PP09. INTEGRIN AVB3 AS A POTENTIAL TARGET FOR IMAGING ANGIOGENESIS IN BRAIN METASTASES
Authors: Ms Jessica Buck, Dr Manuel Sarmiento Soto, Prof Nicola R. Sibson
Journal: Neuro-Oncology (2017)

Molecular imaging with probes targeting integrin avb3 have shown promise in imaging angiogenesis in subcutaneous tumours across different modalities. However, brain metastases have been shown to grow co-optively along the existing vasculature in the early stages of development, and do not exhibit angiogenesis until later stages. This angiogenic switch could represent an important point in metastasis for the application of therapeutic strategies. The purpose of this study was to determine whether avb3 expression increases over time in a mouse model of brain metastasis and presents a potential molecular imaging target. 10,000 4T1-GFP murine metastatic mammary carcinoma cells were injected into the left striatum of BALB/c mice (n=14), with 3 mice receiving a control injection of saline. Mice underwent perfusion-fixation at Day 7 (n=4), 14 (n=6), 28 (n=2) and 35 (n=2) after intracerebral injection. The brains were extracted, sectioned, and consecutive sections were stained for avb3, CD31 (endothelial marker) and angiogenesis marker TIE2 (angiopoietin receptor). The total stained area and the number of blood vessels positive for each marker was determined for each section. Four large tumours had a clear core structure and in these tumours the same parameters were analysed in both core and invasive edge regions of the tumour. The expression of angiogenic markers was found to increase with tumour size. A strong correlation was found between the total stained area and the percentage of tumour area stained for avb3 (R2=0.9613;Fig. 1D), CD31 (R2=0.9441; Fig 1E) and TIE2 (R2=0.7; Fig 1F). The number of vessels per mm2 of tumour that expressed avb3 appeared highest at the Day 7 timepoint, and did not correlate with tumour size. The number of microvessels (CD31) was found to increase with tumour size. In contrast, the number of TIE2 positive vessels was low in the Day 7 and 14 timepoint tumours, appearing only at later timepoints. In four large tumours where core and invasive edge regions were analysed, greater expression of all markers was seen in the core compared to the edge region (Fig. 1G-I), with no necrosis observed. Figure 1: Expression of angiogenic markers (A) avb3, (B) CD31 and (C) TIE2 in consecutive sections of brain metastases (scale bar 20µm). The expression of each marker correlates strongly with the size of the tumour (D, E, F), and expression is higher in the tumour core than at the invasive edge (G, H, I). The expression of the angiogenic markers avb3 and TIE2, and microvessel density, increased with tumour size, which is consistent with results observed in solid tumours. This effect was more pronounced in the core of the tumour compared to the invasive edge, where co-optive growth of micrometastases along existing vasculature showed lower levels of angiogenic markers. The high levels of avb3 expression seen in the Day 7 timepoint tumours likely reflects inflammation of existing vasculature, as avb3 is involved in mediating both inflammation and angiogenesis. This lack of specificity of avb3 for angiogenic vessels suggests that it may not be the optimal molecular target for the imaging of angiogenesis in brain metastasis.