Opiate aromatic pharmacophore structure-activity relationships in CTAP analogues determined by topographical bias, two-dimensional NMR, and biological activity assays

Topographically constrained analogues of the highly μ-opioid-receptor- selective antagonist CTAP (H-D-Phe-c[Cys-Tyr-D-Trp-Arg-Thr-Pen]-Thr-NH₂, 1) were prepared by solid-phase peptide synthesis. Replacement of the D-Phe residue with conformationally biased β-methyl derivatives of phenylalanine or tryptophan (2R,3R; 2R,3S; 2S,3R; 2S,3S) yielded peptides that displayed widely varying types of biological activities. In an effort to correlate the observed biological activities of these analogues with their structures, two- dimensional ¹H NMR and molecular modeling was performed. Unlike the parent (1), which is essentially a pure μ antagonist with weak δ agonist activities in the MVD bioassay, the diastereomeric β-MePhe¹-containing peptides exhibited simultaneous δ agonism and μ antagonism by the (2R,3R)- containing isomer 2; μ antagonism by the (2R,3S)-containing isomer 3; weak μ agonism by the (2S,3R)-containing isomer 4; and δ agonism by the (2S,3S)- containing isomer 5. Incorporation of β-MeTrp isomers into position i led to peptides that were μ antagonists (2R,3R), 8; (2R,3S), 9, or essentially inactive (<10%) in the MVD and GPI assays (2S,3R), 10; (2S,3S), 11. Interestingly, in vivo antinociceptive activity was predicted by neither MVD nor GPI bioactivity. When D-Trp was incorporated in position 1, the result (7) is a partial, yet relatively potent μ agonist which also displayed weak δ agonist activity. Molecular modeling based on 2D NMR revealed that low energy conformers of peptides with similar biological activities had similar aromatic pharmacophore orientations and interaromatic distances. Peptides that exhibit μ antagonism have interaromatic distances of 7.0-7.9 A and have their amino terminal aromatic moiety pointing in a direction opposite to the direction that the amino terminus points. Peptides with δ opioid activity displayed an interaromatic distance of < 7 Å and had their amino terminal aromatic moiety pointing in the same direction as the amino terminus.