A diabetic patient on metformin develops lactic acidosis after contrast-enhanced CT scan. Which pharmacokinetic property of metformin is MOST responsible for the accumulation leading to this adverse effect?

• A Contrast media inhibits hepatic CYP2C8-mediated metabolism of metformin, increasing plasma levels
• B Iodinated contrast displaces metformin from plasma protein binding, raising free drug levels acutely
• C Metformin is excreted unchanged by the kidney; contrast-induced AKI reduces renal clearance, causing drug accumulation ✓
• D Contrast media inhibits mitochondrial complex I, synergizing with metformin's biguanide mechanism

Explanation

Metformin is not metabolized hepatically; it is excreted entirely unchanged via active renal tubular secretion (organic cation transporters OCT2/MATE1) and glomerular filtration. It has no hepatic first-pass effect and is not bound to plasma proteins. Contrast-induced nephropathy reduces GFR and tubular secretion, causing metformin to accumulate. Elevated metformin inhibits mitochondrial complex I in hepatocytes and intestinal enterocytes, impairing hepatic gluconeogenesis from lactate—leading to type B lactic acidosis. Guidelines recommend withholding metformin at the time of contrast administration and restarting only after confirming stable renal function (48 hours later).

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.