A 38-year-old woman with severe hepatic cirrhosis (Child-Pugh Class C) is started on metoprolol 50 mg twice daily for portal hypertension. Her plasma albumin is 2.8 g/dL (normal 3.5–5.0), and INR is 2.1. After 3 days, she develops severe bradycardia (heart rate 42/min) and hypotension (BP 88/54 mmHg). Serum metoprolol concentration is 450 ng/mL (therapeutic range 50–200 ng/mL). Which combination of pharmacokinetic factors best explains this toxicity?

Explanation

## Pharmacokinetic Alterations in Hepatic Cirrhosis **Key Point:** Severe liver disease impairs both drug metabolism and protein binding, resulting in elevated free (active) drug concentrations and increased risk of toxicity, even at standard doses. ### Pathophysiology in Cirrhosis **High-Yield:** Two critical pharmacokinetic changes occur in hepatic cirrhosis: 1. **Decreased hepatic metabolism** - Reduced hepatocyte mass and enzyme activity (CYP450 downregulation) - Impaired first-pass metabolism due to portal-systemic shunting - Metoprolol is extensively metabolized by hepatic CYP2D6; clearance is reduced by 50–80% in cirrhosis 2. **Decreased protein binding** - Hypoalbuminemia (albumin 2.8 g/dL, normal 3.5–5.0) - Metoprolol is 12% protein-bound normally, but in cirrhosis with low albumin, protein binding decreases further - **Result:** Increased free (unbound) drug concentration → increased pharmacological effect ### Quantitative Impact | Parameter | Normal | Cirrhosis (Child C) | Effect | |-----------|--------|---------------------|--------| | Hepatic clearance | 5–7 mL/kg/min | 1–2 mL/kg/min | ↓↓ Severely reduced | | Elimination half-life | 3–4 hours | 10–20 hours | ↑↑ Prolonged | | Protein binding | 12% | 8–10% | ↓ Reduced | | Free drug fraction | 88% | 90–92% | ↑ Increased | | Serum concentration at steady state | 50–200 ng/mL | 300–500+ ng/mL | ↑↑ Elevated | **Clinical Pearl:** The serum metoprolol concentration of 450 ng/mL (2.25× the upper therapeutic limit) combined with severe bradycardia and hypotension reflects **excessive free drug concentration** due to both reduced metabolism and reduced protein binding. ### Why This Combination Matters **Mnemonic: HEPATIC DRUG TOXICITY = Metabolism ↓ + Binding ↓ = Free drug ↑↑** - Decreased metabolism alone → prolonged half-life, accumulation - Decreased protein binding alone → increased free fraction - **Both together** → synergistic increase in free drug concentration and toxicity Metoprolol's negative inotropic and chronotropic effects are exaggerated when free concentration is elevated, causing the observed bradycardia and hypotension. ### Clinical Management 1. **Dose reduction:** Standard dose contraindicated; use 25% of normal dose or extend dosing interval 2. **Therapeutic drug monitoring:** Aim for lower serum concentrations (25–100 ng/mL) 3. **Monitor for toxicity:** Bradycardia, hypotension, worsening hepatic encephalopathy [cite:KD Tripathi 8e Ch 6; Harrison 21e Ch 297]