Massive acetaminophen overdose with metabolic acidosis refractory to N-acetylcysteine, fomepizole, and renal replacement therapy

Massive acetaminophen ingestion managed successfully with N-acetylcysteine, fomepizole, and renal replacement therapy

Acetaminophen ingestion is routinely managed with the antidote, N-acetylcysteine (NAC). Massive acetaminophen poisoning has been treated successfully with adjunctive therapies such as fomepizole and hemodialysis. Fomepizole functions by inhibiting cytochrome p560, which prevents tylenol from forming its toxic metabolite, NAPQI. Prior cases have demonstrated favorable outcomes and a significant drop in acetaminophen levels after a single session of intermittent hemodialysis and continuous veno-venous hemofiltration (CVVH). However, the recommended dosage adjustments of NAC and fomepizole while a patient is undergoing CVVH has not been well reported. We present a case of an 18-year-old male who presented after ingesting 125 g of tylenol. His 4-hour acetaminophen level was 738.6 µg/mL. He was treated with NAC, fomepizole, and a single 4-hour session of hemodialysis. His acetaminophen level remained elevated at 730 µg/mL despite the hemodialysis session. CVVH was initiated, and he was given intravenous NAC at 12.5 mg/kg/h, oral NAC at 70 mg/kg every 4 hours, and intravenous fomepizole at 10 mg/kg every 6 hours. His tylenol levels became undetectable 57 hours after ingestion, and he did not develop permanent liver toxicity. This case encourages the use of CVVH for massive tylenol ingestion when a single run of intermittent hemodialysis is not effective in lowering the tylenol level. NAC, fomepizole, and CVVH can prevent unfavorable outcomes in massive acetaminophen ingestion when provided at an appropriate dose and frequency.

Comparing N-acetylcysteine and 4-methylpyrazole as antidotes for acetaminophen overdose

Acetaminophen (APAP) overdose can cause hepatotoxicity and even liver failure. N-acetylcysteine (NAC) is still the only FDA-approved antidote against APAP overdose 40 years after its introduction. The standard oral or intravenous dosing regimen of NAC is highly effective for patients with moderate overdoses who present within 8 h of APAP ingestion. However, for late-presenting patients or after ingestion of very large overdoses, the efficacy of NAC is diminished. Thus, additional antidotes with an extended therapeutic window may be needed for these patients. Fomepizole (4-methylpyrazole), a clinically approved antidote against methanol and ethylene glycol poisoning, recently emerged as a promising candidate. In animal studies, fomepizole effectively prevented APAP-induced liver injury by inhibiting Cyp2E1 when treated early, and by inhibiting c-jun N-terminal kinase (JNK) and oxidant stress when treated after the metabolism phase. In addition, fomepizole treatment, unlike NAC, prevented APAP-induced kidney damage and promoted hepatic regeneration in mice. These mechanisms of protection (inhibition of Cyp2E1 and JNK) and an extended efficacy compared to NAC could be verified in primary human hepatocytes. Furthermore, the formation of oxidative metabolites was eliminated in healthy volunteers using the established treatment protocol for fomepizole in toxic alcohol and ethylene glycol poisoning. These mechanistic findings, together with the excellent safety profile after methanol and ethylene glycol poisoning and after an APAP overdose, suggest that fomepizole may be a promising antidote against APAP overdose that could be useful as adjunct treatment to NAC. Clinical trials to support this hypothesis are warranted.

Use of fomepizole (4-methylpyrazole) for acetaminophen poisoning: A scoping review

INTRODUCTION

Acetaminophen (paracetamol, APAP) poisoning is a prominent global cause of drug-induced liver injury. While N-acetylcysteine (NAC) is an effective antidote, it has therapeutic limitations in massive overdose or delayed presentation. The objective is to comprehensively review the literature on fomepizole as a potential adjunct antidote for acetaminophen toxicity.

METHODS

A scoping review was performed using standardized search terms from inception through July 2021.

RESULTS

Reports on fomepizole as a therapeutic adjunct for APAP toxicity span heterogeneous types of evidence. Eleven preclinical studies (in vitro and animal), fourteen case reports/series, and one human volunteer study were included. Fomepizole's action is mediated by inhibition of CYP2E1 to prevent oxidant stress generation, and inhibition of c-Jun N-terminal kinase (JNK) to decrease amplification of oxidant stress signaling to mitochondria. Studies have shown a reduction in oxidative metabolites likely by shunting metabolism away from CYP2E1 and a resultant decrease in liver injury in animals, independent of CYP2E1 interactions. Fomepizole has been linked to few adverse effects.

CONCLUSION

Based on in vitro and animal studies, and bolstered by case reports, fomepizole likely offers benefit as an adjunct antidote for APAP toxicity, however this remains to be shown in a human trial. NAC remains the standard of care antidote, but given that fomepizole is approved and generally safe, it may be considered for APAP toxicity as off-label use by experienced clinicians, in rare circumstances associated with increased risk of hepatotoxicity despite standard NAC dosing. The marginal clinical benefit of fomepizole adjunct therapy beyond NAC monotherapy remains to be clearly defined, and routine use for APAP overdose is premature based on current evidence.

The Effects of N-Acetylcysteine on the Rat Mesocorticolimbic Pathway: Role of mGluR5 Receptors and Interaction with Ethanol

N-acetylcysteine (NAC) is a prodrug that is marketed as a mucolytic agent and used for the treatment of acetaminophen overdose. Over the last few decades, evidence has been gathered that suggests the potential use of NAC as a new pharmacotherapy for alcohol use disorder (AUD), although its mechanism of action is already being debated. In this paper, we set out to assess both the potential involvement of the glutamate metabotropic receptors (mGluR) in the possible dual effect of NAC administered at two different doses and NAC's effect on ethanol-induced activation. To this aim, 30 or 120 mg/kg of NAC was intraperitoneally administered to rats with the presence or absence of the negative allosteric modulator of mGluR5 (MTEP 0.1 mg/kg). Thereafter, the cFOS IR-cell expression was analyzed. Secondly, we explored the effect of 120 mg/kg of NAC on the neurochemical and behavioral activation induced by intra-VTA ethanol administration (150 nmol). Our results showed that the high NAC dose stimulated cFOS expression in the NAcc, and that this effect was suppressed in the presence of MTEP, thus suggesting the implication of mGluR5. Additionally, high doses could attenuate the ethanol-induced increase in cFOS-expression in the NAcc, probably due to a phenomenon based on the long-term depression of the MSNs. Additional experiments are required to corroborate our hypothesis.