1. Overview of Antidotal Therapy

Antidotal therapy for toxic alcohol poisoning is a critical intervention aimed at preventing the formation of highly toxic acidic metabolites. The strategy hinges on inhibiting the enzyme alcohol dehydrogenase (ADH), thereby halting the metabolic cascade that leads to severe end-organ damage and profound metabolic acidosis.

Rationale and Goals

The primary mechanism of toxicity from methanol and ethylene glycol is not the parent alcohol but their metabolites: formic acid (from methanol) and glycolic/oxalic acids (from ethylene glycol). By blocking ADH, we can achieve the following goals:

• Prevent Toxic Metabolite Formation: This is the cornerstone of therapy, averting severe acidosis and organ injury.
• Enhance Elimination: Blocking metabolism allows the parent compound to be eliminated unchanged via the kidneys or, more rapidly, through hemodialysis.
• Correct Metabolic Disturbances: Halting acid production is the first step in correcting severe metabolic acidosis.

Indications for ADH Inhibition

Initiation of an ADH inhibitor is indicated when there is a confirmed or suspected significant ingestion, typically defined by one or more of the following criteria:

• A documented serum methanol or ethylene glycol concentration > 20 mg/dL.
• A calculated osmolar gap > 10 mOsm/kg in the context of a suspected ingestion.
• Unexplained anion-gap metabolic acidosis (arterial pH < 7.3).
• Clinical evidence of end-organ damage, such as visual disturbances (methanol) or acute kidney injury (ethylene glycol).

2. Fomepizole (First-Line Antidote)

Fomepizole is the preferred, first-line competitive inhibitor of ADH. Its high affinity for the enzyme, predictable pharmacokinetics, and favorable safety profile make it the standard of care in most settings.

Dosing Regimen

• Loading Dose: 15 mg/kg IV infused over 30 minutes.
• Maintenance Doses: 10 mg/kg IV every 12 hours for 4 doses.
• Subsequent Doses: After the initial maintenance doses, increase to 15 mg/kg IV every 12 hours to account for auto-induction of its own metabolism. Continue until the toxic alcohol level is < 20 mg/dL and metabolic acidosis has resolved.

Adjustments for Hemodialysis

Fomepizole is efficiently removed by dialysis, necessitating dose adjustments to maintain therapeutic concentrations.

• Intermittent Hemodialysis (IHD): Increase dosing frequency to every 4 hours (e.g., 15 mg/kg IV q4h). Alternatively, a continuous infusion of 1 mg/kg/hour can be used for the duration of the dialysis session.

3. Ethanol (Alternative Antidote)

When fomepizole is unavailable or prohibitively expensive, intravenous or enteral ethanol can be used as an alternative ADH inhibitor. It acts as a competitive substrate for ADH, but its use is complicated by difficult-to-manage pharmacokinetics and significant adverse effects.

Dosing and Monitoring

The goal is to achieve and maintain a serum ethanol concentration of 100–150 mg/dL to effectively compete with methanol or ethylene glycol at the ADH enzyme.

• Loading Dose: 600–800 mg/kg IV (e.g., 8-10 mL/kg of 10% ethanol solution) over 30-60 minutes.
• Maintenance Infusion: Requires continuous infusion, with rates adjusted based on frequent serum ethanol monitoring. Rates must be doubled or tripled during hemodialysis to compensate for its rapid clearance.
• Intensive Monitoring: Serum ethanol levels must be checked hourly until stable, then every 2-4 hours. Frequent blood glucose monitoring (q1-2h) is essential to prevent hypoglycemia.

4. Adjunctive Therapies

Adjunctive therapies are crucial to enhance the elimination of toxic metabolites that have already formed and to support organ function. These cofactors should be administered empirically in all suspected cases.

5. Dosing in Special Populations

Pharmacokinetics of ADH inhibitors can be significantly altered in critically ill patients, requiring specific considerations.

• Hemodialysis (IHD): As noted, both fomepizole and ethanol are readily dialyzable. Fomepizole requires more frequent dosing (q4h) or a continuous infusion, while ethanol infusion rates must be significantly increased.
• Continuous Renal Replacement Therapy (CRRT): Clearance is less predictable than with IHD. While standard fomepizole dosing is often sufficient, higher maintenance doses or continuous infusions may be needed depending on effluent rates. Measuring drug levels is ideal if available.
• Critical Illness: Patients with significant fluid resuscitation may have an increased volume of distribution (Vd). Consider recalculating weight-based doses after major fluid shifts.
• Pregnancy: Fomepizole is preferred (Pregnancy Category C). Its benefits in preventing maternal and fetal toxicity from metabolic acidosis are thought to outweigh potential risks. Ethanol readily crosses the placenta and can cause fetal intoxication and hypoglycemia, making it a less desirable option.

6. Safety and Drug Interactions

Both antidotes have important safety profiles and potential drug interactions that must be managed.

Fomepizole

• Safety: Generally well-tolerated. Minor adverse effects include headache, nausea, and dizziness. Transient, mild elevation of liver transaminases can occur but is rarely clinically significant.
• Interactions: Fomepizole is an inhibitor of CYP2E1. While clinically significant interactions are uncommon in the acute setting, it may affect the metabolism of co-administered substrates like acetaminophen.

Ethanol

• Safety: Carries significant risks, including CNS and respiratory depression, hypoglycemia, hypotension, electrolyte disturbances (hypokalemia, hypomagnesemia), and pancreatitis.
• Interactions: Acutely, ethanol competitively inhibits CYP2E1, which can slow the metabolism of other drugs like benzodiazepines or phenytoin, potentially potentiating their effects.

7. Pharmacoeconomic Analysis

The choice between fomepizole and ethanol often involves a pharmacoeconomic trade-off between drug acquisition cost and overall resource utilization.

8. Comprehensive Monitoring Plan

A structured monitoring plan is essential to assess therapeutic efficacy and ensure patient safety.

Laboratory Endpoints (q4-6h initially)

• Efficacy: Anion gap, osmolar gap, arterial blood gas (pH), serum bicarbonate. Direct measurement of toxic alcohol levels if available.
• Safety: Serum electrolytes (especially K+, Mg2+, Ca2+), creatinine, and BUN.

Antidote-Specific Monitoring

• Fomepizole: Monitor liver function tests (AST/ALT) daily.
• Ethanol: Monitor serum ethanol levels hourly until stable, then q2-4h. Monitor blood glucose q1-2h.

Clinical Surveillance

• Continuous assessment of mental status, visual acuity/fields (for methanol), and urine output.
• Hemodynamic monitoring for hypotension.

9. Decision Algorithm

A standardized algorithm helps ensure timely and appropriate initiation of antidotal therapy and consideration for extracorporeal treatment based on key clinical and laboratory criteria.