Selection of Radiolabeled Gastrin Analogs for Peptide Receptor-Targeted Radionuclide Therapy

Abstract

The gastrin/cholecystokinin-2 (CCK-2) receptor has been identified as a possible target for peptide receptor radionuclide imaging and therapy. Several radiolabeled peptides binding to this receptor have been explored in animal models and clinical trials but either low tumor uptake or high renal retention has been found. The aim of this study was to identify a peptide with improved tumor-to-kidney pharmacodynamics when compared with current candidates. Methods: A small peptide-chelator library of 34 compounds based on the C-terminal sequences of CCK-8 or minigastrin was constructed. The peptides were radiolabeled with 111 In with high labeling efficiency (.90%), as determined by high-performance liquid chromatographic analysis. The labeled peptides were screened by assessing tumor and kidney uptake in pancreatic xenograft nude mouse models, including AR42J. An extensive biodistribution analysis was performed on the lead candidate from the library. Results: Minigastrin analogs containing a pentaglutamate sequence showed the highest tumor uptake but very high renal retention. CCK analogs showed the lowest tumor and renal uptake. Deletion of the pentaglutamate sequence in the gastrin analogs lowered the tumor uptake by a factor of 3 but decreased the kidney uptake by a factor of 20. Insertion of histidine residues in the sequence reduced kidney uptake by a further factor of almost 2-fold. In AR42J tumor-bearing mice, the peptide with the sequence DOTA-HHEAYGWMDF-NH 2 (DOTA is tetraazacyclododecane tetraacetic acid) showed the highest tumor-to-kidney ratio of all peptides studied, with saturable uptake in target organs and low uptake by nontarget tissues other than the kidney. Conclusion: This peptide is a worthwhile candidate for clinical studies to determine whether it is suitable for use in peptide receptor-targeted radionuclide therapy.

Biodistribution of 177Lu-octreotate and 111In-minigastrin in female nude mice transplanted with human medullary thyroid carcinoma GOT2

“…In comparison with other studies performed with 111 In-labeled MG0 on BALB/c mice, bearing various tumor types, the highest tumor uptake was found in the CCK2/gastrin receptor transfected cell lines (6.1 and 13%IA/g vs. 0.79%IA/g for GOT2 after 1 h) (31,32,50) (Table IV). The uptake in the rat endocrine pancreatic tumor model AR42J was higher than in GOT2, but lower than in the transfected tumors (50).…”

“…The uptake in the rat endocrine pancreatic tumor model AR42J was higher than in GOT2, but lower than in the transfected tumors (50).…”

Renal Toxicity of Radiolabeled Peptides and Antibody Fragments: Mechanisms, Impact on Radionuclide Therapy, and Strategies for Prevention

“…Removal of the anionic Nterminal pentaglutamic acid sequence of minigastrin analogs, or replacement by a histidine-histidine-glutamic acid sequence, resulted in a marked decrease in kidney uptake. Again this was accompanied by a decrease in tumor uptake but an increased tumor-to-kidney ratio (48). Replacing minigastrin's pentaglutamic acid sequence by a neutral pentaglutamine sequence reduced uptake in tumors and organs positive for the cholecystokinin 2 receptor more than it reduced renal uptake (3).…”

99mTc-HYNIC-Gastrin Peptides: Assisted Coordination of 99mTc by Amino Acid Side Chains Results in Improved Performance Both In Vitro and In Vivo

“…In an acidic environment, however, such as occurs within the lysosomal compartment, the net negative charge of the peptide will be reduced and this may affect intracellular metabolism. In a previous study, we found that the insertion of histidine can also enhance the performance of indium- 1,4,7,10-tetraazacyclododecane-N,N9,N$,N9$-tetraacetic acidlabeled peptides (17), and this phenomenon is therefore worthy of further investigation. Two other groups have explored the use of 99m Tc-labeled minigastrin analogs in the nude mouse AR42J xenograft model.…”

“…Von Guggenberg et al have compared the biodistribution of peptides labeled using both the HYNIC and the tricarbonyl labeling systems (18,19), and Nock et al have studied that of a tetraamine conjugate (20). However, most of these studies used peptides containing the gastrin pentaglutamate motif, which results in moderate increases in tumor uptake but high renal retention, compared with the truncated sequence studied herein (17). In our experience, caution should, in any case, be exercised in directly comparing results from different centers-even when apparently the same tumor cell line is used-because of differences in levels of receptor expression observed in different laboratories.…”