Role of peripheral μ-opioid receptors in arthritis: Are they potential targets for therapy?

Anti-inflammatory and antinociceptive action of the dimeric enkephalin peptide biphalin in the mouse model of colitis: new potential treatment of abdominal pain associated with inflammatory bowel diseases

Biphalin, a mixed MOP/DOP agonist, displays a potent antinociceptive activity in numerous animal models of pain. The aim of the study was to characterize the anti-inflammatory and antinociceptive action of biphalin in the mouse models of colitis. The anti-inflammatory effect of biphalin (5mg/kg, twice daily, i.c. and i.p.) was characterized in a semi-chronic mouse model of colitis, induced by i.c. injection of trinitrobenzenesulfonic acid (TNBS). The antinociceptive action of biphalin (5mg/kg, i.p. and i.c.) in inflamed mice was assessed in mustard oil-induced model of visceral pain and in the hot plate test. In the semi-chronic mouse model of colitis, biphalin i.c. (5mg/kg), but not i.p. improved colitis macroscopic score (2.88±0.19 and 4.99±0.80 units for biphalin and vehicle treated animals, respectively). Biphalin injected i.p. and i.c. (5mg/kg) displayed a potent antinociceptive action in the mustard oil-induced pain test. In the hot plate test, biphalin (5mg/kg, i.p.) produced a potent antinociceptive activity in inflamed mice, suggesting central site of action. Our data suggest that biphalin may become a novel opioid-based analgesic agent in IBD therapy and warrant further investigation of its pharmacological profile.

Synthesis and Evaluation of Potentially Transdermal Morphine Derivatives

Three new morphine-based compounds bearing arginyl moieties, compounds 7, 9a, and 9b, have been synthesized using solid phase and solution phase techniques with the aim of obtaining new transdermal analgesics. Preliminary biological assays have shown that these compounds have a relatively high affinity for opioid receptors, achieving ≥94 % inhibition of radioligand binding at a concentration of 10 µM in non-selective opioid binding assays. Further testing on two of the analogues, 9a and 9b, demonstrated that these compounds were acting as agonists, rather than antagonists, at the opioid receptors and 9b achieved the significant result of 73 % inhibition of contractile responses in the electrically stimulated guinea pig ileum assay at a concentration of 30 µM. Unfortunately, none of the molecules showed evidence of transdermal activity.

Endomorphins in rheumatoid arthritis, osteoarthritis, and experimental arthritis

The opioid tetrapeptides endomorphins (EM)-1 and EM-2 are widely expressed in central nervous system and immune tissues of rats and humans. Their analgesic properties are well characterized but they also have anti-inflammatory properties. EM-1 significantly attenuated the onset of hindpaw inflammation in adjuvant-induced arthritis in rats. Immunohistochemical staining demonstrated the presence of EMs in T cells, macrophages, and fibroblasts in synovial tissues from patients with osteo- or rheumatoid arthritis (RA). In an ex vivo superfusion system, EM-1 potently inhibited the release of proinflammatory cytokines interleukin (IL)-6 and IL-8 from synovial tissues from patients with osteo- or RA. These results demonstrate that EMs are endogenously synthesized within human immune cells and have the potential to act as potent therapeutic agents in the treatment of chronic inflammatory disease. We discuss the clinical potential for EM analogues chemically modified to resist proteolytic degradation and identify modified protease-resistant analogues with enhanced bioactivity.