metformin + pyrimethamine

The combination of metformin and pyrimethamine represents a significant drug-drug interaction rather than an intentional therapeutic combination. Pyrimethamine is a potent inhibitor of multidrug and toxin extrusion (MATE) proteins, which are transporters involved in the renal elimination of metformin[1][2]. ## Pharmacokinetic Interaction When pyrimethamine is co-administered with metformin, it significantly reduces the renal clearance of metformin by 23-35%[1]. This interaction leads to increased systemic exposure of metformin, with studies showing that pyrimethamine causes a 2.58-fold increase in metformin's area under the concentration-time curve (AUC)[2]. The maximum concentration (Cmax) of metformin also increases to 142% of control values when administered with pyrimethamine[1]. ## Mechanism of Interaction The interaction occurs primarily through pyrimethamine's inhibition of MATE transporters. MATE proteins are responsible for the excretion of metformin through the kidneys, and when inhibited by pyrimethamine, metformin's elimination is reduced[1][2]. This has been demonstrated in studies using human canalicular membrane vesicles, which showed pyrimethamine-inhibitable metformin uptake[1]. ## Clinical Effects Interestingly, despite increasing metformin plasma concentrations, co-administration with pyrimethamine actually decreased metformin's antihyperglycemic effects in healthy subjects[2]. This suggests that the response to metformin is not solely related to plasma exposure but also involves other factors such as tissue distribution and gastrointestinal-based pharmacology[2]. Pyrimethamine also affects renal function, reducing the renal clearance of creatinine and causing elevation of plasma creatinine levels[1]. However, it does not appear to affect plasma lactate/pyruvate levels after therapeutic doses of metformin[1]. ## Clinical Implications This interaction is significant for clinical practice as it affects both the pharmacokinetics and potentially the efficacy of metformin. The combination should be used with caution, and metformin dosage may need adjustment when co-administered with pyrimethamine[5]. Pyrimethamine is considered a useful in vivo inhibitor of MATE proteins and has been used in research to study the role of these transporters in drug interactions[1].