The steroid 17alpha-acetoxy-6-dimethylaminomethyl-21-fluoro-3-ethoxy-pregna-3, 5-dien-20-one (SC17599) is a selective mu-opioid agonist: implications for the mu-opioid pharmacophore

The steroid SC17599 (17alpha-acetoxy-6-dimethylaminomethyl-21-fluoro-3-ethoxypregna -3, 5-dien-20-one) has mu-opioid actions in vivo. The ability of SC17599 to interact with opioid receptors has been studied using radioligand and [(35)S]guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) binding assays. SC17599 bound to mu-opioid receptors in SH-SY5Y neuroblastoma cells and to recombinant receptors expressed in rat C6 glioma cells and Chinese hamster ovary cells with good affinity and with greater than 100-fold selectivity for mu- over both delta- and kappa-opioid receptors. Binding was much reduced when aspartate 147 in the wild-type mu-opioid receptor was replaced with asparagine. The affinity of SC17599 for the mu-opioid receptor was decreased in the presence of sodium ions, indicating agonist activity. SC17599 stimulated the binding of [(35)S]GTPgammaS in a naloxone-reversible manner with good potency and maximal effect equivalent to that of the mu-opioid agonists fentanyl and [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin. In rat brain membranes, SC17599-mediated stimulation of [(35)S]GTPgammaS binding was reversed by the antagonist naltrexone. SC17599 lacks an aromatic ring and para-hydroxyl substituent considered critical in the pharmacophore for mu-opioids. The structural relationship between SC17599 and more traditional opioid ligands was investigated through genetic algorithm-based modeling techniques for pharmacophore generation (GASP) and ligand-receptor docking (GOLD). The relatively planar and electron-rich A ring of the steroid compensated for the lack of aromaticity. Modeling of ligand-receptor docking showed that both morphine and SC17599 occupy the same binding pocket within the transmembrane helix bundle of the mu-opioid receptor and that the relationship between their binding modes largely mimicked the pharmacophore alignment.