Acetaminophen (APAP) is an analgesic and fever reducer that is largely metabolized in the liver. At therapeutic doses, APAP is metabolized through non-toxic pathways, but at toxic doses, approximately 5-10% is metabolized by CYP2E1 to the toxic metabolite NAPQI. NAPQI depletes glutathione and forms adducts by binding to proteins inside hepatocytes. These protein adducts cause oxidative stress and mitochondrial dysfunction, triggering necrosis in hepatocytes. The only approved treatment for APAP toxicity is N-acetylcysteine, which restores glutathione levels and neutralizes remaining NAPQI.

1. Toxic Metabolites of
Acetaminophen
Maryl Wright

2. Acetaminophen (APAP)
 Acetaminophen (APAP) is an analgesic
and fever reducer.
 Initially classified as NSAID
 APAP largely lacks peripheral anti-
inflammatory properties
 Weak anti inflammatory properties--IV
From Propublica.org

3. APAP is largely metabolized in the liver
 CYP2E1 is main Cytochrome P450 enzyme involved in APAP
metabolism
 CYP3A4
 CYP1A2
 CYP2D6

4. C. Ghanem, Pharmacological
Research, 2016

5. Acetaminophen Protein Adducts
 Complex that forms when NAPQI binds to protein
 Confined to centrilobular hepatocytes
 Don’t occur in periportal hepatocytes
 At least 20 molecular adducts have been detected
inside mitochondria, nucleus, and hepatocyte cytosol
From slideshare.net,
occupational-liver-disorders

6. Evidence for Protein Adducts in
Hepatocytes with Toxic Levels of APAP
J.A. Hinson, Adverse
Drug Reactions, 2010
• Acetaminophen cysteine adduct
• At 2–6 h  hepatocytes undergoing
necrosis
• By 24 h  all the necrotic cells contained
adducts.

7. Evidence for Protein Adducts in
Hepatocytes with Toxic Levels of APAP
J.A. Hinson, Adverse Drug Reactions, 2010
acetaminophen nitrotyrosine
adducts

8. Protein Adducts Cause Oxidative Stress in
Hepatocytes
J.A. Hinson, Chemical
Research in Toxicology,
1998
• Acetaminophen nitrotyrosine
adducts
• Serum samples were analyzed
for ALT (marker of hepatic cell
lysis) and nitrate plus nitrite
levels (marker of Nitric oxide
synthesis).

9. Protein Adducts Trigger Mitochondria
Dysfunction in Hepatocytes
Yuchao Xie,
Toxicological Applied
Pharmacology, 2015
• Human hepatocytes
• Acetaminophen cysteine adducts

10. Toxic Rescue
 Only approved antidote for APAP toxicity is oral or intravenous administration of
N-acetyl-cysteine (NAC).
 NAC is a precursor in the synthesis of GSH.
 Restores the intracellular pool of GSH and neutralizes any residual NAPQI still
present in the liver.
 Very effective if administered within the first 8 hours after toxic APAP ingestion.
 Many patients wait too long after overdosing.
 Liver transplant.
 Activation of autophagy may protect against APAP liver toxicity.

11. Conclusion
• APAP is largely metabolized in the liver, predominantly by sulfation and
glucuronidation.
• Approximately 5%–10% of APAP is metabolized primarily by CYP2E1 to the toxic
metabolite N-acetyl-p-benzoquinone imine (NAPQI).
• At therapeutic doses of APAP, NAPQI is immediately inactivated by conjugation with
glutathione (GSH).
• At toxic does of APAP, NAPQI depletes GSH, modifies cellular proteins, and forms
protein adducts.
• Hepatotoxicity caused by liver cell necrosis.
• NAC is only approved antidote for early hepatoxicity,

12. Questions?