A 28-year-old woman with newly diagnosed pulmonary tuberculosis is counseled about her anti-tuberculous regimen. She asks about the most common hepatotoxic drug in the standard first-line therapy. Which drug is most frequently responsible for drug-induced liver injury during TB treatment?

Explanation

## Most Common Hepatotoxic Agent in TB Treatment **Key Point:** Isoniazid (INH) is the most common cause of drug-induced liver injury (DILI) during standard anti-tuberculous therapy, consistently cited across major references including Harrison's Principles of Internal Medicine and KD Tripathi's Essentials of Medical Pharmacology. ### Mechanism of Isoniazid Hepatotoxicity **High-Yield:** Isoniazid is metabolized by hepatic N-acetyltransferase 2 (NAT2) to acetylisoniazid, which is further hydrolyzed to acetylhydrazine and then to reactive toxic intermediates (via CYP2E1). These intermediates cause: - Direct hepatocellular necrosis - Centrilobular necrosis (histologically similar to viral hepatitis) - Elevated transaminases (ALT > AST) - Rarely, fulminant hepatic failure ### Hepatotoxicity Profile of First-Line Drugs | Drug | Hepatotoxicity | Frequency | Mechanism | |------|-----------------|-----------|-----------| | **Isoniazid** | **Most common** | **0.5–3% clinical; up to 20% asymptomatic transaminase rise** | Toxic metabolite (acetylhydrazine) via NAT2/CYP2E1 | | Pyrazinamide | Common (dose-dependent) | 1–5% (dose-dependent) | Pyrazinoic acid accumulation | | Rifampicin | Less common alone | <1% alone; potentiates INH toxicity | Enzyme induction accelerates INH → toxic metabolite | | Ethambutol | Very rare | <0.5% | Minimal hepatic metabolism | **Clinical Pearl:** Rifampicin, while rarely hepatotoxic alone, significantly potentiates INH-induced hepatotoxicity by inducing CYP2E1, increasing production of the toxic acetylhydrazine metabolite. This is why the INH + RIF combination carries higher hepatotoxic risk than either drug alone. ### Risk Factors for INH-Induced Hepatotoxicity 1. **Slow acetylator phenotype** (NAT2 polymorphism — more acetylhydrazine accumulates) 2. **Age >35 years** (risk increases with age) 3. **Pre-existing liver disease** (chronic hepatitis B/C, cirrhosis) 4. **Concurrent alcohol use** (induces CYP2E1) 5. **HIV co-infection** 6. **Malnutrition** 7. **Concurrent rifampicin use** (enzyme induction) **Mnemonic: SHARP** — **S**low acetylators, **H**IV+, **A**lcohol, **R**ifampicin co-administration, **P**re-existing liver disease. ### Why Pyrazinamide Is Sometimes Cited Pyrazinamide causes dose-dependent hepatotoxicity and at high doses (>35 mg/kg/day, now obsolete) was the most hepatotoxic drug. At current standard doses (15–30 mg/kg/day), **isoniazid remains the most frequently implicated drug** in DILI during TB treatment in standard regimens. Most authoritative sources (Harrison 21e, KD Tripathi 8e, WHO guidelines) identify INH as the primary hepatotoxic agent. ### Clinical Management **Warning:** Baseline LFTs are mandatory before starting TB therapy. If AST/ALT rises >3× ULN with symptoms or >5× ULN without symptoms, all hepatotoxic drugs should be stopped and reintroduced sequentially after normalization. ### Monitoring Strategy 1. **Baseline:** AST, ALT, bilirubin, albumin 2. **Monthly:** LFTs if baseline abnormal or symptoms develop 3. **Symptoms:** Nausea, jaundice, dark urine → immediate LFTs and drug review [cite: Harrison 21e Ch 205; KD Tripathi 8e Ch 49; WHO Treatment of Tuberculosis Guidelines 4th ed.]