Anti-cancer therapies combining antiangiogenic and tumor cell cytotoxic effects reduce the tumor stem-like cell fraction in glioma xenograft tumors Free

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Vascular endothelial cells have been identified as a critical component of the neural stem cell niche, raising the possibility that brain tumor stem-like cells (TSLC) may also rely on signaling interactions with nearby tumor vasculature to maintain their stem-like state. The disruption of such a TSLC vascular niche by an antiangiogenic therapy could result in loss of stemness characteristics associated with intrinsic drug resistance and thus preferentially sensitize TSLC to the effects of chemotherapy. Considering these possibilities, we investigated the impact of antiangiogenic anti-cancer therapy on the TSLC fraction of glioma tumors. Athymic nude mice bearing subcutaneous tumor xenografts of the C6 rat glioma cell line were treated with either a conventional maximum tolerated dose (MTD) schedule of cyclophosphamide (CTX), frequent low dose “metronomic” (LDM) CTX (which is known to work in part by an anti-angiogenic mechanism), the anti-mouse vascular endothelial growth factor receptor-2 targeting antibody DC101, combinations of DC101 with MTD or LDM CTX, or a variant anti-angiogenic schedule of LDM CTX that includes an up-front bolus dose of CTX. After 2 weeks of treatment, tumors were dissociated enzymatically and subjected to primary and secondary tumor sphere forming assays as a surrogate measurement of TSLC fraction. Targeted antiangiogenic therapy (DC101) or cytotoxic chemotherapy (MTD CTX) did not reduce the fraction of tumor sphere-forming units (SFU) in the tumor, whereas all other treatment groups, which combine both antiangiogenic and cytotoxic drug effects, caused a significant reduction in SFU. Notably, DC101 enhanced the SFU-reducing capacity of LDM CTX, and conferred SFU-reducing capacity to MTD CTX, demonstrating that inhibition of angiogenesis plays a significant role in the observed reductions in SFU fraction. The greatest decline in SFU was caused by LDM CTX + DC101, suggesting that low-level cytotoxic effects (as opposed to the stronger cytotoxic effects of bolus CTX doses) are optimal for reducing SFU fraction when combined with inhibition of angiogenesis. These results highlight the possibility that selective eradication of TSLC may be achieved by targeting the tumor microenvironment (and potentially a supportive TSLC niche) rather than the TSLC directly. Furthermore, this work suggests a possible novel effect of antiangiogenic therapy, namely as a chemosensitizer of TSLC, and thus represents a possible new mechanism to explain the ability of antiangiogenic therapy to enhance the efficacy of chemotherapy.