STMC-30. TRAFFICKING AND EFFECT ON SURVIVAL OF BEVACIZUMAB IN GLIOBLASTOMA

Abstract

Most patients with recurrent glioblastoma (GBM) are treated with bevacizumab, a humanized monoclonal antibody (mAb) that binds VEGF-A and inhibits its binding to VEGFR. Approximately 30% of GBM patients are non-responsive to bevacizumab and the underlying mechanism for the lack of response is unknown. It has been assumed that bevacizumab solely targets circulating VEGF-A. We hypothesized that bevacizumab and human IgGs in general gain access to the perivascular niche that contains cancer stem cells (CSCs) in GBM. We found that bevacizumab gains access to the perivascular tumor area through leaky blood vessels and was internalized by tumor cells in an orthotopic established xenograft mouse model of GBM. In vitro, CSCs (CD133+) from GBM rapidly internalized bevacizumab into membrane protrusions that contained actin and internalization was significantly inhibited by a macropinocytosis inhibitor (EIPA), suggesting CSCs internalize bevacizumab via macropinocytosis. Furthermore, bevacizumab or human IgG were largely detected in the Rab4+ “fast” recycling compartment at 5 min, and in the LAMP1+ compartment (late endosome/lysosome) at 3 hr in the CSCs. We observed similar trafficking patterns for bevacizumab in our in vivo model. CSCs from GBM do not express the neonatal Fc receptor, the canonical pathway for recycling of IgG. Bevacizumab induces autophagy in CSCs due to VEGF deprivation and this mechanism is blocked with the addition of growth factors. Taken together, our data show that in GBM, bevacizumab gains access to the perivascular tumor space, is macropinocytosed by CSCs where it is trafficked to a recycling compartment or to the late endosome/lysosome, and that it induces autophagy promoting survival of these tumor cells. These data suggest that alterations in endocytosis or recycling in the CSCs could impact the fate of therapeutic IgGs like bevacizumab and ultimately influence a patients’ response to GBM therapy.