Fomepizole Explained

Width:125
Width2:125
Alt2:Ball-and-stick model of the fomepizole molecule
Tradename:Antizol, others
Dailymedid:Fomepizole
Routes Of Administration:Intravenous
Atc Prefix:V03
Atc Suffix:AB34
Legal Au:S4
Legal Au Comment:[1]
Legal Us:Rx-only
Legal Us Comment:[2]
Cas Number:7554-65-6
Pubchem:3406
Drugbank:DB01213
Chemspiderid:3289
Unii:83LCM6L2BY
Kegg:D00707
Chebi:5141
Chembl:1308
Synonyms:4-Methylpyrazole
Iupac Name:4-Methyl-1H-pyrazole
C:4
H:6
N:2
Smiles:CC1=CNN=C1
Stdinchi:1S/C4H6N2/c1-4-2-5-6-3-4/h2-3H,1H3,(H,5,6)
Stdinchikey:RIKMMFOAQPJVMX-UHFFFAOYSA-N
Density:0.99
Boiling Point:204 to 207
Boiling Notes:(at 97,3 kPa)

Fomepizole, also known as 4-methylpyrazole, is a medication used to treat methanol and ethylene glycol poisoning. It may be used alone or together with hemodialysis. It is given by injection into a vein.[3]

Common side effects include headache, nausea, sleepiness, and unsteadiness.[3] It is unclear if use during pregnancy causes risk to a fetus.[3] Fomepizole works by blocking the enzyme that converts methanol and ethylene glycol to their toxic breakdown products.[3]

Fomepizole was approved for medical use in the United States in 1997.[3] It is on the World Health Organization's List of Essential Medicines.[4]

Medical use

Fomepizole is used to treat ethylene glycol and methanol poisoning. It acts to inhibit the breakdown of these toxins into their active toxic metabolites. Fomepizole is a competitive inhibitor of the enzyme alcohol dehydrogenase,[5] found in the liver. This enzyme plays a key role in the metabolism of ethylene glycol, and of methanol.

By competitively inhibiting the first enzyme, alcohol dehydrogenase, in the metabolism of ethylene glycol and methanol, fomepizole slows the production of the toxic metabolites. The slower rate of metabolite production allows the liver to process and excrete the metabolites as they are produced, limiting the accumulation in tissues such as the kidney and eye. As a result, much of the organ damage is avoided.[7]

Fomepizole is most effective when given soon after ingestion of ethylene glycol or methanol. Delaying its administration allows for the generation of harmful metabolites.[7]

Interaction with alcohol

Concurrent use with ethanol is contraindicated because fomepizole is known to prolong the half-life of ethanol via inhibiting its metabolism. Extending the half-life of ethanol may increase and extend the intoxicating effects of ethanol, allowing for greater (potentially dangerous) levels of intoxication at lower doses. Fomepizole slows the production of acetaldehyde by inhibiting alcohol dehydrogenase, which in turn allows more time to further convert acetaldehyde into acetic acid by acetaldehyde dehydrogenase. The result is a patient with a prolonged and deeper level of intoxication for any given dose of ethanol, and reduced "hangover" symptoms (since these adverse symptoms are largely mediated by acetaldehyde build up).

In a chronic alcoholic who has built up a tolerance to ethanol, this removes some of the disincentives to ethanol consumption ("negative reinforcement") while allowing them to become intoxicated with a lower dose of ethanol. The danger is that the alcoholic will then overdose on ethanol (possibly fatally). If alcoholics instead very carefully reduce their doses to reflect the now slower metabolism, they may get the "rewarding" stimulus of intoxication at lower doses with less adverse "hangover" effects - leading potentially to increased psychological dependency. However, these lower doses may therefore produce less chronic toxicity and provide a harm minimization approach to chronic alcoholism.

It is, in essence, the antithesis of a disulfiram approach which tries to increase the buildup of acetaldehyde resulting in positive punishment for the patient. Compliance, and adherence, is a substantial problem in disulfiram-based approaches. Disulfiram also has a considerably longer half-life than that of fomepizole, requiring the person to not drink ethanol in order to avoid severe effects. If the person is not adequately managed on a benzodiazepine, barbiturate, acamprosate, or another GABAA receptor agonist, the alcohol withdrawal syndrome, and its attendant, life-threatening risk of delirium tremens "DT", may occur. Disulfiram treatment should never be initiated until the risk of DT has been evaluated, and mitigated appropriately. Fomepizole treatment may be initiated while the DT de-titration sequence is still being calibrated based upon the person's withdrawal symptoms and psychological health.

Adverse effects

Common side effects associated with fomepizole use include headache and nausea.[8]

Kinetics

Absorption and distribution

Fomepizole distributes rapidly into total body water. The volume of distribution is between 0.6 and 1.02 L/kg. The therapeutic concentration is from 8.2 to 24.6 mg (100 to 300 micromoles) per liter. Peak concentration following single oral doses of 7 to 50 mg/kg of body weight occurred in 1 to 2 hours. The half-life varies with dose, so has not been calculated.

Metabolism and elimination

Hepatic

the primary metabolite is 4-carboxypyrazole (about 80 to 85% of an administered dose). Other metabolites include the pyrazoles 4-hydroxymethylpyrazole and the N -glucuronide conjugates of 4-carboxypyrazole and 4-hydroxymethylpyrazole.

Following multiple doses, fomepizole rapidly induces its own metabolism via the cytochrome P450 mixed-function oxidase system.

In healthy volunteers, 1.0 to 3.5% of an administered dose was excreted unchanged in the urine. The metabolites also are excreted unchanged in the urine.

Fomepizole is dialyzable.

Other uses

Apart from medical uses, the role of 4-methylpyrazole in coordination chemistry has been studied.[9]

Notes and References

  1. Web site: Prescription medicines: registration of new chemical entities in Australia, 2016 . Therapeutic Goods Administration (TGA) . 21 June 2022 . 10 April 2023.
  2. Web site: Antizol- fomepizole injection . DailyMed . U.S. National Library of Medicine . 24 December 2020.
  3. Web site: Fomepizole. The American Society of Health-System Pharmacists. 8 December 2016. live. https://web.archive.org/web/20161221003710/https://www.drugs.com/monograph/fomepizole.html. 21 December 2016.
  4. Book: ((World Health Organization)) . World Health Organization model list of essential medicines: 21st list 2019 . 2019 . 10665/325771 . World Health Organization . World Health Organization . Geneva . WHO/MVP/EMP/IAU/2019.06. License: CC BY-NC-SA 3.0 IGO . free .
  5. Casavant MJ . Fomepizole in the treatment of poisoning . Pediatrics . 107 . 1 . 170 . January 2001 . 11134450 . 10.1542/peds.107.1.170 .
  6. Web site: Methanol poisoning . Forensic Pathology . The Internet Pathology Laboratory for Medical Education, The University of Utah Eccles Health Sciences Library . live . https://web.archive.org/web/20080917031229/http://library.med.utah.edu/WebPath/FORHTML/FOR011.html . 17 September 2008 .
  7. Brent J . Fomepizole for ethylene glycol and methanol poisoning . The New England Journal of Medicine . 360 . 21 . 2216–2223 . May 2009 . 19458366 . 10.1056/NEJMct0806112 .
  8. Lepik KJ, Levy AR, Sobolev BG, Purssell RA, DeWitt CR, Erhardt GD, Kennedy JR, Daws DE, Brignall JL . 6 . Adverse drug events associated with the antidotes for methanol and ethylene glycol poisoning: a comparison of ethanol and fomepizole . Annals of Emergency Medicine . 53 . 4 . 439–450.e10 . April 2009 . 18639955 . 10.1016/j.annemergmed.2008.05.008 .
  9. 10.1007/BF00619054 . Pyrazolato and related anions. Part V. Transition metal salts of 4-methylpyrazole . 1979 . Vos JG, Groeneveld WL . Transition Metal Chemistry . 4 . 3 . 137–141 . 93580021 .