In time, the primary tumor reappears, but a repeat injection causes it to regress again. Each time it reappears, the tumor is just as susceptible to treatment as before. This is not seen with conventional chemotherapy. Repeated exposure to chemotherapeutic drugs selects for the appearance of drug-resistant tumor cells. Eventually, further drug treatment is worthless.
Chemotherapy works directly on tumor cells which mutate easily .
Angiogenesis inhibitors don't work on the tumor cells but on normal cells involved in the formation of blood vessels. Some human tumors can be grown in immunodeficient mice. Treatment with endostatin caused these human tumor masses to shrink in their mouse host. Combined treatment with both angiostatin and endostatin has caused some primary mouse tumors to disappear entirely. The cells within plaque need a capillary supply and endostatin blocks its formation - at least in mice.