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

The steroid SC17599 (17 alpha-acetoxy-6-dimethylaminomethyl-21-fluoro- 3-et hoxypregna-3,5-dien-20-one) has mu-opioid actions in vivo. The ability of S C17599 to interact with opioid receptors has been studied using radioligand and [S-35]guanosine-5'-O-(3-thio) triphosphate (GTP gamma S) binding assay s. SC17599 bound to mu-opioid receptors in SH-SY5Y neuroblastoma cells and to recombinant receptors expressed in rat C6 glioma cells and Chinese hamst er ovary cells with good affinity and with greater than 100-fold selectivit y for mu- over both delta- and kappa-opioid receptors. Binding was much red uced when aspartate 147 in the wild-type mu-opioid receptor was replaced wi th asparagine. The affinity of SC17599 for the mu-opioid receptor was decre ased in the presence of sodium ions, indicating agonist activity. SC17599 s timulated the binding of [S-35]GTP gamma S in a naloxone-reversible manner with good potency and maximal effect equivalent to that of the mu-opioid ag onists fentanyl and [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin. In rat bra in membranes, SC17599-mediated stimulation of [S-35]GTP gamma S binding was reversed by the antagonist naltrexone. SC17599 lacks an aromatic ring and para-hydroxyl substituent considered critical in the pharmacophore for mu-o pioids. The structural relationship between SC17599 and more traditional op ioid ligands was investigated through genetic algorithm-based modeling tech niques for pharmacophore generation (GASP) and ligand-receptor docking (GOL D). The relatively planar and electron-rich A ring of the steroid compensat ed for the lack of aromaticity. Modeling of ligand-receptor docking showed that both morphine and SC17599 occupy the same binding pocket within the tr ansmembrane helix bundle of the mu-opioid receptor and that the relationshi p between their binding modes largely mimicked the pharmacophore alignment.