Piritrexim (BW 301U) is an orally available fat-soluble dihydrofolate reductase inhibitor with pulmonary toxicity used in the study of uroepithelial carcinoma and metastatic breast cancer.

Piritrexim (0.1 to 1.0 microM) was able to inhibit the replication of T.gondii in a mouse peritoneal macrophage model. The addition of sulfadiazine, which alone was ineffective, to piritrexim allowed inhibition of T.gondii replication at lower concentrations of piritrexim than when piritrexim was used alone.[5]

Piritrexim (25-mg/m2/dose; oral) occurred the myelosuppression and mucositis in 4 of 4 patients but in none of the patients treated at the 15- and 20-mg/m2/dose levels.[6]
Piritrexim (20 mg/m2/dose; oral) was rapidly absorbed, with the median time to peak level occurring 1.5 h after an oral dose, and the area under the concentration-time curve (AUC) was linearly related to the dose administered. Trough plasma piritrexim concentration strongly correlated with DLT (P = 0.0016). A trough plasma piritrexim concentration greater than 0.5 microM appeared to be predictive of toxicity. Eleven of 15 patients with trough concentrations exceeding this threshold experienced DLTs.[6]