The discovery that angiogenesis inhibitors such as endostatin can restrain the growth of primary tumors raises the possibility that such inhibitors might also be able to slow tumor metastasis.

To test this hypothesis, researchers injected several kinds of mouse cancer cells beneath the animals' skin and allowed the cells to grow for about two weeks. The primary tumors were then removed, and the animals checked for several weeks. Typically, mice developed about 50 visible tumors from individual cancer cells that had spread to the lungs prior to removal of the primary tumor. But mice treated with angiostatin developed an average of only 2-3 tumors in their lungs. Inhibition of angiogenesis by angiostatin had reduced the rate of spread (metastasis) by about 20-fold.

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