Pyrimethamine (Daraprim) is a medication used for protozoal infections. It is commonly used as an antimalarial drug (for both treatment and prevention), and is also used (combined with sulfadiazine) in the treatment of Toxoplasma gondii infections in immunocompromised patients, such as HIV-positive individuals.
Mechanism of actionPyrimethamine interferes with folic acid synthesis by inhibiting the enzyme dihydrofolate reductase. Folic acid is needed for DNA and RNA synthesis in many species, including protozoa.
Mechanism of resistanceResistance to pyrimethamine is widespread. Mutations in the gene for dihydrofolate reductase may reduce the effectiveness of pyrimethamine (PMID 15155209). These mutations decrease the binding affinity between pyrimethamine and dihydrofolate reductase by steric interactions (PMID 14711307).
Clinical usePyrimethamine is typically given with a sulfonamide and folinic acid:
- Sulfonamides inhibit dihydropteroate synthetase, an enzyme that participates in folic acid synthesis from para-aminobenzoic acid. Hence, sulfonamides work synergistically with pyrimethamine by blocking a different enzyme needed for folic acid synthesis.
- Folinic acid (Leucovorin) is a compound that can be converted into folic acid by the human body without relying on dihydrofolate reductase. By doing so, folinic acid reduces side effects related to folate deficiency.
Side effectsPyrimethamine may deplete folic acid in humans, resulting in hematologic side effects associated with folate deficiency.
Side effects include:
- hypersensitivity reactions
- megaloblastic anemia
- atrophic glossitis
- cardiac arrhythmias
- pulmonary eosinophilia (rare)
- hyperphenylalaninemia (particularly when used with a sulfonamide)
- Steven-Johnson syndrome (particularly when used with a sulfonamide)
- toxic epidermal necrolysis (particularly when used with a sulfonamide)
ContraindicationsPyrimethamine is contraindicated in patients with:
- hypersensitivity to pyrimethamine
- megaloblastic anemia - depletion of folic acid may aggravate this condition
pyrimethamine in German: Pyrimethamin
pyrimethamine in Polish: Pirymetamina