1. Clinical Overview
Pralidoxime is a cholinesterase reactivator used as an antidote in the treatment of poisoning by organophosphorus compounds (pesticides and nerve agents). It is a quaternary ammonium oxime that reactivates acetylcholinesterase (AChE) inhibited by organophosphates by removing the phosphoryl group from the enzyme's active site. It is most effective when administered within 24-36 hours of exposure, before the phosphorylated enzyme undergoes 'aging' (irreversible dealkylation). In the Indian context, it is a critical component of hospital emergency protocols, especially in agricultural regions with high incidence of pesticide poisoning.
| Onset | Duration | Bioavailability |
|---|---|---|
| Intravenous administration: Effects on neuromuscular junction begin within 10-30 minutes. Peak reactivation occurs within 5-15 minutes post-IV bolus. | Approximately 1.5 to 2 hours. Due to its short duration, repeated doses or a continuous infusion are often required. | Less than 10% orally due to its quaternary ammonium structure and poor absorption from the GI tract. Intramuscular bioavailability is approximately 90-100%. |
2. Mechanism of Action
Organophosphorus compounds (OPs) phosphorylate the serine hydroxyl group at the active site of acetylcholinesterase (AChE), leading to irreversible inhibition and accumulation of acetylcholine. Pralidoxime acts as a nucleophile. Its oxime group (-CH=NOH) attacks the phosphorus atom of the phosphorylated AChE, forming an oxime-phosphonate complex. This complex then splits, regenerating the active enzyme and releasing the phosphorylated oxime.
3. Indications & Uses
- Antidote in poisoning by organophosphorus insecticides (e.g., chlorpyrifos, malathion, parathion, diazinon)
- Antidote in poisoning by nerve agents (chemical warfare agents like sarin, soman, tabun, VX) in military/civil defense settings
4. Dosage & Administration
Adult Dosage: Initial: 1-2 g (30-40 mg/kg) IV, administered slowly over 15-30 minutes. Alternatively, can be given as an IV infusion in 100 ml normal saline over 15-30 min. Maintenance: Either repeat the initial dose every 4-6 hours if symptoms recur, OR administer a continuous IV infusion of 500 mg/hour (approx. 8-10 mg/kg/hour). Total daily dose should not exceed 12 g. IM route can be used if IV access is unavailable.
Administration: For 25mg/ml solution: Dilute the required dose (e.g., 2g = 80ml) in 100-150 ml of 0.9% Sodium Chloride Injection. Administer IV over 15-30 minutes. Rapid IV injection can cause tachycardia, laryngospasm, muscle rigidity. IM injection can be given undiluted into a large muscle mass. Incompatible with alkaline solutions. Protect from light.
5. Side Effects
Common side effects may include:
- Transient dizziness, blurred vision, diplopia (double vision)
- Headache
- Drowsiness, nausea
- Injection site pain (IM route)
- Tachycardia (if injected too rapidly)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Atropine | Synergistic therapeutic effect. Atropine MUST be given first to block muscarinic effects before pralidoxime is administered. | Major |
| Barbiturates (e.g., Phenobarbital) | Pralidoxime may potentiate the effects of barbiturates. Use with caution. | Moderate |
| Aminophylline, Theophylline | Increased risk of seizures. Monitor closely. | Moderate |
| Succinylcholine, other neuromuscular blockers | Effects may be prolonged in organophosphate poisoning. Pralidoxime may reverse blockade. | Major |
| Reserpine | Theoretical risk of increased hypertension. Avoid concomitant use. | Moderate |
7. Patient Counselling
- DO seek immediate emergency medical help in case of suspected pesticide poisoning.
- DO inform the doctor of any kidney problems or history of myasthenia gravis.
- DO NOT attempt to self-medicate with this drug; it is for hospital use only.
- DO NOT drive or operate heavy machinery for 24 hours after treatment due to dizziness.
8. Toxicology & Storage
Overdose: Symptoms are an extension of side effects: Severe hypertension, tachycardia, muscle rigidity, laryngospasm, seizures, respiratory arrest. Paradoxically, it can itself cause weakness and neuromuscular blockade at very high doses.
Storage: Store at controlled room temperature (15°C to 30°C). Protect from light. The solution should be clear and colorless to pale yellow. Do not use if discolored or if precipitate is present. After dilution in saline, use immediately. Do not freeze.