Inhibitors of angiogenesis selectively reduce the malignant cell load in rodent models of human myeloid leukemias

Angiogenesis is essential for growth and metastasis of solid tumors and probably also for hematological malignancies. Angiogenic inhibitors, like endostatin (ES) and PI-88, retard cancer growth. We tested these in mice with juvenile myelomonocytic leukemia (JMML), and in rats with acute myelold leukemia (BNML). Eight weeks after transplantation and with a continuous drug treatment for the last 4 weeks, the leukemic cell mass decreased from almost 90% of all bone marrow cells to about 15 and 45% with ES, to about 35 and 55% with PI-88, and to about 10 and 25% with ES + PI-88 in the leukemic mice and rats, respectively. The numbers of normal human bone marrow cells transplanted into mice were unchanged by the treatments. The microvessel density in leukemic animals given ES or PI-88 was 10-50% of that in untreated animals. Notably, simultaneous treatment with ES and PI-88 led to a reduction of about 95% in JMML mice and 85% in BNML rats. In vitro proliferation of either JMML or BNML cells was not significantly altered by either drug, demonstrating the selectivity of ES and PI-88 as angiogenic inhibitors. In conclusion, anti-angiogenic therapy may be a valuable adjunct to conventional treatment of leukemia.