It is produced by or near a solid tumor. Angiostatin is a relatively long-lasting protein, staying in the circulation for up to 5 days. It does not normally seem to stop the growth of the primary tumor. When the primary tumor is removed, so is the source of angiostatin. That opens the floodgates, allowing metastatic tumors to attract blood supplies and grow.
Primary tumors produce angiogenic factors in much greater quantities than angiostatic factors.
But the angiogenic factors stay in the bloodstream for only a few minutes, usually no more than four. Thus, these factors act only near the primary tumor, and they act fast. They don't travel far enough to have an influence on metastatic tumors. Only angiostatin makes it to the distant sites. By stopping blood vessel growth, it could stop tumor growth and also the spread of tumors.