Behavioral effects and receptor binding affinities of fentanyl derivatives in rhesus monkeys

These studies examined the opioid receptor binding affinities and behavioral effects of several fentanyl derivatives in rhesus monkeys. OHM3295, OHM3296, OHM3326 and OHM3463 displayed high affinity for μ (IC₅₀ = 7-66 nM) as compared to κ (IC₅₀ = 263-3255 nM) or δ (IC₅₀ = 480-4500 nM) receptors as measured by their ability to displace [³H](D-Ala²-Me- Phe⁴, Glyol⁵)enkephalin, [³H](5,7,8[beta])-N-[2-(1-pyrrolidinyl)1- oxaspiro[4,5]dec-8-yl]benzeneacetamide and [³H](D-Pen²-D-Pen⁶)enkephalin, respectively. All four compounds maintained i.v. self-administration responding at rates above those maintained by saline and below those maintained by the μ agonist alfentanil. In drug discrimination studies, OHM3463, OHM3326 and OHM3296 substituted completely for nalbuphine whereas OHM3295, and a related compound, mirfentanil, substituted partially for nalbuphine. In morphine-treated monkeys, OHM3295 substituted for naltrexone; in monkeys acutely deprived of morphine, only OHM3463 reversed naltrexone- lever responding. All four compounds had antinociceptive effects, although the extent to which these effects were accompanied by respiratory depression or modified by naltrexone, as well as the interactions between antinociceptive effects of fentanyl derivatives and alfentanil, varied markedly among compounds. Thus, OHM3463 shared effects with μ agonists (e.g., alfentanil) under all conditions; the other three compounds had opioid agonist effects under only a subset of conditions. Moreover, one of these compounds (OHM3295) antagonized the discriminative stimulus and antinociceptive effects of other μ agonists. Collectively, these compounds appear to vary on two dimensions: opioid efficacy and the contribution of nonopioid actions to their antinociceptive effects. Together with results obtained with other fentanyl derivatives (mirfentanil) under similar conditions, results of the current study suggest this chemical class might be especially fertile for the development of novel analgesics that might have reduced toxicity and abuse liability as compared to fentanyl and related compounds that are currently used in medicine.