Persistence of FK1012-mediated cell proliferation after drug withdrawal. (A) A clonal population of Ba/F3 cells expressing F3c-kit432 proliferates in IL-3–deficient medium supplemented with either FK1012 (○) or AP1510 (◊) (both 100 nmol/L). In the absence of either drug or IL-3 (•), cells died over a period of 2 to 3 days. (B) The same cells grown initially in FK1012 or in AP1510 were washed extensively and then cultured either in the presence or absence of added drug. In the continued presence of FK1012 (○) or AP1510 (◊), continued cell proliferation was observed. After withdrawal of AP1510 (⧫), cell death occurred over 1 to 2 days. In contrast, cells grown initially in FK1012 continued to proliferate for up to 1 week after FK1012 was withdrawn (•). Very similar findings were observed in a repeat experiment.