CYX-5, a G-protein biassed MOP receptor agonist, DOP receptor antagonist and KOP receptor agonist, evokes constipation but not respiratory depression relative to morphine in rats.

BACKGROUND: Strong opioid analgesics such as morphine alleviate moderate to severe acute nociceptive pain (e.g. post-surgical or post-trauma pain) as well as chronic cancer pain. However, they evoke many adverse effects and so there is an unmet need for opioid analgesics with improved tolerability. Recently, a prominent hypothesis has been that opioid-related adverse effects are mediated by ß-arrestin2 recruitment at the µ-opioid (MOP) receptor and this stimulated research on discovery of G-protein biassed opioid analgesics. In other efforts, opioids with MOP agonist and δ-opioid (DOP) receptor antagonist profiles are promising for reducing side effects c.f. morphine. Herein, we report on the in vivo pharmacology of a novel opioid peptide (CYX-5) that is a G-protein biassed MOP receptor agonist, DOP receptor antagonist and kappa opioid (KOP) receptor agonist. METHODS: Male Sprague-Dawley received intracerebroventricular bolus doses of CYX-5 (3, 10, 20 nmol), morphine (100 nmol) or vehicle, and antinociception (tail flick) was assessed relative to constipation (charcoal meal and castor oil-induced diarrhoea tests) and respiratory depression (whole body plethysmography). RESULTS: CYX-5 evoked naloxone-sensitive, moderate antinociception, at the highest dose tested. Although CYX-5 did not inhibit gastrointestinal motility, it reduced stool output markedly in the castor oil-induced diarrhoea test. In contrast to morphine that evoked respiratory depression, CYX-5 increased tidal volume, thereby stimulating respiration. CONCLUSION: Despite its lack of recruitment of ß-arrestin2 at MOP, DOP and KOP receptors, CYX-5 evoked constipation, implicating a mechanism other than ß-arrestin2 recruitment at MOP, DOP and KOP receptors, mediating constipation evoked by CYX-5 and potentially other opioid ligands.

Synthesis and Biological Evaluation of Fentanyl Analogues Modified at Phenyl Groups with Alkyls.

A series of fentanyl analogues modified at the phenyl group of the phenethyl with alkyl and/or hydroxyl and alkoxy, and the phenyl group in the anilido moiety replaced with benzyl or substituted benzyl, were synthesized. The in vitro opioid receptor functional activity of these compounds was evaluated by assessment of their ability to modulate forskolin-stimulated cAMP accumulation and by their ability to induce ß-arrestin2 recruitment. Compound 12 is a potent µ-opioid (MOP) receptor agonist, a potent κ-opioid (KOP) receptor antagonist with weak ß-arrestin2 recruitment activity. Compounds 10 and 11 are potent MOP receptor agonists with weak δ-opioid (DOP) receptor antagonist activity and moderate KOP receptor antagonist activity as well as weak ß-arrestin2 recruitment activity at the MOP receptor. These compounds are promising leads for discovery of potent opioid analgesics with reduced side effects relative to clinically available strong opioid analgesics.

Differentiation of opioid receptor preference by [Dmt1]endomorphin-2-mediated antinociception in the mouse.

The potent opioid [Dmt1]endomorphin-2 (Dmt-Pro-Phe-Phe-NH2) differentiated between the opioid receptor subtypes responsible for the antinociception elicited by endomorphin-2 in mice. Antinociception, induced by the intracerebroventricular administration of [Dmt1]endomorphin-2 and inhibited by various opioid receptor antagonists [naloxone, naltrindole, beta-funaltrexamine, naloxonazine], was determined by the tail-flick (spinal effect) and hot-plate (supraspinal effect) tests. The opioid receptor subtypes involved in [Dmt1]endomorphin-2-induced antinociception differed between these in vivo model paradigms: naloxone (non-specific opioid receptor antagonist) and beta-funaltrexamine (irreversible mu1/mu2-opioid receptor antagonist) blocked antinociception in both tests, although stronger inhibition occurred in the hot-plate than the tail-flick test suggesting involvement of other opioid receptors. Consequently, we applied naloxonazine (mu1-opioid receptor antagonist) that significantly blocked the effect in the hot-plate test and naltrindole (delta-opioid receptor antagonist), which was only effective in the tail-flick test. The data indicated that [Dmt1]endomorphin-2-induced spinal antinociception was primarily mediated by both mu2- and delta-opioid receptors, while a supraspinal mechanism involved only mu1/mu2-subtypes.