Fomepizole’s Role in Acetaminophen Toxicity
by Tyler F. Thao, 2025 PharmD Candidate, Devonta L. Parker, 2025 PharmD Candidate, Matthew T. Stanton, PharmD, DABAT
Abstract:
Acetaminophen (APAP) toxicity is the leading cause of acute liver failure and results from overproduction of N-acetyl-p benzoquinone-imine (NAPQI), a toxic metabolite formed via cytochrome P450 2E1 (CYP2E1). While N-acetylcysteine (NAC) is the standard treatment, its efficacy is limited in delayed or massive overdoses, necessitating adjunctive therapies. NAC works by replenishing glutathione stores, enhancing non-toxic metabolism of NAPQI, and improving hepatic perfusion, but its protective effect diminishes once significant liver injury has occurred. Therefore, alternative or complementary interventions are being explored to address these limitations. Fomepizole (4MP), a CYP2E1 inhibitor, shows promise in mitigating APAP-induced hepatotoxicity and nephrotoxicity. Animal studies demonstrate that 4MP reduces hepatic injury by more than 80%, largely through CYP2E1 inhibition and suppression of c-Jun N-terminal kinase (JNK)-mediated mitochondrial dysfunction, preventing oxidative stress and apoptosis. Additionally, 4MP attenuates endoplasmic reticulum (ER) stress-induced apoptosis in proximal tubular cells, reducing renal injury. Studies suggest initiating 4MP when plasma APAP levels >300 mcg/mL, with efficacy observed even in delayed treatment scenarios. Human data corroborate these findings. A case report showed that adding 4MP to NAC prevented hepatotoxicity in a 33-year-old with severe APAP overdose. In another case, a 7-month-old with acute liver failure from repeated APAP dosing avoided transplantation with 4MP-NAC therapy. Larger case series, including Toxicology Investigators Consortium (ToxIC) data, report favorable outcomes, with reduced transaminase levels and avoidance of liver failure in critically ill patients treated with 4MP. Further randomized controlled trials are essential to establish its role in clinical practice.
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2025 September/October Table of Contents
Abstract:
Acetaminophen (APAP) toxicity is the leading cause of acute liver failure and results from overproduction of N-acetyl-p benzoquinone-imine (NAPQI), a toxic metabolite formed via cytochrome P450 2E1 (CYP2E1). While N-acetylcysteine (NAC) is the standard treatment, its efficacy is limited in delayed or massive overdoses, necessitating adjunctive therapies. NAC works by replenishing glutathione stores, enhancing non-toxic metabolism of NAPQI, and improving hepatic perfusion, but its protective effect diminishes once significant liver injury has occurred. Therefore, alternative or complementary interventions are being explored to address these limitations. Fomepizole (4MP), a CYP2E1 inhibitor, shows promise in mitigating APAP-induced hepatotoxicity and nephrotoxicity. Animal studies demonstrate that 4MP reduces hepatic injury by more than 80%, largely through CYP2E1 inhibition and suppression of c-Jun N-terminal kinase (JNK)-mediated mitochondrial dysfunction, preventing oxidative stress and apoptosis. Additionally, 4MP attenuates endoplasmic reticulum (ER) stress-induced apoptosis in proximal tubular cells, reducing renal injury. Studies suggest initiating 4MP when plasma APAP levels >300 mcg/mL, with efficacy observed even in delayed treatment scenarios. Human data corroborate these findings. A case report showed that adding 4MP to NAC prevented hepatotoxicity in a 33-year-old with severe APAP overdose. In another case, a 7-month-old with acute liver failure from repeated APAP dosing avoided transplantation with 4MP-NAC therapy. Larger case series, including Toxicology Investigators Consortium (ToxIC) data, report favorable outcomes, with reduced transaminase levels and avoidance of liver failure in critically ill patients treated with 4MP. Further randomized controlled trials are essential to establish its role in clinical practice.
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2025 September/October Table of Contents