Chemistry - A semi-synthetic phenanethrene narcotic agonist, oxymorphone HCl occurs as odorless white crystals or white to off-white powder. It will darken in color with prolonged exposure to light. One gram of oxymorphone HCl is soluble in 4 ml of water and it is sparingly soluble in alcohol and ether. The commercially available injection has a pH of 2.7 - 4.5.
Storage/Stability/Compatibility - The injection should be stored protected from light and at room temperature (15-30° C); avoid freezing. The commercially available suppositories should be stored at temperatures between 2° and 15° C. Oxymorphone has been reported to be compatible when mixed with acepromazine, atropine, and glycopyrrolate. It is incompatible when mixed with barbiturates, and diazepam.
Pharmacology - See the monograph: Narcotic (Opiate) Analgesics for more infortmation. Oxymorphone is approximately 10 times more potent an analgesic on a weight basis when compared to morphine. It has less antitussive activity than does morphine. In humans, it has more of a tendency to cause increased nausea and vomiting than does morphine, while in dogs the opposite appears to be true. At the usual doses employed, oxymorphone alone has good sedative qualities in dog. Respiratory depression can occur especially in debilitated, neonatal or geriatric patients. Bradycardia, as well as a slight decrease in cardiac contractility and blood pressure may also be seen. Like morphine, oxymorphone does initially increase the respiratory rate (panting in dogs) while actual oxygenation may be decreased and blood CO2 levels may increase by 10 mmHg or more. Gut motility is decreased with resultant increases in stomach emptying times. Unlike either morphine or meperidine, oxymorphone does not appear to cause histamine release.
Pharmacokinetics - Oxymorphone is absorbed when given by IV, IM , SQ, and rectal routes. Although absorbed when given orally bioavailability is reduced, probably from a high first-pass effect. After IV administration, analgesic efficacy usually occurs within 3-5 minutes.
Like morphine, oxymorphone concentrates in the kidney, liver, and lungs; lower levels are found in the CNS. Oxymorphone crosses the placenta and narcotized newborns can result if mothers are given the drug before giving birth, but these effects can be rapidly reversed with naloxone.
The drug is metabolized in the liver; primarily by glucuronidation. Because cats are deficient in this metabolic pathway, half-lives in cats are probably prolonged. The glucuronidated metabolite is excreted by the kidney.
Uses/Indications - Oxymorphone is used in dogs and cats as a sedative/restraining agent, analgesic and preanesthetic and occassionally in horses as an analgesic and anesthesia induction agent. It may also be used in swine as an adjunctive analgesic with ketamine/xylazine anesthesia and in small rodents as an analgesic/anesthetic for minor surgical procedures.
Contraindications/Precautions - All opiates should be used with caution in patients with hypothyroidism, severe renal insufficiency, adrenocortical insufficiency (Addison’s), and in geriatric or severely debilitated patients. Oxymorphone is contraindicated in patients hypersensitive to narcotic analgesics, and in patients taking monamine oxidase inhibitors (MAOIs). It is also contraindicated in patients with diarrhea caused by a toxic ingestion until the toxin is eliminated from the GI tract.
Oxymorphone should be used with extreme caution in patients with head injuries, increased intracranial pressure and acute abdominal conditions (e.g., colic) as it may obscure the diagnosis or clinical course of these conditions. It should be used with extreme caution in patients suffering from respiratory disease or from acute respiratory dysfunction (e.g., pulmonary edema secondary to smoke inhalation).
Oxymorphone can cause bradycardia and therefore should be used cautiously in patients with preexisting bradyarrhythmias.
Neonatal, debilitated or geriatric patients may be more susceptible to the effects of oxymorphone and may require lower dosages. Patients with severe hepatic disease may have prolonged durations of action of the drug. If used in cats at high dosages, the drug has been recommended to be given along with a tranquilizing agent, as oxymorphone can produce bizarre behavioral changes in this species. This also is true in cats also for the other opiate agents, such as morphine.
Opiate analgesics are also contraindicated in patients who have been stung by the scorpion species Centruroides sculpturatus Ewing and C. gertschi Stahnke as it may potentiate these venoms.
Adverse Effects/Warnings - Oxymorphone may cause respiratory depression and bradycardia (see above). When used in cats at high dosages, oxymorphone may cause ataxia, hyperesthesia and behavioral changes (without concomitant tranquilization). Decreased GI motility with resultant constipation has also been described.
Overdosage - Massive overdoses may produce profound respiratory and/or CNS depression in most species. Other effects may include cardiovascular collapse, hypothermia, and skeletal muscle hypotonia. Naloxone is the agent of choice in treating respiratory depression. In massive overdoses, naloxone doses may need to be repeated, and animals should be closely observed as naloxone’s effects may diminish before sub-toxic levels of oxymorphone are attained.
Mechanical respiratory support should also be considered in cases of severe respiratory depression.
Drug Interactions - Other CNS depressants (e.g., anesthetic agents, antihistamines, phenothiazines, barbiturates, tranquilizers, alcohol, etc.) may cause increased CNS or respiratory depression when used with oxymorphone. Oxymorphone is contraindicated in patients receiving monoamine oxidase (MOA) inhibitors (rarely used in veterinary medicine) for at least 14 days after receiving MOA inhibitors in humans. Some human patients have exhibited signs of opiate overdose after receiving therapeutic doses of oxymorphone while on these agents.
Laboratory Interactions - Plasma amylase and lipase values may be increased for up to 24 hours following administration of opiate analgesics as they may increase biliary tract pressure.
As an analgesic:
a) 0.01 - 0.02 mg/kg IV (Muir 1987)
b) 0.01 - 0.022 mg/kg IV; up to 15mg total (divide dose into 3-4 increments and give several minutes apart (Shaw et al. 1986)
c) 0.02 - 0.03 mg/kg IM (Robinson 1987)
d) 0.015 - 0.03 mg/kg IV (Thurmon and Benson 1987)
Anesthetic induction in severely compromised horses:
a) 0.01 - 0.022 mg/kg IV (after approx. 45 minutes, may be necessary to “top off” with another 1/3 of the original dose) (Shaw et al. 1986)
Note: Narcotics (oxymorphone included) may cause CNS excitement in the horse. Some clinicians recommend pretreatment with acepromazine (0.02 - 0.04 mg/kg IV), or xylazine (0.3 - 0.5 mg/kg IV) to reduce the behavioral changes these drugs can cause.
Warning: Narcotic analgesics can mask the behavioral and cardiovascular symptoms associated with mild colic.
Monitoring Parameters -
1) Respiratory rate/depth
2) CNS level of depression/excitation
3) Blood pressure if possible and indicated (especially with IV use)
4) Analgesic activity
5) Cardiac rate
Client Information - When given parenterally, this agent should be used in an inpatient setting or with direct professional supervision.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Veterinary-Approved Products: The veterinary labeled product is reportedly discontinued.
Oxymorphone HCl for Injection 1 mg/ml in 1 ml amps; 1.5 mg/ml in 1 ml amps and 10 ml vials; Numorphan® (Du Pont); (Rx)
Oxymorphone HCl 5 mg suppositories in 6s.; Numorphan® (Du Pont); (Rx)
Note: Oxymorphone is a Class-II controlled substance. Very accurate record keeping is required as to use and disposition of stock.
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