Antiangiogenic therapy inhibits the recruitment of vascular accessory cells to the perivascular niche in glioma angiogenesis.

OBJECTIVE: Antiangiogenic therapies could be limited by various escape mechanisms including bone marrow-derived myeloid cell-induced vasculogenesis. The recruitment of vascular accessory cells (VACs) to the tumor neovasculature is as a multistep process. However, the recruitment process of these cells during antiangiogenic treatment remains unknown. The aim of our study was to characterize the recruitment of VACs during antiangiogenic therapy using sunitinib. METHODS: C6 glioma cells were implanted into dorsal skinfold chambers. Animals received antiangiogenic therapy intraperitoneally for 5 days prior to VAC application intra-arterially. Intravital microscopy was performed during VAC injection and 1 and 48 h after injection. Analyses included total (TVD) and functional vessel densities (FVD), the perfusion index (PI), microvascular permeability, blood flow rate (Q), microvascular diameter (D), red blood cell velocity (RBCV), wall shear rate (γ), wall shear stress (τ), first and firm adhesions of VACs, and accumulation in the perivascular niche. RESULTS: Antiangiogenic therapy resulted in a significant reduction in TVD (365 ± 47 cm/cm(2) vs. 183 ± 37 cm/cm(2)) and FVD (227 ± 65 cm/cm(2) vs. 147 ± 25 cm/cm(2)) and an increase in PI, Q, D, and RBCV. γ and τ remained unaltered. Initial adhesion and firm adhesion were unaffected by antiangiogenic therapy; however, the accumulation in the perivascular niche was significantly diminished in treated tumors (53.7 ± 8% vs. 24.0 ± 17%). CONCLUSIONS: Antiangiogenic treatment inhibits the accumulation of VACs in the perivascular niche and therefore interferes with consecutive neovascularization.

Combined temozolomide and sunitinib treatment leads to better tumour control but increased vascular resistance in O6-methylguanine methyltransferase-methylated gliomas.

INTRODUCTION: Combined antiangiogenic and cytotoxic treatment represents an appealing treatment approach for malignant glioma. In this study we characterised the antitumoural and microvascular consequences of sunitinib (Su) and temozolomide (TMZ) therapy and verified the ideal treatment protocol, with special focus on a potential therapeutic window for combined scheduling. MATERIALS AND METHODS: O(6)-Methylguanine methyltransferase (MGMT) status was analysed by pyrosequencing. Tumour growth of subcutaneous xenografts was assessed under different treatment protocols (TMZ, SU, SU followed by TMZ, TMZ followed by SU, combined TMZ/SU). Intravital microscopy (dorsal skinfold chamber model) assessed microvascular consequences. Immunohistochemistry included tumour and endothelial cell proliferation, apoptosis and vascular pericyte coverage. Real-time polymerase chain reaction (RT-PCR) analysed the expression of angiogenesis-related pathways in response to therapy. RESULTS: Combined TMZ/SU resulted in significantly reduced tumour growth compared to either monotreatment (TMZ: 106 ± 13 mm(3); SU: 114 ± 53 mm(3); TMZ/SU: 34 ± 7 mm(3)) by additional antiangiogenic effects and synergistic induction of apoptosis versus TMZ monotreatment. Sequential treatment protocols did not show additive antitumour responses. TMZ/SU aggravated vascular resistance mechanisms characterised by significantly higher blood flow rate (TMZ: 74 ± 34 µl/s; SU: 164 ± 36 µl/s; TMZ/SU: 254 ± 95 µl/s), reduced permeability (TMZ: 1.05 ± 0.02; SU: 0.99 ± 0.07; TMZ/SU: 0.89 ± 0.05) and recovery of pericyte-endothelial interactions (TMZ: 89 ± 7%; SU: 67 ± 9%, TMZ/SU:80 ± 10%) versus either monotreatment. Vascular resistance was paralleled by an increase in Ang-1 and Tie-2 and by the downregulation of Dll4. CONCLUSION: Sequential application of TMZ and SU in the angiogenic window does not add antitumour efficacy to monotherapy. Simultaneous application yields beneficial tumour control due to additive antiangiogenic and proapoptotic effects. Combined treatment may aggravate pericyte-mediated vascular resistance mechanisms by altering Ang-1-Tie-2 and Dll4/Notch pathways.