As metformin is not metabolized in the liver, drug–drug interactions through the inhibition of metformin transporters (OCTs and MATEs) are clinically relevant. Genetic polymorphisms in these transporter genes are also likely to have a direct impact on metformin pharmacokinetics and variability in drug responses (see the Pharmacogenomics section). Recent drug–drug interaction studies suggest that proton-pump inhibitors inhibit metformin uptake in vitro by inhibiting OCT1, OCT2, and OCT3 [25]. Oral antidiabetic drugs repaglinide and rosiglitazone also inhibited OCT1-mediated metformin transport in vitro [26]. The H2 blocker cimetidine is associated with reduced renal tubular secretion and increased systemic exposure to metformin when both drugs are coadministered [27]. Inhibition of MATEs, but not OCT2 [28], is the likely mechanism underlying the drug–drug interactions with cimetidine in renal elimination [20]. A recent study suggests the potential for a transporter-mediated drug–drug interaction between metformin and specific tyrosine kinase inhibitors (e.g. imatinib, nilotinib, gefitinib, and erlotinib), which may have clinical implications in the disposition, efficacy, and toxicity of metformin [29].