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.

FIGURE 1

Biodistribution of H2-Met in AR42J-bearing nude mice, between 1 and 72 h after intravenous administration. Greatest initial uptake is observed in tumor and kidney but washout from kidney is faster than that from tumor. Four-hour blocking data indicate biodistribution when 100 μg of unlabeled peptide were coadministered with radiolabeled peptide. Significant blocking effect (P < 0.0005) is seen in blood, tumor, and stomach, but no other organs. int = intestine.

FIGURE 2

Effect of peptide dose on biodistribution. Results are expressed as percentage change in uptake compared with that obtained with “standard” dose of 0.2 μg. No significant dose effect was observed in any tissue other than tumor and stomach. Increasing the dose above 0.2 μg reduced uptake in both tissues. Decreasing the dose had no effect on tumor uptake but increased uptake in the stomach.

FIGURE 3

Fused SPECT/CT image shows biodistribution of ¹¹¹In-H2-Met in AR42J-bearing nude mouse, 4 h after intravenous injection. Heterogeneous uptake can be observed in tumor with little uptake in central organs.

FIGURE 4

HPLC chromatograms of ¹¹¹In-H2-Met prepared with and without addition of antioxidant MTG, where trace a contained no MTG (resulting in 12.3% oxidized peptide, R_t = 19.6 min) and trace b contained 0.04 mg MTG (resulting in 5.9% oxidized peptide, R_t = 19.2 min).