TNP-470 promotes initial vascular sprouting in xenograft tumors

TNP-470 (AGM-1470), an analogue of fumagillin, was one of the first molecules proposed to have antiangiogenic properties. This concept was based on its ability to inhibit both endothelial proliferation in vitro and tumor growth in vivo in a number of xenograft models. Yet, subsequent investigations indicated that the biochemical activities associated with TNP-470 are not selective for endothelial cells. Moreover, recent evidence suggests that this agent inhibits tumor growth in vivo, but without a corresponding decrease in angiogenesis. Therefore, we performed a detailed comparison of TNP-470 to a validated antiangiogenic agent, a VEGF inhibitor termed VEGF-Trap, using a xenograft model of Wilms tumor. Treatment with TNP-470 for 5 weeks significantly suppressed xenograft growth (83%). Surprisingly, this inhibition was not associated with a decrease in angiogenesis, but instead with an increase in tiny neovessels. To determine whether this was a direct effect of TNP-470 on tumor vessels, we examined its effect in a short-term assay using large tumors with established vasculature. In contrast to treatment with VEGF-Trap, which led to rapid vessel regression and tumor hypoxia, tumors exposed to TNP-470 for 1 day displayed increased capillary sprouting, with significantly increased microvessel density, vessel length, and branch points. TNP-470 did not induce tumor hypoxia as demonstrated by minimal pimonidazole staining and VEGF expression. TNP-470 did, however, cause a marked increase in apoptosis of tumor cells. Our results indicate that the antitumor effects of TNP-470 cannot be attributed to prevention of neoangiogenesis, but instead to its direct action on tumor cells.