Imaging of alpha(v)beta(3) Expression by a Bifunctional Chimeric RGD Peptide not Cross-Reacting with alpha(v)beta(5)

Purpose: To test whether a novel bifunctional chimeric peptide comprising a cyclic Arg- Gly-Asp pentapeptide covalently bound to an echistatin domain can discriminate alpavbeta3 from alpavbeta5 integrin, thus allowing the in vivo selective visualization of alpavbeta3 expression by single-photon and positron emission tomography (PET) imaging. Experimental Design: The chimeric peptide was preliminarily tested for inhibition of alphavbeta3-dependent cell adhesion and competition of 125I-echistatin binding to membrane of stably transfected K562 cells expressing alpavbeta3 (Kalpavbeta3 ) or alphavbeta5 (Kalpavbeta5) integrin. The chimeric peptide was then conjugated with diethylenetriaminepentaacetic acid and la- beled 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-termi- nal amino acids, with 18F for PET imaging. Nude mice bearing tumors from Kalpavbeta3 , Kalpavbeta5, 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 pep- tide selectively binds to alpavbeta3 integrin and does not cross-react with alpavbeta5. In agreement with in vitro findings, single-photon and PET imaging studies showed that the radiola- beled chimeric peptide selectively localizes in tumor xenografts expressing alpavbeta3 and fails to accumulate in those expressing alpavbeta5 integrin. When 18F-labeled truncated derivative was used for PET imaging, alpavbeta3 - and alpavbeta5-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 alpavbeta5 integrin, allows highly selective alpavbeta3 expression imaging and monitoring. (Clin Cancer Res 2009;15(16):5224-33)