A patient on metformin for type 2 diabetes develops lactic acidosis after receiving IV contrast for CT angiography. The metformin-contrast interaction causing lactic acidosis is mediated by:
- A Contrast dye directly inhibits complex I of the mitochondrial ETC, combining with metformin's ETC inhibition
- B Contrast dye displaces metformin from albumin binding sites, increasing free drug concentration
- C Contrast-induced nephropathy reduces renal metformin clearance, causing drug accumulation and subsequent lactate accumulation via enhanced anaerobic glycolysis ✓
- D Iodinated contrast activates hepatic CYP3A4, metabolizing metformin to a toxic lactate-producing metabolite
Explanation
Metformin is entirely renally cleared (not metabolized). Its principal mechanism of lactic acidosis involves inhibition of mitochondrial Complex I (NADH dehydrogenase), shifting hepatic metabolism toward anaerobic glycolysis. Iodinated contrast agents can cause contrast-induced nephropathy (AKI), dramatically reducing metformin clearance and causing accumulation. Elevated metformin levels markedly increase Complex I inhibition and lactate production. Current guidelines recommend withholding metformin 48 hours before and after contrast administration if renal function is borderline (eGFR 30–60 mL/min).
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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