Liver Injuries

 

Drug-Induced Liver Injury (DILI)

How Tylenol (Acetaminophen) and Other Medications Damage the Liver

Clinician-Level Study Notes

1. Big Picture: Why the Liver Is Vulnerable

• Liver = main site of drug metabolism

• High blood exposure:

  • Portal vein brings absorbed drugs directly from gut

  • Hepatic artery supplies systemic drugs

• Hepatocytes contain:

  • Phase I enzymes (CYP450)

  • Phase II conjugation systems

• Injury occurs when:

  • Toxic metabolites form

  • Detox pathways are overwhelmed

  • Immune reactions are triggered

---

2. Acetaminophen (Tylenol): Prototype of Predictable Hepatotoxicity

Normal Metabolism

Acetaminophen is metabolized by 3 pathways:

1. Glucuronidation (Phase II) – ~60%

2. Sulfation (Phase II) – ~30%

3. CYP450 (Phase I, mainly CYP2E1) – ~5–10%

The CYP pathway produces a toxic metabolite:

• NAPQI (N-acetyl-p-benzoquinone imine)

• Normally neutralized by glutathione → harmless excretion

---

Mechanism of Toxicity

When dose is too high:

• Phase II pathways become saturated

• More drug shunted to CYP2E1

• Excess NAPQI formed

• Glutathione stores depleted

• NAPQI binds to:

  • Mitochondrial proteins

  • Cellular membranes

  • DNA
    → Hepatocyte necrosis

---

Risk Factors for Acetaminophen Toxicity

• Dose > 3–4 g/day (lower in liver disease, elderly, malnourished)

• Chronic alcohol use:

  • Induces CYP2E1 → more NAPQI

  • Depletes glutathione

• Fasting/malnutrition

• Liver disease

• Concomitant enzyme-inducing drugs

---

Pattern of Injury

• Zone 3 (centrilobular) necrosis

• AST/ALT often > 3000–10,000

• Early: nausea, vomiting, diaphoresis

• Later: RUQ pain, jaundice, encephalopathy, coagulopathy

---

Antidote

• N-acetylcysteine (NAC)

  • Replenishes glutathione

  • Directly detoxifies NAPQI

  • Improves mitochondrial function

• Most effective within 8–10 hours

---

3. Other Mechanisms of Drug-Induced Liver Injury

A. Direct (Intrinsic) Hepatotoxicity

• Dose-dependent

• Predictable

• Reproducible in animals

Examples:

• Acetaminophen

• Carbon tetrachloride

• Some chemotherapy drugs

Mechanism:

• Toxic metabolite

• Oxidative stress

• Mitochondrial failure

• Cell necrosis or apoptosis

---

B. Idiosyncratic Hepatotoxicity

• Not dose-dependent

• Unpredictable

• Rare

• Often immune-mediated

Mechanisms:

1. Immune reaction to drug-protein adduct

2. Genetic variation in metabolism

3. Mitochondrial susceptibility

Latency:

• Days to months after starting drug

---

4. Major Drug Classes That Injure the Liver

1. Antibiotics

Common offenders:

• Amoxicillin-clavulanate (most common DILI cause in West)

• Isoniazid

• Rifampin

• Nitrofurantoin

• Sulfonamides

Patterns:

• Hepatocellular

• Cholestatic

• Mixed

Mechanisms:

• Immune-mediated injury

• Toxic metabolites

• Mitochondrial injury

---

2. Statins

• Usually safe

• Mild ALT elevation common

• True liver failure extremely rare

Mechanism:

• Mitochondrial dysfunction

• Altered membrane lipid composition

Management:

• Monitor, don’t stop for mild asymptomatic ALT rise

---

3. Anti-Seizure Drugs

Examples:

• Valproate

• Phenytoin

• Carbamazepine

Mechanisms:

• Toxic metabolites

• Mitochondrial toxicity

• Immune reactions

Valproate:

• Inhibits mitochondrial β-oxidation

• Causes microvesicular steatosis

---

4. TB Medications

• Isoniazid

• Rifampin

• Pyrazinamide

Mechanisms:

• Isoniazid → toxic metabolites via acetylation

• Risk higher in:

  • Older age

  • Alcohol use

  • Liver disease

---

5. Chemotherapy

• Methotrexate → fibrosis

• Azathioprine → cholestasis

• Checkpoint inhibitors → autoimmune hepatitis

---

6. Hormones & Steroids

• Oral contraceptives

• Anabolic steroids

Effects:

• Cholestasis

• Hepatic adenomas

• Peliosis hepatis

---

7. Herbal & Dietary Supplements

High-risk:

• Kava

• Green tea extract (concentrated)

• Bodybuilding supplements

• Black cohosh

• Chaparral

Mechanisms:

• Unknown toxins

• CYP interactions

• Immune reactions

---

5. Patterns of Liver Injury

Hepatocellular

• High AST/ALT

• Modest ALP

• Examples: acetaminophen, isoniazid

Cholestatic

• High ALP, bilirubin

• Pruritus prominent

• Examples: OCPs, amox-clav

Mixed

• Both elevated

---

6. Pathophysiologic Mechanisms Summary

Mechanism	Result
Toxic metabolites	Cell necrosis
Oxidative stress	Mitochondrial failure
Immune reaction	Hepatitis-like injury
Cholestasis	Bile flow obstruction
Mitochondrial inhibition	Steatosis, lactic acidosis

---

7. Clinical Approach to Suspected DILI

Step 1: History

• All meds (including OTC, herbs)

• Timing of initiation

• Alcohol use

• Viral risk

Step 2: Labs

• AST, ALT, ALP, bilirubin

• INR, albumin if severe

Step 3: Pattern Recognition

• Hepatocellular vs cholestatic vs mixed

Step 4: Exclude Other Causes

• Viral hepatitis

• Autoimmune

• Ischemic injury

• Biliary obstruction

Step 5: Stop Suspected Drug

---

8. Red Flags

• AST/ALT > 1000

• Rising bilirubin + ALT

• INR prolongation

• Encephalopathy

• Hypoglycemia

These suggest acute liver failure.

---

9. Prevention Principles

• Avoid polypharmacy

• Check interactions

• Dose adjust in liver disease

• Avoid alcohol with hepatotoxic drugs

• Educate patients about OTC + herb risk

---

10. Key Takeaways

• Tylenol kills liver cells by:

  • Overproducing NAPQI

  • Depleting glutathione

  • Causing centrilobular necrosis

• Many drugs injure liver by:

  • Toxic metabolites

  • Immune reactions

  • Mitochondrial injury

• DILI is:

  • Common

  • Underrecognized

  • Preventable with vigilance