Design and synthesis of topographically constrained amino acids, and bioactive peptides for studies of ligand-receptor interaction, and for de novo design of delta-opioid selective non-peptide mimetics as potential therapeutics

Abstract

Topographical constraint is the most powerful approach for the design of bioactive peptides to explore the bioactive conformation of crucial side-chain pharmacophores of amino acid residues in peptide-receptor recognition and signal transduction. Novel topographically constrained amino acids β-isopropylphenylalanine and 2',6'-dimethyl-2,3-methanophenylalanine have been designed and synthesized. Incorporation of the four optically pure β-isopropylphenylalanine stereoisomers into deltorphin I produced four peptide analogues of [β-iPrPhe]Deltorphin I with differentiated bioactivities. The most potent and selective analogue, [(2S,3R)-β-iPrPhe]Deltorphin I showed an IC₅₀ nM binding affinity, and a 29000 fold selectivity for the δ-opioid receptor over the μ opioid receptor. Combined molecular modeling and NMR studies indicated that the (2S,3R)-β-iPrPhe³ residue in the analogue favors the trans rotamer, and can induce the linear peptide to form a low-energy folded conformation which was proposed as the bioactive conformation for the δ-opioid receptor. Coupling four optically pure, conformationally constrained β-methyl-2',6'-dimethyltyrosine (TMT) with L-Tic formed four dipeptide analogues of TMT-L-Tic. The most potent and selective analogue, (2S,3R)-TMT-L-Tic showed 9 nM binding affinity and 4000 fold selectivity to the δ vs μ opioid receptor. The lowest-energy conformation of (2S,3R)-TMT-L-Tic was suggested to be the bioactive one in which TMT side chain is trans and Tic side chain is in a gauche (+) conformation. Bicyclic oxytocin antagonist [dPen¹, cyclo(Glu⁴ Lys⁸)]OT (BC-OT) (pA₂ = 8.10) is an excellent template to examine further topographical ideas. Substitution of Tyr² with the topographically constrained para-methoxy-β-methyl-2',6'-dimethyltyrosine (p-MeOTMT) amino acids produced two very potent antagonists [(2S,3S)-p-MeOTMT²]BC-OT (pA₂ = 8.26) and [(2R,3R)-p-MeOTMT²]BC-OT(pA₂ = 7.80), and two inactive analogues [(2S,3R)-p-MeOTMT²]BC-OT and [(2R,3S)-p-MeOTMT²]BC-OT. These interesting results can be attributed to the biased side-chain conformation, gauche(+) and gauche(-) in (2S,3S)-p-MeOTMT and (2R,3R)-p-MeOTMT respectively, and trans in both (2S,3R)-p-MeOTMT and (2R,3S)-p-MeOTMT residues. Rational design of non-peptide mimetics from peptide leads is still elusive. Based on the δ-opioid selective lead [(2S,3R)-TMT¹]DPDPE and SAR of δ-opioid selective ligands, the first generation of non-peptide mimetics have been designed and synthesized. The new lead SL-3111 showed binding affinity IC₅₀ = 8 nM, and over 2000 fold selectivity for the δ-opioid receptor over the μ receptor