Pyrimethamine (Pirimecidan) is a competitive inhibitor of dihydrofolate reductase (DHFR), used as an antimalarial drug.

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]