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 not known. It has been assumed that bevacizumab solely targets circulating VEGF-A in blood. 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 xenograft mouse model of GBM. In vitro, CSCs (CD133+) from GBM rapidly internalized either bevacizumab or human IgG into membrane protrusions that contained actin and internalization was significantly inhibited by a macropinocytosis inhibitor (EIPA), suggesting CSCs internalize bevacizumab or human IgG via macropinocytosis. Furthermore, bevacizumab or human IgG was largely detected in the Rab4+ “fast” recycling compartment at 5 min, and both were largely detected in the LAMP1+ compartment (late endosome/lysosome) at 3 hr in the CSCs. CSCs (CD133+) from GBM do not express the neonatal Fc receptor, the canonical pathway for recycling of IgG. Administration of bevacizumab to an orthotopic xenograft mouse model of established GBM showed that bevacizumab was partially co-localized with Rab4+ or with LAMP1+ in perivascular tumor cells, consistent with our in vitro findings. Taken together, our data show that in GBM, humanized IgG, including bevacizumab, gains access to the perivascular tumor space and is then macropinocytosed by CSCs and trafficked to a recycling compartment or to the late endosome/lysosome. 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.
Citation Format: Gaelle Muller-Greven, Cathleen Carlin, Steven Toms, Manmeet Ahluwalia, Markus Bredel, Justin Lathia, Jeremy Rich, Petra Hamerlik, Candece L. Gladson. The tissue and cellular destination of therapeutic IgGs in glioblastoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3276.