99mTc Radiotracers Based on Human GRP(18-27): Synthesis and Comparative Evaluation

Synthesis and Evaluation of 68Ga- and 177Lu-Labeled [diF-Pro14]Bombesin(6-14) Analogs for Detection and Radioligand Therapy of Gastrin-Releasing Peptide Receptor-Expressing Cancer

Background/Objectives: Overexpressed in various solid tumors, the gastrin-releasing peptide receptor (GRPR) is a promising target for cancer diagnosis and therapy. However, the high pancreas uptake of the current clinically evaluated GRPR-targeted radiopharmaceuticals limits their applications. In this study, we replaced the Pro14 residue in our previously reported GRPR-targeted LW02056 and ProBOMB5 with 4,4-difluoroproline (diF-Pro) to obtain an agonist LW02060 (DOTA-Pip-[D-Phe6,Tle10,NMe-His12,diF-Pro14]Bombesin(6-14)) and an antagonist LW02080 (DOTA-Pip-[D-Phe6,NMe-Gly11,Leu13(ψ)diF-Pro14]Bombesin(6-14)), respectively. Methods/Results: The binding affinities (Ki) of Ga-LW02060, Ga-LW02080, Lu-LW02060, and Lu-LW02080 were measured by in vitro competition binding assays using PC-3 cells and were found to be 5.57 ± 2.47, 21.7 ± 6.69, 8.00 ± 2.61, and 32.1 ± 8.14 nM, respectively. The 68Ga- and 177Lu-labeled ligands were obtained in 36-75% decay-corrected radiochemical yields with >95% radiochemical purity. PET imaging, SPECT imaging, and ex vivo biodistribution studies were conducted in PC-3 tumor-bearing mice. Both [68Ga]Ga-LW02060 and [68Ga]Ga-LW02080 enabled clear tumor visualization in PET images at 1 h post-injection (pi). Tumor uptake values of [68Ga]Ga-LW02060 and [68Ga]Ga-LW02080 at 1 h pi were 16.8 ± 2.70 and 7.36 ± 1.33 %ID/g, respectively, while their pancreas uptake values were 3.12 ± 0.89 and 0.38 ± 0.04 %ID/g, respectively. Compared to [177Lu]Lu-LW02080, [177Lu]Lu-LW02060 showed higher tumor uptake at all time points (1, 4, 24, 72, and 120 h pi). However, fast tumor clearance was observed for both [177Lu]Lu-LW02060 and [177Lu]Lu-LW02080. Conclusions: Our data demonstrate that [68Ga]Ga-LW02060 is promising for clinical translation for the detection of GRPR-expressing tumor lesions. However, further optimizations are needed for [177Lu]Lu-LW02060 and [177Lu]Lu-LW02080 to prolong tumor retention for therapeutic applications.

Radiopharmaceuticals Targeting Gastrin-Releasing Peptide Receptor for Diagnosis and Therapy of Prostate Cancer

The high incidence and heavy disease burden of prostate cancer (PC) require accurate and comprehensive assessment for appropriate disease management. Prostate-specific membrane antigen (PSMA) positron emission tomography (PET) cannot detect PSMA-negative lesions, despite its key role in PC disease management. The overexpression of gastrin-releasing peptide receptor (GRPR) in PC lesions reportedly performs as a complementary target for the diagnosis and therapy of PC. Radiopharmaceuticals derived from the natural ligands of GRPR have been developed. These radiopharmaceuticals enable the visualization and quantification of GRPR within the body, which can be used for disease assessment and therapeutic guidance. Recently developed radiopharmaceuticals exhibit improved pharmacokinetic parameters without deterioration in affinity. Several heterodimers targeting GRPR have been constructed as alternatives because of their potential to detect tumor lesions with a low diagnostic efficiency of single target detection. Moreover, some GRPR-targeted radiopharmaceuticals have entered clinical trials for the initial staging or biochemical recurrence detection of PC to guide disease stratification and therapy, indicating considerable potential in PC disease management. Herein, we comprehensively summarize the progress of radiopharmaceuticals targeting GRPR. In particular, we discuss the impact of ligands, chelators, and linkers on the distribution of radiopharmaceuticals. Furthermore, we summarize a potential design scheme to facilitate the advancement of radiopharmaceuticals and, thus, prompt clinical translation.

Synthesis and Evaluation of Novel 68Ga-Labeled [D-Phe6,Leu13ψThz14]bombesin(6-14) Analogs for Cancer Imaging with Positron Emission Tomography

Gastrin-releasing peptide receptor (GRPR) is overexpressed in various cancers and is a promising target for cancer diagnosis and therapy. However, the high pancreas uptake and/or metabolic instability observed for most reported GRPR-targeted radioligands might limit their clinical applications. Our group recently reported a GRPR-targeted antagonist tracer, [68Ga]Ga-TacsBOMB2 ([68Ga]Ga-DOTA-Pip-D-Phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Leu13ψThz14-NH2), which showed a minimal pancreas uptake in a preclinical mouse model. In this study, we synthesized four derivatives with unnatural amino acid substitutions (Tle10-derived Ga-LW01158, NMe-His12-derived Ga-LW01160, α-Me-Trp8- and Tle10-derived Ga-LW01186, and Tle10- and N-Me-Gly11-derived Ga-LW02002) and evaluated their potential for detecting GRPR-expressing tumors with positron emission tomography (PET). The binding affinities (Ki(GRPR)) of Ga-LW01158, Ga-LW01160, Ga-LW01186, and Ga-LW02002 were 5.11 ± 0.47, 187 ± 17.8, 6.94 ± 0.95, and 11.0 ± 0.39 nM, respectively. [68Ga]Ga-LW01158, [68Ga]Ga-LW01186, and [68Ga]Ga-LW02002 enabled clear visualization of subcutaneously implanted human prostate cancer PC-3 tumor xenografts in mice in PET images. Ex vivo biodistribution studies showed that [68Ga]Ga-LW01158 had the highest tumor uptake (11.2 ± 0.65 %ID/g) and good tumor-to-background uptake ratios at 1 h post-injection. Comparable in vivo stabilities were observed for [68Ga]Ga-LW01158, [68Ga]Ga-LW01186, and [68Ga]Ga-LW02002 (76.5-80.7% remaining intact in mouse plasma at 15 min post-injection). In summary, the Tle10 substitution, either alone or combined with α-Me-Trp8 or NMe-Gly11 substitution, in Ga-TacsBOMB2 generates derivatives that retained good GRPR binding affinity and in vivo stability. With good tumor uptake and tumor-to-background imaging contrast, [68Ga]Ga-LW01158 is promising for detecting GRPR-expressing lesions with PET.

Unnatural amino acid substitutions to improve in vivo stability and tumor uptake of 68Ga-labeled GRPR-targeted TacBOMB2 derivatives for cancer imaging with positron emission tomography

BACKGROUND

Overexpressed in various solid tumors, gastrin-releasing peptide receptor (GRPR) is a promising cancer imaging marker and therapeutic target. Although antagonists are preferable for the development of GRPR-targeted radiopharmaceuticals due to potentially fewer side effects, internalization of agonists may lead to longer tumor retention and better treatment efficacy. In this study, we systematically investigated unnatural amino acid substitutions to improve in vivo stability and tumor uptake of a previously reported GRPR-targeted agonist tracer, [68Ga]Ga-TacBOMB2 (68Ga-DOTA-Pip-D-Phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Leu13-Thz14-NH2).

RESULTS

Unnatural amino acid substitutions were conducted for Gln7, Trp8, Ala9, Val10, Gly11 and His12, either alone or in combination. Out of 25 unnatural amino acid substitutions, tert-Leu10 (Tle10) and NMe-His12 substitutions were identified to be preferable modifications especially in combination. Compared with the previously reported [68Ga]Ga-TacBOMB2, the Tle10 and NMe-His12 derived [68Ga]Ga-LW01110 showed retained agonist characteristics and improved GRPR binding affinity (Ki = 7.62 vs 1.39 nM), in vivo stability (12.7 vs 89.0% intact tracer in mouse plasma at 15 min post-injection) and tumor uptake (5.95 vs 16.6 %ID/g at 1 h post-injection).

CONCLUSIONS

Unnatural amino acid substitution is an effective strategy to improve in vivo stability and tumor uptake of peptide-based radiopharmaceuticals. With excellent tumor uptake and tumor-to-background contrast, [68Ga]Ga-LW01110 is promising for detecting GRPR-expressing cancer lesions with PET. Since agonists can lead to internalization upon binding to receptors and foreseeable long tumor retention, our optimized GRPR-targeted sequence, [Tle10,NMe-His12,Thz14]Bombesin(7-14), is a promising template for use for the design of GRPR-targeted radiotherapeutic agents.

Peptide Radioligands in Cancer Theranostics: Agonists and Antagonists

The clinical success of radiolabeled somatostatin analogs in the diagnosis and therapy-"theranostics"-of tumors expressing the somatostatin subtype 2 receptor (SST₂R) has paved the way for the development of a broader panel of peptide radioligands targeting different human tumors. This approach relies on the overexpression of other receptor-targets in different cancer types. In recent years, a shift in paradigm from internalizing agonists to antagonists has occurred. Thus, SST₂R-antagonist radioligands were first shown to accumulate more efficiently in tumor lesions and clear faster from the background in animal models and patients. The switch to receptor antagonists was soon adopted in the field of radiolabeled bombesin (BBN). Unlike the stable cyclic octapeptides used in the case of somatostatin, BBN-like peptides are linear, fast to biodegradable and elicit adverse effects in the body. Thus, the advent of BBN-like antagonists provided an elegant way to obtain effective and safe radiotheranostics. Likewise, the pursuit of gastrin and exendin antagonist-based radioligands is advancing with exciting new outcomes on the horizon. In the present review, we discuss these developments with a focus on clinical results, commenting on challenges and opportunities for personalized treatment of cancer patients by means of state-of-the-art antagonist-based radiopharmaceuticals.

68Ga-Labeled [Thz14]Bombesin(7-14) Analogs: Promising GRPR-Targeting Agonist PET Tracers with Low Pancreas Uptake

With overexpression in various cancers, the gastrin-releasing peptide receptor (GRPR) is a promising target for cancer imaging and therapy. However, the high pancreas uptake of reported GRPR-targeting radioligands limits their clinical application. Our goal was to develop ⁶⁸Ga-labeled agonist tracers for detecting GRPR-expressing tumors with positron emission tomography (PET), and compare them with the clinically validated agonist PET tracer, [⁶⁸Ga]Ga-AMBA. Ga-TacBOMB2, TacBOMB3, and TacBOMB4, derived from [Thz¹⁴]Bombesin(7-14), were confirmed to be GRPR agonists by a calcium mobilization study, and their binding affinities (K_i(GRPR)) were determined to be 7.62 ± 0.19, 6.02 ± 0.59, and 590 ± 36.5 nM, respectively, via in vitro competition binding assays. [⁶⁸Ga]Ga-TacBOMB2, [⁶⁸Ga]Ga-TacBOMB3, and [⁶⁸Ga]Ga-AMBA clearly visualized PC-3 tumor xenografts in a PET imaging study. [⁶⁸Ga]Ga-TacBOMB2 showed comparable tumor uptake but superior tumor-to-background contrast ratios when compared to [⁶⁸Ga]Ga-AMBA. Moreover, [⁶⁸Ga]Ga-TacBOMB2 and [⁶⁸Ga]Ga-TacBOMB3 showed a much lower rate of uptake in the pancreas (1.30 ± 0.14 and 2.41 ± 0.72%ID/g, respectively) than [⁶⁸Ga]Ga-AMBA (62.4 ± 4.26%ID/g). In conclusion, replacing Met¹⁴ in the GRPR-targeting sequence with Thz¹⁴ retains high GRPR-binding affinity and agonist properties. With good tumor uptake and tumor-to-background uptake ratios, [⁶⁸Ga]Ga-TacBOMB2 is promising for detecting GRPR-expressing tumors. The much lower pancreas uptake of [⁶⁸Ga]Ga-TacBOMB2 and [⁶⁸Ga]Ga-TacBOMB3 suggests that [Thz¹⁴]Bombesin(7-14) is a promising targeting vector for the design of GRPR-targeting radiopharmaceuticals, especially for radioligand therapy application.

From Bench to Bedside-The Bad Berka Experience With First-in-Human Studies

Precision oncology is being driven by rapid advances in novel diagnostics and therapeutic interventions, with treatments targeted to the needs of individual patients on the basis of genetic, biomarker, phenotypic, or psychosocial characteristics that distinguish a given patient from other patients with similar clinical presentations. Inherent in the theranostics paradigm is the assumption that diagnostic test results can precisely determine whether an individual is likely to benefit from a specific treatment. As part and integral in the current era of precision oncology, theranostics in the context of nuclear medicine aims to identify the appropriate molecular targets in neoplasms (diagnostic tool), so that the optimal ligands and radionuclides (therapeutic tool) with favorable labeling chemistry can be selected for personalized management of a specific disease, taking into consideration the specific patient, and subsequently monitor treatment response. Over the past two decades, the use of gallium-68 labeled peptides for somatostatin receptor (SSTR)-targeted PET/CT (or PET/MRI) imaging followed by lutetium-177 and yttrium-90 labeled SSTR-agonist for peptide receptor radionuclide therapy has demonstrated remarkable success in the management of neuroendocrine neoplasms, and paved the way to other indications of theranostics. Rapid advances are being made in the development of other peptide-based radiopharmaceuticals, small molecular-weight ligands and with newer radioisotopes with more favorable kinetics, potentially useful for theranostics strategies for the clinical application. The present review features the Bad Berka experience with first-in-human studies of new radiopharmaceuticals, for example, prostate-specific membrane antigen ligand, gastrin-releasing peptide receptor, neurotensin receptor 1 ligand, novel SSTR-targeting peptides and nonpeptide, and bone-seeking radiopharmaceuticals. Also new radioisotopes, for example, actinium (²²⁵Ac), copper (⁶⁴Cu), scandium (⁴⁴Sc), and terbium (¹⁵²Tb/¹⁶¹Tb) will be discussed briefly demonstrating the development from basic science to precision oncology in the clinical setting.

Localization of 99mTc-GRP Analogs in GRPR-Expressing Tumors: Effects of Peptide Length and Neprilysin Inhibition on Biological Responses

The overexpression of gastrin-releasing peptide receptors (GRPRs) in frequently occurring human tumors has provided the opportunity to use bombesin (BBN) analogs as radionuclide carriers to cancer sites for diagnostic and therapeutic purposes. We have been alternatively exploring human GRP motifs of higher GRPR selectivity compared to frog BBN sequences aiming to improve pharmacokinetic profiles. In the present study, we compared two differently truncated human endogenous GRP motifs: GRP(14–27) and GRP(18–27). An acyclic tetraamine was coupled at the N-terminus to allow for stable binding of the SPECT radionuclide ^99mTc. Their biological profiles were compared in PC-3 cells and in mice without or with coinjection of phosphoramidon (PA) to induce transient neprilysin (NEP) inhibition in vivo. The two ^99mTc-N₄-GRP(14/18–27) radioligands displayed similar biological behavior in mice. Coinjection of PA exerted a profound effect on in vivo stability and translated into notably improved radiolabel localization in PC-3 experimental tumors. Hence, this study has shown that promising ^99mTc-radiotracers for SPECT imaging may indeed derive from human GRP sequences. Radiotracer bioavailability was found to be of major significance. It could be improved during in situ NEP inhibition resulting in drastically enhanced uptake in GRPR-expressing lesions.

From Bench to Bed: New Gastrin-Releasing Peptide Receptor-Directed Radioligands and Their Use in Prostate Cancer

Gastrin-releasing peptide receptors (GRPRs) are overexpressed in prostate and breast cancer, and are therefore attractive molecular targets for diagnosis and therapy with radiolabeled GRPR-directed peptide probes. The amphibian tetradecapeptide bombesin or the mammalian gastrin-releasing peptide and neuromedin C have been modified with a variety of chelators. As a result, labeling with radiometals attractive for SPECT or PET imaging and for radionuclide therapy has led to the development of peptide radioligands suitable for in vivo targeting of prostate cancer. A shift of paradigm from internalizing GRPR-agonists to antagonists has occurred owing to the higher biosafety and superior pharmacokinetics of radioantagonists.

Structure-activity relationship study towards non-peptidic positron emission tomography (PET) radiotracer for gastrin releasing peptide receptors: Development of [18F] (S)-3-(1H-indol-3-yl)-N-[1-[5-(2-fluoroethoxy)pyridin-2-yl]cyclohexylmethyl]-2-methyl-2-[3-(4-nitrophenyl)ureido]propionamide

Gastrin-releasing peptide receptors (GRP-Rs, also known as bombesin 2 receptors) are overexpressed in a variety of human cancers, including prostate cancer, and therefore they represent a promising target for in vivo imaging of tumors using positron emission tomography (PET). Structural modifications of the non-peptidic GRP-R antagonist PD-176252 ((S)-1a) led to the identification of the fluorinated analog (S)-3-(1H-indol-3-yl)-N-[1-[5-(2-fluoroethoxy)pyridin-2-yl]cyclohexylmethyl]-2-methyl-2-[3-(4-nitrophenyl)ureido]propionamide ((S)-1m) that showed high affinity and antagonistic properties for GRP-R. This antagonist was stable in rat plasma and towards microsomal oxidative metabolism in vitro. (S)-1m was successfully radiolabeled with fluorine-18 through a conventional radiochemistry procedure. [¹⁸F](S)-1m showed high affinity and displaceable interaction for GRP-Rs in PC3 cells in vitro.

Improving the stability of peptidic radiotracers by the introduction of artificial scaffolds: which structure element is most useful?

Peptidic radiotracers are highly potent substances for the specific in vivo imaging of various biological targets with Single Photon Emission Computed Tomography and Positron Emission Tomography. However, some radiolabeled peptides such as bombesin analogs were shown to exhibit only a limited stability, hampering a successful target visualization. One option to positively influence the stability of radiolabeled peptides is the introduction of certain artificial molecular scaffolds. In order to comparatively assess the influence of different structure elements on the stability of radiolabeled peptides and to identify those structure elements being most useful for peptide radiotracer stabilization, several monomeric and dimeric bombesin derivatives were synthesized, exhibiting differing molecular designs and the chelator NODAGA for (68) Ga-labeling. The radiolabeled peptides were evaluated regarding their in vitro stability in human serum to determine the influence of the introduced molecular scaffolds on the peptides' serum stabilities. The results of the evaluations showed that the introduction of scaffold structures and the overall molecular design have a substantial impact on the stabilities of the resulting peptidic radiotracers. But besides some general trends found using certain scaffold structures, the obtained results point to the necessity to empirically assess their influence on stability for each susceptible peptidic radiotracer individually.

In vivo enzyme inhibition improves the targeting of [177Lu]DOTA-GRP(13-27) in GRPR-positive tumors in mice

INTRODUCTION

Gastrin-releasing peptide receptors (GRPR) and GRP-derived analogs have attracted attention due to high receptor expression in frequently occurring human neoplasia. The authors recently synthesized a series of GRPR-affine peptide analogs based on the 27-mer GRP and derivatized with the DOTA chelator at the N-terminus for (111)In-labeling. In this study, the authors evaluated the most promising from these series, DOTA-GRP(13-27), after radiolabeling with (177)Lu for future therapeutic applications. In addition, to improve in vivo stability of the peptide against in vivo degradation by the protease neutral endopeptidase (NEP), the authors coinjected [(177)Lu]DOTA-GRP(13-27) with the potent NEP inhibitor phosphoramidon (PA). The authors also aimed at reducing renal uptake by coadministration of lysine.

METHODS

In vivo stability studies were performed in Swiss albino mice. Biodistribution studies were conducted in NMRI nu/nu mice bearing prostate cancer (PC)-3 xenografts. Ex vivo autoradiography was performed using frozen sections from PC-3 xenografts and kidneys.

RESULTS AND DISCUSSION

Coadministration of PA significantly increased the percentage of intact radiopeptide in the mouse circulation. From biodistribution and ex vivo autoradiography studies, coadministration of both lysine and PA with [(177)Lu]DOTA-GRP(13-27) appeared to induce a clear improvement of tumor uptake as well as lower levels of renal radioactivity, causing a promising ninefold increase in tumor/kidney ratios.

GEP-NETS update: functional localisation and scintigraphy in neuroendocrine tumours of the gastrointestinal tract and pancreas (GEP-NETs)

For patients with neuroendocrine tumours (NETs) of the gastrointestinal tract and pancreas (GEP) (GEP-NETs), excellent care should ideally be provided by a multidisciplinary team of skilled health care professionals. In these patients, a combination of nuclear medicine imaging and conventional radiological imaging techniques is usually mandatory for primary tumour visualisation, tumour staging and evaluation of treatment. In specific cases, as in patients with occult insulinomas, sampling procedures can provide a clue as to where to localise the insulin-hypersecreting pancreatic NETs. Recent developments in these fields have led to an increase in the detection rate of primary GEP-NETs and their metastatic deposits. Radiopharmaceuticals targeted at specific tumour cell properties and processes can be used to provide sensitive and specific whole-body imaging. Functional imaging also allows for patient selection for receptor-based therapies and prediction of the efficacy of such therapies. Positron emission tomography/computed tomography (CT) and single-photon emission CT/CT are used to map functional images with anatomical localisations. As a result, tumour imaging and tumour follow-up strategies can be optimised for every individual GEP-NET patient. In some cases, functional imaging might give indications with regard to future tumour behaviour and prognosis.