2,6-Dimethyl-L-tyrosine

This study reports the synthesis and biological evaluation of a series of new side-chain-to-side-chain cyclized endomorphin-2 (EM-2) and morphiceptin analogs of a general structure Tyr-c(Xaa-Phe-Phe-Yaa)NH(2) or Tyr-c(Xaa-Phe-D-Pro-Yaa)NH(2), respectively, where Xaa and Yaa were L/D Asp or L/D Lys. Further modification of these analogs was achieved by introduction of 2',6'-dimethyl-L-tyrosine (Dmt) instead of Tyr in position 1. Peptides were synthesized by solid phase method and cleaved from the resin by a microwave-assisted procedure. Dmt(1)-substituted analogs displayed high affinity at the μ-opioid receptors, remained intact after incubation with the rat brain homogenate and showed remarkable, long-lasting μ-opioid receptor-mediated antinociceptive activity after central, but not peripheral administration. Our results demonstrate that cyclization is a promising strategy in the development of new opioid analgesics, but further modifications are necessary to enhance the blood-brain barrier permeability.

The unnatural amino acid 2',6'-dimethyl-l-tyrosine has found widespread use in the development of synthetic opioid ligands. Opioids featuring this residue at the N-terminus often display superior potency at one or more of the opioid receptor types, but the availability of the compound is hampered by its cost and difficult synthesis. We report here a short, three-step synthesis of Boc-2',6'-dimethyl-l-tyrosine (3a) utilizing a microwave-assisted Negishi coupling for the key carbon-carbon bond forming step, and employ this chemistry for the expedient synthesis of other unnatural tyrosine derivatives. Three of these derivatives (3c, 3d, 3f) have not previously been examined as Tyr(1) replacements in opioid ligands. We describe the incorporation of these tyrosine derivatives in a series of opioid peptidomimetics employing our previously reported tetrahydroquinoline (THQ) scaffold, and the binding and relative efficacy of each of the analogues at the three opioid receptor subtypes: mu (MOR), delta (DOR), and kappa (KOR).

Here we evaluated how the interchange of the amino acids 2',6'-dimethyl-L-tyrosine (Dmt), 2',6'-difluoro-L-tyrosine (Dft), and tyrosine in position 1 can affect the pharmacological characterization of some reference opioid peptides and pseudopeptides. Generally, Dft and Tyr provide analogues with a similar pharmacological profile, despite different pK(a) values. Dmt/Tyr(Dft) replacement gives activity changes depending on the reference opioid in which the modification was made. Whereas, H-Dmt-Tic-Asp *-Bid is a potent and selective delta agonist (MVD, IC(50)=0.12nM); H-Dft-Tic-Asp *-Bid and H-Tyr-Tic-Asp *-Bid are potent and selective delta antagonists (pA(2)=8.95 and 8.85, respectively). When these amino acids are employed in the synthesis of deltorphin B and its Dmt(1) and Dft(1) analogues, the three compounds maintain a very similar delta agonism (MVD, IC(50) 0.32-0.53 nM) with a decrease in selectivity relative to the Dmt(1) analogue.

Multivalent ligands with δ/μ opioid agonist and NK1 antagonist activities have shown promising analgesic potency without detectable sign of toxicities, including motor skill impairment and opioid-induced tolerance. To improve their biological activities and metabolic stability, structural optimization was performed on our peptide-derived lead compounds by introducing 2',6'-dimethyl-L-tyrosine (Dmt) instead of Tyr at the first position. The compound 7 (Dmt-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)-Bzl]) showed improved multivalent bioactivities compared to those of the lead compounds, had more than 6 h half-life in rat plasma, and had significant antinociceptive efficacy in vivo. The NMR structural analysis suggested that Dmt(1) incorporation in compound 7 induces the structured conformation in the opioid pharmacophore (N-terminus) and simultaneously shifts the orientation of the NK1 pharmacophore (C-terminus), consistent with its affinities and activities at both opioid and NK1 receptors.