In other tumor models, antiangiogenic agents have failed to normalize the vasculature and have induced hypoxia [10, 11]. In the current study, sunitinib treatment reduced microvascular density, increased hypoxic fraction, induced necrosis, and did not alter IFP. Consequently, the treatment schedule applied here resulted in changes in the tumor microenvironment that argue against treatment-induced normalization. This observation is in line with our previous experience with A-07 and R-18 human melanoma xenografts growing in dorsal window chambers [11]. PD-332991 In that study, tumors were treated with two different

sunitinib doses and the Z-VAD-FMK molecular weight effect was assessed multiple times during the treatment period. The treatments did not improve vascular function at any time point, suggesting that sunitinib cannot normalize tumor vasculature in these melanoma xenografts. In tumors where antiangiogenic treatment induces hypoxia, neoadjuvant antiangiogenic therapy is expected to reduce the effect of radiation and chemotherapy [7, 8]. In contrast, neoadjuvant antiangiogenic therapy has been shown to enhance the effect of radiation or chemotherapy in preclinical tumors where antiangiogenic treatment normalizes the vasculature and the microenvironment [2, 3]. The current study suggests that DW-MRI and DCE-MRI can be used to identify tumors where antiangiogenic treatment does not normalize the microenvironment. These tumors

APR-246 purchase respond to antiangiogenic treatment with reduced K trans and increased ADC. Interestingly, increased K trans and reduced ADC have been reported in tumors where antiangiogenic treatment has normalized the vasculature and the microenvironment [14, 32]. Vascular normalization is a transient effect because tumors can switch to other angiogenesis pathways and become resistant to antiangiogenic agents. The duration of improved tumor oxygenation is also expected to be limited because the beneficial effects of vascular normalization may be balanced by severe vascular regression after prolonged exposure to antiangiogenic agents [31]. Winkler et al. demonstrated that VEGFR-2 blockade enhanced

the effect of radiation when the tumors were irradiated during the time window when the antiangiogenic agent normalized the vasculature and improved oxygenation [3]. They also showed that VEGFR-2 blockade did not enhance the effect of oxyclozanide radiation when tumors were irradiated before or after this time window, suggesting that the timing of combination therapies may be crucial to achieve maximal antitumor effect. Previous studies suggest that DW-MRI and DCE-MRI are sensitive to vascular normalization [14, 32], and the current study suggests that these techniques are also sensitive to microenvironmental effects that indicate no normalization. Taken together, these studies suggest that DW-MRI and DCE-MRI may be used to monitor the effect of antiangiogenic treatment to identify a potential normalization window.