Positron Emission Tomography Imaging of Prostate Cancer with Ga-68-Labeled Gastrin-Releasing Peptide Receptor Agonist BBN_7–14 and Antagonist RM26

Abstract

Radiolabeled bombesin (BBN) analogs have long been used for developing gastrin-releasing peptide receptor (GRPR) targeted imaging probes, and tracers with excellent in vivo performance including high tumor uptake, high contrast, and favorable pharmacokinetics are highly desired. In this study, we compared the 68Ga-labeled GRPR agonist (Gln–Trp–Ala–Val–Gly–His–Leu–Met–NH2, BBN7–14) and antagonist (d-Phe–Gln–Trp–Ala–Val–Gly–His–Sta–Leu–NH2, RM26) for the positron emission tomography (PET) imaging of prostate cancer. The in vitro stabilities, receptor binding, cell uptake, internalization, and efflux properties of the probes 68Ga–1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)–Aca–BBN7–14 and 68Ga–NOTA–poly(ethylene glycol)3 (PEG3)–RM26 were studied in PC-3 cells, and the in vivo GRPR targeting abilities and kinetics were investigated using PC-3 tumor xenografted mice. BBN7–14, PEG3-RM26, NOTA–Aca–BBN7–14, and NOTA–PEG3–RM26 showed similar binding affinity to GRPR. In PC-3 tumor-bearing mice, the tumor uptake of 68Ga–NOTA–PEG3–RM26 remained at around 3.00 percentage of injected dose per gram of tissue within 1 h after injection, in contrast with 68Ga–NOTA–Aca–BBN7–14, which demonstrated rapid elimination and high background signal. Additionally, the majority of the 68Ga–NOTA–PEG3–RM26 remained intact in mouse serum at 5 min after injection, while almost all of the 68Ga–NOTA–Aca–BBN7–14 was degraded under the same conditions, demonstrating more-favorable in vivo pharmacokinetic properties and metabolic stabilities of the antagonist probe relative to its agonist counterpart. Overall, the antagonistic GRPR targeted probe 68Ga–NOTA–PEG3–RM26 is a more-promising candidate than the agonist 68Ga–NOTA–Aca–BBN7–14 for the PET imaging of prostate cancer patients.

Development of Improved Tumor-Residualizing, GRPR-Targeted Agents: Preclinical Comparison of an Endolysosomal Trapping Approach in Agonistic and Antagonistic Constructs

“…Examination of the structure-activity profile of this fragment has revealed numerous highly GRPR-binding derivatives with different pharmacologic profiles in terms of agonistic versus antagonistic character (4)(5)(6). Exceptional work by Maecke's group and others has identified antagonistic analogs (e.g., RM26) that have substantially higher tumor uptake and better nontarget clearance profiles than agonistic analogs (7)(8)(9). Despite encouraging clinical and preclinical results (10,11), the relatively short tumor residence time of low-molecular-weight constructs such as GRPR-targeted peptides still represents a translational hurdle, particularly with regard to radiotherapeutic uses (12).…”

Development and Characterization of a Novel, High-Affinity, Specific, Radiolabeled Ligand for BRS-3 Receptors

“…Generally, GPCR peptide antagonists are poorly and slowly internalized, if at all, whereas agonists are extensively and rapidly internalized (Mantey et al, 1993;Ginj et al, 2006;Moreno et al, 2016;Dalm et al, 2017). The above generalization is also true for BnRs, where numerous studies have reported that most BnR peptide antagonists are poorly internalized, if at all (Mantey et al, 1993;Cescato et al, 2008;Varasteh et al, 2013;Nock et al, 2017;Popp et al, 2017;Cheng et al, 2018). The internalization of radiolabeled Bantag-1 in our studies was not due to the possibility that Bantag-1 was functioning as a partial agonist, because in previous studies it has been reported to have no agonist activity at the BRS-3 receptor for the activation of phospholipase C, as assessed by either stimulation of the generation of phosphoinositides (Moreno et al, 2013) or mobilization of cellular calcium (Guan et al, 2010).…”

“…Recently, numerous studies have demonstrated that a number of common neoplasms overexpress BnRs (i.e., prostate, breast, lung, pancreas, and CNS tumors) (Reubi et al, 2002;Reubi, 2013;Moreno et al, 2016), and in some tumors such as lung cancers, BRS-3 is the most frequently overexpressed and is expressed in the highest numbers (Moreno et al, 2018). A number of recent studies report the ability of BnR radioligands to image various human tumors in humans, particularly breast and prostate, using high-affinity GRPR ligands (Sancho et al, 2011;Moreno et al, 2016;Ferreira et al, 2017;Cheng et al, 2018;Zang et al, 2018), and with the high expression of BRS-3 in lung tumors (Moreno et al, 2018) a similar approach might be used with radiolabeled Bantag-1. Furthermore, using 177 Lu-or 90 Y-labeled peptides, peptide-radioreceptor therapy has been shown to be effective and has been recently approved for the treatment of neuroendocrine tumors with radiolabeled somatostatin receptor ligands (Strosberg et al, 2016;Ito and Jensen, 2017).…”

Comparative evaluation of the new GRPR‐antagonist 111In‐SB9 and 111In‐AMBA in prostate cancer models: Implications of in vivo stability

“…Moreover, the localization of 68 Ga‐SB3 in a small number of therapy‐naïve prostate cancer patients could be well correlated with GRPR‐expression levels in the lesions, determined by receptor autoradiography on patient biopsy samples . We and others have previously compared GRPR‐radioantagonist and GRPR‐radioagonist pairs . However, selection of individual analogs for comparison in these studies has not taken into account important structural differences at the N‐terminus of peptide conjugates.…”

“…21 We and others have previously compared GRPR-radioantagonist and GRPR-radioagonist pairs. 19,[22][23][24][25] However, selection of individual analogs for comparison in these studies has not taken into account important structural differences at the N-terminus of peptide conjugates. Hence, the observed response differences could not be attributed exclusively to agonism/ antagonism at the GRPR.…”