CYCLIC BETA-CASOMORPHIN ANALOGS WITH MIXED MU-AGONIST DELTA-ANTAGONIST PROPERTIES - SYNTHESIS, PHARMACOLOGICAL CHARACTERIZATION, AND CONFORMATIONAL ASPECTS

Analogues of the potent and moderately mu-opioid-receptor-selective cy clic beta-casomorphin-5 derivative H-Tyr-c -D-Orn-Phe-D-Pro-Gly- (2) were prepared by conventional solution synthesis. Replacement of the Phe(3) residue by 2-naphthylalanine (2-Nal) led to a peptide (4) with high affinity for both mu and delta opioid receptors. This compound tu rned out to be an agonist in the mu-receptor-representative guinea pig ileum (GPI) assay but a moderately potent antagonist against various delta agonists in the 6-receptor-representative mouse vas deferens (MV D) assay. It thus represents the first known cyclic opioid peptide ana logue with mixed mu agonist/delta antagonist properties. Interestingly , replacement of 2-Nal(3) in compound 4 with 1-naphthylalanine (1-Nal) resulted in an analogue (5) showing high affinity for mu receptors an d a full agonist effect in the MVD assay that was mediated by both mu and delta receptors. Substitution of Trp for Phe(3) in 2 (compound 8) was well tolerated at both receptors and led to an analogue with agoni st activity in both the GPI and MVD assays. Variation of the peptide r ing size in 4 was achieved by substitution of D-Orn(2) with D-Lys (com pound 6) or D-2,4-diaminobutyric acid (compound 7). Analogue 6 was als o a mixed mu agonist/delta antagonist with somewhat lower potency than 4, whereas compound 7 displayed mu agonist and partial delta agonist properties. Further reduction of the peptide ring size, as achieved by deletion of the Gly(5) residue, produced a compound (9) which was a f ull agonist in both bioassays. Conformational analysis of analogues 2, 4, and 5 by H-1 NMR spectroscopy and molecular mechanics studies sugg ested that the overall conformation of parent compound 2 and the 2-Nal -containing peptide 4 was similar, while the side-chain orientation of 1-Nal in peptide 5 was different. These results suggest that the 6 an tagonist properties of analogue 4 may not be due to a difference in it s overall conformation as compared to the agonist 2 but may be a direc t effect of the 2-naphthyl moiety per se preventing proper alignment o f the peptide for receptor activation.