Drug-induced liver injury

Mechanisms of Injury - When Good Drugs Go Bad

• Intrinsic (Predictable): Dose-related, direct hepatocyte damage from a toxic metabolite.
  • Classic example: Acetaminophen.
• Idiosyncratic (Unpredictable): Not dose-related; variable latency.
  • Immune-mediated: Allergic features like fever, rash, eosinophilia (e.g., Halothane, Phenytoin).
  • Non-immune: Metabolic abnormality (e.g., Isoniazid).

⭐ Acetaminophen overdose depletes glutathione (GSH), causing accumulation of the toxic metabolite NAPQI, which leads to centrilobular necrosis.

Patterns of Injury - Hepatocellular vs. Cholestatic

• Classification is based on the pattern of liver enzyme elevation. The R-value quantifies the type of injury:
  • $R = (ALT / ULN_{ALT}) / (ALP / ULN_{ALP})$

⭐ Hy's Law: A key prognostic indicator for fatal DILI. It combines hepatocellular injury (jaundice with ↑ bilirubin) and significant ↑ aminotransferase levels, without initial cholestasis (normal ALP). This pattern signals a high risk of acute liver failure with >10% mortality.

Causative Agents - The Usual Suspects

• Predictable (Intrinsic/Dose-Dependent):

  • Acetaminophen: The classic example. Overdose depletes glutathione, leading to accumulation of toxic metabolite NAPQI and centrilobular necrosis.

• Idiosyncratic (Unpredictable/Host-Dependent):

  • Hepatocellular Pattern (↑↑ ALT):
    • Anti-TB drugs (Isoniazid, Rifampin, Pyrazinamide)
    • Antiepileptics (Valproate, Phenytoin)
    • Statins, Ketoconazole, Halothane, Allopurinol
  • Cholestatic Pattern (↑↑ ALP):
    • Antibiotics (Amoxicillin-clavulanate, Erythromycin)
    • Hormones (Anabolic steroids, Oral contraceptives)
    • Antipsychotics (Chlorpromazine)
  • Mixed Pattern:
    • Carbamazepine, Phenytoin, Sulfonamides

⭐ Amoxicillin-clavulanate is a leading cause of idiosyncratic DILI, typically presenting as cholestatic or mixed hepatitis. The injury can have a delayed onset of weeks to months after the drug is stopped.

Diagnosis & Management - Spotting & Stopping Toxins

• History is Key: Detailed review of all medications, supplements, & herbal products (timing is crucial).
• Pattern Recognition (R-Value):
  • Calculate: $R = (\text{ALT} / \text{ULN}) / (\text{ALP} / \text{ULN})$
  • Hepatocellular: $R \ge \textbf{5}$
  • Cholestatic: $R \le \textbf{2}$
  • Mixed: $R > \textbf{2}$ and $< \textbf{5}$

• Core Management:
  • Discontinue the suspected offending agent immediately.
  • Primarily supportive care.
  • N-acetylcysteine (NAC) for acetaminophen toxicity.
  • Consider liver biopsy if diagnosis is uncertain.

⭐ Amoxicillin-clavulanate is a classic cause of DILI, often presenting with a delayed cholestatic picture weeks after drug cessation.

High‑Yield Points - ⚡ Biggest Takeaways

• Acetaminophen toxicity is the most common cause of DILI; the antidote is N-acetylcysteine.
• DILI is classified as intrinsic (predictable, dose-related) or idiosyncratic (unpredictable).
• Centrilobular (Zone 3) necrosis is characteristic of injury from toxic metabolites, like with acetaminophen.
• Reye syndrome is a pediatric DILI caused by aspirin use during a viral infection.
• Key hepatotoxic drugs include anti-tuberculosis agents, amiodarone, methotrexate, and valproic acid.
• A thorough drug history is the most critical step in diagnosing DILI.