Severe 6-thioguanine-induced marrow aplasia in a child with acute lymphoblastic leukemia and inherited thiopurine methyltransferase deficiency.

6-thioguanine (6TG) is undergoing investigation for use in the maintenance phase of acute lymphoblastic leukemia (ALL). Just as with 6-mercaptopurine (6MP), it is be expected that 6TG would cause pancytopenia in individuals with inherited thiopurine methyltransferase (TPMT) deficiency. We report the first case of severe and prolonged pancytopenia caused by 6-thioguanine in an 8-year-old boy with ALL and inherited TPMT deficiency. Neutropenia lasted 67 days, whereas anemia and thrombocytopenia did not recover for 96 days. To obviate this life-threatening complication, clinicians should consider assaying TPMT activity before initiating therapy with 6MP and, particularly, 6TG in children with ALL.