Pyrimethamine has an IC50 of 5–13 μM for the Hex isozymes at pH 4.3. Pyrimethamine increases the enzyme activity and protein level of the α and β subunits of Hex A in the βR505Q/Δ16kb cell line. [1] Pyrimethamine-sulfadoxine is an inhibitor of dihydrofolate reductase(DHFR) that has been widely used to treat chloroquine-resistant Plasmodium falciparum malaria. [2] Pyrimethamine is a potent inhibitor of mouse (m)Mate1 (K(i) = 145 nM) among renal organic cation transporters mOctn1 and mOctn2 (K(i) > 30 mM), mOct1 (K(i) = 3.6 mM), and mOct2 (K(i) = 6.0 mM). Pyrimethamine inhibits the uptake of metformin by kidney brush-border membrane vesicles (BBMVs) (K(i) = 41 nM) and canalicular membrane vesicles in the presence of outward gradient of H+. Pyrimethamine treatment significantly increases the kidney-to-plasma ratio of tetraethylammonium, and both the liver- and kidney-to-plasma ratios of metformin in mice, whereas it does not affect their plasma concentrations and urinary excretion rates. Pyrimethamine is a potent inhibitor of human (h)MATE1 and hMATE2-K (K(i) = 77 and 46 nM, respectively) and H+ and organic cation exchanger in human kidney BBMVs (K(i) = 31 nM) in the presence of outward gradient of H+. [3]