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Effects of the solid tumour microenvironment on the penetration and distribution of trastuzumab Sy, Jonathan Tiu


The abnormal development of the solid tumour microenvironment may result in populations of tumour cells that receive sub-optimal drug exposure and as a consequence experience diminished efficacy of treatment. Antibody-based therapies may face additional limitations from their inherent molecular size and antigen binding specificity. Trastuzumab, a monoclonal antibody that targets the HER2 receptor, has been shown to have incomplete distribution in HER2 over-expressing tumour xenografts. The objective of this research was to examine the roles of various tumour microenvironment components, namely pericytes and epithelial tight junctions, in modulating extravasation of trastuzumab and its distribution throughout the tumour. To examine pericyte coverage, tumour bearing mice were administered imatinib to reduce pericyte recruitment. Fluorescent microscopy was used to assess blood vessel changes and directly visualized the effects these changes had on systemically administered trastuzumab. Imatinib treatment resulted in increased necrosis, decreased microvessel density and pericyte staining, and a reduction in the fraction of blood vessels in close proximity to pericytes. Interestingly, trastuzumab distribution became significantly reduced in imatinib treated tumours and blood vessel analysis revealed vessels possessing pericyte coverage to have considerably greater trastuzumab penetration. These results indicate that combinations of imatinib prior to trastuzumab dosing may not be optimal for purposes of increased drug distribution. However, the degree of pericyte coverage may contribute to the heterogeneous distribution of trastuzumab. To investigate the role of epithelial tight junctions (TJ), MCF7HER² multilayered cell cultures (MCC) were exposed to trastuzumab and bound antibody was visualized by immunohistochemistry. Trastuzumab was found to penetrate effectively from the “basolateral” side of the MCC attached to the anchoring membrane, whereas penetration into the apical, or “luminal,” cell layer was limited to the outermost row of cells even after prolonged exposure. Transmission EM revealed confluent TJ in the luminal cell layer and immunohistochemical staining of the peripheral TJ marker ZO-1 revealed positive staining in a continuous band along the luminal cell layer that was consistent with the localization of bound trastuzumab. Taken together, these data suggest that abnormal pericyte coverage and TJ localization are obstacles in the tumour microenvironment preventing the effective distribution of anti-cancer agents such as trastuzumab.

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