4-Methylpyrazole--present status

Current recommendations for treatment of severe toxic alcohol poisonings

BACKGROUND

Ethylene glycol (EG) and methanol are responsible for accidental, suicidal, and epidemic poisonings, resulting in death or permanent sequelae. Toxicity is due to the metabolic products of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase. Conventional management of these intoxications consists of ethanol and hemodialysis. Fomepizole, a potent ADH inhibitor, has largely replaced antidotal ethanol use in France and two recent prospective U.S. trials definitively established its efficacy. Fomepizole appears safer than ethanol and while no comparative study of efficacy exists, fomepizole is recommended as the first-line antidote.

FOCUS

Fomepizole, administered early in EG intoxication, prevents renal injury. In the absence of renal failure, EG clearance is rapid, avoiding the need for prolonged fomepizole administration. The long elimination half-life of methanol poisonings, with absent hemodialysis, necessitates prolonged administration of fomepizole. In the U.S. trials, patients were dialyzed when plasma EG or methanol concentrations were >/=0.5 g/l. However, EG-poisoned patients treated with fomepizole prior to the onset of significant acidosis may not require hemodialysis. Indeed, fomepizole may also obviate the need for hemodialysis in selected methanol-poisoned patients, in the absence of neurological and ocular impairment or severe acidosis. When dialysis is indicated, 1 mg.kg.h continuous infusion of fomepizole should be provided to compensate for its elimination.

CONCLUSIONS

Fomepizole is an effective and safe first-line recommended antidote for EG and methanol intoxication. In selected patients, fomepizole may obviate the need for hemodialysis.

Methanol ingestion: prevention of toxic sequelae after massive ingestion

Methanol ingestion, a rare but potentially fatal poisoning, is often difficult to diagnose in the emergency department (ED) and historically has been difficult to treat. In this article, we report a methanol ingestion with a blood concentration of 692 mg/dL, which was treated with 4-methylpyrazole (Fomepizole) and dialysis, without sequelae. To our knowledge, such a massive ingestion has never been treated with this modality without development of long-term disability. Another unusual feature of this case is the significantly elevated serum osmolal gap at presentation without elevation in anion gap, demonstrating the effects of co-ingestion of ethanol. Additionally, there was a marked disparity between the patient's breath alcohol analyzer level and the blood ethanol concentration, illustrating the inability of the breath alcohol analyzer to differentiate between volatile alcohols. Treatment of the methanol-poisoned patient with Fomepizole is discussed.

Antidote review: fomepizole for methanol poisoning

Fomepizole (Antizol) was recently approved by the US Food and Drug Administration for treatment of methanol poisoning. By inhibiting the hepatic enzyme alcohol dehydrogenase, it presents formation of toxic metabolites with far fewer consequences than traditional ethanol therapy. It appears that fomepizole will become standard therapy for methanol intoxication as it is for ethylene glycol poisoning.

Role of fomepizole in the management of ethylene glycol toxicity

OBJECTIVE

To systematically review English-language articles on fomepizole administration in patients with ethylene glycol poisoning.

DATA SOURCES

MEDLINE, EMBASE, Current Contents, and PubMed. Search terms were fomepizole, 4-methylpyrazole, and ethylene glycol. The search was supplemented with a bibliographic review of all relevant articles.

STUDY SELECTION

All published reports of fomepizole administration in patients with ethylene glycol poisoning were reviewed, irrespective of study design. We identified one clinical trial and subsequent pharmacokinetic study, one case series, and 13 case reports.

RESULTS

Fomepizole has been investigated in 70 patients in open, unblinded studies. Most patients received an intravenous loading dose, with subsequent variable maintenance doses every 12 hours until plasma ethylene glycol levels became undetectable. Additional hemodialysis treatment generally was administered when patients had renal insufficiency or ethylene glycol levels above 50 mg/dl. Many patients had detectable ethanol levels either because of coadministration or as a result of adjunctive treatment at a referring center. Poorer patient outcomes, such as death and renal insufficiency, were associated with later clinical presentation time after ingestion. At therapeutic fomepizole levels (> 8.6 mg/ml), the half-life of ethylene glycol was prolonged to over 19 hours. Fomepizole appeared to be well tolerated by most patients.

CONCLUSION

Fomepizole is an effective alcohol dehydrogenase inhibitor that decreases production of ethylene glycol metabolites. Reduced mortality and morbidity are undetermined because of the small number of patients evaluated to date. Data on comparative efficacy of fomepizole versus ethanol and data on administration of fomepizole in children are limited.

Toxicokinetics of ethylene glycol during fomepizole therapy: implications for management. For the Methylpyrazole for Toxic Alcohols Study Group

STUDY OBJECTIVE

The elimination kinetics of ethylene glycol (EG) in human subjects treated with fomepizole (4-methylpyrazole) were analyzed to establish the efficacy of alcohol dehydrogenase (ADH) inhibition and to characterize elimination pathways.

METHODS

Drug concentration data from patients enrolled in the EG arm of the Methylpyrazole for Toxic Alcohols trial, a prospective, multicenter, open-label trial of fomepizole, were analyzed and compared with published estimates.

RESULTS

In 19 patients analyzed (EG concentrations of 3.5 to 211 mg/dL), elimination was first order during fomepizole monotherapy (half-life of 19.7+/-1.3 hours) and was not affected by the presence of ethanol. The elimination rate was significantly faster (half-life of <8.6+/-1.1 hours, P <.001) in the absence of fomepizole and ethanol. EG elimination by the kidneys was directly proportional to remaining renal function as estimated by creatinine clearance, with a fractional excretion of 25.5%+/-9.4%. Renal elimination and hemodialysis were the only significant routes of EG elimination as long as fomepizole concentrations were maintained well above 10 micromol/L (EG/fomepizole molar ratio, <100:1). All patients with normal serum creatinine concentrations at the initiation of fomepizole treatment had rapid rates of renal elimination (half-life of 16.8+/-0.8 hours).

CONCLUSION

At doses used, fomepizole effectively inhibits ADH-mediated metabolism of EG. Serum creatinine concentration at presentation and creatinine clearance can be used to predict EG elimination during fomepizole therapy and can help determine which patients will require hemodialysis to expedite EG elimination. An absolute EG concentration above 50 mg/dL should no longer be used as an independent criterion for hemodialysis in patients treated with fomepizole.