyl‐ N ‐(5,5,8,8‐tetramethyl‐5,6,7,8‐tetrahydro‐2‐naphthyl)amino]benzoic acid ( 6 a ). The RXR agonistic activity is much less than that of well‐known RXR agonists. To develop potent, less‐lipophilic, and subtype‐selective RXR agonists, we created new RXR agonists possessing alkoxy and isopropyl groups as a lipophilic domain of the common structure of well‐known RXR agonists. As a result, compounds possessing branched alkoxy groups, 6‐[ N ‐ethyl‐ N ‐(3‐isopropoxy‐4‐isopropylphenyl)amino]nicotinic acid (NEt‐3IP: 7 a ) and 6‐[ N ‐ethyl‐ N ‐(3‐isobutoxy‐4‐isopropylphenyl)amino]nicotinic acid (NEt‐3IB: 7 c ), showed RXR agonistic activity as potent as, or more potent than, the activities of representative RXR agonists. Moreover, NEt‐3IP ( 7 a ) was found to be the first RXRα/β‐selective (or RXRα/β‐dual) agonist. Being potent, less lipophilic, and having RXR subtype‐selective activity, NEt‐3IP ( 7 a ) is expected to become a new drug candidate and to be a useful biological tool for clarifying each RXR subtype function.