N-terminal modifications improve the receptor affinity and pharmacokinetics of radiolabeled peptidic gastrin-releasing peptide receptor antagonists: Examples of 68Ga-and 64Cu-labeled peptides for PET imaging

摘要

Conclusion: We have developed 2 GRPr antagonistic radioligands, which are improved in terms of binding affinity and overall biodistribution profile. Their promising in vivo pharmacokinetic performance may contribute to the improvement of the diagnostic imaging of tumors overexpressing GRPr.Methods: The potent GRPr antagonist MJ9, Pip-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (Pip, 4-amino-1-carboxymethyl-piperidine), was conjugated to 1,4,7-triazacyclononane, 1-glutaric acid-4,7 acetic acid (NODAGA), and 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) and radiolabeled with 68Ga and 64Cu. The GRPr affinity of the corresponding metalloconjugates was determined using 125I-Tyr4-BN as a radioligand. The labeling efficiency of 68Ga3+ was compared between NODAGA-MJ9 and NOTA-MJ9 in acetate buffer, at room temperature and at 95°C. The 68Ga and 64Cu conjugates were further evaluated in vivo in PC3 tumor xenografts by biodistribution and PET imaging studies.Results: The half maximum inhibitory concentrations of all the metalloconjugates are in the high picomolar-low nanomolar range, and these are the most affine-radiolabeled GRPr antagonists we have studied so far in our laboratory. NODAGA-MJ9 incorporates 68Ga3+ nearly quantitatively (98%) at room temperature within 10 min and at much lower peptide concentrations (1.4 × 10-6 M) than NOTA-MJ9, for which the labeling yield was approximately 45% under the same conditions and increased to 75% at 95°C for 5 min. Biodistribution studies showed high and specific tumor uptake, with a maximum of 23.3 ± 2.0 percentage injected activity per gram of tissue (%IA/g) for 68Ga-NOTA-MJ9 and 16.7 ± 2.0%IA/g for 68Ga-NODAGA-MJ9 at 1 h after injection. The acquisition of PET images with the 64Cu-MJ9 conjugates at later time points clearly showed the efficient clearance of the accumulated activity from the background already at 4 h after injection, whereas tumor uptake still remained high. The high pancreas uptake for all radiotracers at 1 h after injection was rapidly washed out, resulting in an increased tumor-to-pancreas ratio at later time points.Gastrin-releasing peptide receptors (GRPrs) are overexpressed on a variety of human cancers, providing the opportunity for peptide receptor targeting via radiolabeled bombesin-based peptides. As part of our ongoing investigations into the development of improved GRPr antagonists, this study aimed at verifying whether and how Nterminal modulations improve the affinity and pharmacokinetics of radiolabeled GRPr antagonists.