Design, Synthesis, and Pharmacological Characterization of Novel Endomorphin-1 Analogues as Extremely Potent μ-Opioid Agonists

Recently we reported the synthesis and structure activity study of endomorphin-1 (EM-1) analogues containing novel, unnatural alpha-methylene-beta-aminopropanoic acids (Map). In the present study, we describe new EM-1 analogues containing Dmt(1), (R/S)-beta Pro(2), and (ph)Map(4)/(2-furyl)Map(4). All of the analogues showed a high affinity for the mu-opioid receptor (MOR) and increased stability in mouse brain homogenates. Of the new compounds, Dmt(1)-(R)-beta Pro(2)-Trp(3)-(2-furyl)Map(4) (analogue 12) displayed the highest affinity toward MOR, in the picomolar range (K-i(mu) = 3.72 pM). Forskolin-induced cAMP accumulation assays indicated that this analogue displayed an extremely high agonistic potency, in the subpicomolar range (EC50 = 0.0421 pM, E-max = 99.5%). This compound also displayed stronger in vivo antinociceptive activity after iv administration when compared to morphine in the tail-flick test, which indicates that this analogue was able to cross the blood-brain barrier.