Introduction: 

A major goal in opioid peptide research is the development of novel analgesics that could substitute for morphine without its well-known side effects. This study investigated the antinociceptive effects of four new endomorphin-2 (EM-2) derivatives (EMD1-4) containing unnatural amino acids, i.e. 2-aminocyclohexanecarboxylic acid (Achc2), para-fluorophenylalanine (pFPhe4), b methylphenylalanine (bMePhe4) and/or 2',6'-dimethyltyrosine (Dmt1): EMD1:

Tyr-(1S,2R)Achc-Phe-pFPhe-NH₂, Achc2 +); EMD2:

Tyr-(1S,2R)Achc-Phe-(2S,3S)?MePhe-NH₂; EMD3:

Dmt-(1S,2R)Achc-Phe-pFPhe-NH₂ and EMD4:

Dmt-(1S,2R)Achc-Phe-(2S,3S)?MePhe-NH₂ at spinal level.

Methods: 

After baseline determination of joint diameter and mechanical paw withdrawal threshold (by a dynamic plantar aesthesiometer), osteoarthritis was induced by monosodium iodoacetate (MIA; 2x1 mg) into one of the ankle joint of male Wistar rats. These measurements were repeated 7 and 14 days later, and then the intrathecal catheterization was inserted for spinal drug delivery. One week later the non-paralysed rats were selected and after the post-MIA baseline value determination, the effects of intrathecal injection of EM-2 and the ligands (0.3–10 mg) were observed. The control group received physiological saline. In the positive control group, animals were treated with 10 mg morphine. The pain thresholds were registered 10, 20, 30, 45, 60, 70, 90 and 120 min after the intrathecal injections, and the mean of the values obtained between 10–30, 45–70 and 90–120 min were analyzed.

Results: 

The MIA injection caused a permanent increase in joint cross-section area which accompanied with decreased mechanical threshold (allodynia). Intrathecal injection of EM-2 and the derivatives caused dose-dependent antiallodynic effects without changes in joint diameter. The comparison of the different substances revealed that EMD3 and EMD4 showed more prolonged antinociception than EM-2 and the highest dose of EMD3 (10mg) caused paralysis. Morphine, as a positive control, produced long-lasting and highly effective antinociception. EM-2, EMD1 and EMD2 were effective as morphine only at the first investigated phase (10–30 min), while the effect of EMD4 did not differ significantly from morphine during the whole period. The potency of the different ligands did not differ from EM-2. Conclusion: 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 a long-lasting stability of these drugs in vivo.

This work was supported by OTKA (K83810) and TAMOP 4.2.2.-08/01-2008-0002.