Vol. 58, Issue 4, 669-676, October 2000

The Steroid 17-Acetoxy-6-dimethylaminomethyl-21-fluoro-3-ethoxy-pregna-3,5-dien-20-one (SC17599) Is a Selective µ-Opioid Agonist: Implications for the µ-Opioid Pharmacophore

Iain J. McFadyen, Hani Houshyar, Lee-Yuan Liu-Chen, James H. Woods, and John R. Traynor

Departments of Pharmacology (I.J.M., H.H., J.H.W., J.R.T.) and Psychology (J.H.W.), University of Michigan, Ann Arbor, Michigan; and Department of Pharmacology, Temple University School of Medicine, Philadelphia, Pennsylvania (L.-Y.L.-C.)

The steroid SC17599 (17-acetoxy-6-dimethylaminomethyl-21-fluoro-3-ethoxypregna-3,5-dien-20-one) has µ-opioid actions in vivo. The ability of SC17599 to interact with opioid receptors has been studied using radioligand and [³⁵S]guanosine-5'-O-(3-thio)triphosphate (GTPS) binding assays. SC17599 bound to µ-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 µ- over both - and -opioid receptors. Binding was much reduced when aspartate 147 in the wild-type µ-opioid receptor was replaced with asparagine. The affinity of SC17599 for the µ-opioid receptor was decreased in the presence of sodium ions, indicating agonist activity. SC17599 stimulated the binding of [³⁵S]GTPS in a naloxone-reversible manner with good potency and maximal effect equivalent to that of the µ-opioid agonists fentanyl and [D-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin. In rat brain membranes, SC17599-mediated stimulation of [³⁵S]GTPS binding was reversed by the antagonist naltrexone. SC17599 lacks an aromatic ring and para-hydroxyl substituent considered critical in the pharmacophore for µ-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 µ-opioid receptor and that the relationship between their binding modes largely mimicked the pharmacophore alignment.