In this study the μ opioid receptor (MOR) ligands DALDA (Tyr-d-Arg-Phe-Lys-NH2) and Dmt1-DALDA (Dmt-d-Arg-Phe-Lys-NH2, Dmt = 2′,6′-dimethyltyrosine) were glycosylated at the N- or C-terminus. Subsequently, the modified peptides were subjected to in vitro and in vivo evaluation. In contrast to the N-terminally modified peptide (3), all peptide analogues derivatized at the C-terminus (4–7) proved to possess high affinity and agonist potency at both MOR and DOR (δ opioid receptor). Results of the Caco-2 monolayer permeation, as well as in vitro blood–brain barrier model experiments, showed that, in the case of compound 4, the glycosylation only slightly diminished the lumen-to-blood and blood-to-lumen transport. Altogether, these experiments were indicative of transcellular transport but not active transport. In vivo assays demonstrated that the peptides were capable of (i) crossing the blood–brain barrier (BBB) and (ii) activating both the spinal ascending as well as the descending opioid pathways, as determined by the tail-flick and hot-plate assays, respectively. In contrast to the highly selective MOR agonist Dmt1-DALDA 1, compounds 4–7 are mixed MOR/DOR agonists, expected to produce reduced opioid-related side effects.
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