Synthesis and Evaluation of a Novel Bivalent Selective Antagonist for the Mu-Delta Opioid Receptor Heterodimer that Reduces Morphine Withdrawal in Mice. 
A major limitation in the study of the mu-delta opioid receptor heterodimer (MDOR) is that few selective pharmacological tools exist and no heteromer-selective antagonists.  We thus designed a series of variable-length (15-41 atoms) bivalent linked peptides with selective but moderate/low-affinity pharmacophores for the mu and delta opioid receptors.  We observed a U-shaped MDOR potency/affinity profile in vitro, with the 24-atom spacer length (D24M) producing the highest MDOR potency/affinity (<1 nM) and selectivity (≥89-fold).  We further evaluated D24M in mice and observed that D24M dose-dependently antagonized tail flick antinociception produced by the MDOR agonists CYM51010 and Deltorphin-II, without antagonizing the monomer agonists DAMGO and DSLET.  We also observed that D24M sharply reduced withdrawal behavior in models of acute and chronic morphine dependence.  These findings suggest that D24M is a first-in-class high-potency MDOR-selective antagonist both in vitro and in vivo.