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Mechanism of Action
Oxymorphone, a pure opioid agonist, is relatively selective for the mu receptor, although it can interact with other opioid receptors at higher doses.
The precise mechanism of analgesia, the principal therapeutic action of oxymorphone, is unknown. Specific CNS opiate receptors and endogenous compounds with morphine-like activity have been identified throughout the brain and spinal cord and are likely to play a role in the expression and perception of analgesic effects.
Pharmacological effects of opioid agonists include analgesia, anxiolysis, euphoria, feelings of relaxation, respiratory depression, constipation, miosis, and cough suppression. Like all pure opioid agonist analgesics, with increasing doses there is increasing analgesia, unlike with mixed agonist/antagonists or non-opioid analgesics, where there is a limit to the analgesic effect with increasing doses. With pure opioid agonist analgesics, there is no defined maximum dose; the ceiling to analgesic effectiveness is imposed only by side effects, the more serious of which may include somnolence and respiratory depression.
The minimum effective plasma concentration of oxymorphone for analgesia varies widely among patients, especially among patients who have been previously treated with potent agonist opioids. As a result, individually titrate patients to achieve a balance between therapeutic and adverse effects. The minimum effective analgesic concentration of oxymorphone for any individual patient may increase over time due to an increase in pain, progression of disease, development of a new pain syndrome and/or development of analgesic tolerance.
Concentration-Adverse Experience Relationships
There is a general relationship between increasing opioid plasma concentration and increasing frequency of adverse experiences such as nausea, vomiting, CNS effects, and respiratory depression.
As with all opioids, the dose of OPANA must be individualized [see DOSAGE AND ADMINISTRATION]. The effective analgesic dose for some patients will be too high to be tolerated by other patients.
Effects on the Central Nervous System (CNS)
The principal therapeutic action of oxymorphone is analgesia. In common with other opioids, oxymorphone causes respiratory depression, in part by a direct effect on the brainstem respiratory centers. The respiratory depression involves a reduction in the responsiveness of the brain stem respiratory centers to both increases in carbon dioxide tension and electrical stimulation. Opioids depress the cough reflex by direct effect on the cough center in the medulla. Antitussive effects may occur with doses lower than those usually required for analgesia. Oxymorphone causes miosis, even in total darkness. Pinpoint pupils are a sign of opioid overdose but are not pathognomonic (e.g., pontine lesions of hemorrhagic or ischemic origin may produce similar findings). Marked mydriasis rather than miosis may be seen with hypoxia in overdose situations [see OVERDOSAGE]. Other therapeutic effects of oxymorphone include anxiolysis, euphoria and feelings of relaxation.
In addition to analgesia, the widely diverse effects of oxymorphone include drowsiness, changes in mood, decreased gastrointestinal motility, nausea, vomiting, and alterations of the endocrine and autonomic nervous system.
Effects on the Gastrointestinal Tract and on Other Smooth Muscle
Gastric, biliary and pancreatic secretions are decreased by oxymorphone. Oxymorphone causes a reduction in motility and is associated with an increase in tone in the antrum of the stomach and duodenum. Digestion of food in the small intestine is delayed and propulsive contractions are decreased. Propulsive peristaltic waves in the colon are decreased, while tone is increased to the point of spasm. The end result may be constipation. Oxymorphone can cause a marked increase in biliary tract pressure as a result of spasm of the sphincter of Oddi; and transient elevations in serum amylase. Oxymorphone may also cause spasm of the sphincter of the urinary bladder.
Cardiovascular System Effects
Opioids produce peripheral vasodilation which may result in orthostatic hypotension. Release of histamine can occur and may contribute to opioid-induced hypotension. Manifestations of histamine release may include orthostatic hypotension, pruritus, flushing, red eyes, and sweating. Animal studies have shown that oxymorphone has a lower propensity to cause histamine release than other opioids.
Endocrine System Effects
Opioid agonists have been shown to have a variety of effects on the secretion of hormones. Opioids inhibit the secretion of ACTH, cortisol, and luteinizing hormone (LH) in humans. They also stimulate prolactin, growth hormone (GH) secretion, and pancreatic secretion of insulin and glucagon in humans and other species, rats and dogs. Thyroid stimulating hormone (TSH) has been shown to be both inhibited and stimulated by opioids.
Immune System Effects
Opioids have been shown to have a variety of effects on components of the immune system in in vitro and animal models. The clinical significance of these findings is unknown.
The absolute oral bioavailability of oxymorphone is approximately 10%. Studies in healthy volunteers reveal predictable relationships between OPANA (oxymorphone hydrochloride) dosage and plasma oxymorphone concentrations.
Steady-state levels were achieved after three days of multiple dose administration. Under both single-dose and steady-state conditions, dose proportionality has been established for 5 mg, 10 mg and 20 mg doses of OPANA (oxymorphone hydrochloride) , for both peak plasma levels (Cmax) and extent of absorption (AUC) (see Table 2).
Table 2 : Mean ( ± SD) OPANA (oxymorphone hydrochloride) Pharmacokinetic Parameters
|Regimen||Dosage||Cmax (ng/mL)||AUC (ng•hr/mL)||T½ (hr)|
|Single Dose||5 mg||1.10 ± 0.55||4.48 ± 2.07||7.25 ± 4.40|
|10 mg||1.93 ± 0.75||9.10 ± 3.40||7.78 ± 3.58|
|20 mg||4.39 ± 1.72||20.07 ± 5.80||9.43 ± 3.36|
|Multiple Dosea||5 mg||1.73 ± 0.62||4.63 ± 1.49||NA|
|10 mg||3.51 ± 0.91||10.19 ± 3.34||NA|
|20 mg||7.33 ± 2.93||21.10 ± 7.59||NA|
|NA = not applicable
a Results after 5 days of every 6 hours dosing.
After oral dosing with 40 mg of OPANA (oxymorphone hydrochloride) in healthy volunteers under fasting conditions or with a high-fat meal, the Cmax and AUC were increased by approximately 38% in fed subjects relative to fasted subjects. As a result, OPANA (oxymorphone hydrochloride) should be dosed at least one hour prior to or two hours after eating [see DOSAGE AND ADMINISTRATION].
The effect of co-ingestion of alcohol with OPANA (oxymorphone hydrochloride) has not been evaluated. However, an in vivo study was performed to evaluate the effect of alcohol (40%, 20%, 4% and 0%) on the bioavailability of a single dose of 40 mg of extended-release oxymorphone tablets in healthy, fasted volunteers. Following concomitant administration of 240 mL of 40% ethanol the Cmax increased on average by 70% and up to 270% in individual subjects. Following the concomitant administration of 240 mL of 20% ethanol, the Cmax increased on average by 31% and up to 260% in individual subjects. In some individuals there was also a decrease in oxymorphone peak plasma concentrations. No effect on the release of oxymorphone from the extended-release tablet was noted in an in vitro alcohol interaction study. The mechanism of the in vivo interaction is unknown. Therefore, avoid co-administration of oxymorphone and ethanol.
Formal studies on the distribution of oxymorphone in various tissues have not been conducted. Oxymorphone is not extensively bound to human plasma proteins; binding is in the range of 10% to 12%.
Oxymorphone is highly metabolized, principally in the liver, and undergoes reduction or conjugation with glucuronic acid to form both active and inactive products. The two major metabolites of oxymorphone are oxymorphone-3-glucuronide and 6-OH-oxymorphone. The mean plasma AUC for oxymorphone-3-glucuronide is approximately 90-fold higher than the parent compound. The pharmacologic activity of the glucuronide metabolite has not been evaluated. 6-OH-oxymorphone has been shown in animal studies to have analgesic bioactivity. The mean plasma 6-OH-oxymorphone AUC is approximately 70% of the oxymorphone AUC following single oral doses but is essentially equivalent to the parent compound at steady-state.
Because oxymorphone is extensively metabolized, < 1% of the administered dose is excreted unchanged in the urine. On average, 33% to 38% of the administered dose is excreted in the urine as oxymorphone-3-glucuronide and 0.25% to 0.62% is excreted as 6-OH-oxymorphone in subjects with normal hepatic and renal function. In animals given radiolabeled oxymorphone, approximately 90% of the administered radioactivity was recovered within 5 days of dosing. The majority of oxymorphone-derived radioactivity was found in the urine and feces.
Pharmacokinetics in Special Populations
The plasma levels of oxymorphone administered as an extended-release tablet were about 40% higher in elderly ( ≥ 65 years of age) than in younger subjects [see Use In Specific Populations].
The effect of gender on the pharmacokinetics of OPANA (oxymorphone hydrochloride) has not been studied. In a study with an extended-release formulation of oxymorphone, there was a consistent tendency for female subjects to have slightly higher AUCss and Cmax values than male subjects. However, gender differences were not observed when AUCss and Cmax were adjusted by body weight.
The liver plays an important role in the pre-systemic clearance of orally administered oxymorphone. Accordingly, the bioavailability of orally administered oxymorphone may be markedly increased in patients with moderate to severe liver disease. The effect of hepatic impairment on the pharmacokinetics of OPANA (oxymorphone hydrochloride) has not been studied. However, in a study with an extended-release formulation of oxymorphone, the disposition of oxymorphone was compared in 6 patients with mild, 5 patients with moderate, and one patient with severe hepatic impairment, and 12 subjects with normal hepatic function. The bioavailability of oxymorphone was increased by 1.6-fold in patients with mild hepatic impairment and by 3.7-fold in patients with moderate hepatic impairment. In one patient with severe hepatic impairment, the bioavailability was increased by 12.2-fold. The half-life of oxymorphone was not significantly affected by hepatic impairment.
The effect of renal impairment on the pharmacokinetics of OPANA (oxymorphone hydrochloride) has not been studied. However, in a study with an extended-release formulation of oxymorphone, an increase of 26%, 57%, and 65% in oxymorphone bioavailability was observed in mild (creatinine clearance 51-80 mL/min; n=8), moderate (creatinine clearance 3050 mL/min; n=8), and severe (creatinine clearance < 30 mL/min; n=8) patients, respectively, compared to healthy controls.
In vitro studies revealed little to no biotransformation of oxymorphone to 6-OH-oxymorphone by any of the major cytochrome P450 (CYP P450) isoforms at therapeutically relevant oxymorphone plasma concentrations.
No inhibition of any of the major CYP P450 isoforms was observed when oxymorphone was incubated with human liver microsomes at concentrations of ≤ 50 μM. An inhibition of CYP 3A4 activity occurred at oxymorphone concentrations ≥ 150 μM. Therefore, it is not expected that oxymorphone, or its metabolites will act as inhibitors of any of the major CYP P450 enzymes in vivo.
Increases in the activity of the CYP 2C9 and CYP 3A4 isoforms occurred when oxymorphone was incubated with human hepatocytes. However, clinical drug interaction studies with OPANA (oxymorphone hydrochloride) ER showed no induction of CYP450 3A4 or 2C9 enzyme activity, indicating that no dose adjustment for CYP 3A4- or 2C9-mediated drug-drug interactions is required.
Two double-blind, placebo-controlled, dose-ranging studies in patients with acute moderate to severe pain following orthopedic surgery evaluated the doses of OPANA (oxymorphone hydrochloride) 10 mg and 20 mg, and 30 mg was included in one study. Both studies demonstrated that OPANA (oxymorphone hydrochloride) 20 mg provided greater analgesia as measured by total pain relief based on a weighted analysis over 8 hours using a 0-4 categorical, compared to placebo. OPANA (oxymorphone hydrochloride) 10 mg provided greater analgesia as compared to placebo in one of the two studies. There was no evidence of superiority of the 30 mg dose over the 20 mg dose. However, there was a high rate of naloxone use in patients receiving the OPANA 30 mg dose in the post-operative period [see DOSAGE AND ADMINISTRATION].
In a randomized, double-blind, placebo-controlled, multiple-dose study, the efficacy of OPANA (oxymorphone hydrochloride) 10 mg and 20 mg was assessed in patients with moderate to severe acute pain following abdominal surgery. In this study, patients were dosed every 4 to 6 hours over a 48-hour treatment period. OPANA (oxymorphone hydrochloride) 10 and 20 mg provided greater analgesia, as measured by the mean average pain intensity on a 0-100 mm visual analog scale, over 48 hours, compared to placebo [see DOSAGE AND ADMINISTRATION].
Last reviewed on RxList: 3/28/2011
This monograph has been modified to include the generic and brand name in many instances.
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