A new class of highly potent and selective endomorphin-1 analogues containing α-methylene-β-aminopropanoic acids...

A new class of endomorphin-1 (EM-1) analogues were synthesized by introduction of novel unnatural α-methylene-β-amino acids (Map) at position 3 or/and position 4. Their binding and functional activity, metabolic stability, and antinociceptive activity were determined and compared. Most of these analogues showed high affinities for the μ-opioid receptor and an increased stability in mouse brain homogenates compared with EM-1. Examination of cAMP accumulation and ERK1/2 phosphorylation in HEK293 cells confirmed the agonist properties of these analogues. Among these new analogues, H-Tyr-Pro-Trp-(2-furyl)Map-NH(2) (analogue 12) exhibited the highest binding potency (K(i)(μ) = 0.221 nM) and efficacy (EC(50) = 0.0334 nM, E(max) = 97.14%). This analogue also displayed enhanced antinociceptive activity in vivo in comparison to EM-1. Molecular modeling approaches were then carried out to demonstrate the interaction pattern of these analogues with the opioid receptors. We found that, compared to EM-1, the incorporation of our synthesized Map at position 4 would bring the analogue to a closer binding mode with the μ-opioid receptor.