Pharmacodynamic response of 2 representative patients. FLT3 was immunoprecipitated and Western blotting was performed using 4G10 antiphosphotyrosine antibodies and anti-FLT3 antibodies as described in “Patients, materials, and methods.” (A) Patient 501-0008 had a significant decrease in relative phosphotyrosine content of FLT3 by 48 hours after initiation of therapy. The absolute blast count (ABC) was 15 600 on day 1 and had decreased to 200 by day 8 of therapy. (B) Patient 501-00017 started therapy with an absolute blast count of 4530 on day 1, which decreased to 100 by day 8.