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This study reports on the in vivo effects of four endomorphin-2 (EM-2) derivatives (EMD1-4) containing unnatural amino acids, i.e. 2-aminocyclohexanecarboxylic acid (Achc2), para-fluorophenylalanine (pFPhe4), β-methylphenylalanine (βMePhe4) and/or 2',6'-dimethyltyrosine (Dmt1). After induction of osteoarthritis by monosodium iodoacetate into the ankle joint of male Wistar rats, a chronic intrathecal catheter was inserted for spinal drug delivery. The mechanical threshold was assessed by a dynamic aesthesiometer. Intrathecal injection of the original EM-2 and the ligands (0.3-10 μg) caused dose-dependent antiallodynic effects. The comparison of the different substances revealed that EMD3 and EMD4 showed more prolonged antinociception than EM-2, and the effects of the highest dose of EMD4 were comparable to morphine, while EMD3 caused paralysis at this dose. The potency of the different ligands did not differ from EM-2. The results show that the derivatives of EM-2 have similar in vivo potency to the original ligand, but their effects were more prolonged suggesting that these structural modifications may play a role in the development of novel endomorphin analogues with increased therapeutic potential.

Characterization of antinociceptive potency of endomorphin-2 derivatives with unnatural amino acids in rats