In vivo tumor angiogenesis imaging with site-specific labeled 99mTc-HYNIC-VEGF

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Abstract
Purpose We recently developed a cysteine-containing peptide tag (C-tag) that allows for site-specific modification of C-tag-containing fusion proteins with a bifunctional chelator, HYNIC (hydrazine nicotinamide)-maleimide. We then constructed and expressed C-tagged vascular endothelial growth factor (VEGF) and labeled it with HYNIC. We wished to test 99mTc-HYNIC-C-tagged VEGF (99mTc-HYNIC-VEGF) for the imaging of tumor vasculature before and after antiangiogenic (low continuous dosing, metronomic) and tumoricidal (high-dose) cyclophosphamide treatment. Methods HYNIC-maleimide was reacted with the two thiol groups of C-tagged VEGF without any effect on biologic activity in vitro. 99mTc-HYNIC-VEGF was prepared using tin/tricine as an exchange reagent, and injected via the tail vein (200–300 μCi, 1–2 μg protein) followed by microSPECT imaging 1 h later. Results Sequencing analysis of HYNIC-containing peptides obtained after digestion confirmed the site-specific labeling of the two accessible thiol groups of C-tagged VEGF. Tumor vascularity was easily visualized with 99mTc/VEGF in Balb/c mice with 4T1 murine mammary carcinoma 10 days after implantation into the left axillary fat pad in controls (12.3±5.0 tumor/bkg, n=27) along with its decrease following treatment with high (150 mg/kg q.o.d. ×4; 1.14±0.48 tumor/bkg, n=9) or low (25 mg/kg q.d. ×7; 1.03±0.18 tumor/bkg, n=9) dose cyclophosphamide. Binding specificity was confirmed by observing a 75% decrease in tumor uptake of 99mTc/biotin-inactivated VEGF, as compared with 99mTc-HYNIC-VEGF. Conclusion 99mTc can be loaded onto C-tagged VEGF in a site-specific fashion without reducing its bioactivity. 99mTc-HYNIC-VEGF can be rapidly prepared for the imaging of tumor vasculature and its response to different types of chemotherapy.