Para-substituted phenylalanine-4 analogues of [L-Ala3]DPDPE: highly selective delta opioid receptor ligands

Several para-substituted Phe4 analogues of the delta 1-selective antagonist [L-Ala3]DPDPE (DPADPE) were prepared and evaluated for their brain-binding and in vitro pharmacological effects. Unlike the p-haloPhe4 analogues of DPDPE and the deltorphins, similar analogues of DPADPE with electron-withdrawing groups substituted at the para-position of the Phe4 aromatic ring did not all have increased potency and selectivity for delta opioid receptors, but all retained high potency and selectivity for delta opioid receptors greater than DPDPE.