Therapy resistance in solid tumors is of growing concern due to the failure of multiple therapeutic approaches. Targeting tumor cells alone with chemotherapy, tumor vasculature with antiangiogenic therapies (AAT) and tumor-infiltrating myeloid cells with CSF1R inhibitor have all lead to the development of refractory tumors with greater relapse rates. There is an urgent need to understand molecular mechanisms of therapy resistance in cancer. There could be tumor cell extrinsic and intrinsic mechanisms in the tumor microenvironment. We focused our study to investigate tumor cell intrinsic pathways using glioblastoma (GBM) as a model tumor and AAT (antiVEGF-VEGFR) as a model therapy. The benefits of AAT are transient with increased relapse owing to adaptive responses by the GBM. Our preclinical study and in vitro data, for the very first time, identified that AAT induces transdifferentiation of tumor cells into endothelial-like cells, capable of forming functional blood vessels in the growing tumors, termed as vasculogenic mimicry (VM). We observed that anti-VEGFR2 (Vatalanib) induced VM vessels are positive for periodic acid-Schiff (PAS) matrix but devoid of any endothelium on the inner side and lined by tumor cells on the outer-side. The PAS+ matrix is positive for basal laminae (laminin) indicating vascular structures. Vatalanib treated GBM displayed various stages of VM such as initiation (mosaic), sustenance, and full-blown VM. In addition to this, vatalanib treated tumors show significantly increased Laminin positive loops characteristic of VM in tumor center as well as at the periphery. A positive correlation was observed between the VM-like structures and the tumor size. We also performed in vitro tube formation assays with AAT treated GBM cells alone and HUVEC cells (co-culture) to confirm the role of GBM cells in the formation of mosaic vessels in normoxic conditions. Interestingly, tumor cells are incorporated into the tubes formed by HUVEC cells. We found a higher number of complete tube like structures with AAT treated tumor cells as compared to control. Cytokine array with the condition media from tumor cells treated with AAT showed a significant upregulation in the levels of IL8. We observed a significant increase in the CXCR1+ and CXCR2+ endothelial-like GBM cells following treatment with AAT. Ongoing investigations are focused on study of IL8-CXCR1/2 pathway in VM regulation using loss or gain of function approaches. The study will identify critical mediators of VM in GBM. In clinics, discovering novel targets causing VM associated therapy resistance is essential for identifying subset of patients that could be treated with alternate regimens.

Citation Format: Kartik Angara, Mohammad Rashid, Thaiz Borin, Bhagelu Achyut, Meenu Jain, ASM Iskander, Roxan Ara, Ali Arbab. Vascular mimicry mediated mechanisms drive therapy resistance in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 787. doi:10.1158/1538-7445.AM2017-787