Antizol

MIMS Revision Date: 01 December 2020

1 Name of Medicine

Fomepizole.

2 Qualitative and Quantitative Composition

Each vial contains 1.5 mL (1 g/mL) of fomepizole. It is supplied sterile. Fomepizole injection does not contain any excipients.

3 Pharmaceutical Form

A clear to yellow liquid (or solid) sterile, preservative-free solution for intravenous use.
Solidification does not affect the stability of Antizol.

4 Clinical Particulars

4.1 Therapeutic Indications

Antizol (fomepizole) is indicated for the treatment of ethylene glycol or methanol poisoning (see Section 4.2 Dose and Method of Administration).

4.2 Dose and Method of Administration

Caution. Must be diluted prior to use.
Treatment guidelines. If ethylene glycol or methanol poisoning is left untreated, the natural progression of the poisoning leads to accumulation of toxic metabolites, including glycolic and oxalic acids (ethylene glycol intoxication) and formic acid (methanol intoxication). These metabolites can induce metabolic acidosis, nausea/vomiting, seizures, stupor, coma, calcium oxaluria, acute tubular necrosis, blindness and death. The diagnosis of these poisonings may be difficult because ethylene glycol and methanol concentrations diminish in the blood as they are metabolized to their respective metabolites. Hence, if the fomepizole solution has become a solid in the vial, the solution should be liquefied by running the vial under warm water or by holding in the hand. Solidification does not affect the stability of Antizol.
Treatment with fomepizole. Begin fomepizole treatment immediately upon suspicion of ethylene glycol or methanol ingestion based on patient history and/or anion gap metabolic acidosis, increased osmolar gap, visual disturbances, or oxalate crystals in the urine; or a documented serum ethylene glycol or methanol concentration greater than 20 mg/dL (ethylene glycol 3.22 mmol/L, methanol 6.24 mmol/L).
Discontinuation of fomepizole treatment. Treatment with fomepizole may be discontinued when ethylene glycol or methanol concentrations are undetectable or have been reduced below 20 mg/dL (ethylene glycol 3.22 mmol/L, methanol 6.24 mmol/L), and the patient is asymptomatic with normal pH.
Dose. Dosing of fomepizole. A loading dose of 15 mg/kg should be administered, followed by doses of 10 mg/kg every 12 hours for 4 doses, then 15 mg/kg every 12 hours thereafter until ethylene glycol or methanol concentrations are undetectable or have been reduced below 20 mg/dL (ethylene glycol 3.22 mmol/L, methanol 6.24 mmol/L), and the patient is asymptomatic with normal pH. All doses should be administered as a slow intravenous infusion over 30 minutes (see Treatment guidelines).
Haemodialysis. Haemodialysis should be considered in addition to fomepizole in the case of renal failure (Stage 5 CKD, GFR < 15 mL/min/1.73 m²), significant or worsening metabolic acidosis, or a measured ethylene glycol or methanol concentration of greater than or equal to 50 mg/dL (ethylene glycol 8.06 mmol/L, methanol 15.61 mmol/L). Patients should be dialyzed to correct metabolic abnormalities and to lower the ethylene glycol concentrations below 50 mg/dL (ethylene glycol 8.06 mmol/L). See Table 1.

Dosage with renal dialysis. Fomepizole injection is dialyzable and the frequency of dosing should be increased to every 4 hours during haemodialysis.
Both ethylene glycol and methanol concentrations, acid base balance [as determined by serum electrolyte (anion gap) and/or blood gas analysis], should be frequently monitored and used to guide treatment.
Method of administration. Using sterile technique, the appropriate dose of fomepizole should be drawn from the vial with a syringe and injected into at least 100 mL of sterile 0.9% sodium chloride injection or dextrose 5% injection. Mix well. The entire contents of the resulting solution should be infused over 30 minutes. Fomepizole, like all parenteral products, should be inspected for particulate matter prior to administration.

4.3 Contraindications

Fomepizole should not be administered to patients with a documented serious hypersensitivity reaction to fomepizole or other pyrazoles.
Fomepizole should not be used in patients with ethanol intoxication.
Fomepizole should not be used in patients exposed to poisons other than ethylene glycol or methanol alone or in combination (including diethylene glycol).

4.4 Special Warnings and Precautions for Use

General. Fomepizole should not be given undiluted or by bolus injection. Venous irritation and phlebosclerosis were noted in two of six normal volunteers given bolus injections (over 5 minutes) fomepizole at a concentration of 25 mg/mL (0.3 mol/L).
Major allergic reactions (anaphylaxis) and minor allergic reactions (mild rash, eosinophilia) have been reported in a few patients receiving fomepizole (see Section 4.8 Adverse Effects (Undesirable Effects)). Therefore, patients should be monitored for signs of allergic reactions.
There is a potential risk of muscle damage resulting from creatine kinase elevation with use of fomepizole.
Medical professionals are warned of the potential risk of hypoglycaemia and/or seizures with use of fomepizole.
Fomepizole has not been studied sufficiently to determine whether the pharmacokinetics differ between the genders.
Use in hepatic impairment. Fomepizole is metabolized through the liver, but no definitive pharmacokinetic studies have been done in subjects with hepatic disease.
Use in renal impairment. The metabolites of fomepizole are excreted renally. Definitive pharmacokinetic studies have not been done to assess pharmacokinetics in patients with renal impairment.
Use in the elderly. Fomepizole injection has not been studied sufficiently to determine whether the pharmacokinetics differ for an elderly population.
Paediatric use. Safety and effectiveness in paediatric patients have not been established.
Effects on laboratory tests. No data available.
Patient management. In addition to specific antidote treatment with fomepizole, patients intoxicated with ethylene glycol or methanol must be managed for metabolic acidosis, acute renal failure (ethylene glycol), adult respiratory distress syndrome, visual disturbances (methanol) and hypocalcaemia. Fluid therapy and sodium bicarbonate administration are potential supportive therapies. In addition, potassium and calcium supplementation and oxygen administration are usually necessary.
Haemodialysis is necessary in the anuric patient, or in patients with severe metabolic acidosis or azotaemia. Treatment success should be assessed by frequent measurements of blood gases, pH, electrolytes, BUN, creatinine and urinalysis, in addition to other laboratory tests as indicated by individual patient conditions. At frequent intervals throughout the treatment, patients poisoned with ethylene glycol or methanol should be monitored.
Electrocardiography should be performed because acidosis and electrolyte imbalances can affect the cardiovascular system. In addition, hepatic enzymes and white blood cell counts should be monitored during treatment, as transient increases in serum transaminase concentrations and eosinophilia have been noted with Antizol dosing.

4.5 Interactions with Other Medicines and Other Forms of Interactions

Oral doses of fomepizole (10-20 mg/kg), via alcohol dehydrogenase inhibition, significantly reduced the rate of elimination of ethanol (by approximately 40%) given to healthy volunteers in moderate doses. Similarly, ethanol decreased the rate of elimination of fomepizole (by approximately 50%) by the same mechanism. Fomepizole is ineffective at treating ethanol intoxication, and would conversely prolong ethanol intoxication if administered.
Reciprocal interactions may occur with concomitant use of fomepizole and drugs that increase or inhibit the cytochrome P450 system (e.g. phenytoin, carbamazepine, cimetidine, ketoconazole), though this has not been studied.
Fomepizole has been shown to induce the expression of CYP2E1 and to inhibit its activity. These effects were enhanced in rats that had been exposed to ethanol. Fomepizole may also inhibit other CYP enzymes and therefore may alter the exposure to other drugs that are metabolised by CYP enzymes.
Interactions with disulfiram and medications for psychiatric disorders (anti-depressants, anxiolytics and antipsychotics) have not been established.

4.6 Fertility, Pregnancy and Lactation

Effects on fertility. The effects of fomepizole on male and female fertility have not been adequately studied in animals. In rats, fomepizole (110 mg/kg) administered orally for 40 to 42 days resulted in decreased testicular mass (approximately 8% reduction). This dose is approximately 0.1 times the human maximum daily exposure based on surface area (mg/m²). Reductions in testicular mass were significantly greater when fomepizole was given in combination with ethanol (approximately 30% reduction).
Use in pregnancy. (Category B2)
Animal reproduction studies have not been conducted with fomepizole. It is also not known whether fomepizole can cause foetal harm when administered to pregnant women or can affect reproduction capacity. Fomepizole has been shown to cross the placenta in pregnant rats, with the concentration higher in foetal tissues that that observed in maternal serum. Fomepizole should be given to pregnant women only if clearly needed.
Use in lactation. It is not known whether fomepizole is excreted in human milk. Because many drugs are excreted in human milk, caution should be exercised when fomepizole is administered to a breastfeeding woman.

4.7 Effects on Ability to Drive and Use Machines

The effects of this medicine on a person's ability to drive and use machines were not assessed as part of its registration.

4.8 Adverse Effects (Undesirable Effects)

A total of 141 patients were exposed to fomepizole during the clinical trial period, including both patients (n=78) and healthy volunteers (n=63). See Tables 2 and 3.

Post-marketing experience. Cyanosis, anaphylactic reaction, brain herniation, drug administration error, decreased platelet count, hypoglycaemia, brain oedema, renal failure, renal necrosis, pharyngeal oedema, respiratory distress and superficial thrombophlebitis have been reported in post-market data.
Reporting suspected adverse effects. Reporting suspected adverse reactions after registration of the medicinal product is important. It allows continued monitoring of the benefit-risk balance of the medicinal product. Healthcare professionals are asked to report any suspected adverse reactions at www.tga.gov.au/reporting-problems (Australia) or nzphvc.otago.ac.nz/reporting/ (New Zealand).

4.9 Overdose

Nausea, dizziness, and vertigo were noted in healthy volunteers receiving 50-100 mg/kg doses of fomepizole (at plasma concentrations of 290-520 micromol/L, 23.8-42.6 mg/L). These doses are 3-6 times the recommended dose. This dose-dependent CNS effect was short-lived in most subjects and lasted up to 30 hours in one subject.
Fomepizole is dialyzable, and haemodialysis may be useful in treating cases of overdosage.
For information on the management of overdose, contact the Poisons Information Centre on 13 11 26 (Australia) or 0800 Poison (0800 764 766) (New Zealand).

5 Pharmacological Properties

5.1 Pharmacodynamic Properties

Mechanism of action. Mechanism of action: fomepizole is a competitive inhibitor of alcohol dehydrogenase. Alcohol dehydrogenase catalyses the oxidation of ethanol to acetaldehyde. Alcohol dehydrogenase also catalyses the initial steps in the metabolism of ethylene glycol and methanol to their toxic metabolites.
Ethylene glycol, the main component of most antifreezes and coolants, is metabolized to glycoaldehyde, and oxalate. Glycolate and oxalate are the metabolic by-products primarily responsible for the metabolic acidosis and renal damage seen ethylene glycol toxicosis. The lethal dose of ethylene glycol in humans is approximately 1.4 mL/kg.
Methanol, the main component of windshield wiper fluid, is slowly metabolized via alcohol dehydrogenase to yield formic acid. Formic acid is primarily responsible for the metabolic acidosis and visual disturbances (e.g. decreased visual activity and potential blindness) associated with methanol poisoning. A lethal dose of methanol in humans is approximately 1-2 mL/kg.
Fomepizole has been shown in vitro to block alcohol dehydrogenase enzyme activity in dog, monkey and human liver. The concentration of fomepizole at which alcohol dehydrogenase is inhibited by 50% in vitro is approximately 0.1 micromol/L. In animal studies, fomepizole has been shown to inhibit the formation of toxic metabolites of ethylene glycol and methanol, to reverse acidosis and to prevent mortality and ethylene-glycol induced renal damage.
Fomepizole in the range of 100 to 300 micromol/L (8.6-24.6 mg/L) has been targeted to assure adequate plasma concentrations in humans for the effective inhibition of alcohol dehydrogenase.
In healthy volunteers, oral doses of fomepizole (10-20 mg/kg) significantly reduced the rate of elimination of moderate doses of ethanol, which is also metabolized through the action of alcohol dehydrogenase (see Section 4.4 Special Warnings and Precautions for Use; Section 4.5 Interactions with Other Medicines and Other Forms of Interactions).
Clinical trials. The efficacy of fomepizole in the treatment of ethylene glycol and methanol intoxication was studied in two prospective, U.S. clinical trials without concomitant control groups. The clinical trials described below were supplemented by open-label studies and case reports from the published literature.
An open-label, multi-dose, multi-centre, phase III pivotal trial for treatment of ethylene glycol poisoning with fomepizole has been conducted. Eligible subjects were male or female over 12 years of age who presented with a documented serum ethylene glycol concentration of > 20 mg/dL (3.2 mmol/L); or a history (or strong clinical suspicion) of ethylene glycol ingestion along with arterial pH < 7.3, serum bicarbonate < 20 mEq/L, osmolar gap by freezing point depression > 10 mOsm/L, and/or oxalate crystals in urine; or recent (< 1 hour) documented history of a potentially toxic amount of ethylene glycol and osmolar gap > 10 mOsm/L. All patients received fomepizole shortly after admission with a loading dose of 15 mg/kg body weight fomepizole followed by supplemental doses of 10 mg/kg over 30 minutes at intervals of 12 hours (or 15 mg/kg after 48 hours to compensate for increased elimination 30-40 hours after multiple doses) until the serum ethylene glycol level was reduced to ≤ 20 mg/dL (3.2 mmol/L).
Fourteen of 16 patients being treated with fomepizole for ethylene glycol poisoning underwent haemodialysis because of severe intoxication (see Section 4.2 Dose and Method of Administration). There were no differences in outcomes between those who did and did not undergo haemodialysis. Fomepizole levels fell faster during haemodialysis than when due to endogenous clearance alone. During haemodialysis, fomepizole levels fell at a median rate of 0.8 micromol/L/minute (range 0.27 to 4.4). In the absence of haemodialysis, levels fell at a median rate of 0.33 micromol/L/minute (range 0.1 to 0.83). Median clearance of fomepizole by dialysis was 183 mL/min (range 129-218 mL/min).
The results of the ethylene glycol study provide evidence that fomepizole blocks ethylene glycol metabolism mediated by alcohol dehydrogenase in the clinical setting. Plasma concentrations of toxic metabolites of ethylene glycol failed to rise in the initial phases of treatment. The relationship to fomepizole therapy, however, was confounded by haemodialysis and significant blood ethanol concentrations in many of the patients. Nevertheless, in the post-dialysis period(s), when ethanol concentrations were insignificant and the concentrations of ethylene glycol > 20 mg/dL (> 3.2 mmol/L), the administration of fomepizole alone blocked any rise in glycolate concentrations.
An open-label, multi-dose, multi-centre, phase III pivotal trial for treatment of methanol poisoning has been conducted. Eligible subjects were male or female patients over 12 years of age who presented with a documented serum methanol level of > 20 mg/dL (> 6.24 mmol/L) OR a history (or strong clinical suspicion) of methanol intoxication and presented with at least two of the four following criteria: arterial pH < 7.3, serum bicarbonate < 20 mEq/L, osmolar gap by freezing point depression > 10 mOsm/L, and/or recent (less than 1 hour) documented history of an ingestion of a potentially toxic amount of methanol).
All patients received fomepizole shortly after admission with a loading dose of 15 mg/kg body weight fomepizole followed by supplemental doses of 10 mg/kg at intervals of 12 hours (or 15 mg/kg after 48 hours) until the serum ethylene glycol level was reduced to ≤ 20 mg/dL (6.24 mmol/L).
Seven of 11 patients in the trial for treatment of methanol poisoning underwent haemodialysis because of severe intoxication (see Section 4.2 Dose and Method of Administration). There was no difference in outcome for these patients compared to the non-dialysis patients.
The results of the methanol study provide evidence that fomepizole blocks methanol metabolism mediated by alcohol dehydrogenase in the clinical setting. Plasma concentrations of toxic metabolites of methanol failed to rise in the initial phases of treatment. The relationship to fomepizole therapy, however, was confounded by haemodialysis and significant blood ethanol concentrations in many of the patients. Nevertheless, in the post-dialysis period(s), when ethanol concentrations were insignificant and the concentrations of methanol were > 20 mg/dL (6.24 mmol/L), the administration of fomepizole alone blocked any rise in formate concentrations.

5.2 Pharmacokinetic Properties

Distribution. After intravenous infusion, fomepizole rapidly distributes to total body water. The volume of distribution is between 0.6 L/kg and 1.02 L/kg.
Metabolism. In healthy volunteers, only 1-3.5% of the administered dose of fomepizole (7-20 mg/kg oral and IV) was excreted unchanged in the urine, indicating that metabolism is the major route of elimination. In humans, the primary metabolite of fomepizole is 4-carboxypyrazole (approximately 80-85% of administered dose), which is excreted in the urine. Other metabolites of fomepizole observed in the urine are 4-hydroxymethylpyrazole and the N-glucuronide conjugates of 4-carboxypyrazole and 4-hydroxymethylpyrazole.
With multiple doses, fomepizole rapidly induces its own metabolism via the cytochrome P450 mixed-function oxidase system, which produces a significant increase in the elimination rate after about 30-40 hours. After enzyme induction, elimination follows first-order kinetics.
Excretion. The plasma half-life of fomepizole varies with dose, even in patients with normal renal function (GFR ≥ 90 mL/min/1.73 m²), and has not been calculated.
The elimination of fomepizole is best characterized by Michaelis-Menten kinetics after acute doses, with saturable elimination occurring at therapeutic blood concentrations [100-300 micromol/L, 8.2-24.6 mg/L]. Fomepizole and its metabolites are renally excreted.
With multiple doses, fomepizole rapidly induces its own metabolism via the cytochrome P450 mixed-function oxidase system, which produces a significant increase in the elimination rate after about 30-40 hours. After enzyme induction, elimination follows first-order kinetics.

5.3 Preclinical Safety Data

Genotoxicity. The genotoxic potential of fomepizole has only been partially investigated. Fomepizole was shown to be mutagenic in bacteria. Fomepizole did not appear to cause chromosomal aberrations in mouse micronucleus assay. The available evidence indicates that fomepizole is genotoxic.
Carcinogenicity. There have been no long-term studies performed in animals to evaluated carcinogenic potential.

6 Pharmaceutical Particulars

6.1 List of Excipients

Fomepizole injection does not contain any excipients.

6.2 Incompatibilities

Incompatibilities were either not assessed or not identified as part of the registration of this medicine.

6.3 Shelf Life

In Australia, information on the shelf life can be found on the public summary of the Australian Register of Therapeutic Goods (ARTG). The expiry date can be found on the packaging.

6.4 Special Precautions for Storage

Store below 25°C. Protect from light.
Contains no antimicrobial preservative. Product is for single use in one patient only. Discard any residue.
To reduce microbiological hazard, use as soon as practicable after reconstitution/preparation. If storage is necessary, hold at 2-8°C for not more than 24 hours.

6.5 Nature and Contents of Container

Antizol (fomepizole 1.5 g/1.5 mL injection) is supplied in type I, 2 mL vials.
Each 1.5 mL vial contains 1.5 g (1 g/mL) of fomepizole.
Supplied in packages of four vials or one vial.

6.6 Special Precautions for Disposal

In Australia, any unused medicine or waste material should be disposed of in accordance with local requirements.

6.7 Physicochemical Properties

Fomepizole injection is a competitive inhibitor of alcohol dehydrogenase. The chemical name of fomepizole is 4-methylpyrazole. It has the molecular formula C₄H₆N₂ and a molecular weight of 82.1 g/mol. It is a clear to yellow liquid at room temperature. Its melting point is 25°C and it may present as a solid at room temperature. Fomepizole is soluble in water and very soluble in ethanol, diethyl ether, and chloroform.
Chemical structure.

C₄H₆N₂. 82.1 g mol^-1.
CAS number. 7554-65-6.

7 Medicine Schedule (Poisons Standard)

Prescription Medicine S4.

Date of First Approval

01 December 2016

Date of Revision

26 October 2020

Summary Table of Changes