Imaging of α_vβ₃ expression by a bifunctional chimeric RGD peptide not cross-reacting with α _vβ⁵

Purpose: To test whether a novel bifunctional chimeric peptide comprising a cyclic Arg-Gly-Asp pentapeptide covalently bound to an echistatin domain can discriminate α_vβ₃ from α _vβ₅ integrin, thus allowing the in vivo selective visualization of α_vβ₃ expression by single-photon and positron emission tomography (PET) imaging. Experimental Design: The chimeric peptide was preliminarily tested for inhibition of α_vβ₃-dependent cell adhesion and competition of 125I-echistatin binding to membrane of stably transfected K562 cells expressing α_vβ₃ (Kα_vλ ₃) or α_vβ₅ (Kα _vβ₅) integrin. The chimeric peptide was then conjugated with diethylenetriaminepentaacetic acid and labeled with ¹¹¹In 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 ¹⁸F for PET imaging. Nude mice bearing tumors from Kα_vβ₃, Kα_vβ₅, 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 α_vβ₃ integrin and does not cross-react with α_vβ₅. In agreement with in vitro findings, single-photon and PET imaging studies showed that the radiolabeled chimeric peptide selectively localizes in tumor xenografts expressing α_vβ₃ and fails to accumulate in those expressing α_vβ₅ integrin. When ¹⁸F-labeled truncated derivative was used for PET imaging, α_vβ₃- and α_vβ 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 α_vβ₅ integrin, allows highly selective α_vβ₃ expression imaging and monitoring.