The emerging role of angiogenesis inhibitors in hematologic malignancies

Angiogenesis is an important component of the pathogenesis of hematologic malignancies. A negative prognostic implication of increased angiogenesis has been established for acute and chronic myeloid and lymphocytic leukemias, myeloproliferative diseases, multiple myeloma, non-Hodgkin's lymphoma (NHL), and hairy cell leukemia. An association between the return of increased marrow vascularity to normal levels and durability of response has been established in some of these diseases. Elevated levels ofproangiogenic factors have been associated with a poor prognosis in the acute and chronic leukemias, multiple myeloma, and NHL. These data lend support to the reduction of activity of proangiogenic factors as a therapeutic modality. Vascular endothelial growth factor (VEGF) has been implicated as the major proangiogenic factor that regulates multiple endothelial cell functions, including mitogenesis. A direct relationship between VEGF and leukemic blasts and malignant plasma cells has been established, but VEGF may have a function distinct from its role in angiogenesis. Current protocols with anti- VEGF agents in patients with hematologic malignancies involve the use of monoclonal antibody, blockers of the VEGF-receptor tyrosine kinase pathway, thalidomide (Thalomid) and its analogs, and cyclooxygenase inhibitors. The receptor tyrosine kinase inhibitors also affect platelet-derived growthfactor, c-kit, and Flt-3 to varying degrees, considerably broadening their potential efficacy. This review will summarize several angiogenesis inhibitors in clinical development.