Imaging of αvβ3 Expression by a Bifunctional Chimeric RGD Peptide not Cross-Reacting with αvβ5

Purpose: To test whether a novel bifunctional chimeric peptide comprising a cyclic Arg-Gly-Asp pentapeptide covalently bound to an echistatin domain can discriminate alpha(v)beta(3) from alpha(v)beta(5) integrin, thus allowing the in vivo selective visualization of alpha(v)beta(3) expression by single-photon and positron emission tomography (PET) imaging. Experimental Design: The chimeric peptide was preliminarily tested for inhibition of alpha(v)beta(3)-dependent cell adhesion and competition of I-125-echistatin binding to membrane of stably transfected K562 cells expressing alpha(v)beta(3) (K alpha(v)beta(3)) or alpha(v)beta(5) (K alpha(v)beta(5)) integrin. The chimeric peptide was then conjugated with diethylenetriaminepentaacetic acid and labeled with In-111 for single-photon imaging, whereas a one-step procedure was used for labeling the full-length peptide and a truncated derivative, lacking the last five C-terminal amino acids, with F-18 for PET imaging. Nude mice bearing tumors from K alpha(v)beta(3), K alpha(v)beta(5), U87MG human glioblastoma, and A431 human epidermoid cells were subjected to single-photon and PET imaging. Results: Adhesion and competitive binding assays showed that the novel chimeric peptide selectively binds to alpha(v)beta(3) integrin and does not cross-react with alpha(v)beta(5). In agreement with in vitro findings, single-photon and PET imaging studies showed that the radiolabeled chimeric peptide selectively localizes in tumor xenografts expressing alpha(v)beta(3) and fails to accumulate in those expressing alpha(v)beta(5) integrin. When F-18-labeled truncated derivative was used for PET imaging, alpha(v)beta(3)- and alpha(v)beta(5)-expressing tumors were visualized, indicating that the five C-terminal amino acids are required to differentially bind the two integrins. Conclusion: Our findings indicate that the novel chimeric Arg-Gly-Asp peptide, having no cross-reaction with alpha(v)beta(5) integrin, allows highly selective alpha(v)beta(3) expression imaging and monitoring. (Clin Cancer Res 2009;15(16):5224-33)