In vivo and in vitro characterization of CCK8 bearing a histidinebased chelator labeled with 99mTctricarbonyl

The development of receptor targeting radiolabeled ligands has gained much interest in recent years for diagnostic and therapeutic applications in nuclear medicine. Cholecystokinin CCK receptors have been shown to be overexpressed in a subset of neuroendocrine and other tumors. We are evaluating binding and biodistribution properties of a CCK8 peptide derivative labeled with 99mTcItricarbonyl. The CCK8 peptide was modified at its Nterminus by adding to its Nterminus two lysine–histidine modules KH, where histidine is coupled to the side chain of the lysine KH2CCK8. 99mTcItricarbonyl was generated with the IsoLink™ kit. A431 cells stably transfected with a cDNA encoding for the human CCK2 receptor were utilized to determine binding affinity, internalization, and retention of the labeled peptide, in comparison with wildtype A431 cells. A nude mouse tumor model was obtained by generating A431CCK2R and A431control tumors in opposite flanks of the animals. High specific activity labeling with 99mTc was achieved. In A431CCK2R cells, specific saturable binding was observed as well as evident internalization of the radiolabeled peptide after binding. Biodistribution experiments showed rapid, specific localization of KH2CCK8 on A431CCK2R xenografts compared with control tumors, although absolute uptake values were not markedly higher compared with background activity. Clearance of unbound radioactivity was both urinary and hepatobiliary. In imaging experiments, while targeting to CCK2R positive tumors could be appreciated, there was poor contrast between target and nontarget areas. KH2CCK8 shows adequate in vitro and in vivo properties for CCK2R targeting although improvement of biodistribution warrant further development. © 2008 Wiley Periodicals, Inc. Biopolymers Pept Sci 90: 707–712, 2008.This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymerswiley.com