Acute myeloid leukemia (AML) arises from neoplastic transformation of hematopoietic stem and progenitor cells, and relapsed disease remains one of the greater challenges in treating this hematologic malignancy. This paper focuses on angiogenic aspects of AML including the significance and prognostic value of bone marrow microvessel density and circulating cytokine levels. We show three general mechanisms whereby AML exploits angiogenic pathways, including direct induction of angiogenesis, paracrine regulation, and autocrine stimulation. We also present early evidence that leukemia cells contribute directly to vascular endothelia. Novel treatment strategies are proposed, and a review of relevant antiangiogenic clinical trials is presented. By understanding how blood vessels can serve as a reservoir for refractory and relapsed AML, new diagnostics and promising treatment strategies can be developed.

In acute myeloid leukemia (AML), refractory disease is a major challenge and the leukemia microenvironment may harbor refractory disease. Human AML cell lines KG-1 and HL-60 expressed receptors also found on endothelial cells (ECs) such as VEGFRs, PDGFRs, and cKit. When human AML cells were co-cultured with human umbilical vein endothelial cells (HUVECs) and primary bone marrow endothelial cell (BMECs), the AML cells were more resistant to cytarabine chemotherapy, even in transwell co-culture suggesting angiocrine regulation. Primary BMECs secreted significantly increased levels of VEGF-A and PDGF-AB after exposure to cytarabine. Pazopanib, a receptor tyrosine kinase inhibitor (RTKI) of VEGFRs, PDGFRs, and cKit, removed EC protection of AML cells and enhanced AML cell sensitivity to cytarabine. Xenograft modeling showed significant regression of AML cells and abrogation of BM hypervascularity in RTKI treated cohorts. Together, these results show direct cytotoxicity of RTKIs on AML cells and reversal of EC protection. Combining RTKIs with chemotherapy may serve as promising therapeutic strategy for patients with AML.

1057 Acute myeloid leukemia (AML) cells depend on endothelial cells for survival and proliferation. By targeting endothelial cells with a novel vascular disrupting agent, we recently demonstrated regression of AML (Madlambayan, et al., Blood 2010). In an effort to discover a more selective, anti-vascular therapy for leukemia, we hypothesized that targeting endothelial cell-derived paracrine and leukemia cell-derived autocrine growth factors would result in regression of disease. In particular, our strategy focused on vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF) and stem cell factor (SCF), which are known to promote leukemia cell proliferation. Receptors for each of these growth factors are potently inhibited by pazopanib, which is an orally available tyrosine kinase inhibitor. The aim of our study was to determine the dependence of leukemia cell survival and proliferation on the combined receptor signaling of VEGF, PDGF and SCF. Leukemia cell lines (KG-1, HL60 and K562) were incubated at various durations with varying concentrations of pazopanib. Leukemia cell proliferation was quantified using XTT assay. Apoptosis induction was analysed by flow cytometry (Annexin V and PI staining). Pazopanib effectively impaired proliferation in leukemia cells in vitro in a dose and time dependent fashion. During a 16-hour incubation of leukemia cells, pazopanib showed a 50% lethal concentration (LC50) of 22.57 μM (r = .986), 41.6 μM (r = .991), and 81.97 μM (r = .996) for HL60, K562, and KG-1 cells, respectively. Staining with Annexin V and PI identified apoptosis as the main cause of cell death after exposure to pazopanib. The IC50 of apoptosis for HL60 cells was 132.5 ± 2.7 μM (p = 0.0002). These results indicate that leukemia cells depend on the combined signaling of VEGF, PDGF, and SCF, and suggest that selective inhibition by pazopanib may be a promising therapeutic for AML. Furthermore, pazopanib is orally available and associated with minimal side effects, thus representing an attractive candidate for further testing in AML. Disclosures: No relevant conflicts of interest to declare.

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