Jerome R. Bagley

Mirfentanil antagonizes morphine-induced suppression of splenic NK activity in mice

Mirfentanil [N-(2-pyrazinyl)-N-(1-phenethyl-4-piperidinyl)-2-furamide] was studied for its antinociceptive and immunomodulatory effects in mice Mirfentanil (1.0-32.0 mg/kg) increased tail-flick latency to a thermal stimulus and this effect was antagonized (94 +/- 2%) by naltrexone (10.0 mg/kg). Unlike naltrexone, the delta opioid selective antagonist naltrindole (20.0 mg/kg) had no effect on mirfentanil-induced analgesia. In a dose-dependent fashion, the mu-selective antagonists beta-funaltrexamine (1.0-40.0 mg/kg) and naloxonazine (1.0-35.0 mg/kg) blocked mirfentanil (10.0 mg/kg)-induced analgesia up to 75% of the maximum analgesic effect. Norbinaltorphimine (10.0 mg/kg) partially blocked (35%) the maximum analgesic effect following mirfentanil (10.0 mg/kg) administration. Single doses of mirfentanil (0.1-32.0 mg/kg) had no effect on splenic NK activity. However, preadministration of mirfentanil (1.0-10.0 mg/kg) blocked morphine-induced suppression of splenic NK activity. Collectively, the results suggest that mirfentanil is a novel opioid that induces antinociception predominately through mu opioid receptors but, unlike morphine or fentanyl, does not suppress splenic NK activity.

Behavioral Effects and Binding Affinities of the Fentanyl Derivative OHM3507

Several fentanyl derivatives have been reported to have novel pharmacologies that might be exploited for developing alternate approaches to the treatment of pain. The purpose of the current series of studies was to evaluate OHM3507, a novel fentanyl derivative reported to have an unusual pharmacological profile in nonprimate species. Similar to several other fentanyl derivatives with clinical potential, OHM3507 had the highest affinity (IC50 = 10 nM) for mu ([3H]D-Ala2,N-Me-Phe4,Gly5-OH-labeled) receptors with 6- and 176-fold lower affinity for delta ([3H]D-Pen2-D-Pen5-labeled), and kappa ([3H]ethylketocyclazocine-labeled) receptors, respectively. In rhesus monkeys, OHM3507 shared discriminative stimulus effects with morphine, increased tail-withdrawal latencies in a warm-water procedure of antinociception, decreased ventilation in monkeys breathing normal air or 5% CO2, and failed to modify accuracy on acquisition and performance tasks up to doses that decreased rates of food-maintained responding. The opioid antagonists naltrexone and naltrindole antagonized the behavioral effects of OHM3507 in a manner that was consistent with mu-receptor mediation. Although OHM3507 appeared to have low efficacy opioid actions in nonprimate species, results from the current studies clearly show this compound to have strong, fentanyl-like mu agonist actions in rhesus monkeys. These results provide another example of the sometimes poor predictability in the behavioral pharmacology of fentanyl derivatives among species, in this case between monkeys and rats, mice and rabbits, and demonstrates the need for evaluating new drugs under a broad range of conditions to increase the probability of identifying novel compounds that can be used to treat pain.

The rate-decreasing effects of fentanyl derivatives in pigeons before, during and after chronic morphine treatment

Abstract Mirfentanil is a fentanyl derivative with non-opioid actions, including non-opioid antinociceptive effects in rhesus monkeys. The current study examined the rate-altering effects of mirfentanil and several other compounds in pigeons to assess: 1) the opioid and non-opioid actions of acutely-administered fentanyl derivatives; and 2) the development of cross-tolerance between each of these compounds and morphine. Seven pigeons responded under a fixed-ratio 20 (FR20) schedule of food delivery. In untreated pigeons, fentanyl, morphine, naltrexone, ketamine and three fentanyl derivatives (mirfentanil, OHM3463 and OHM3295) decreased rates of key pecking in a dose-related manner. Naltrexone (0.1–1.0 mg/kg) attenuated the effects of OHM3463 and not mirfentanil or OHM3295, suggesting non-opioid mediation of the rate-decreasing effects for the latter two fentanyl derivatives. Subjects were treated daily with morphine for 9 weeks, up to a dose of 100 mg/kg per day, during which time the dose-effect curves for morphine, fentanyl and OHM3463 shifted rightward 6-, 10- and 2-fold, respectively, indicating the development of tolerance to morphine and cross-tolerance to fentanyl and OHM3463. Dose-effect curves for ketamine, OHM3295 and mirfentanil were not shifted to the right during morphine treatment, and the dose-effect curve for naltrexone was shifted leftward 180-fold. To the extent that rate-decreasing effects are predictive of antinociceptive effects, these data suggest that some fentanyl derivatives might be useful therapeutics under conditions where tolerance develops to morphine-like opioids.

Effects of a novel fentanyl derivative on drug discrimination and learning in rhesus monkeys.

Three monkeys discriminated 1.78 mg/kg of mirfentanil while responding under a fixed-ratio 5 schedule of stimulus-shock termination. Two mirfentanil derivatives, OHM3295 and OHM10579, substituted for mirfentanil in all subjects. However, other drugs produced variable effects among monkeys; for example, mu and kappa opioid agonists and clonidine substituted for mirfentanil on some occasions in two monkeys. Cocaine, amphetamine, and ketamine did not substitute in any subject. Opioid antagonists did not attenuate the effects of mirfentanil. In monkeys responding under a repeated acquisition and performance procedure, errors increased only during the acquisition phase at doses of mirfentanil that decreased response rates. Thus, unlike fentanyl, the discriminative stimulus effects of mirfentanil do not appear to be mediated exclusively through opioid receptors. Finally, mirfentanil does not appear to disrupt complex behavioral processes.

Pharmacological Profile of a Deuterium-Substituted Mirfentanil Derivative, OHM10579, in Rhesus Monkeys

The discriminative-stimulus, respiratory, and antinociceptive effects of OHM10579, an isotopic isomer of mirfentanil, were characterized in rhesus monkeys. In monkeys discriminating nalbuphine, 0.32 mg/kg of OHM10579 partially substituted for nalbuphine. In monkeys treated daily with 3.2 mg/kg of morphine and discriminating 0.01 mg/kg of naltrexone, 0.32 mg/kg of OHM10579 substituted for naltrexone. In morphine-abstinent monkeys, morphine reversed naltrexone-lever responding, an effect attenuated by OHM10579. The shift to the right in the morphine dose-effect curve was greater 2 h after 0.32 mg/kg of OHM10579 compared to 0.32 mg/kg of mirfentanil, indicating that OHM10579 has a longer duration of action than mirfentanil. In a warm-water tail-withdrawal procedure, 10 and 17.8 mg/kg of OHM10579 had antinociceptive effects that were not antagonized by naltrexone. Morphine decreased breathing in air to 48%, whereas the maximal decrease with OHM10579 was to 75% of control. OHM10579 attenuated hyperventilation induced by 5% CO2 and partially antagonized the respiratory-depressant effects of morphine. OHM10579 can be classified as a low-efficacy mu-opioid agonist with some nonopioid actions. These results indicate that the pharmacology of the mirfentanil isotope OHM10579 is similar to that of mirfentanil, but that OHM10579 might have a longer duration of action.