[Dmt1]DALDA analogues with enhanced μ opioid agonist potency and with a mixed μ/κ opioid activity profile

Analogues of [Dmt1]DALDA (H-Dmt-d-Arg-Phe-Lys-NH2; Dmt = 2′,6′-dimethyltyrosine), a potent μ opioid agonist peptide with mitochondria-targeted antioxidant activity, were prepared by replacing Phe3 with various 2′,6′-dialkylated Phe analogues, including 2′,6′-dimethylphenylalanine (Dmp), 2′,4′,6′-trimethylphenylalanine (Tmp), 2′-isopropyl-6′-methylphenylalanine (Imp) and 2′-ethyl-6′-methylphenylalanine (Emp), or with the bulky amino acids 3′-(1-naphthyl)alanine (1-Nal), 3′-(2-naphthyl)alanine (2-Nal) or Trp. Several compounds showed significantly increased μ agonist potency, retained μ receptor selectivity and are of interest as drug candidates for neuropathic pain treatment. Surprisingly, the Dmp3-, Imp3-, Emp3- and 1-Nal3-containing analogues showed much increased κ receptor binding affinity and had mixed μ/κ properties. In these cases, molecular dynamics studies indicated conformational preorganization of the unbound peptide ligands due to rotational restriction around the CβCγ bond of the Xxx3 residue, in correlation with the observed κ receptor binding enhancement. Compounds with a mixed μ/κ opioid activity profile are known to have therapeutic potential for treatment of cocaine abuse.