Characterization of opiates, neuroleptics, and synthetic analogs at ORL1 and opioid receptors

Abstract

Nociceptin/orphanin FQ (N/OFQ) was recently identified as the endogenous ligand for the opioid-receptor like (ORL1) receptor. Although the ORL1 receptor shows sequence homology with the opioid receptors, the nociceptin/ORL1 ligand-receptor system has very distinct pharmacological actions compared to the opioid receptor system. Recently, several small-molecule ORLI receptor ligands were reported by pharmaceutical companies. Most of these ligands had close structural similarities with known neuroleptics and opiates. In this study, we screened several available neuroleptics and opiates for their binding affinity and functional activity at ORL1 and the opioid receptors. We also synthesized several analogs of known opiates with modified piperidine N-substituents in order to characterize the ORL1 receptor ligand binding pocket. Substitution with the large, lipophilic cyclooctylmethyl moiety increased ORL1 receptor affinity and decreased mu receptor affinity and efficacy in the fentanyl series of ligands but had a different effect in the oripavine class of opiate ligands. Our results indicate that opiates and neuroleptics may be good starting points for ORL1 receptor ligand design, and the selectivity may be modulated by appropriate structural modifications.

Fig. 1

Structures of recently discovered ORL1 receptor ligands.

Fig. 2

Structures of opiates and neuroleptics tested for ORL1 receptor affinity (top) and structures of SRI compounds based on ORL1 receptor-binding opiates (bottom).

Fig. 3

Stimulation of [³⁵S]GTPγS binding to hORL1 receptor-containing CHO cell membranes. The experiment was conducted as described in Materials and methods. Data are from a single experiment conducted in triplicate that was repeated with very similar results. Basal [³⁵S]GTPγS binding was 71 fmol/mg and maximal stimulation with nociceptin was 240 fmol/mg protein. N-cyclooctylmethylfentanyl, N-cyclooctylmethylcarfentanil, and N-cyclooctylmethyl-normeperidine were tested concurrently and were found to produce no stimulation up to 10 μM.

Fig. 4

Stimulation of [³⁵S]GTPγS binding to human μ receptor-containing CHO cell membranes. The experiment was conducted as described in Materials and methods. Data are from a single experiment conducted in triplicate that was repeated at least once with very similar results. (A) Stimulation with the full agonist DAMGO(■), N-cyclooctylmethyl(com) fentanyl (▲), or fentanyl (▼). (B) Stimulation with DAMGO (■), N-cyclooctylmethylnor(com)-buprenorphine(▲), or buprenorphine(▼). Basal [³⁵S]GTPγS binding was 113 fmol/mg and maximal stimulation with DAMGO was 590 fmol/mg protein.

Fig. 5

Structures of the anilidopiperidine class of opiate ligands.