P02.24.A MOLECULAR AND MORPHOLOGICAL FEATURES OF BLOOD VESSELS IN TUMOUR MARGIN REGIONS OF GLIOBLASTOMA ARE DISTINCT FROM TUMOUR CORE AND NORMAL BRAIN, AND DRUG DELIVERY IS REDUCED

Abstract BACKGROUND Glioblastoma (GBM) is an aggressive cancer that infiltrates the surrounding brain. Tumour margin regions include functioning brain hence cannot be surgically resected and are a crucial target for systemic therapies. While the blood-brain barrier (BBB) is compromised in contrast-enhancing ‘tumour core’ regions of GBM, little is known about its composition and function in tumour margins. To address this knowledge gap we compared functional and structural features of the BBB in GBM core and margin regions and adjacent normal brain. MATERIAL AND METHODS Longitudinal measurement (11 samples, 8 hours) of free extracellular levels of PARP inhibitors pamiparib (high brain penetrance) and olaparib (low penetrance) was performed by microdialysis in tumour core, margin and contralateral brain in GBM xenografted CD1 nude mice. Human GBM samples of core, margin and adjacent brain underwent multiplex immunohistochemical (IHC) staining for CD31 (endothelium), laminin (basement membrane), claudin-5 (tight junctions), PDGFRβ (pericytes), actin (smooth muscle) for quantitative morphological and molecular analysis. RESULTS Olaparib and pamiparib concentrations peaked at 60-90 minutes. By 8 hours, olaparib was undetectable while pamiparib levels had fallen to c.50% of peak. While no difference was observed between mean pamiparib levels in core and margin, olaparib concentrations were significantly lower in margin than core. Morphological analysis of CD31 stained vessels showed those in margin regions to be distinct from core and more similar to normal brain. Vessel density was lowest in margin regions and highest in core. From morphological and IHC data we generated a Linear Discriminant Analysis model which revealed 4 categories of vessels: a) tumor core specific, b) margin specific, c) normal brain specific and d) common to all regions. Core vessels were large with high levels of laminin, claudin-5 and PDGFRβ; normal brain vessels were smaller with lower levels and tightly defined distributions of CD31, laminin, claudin-5 and PDGFRβ. Margin vessels were morphologically similar to normal but exhibited higher levels of claudin-5 (no difference in other markers). Levels of CD31, laminin and PDGFRβ were twofold higher in core than margin vessels whereas claudin-5 staining was twofold higher in margin. The common type accounted for c.50% of margin, 30% of normal and 2% of core vessels; these exhibited diffuse laminin staining and varying levels of claudin-5. CONCLUSION Dynamic microdialysis studies show that the PARP inhibitor olaparib penetrates tumour margin regions at lower concentrations than core. Towards understanding this effect, multiplex IHC has revealed two types of vessels in margin regions: almost 50% resembled normal brain vessels but with altered levels of claudin-5, while the others shared morphological properties with a subpopulation of tumour core vessels.