Blockade of internal VEGF-KDR autocrine loop induces leukemia apoptosis: synergistic effects with the external blocker and with chemotherapy. (A) HEL cells were left untreated (in serum-free conditions), treated with the Ab 4.6.1 or/and with the internal KDR inhibitor (KDRi), stained with annexin V and PI, and analyzed by FACS. Results show the percentage of viable cells (nonapoptotic, annexin V–negative) cells in the different conditions. As shown by the data, treatment with the internal KDRi has a clear effect at reducing cell viability, in contrast to those treated with the 4.6.1 Ab. Importantly, the 2 agents had synergistic effects and reduced cell viability significantly (P < .05) more than either agent alone. These results are representative of 3 independent experiments, and were repeated 3 times. (B) Combination of chemotherapy (etoposide) with the 4.6.1 Ab results in synergistic effects, decreasing cell visibility significantly (P < .05). Results are shown as the percentage of viable (annexin V–negative) cells. These results are representative of 3 independent experiments, and were repeated 3 times.