The Meta-Position of Phe4 in Leu-enkephalin Regulates Potency, Selectivity, Functional Activity, and Signaling Bias at the Delta and Mu Opioid Receptors

Abstract

As tool compounds to study cardiac ischemia, the endogenous {delta}-opioid receptors ({delta}OR) agonist Leu5-enkephalin and the more metabolically stable synthetic peptide [D-Ala2, D-Leu5]-enkephalin are frequently employed. However, both peptides have similar pharmacological profiles that restrict detailed investigation of the cellular mechanism of the {delta}ORs protective role during ischemic events. Thus, a need remains for {delta}OR peptides with improved selectivity and unique signaling properties for investigating the specific roles for {delta}OR signaling in cardiac ischemia. To this end, we explored substitution at the Phe4 position of Leu5-enkephalin for its ability to modulate receptor function and selectivity. Peptides were assessed for their affinity to bind to {delta}ORs and -opioid receptors (ORs) and potency to inhibit cAMP signaling and to recruit {beta}-arrestin 2. Additionally, peptide stability was measured in rat plasma. Substitution of the meta-position of Phe4 of Leu5-enkephalin provided high-affinity ligands with varying levels of selectivity and bias at both the {delta}OR and OR and improved peptide stability, while substitution with picoline derivatives produced lower-affinity ligands with G protein biases at both receptors. Overall, these favorable substitutions at the meta-position of Phe4 may be combined with other modifications to Leu5-enkephalin to deliver improved agonists with finely tuned potency, selectivity, bias and drug-like properties. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=51 SRC="FIGDIR/small/750794v2_ufig1.gif" ALT="Figure 1"> View larger version (9K): org.highwire.dtl.DTLVardef@143ee98org.highwire.dtl.DTLVardef@e55c94org.highwire.dtl.DTLVardef@2114b4org.highwire.dtl.DTLVardef@1d63ba4_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOTOC FIGUREC_FLOATNO C_FIG