Serial blood counts obtained in 2 delayed responders to lestaurtinib. (A) Patient 5, a 75-year-old man with an I836 deletion point mutation, presented with pancytopenia. No peripheral-blood or bone marrow response was seen in 56 days of lestaurtinib therapy. Transfusion independence was subsequently achieved for a period of 4 months from trial day 104, which was associated with transient normalization of WCC and neutrophil and platelet counts. There was no second response following hematologic deterioration and reinstitution of lestaurtinib therapy. (B) Patient 17, a 72-year-old woman with WT FLT3, also presented with pancytopenia. Bone marrow blast numbers fell from 80% to 30% over 56 days of lestaurtinib treatment, after which the drug was withdrawn following an episode of febrile neutropenia. The patient made a marked delayed hematologic response, with improvements in platelet and neutrophil counts and sustained transfusion independence between days 123 and 284. At the time of subsequent disease progression, the patient declined the option of a second course of lestaurtinib treatment and died on trial day 364.