Pralidoxime Chloride

Cholinesterase Reactivator (Oxime) Antidote for Organophosphate Poisoning

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog IV (slow), IM, or SC 10-20 mg/kg (or 20-50 mg/kg in severe cases) q12h or as needed; may repeat in 1-2 hours if muscle weakness persists Duration: Until clinical signs resolve (usually 24-48 hours)
Notes: Administer slowly IV over 5-10 minutes. Use in conjunction with atropine (0.02-0.05 mg/kg IV, repeated as needed).
Cat IV (slow), IM, or SC 10-20 mg/kg q12h or as needed; may repeat in 1-2 hours if muscle weakness persists Duration: Until clinical signs resolve (usually 24-48 hours)
Notes: Administer slowly IV over 5-10 minutes. Use in conjunction with atropine (0.02-0.05 mg/kg IV, repeated as needed).
Horse IV (slow) 10-20 mg/kg q12h or as needed; may repeat in 1-2 hours if muscle weakness persists Duration: Until clinical signs resolve (usually 24-48 hours)
Notes: Administer slowly IV. Use in conjunction with atropine (0.02-0.05 mg/kg IV, repeated as needed).
Cattle IV (slow) 10-20 mg/kg q12h or as needed; may repeat in 1-2 hours if muscle weakness persists Duration: Until clinical signs resolve (usually 24-48 hours)
Notes: Administer slowly IV. Use in conjunction with atropine (0.02-0.05 mg/kg IV, repeated as needed).
Small Ruminants (sheep, goats) IV (slow) 10-20 mg/kg q12h or as needed; may repeat in 1-2 hours if muscle weakness persists Duration: Until clinical signs resolve (usually 24-48 hours)
Notes: Administer slowly IV. Use in conjunction with atropine (0.02-0.05 mg/kg IV, repeated as needed).
Rabbit IV, IM, or SC 10-20 mg/kg q12h or as needed Duration: Until clinical signs resolve
Notes: Limited data; use cautiously.
Birds/Poultry IM or IV 10-20 mg/kg q12h or as needed Duration: Until clinical signs resolve
Notes: Limited data; use cautiously.

Clinical Indications & Species Uses

General Indications
  • Antidote for organophosphate and carbamate poisoning in all species
Dog (Canine)
  • Adjunct to atropine in the treatment of organophosphate insecticide poisoning
  • Treatment of carbamate poisoning (e.g., carbaryl) when atropine is insufficient
  • Reactivation of cholinesterase in cases of nerve agent exposure
Cat (Feline)
  • Adjunct to atropine in the treatment of organophosphate insecticide poisoning
  • Treatment of carbamate poisoning (e.g., carbaryl) when atropine is insufficient
Horse (Equine)
  • Adjunct to atropine in the treatment of organophosphate poisoning (e.g., from anthelmintics or insecticides)
Cattle (Bovine)
  • Adjunct to atropine in the treatment of organophosphate poisoning (e.g., from insecticides or anthelmintics)
Small Ruminants (Sheep / Goat)
  • Adjunct to atropine in the treatment of organophosphate poisoning (e.g., from insecticides)

Pharmacology & Mechanism of Action

Drug Class: Cholinesterase Reactivator (Oxime) | Pharmacological Group: Antidote for Organophosphate Poisoning

Mechanism of Action: Pralidoxime chloride is a nucleophilic oxime that reactivates cholinesterase (acetylcholinesterase) enzymes that have been phosphorylated by organophosphate (OP) compounds. It does so by binding to the phosphorylated enzyme and removing the phosphate group, thereby restoring enzyme activity. This reactivation is most effective if administered before 'aging' of the enzyme occurs (i.e., before the phosphorylated enzyme loses an alkyl group, making it resistant to reactivation). Pralidoxime also directly detoxifies some organophosphates by chemical combination, and it may have a mild antimuscarinic effect. It does not antagonize the muscarinic effects of acetylcholine at receptors; therefore, atropine is typically co-administered to block the effects of accumulated acetylcholine.

Pharmacodynamics: Pralidoxime reactivates cholinesterase, leading to a reduction in the accumulation of acetylcholine at cholinergic synapses. This results in improvement of both muscarinic and nicotinic signs of OP poisoning, such as excessive salivation, lacrimation, urination, defecation, gastrointestinal distress, muscle fasciculations, and respiratory depression. The onset of action is rapid (within minutes) when given IV, but the duration of action is short (1-2 hours) due to rapid renal excretion. The clinical effect is dependent on the degree of enzyme reactivation and the extent of aging of the OP-enzyme complex.

⚑ Pharmacokinetics Summary

Absorption: Pralidoxime is poorly absorbed orally; therefore, parenteral administration is required for systemic effects. After IM or SC injection, absorption is rapid and complete. Peak plasma concentrations are achieved within 10-20 minutes after IM administration.
Distribution: Pralidoxime distributes widely into extracellular fluids. It does not penetrate the blood-brain barrier to a significant extent, so it has limited effect on central nervous system signs. It crosses the placenta and is distributed into milk.
Metabolism: Pralidoxime undergoes minimal metabolism. It is not significantly metabolized in the liver.
Excretion: Pralidoxime is rapidly excreted unchanged in the urine via glomerular filtration and tubular secretion. The elimination half-life is approximately 1-2 hours in most species. Renal clearance is high, and excretion is reduced in animals with renal impairment.
Half-Life: Dog: ~1.7 hours; Cat: ~1.2 hours; Horse: ~1.5 hours; Cattle: ~1.4 hours (approximate values)
Bioavailability: Oral bioavailability is low (<10%); IM and SC bioavailability is essentially complete (near 100%).
Protein Binding: Low (<10%)

Available Formulations & Strengths

Injectable Solution 300 mg/mL (1 g/2 mL vial) (IV, IM, SC)
Powder for Injection 1 g/vial (reconstitute with 20 mL sterile water to 50 mg/mL) (IV, IM, SC)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to pralidoxime or any component of the formulation
  • Use in patients with known myasthenia gravis (may precipitate cholinergic crisis)
  • Do not use in patients with severe renal impairment without dose adjustment
  • Do not use as a sole antidote; atropine must be administered concurrently for muscarinic signs
Warnings & Clinical Precautions:
  • Use with caution in patients with renal impairment; dose adjustment may be necessary.
  • Administer slowly IV to avoid adverse effects such as tachycardia, laryngospasm, and muscle rigidity.
  • Pralidoxime is most effective when given early after OP exposure; delay may reduce efficacy due to enzyme aging.
  • Monitor cholinesterase levels if available.
  • Use in pregnant animals only if clearly needed; safety not established.
  • May cause transient increases in blood pressure and heart rate.
  • In cases of carbamate poisoning, pralidoxime may be less effective and should be used with caution, as it may increase toxicity in some carbamate poisonings (e.g., carbaryl).

Adverse Effects & Reactions

Common:

  • Dizziness
  • Blurred vision
  • Headache
  • Nausea
  • Tachycardia
  • Mild hypertension
  • Pain at injection site

Serious / Severe:

  • Laryngospasm
  • Muscle rigidity
  • Neuromuscular blockade (with rapid IV administration)
  • Respiratory arrest (with rapid IV administration)
  • Seizures (rare)

Rare:

  • Hypersensitivity reactions
  • Hepatotoxicity (with high doses)
  • Renal toxicity (with high doses)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Atropine Additive antimuscarinic effects; atropine is used concurrently to control muscarinic signs. Pralidoxime may potentiate the effects of atropine, so atropine dosage should be adjusted to avoid over-atropinization. Moderate
Succinylcholine Pralidoxime may prolong the neuromuscular blocking effects of succinylcholine. Moderate
Other cholinesterase inhibitors (e.g., neostigmine, physostigmine) Antagonistic effects; pralidoxime may reverse the effects of these drugs. Moderate
Morphine and other opioids May cause additive respiratory depression; use with caution. Mild
Theophylline and aminophylline May increase the risk of cardiac arrhythmias. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Muscle weakness
  • Fasciculations
  • Neuromuscular blockade
  • Respiratory depression
  • Seizures
  • Cardiac arrhythmias
  • Hypotension or hypertension

Emergency Treatment Protocol: Discontinue the drug immediately. Provide supportive care including respiratory support (mechanical ventilation if needed), cardiovascular monitoring, and seizure management (e.g., benzodiazepines). There is no specific antidote for pralidoxime overdose. Hemodialysis may be considered in severe cases with renal impairment.

Food Animal Withdrawal Times

πŸ₯© Meat: 7 daysπŸ₯› Milk: 3 days

Withdrawal times are not officially established for pralidoxime in food animals. The values provided are conservative estimates based on the drug's short half-life and are intended as a guide. Always consult regulatory authorities and follow label directions. For extra-label use, follow veterinary oversight and ensure appropriate withdrawal periods.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25Β°C, excursions permitted to 15-30Β°C).

Handling & Special Conditions: Protect from freezing. Reconstituted solutions should be used within 24 hours if stored at room temperature, or up to 48 hours if refrigerated. Discard any unused solution.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use. Available for veterinary use via extra-label drug use.

Extra-Label (Off-Label) Use: In the US, pralidoxime is not FDA-approved for veterinary species, but it can be used extra-label under the Animal Medicinal Drug Use Clarification Act (AMDUCA) when prescribed by a veterinarian within a valid VCPR. For food animals, a withdrawal period must be established by the veterinarian, and residues must be within tolerance.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Pralidoxime chloride is a critical antidote for organophosphate (OP) poisoning in veterinary medicine. It is most effective when administered early, before aging of the cholinesterase enzyme occurs. It should always be used in conjunction with atropine to control muscarinic signs, as pralidoxime does not directly antagonize acetylcholine at muscarinic receptors. The combination of atropine and pralidoxime is synergistic and improves survival rates. Clinical monitoring should include heart rate, respiratory rate, pupil size, and muscle strength. Cholinesterase levels can be measured to guide therapy. In cases of carbamate poisoning, pralidoxime is generally not recommended for carbaryl poisoning, as it may worsen toxicity; however, it may be used for other carbamates if atropine is insufficient. Due to its short half-life, repeated dosing may be necessary. Always administer slowly IV to avoid adverse effects. For food animals, adhere to withdrawal times and regulatory requirements.

References & Literature

  • πŸ“š Plumb's Veterinary Drug Handbook
  • πŸ“š Papich Veterinary Pharmacology and Therapeutics
  • πŸ“š Compendium of Veterinary Products (CVP)