Ch 55 is a synthetic analog of retinoic acid that binds to retinoic acid receptors with similar efficiency as retinoic acid (Kis in the nM range) yet does not display affinity for cellular retinoic acid-binding protein (Ki = 540 μM). Ch 55 has been shown to inhibit squamous cell differentiation of rabbit tracheal epithelial cells by inhibiting type I transglutaminase activity (EC50 = 0.02 nM) and increasing cholesterol sulfate levels (EC50 = 0.03 nM). In contrast, it has also been shown to induce differentiation of embryonic carcinoma F9 cells and melanoma S91 cells (EC50s = 0.26 and 0.5 nM, respectively), as well as inhibit the induction of ornithine decarboxylase activity in 3T6 fibroblasts (EC50 = 1 nM).
