naloxone and oxymorphazone

Oxymorphazone (at doses of 50-200 mg/kg) was found to be a relatively weak antinociceptive drug in intact frog (Rana esculenta) when acetic acid was used as pain stimulus. Frogs remained analgesic for at least 48 hrs following oxymorphazone (200 mg/kg) administration. The ligand increased the latency of wiping reflex in spinal frogs too. These effects were blocked by naloxone. In equilibrium binding studies (3H)oxymorphazone had high affinity to the opioid receptors of frog brain and spinal cord as well (apparent Kd values were 8.9 and 10.6 nM, respectively). Kinetic experiments show that only 25% of the bound (3H)oxymorphazone is readily dissociable. Preincubation of the membranes with labeled oxymorphazone results in a washing resistant inhibition of the opioid binding sites. At least 70% of the (3H)oxymorphazone specific binding is apparently irreversible after reaction at 5 nM ligand concentration, and this can be enhanced by a higher concentration of tritiated ligand.

Topics: Analgesics; Animals; Binding, Competitive; Brain; Cell Membrane; Dose-Response Relationship, Drug; Hydromorphone; Kinetics; Naloxone; Oxymorphone; Rana esculenta; Receptors, Opioid; Spinal Cord

Several phenylhydrazone derivatives of oxymorphone [phenylhydrazone and p-nitrophenylhydrazone (OxyPNPH)] as well as oxymorphonazine produce a wash-resistant inhibition of radiolabeled opioid binding, suggesting nonequilibrium binding to opiate receptors. All are agonists and, in an effort to correlate their prolonged inhibition of binding with their pharmacology, we examined their analgesic actions in vivo. Dose-response curves at 1 hr revealed similar potencies of oxymorphone and the derivatives, with the exception of OxyPNPH which was significantly less potent. After 10 hr, oxymorphone at doses up to 50 mg/kg did not demonstrate any effect. In contrast, OxyPNPH at 25 mg/kg elevated tail-flick latencies from 2 to over 8 sec after 10 hr. The 50-mg/kg dose elevated latencies to approximately 5 sec after 24 hr. Oxymorphonazine and oxymorphone phenylhydrazone also produced a prolonged analgesia, although not as effectively as OxyPNPH. The prolonged analgesic actions of OxyPNPH were highly dependent upon a critical period of 2 to 3 h immediately after injection. Blockade of receptors during this period with naloxone prevented analgesia at all time points examined. If the long duration of action of OxyPNPH resulted simply from a long half-life and persistent-free compound within the brain, analgesia should have returned by 8 hr, at which time naloxone has been eliminated. The absence of analgesia 8 hr after both OxyPNPH and naloxone argues against simple pharmacokinetic mechanisms for the prolonged analgesia and is consistent with persistent receptor occupation. OxyPNPH (25 mg/kg) administered in vivo lowered radiolabeled opioid binding effectively in brain membranes despite extensive washing. OxyPNPH lowered mu1 binding by approximately 60% and mu2 binding by 35% whereas delta binding was not lowered significantly.

Topics: Analgesia; Animals; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Enkephalins; Hydrazones; Hydromorphone; Male; Mice; Naloxone; Oxymorphone; Receptors, Opioid; Time Factors

Oxymorphazone is a 14-hydroxydihydromorphinone derivative which contains a C-6 hydrazone group and hence could serve as an irreversible label for opioid receptors. 3H-oxymorphazone was synthesized by the reaction of 3H-oxymorphone with excess hydrazine. A specific radioactivity of 640 GBq/mmol (17,3 Ci/mmol) was achieved. Both the unlabelled compound and the tritiated ligand show high affinity to mu and kappa opiate receptor subtypes in rat brain membranes. Two binding sites were detected by equilibrium binding studies, with apparent Kd values of 0.62 nM and 28 nM. About 20% of the H-oxymorphazone specific binding is irreversible after reaction at 1 nM ligand concentration, and this can be enhanced by a higher concentration of tritiated ligand. No azine formation was detected. Preincubation of the membranes with unlabelled oxymorphazone resulted in an irreversible blockade of the high affinity 3H-naloxone binding sites.

Topics: Animals; Binding, Competitive; Brain; Hydromorphone; In Vitro Techniques; Isotope Labeling; Kinetics; Naloxone; Oxymorphone; Radioligand Assay; Rats; Receptors, Opioid; Tritium

Treating rat brain homogenates in vitro with oxymorphazone, the hydrazone derivative of oxymorphone, selectively inhibited in a long-acting manner the high-affinity (mu 1) binding of a number of 3H-opioids. This inhibition was not affected by extensive wash procedures which did effectively reverse classical opiates such as morphine and naloxone. A similar, persistent inhibition of binding was observed following in vivo administration of the drug. Both systemically and intracerebroventricularly, oxymorphazone produced a dose-dependent analgesia. Acutely, oxymorphazone (ED50, 0.6 mg/kg, sc) was approximately half as potent as oxymorphone (ED50, 0.3 mg/kg, sc) in the tail-flick assay. Administered at their ED50 doses, both compounds had the same durations of action. As the doses of drug were increased, however, the time course of oxymorphazone's analgesia became far more prolonged than that of oxymorphone. Following the administration of oxymorphazone (100 mg/kg), over 50% of the mice remained analgesic for greater than 24 hr, as opposed to none of the mice given oxymorphone (100 mg/kg). Oxymorphazone was far more potent intraventricularly (icv) than systemically. Fifty percent of the mice remained analgesic for greater than 20 hr following the injection of 40 micrograms/mouse (icv), whereas no mice remained analgesic after 20 hr following doses of oxymorphone as high as 50 micrograms/mouse (icv). These long-lasting analgesic actions of oxymorphazone could not be easily explained on pharmacokinetic grounds. Repeated administration of oxymorphazone daily for 3 days resulted in significant tolerance.

Topics: Analgesia; Animals; Brain; Dose-Response Relationship, Drug; Drug Tolerance; Enkephalin, Methionine; Hydromorphone; Injections, Intraventricular; Morphine; Naloxone; Oxymorphone; Rats; Receptors, Opioid; Time Factors

Further investigations into the molecular actions of the 14-hydroxydihydromorphinone hydrazones (naloxazone, oxymorphazone, and naltrexazone) have suggested that their irreversible actions can be explained by the formation of their azines. These azines, naloxonazine, naltrexonazine, and oxymorphonazine, irreversibly block opiate binding in vitro 20- to 40-fold more potently than their corresponding hydrozones, naloxazone, naltrexazone, and oxymorphazone. The blockade of binding by naloxonazine shows the same selectivity for high affinity, or mu1, sites as naloxazone.

Topics: Animals; Brain; Cell Membrane; Dihydromorphine; Drug Stability; Hydromorphone; Kinetics; Naloxone; Naltrexone; Oxymorphone; Receptors, Opioid; Structure-Activity Relationship

Differences in the receptor mechanisms of opiate analgesia and respiratory depression have been studied with three novel irreversible opiates. A single injection of the irreversible agonist oxymorphazone produces analgesia in mice, lasting over 24 hours. Conversely, the irreversible antagonist naloxazone dramatically reduces the analgesic effectiveness of a variety of opiate alkaloids and enkephalin analogs for over a day. Despite this marked reduction in analgesia after naloxazone treatment, morphine lethality (LD50) is unchanged in similarly treated mice. Receptor binding studies show that naloxazone irreversibly and selectively blocks a subpopulation of opiate receptors (the mu1 sites) to which all classes of opiates and enkephalins bind with highest affinity, whereas the drug has little to no effect on their lower-affinity sites (mu, and delta). The return of high-affinity receptor (mu1) binding to normal levels corresponds closely to the return of analgesic sensitivity and possibly represents receptor turnover in the central nervous system. These studies suggest that both opiate and opioid peptide analgesia is mediated through a single receptor subpopulation distinct from those involved with respiratory depression, and raise the possibility of specific opiate analgesics without respiratory depression.

Topics: Analgesia; Animals; Humans; Lethal Dose 50; Morphine; Muridae; Naloxone; Naltrexone; Oxymorphone; Receptors, Opioid; Respiration; Respiratory Insufficiency