A 42-year-old man with chronic kidney disease (eGFR 28 mL/min/1.73 m²) is prescribed clarithromycin 500 mg twice daily for 7 days to treat a respiratory tract infection. He is also on simvastatin 40 mg daily and verapamil 120 mg twice daily for hypertension. On day 3, he develops severe myalgia, dark urine, and elevated creatine kinase (CK 2800 U/L). What is the most likely mechanism of this adverse event?

Explanation

## Clinical Scenario Interpretation A patient on **clarithromycin + simvastatin + verapamil** develops myalgia, myoglobinuria (dark urine), and elevated CK — classic features of **statin-induced myopathy**. The timing (day 3) and drug combination point to a **drug–drug interaction**. ## Macrolide–Statin Interaction: CYP3A4 Inhibition **Key Point:** Macrolides (especially clarithromycin and erythromycin) are potent **CYP3A4 inhibitors**. Statins metabolized by CYP3A4 (simvastatin, atorvastatin) accumulate when co-prescribed with macrolides, increasing myopathy risk. **High-Yield:** Statin myopathy risk hierarchy with macrolides: 1. **Simvastatin + clarithromycin** — highest risk (both CYP3A4-dependent) 2. Atorvastatin + clarithromycin — moderate risk 3. Pravastatin + clarithromycin — minimal risk (not CYP3A4-metabolized) 4. Azithromycin — weaker CYP3A4 inhibitor; lower interaction risk ## CYP3A4 Inhibition Mechanism ```mermaid flowchart TD A[Clarithromycin started]:::action --> B[CYP3A4 inhibition]:::outcome B --> C[Simvastatin metabolism ↓]:::outcome C --> D[Simvastatin accumulation]:::outcome D --> E{Statin concentration}:::decision E -->|Exceeds myotoxic threshold| F[Myopathy: myalgia, myoglobinuria]:::urgent E -->|Normal| G[No adverse effect]:::outcome F --> H[Elevated CK, dark urine, AKI risk]:::urgent ``` **Clinical Pearl:** Renal impairment (eGFR 28) compounds the problem by: - Reducing macrolide clearance → higher drug levels → stronger CYP3A4 inhibition - Reducing statin clearance → further accumulation - Increasing myoglobinuria-induced acute kidney injury risk ## Why Not the Other Options? | Mechanism | Why Incorrect | |-----------|---------------| | **Acute interstitial nephritis (AIN)** | Clarithromycin can cause AIN, but it presents with fever, rash, eosinophilia, and acute rise in creatinine — not myalgia or elevated CK. | | **Direct clarithromycin nephrotoxicity** | Clarithromycin is not directly nephrotoxic; it does not cause myopathy. | | **Verapamil-induced rhabdomyolysis** | Verapamil does not cause myopathy or rhabdomyolysis; it is a calcium channel blocker used for hypertension. | **Mnemonic: CYP3A4 Macrolides — **CREAM** - **C** — Clarithromycin (potent inhibitor) - **R** — Erythromycin (potent inhibitor) - **E** — Erythromycin (repeat for emphasis) - **A** — Azithromycin (weak inhibitor) - **M** — Macrolides ↑ statin levels → myopathy ## Management 1. **Stop clarithromycin immediately** 2. **Stop simvastatin** (until CK normalizes) 3. **Monitor CK, urine myoglobin, creatinine** (risk of acute kidney injury) 4. **Hydration** to prevent myoglobin precipitation in renal tubules 5. **Switch macrolide** to azithromycin (if antibiotic still needed) or choose alternative antibiotic 6. **Resume statin** only after CK normalizes, using pravastatin (CYP3A4-independent) or a lower simvastatin dose