5.4 Toxicology and Antidotes
Key Takeaways
- NOAEL is the highest exposure without observed adverse effect and LOAEL is the lowest exposure with a detectable adverse effect; both anchor regulatory risk assessment.
- Xenobiotic metabolism may detoxify or bioactivate a compound — acetaminophen's NAPQI, methanol's formic acid, and ethylene glycol's oxalic acid are classic bioactivation toxins.
- Antidote selection depends on the agent, mechanism, and time since exposure; naloxone for opioids, N-acetylcysteine for acetaminophen, and fomepizole for toxic alcohols are high-yield pairings.
- Decontamination and enhanced elimination — activated charcoal, gastric lavage, whole-bowel irrigation, hemodialysis, urinary alkalinization — are adjuncts whose use depends on agent properties and time window.
- The Poison Prevention Packaging Act requires child-resistant packaging for hazardous household substances, and poison control centers (1-800-222-1222) should guide management of suspected poisoning.
Toxicology Principles: NOAEL, LOAEL, and Dose-Response
Toxicology applies the dose-response framework to adverse effects. The NOAEL (No Observed Adverse Effect Level) is the highest exposure producing no detectable adverse effect in animal studies; the LOAEL (Lowest Observed Adverse Effect Level) is the lowest exposure producing a detectable adverse effect. These thresholds inform reference doses and regulatory limits. The margin of safety in animal-to-human extrapolation incorporates uncertainty factors (typically 10-fold for interspecies differences and 10-fold for human variability).
Acute toxicity results from a single or short-duration exposure, while chronic toxicity arises from repeated lower-dose exposure (e.g., lead neurotoxicity, methotrexate hepatic fibrosis). Both follow dose-response relationships, but chronic toxicity often involves cumulative tissue injury and bioaccumulation.
Xenobiotic Metabolism: Detoxification and Bioactivation
Xenobiotics are foreign chemicals metabolized primarily in the liver via phase I (oxidation, reduction, hydrolysis — mostly CYP450) and phase II (conjugation — glucuronidation, sulfation, acetylation, glutathione). Metabolism usually detoxifies, but some compounds undergo bioactivation to reactive metabolites that cause injury:
- Acetaminophen — minor CYP2E1 oxidation forms NAPQI, a reactive quinone that depletes glutathione and covalently binds hepatocytes. Overdose overwhelms glutathione reserves, producing centrilobular necrosis.
- Methanol — alcohol dehydrogenase oxidizes methanol to formaldehyde and then formic acid, which causes retinal toxicity and severe metabolic acidosis.
- Ethylene glycol — alcohol dehydrogenase produces glycolate and oxalic acid, the latter precipitating as calcium oxalate crystals in renal tubules, causing acute kidney injury and severe acidosis.
The therapeutic strategy for toxic alcohols is to block alcohol dehydrogenase with fomepizole (or ethanol) to prevent formation of the toxic metabolite.
Organ-Specific Toxic Mechanisms and Antidotes
| Poisoning or overdose | Antidote | Mechanism |
|---|---|---|
| Acetaminophen | N-acetylcysteine | Replenishes glutathione; detoxifies NAPQI; most effective within 8 hours |
| Opioids (morphine, heroin, fentanyl) | Naloxone | Competitive mu receptor antagonist; reverses respiratory depression |
| Benzodiazepines | Flumazenil | Competitive GABA-A antagonist; risk of seizure in chronic users |
| Beta-blockers and calcium-channel blockers | Calcium (chloride or gluconate), glucagon, high-dose insulin/euglycemia | Overcome channel blockade; glucagon increases cAMP and contractility; HDI enhances myocardial carbohydrate metabolism |
| Digoxin | Digoxin Fab antibody fragments (DigiFab) | Bind digoxin in plasma; shift drug from tissue receptors into blood for renal elimination |
| Anticholinergics (atropine, antihistamines) | Physostigmine | Acetylcholinesterase inhibitor that crosses the blood-brain barrier; reverses central and peripheral effects |
| Cholinergics / organophosphates (nerve agents, pesticides) | Atropine plus pralidoxime (2-PAM) | Atropine blocks muscarinic effects; 2-PAM reactivates acetylcholinesterase before the enzyme-phosphate complex ages |
| Iron | Deferoxamine | Chelates iron; forms ferrioxamine excreted in urine (vin rose urine) |
| Lead (acute and chronic) | Succimer (DMSA) oral; EDTA or dimercaprol in severe cases | Chelation; dimercaprol for encephalopathy, EDTA for blood lead reduction, succimer for moderate poisoning |
| Methanol and ethylene glycol | Fomepizole or ethanol, plus hemodialysis | Block alcohol dehydrogenase to prevent toxic metabolites; dialysis removes parent alcohol and metabolites |
| Heparin (unfractionated) | Protamine sulfate | Positively charged protein that binds negatively charged heparin, neutralizing anticoagulant effect (1 mg per 100 units heparin) |
| Warfarin | Vitamin K1 (phytomenadione) plus four-factor prothrombin complex concentrate for serious bleeding | Replenishes vitamin K-dependent factors; PCC provides immediate replacement |
| Cyanide | Hydroxocobalamin or sodium thiosulfate (plus sodium nitrite in the traditional kit) | Hydroxocobalamin binds cyanide to form cyanocobalamin (vitamin B12); thiosulfate donates sulfur to convert cyanide to thiocyanate via rhodanese |
| Carbon monoxide | 100% oxygen; hyperbaric oxygen for severe poisoning | Oxygen displaces CO from hemoglobin; hyperbaric oxygen accelerates dissociation and reduces neurologic sequelae |
| Methotrexate | Leucovorin (folinic acid) | Bypasses dihydrofolate reductase blockade; rescues normal cells (time-sensitive) |
| Heparin-induced thrombocytopenia | Argatroban or bivalirudin (direct thrombin inhibitors) | Provide anticoagulation without heparin exposure |
Decontamination and Enhanced Elimination
Decontamination reduces absorption after exposure:
- Activated charcoal — adsorbs most drugs; most effective within 1 hour of ingestion; dose 1 g/kg. Not useful for alcohols, metals, lithium, or hydrocarbons.
- Gastric lavage — rarely used; considered only within 1 hour of a life-threatening ingestion when the airway is protected.
- Whole-bowel irrigation — polyethylene glycol solution for sustained-release drugs, iron, lithium, or body-packing ("packers").
- Skin and ocular irrigation — copious water or saline for corrosives and pesticides.
Enhanced elimination accelerates removal of absorbed drug:
- Hemodialysis — effective for small, water-soluble, low-protein-bound, low-Vd drugs (salicylates, methanol, ethylene glycol, lithium, metformin, valproate, carbamazepine).
- Urinary alkalinization — intravenous sodium bicarbonate to maintain urine pH > 7.5 traps weak acids (salicylates, methotrexate, phenobarbital) in the tubule.
- Hemoperfusion — rarely used; effective for theophylline, carbamazepine, and valproate.
The decision to use enhanced elimination depends on the agent's properties (volume of distribution, protein binding), the patient's clinical status, and the availability of supportive care.
Poison Prevention and Public Health
The Poison Prevention Packaging Act (PPPA, 1970) requires child-resistant packaging for hazardous household substances, including most oral prescription drugs. Compliance has markedly reduced pediatric poisoning deaths. Poison control centers (US nationwide hotline 1-800-222-1222) provide 24/7 expert consultation for poisonings and should be contacted early in the management of any serious exposure. Pharmacists play a central role in patient education, identifying at-risk products in the home, and counseling caregivers on safe storage.
A 19-year-old presents 6 hours after an intentional acetaminophen ingestion of approximately 15 g. Which antidote is most appropriate, and what is its primary mechanism?
A farm worker presents with profuse salivation, lacrimation, urination, defecation, and muscle fasciculations after accidental organophosphate exposure. Which combination of antidotes is most appropriate, and why?
Which patient is the best candidate for hemodialysis as an enhanced-elimination strategy after an overdose?