Metabolically Stabilized (68)Ga-NOTA-Bombesin for PET Imaging of Prostate Cancer and Influence of Protease Inhibitor Phosphoramidon

Tackling Prostate Cancer with Theranostic E5B9-Bombesin Target Modules (TMs): From Imaging to Treatment with UniCAR T-Cells

Target modules (TMs), intermediate molecules required for UniCAR T-cell therapy, are promising molecules for immunotheranostic approaches. In the current work, we developed TMs containing a monomeric or dimeric form of the antagonist bombesin peptide (BBN2) and assessed their potential for diagnostic imaging using positron emission tomography (PET) as well as immunotherapy in combination with UniCAR T-cells to target and image GRPR expression in prostate cancer. Synthesized monomeric and dimeric BBN2 TMs retained binding to GRPR in vitro. Both BBN2 TMs specifically activated and redirected UniCAR T-cells to eradicate PC3 and LNCaP cancer cells with high efficiency and in a comparable manner. UniCAR T-cells retained a non-exhausted memory phenotype favorable to their persistence and fitness. The 68Ga-labeled BBN2 TMs showed proof-of-target towards GRPR in PC3 and LNCaP xenografts with similar uptake profiles for both BBN2 TMs in dynamic PET experiments. Clearance occurred exclusively through renal elimination. A tremendously increased in vivo metabolic stability of the BBN2 TMs was observed compared to their counterparts without E5B9. Both monomeric and dimeric BBN2 TMs represent novel and promising immunotheranostic tools for application in prostate cancer with exceptionally high in vivo metabolic stability.

First-Generation Radiolabeled Cyclic Peptides for Molecular Imaging of Platelet-Derived Growth Factor Receptor α

Occult nodal spread and metastatic disease require longstanding imaging and biochemical assessments for thyroid cancer, a disease that has a propensity for diffuse, small-volume disease. We have developed a 64Cu-labeled platelet-derived growth factor receptor α (PDGFRA) antibody for immuno-PET of PDGFRA in metastatic papillary thyroid cancer (PTC). The present work describes the discovery of small cyclic PDGFRA-targeting peptides, their binding features, and radiolabeling with positron emitter gallium-68 (68Ga) for in vitro and in vivo characterization in thyroid cancer models. Phage-display technology with two separate libraries and seven different cell lines was used through three rounds of biopanning as well as flow cytometry and comparative analysis with recombinant protein to select specific peptide sequences. Phenotypic binding analysis was completed by using phosphorylation and cell migration assays. In vitro protein binding was analyzed with thermophoresis and flow cytometry using the fluorescent-labeled PDGFRA peptide. Peptide candidates were modified with the NOTA chelator for radiolabeling with 68Ga. In vitro cell uptake was studied in various thyroid cancer cell lines. In vivo studies of 68Ga-labeled peptides included metabolic stability and PET imaging. From the original library (1013 compounds), five different peptide groups were identified based on biopanning experiments with and without the α subunit of PDGFR, leading to ∼50 peptides. Subsequent phenotypic screening revealed two core peptide sequences (CP16 and CP18) that demonstrated significant changes in the level of PDGFRA phosphorylation and cell migration. Alanine scan sublibraries were created from these two lead peptide sequences, and peptides were radiolabeled using 68Ga-GaCl3 at pH 4.5, resulting in RCP > 95% within 34-40 min, including SPE purification. Cyclic peptide CP18.5 showed the strongest effects on cell migration, flow cytometry, and binding by visual interference color assay. 68Ga-labeled PDGFRA-targeting peptides showed elevated cell and tumor uptake in models of thyroid cancer, with 68Ga-NOTA-CP18.5 being the lead candidate. However, metabolic stability in vivo was compromised for 68Ga-NOTA-CP18.5 vs 68Ga-NOTA-CP18 but without impacting tumor uptake or clearance profiles. First-generation radiolabeled cyclic peptides have been developed as novel radiotracers, particularly 68Ga-NOTA-CP18.5, for the molecular imaging of PDGFRA in thyroid cancer.

Gastrin Releasing Peptide Receptors-targeted PET Diagnostics and Radionuclide Therapy for Prostate Cancer Management: Preclinical and Clinical Developments of the Past 5 Years

Each tumor has its own distinctive molecular identity. Treatment, therefore, should be tailored to this unique cancer phenotype. Theragnostics uses the same compound for targeted imaging and treatment, radiolabeled to an appropriate radionuclide, respectively. Gastrin-releasing peptide receptors (GRPRs) are overexpressed in prostate cancer, and radiolabeled GRPR antagonists have shown high diagnostic performance at staging and biochemical recurrence. Several GRPR-targeting theragnostic compounds have been developed preclinically. Their translation into clinics is underway with 4 clinical trials recruiting participants with GRPR-expressing tumors.

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.

Novel GRPR-Targeting Peptide for Pancreatic Cancer Molecular Imaging in Orthotopic and Liver Metastasis Mouse Models

Despite advancements in pancreatic cancer treatment, it remains one of the most lethal malignancies with extremely poor diagnosis and prognosis. Herein, we demonstrated the efficiency of a novel peptide GB-6 labeled with a near-infrared (NIR) fluorescent dye 3H-indolium, 2-[2-[2-[(2-carboxyethyl)thio]-3-[2-[1,3-dihydro-3,3-dimethyl-5-sulfo-1-(3-sulfopropyl)-2H-indol-2-ylidene]ethylidene]-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-5-sulfo-1-(3-sulfopropyl)-, inner salt (MPA) and radionuclide technetium-99m (99mTc) as targeting probes using the gastrin-releasing peptide receptor (GRPR) that is overexpressed in pancreatic cancer as the target. A short linear peptide with excellent in vivo stability was identified, and its radiotracer [99mTc]Tc-HYNIC-PEG4-GB-6 and the NIR probe MPA-PEG4-GB-6 exhibited selective and specific uptake by tumors in an SW1990 pancreatic cancer xenograft mouse model. The favorable biodistribution of the tracer [99mTc]Tc-HYNIC-PEG4-GB-6 in vivo afforded tumor-specific accumulation with high tumor-to-muscle and -bone contrasts and renal body clearance at 1 h after injection. The biodistribution analysis revealed that the tumor-to-pancreas and -intestine fluorescence signal ratios were 5.2 ± 0.3 and 6.3 ± 1.5, respectively, in the SW1990 subcutaneous xenograft model. Furthermore, the high signal accumulation in the orthotopic pancreatic and liver metastasis tumor models with tumor-to-pancreas and -liver fluorescence signal ratios of 7.66 ± 0.48 and 3.94 ± 0.47, respectively, enabled clear tumor visualization for intraoperative navigation. The rapid tumor targeting, precise tumor boundary delineation, chemical versatility, and high potency of the novel GB-6 peptide established it as a high-contrast imaging probe for the clinical detection of GRPR, with compelling additional potential in molecular-targeted therapy.

Peptide-based diagnostic and therapeutic agents: Where we are and where we are heading?

Peptides are increasingly present in all branches of medicine as innovative drugs, imaging agents, theragnostic, and constituent moieties of other sophisticated drugs such as peptide-drug conjugates. Due to new developments in chemical synthesis strategies, computational biology, recombinant technology, and chemical biology, peptide drug development has made a great progress in the last decade. Numerous natural peptides and peptide mimics have been obtained and studied, covering multiple therapeutic areas. Even though peptides have been investigated across the wide therapeutic spectrum, oncology, metabolism, and endocrinology are the most frequent medical indications of them. This review summarizes the current use of and the emerging new opportunities of peptides for diagnosis and treatment of various diseases.

Good practices for 68Ga radiopharmaceutical production

Background

The radiometal gallium-68 (⁶⁸Ga) is increasingly used in diagnostic positron emission tomography (PET), with ⁶⁸Ga-labeled radiopharmaceuticals developed as potential higher-resolution imaging alternatives to traditional ^99mTc agents. In precision medicine, PET applications of ⁶⁸Ga are widespread, with ⁶⁸Ga radiolabeled to a variety of radiotracers that evaluate perfusion and organ function, and target specific biomarkers found on tumor lesions such as prostate-specific membrane antigen, somatostatin, fibroblast activation protein, bombesin, and melanocortin.

Main body

These ⁶⁸Ga radiopharmaceuticals include agents such as [⁶⁸Ga]Ga-macroaggregated albumin for myocardial perfusion evaluation, [⁶⁸Ga]Ga-PLED for assessing renal function, [⁶⁸Ga]Ga-t-butyl-HBED for assessing liver function, and [⁶⁸Ga]Ga-PSMA for tumor imaging. The short half-life, favourable nuclear decay properties, ease of radiolabeling, and convenient availability through germanium-68 (⁶⁸Ge) generators and cyclotron production routes strongly positions ⁶⁸Ga for continued growth in clinical deployment. This progress motivates the development of a set of common guidelines and standards for the ⁶⁸Ga radiopharmaceutical community, and recommendations for centers interested in establishing ⁶⁸Ga radiopharmaceutical production.

Conclusion

This review outlines important aspects of ⁶⁸Ga radiopharmacy, including ⁶⁸Ga production routes using a ⁶⁸Ge/⁶⁸Ga generator or medical cyclotron, standardized ⁶⁸Ga radiolabeling methods, quality control procedures for clinical ⁶⁸Ga radiopharmaceuticals, and suggested best practices for centers with established or upcoming ⁶⁸Ga radiopharmaceutical production. Finally, an outlook on ⁶⁸Ga radiopharmaceuticals is presented to highlight potential challenges and opportunities facing the community.

68Ga-Labeled [Leu13ψThz14]Bombesin(7-14) Derivatives: Promising GRPR-Targeting PET Tracers with Low Pancreas Uptake

The gastrin-releasing peptide receptor (GRPR) is a G-protein-coupled receptor that is overexpressed in many solid cancers and is a promising target for cancer imaging and therapy. However, high pancreas uptake is a major concern in the application of reported GRPR-targeting radiopharmaceuticals, particularly for targeted radioligand therapy. To lower pancreas uptake, we explored Ga-complexed TacsBOMB2, TacsBOMB3, TacsBOMB4, TacsBOMB5, and TacsBOMB6 derived from a potent GRPR antagonist sequence, [Leu¹³ψThz¹⁴]Bombesin(7-14), and compared their potential for cancer imaging with [⁶⁸Ga]Ga-RM2. The K_i(GRPR) values of Ga-TacsBOMB2, Ga-TacsBOMB3, Ga-TacsBOMB4, Ga-TacsBOMB5, Ga-TacsBOMB6, and Ga-RM2 were 7.08 ± 0.65, 4.29 ± 0.46, 458 ± 38.6, 6.09 ± 0.95, 5.12 ± 0.57, and 1.51 ± 0.24 nM, respectively. [⁶⁸Ga]Ga-TacsBOMB2, [⁶⁸Ga]Ga-TacsBOMB3, [⁶⁸Ga]Ga-TacsBOMB5, [⁶⁸Ga]Ga-TacsBOMB6, and [⁶⁸Ga]Ga-RM2 clearly show PC-3 tumor xenografts in positron emission tomography (PET) images, while [⁶⁸Ga]Ga-TacsBOMB5 shows the highest tumor uptake (15.7 ± 2.17 %ID/g) among them. Most importantly, the pancreas uptake values of [⁶⁸Ga]Ga-TacsBOMB2 (2.81 ± 0.78 %ID/g), [⁶⁸Ga]Ga-TacsBOMB3 (7.26 ± 1.00 %ID/g), [⁶⁸Ga]Ga-TacsBOMB5 (1.98 ± 0.10 %ID/g), and [⁶⁸Ga]Ga-TacsBOMB6 (6.50 ± 0.36 %ID/g) were much lower than the value of [⁶⁸Ga]Ga-RM2 (41.9 ± 10.1 %ID/g). Among the tested [Leu¹³ψThz¹⁴]Bombesin(7-14) derivatives, [⁶⁸Ga]Ga-TacsBOMB5 has the highest tumor uptake and tumor-to-background contrast ratios, which is promising for clinical translation to detect GRPR-expressing tumors. Due to the low pancreas uptake of its derivatives, [Leu¹³ψThz¹⁴]Bombesin(7-14) represents a promising pharmacophore for the design of GRPR-targeting radiopharmaceuticals, especially for targeted radioligand therapy application.

Nano-Theranostics for the Sensing, Imaging and Therapy of Prostate Cancers

We highlight hereby recent developments in the emerging field of theranostics, which encompasses the combination of therapeutics and diagnostics in a single entity aimed for an early-stage diagnosis, image-guided therapy as well as evaluation of therapeutic outcomes of relevance to prostate cancer (PCa). Prostate cancer is one of the most common malignancies in men and a frequent cause of male cancer death. As such, this overview is concerned with recent developments in imaging and sensing of relevance to prostate cancer diagnosis and therapeutic monitoring. A major advantage for the effective treatment of PCa is an early diagnosis that would provide information for an appropriate treatment. Several imaging techniques are being developed to diagnose and monitor different stages of cancer in general, and patient stratification is particularly relevant for PCa. Hybrid imaging techniques applicable for diagnosis combine complementary structural and morphological information to enhance resolution and sensitivity of imaging. The focus of this review is to sum up some of the most recent advances in the nanotechnological approaches to the sensing and treatment of prostate cancer (PCa). Targeted imaging using nanoparticles, radiotracers and biomarkers could result to a more specialised and personalised diagnosis and treatment of PCa. A myriad of reports has been published literature proposing methods to detect and treat PCa using nanoparticles but the number of techniques approved for clinical use is relatively small. Another facet of this report is on reviewing aspects of the role of functional nanoparticles in multimodality imaging therapy considering recent developments in simultaneous PET-MRI (Positron Emission Tomography-Magnetic Resonance Imaging) coupled with optical imaging in vitro and in vivo, whilst highlighting feasible case studies that hold promise for the next generation of dual modality medical imaging of PCa. It is envisaged that progress in the field of imaging and sensing domains, taken together, could benefit from the biomedical implementation of new synthetic platforms such as metal complexes and functional materials supported on organic molecular species, which can be conjugated to targeting biomolecules and encompass adaptable and versatile molecular architectures. Furthermore, we include hereby an overview of aspects of biosensing methods aimed to tackle PCa: prostate biomarkers such as Prostate Specific Antigen (PSA) have been incorporated into synthetic platforms and explored in the context of sensing and imaging applications in preclinical investigations for the early detection of PCa. Finally, some of the societal concerns around nanotechnology being used for the detection of PCa are considered and addressed together with the concerns about the toxicity of nanoparticles–these were aspects of recent lively debates that currently hamper the clinical advancements of nano-theranostics. The publications survey conducted for this review includes, to the best of our knowledge, some of the most recent relevant literature examples from the state-of-the-art. Highlighting these advances would be of interest to the biomedical research community aiming to advance the application of theranostics particularly in PCa diagnosis and treatment, but also to those interested in the development of new probes and methodologies for the simultaneous imaging and therapy monitoring employed for PCa targeting.

Dual Probes for Positron Emission Tomography (PET) and Fluorescence Imaging (FI) of Cancer

Dual probes that possess positron emission tomography (PET) and fluorescence imaging (FI) capabilities are precision medicine tools that can be used to improve patient care and outcomes. Detecting tumor lesions using PET, an extremely sensitive technique, coupled with fluorescence-guided surgical resection of said tumor lesions can maximize the removal of cancerous tissue. The development of novel molecular probes is important for targeting different biomarkers as every individual case of cancer has different characteristics. This short review will discuss some aspects of dual PET/FI probes and explore the recently reported examples.

Water-Soluble Chitosan Conjugated DOTA-Bombesin Peptide Capped Gold Nanoparticles as a Targeted Therapeutic Agent for Prostate Cancer

Introduction

Functionalization of water-soluble chitosan (WSCS) nanocolloids with, gold nanoparticles (AuNPs), and LyslLys3 (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-bombesin 1–14 (DOTA-BBN) peptide affords an innovative pathway to produce prostate tumor cell-specific nanomedicine agents with potential applications in molecular imaging and therapy.

Methods

The preparation involves the production and full characterization of water-soluble chitosan (WSCS), via gamma (γ) rays (80 kGy) irradiation, followed by DOTA-BBN conjugation for subsequent use as an effective template toward the synthesis of tumor cell-specific AuNPs-WSCS-DOTA-BBN.

Results

The WSCS-DOTA-BBN polymeric nanoparticles (86 ± 2.03 nm) served multiple roles as reducing and stabilizing agents in the overall template synthesis of tumor cell-targeted AuNPs. The AuNPs capped with WSCS and WSCS-DOTA-BBN exhibited average Au-core diameter of 17 ± 8 nm and 20 ± 7 nm with hydrodynamic diameters of 56 ± 1 and 67± 2 nm, respectively. The AuNPs-WSCS-DOTA-BBN showed optimum in vitro stability in biologically relevant solutions. The targeted AuNPs showed selective affinity toward GRP receptors overexpressed in prostate cancer cells (PC-3 and LNCaP).

Discussion

The AuNPs-WSCS-DOTA-BBN displayed cytotoxicity effects against PC-3 and LNCaP cancer cells, with concomitant safety toward the HAECs normal cells. The AuNPs-WSCS-DOTA-BBN showed synergistic targeting toward tumor cells with selective cytotoxicity of AuNPs towards PC-3 and LNCaP cells. Our investigations provide compelling evidence that AuNPs functionalized with WSCS-DOTA-BBN is an innovative nanomedicine approach for use in molecular imaging and therapy of GRP receptor-positive tumors. The template synthesis of AuNPs-WSCS-DOTA-BBN serves as an excellent non-radioactive surrogate for the development of the corresponding ¹⁹⁸AuNPs theragnostic nanoradiopharmaceutical for use in cancer diagnosis and therapy.

Design, synthesis, and evaluation of positron emission tomography/fluorescence dual imaging probes for targeting facilitated glucose transporter 1 (GLUT1)

Increased energy metabolism followed by enhanced glucose consumption is a hallmark of cancer. Most cancer cells show overexpression of facilitated hexose transporter GLUT1, including breast cancer. GLUT1 is the main transporter for 2-deoxy-2-[¹⁸F]fluoro-d-glucose (2-[¹⁸F]FDG), the gold standard of positron emission tomography (PET) imaging in oncology. The present study's goal was to develop novel glucose-based dual imaging probes for their use in tandem PET and fluorescence (Fl) imaging. A glucosamine scaffold tagged with a fluorophore and an ¹⁸F-label should confer selectivity to GLUT1. Out of five different compounds, 2-deoxy-2-((7-sulfonylfluoro-2,1,3-benzoxadiazol-4-yl)amino)-d-glucose (2-FBDG) possessed favorable fluorescent properties and a similar potency as 2-deoxy-2-((7-nitro-2,1,3-benzoxadiazol-4-yl)amino)-d-glucose (2-NBDG) in competing for GLUT1 transport against 2-[¹⁸F]FDG in breast cancer cells. Radiolabeling with ¹⁸F was achieved through the synthesis of prosthetic group 7-fluoro-2,1,3-benzoxadiazole-4-sulfonyl [¹⁸F]fluoride ([¹⁸F]FBDF) followed by the reaction with glucosamine. The radiotracer was finally analyzed in vivo in a breast cancer xenograft model and compared to 2-[¹⁸F]FDG. Despite favourable in vitro fluorescence imaging properties, 2-[¹⁸F]FBDG was found to lack metabolic stability in vivo, resulting in radiodefluorination. Glucose-based 2-[¹⁸F]FBDG represents a novel dual-probe for GLUT1 imaging using FI and PET with the potential for further structural optimization for improved metabolic stability in vivo.

A comparative PET imaging study of 44gSc- and 68Ga-labeled bombesin antagonist BBN2 derivatives in breast and prostate cancer models

INTRODUCTION

Radiolabeled peptides play a central role in nuclear medicine as radiotheranostics for targeted imaging and therapy of cancer. We have recently proposed the use of metabolically stabilized GRPR antagonist BBN2 for radiolabeling with 18F and 68Ga and subsequent PET imaging of GRPRs in prostate cancer. The present work studied the impact of 44gSc- and 68Ga-labeled DOTA complexes attached to GRPR antagonist BBN2 on the in vitro GRPR binding affinity, and their biodistribution and tumor uptake profiles in MCF7 breast and PC3 prostate cancer models.

METHODS

DOTA-Ava-BBN2 was radiolabeled with radiometals 68Ga and 44gSc. Gastrin-releasing peptide receptor (GRPR) affinities of peptides were assessed in PC3 prostate cancer cells. GRPR expression profiles were studied in human breast cancer tissue samples and MCF7 breast cancer cells. PET imaging of 68Ga- and 44gSc-labeled peptides was performed in MCF7 and PC3 xenografts as breast and prostate cancer models.

RESULTS

Radiopeptides [68Ga]Ga-DOTA-Ava-BBN2 and [44gSc]Sc-DOTA-Ava BBN2 were prepared in radiochemical yields of 70-80% (decay-corrected), respectively. High binding affinities were found for both peptides (IC50 = 15 nM (natGa) and 5 nM (natSc)). Gene expression microarray analysis revealed high GRPR mRNA expression levels in estrogen receptor (ER)-positive breast cancer, which was further confirmed with Western blot and immunohistochemistry. However, PET imaging showed only low tumor uptake of both radiotracers in MCF7 xenografts ([68Ga]Ga-DOTA-BBN2 (SUV60min 0.27 ± 0.06); [44gSc]Sc-DOTA-BBN2 (SUV60min 0.20 ± 0.03)). In contrast, high tumor uptake and retention were found for both radiopeptides in PC3 tumors ([68Ga]Ga-DOTA-BBN2 (SUV60min 0.46 ± 0.07); [44gSc]Sc-DOTA-BBN2 (SUV60min 0.51 ± 0.11)).

CONCLUSIONS

Comparison of 68Ga- and 44gSc-labeled DOTA-Ava-BBN2 peptides revealed slight but noticeable differences of the radiometal with an impact on the in vitro GRPR receptor binding properties in PC3 cells. No differences were found in their in vivo biodistribution profiles in MCF7 and PC3 xenografts. Radiopeptides [68Ga]Ga-DOTA-Ava-BBN2 and [44gSc]Sc-DOTA-Ava-BBN2 displayed comparable tumor uptake and retention profiles with rapid blood and renal clearance profiles in both tumor models.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The favorable PET imaging performance of [44gSc]Sc-DOTA-Ava-BBN2 in prostate cancer should warrant the development of an [43Sc]Sc-DOTA-Ava-BBN2 analog for clinical translation which comes with a main γ-line of much lower energy and intensity compared to 44gSc.

Identification of a Suitable Peptidic Molecular Platform for the Development of NPY(Y1 )R-Specific Imaging Agents

NPY(Y1 )R (neuropeptide Y receptor subtype 1) is an important target structure for tumor-specific imaging and therapy as this receptor subtype is overexpressed in very high density and incidence especially in human breast cancer. Targeting this receptor with radiolabeled truncated analogues of the endogenous ligand NPY (neuropeptide Y) has, however, not yet resulted in satisfactory imaging results when using positron emission tomography (PET). This can be attributed to the limited stability of these PET imaging agents caused by their fast proteolytic degradation. Although highly promising NPY analogues were developed, their stability has only been investigated in very few cases. In this systematical work, we comparatively determined the stability of the five most promising truncated analogues of NPY that were developed over the last years, showing the highest receptor affinities and subtype selectivities. The stability of the peptides was assessed in human serum as well as in a human liver microsomal stability assay; these gave complementary results, thus demonstrating the necessity to perform both assays and not just conventional serum stability testing. Of the tested peptides, only [Lys(lauroyl)27 ,Pro30 ,Lys(DOTA)31 ,Bip32 ,Leu34 ]NPY27-36 showed high stability against peptidase degradation; thus this is the best-suited truncated NPY analogue for the development of NPY(Y1 )R-specific imaging agents.

Interactions Between Tumor Biology and Targeted Nanoplatforms for Imaging Applications

Although considerable efforts have been conducted to diagnose, improve, and treat cancer in the past few decades, existing therapeutic options are insufficient, as mortality and morbidity rates remain high. Perhaps the best hope for substantial improvement lies in early detection. Recent advances in nanotechnology are expected to increase the current understanding of tumor biology, and will allow nanomaterials to be used for targeting and imaging both in vitro and in vivo experimental models. Owing to their intrinsic physicochemical characteristics, nanostructures (NSs) are valuable tools that have received much attention in nanoimaging. Consequently, rationally designed NSs have been successfully employed in cancer imaging for targeting cancer-specific or cancer-associated molecules and pathways. This review categorizes imaging and targeting approaches according to cancer type, and also highlights some new safe approaches involving membrane-coated nanoparticles, tumor cell-derived extracellular vesicles, circulating tumor cells, cell-free DNAs, and cancer stem cells in the hope of developing more precise targeting and multifunctional nanotechnology-based imaging probes in the future.

Taking cyclotron 68Ga production to the next level: Expeditious solid target production of 68Ga for preparation of radiotracers

INTRODUCTION

Gallium-68 is an important radionuclide for positron emission tomography (PET) with steadily increasing applications of ⁶⁸Ga-based radiopharmaceuticals for clinical use. Current ⁶⁸Ga sources are primarily ⁶⁸Ge/⁶⁸Ga-generators, along with successful attempts of ⁶⁸Ga production using a cyclotron. This study evaluated cyclotron ⁶⁸Ga production and automated separation using expeditiously manufactured solid targets, demonstrates an order of magnitude improvement in yield compared to ⁶⁸Ge/⁶⁸Ga generators, and presents a convenient alternative to existing cyclotron production processes. A comparison of radiolabeling and preclinical PET imaging was performed with both cyclotron and generator produced ⁶⁸Ga.

METHODS

100 mg enriched ⁶⁸Zn (99.3% ⁶⁸Zn, 0.48% ⁶⁷Zn, 0.1% ⁶⁶Zn) pellets pressed on silver discs were bombarded for 20-75 min using 12.5 MeV proton beam energies and 10-30 μA currents. ⁶⁸Ga was separated using an automated TRASIS AllinOne synthesizer employing AG 50W-X8 and UTEVA resins. Post-separation recovery of the ⁶⁸Zn by electrolysis yielded 76.7 ± 4.3%. Radionuclidic purity of cyclotron-produced ⁶⁸Ga was investigated with gamma spectroscopy using a HPGe-detector. Radiolabeling was investigated using the macrocyclic chelator DOTA and the bombesin-derived peptide NOTA-BBN2. PET imaging was performed using [⁶⁸Ga]Ga-NOTA-BBN2 in a PC3 xenograft model.

RESULTS

600 μA·min fresh and recycled quadruplet ⁶⁸Zn target irradiations (n = 8) at 12.5 MeV and 30 μA yielded 13.9 ± 1.0 GBq ⁶⁸Ga; 2200 μA·min irradiations (n = 3) yielded 37.5 ± 1.9 GBq ⁶⁸Ga. HPGe analysis showed EOB 0.0074% and 0.0084% of total activity of ⁶⁶Ga and ⁶⁷Ga, respectively. Metal impurities were 0.06 ± 0.03 μg/GBq Zn, 0.13 ± 0.007 μg/GBq Fe, and 0.02 ± 0.01 μg/GBq Al for cyclotron ⁶⁸Ga. Cyclotron and ⁶⁸Ge/⁶⁸Ga generator ⁶⁸Ga respective DOTA and NOTA-BBN2 labeling incorporations were 99.4 ± 0.0% and 99.3 ± 0.2%, and 90.4 ± 1.5% and 93.0 ± 3.6% determined by radio-thin layer chromatography (radio-TLC). Preclinical PET imaging comparison between generator and cyclotron produced ⁶⁸Ga showed identical radiotracer tumor uptake and biodistribution profiles in PC3 tumor bearing mice.

CONCLUSIONS

Cyclotron ⁶⁸Ga production provides highly scalable production with equivalent or superior quality ⁶⁸Ga to a ⁶⁸Ge/⁶⁸Ga generator, while providing identical biodistribution and tumor uptake profiles. Our described targetry is simpler and more cost-effective than existing liquid and solid targetry, enabling a turnkey production system for multi-facility distribution of cyclotron produced ⁶⁸Ga. The manufacturing simplicity described has potential applications for producing other radiometals such as ⁴⁴Sc.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Our cost-effective method of solid target ⁶⁸Ga production can enhance ⁶⁸Ga production capabilities to meet the high demand for ⁶⁸Ga-radiopharmaceuticals for research and clinical use.

Preliminary studies of 68Ga-NODA-USPION-BBN as a dual-modality contrast agent for use in positron emission tomography/magnetic resonance imaging

The aim of this study was to propose a new dual-modality nanoprobe for positron emission tomography/magnetic resonance imaging (PET/MRI) for the early diagnosis of breast cancer. For synthesis of the nanoprobe, polyethylene glycol-coated ultra-small superparamagnetic iron-oxide nanoparticles (USPION) armed with NODA-GA chelate and grafted with bombesin (BBN) were radiolabeled with ⁶⁸Ga. After characterization, in vitro studies to evaluate the cell binding affinity of the nanoprobe were done by performing Perl's Prussian blue cell staining and MRI imaging. Finally, for in vivo studies, magnetic resonance images were taken in SCID mice bearing breast cancer tumor pre- and post-injection, and a multimodal nanoScan PET/computed tomography was used to perform preclinical imaging of the radiolabeled nanoparticles. Afterwards, a biodistribution study was done on sacrificed mice. The results showed that the highest r₁ and r₂ values were measured for USPIONs at 20 and 60 MHz, respectively. From the in vitro studies, the optical density of the cells after incubation increased with the increase of the iron concentration and the duration of incubation. However, the T₂ values decreased when the iron concentration increased. Furthermore, from in vivo studies, the T₂ and signal intensity decreased during the elapsed time post-injection in the tumor area. In this study, the in vitro studies showed that the affinity of cancer cells to nanoprobe increases meaningfully after conjugation with BBN, and also by increasing the duration of incubation and the iron concentration. Meanwhile, the in vivo results confirmed that the blood clearance of the nanoprobe happened during the first 120 min post-injection of the radiolabeled nanoprobe and also confirmed the targeting ability of that to a gastrin-releasing peptide receptor positive tumor.

Positron Emission Tomography Imaging of the Gastrin-Releasing Peptide Receptor with a Novel Bombesin Analogue

The gastrin-releasing peptide receptor (GRPR), a G protein-coupled receptor, is overexpressed in solid malignancies and particularly in prostate cancer. We synthesized a novel bombesin derivative, [⁶⁸Ga]Ga-ProBOMB1, evaluated its pharmacokinetics and potential to image GRPR expression with positron emission tomography (PET), and compared it with [⁶⁸Ga]Ga-NeoBOMB1. ProBOMB1 (DOTA-pABzA-DIG-d-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ(CH₂N)-Pro-NH₂) was synthesized by solid-phase peptide synthesis. The polyaminocarboxylate chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was coupled to the N-terminal and separated from the GRPR-targeting sequence by a p-aminomethylaniline-diglycolic acid (pABzA-DIG) linker. The binding affinity to GRPR was determined using a cell-based competition assay, whereas the agonist/antagonist property was determined with a calcium efflux assay. ProBOMB1 was radiolabeled with ⁶⁸GaCl₃. PET imaging and biodistribution studies were performed in male immunocompromised mice bearing PC-3 prostate cancer xenografts. Blocking experiments were performed with coinjection of [d-Phe⁶,Leu-NHEt¹³,des-Met¹⁴]bombesin(6-14). Dosimetry calculations were performed with OLINDA software. ProBOMB1 and the nonradioactive Ga-ProBOMB were obtained in 1.1 and 67% yield, respectively. The K _i value of Ga-ProBOMB1 for GRPR was 3.97 ± 0.76 nM. Ga-ProBOMB1 behaved as an antagonist for GRPR. [⁶⁸Ga]Ga-ProBOMB1 was obtained in 48.2 ± 10.9% decay-corrected radiochemical yield with 121 ± 46.9 GBq/μmol molar activity and >95% radiochemical purity. Imaging/biodistribution studies showed that the excretion of [⁶⁸Ga]Ga-ProBOMB1 was primarily through the renal pathway. At 1 h postinjection (p.i.), PC-3 tumor xenografts were clearly delineated in PET images with excellent contrast. The tumor uptake for [⁶⁸Ga]Ga-ProBOMB1 was 8.17 ± 2.57 percent injected dose per gram (% ID/g) and 9.83 ± 1.48% ID/g for [⁶⁸Ga]Ga-NeoBOMB1, based on biodistribution studies at 1 h p.i. This corresponded to tumor-to-blood and tumor-to-muscle uptake ratios of 20.6 ± 6.79 and 106 ± 57.7 for [⁶⁸Ga]Ga-ProBOMB1 and 8.38 ± 0.78 and 39.0 ± 12.6 for [⁶⁸Ga]Ga-NeoBOMB1, respectively. Blockade with [d-Phe⁶,Leu-NHEt¹³,des-Met¹⁴]bombesin(6-14) significantly reduced the average uptake of [⁶⁸Ga]Ga-ProBOMB1 in tumors by 62%. The total absorbed dose was lower for [⁶⁸Ga]Ga-ProBOMB1 in all organs except for bladder compared with [⁶⁸Ga]Ga-NeoBOMB1. Our data suggest that [⁶⁸Ga]Ga-ProBOMB1 is an excellent radiotracer for imaging GRPR expression with PET. [⁶⁸Ga]Ga-ProBOMB1 achieved a similar uptake as [⁶⁸Ga]Ga-NeoBOMB1 in tumors, with enhanced contrast and lower whole-body absorbed dose.

Radiolabeled bombesin derivatives for preclinical oncological imaging

Despite efforts, cancer is still one of the leading causes of morbidity and mortality worldwide, with approximately 14 million new cases and 8.2 million cancer-related deaths each year, according to the World Health Organization. Among the strategies to reduce cancer progression and improving its management, implementing early detection technologies is crucial. Based on the fact that several types of cancer cells overexpress surface receptors, small molecule ligands, such as peptides, have been developed to allow tumor identification at earlier stages. Allied with imaging techniques such as PET and SPECT, radiolabeled peptides play a pivotal role in nuclear medicine. Bombesin, a peptide of 14 amino acids, is an amphibian homolog to the mammalian gastrin-releasing peptide (GRP), that has been extensively studied as a targeting ligand for diagnosis and therapy of GRP positive tumors, such as breast, pancreas, lungs and prostate cancers. In this context, herein we provide a review of reported bombesin derivatives radiolabeled with a multitude of radioactive isotopes for diagnostic purposes in the preclinical setting. Moreover, since animal models are highly relevant for assessing the potential of clinical translation of this radiopeptides, a brief report of the currently used GRP-positive tumor-bearing animal models is described.