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In vivo Optical Molecular Imaging of Vascular Endothelial Growth Factor for Monitoring Cancer Treatment

Authors' Affiliation: Wellman Center for Photomedicine, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts

Requests for reprints: Tayyaba Hasan, Wellman Center for Photomedicine, Massachusetts General Hospital, Harvard Medical School, 40 Blossom Street, Boston MA 02114. Phone: 617-726-6996; Fax: 617-726-8566; E-mail: thasan{at}partners.org.

Purpose: Vascular endothelial growth factor (VEGF) expression is a critical component in tumor growth and metastasis. Capabilities to monitor VEGF expression in vivo can potentially serve as a useful tool for diagnosis, prognosis, treatment planning, monitoring, and research. Here, we present the first report of in vivo hyperspectral molecular imaging strategy capable of monitoring treatment-induced changes in VEGF expression.

Experimental Design: VEGF was targeted with an anti-VEGF antibody conjugated with a fluorescent dye and was imaged in vivo using a hyperspectral imaging system. The strategy was validated by quantitatively monitoring VEGF levels in three different tumors as well as following photodynamic treatment. Specificity of the molecular imaging strategy was tested using in vivo competition experiments and mathematically using a quantitative pharmacokinetic model.

Results: The molecular imaging strategy successfully imaged VEGF levels quantitatively in three different tumors and showed concordance with results from standard ELISA. Changes in tumoral VEGF concentration following photodynamic treatment and Avastin treatment were shown. Immunohistochemistry shows that (a) the VEGF-specific contrast agent labels both proteoglycan-bound and unbound VEGF in the extracellular space and (b) the bound VEGF is released from the extracellular matrix in response to photodynamic therapy. In vivo competition experiments and quantitative pharmacokinetic model-based analysis confirmed the high specificity of the imaging strategy.

Conclusion: This first report of in vivo quantitative optical molecular imaging-based monitoring of a secreted cytokine in tumors may have implications in providing tools for mechanistic investigations as well as for improved treatment design and merits further investigation.