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Noninvasive Monitoring of Gene Transfer Using a Reporter Receptor Imaged with a High-Affinity Peptide Radiolabeled with ^99mTc or ¹⁸⁸Re

Departments of Radiology, Radiation Oncology, and Pathology, University of Alabama at Birmingham, Birmingham, Alabama

Correspondence: For correspondence or reprints contact: Kurt R. Zinn, DVM, PhD, University of Alabama at Birmingham, Dept. of Radiology, Division of Nuclear Medicine, BDB 11, 1530 3rd Ave. S., Birmingham, AL 35294-0012.

ABSTRACT

Gene therapy protocols require better modalities to monitor the location and level of transferred gene expression. One potential in vivo mechanism to assess gene expression would be to image the binding of a radiolabeled peptide to a reporter receptor that is expressed in targeted tissues. This concept was tested in a tumor model using a replication-incompetent adenoviral vector encoding the human type 2 somatostatin receptor (Ad5-CMVhSSTr2). Expression of the hSSTr2 reporter was imaged using a radiolabeled, somatostatin-avid peptide (P829). Methods: Bilateral subcutaneous A427 tumor xenografts were established on the flanks of athymic nude mice. These human-origin, non-small cell lung tumors are normally negative for hSSTr2 expression. One tumor was injected directly with Ad5-CMVhSSTr2, whereas the second tumor was injected directly with a control Ad5 vector. The mice were injected intravenously 48 h later with P829 peptide that was radiolabeled to high specific activity with ^99mTc (half-life, 6 h) or ¹⁸⁸Re (half-life, 17 h). Tumors were frozen and evaluated for somatostatin receptor expression using fluorescein-labeled somatostatin. Results: The accumulation of radiolabeled P829 in hSSTr2-expressing tumors was easily visualized by camera imaging 3 h after injection. Imaging region of interest analyses and biodistribution studies confirmed a 5- to 10-fold greater accumulation of both radiolabeled P829 peptides in the Ad5-CMVhSSTr2-injected tumors versus control tumors injected with control Ad5 vectors. Ad5-CMVhSSTr2-injected tumors accumulated 2.5-3.8 percentage injected dose per gram 3 h after injection. Only Ad5-CMVhSSTr2-injected tumors expressed somatostatin receptors, as determined by immunohistochemistry. Conclusion: These studies show the feasibility of imaging a ^99mTc-labeled peptide's binding to a reporter receptor after in vivo gene transfer to tumor cells. The ¹⁸⁸Re-labeled peptide worked equally well for this imaging approach and offers the additional advantage of energetic p decay with potential therapeutic efficacy. ^99mTc and ¹⁸⁸Re are generator produced, an advantage for widespread availability and low cost, and both radioisotopes can be imaged with existing, high-resolution modalities. There is great potential for using ^99mTc-labeled peptides for imaging gene transfer with the hSSTr2 reporter receptor, especially when the reporter correlates with the expression of therapeutic genes that can be included simultaneously in the gene therapy vector.

Key Words: imaging • gene transfer • somatostatin receptor • peptide • ^99mTc • ¹⁸⁸Re

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