Gastrin releasing peptide (GRP) receptor targeted radiopharmaceuticals: a concise update

Scandium-44: Diagnostic Feasibility in Tumor-Related Angiogenesis

Angiogenesis-related cell-surface molecules, including integrins, aminopeptidase N, vascular endothelial growth factor, and gastrin-releasing peptide receptor (GRPR), play a crucial role in tumour formation. Radiolabelled imaging probes targeting angiogenic biomarkers serve as valuable vectors in tumour identification. Nowadays, there is a growing interest in novel radionuclides other than gallium-68 (⁶⁸Ga) or copper-64 (⁶⁴Cu) to establish selective radiotracers for the imaging of tumour-associated neo-angiogenesis. Given its ideal decay characteristics (E_β⁺_average: 632 KeV) and a half-life (T_1/2 = 3.97 h) that is well matched to the pharmacokinetic profile of small molecules targeting angiogenesis, scandium-44 (⁴⁴Sc) has gained meaningful attention as a promising radiometal for positron emission tomography (PET) imaging. More recently, intensive research has been centered around the investigation of ⁴⁴Sc-labelled angiogenesis-directed radiopharmaceuticals. Previous studies dealt with the evaluation of ⁴⁴Sc-appended a_vb₃ integrin-affine Arg-Gly-Asp (RGD) tripeptides, GRPR-selective aminobenzoyl-bombesin analogue (AMBA), and hypoxia-associated nitroimidazole derivatives in the identification of various cancers using experimental tumour models. Given the tumour-related hypoxia- and angiogenesis-targeting capability of these PET probes, ⁴⁴Sc seems to be a strong competitor of the currently used positron emitters in radiotracer development. In this review, we summarize the preliminary preclinical achievements with ⁴⁴Sc-labelled angiogenesis-specific molecular probes.

Human Dose Assessment of 68Ga-NODAGA-RGD-BBN Heterodimer Peptide based on Animal Data

Aims

Calculation of the absorbed dose in human organs is one of the first steps for developing new radiopharmaceuticals. The aim of this study is to estimate the human absorbed dose of a newly developed 68Ga-NODAGA-RGD-BBN radiolabeled compound.

Materials and Methods

68Ga-NODAGA-RGD-BBN was prepared by varying different parameters at optimized conditions. The stability of the radiolabeled peptide in phosphate-buffered saline (PBS) and in human serum was evaluated for 120 min. Afterward, the biodistribution of the complex was assessed in normal and tumor-bearing mice, at least for 120 min postinjection. Finally, the human absorbed dose of 68Ga-NODAGA-RGD-BBN was estimated based on mice data using Radiation Dose Assessment Resource and Spark method.

Results

68Ga-NODAGA-RGD-BBN was produced with radiochemical purity of more than 98% (high-performance liquid chromatography/ radio thin layer chromatography (RTLC)) with high stability in PBS buffer and in human serum at least for 2 h. The complex demonstrated high uptake in gastrin-releasing peptide receptor-expressing tumors compared to other nontarget organs. Furthermore, the dose assessment for the complex showed that the kidneys receive the highest absorbed dose in comparison with other organs.

Conclusion

The result of this study showed that 68Ga-NODAGA-RGD-BBN is an effective and radiolabeled ligand for tumor detection, however more studies are still needed.

GMP-compliant production of [68Ga]Ga-NeoB for positron emission tomography imaging of patients with gastrointestinal stromal tumor

Background

[⁶⁸Ga]Ga-NeoB is a novel DOTA-coupled Gastrin Releasing Peptide Receptor (GRPR) antagonist with high affinity for GRPR and good in vivo stability. This study aimed at (1) the translation of preclinical results to the clinics and establish the preparation of [⁶⁸Ga]Ga-NeoB using a GMP conform kit approach and a licensed ⁶⁸Ge/⁶⁸Ga generator and (2) to explore the application of [⁶⁸Ga]Ga-NeoB in patients with gastrointestinal stromal tumors (GIST) before and/or after interventional treatment (selective internal radiotherapy, irreversible electroporation, microwave ablation).

Results

Validation of the production and quality control of [⁶⁸Ga]Ga-NeoB for patient use had to be performed before starting the GMP production. Six independent batches of [⁶⁸Ga]Ga-NeoB were produced, all met the quality and sterility criteria and yielded 712 ± 73 MBq of the radiotracer in a radiochemical purity of > 95% and a molar activity of 14.2 ± 1.5 GBq/μmol within 20 min synthesis time and additional 20 min quality control. Three patients (2 females, 1 male, 51–77 yrs. of age) with progressive gastrointestinal stromal tumor metastases in the liver or peritoneum not responsive to standard tyrosine kinase inhibitor therapy underwent both [⁶⁸Ga]Ga-NeoB scans prior and after interventional therapy. Radiosynthesis of ⁶⁸Ga-NeoB was performed using a kit approach under GMP conditions. No specific patient preparation such as fasting or hydration was required for [⁶⁸Ga]Ga-NeoB PET/CT imaging. Contrast-enhanced PET/CT studies were performed. A delayed, second abdominal image after the administration of the of [⁶⁸Ga]Ga-NeoB was acquired at 120 min post injection.

Conclusions

A fully GMP compliant kit preparation of [⁶⁸Ga]Ga-NeoB enabling the routine production of the tracer under GMP conditions was established for clinical routine PET/CT imaging of patients with metastatic GIST and proved to adequately visualize tumor deposits in the abdomen expressing GRPR. Patients could benefit from additional information derived from [⁶⁸Ga]Ga-NeoB diagnosis to assess the presence of GRPR in the tumor tissue and monitor antitumor treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-021-00137-w.

Emerging trends of receptor-mediated tumor targeting peptides: A review with perspective from molecular imaging modalities

Natural peptides extracted from natural components such are known to have a relatively short in-vivo half-life and can readily metabolize by endo- and exo-peptidases. Fortunately, synthetic peptides can be easily manipulated to increase in-vivo stability, membrane permeability and target specificity with some well-known natural families. Many natural as well as synthetic peptides target to their endogenous receptors for diagnosis and therapeutic applications. In order to detect these peptides externally, they must be modified with radionuclides compatible with single photon emission computed tomography (SPECT) or positron emission tomography (PET). Although, these techniques mainly rely on physiological changes and have profound diagnostic strength over anatomical modalities such as MRI and CT. However, both SPECT and PET observed to possess lack of anatomical reference frame which is a key weakness of these techniques, and unfortunately, cannot be available freely in most clinical centres especially in under-developing countries. Hence, it is need of the time to design and develop economic, patient friendly and versatile strategies to grapple with existing problems without any hazardous side effects. Optical molecular imaging (OMI) has emerged as a novel technique in field of medical science using fluorescent probes as imaging modality and has ability to couple with organic drugs, small molecules, chemotherapeutics, DNA, RNA, anticancer peptide and protein without adding chelators as necessary for radionuclides. Furthermore, this review focuses on difference in imaging modalities and provides ample knowledge about reliable, economic and patient friendly optical imaging technique rather radionuclide-based imaging techniques.

Recent Breakthrough in 68Ga-Radiopharmaceuticals Cold Kits for Convenient PET Radiopharmacy

⁶⁸Ga-PET has emerged as an important diagnostic tool for precise detection and monitoring of oncological situations. Availability, cost, and radiosynthesis procedure are determining steps for success of a radioisotope/radiopharmaceutical in nuclear medicine. Availability of ⁶⁸Ga from a ⁶⁸Ge/⁶⁸Ga generator containing a long-lived parent radioisotope (⁶⁸Ge: t_1/2 = 271 days) and an inexpensive, simplified production of ⁶⁸Ga-radiopharmaceuticals through kit methodology has allowed smooth accommodation of ⁶⁸Ga-PET in clinics. The uncomplicated formulation of ⁶⁸Ga-radiopharmaceuticals from a lyophilized, cold kit is an impending breakthrough in clinical PET. The huge success of ⁶⁸Ga in neuroendocrine tumor and prostate cancer imaging along with the regulatory approval of respective cold kits has opened a pathway for development of kits for other evolving radiotracers. There is a definite scope for increased participation of commercial manufacturers and distributors of cold kits to spread the potential of ⁶⁸Ga worldwide across all the geographical locations and satellite centers.

Development of Heterobivalent Theranostic Probes Having High Affinity/Selectivity for the GRPR/PSMA

In this study, we describe the development of heterobivalent [DUPA-6-Ahx-([¹¹¹In]In-DO3A)-8-Aoc-BBN ANT] and [DUPA-6-Ahx-([¹⁷⁷Lu]Lu-DO3A)-8-Aoc-BBN ANT] radiotracers that display very high selectivity/specificity for gastrin-releasing peptide receptor (GRPR)-/prostate-specific membrane antigen (PSMA)-expressing cells. These studies include metallation, purification, characterization, and in vitro and in vivo evaluation of the new small-molecule-/peptide-based radiopharmaceuticals having utility for imaging and potentially therapy. Competitive displacement binding assays using PC-3 cells and LNCaP cell membranes showed high binding affinity for the GRPR or the PSMA. Biodistribution studies showed favorable excretion pharmacokinetics with high tumor uptake in PC-3 or PC-3 prostatic inhibin peptide (PIP) tumor-bearing mice. For example, tumor accumulation at the 1 h time point ranged from (4.74 ± 0.90) to (7.51 ± 2.61)%ID/g. Micro-single-photon emission computed tomography (microSPECT) molecular imaging investigations showed very high uptake in tumors with minimal accumulation of tracers in the surrounding collateral tissues in xenografted mice at 4 h postintravenous injection. In conclusion, [DUPA-6-Ahx-([¹¹¹In]In-DO3A)-8-Aoc-BBN ANT] and [DUPA-6-Ahx-([¹⁷⁷Lu]Lu-DO3A)-8-Aoc-BBN ANT] tracers displayed favorable pharmacokinetic and excretion profiles with high uptake and retention in tumors.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

A novel peptide targeting gastrin releasing peptide receptor for pancreatic neoplasm detection

The designed novel peptide GB-6 with targeted GRPR-binding possesses more favorable pharmacokinetic properties and metabolic stability, as well as superior tumor-targeting ability in pancreatic cancer models, relative to BBN_7–14.

Stability Evaluation and Stabilization of a Gastrin-Releasing Peptide Receptor (GRPR) Targeting Imaging Pharmaceutical

The prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are identified as important targets on prostate cancer. Receptor-targeting radiolabeled imaging pharmaceuticals with high affinity and specificity are useful in studying and monitoring biological processes and responses. Two potential imaging pharmaceuticals, AMBA agonist (where AMBA = DO3A-CH2CO-G-[4-aminobenzyl]- Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH₂) and RM1 antagonist (where RM1 = DO3A-CH₂CO-G-[4-aminobenzyl]-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂), have demonstrated high binding affinity (IC₅₀) to GRP receptors and high tumor uptake. Antagonists, despite the poor tumor cell internalization properties, can show clearer images and pharmacokinetic profiles by virtue of their higher tumor uptake in animal models compared to agonists. For characterization, development, and translation of a potential imaging pharmaceutical into the clinic, it must be evaluated in a series of tests, including in vitro cell binding assays, in vitro buffer and serum stability studies, the biodistribution of the radiolabeled material, and finally imaging studies in preclinical animal models. Data related to acetate buffer, mouse, canine, and human sera stability of ¹⁷⁷Lu-labeled RM1 are presented here and compared with the acetate buffer and sera stability data of AMBA agonist. The samples of ¹⁷⁷Lu-labeled RM1 with a high radioconcentration degrade faster than low-radioconcentration samples upon storage at 2–8 °C. Addition of stabilizers, ascorbic acid and gentisic acid, improve the stability of ¹⁷⁷Lu-labeled RM1 significantly with gentisic acid being more efficient than ascorbic acid as a stabilizer. The degradation kinetics of ¹⁷⁷Lu-labeled AMBA and RM1 in sera follow the order (fastest to slowest): mouse > canine > human sera. Finally, ¹⁷⁷Lu-labeled RM1 antagonist is slower to degrade in mouse, canine, and human sera than ¹⁷⁷Lu-labeled AMBA agonist, further suggesting that an antagonist is a more promising candidate than agonist for the positron emission tomography (PET) imaging and therapy of prostate cancer patients.

Prostate Cancer Theranostics Targeting Gastrin-Releasing Peptide Receptors

Gastrin-releasing peptide receptors (GRPRs), part of the bombesin (BBN) family, are aberrantly overexpressed in many cancers, including those of the breast, prostate, pancreas, and lung, and therefore present an attractive target for cancer diagnosis and therapy. Different bombesin analogs have been radiolabeled and used for imaging diagnosis, staging, evaluation of biochemical recurrence, and assessment of metastatic disease in patients with prostate cancer. Recently, interest has shifted from BBN-like receptor agonists to antagonists, because the latter does not induce adverse effects and demonstrate superior in vivo pharmacokinetics. We review the preclinical and clinical literatures on the use of GRPRs as targets for imaging and therapy of prostate cancer, with a focus on the newer developments and theranostic potential of GRPR peptides.

GPCR Modulation in Breast Cancer

Breast cancer is the most prevalent cancer found in women living in developed countries. Endocrine therapy is the mainstay of treatment for hormone-responsive breast tumors (about 70% of all breast cancers) and implies the use of selective estrogen receptor modulators and aromatase inhibitors. In contrast, triple-negative breast cancer (TNBC), a highly heterogeneous disease that may account for up to 24% of all newly diagnosed cases, is hormone-independent and characterized by a poor prognosis. As drug resistance is common in all breast cancer subtypes despite the different treatment modalities, novel therapies targeting signaling transduction pathways involved in the processes of breast carcinogenesis, tumor promotion and metastasis have been subject to accurate consideration. G protein-coupled receptors (GPCRs) are the largest family of cell-surface receptors involved in the development and progression of many tumors including breast cancer. Here we discuss data regarding GPCR-mediated signaling, pharmacological properties and biological outputs toward breast cancer tumorigenesis and metastasis. Furthermore, we address several drugs that have shown an unexpected opportunity to interfere with GPCR-based breast tumorigenic signals.

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

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.

In vivo biodistribution and imaging studies with a 99mTc-radiolabeled derivative of the C-terminus of prothymosin alpha in mice bearing experimentally-induced inflammation

Prothymosin alpha (ProTα) is a highly conserved mammalian polypeptide (109 amino acids in man) exerting in vitro and in vivo immunoenhancing activities. Recently, our team has developed a ^99mTc-radiolabeled derivative of the C-terminal bioactive decapeptide of ProTα ([^99mTc]C1) and employed it in in vitro studies, the results of which support the existence of binding sites on human neutrophils that recognize [^99mTc]C1, intact ProTα as well as the C-terminal decapeptide of ProTα and presumably involve Toll-like receptor 4. In the present work, [^99mTc]C1 was administered to Swiss albino mice with experimentally-induced inflammation for in vivo biodistribution and imaging studies, in parallel with a suitable negative control, which differs from [^99mTc]C1 only in bearing a scrambled version of the ProTα decapeptide. The biodistribution data obtained with [^99mTc]C1 demonstrated fast clearance of radioactivity from blood, heart, lungs, normal muscle, and predominantly urinary excretion. Most importantly, slow clearance of radioactivity from the inflammation focus was observed, resulting in a high ratio of inflamed/normal muscle tissue (9.15 at 30min post injection, which remained practically stable up to 2h). The inflammation-targeting capacity of [^99mTc]C1 was confirmed by imaging studies and might be attributed to neutrophils, which are recruited at the inflamed areas and bear binding sites for [^99mTc]C1. In this respect, apart from being a valuable tool for further studies on ProTα in in vitro and in vivo systems, [^99mTc]C1 merits further evaluation as a radiopharmaceutical for specific imaging of inflammation foci.

Preclinical Study on GRPR-Targeted (68)Ga-Probes for PET Imaging of Prostate Cancer

Gastrin-releasing peptide receptor (GRPR) targeted positron emission tomography (PET) is a highly promising approach for imaging of prostate cancer (PCa) in small animal models and patients. Developing a GRPR-targeted PET probe with excellent in vivo performance such as high tumor uptake, high contrast, and optimal pharmacokinetics is still very challenging. Herein, a novel bombesin (BBN) analogue (named SCH1) based on JMV594 peptide modified with an 8-amino octanoic acid spacer (AOC) was thus designed and conjugated with the metal chelator 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA). The resulting NODAGA-SCH1 was then radiolabeled with (68)Ga and evaluated for PET imaging of PCa. Compared with (68)Ga-NODAGA-JMV594 probe, (68)Ga-NODAGA-SCH1 exhibited excellent PET/CT imaging properties on PC-3 tumor-bearing nude mice, such as high tumor uptake (5.80 ± 0.42 vs 3.78 ± 0.28%ID/g, 2 h) and high tumor/muscle contrast (16.6 ± 1.50 vs 8.42 ± 0.61%ID/g, 2 h). Importantly, biodistribution data indicated a relatively similar accumulation of (68)Ga-NODAGA-SCH1 was observed in the liver (4.21 ± 0.42%ID/g) and kidney (3.41 ± 0.46%ID/g) suggesting that the clearance is through both the kidney and the liver. Overall, (68)Ga-NODAGA-SCH1 showed promising in vivo properties and is a promising candidate for translation into clinical PET-imaging of PCa patients.

The influence of different metal-chelate conjugates of pentixafor on the CXCR4 affinity

Background

The overexpression of the chemokine receptor 4 (CXCR4) in different epithelial, mesenchymal, and hematopoietic cancers makes CXCR4 an attractive diagnostic and therapeutic target. However, targeting the CXCR4 receptor with small cyclic pentapeptide-based radiopharmaceuticals remains challenging because minor structural modifications within the ligand-linker-chelate structure often significantly affect the receptor affinity. Based on the excellent in vivo properties of CXCR4-directed pentapeptide [⁶⁸Ga]pentixafor (cyclo(-d-Tyr-N-Me-d-Orn(AMB-DOTA)-l-Arg-l-2-Nal-Gly-)), this study aims to broaden the spectrum of applicable (radio)metal-labeled pentixafor analogs.

Methods

Cyclic pentapeptides, based on the pentixafor scaffold, were synthesized by a combined solid- and solution-phase peptide synthesis. The CXCR4 receptor affinities of the cold reference compounds were determined in competitive binding assays using CXCR4-expressing Jurkat T - cell leukemia cells and [¹²⁵I]FC131 as the radioligand.

Results

Metalated pentixafor derivatives with cyclic and acyclic chelators were synthesized by solid-phase peptide synthesis and evaluated in vitro. The resulting CXCR4 affinities (IC₅₀) were highly dependent on the chelator and metal used. Two pentapeptides, Ga-NOTA and Bi-DOTA conjugates, offer an improved affinity compared to [⁶⁸Ga]pentixafor.

Conclusions

Based on the pentapeptide [⁶⁸Ga]pentixafor, a broad range of metal-labeled analogs were investigated. The affinities of the new compounds were found to be strongly dependent on both the chelator and the metal used. Bi-labeled pentixafor showed high receptor affinity and seems to be a promising ligand for further preclinical evaluation and future α-emitter-based endoradiotherapy.

Selection of optimal chelator improves the contrast of GRPR imaging using bombesin analogue RM26

Bombesin (BN) analogs bind with high affinity to gastrin-releasing peptide receptors (GRPRs) that are up-regulated in prostate cancer and can be used for the visualization of prostate cancer. The aim of this study was to investigate the influence of radionuclide-chelator complexes on the biodistribution pattern of the 111In-labeled bombesin antagonist PEG2-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (PEG2-RM26) and to identify an optimal construct for SPECT imaging. A series of RM26 analogs N-terminally conjugated with NOTA, NODAGA, DOTA and DOTAGA via a PEG2 spacer were radiolabeled with 111In and evaluated both in vitro and in vivo. The conjugates were successfully labeled with 111In with 100% purity and retained binding specificity to GRPR and high stability. The cellular processing of all compounds was characterized by slow internalization. The IC50 values were in the low nanomolar range, with lower IC50 values for positively charged natIn-NOTA-PEG2-RM26 (2.6 ± 0.1 nM) and higher values for negatively charged natIn-DOTAGA-PEG2-RM26 (4.8 ± 0.5 nM). The kinetic binding studies showed KD values in the picomolar range that followed the same pattern as the IC50 data. The biodistribution of all compounds was studied in BALB/c nu/nu mice bearing PC-3 prostate cancer xenografts. Tumor targeting and biodistribution studies displayed rapid clearance of radioactivity from the blood and normal organs via kidney excretion. All conjugates showed similar uptake in tumors at 4 h p.i. The radioactivity accumulation in GRPR-expressing organs was significantly lower for DOTA- and DOTAGA-containing constructs compared to those containing NOTA and NODAGA. 111In-NOTA-PEG2-RM26 with a positively charged complex showed the highest initial uptake and the slowest clearance of radioactivity from the liver. At 4 h p.i., DOTA- and DOTAGA-coupled analogs showed significantly higher tumor-to-organ ratios compared to NOTA- and NODAGA-containing variants. The NODAGA conjugate demonstrated the best retention of radioactivity in tumors, and, at 24 h p.i., had the highest contrast to blood, muscle and bones.

Specific in vitro binding of a new (99m)Tc-radiolabeled derivative of the C-terminal decapeptide of prothymosin alpha on human neutrophils

Prothymosin alpha (ProTα) is a conserved mammalian polypeptide with intracellular functions associated with cell proliferation and apoptosis and an extracellular role associated with immunopotentiation. The N-terminal fragment [1-28], which is identical with the immunostimulating peptide thymosin α1 (Tα1), was earlier considered as the immunoactive region of the polypeptide; however, recent data suggest that ProTα may exert a discrete immunomodulating action through its central or C-terminal region, via targeting Toll-like receptor- 4 (TLR4). In this work, a derivative of the C-terminal fragment ProTα[100-109] (ProTα-D1) that can be radiolabeled with (99m)Tc was developed. The biological activity of the non-radioactive (185/187)rhenium-complex of this derivative ([(185/187)Re]ProTα-D1, structurally similar with [(99m)Tc]ProTα-D1) was verified through suitable in vitro bioassays on human neutrophils. Subsequent cell-binding studies revealed specific, time-dependent and saturable binding of [(99m)Tc]ProTα-D1 on neutrophils, which was inhibited by intact ProTα and ProTα[100-109], as well as by a "prototype" TLR4-ligand (lipopolysaccharide from Escherichia coli). Overall, our results support the existence of ProTα-binding sites on human neutrophils, recognizing [(99m)Tc]ProTα-D1, which might involve TLR4. [(99m)Tc]ProTα-D1 may be a useful tool for conducting further in vitro and in vivo studies, aiming to elucidate the extracellular mode of action of ProTα and, eventually, develop ProTα-based immunotherapeutics.

A heterodimeric [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2] αvβ3/GRPr-targeting antagonist radiotracer for PET imaging of prostate tumors

INTRODUCTION

In the present study, we describe a (64)Cu-radiolabeled heterodimeric peptide conjugate for dual αvβ3/GRPr (αvβ3 integrin/gastrin releasing peptide receptor) targeting of the form [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2] (RGD: the amino acid sequence [Arg-Gly-Asp], a nonregulatory peptide used for αvβ3 integrin receptor targeting; Glu: glutamic acid; NO2A: 1,4,7-triazacyclononane-1,4-diacetic acid; 6-Ahx: 6-amino hexanoic acid; and RM2: (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2), an antagonist analogue of bombesin (BBN) peptide used for GRPr targeting).

METHODS

RGD-Glu-6Ahx-RM2] was conjugated to a NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) complexing agent to produce [RGD-Glu-[NO2A]-6-Ahx-RM2], which was purified by reversed-phase high-performance liquid chromatography (RP-HPLC) and characterized by electrospray ionization-mass spectrometry (ESI-MS). Radiolabeling of the conjugate with (64)Cu produced [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2 in high radiochemical yield (≥95%). In vivo behavior of the radiolabeled peptide conjugate was investigated in normal CF-1 mice and in the PC-3 human prostate cancer experimental model.

RESULTS

A competitive displacement receptor binding assay in human prostate PC-3 cells using (125)I-[Tyr(4)]BBN as the radioligand showed high binding affinity of [RGD-Glu-[(nat)Cu-NO2A]-6-Ahx-RM2] conjugate for the GRPr (3.09±0.34 nM). A similar assay in human, glioblastoma U87-MG cells using (125)I-Echistatin as the radioligand indicated a moderate receptor-binding affinity for the αvβ3 integrin (518±37.5 nM). In vivo studies of [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2] showed high accumulation (4.86±1.01 %ID/g, 1h post-intravenous injection (p.i.)) and prolonged retention (4.26±1.23 %ID/g, 24h p.i.) of tracer in PC-3 tumor-bearing mice. Micro-positron emission tomography (microPET) molecular imaging studies produced high-quality, high contrast images in PC-3 tumor-bearing mice at 4h p.i.

CONCLUSIONS

The favorable pharmacokinetics and enhanced tumor uptake of (64)Cu-NOTA-RGD-Glu-6Ahx-RM2 warrant further investigations for dual integrin and GRPr-positive tumor imaging and possible radiotherapy.

Evaluation of a new radiolabeled bombesin derivative with 99mTc as potential targeted tumor imaging agent

Gastrin-releasing peptide (GRP) receptors are over-expressed in various human tumor including breast and prostate which can be targeted with bombesin for diagnosis and targeted therapy. High abdominal accumulation and the poor in vivo stability of radiolabeled bombesin analogues may represent a limitation for diagnostic imaging and targeted therapy. In this study a new bombesin derivative was labeled with ^99mTc via HYNIC and tricine as a coligand and investigated further. The peptide HYNIC conjugate was synthesized on a solid phase using Fmoc strategy. Labeling with ^99mTc was performed at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was checked in the presence of human serum at 37 °C up to 24 h. Internalization was studied with the human GRP receptor cell line PC-3. The Biodistribution was studied in mice. Labeling yield of >98 % was obtained to correspond a specific activity of ~80.9 GBq/μmol. Radioconjugate internalization into PC-3 cells was high and specific (15.6 ± 1.9 % at 4 h). A high and specific uptake in GRP-receptor-positive organs such as mouse tumor and pancreas (2.11 ± 0.18 and 1.78 ± 0.09 % ID/g after 1 h respectively) was also determined.

Nanoparticles and phage display selected peptides for imaging and therapy of cancer

Molecular imaging probes are a special class of pharmaceuticals that target specific biochemical signatures associated with disease and allow for noninvasive imaging on the molecular level. Because changes in biochemistry occur before diseases reach an advanced stage, molecular imaging probes make it possible to locate and stage disease, track the effectiveness of drugs, treat disease, monitor response, and select patients to allow for more personalized diagnosis and treatment of disease. Targeting agents radiolabeled with positron emitters are of interest due to their ability to quantitatively measure biodistribution and receptor expression to allow for optimal dose determinations. (68)Ga is a positron emitter, which allows for quantitative imaging through positron emission chromatography (PET). The availability of (68)Ga from a generator and its ability to form stable complexes with a variety of chelates hold promise for expanding PET utilization to facilities unable to afford their own cyclotron. Nanoparticles conjugated with various proteins and peptides derived from phage display that can be selectively targeted are being developed and evaluated for guided imaging and therapy. Herein we highlight some initial efforts in combining the enhanced selectivity of nanoparticles and peptides with (68)Ga for use as molecular imaging probes.

(68)Ga-labeled bombesin analogs for receptor-mediated imaging

Targeted receptor-mediated imaging techniques have become crucial tools in present targeted diagnosis and radiotherapy as they provide accurate and specific diagnosis of disease information. Peptide-based pharmaceuticals are gaining popularity, and there has been vast interest in developing (68)Ga-labeled bombesin (Bn) analogs. The gastrin-releasing peptide (GRP) family and its Bn analog have been implicated in the biology of several human cancers. The three bombesin receptors GRP, NMB, and BRS-3 receptor are most frequently ectopically expressed by common, important malignancies. The low expression of Bn/GRP receptors in normal tissue and relatively high expression in a variety of human tumors can be of biological importance and form a molecular basis for Bn/GRP receptor-mediated imaging. To develop a Bn-like peptide with favorable tumor targeting and pharmacokinetic characteristics for possible clinical use, several modifications in the Bn-like peptides, such as the use of a variety of chelating agents, i.e., acyclic and macrocyclic agents with different spacer groups and with different metal ions (gallium), have been performed in recent years without significant disturbance of the vital binding scaffold. The favorable physical properties of (68)Ga, i.e., short half-life, and the fast localization of small peptides make this an ideal combination to study receptor-mediated imaging in patients.

Single photon emission computed tomography tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g. peptide, antibody fragment) and a γ-radiation emitting radionuclide (e.g. (99m)Tc, (123)I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Advances in molecular imaging for diagnosis of digestive tract cancers

Digestive tract cancers are common cancer types and have high incidence and mortality. Currently available diagnostic methods have some limitations that make an early and accurate diagnosis and prompt treatment difficult. Molecular imaging, which has been formally defined as visualization, characterization and measurement at the molecular level instead of the anatomic level, significantly increases the sensitivity and specificity of cancer detection. Several modalities have been utilized for molecular imaging in digestive tract cancers, such as endoscopy, scintigraphy (PET/SPECT), magnetic resonance imaging (MRI), and ultrasound (US). Antibodies, peptides, and aptamers are classes of molecular probes that have been extensively used as affinity ligands. After being conjugated with various labels such as radioisotopes, fluorophore, supermagnetic or paramagnetic metals and microbubbles, the probes can specifically target tumor cells and stroma and are used with imaging modalities to detect cancers. Molecular imaging is a methodology for not only the early detection of cancer, but also the judgment of tumor staging and the guidance of therapy. With the development of new instrument and probes, as well as multi-modal platforms, molecular imaging has been gradually perfected and taken from bench to bedside, bringing opportunities for early, accurate and comprehensive diagnosis of digestive tract cancers.

44Sc-DOTA-BN[2-14]NH2 in comparison to 68Ga-DOTA-BN[2-14]NH2 in pre-clinical investigation. Is 44Sc a potential radionuclide for PET?

AIM

In the present study we demonstrate the in vitro and in vivo comparison of the (44)Sc and (68)Ga labeled DOTA-BN[2-14]NH(2). (44)Sc is a positron emitter with a half life of 3.92 h. Hence it could be used for PET imaging with ligands requiring longer observation time than in the case of (68)Ga.

METHODS

The binding affinity of (nat)Sc-DOTA-BN[2-14]NH(2) and (nat)Ga-DOTA-BN[2-14]NH(2) to GRP receptors was studied in competition to [(125)I-Tyr(4)]-Bombesin in the human prostate cancer cell line PC-3. A preliminary biodistribution in normal rats was performed, while first microPET images were assessed in male Copenhagen rats bearing the androgen-independent Dunning R-3327-AT-1 prostate cancer tumor.

RESULTS

The affinity to GRP receptors in the PC-3 cell line was higher for (nat)Ga-DOTA-BN[2-14]NH(2) (IC(50)(nM)=0.85 ± 0.06) than that of (nat)Sc-DOTA-BN[2-14]NH(2) (IC(50) (nM)=6.49 ± 0.13). The internalization rate of (68)Ga labeled DOTA-BN[2-14]NH(2) was slower than that of (44)Sc, but their final internalization percents were comparable. (68)Ga-DOTA-BN[2-14]NH(2) was externalized faster than (44)Sc-DOTA-BN[2-14]NH(2). The biodistribution of (44)Sc-DOTA-BN[2-14]NH(2) and (68)Ga-DOTA-BN[2-14]NH(2) in normal rats revealed a higher uptake in target organs and tissues of the first one while both excreted mainly through urinary tract. In microPET images both tracers were accumulated in the tumor with similar uptake patterns.

CONCLUSIONS

Despite the differences in the receptor affinity both the (68)Ga- and the (44)Sc-labeled DOTA-BN[2-14]NH(2) tracers showed comparable distribution and similar time constants of uptake and elimination. Moreover no differences in tumor accumulation (neither in the overall uptake nor in the dynamics) were observed from the microPet imaging. From that perspective the use of either (44)Sc or (68)Ga for detecting tumors with GRP receptors is equivalent.

Sulfonyl fluoride-based prosthetic compounds as potential 18F labelling agents

Nucleophilic incorporation of [(18)F]F(-) under aqueous conditions holds several advantages in radiopharmaceutical development, especially with the advent of complex biological pharmacophores. Sulfonyl fluorides can be prepared in water at room temperature, yet they have not been assayed as a potential means to (18)F-labelled biomarkers for PET chemistry. We developed a general route to prepare bifunctional 4-formyl-, 3-formyl-, 4-maleimido- and 4-oxylalkynl-arylsulfonyl [(18)F]fluorides from their sulfonyl chloride analogues in 1:1 mixtures of acetonitrile, THF, or tBuOH and Cs[(18)F]F/Cs(2)CO(3(aq.)) in a reaction time of 15 min at room temperature. With the exception of 4-N-maleimide-benzenesulfonyl fluoride (3), pyridine could be used to simplify radiotracer purification by selectively degrading the precursor without significantly affecting observed yields. The addition of pyridine at the start of [(18)F]fluorination (1:1:0.8 tBuOH/Cs(2)CO(3(aq.))/pyridine) did not negatively affect yields of 3-formyl-2,4,6-trimethylbenzenesulfonyl [(18)F]fluoride (2) and dramatically improved the yields of 4-(prop-2-ynyloxy)benzenesulfonyl [(18)F]fluoride (4). The N-arylsulfonyl-4-dimethylaminopyridinium derivative of 4 (14) can be prepared and incorporates (18)F efficiently in solutions of 100 % aqueous Cs(2)CO(3) (10 mg mL(-1)). As proof-of-principle, [(18)F]2 was synthesised in a preparative fashion [88(±8) % decay corrected (n=6) from start-of-synthesis] and used to radioactively label an oxyamino-modified bombesin(6-14) analogue [35(±6) % decay corrected (n=4) from start-of-synthesis]. Total preparation time was 105-109 min from start-of-synthesis. Although the (18)F-peptide exhibited evidence of proteolytic defluorination and modification, our study is the first step in developing an aqueous, room temperature (18)F labelling strategy.

PEGylation, increasing specific activity and multiple dosing as strategies to improve the risk-benefit profile of targeted radionuclide therapy with 177Lu-DOTA-bombesin analogues

Background

Radiolabelled bombesin (BN) conjugates are promising radiotracers for imaging and therapy of breast and prostate tumours, in which BN₂/gastrin-releasing peptide receptors are overexpressed. We describe the influence of the specific activity of a ¹⁷⁷Lu-DOTA-PEG_5k-Lys-B analogue on its therapeutic efficacy and compare it with its non-PEGylated counterpart.

Methods

Derivatisation of a stabilised DOTA-BN(7–14)[Cha¹³,Nle¹⁴] analogue with a linear PEG molecule of 5 kDa (PEG_5k) was performed by PEGylation of the ϵ-amino group of a β³hLys-βAla-βAla spacer between the BN sequence and the DOTA chelator. The non-PEGylated and the PEGylated analogues were radiolabelled with ¹⁷⁷Lu. In vitro evaluation was performed in human prostate carcinoma PC-3 cells, and in vivo studies were carried out in nude mice bearing PC-3 tumour xenografts. Different specific activities of the PEGylated BN analogue and various dose regimens were evaluated concerning their therapeutic efficacy.

Results

The specificity and the binding affinity of the BN analogue for BN₂/GRP receptors were only slightly reduced by PEGylation. In vitro binding kinetics of the PEGylated analogue was slower since steady-state condition was reached after 4 h. PEGylation improved the stability of BN conjugate in vitro in human plasma by a factor of 5.6. The non-PEGylated BN analogue showed favourable pharmacokinetics already, i.e. fast blood clearance and renal excretion, but PEGylation improved the in vivo behaviour further. One hour after injection, the tumour uptake of the PEG_5k-BN derivative was higher compared with that of the non-PEGylated analogue (3.43 ± 0.63% vs. 1.88 ± 0.4% ID/g). Moreover, the increased tumour retention resulted in a twofold higher tumour accumulation at 24 h p.i., and increased tumour-to-non-target ratios (tumour-to-kidney, 0.6 vs. 0.4; tumour-to-liver, 8.8 vs. 5.9, 24 h p.i.). In the therapy study, both ¹⁷⁷Lu-labelled BN analogues significantly inhibited tumour growth. The therapeutic efficacy was highest for the PEGylated derivative of high specific activity administered in two fractions (2 × 20 MBq = 40 MBq) at day 0 and day 7 (73% tumour growth inhibition, 3 weeks after therapy).

Conclusions

PEGylation and increasing the specific activity enhance the pharmacokinetic properties of a ¹⁷⁷Lu-labelled BN-based radiopharmaceutical and provide a protocol for targeted radionuclide therapy with a beneficial anti-tumour effectiveness and a favourable risk-profile at the same time.

PEGylation of (99m)Tc-labeled bombesin analogues improves their pharmacokinetic properties

INTRODUCTION

Radiolabeled bombesin (BN) conjugates are promising radiotracers for imaging and therapy of breast and prostate tumors in which BN(2)/gastrin-releasing peptide (GRP) receptors are overexpressed. However, the low in vivo stability of BN conjugates may limit their clinical application. In an attempt to improve their pharmacokinetics and counteract their rapid enzymatic degradation, we prepared a series of polyethylene glycol (PEG)-ylated BN(7-14) analogues for radiolabeling with (99m)Tc(CO)(3) and evaluated them in vitro and in vivo.

METHODS

Derivatization of a stabilized (N(α)His)Ac-BN(7-14)[Cha(13),Nle(14)] analogue with linear PEG molecules of various sizes [5 kDa (PEG(5)), 10 kDa (PEG(10)) and 20 kDa (PEG(20))] was performed by PEGylation of the ɛ-amino group of a β(3)hLys-βAla-βAla spacer between the stabilized BN sequence and the (N(α)His)Ac chelator. The analogues were then radiolabeled by employing the (99m)Tc-tricarbonyl technique. Binding affinity and internalization/externalization studies were performed in vitro in human prostate carcinoma PC-3 cells. Stability was investigated in vitro in human plasma and in vivo in Balb/c mice. Finally, single photon emission computed tomography (SPECT)/X-ray computed tomography studies were performed in nude mice bearing PC-3 tumor xenografts.

RESULTS

PEGylation did not affect the binding affinity of BN analogues, as the binding affinity for BN(2)/GRP receptors remained high (K(d)<0.9 nM). However, in vitro binding kinetics of the PEGylated analogues were slower. Steady-state condition was reached after 4 h, and the total cell binding was 10 times lower than that for the non-PEGylated counterpart. Besides, PEGylation improved the stability of BN conjugates in vitro and in vivo. The BN derivative conjugated with a PEG(5) molecule showed the best pharmacokinetics in vivo, i.e., faster blood clearance and preferential renal excretion. The tumor uptake of the (99m)Tc-PEG(5)-Lys-BN conjugate was slightly higher compared to that of the non-PEGylated analogue (3.91%±0.44% vs. 2.80%±0.28% injected dose per gram 1 h postinjection, p.i.). Tumor retention was also increased, resulting in a threefold higher amount of radioactivity in the tumor at 24 h p.i. Furthermore, decreased hepatobiliary excretion and increased tumor-to-nontarget ratios (tumor-to-blood: 17.1 vs. 2.1; tumor-to-kidney: 1.1 vs. 0.4; tumor-to-liver: 5.8 vs. 1.0, 24 h p.i.) were observed and further confirmed via small-animal SPECT images 1 h p.i.

CONCLUSION

PEGylation proved to be an effective strategy to enhance the tumor-targeting potential of (99m)Tc-labeled BN-based radiopharmaceuticals and probably other radiolabeled peptides.

In vitro and in vivo characterization of novel 18F-labeled bombesin analogues for targeting GRPR-positive tumors

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a number of human tumors and has been targeted with radiolabeled bombesin analogues for the diagnosis and therapy of these cancers. Seven bombesin analogues containing various linkers and peptide sequences were designed, synthesized, radiolabeled with (18)F, and characterized in vitro and in vivo as potential PET imaging agents. Binding studies displayed nanomolar binding affinities toward human GRPR for all synthesized bombesin analogues. Two high-affinity peptide candidates 6b (K(i) = 0.7 nM) and 7b (K(i) = 0.1 nM) were chosen for further in vivo evaluation. Both tracers revealed specific uptake in GRPR-expressing PC-3 tumors and the pancreas. Compared to [(18)F]6b, compound [(18)F]7b was characterized by superior tumor uptake, higher specificity of tracer uptake, and more favorable tumor-to-nontarget ratios. In vivo PET imaging allowed for the visualization of PC-3 tumor in nude mice suggesting that [(18)F]7b is a promising PET tracer candidate for the diagnosis of GRPR-positive tumors in humans.

Trifunctional 99mTc based radiopharmaceuticals: metal-mediated conjugation of a peptide with a nucleus targeting intercalator

The development of molecular imaging agents with multiple functions has become a major trend in radiopharmaceutical chemistry. We present herein the syntheses of trifunctional compounds, combining an acridine orange (AO) based intercalator with a GRP receptor specific bombesin like peptide (BBN). Metal-mediated conjugation of these two functions via the [2 + 1] approach to the third function, the [M(CO)(3)](+) (M = (99m)Tc, Re) moiety, yielded the final trifunctional molecules. The strongly fluorescent acridine orange, a nuclear targeting agent, has been derivatised with 4-imidazolecarboxylate as a bidentate ligand and bombesin with an isonitrile group as a monodentate ligand. For cell and nuclear uptake studies, [Re(L(1)-BBN)(L(2)-Ical)(CO)(3)] type complexes were synthesized and characterized. For radiopharmaceutical purposes, the (99m)Tc analogues have been prepared in a stepwise synthesis. Fluorescence microscopy studies on PC-3 cells, bearing the BBN receptor, showed high and rapid uptake into the cytoplasm. For the bifunctional molecule, lacking the BBN peptide, no internalization was observed.

Androgen-regulated gastrin-releasing peptide receptor expression in androgen-dependent human prostate tumor xenografts

Human prostate cancer (PC) overexpresses the gastrin-releasing peptide receptor (GRPR). Radiolabeled GRPR-targeting analogs of bombesin (BN) have successfully been introduced as potential tracers for visualization and treatment of GRPR-overexpressing tumors. A previous study showed GRPR-mediated binding of radiolabeled BN analogs in androgen-dependent but not in androgen-independent xenografts representing the more advanced stages of PC. We have further investigated the effect of androgen modulation on GRPR-expression in three androgen-dependent human PC-bearing xenografts: PC295, PC310 and PC82 using the androgen-independent PC3-model as a reference. Effects of androgen regulation on GRPR expression were initially studied on tumors obtained from our biorepository of xenograft tissues performing reverse transcriptase polymerase chain reaction (RT-PCR) and autoradiography ((125)I-universal-BN). A prospective biodistribution study ((111)In-MP2653) and subsequent autoradiography ((125)I-GRP and (111)In-MP2248) was than performed in castrated and testosterone resupplemented tumor-bearing mice. For all androgen-dependent xenografts, tumor uptake and binding decreased drastically after 7 days of castration. Resupplementation of testosterone to castrated animals restored GRPR expression extensively. Similar findings were concluded from the initial autoradiography and RT-PCR studies. Results from RT-PCR, for which human specific primers are used, indicate that variations in GRPR expression can be ascribed to mRNA downregulation and not to castration-induced reduction in the epithelial fraction of the xenograft tumor tissue. In conclusion, expression of human GRPR in androgen-dependent PC xenografts is reduced by androgen ablation and is reversed by restoring the hormonal status of the animals. This knowledge suggests that hormonal therapy may affect GRPR expression in PC tissue making GRPR-based imaging and therapy especially suitable for non-hormonally treated PC patients.

Versatile new bis(thiosemicarbazone) bifunctional chelators: synthesis, conjugation to bombesin(7-14)-NH(2), and copper-64 radiolabeling

New bifunctional derivatives of diacetyl-bis(4-methylthiosemicarbazone) (H(2)atsm) have been prepared by a selective transamination reaction of a new dissymmetric bis(thiosemicarbazone) precursor H(2)L(1). The new derivatives contain an aliphatic carboxylic acid (H(2)L(2) and H(2)L(3)), t-butyl carbamate (H(2)L(4)), or ammonium ion (H(2)L(5)) functional group. The new ligands and copper(II) complexes have been characterized by NMR spectroscopy, mass spectrometry, and microanalysis. The complex Cu(II)(L(4)) was structurally characterized by X-ray crystallography and shows the metal center to be in an N(2)S(2) distorted square planar coordination geometry. Electrochemical measurements show that the copper(II) complexes undergo a reversible reduction attributable to a Cu(II)/Cu(I) process. The ligands and the copper(II) complexes featuring a carboxylic acid functional group have been conjugated to the tumor targeting peptide bombesin(7-14)-NH(2). The bifunctional peptide conjugates were radiolabeled with copper-64 in the interest of developing new positron emission tomography (PET) imaging agents. The conjugates were radiolabeled with copper-64 rapidly in high radiochemical purity (>95%) at room temperature under mild conditions and were stable in a cysteine and histidine challenge study.

Biological evaluation of 153Sm and 166Ho complexes with tetraazamacrocycles containing methylcarboxylate and/or methylphosphonate pendant arms

153Sm and 166Ho complexes with two series of tetraazamacrocyclic ligands containing methylcarboxylate and/or methylphosphonate pendant arms were synthesized and their charge, lipophilicity, protein binding and in vitro and in vivo behaviour evaluated. The first series has the same backbone, a 14-membered tetraazamacrocycle containing a pyridine unit with different pendant arms, namely methylcarboxylates (ac3py14) or methylphosphonates (MeP2py14 and P3py14). The second series comprises 12- to 14-membered tetraazamacrocycles having methylcarboxylates and/or methylphosphonates as pendant arms (trans-DO2A2P, TRITA, TRITP, TETA and TETP). The 153Sm/166Ho complexes with the 14-membered tetraazamacrocycles containing the pyridine unit are neutral, hydrophilic, have a significant plasmatic protein binding, are unstable in vivo and present a slow rate of radioactivity excretion and high hepatic retention. 153Sm/166Ho complexes with the 12- to 14-membered tetraazamacrocycles are quantitatively prepared, except those with TETP. These complexes are hydrophilic, have an overall negative charge and present a medium to low plasmatic protein binding.

The 153Sm/166Ho- trans-DO2A2P, 153Sm/166Ho-TRITA and 166Ho-TRITP complexes are stable in vitro and in vivo, presenting a rapid clearance from main organs and a high rate of whole body radioactivity excretion. Biological profile of 153Sm/166Ho-TRITA complexes makes them promising candidates for therapy when conjugated to a biomolecule, while 166Ho-TRITP is potentially useful for bone targeting due to its considerable uptake by bone.