General The recommended dosage for chemoprophylaxis of malaria should not be exceeded. A small “starting” dose for toxoplasmosis is recommended in patients with convulsive disorders to avoid the potential nervous system toxicity of pyrimethamine. DARAPRIM (pyrimethamine) should be used with caution in patients with impaired renal or hepatic function or in patients with possible folate deficiency, such as individuals with malabsorption syndrome, alcoholism, or pregnancy, and those receiving therapy, such as phenytoin, affecting folate levels (see Pregnancy subsection). Laboratory Tests In patients receiving high dosage, as for the treatment of toxoplasmosis, semiweekly blood counts, including platelet counts, should be performed. Carcinogenesis, Mutagenesis, Impairment of Fertility See WARNINGS section for information on carcinogenesis. Mutagenesis: Pyrimethamine has been shown to be nonmutagenic in the following in vitro assays: the Ames point mutation assay, the Rec assay, and the E. coli WP2 assay. It was positive in the L5178Y/TK +/- mouse lymphoma assay in the absence of exogenous metabolic activation.6 Human blood lymphocytes cultured in vitro had structural chromosome aberrations induced by pyrimethamine. In vivo, chromosomes analyzed from the bone marrow of rats dosed with pyrimethamine showed an increased number of structural and numerical aberrations. Pregnancy Teratogenic Effects: Pregnancy Category C. Pyrimethamine has been shown to be teratogenic in rats when given in oral doses 7 times the human dose for chemoprophylaxis of malaria or 2.5 times the human dose for treatment of toxoplasmosis. At these doses in rats, there was a significant increase in abnormalities such as cleft palate, brachygnathia, oligodactyly, and microphthalmia. Pyrimethamine has also been shown to produce terata such as meningocele in hamsters and cleft palate in miniature pigs when given in oral doses 170 and 5 times the human dose, respectively, for chemoprophylaxis of malaria or for treatment of toxoplasmosis. There are no adequate and well-controlled studies in pregnant women. DARAPRIM (pyrimethamine) should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. Concurrent administration of folinic acid is strongly recommended when used for the treatment of toxoplasmosis during pregnancy. Nursing Mothers Pyrimethamine is excreted in human milk. Because of the potential for serious adverse reactions in nursing infants from pyrimethamine and from concurrent use of a sulfonamide with DARAPRIM (pyrimethamine) for treatment of some patients with toxoplasmosis, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother (see WARNINGS and PRECAUTIONS: Pregnancy ). Pediatric Use See DOSAGE AND ADMINISTRATION section. Geriatric Use Clinical studies of DARAPRIM (pyrimethamine) did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy. REFERENCES 3. Jim RTS, Elizaga FV. Development of chronic granulocytic leukemia in a patient treated with pyrimethamine. Hawaii Med J. 1977;36:173-176. 4. Sadoff L. Antimalarial drugs and Burkitt's lymphoma. Lancet. 1973;2:1262-1263. 5. Bahna L. Pyrimethamine. LARC Monogr Eval Carcinog Risk Chem. 1977;13:233-242. 6. Clive D, Johnson KO, Spector JKS, et al. Validation and characterization of the L5178Y/TK +/- mouse lymphoma mutagen assay system. Mut Res. 1979;59:61-108. Last reviewed on RxList: 1/20/2009
This monograph has been modified to include the generic and brand name in many instances.