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Chemistry – A quaternary ammonium oxime cholinesterase reactivator, pralidoxime chloride occurs as a white to pale yellow, crystalline powder with a pKa of 7.8-8. It is freely soluble in water. The commercially available injection has a pH of 3.5-4.5 after reconstitution. Pralidoxime may also be known as 2-PAM Chloride , or 2-Pyridine Aldoxime Methochloride .
Storage/Stability/Compatibility – Unless otherwise instructed by the manufacturer, pralidoxime chloride powder for injection should be stored at room temperature. After reconstituting with sterile water for injection, the solution should be used within a few hours. Do not use sterile water with preservatives added.
Pharmacology – Pralidoxime reactivates cholinesterase that has been inactivated by phosphorylation secondary to certain organophosphates. Via nucleophilic attack, the drug removes and binds the offending phosphoryl group attached to the enzyme and is then excreted.
Uses/Indications – Pralidoxime is used in the treatment of organophosphate poisoning, often in conjunction with atropine and supportive therapy.
Pharmacokinetics – Pralidoxime is only marginally absorbed after oral dosing and oral dosage forms are no longer available in the United States. It is distributed primarily throughout the extracellular water. Because of its quaternary ammonium structure, it is not believed to enter the CNS in significant quantities, but recent studies and clinical responses have led some to question this. Pralidoxime is thought to be metabolized in the liver and excreted as both metabolite(s) and unchanged drug in the urine.
Contraindications/Precautions/Reproductive Safety – Pralidoxime is contraindicated in patients hypersensitive to it. Pralidoxime is generally not recommended to be used in instances of carbamate poisoning because inhibition is rapidly reversible, but there is some controversy regarding this issue.
Pralidoxime should be used with caution in patients receiving anticholinesterase agents for the treatment of myasthenia gravis as it may precipitate a myasthenic crisis. It should also be used cautiously and at a reduced dosage rate in patients with renal impairment.
Adverse Effects/Warnings – At usual doses, pralidoxime generally is safe and free of significant adverse effects. Rapid IV injection may cause tachycardia, muscle rigidity, transient neuromuscular blockade, and laryngospasm.
Pralidoxime must generally be given within 24 hours of exposure to be effective, but some benefits may occur, particularly in large exposures, if given within 36-48 hours.
Overdosage/Acute Toxicity – The acute LD50 of pralidoxime in dogs is 190 mg/kg and, at high dosages, exhibits symptoms of its own anticholinesterase activity. Symptoms of toxicity in dogs may be exhibited as muscle weakness, ataxia, vomiting, hyperventilation, seizures, respiratory arrest and death.
Drug Interactions – Anticholinesterases can potentiate the action of barbiturates; use with caution. Cimetidinemay potentiate the action of organophosphates by slowing its metabolism.
Use of succinylcholine, theophylline/aminophylline, reserpine, and respiratory depressant drugs (e.g., narcotics, phenothiazines) should be avoided in patients with organophosphate toxicity.
Doses – Note: often used in conjunction with atropine; refer to that monograph and/or the references below for more information.
For organophosphate poisoning:
a) 25 – 50 mg/kg as a 20% solution IV over 6 minutes; or as a maximum of 100 mg/kg/day as an IV drip. (Smith 1986)
For organophosphate poisoning:
a) 20 mg/kg (may require up to 35 mg/kg) IV and repeat q4-6h. (Oehme 1987c)
Monitoring Parameters – Monitoring of pralidoxime therapy is basically by monitoring the signs and symptoms associated with organophosphate poisoning. For more information, refer to one of the references outlined noted below.
Client Information – This agent should only be used with close professional supervision.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times –
Veterinary-Approved Products: None
Pralidoxime Chloride 1 gram cake in containers of six 20 ml vials without diluent or syringes; 600 mg in one 2 ml auto-injector; Protopam Chloride® (Wyeth-Ayerst); (Rx); Pralidoxime Chloride® (Survival Technology) (Rx)