Synthesis Using a Fmoc-Based Strategy and Biological Activities of Some Reduced Peptide Bond Pseudopeptide Analogues of Dynorphin A

Eight analogues of Dyn A(1-11)-NH₂ incorporating the enzymatically stable ψ(CH₂-NH) isosteric peptide bond replacement were synthesized and tested for binding affinity at the central opioid μ, δ, and κ receptors in guinea pig brain (GPB) homogenates and for activity at the peripheral κ (and μ) receptors in the guinea pig ileum (GPI). The peptidic analogues were synthesized by solid phase techniques using a Fmoc/tert-butyl strategy, and the ψ(CH₂-NH) bond, or reduced bond, was introduced via reductive alkylation of the N-terminal amino group of the growing peptide with a Fmoc-N^α-protected amino aldehyde. The synthesis of Fmoc-N^α- protected amino aldehydes also is described. Several other peptides have been previously synthesized incorporating this modification and showed for instance increased enzymatic stability and antagonist properties. Results obtained in the GPB show that modifications of the peptide bond in the address site (analogues 4-9) do not affect the binding at the κ receptor and, with a few exceptions, at the μ and δ receptors. On the other hand, analogues 2 and 3, modified in the message segment of Dyn A(1-11)-NH₂, show a decrease in binding affinity at all three receptors. In the GPI, the results are more varied as the influence of the peptide bond modification seems to be more important than in the GPB. Finally, selected analogues were tested with no indication for antagonist activity at the κ peripheral receptor.