Characterization of [125I]AR-M100613, a high-affinity radioligand for δ opioid receptors

Abstract AR-M100613 ([I]-Dmt-c[- d- Orn-2-Nal- d -Pro- d -Ala-]) is the iodinated analog of a cyclic casomorphin previously shown to be a potent antagonist at the δ opioid receptor. Specific [ 125 I]AR-M100613 binding to rat whole brain membranes was saturable, reversible, and best fit to a one-site model (K d = 0.080 ± 0.008 nM, B max = 45.2 ± 4.4 fmol/mg protein). [ 125 I]AR-M100613 binding was displaced with high affinity by the δ opioid receptor ligands SNC-80, Deltorphin II and DPDPE but not the μ or κ-selective receptor ligands DAMGO and U69593. Residual non-selective binding of [ 125 I]AR-M100613 to μ opioid receptors is blocked by the addition of CTOP to the assay buffer. [ 35 S]GTPγS binding assays indicate that AR-M100613 is a potent, selective, and reversible antagonist for δ opioid receptors in rat brain membranes. The high-affinity, high specific activity, low nonspecific binding and antagonist profile of [ 125 I]AR-M100613 favor its use as a radiochemical probe for δ opioid receptors.