Design of enkephalin modifications protected from brain extracellular peptidases providing long-term analgesia

The main obstacle to the use of many therapeutic peptides in practice is their rapid destruction by extracellular peptidases. Earlier we have found that active in the extracellular medium of mammalian brain exopeptidases are unable to break the bonds formed by β-alanine. We have designed several modified forms of opioid peptide enkephalin (Tyr-Gly-Gly-Phe-Met; Enk) with end βAla: ModEnk1 (βAla-Tyr-Gly-Gly-Phe-Met-βAla), ModEnk2 (βAla-Tyr-Gly-Gly-Phe-NH₂), ModEnk3 (βAla-Tyr-Gly-Phe-NH₂). These modifications are much more stable than Enk in the suspension of isolated axonal endings (synaptosomes) that mimics the brain extracellular medium. ModEnk1-3 have been tested in standard "pain" experiment "tail flick" on rats using intranasal peptide administration. ModEnk1 and ModEnk2 (but not ModEnk3) have fully preserved pain-relieving properties of Enk, but their efficiency was maintained for much longer. Compared to ModEnk1, ModEnk2 is more stable and provides longer analgesia because it is less accessible for endopeptidases. They are potent non-toxic analgesics.