Which pharmacogenomic variant is most strongly associated with lactic acidosis risk in patients on metformin, beyond renal impairment?

• A CYP3A4 poor metabolizer status causes metformin accumulation due to reduced hepatic metabolism
• B MATE1/MATE2-K gain-of-function variants reduce renal tubular secretion of metformin
• C UGT1A3 variants reduce metformin glucuronidation, increasing plasma concentrations
• D SLC22A1 loss-of-function variants (OCT1 transporter) reduce hepatic metformin uptake and increase plasma levels ✓

Explanation

Metformin is not metabolized by CYP450 enzymes but is transported by organic cation transporters. OCT1 (encoded by SLC22A1) is the primary transporter responsible for metformin uptake into hepatocytes — the pharmacological target for its glucose-lowering effect. Loss-of-function variants in SLC22A1 (e.g., *2, *3, *4 alleles, particularly common in European populations) reduce hepatic accumulation of metformin. Paradoxically, this reduces efficacy but also allows higher plasma concentrations if renal clearance is borderline, potentially increasing systemic lactate production. MATE1/2-K efflux transporters (SLC47A1/2) also affect renal elimination; their loss-of-function variants reduce tubular secretion and increase metformin plasma levels, contributing to toxicity risk. Both transporter pharmacogenomics affect metformin response and toxicity.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.