Pancytopenia due to pyrimethamine triggered by transplant-associated microangiopathy after allogeneic bone marrow transplantation.

Toxoplasmosis is one of the life-threatening infections that can occur after hematopoietic stem cell transplantation (HSCT) and also solid organ transplantation. The standard treatment for toxoplasmosis is combination therapy with pyrimethamine and sulfadiazine, both of which inhibit folate metabolism. Therefore, therapy with these agents could result in marrow toxicity including megaloblastic anemia or pancytopenia, which is reversible or preventable with folate supplementation. Transplant-associated microangiopathy (TAM) is another situation where folate is required to compensate for increased erythropoiesis due to hemolysis after allogeneic HSCT. Here, we report a case of severe marrow toxicity manifesting as pancytopenia due to low-dose pyrimethamine, which was triggered by TAM after HSCT.