CDK6 is required for FLT3-ITD–driven tumor formation and leukemogenesis in vivo. (A) Kaplan-Meier plot depicting disease onset of immune-compromised Rag2^−/−γc^−/− recipients injected with FLT3-ITD⁺ cells (MOLM-14). On day 5 after engraftment, mice were randomly divided into 2 groups and dosed once per day with vehicle (n = 5) or palbociclib (n = 6). Mean survival with vehicle, 43 days; with palbociclib, 73.5 days. Log-rank test was used for statistical comparison. (B) Bone marrow–infiltrating MOLM-14 cells isolated from diseased mice (n = 3 for each group) shown in (A) were analyzed for human PIM1 and FLT3 gene expression upon treatment with either vehicle or palbociclib. (C-D) FLT3-ITD⁺ (MOLM-14) cells were subcutaneously (s.c.) injected into both flanks of immune-compromised Rag2^−/−γc^−/− recipients. Mice were treated once per day with vehicle or palbociclib on (C) day 0 (n = 4 mice for each group) or on (D) day 5 (vehicle, n = 3 mice; palbociclib, n = 2 mice) until terminal workup at day 12. (E) Human PIM1 and FLT3 gene expression was analyzed by quantitative RT-PCR in subcutaneously grown tumors shown in (D) after treatment with either vehicle or palbociclib. (F) Scheme of the mechanism of action of palbociclib in FLT3-ITD leukemic cells: blockade of CDK6 kinase activity upon palbociclib exposure impairs cell cycle progression from G₁ phase to S phase and inhibits transcription of FLT3 and PIM1 leading to survival inhibition. *P < .05; **P < .01.