Sunitinib treatment inhibits KIT signaling in the BM and decreases BM progenitors. (A) Representative flow cytometry plots showing diminished frequencies of Lin⁻KIT^hi marrow progenitors in WT mice 4 hours after the fourth daily dose of vehicle (top panel) or sunitinib (bottom panel). (B) Frequency (top panel) and absolute numbers (bottom panel) of total marrow Lin⁻KIT^hi cells (represented by the total of all 3 stacked bar graphs) are reduced after sunitinib therapy administered as in panel A (mean percentage ± SEM: 1.55 ± 0.1 after sunitinib vs 1.03 ± 0.08 after vehicle, P < .001; and mean absolute number ± SEM: 19.2 ± 0.75 × 10⁴ after sunitinib vs 11.42 ± 0.74 × 10⁴ after vehicle, P < .001). Within the Lin⁻KIT^hi population, frequency and absolute numbers of MPPs and Lin⁻KIT^hiSCA-1⁻ progenitors are significantly reduced by sunitinib, whereas changes in HSC frequency and absolute numbers are not significant (P values shown). Data represent pooled results from 4 independent experiments; N = 18 to 20/group. (C) BM cells were harvested from untreated WT mice and then treated with sunitinib (100nM) or vehicle for 2 hours followed by recombinant mouse stem cell factor (250 ng/mL) for 30 minutes; then immunoprecipitation for KIT was performed and blotted with antiphosphotyrosine (left). Densitometric normalized ratios of phosphotyrosine/KIT are shown (right). This experiment was repeated 3 times with similar results. (D) KIT^W/Wv recipients and WT littermates received sunitinib or vehicle as described in panel A. Differences between sunitinib- and vehicle-treated KIT^W/Wv mice were insignificant (P = .18), whereas sunitinib significantly decreased the number of Lin⁻BM progenitors in wild-type littermate controls (P < .001); n = 10 mice per group. Similar results were seen in 2 different experiments. ***P < .001.