A New High Affinity Technetium-99m-Bombesin Analogue with Low Abdominal Accumulation

A Novel Radiotracer for Molecular Imaging and Therapy of Gastrin-Releasing Peptide Receptor Positive Prostate Cancer

The gastrin-releasing peptide receptor (GRPR) is overexpressed in many solid malignancies, particularly in prostate and breast cancers, among others. We synthesized ProBOMB2, a novel bombesin derivative radiolabeled with ⁶⁸Ga and ¹⁷⁷Lu, and evaluated its ability to target GRPR in a preclinical model of human prostate cancer. Methods: ProBOMB2 was synthesized on solid phase using Fmoc chemistry. The chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid was coupled to the N-terminus and separated from the GRPR-targeting sequence by a cationic 4-amino-(1-carboxymethyl)-piperidine spacer. Binding affinity for both human and murine GRPR was determined using a cell-based competition assay, while a calcium efflux assay was used to measure the agonist/antagonist properties of the derivatives. ProBOMB2 was radiolabeled with ¹⁷⁷Lu and ⁶⁸Ga. SPECT and PET imaging, and biodistribution studies were conducted using a preclinical prostate cancer model of male immunocompromised mice bearing GRPR-positive PC-3 human prostate cancer xenografts. Results: Ga-ProBOMB2 and Lu-ProBOMB2 bound to PC-3 cells with a Ki of 4.58±0.67 and 7.29±1.73 nM, respectively. ⁶⁸Ga-ProBOMB2 and ¹⁷⁷Lu-ProBOMB2 were radiolabeled with a radiochemical purity greater than 95%. Both radiotracers were primarily excreted via the renal pathway. PET images of PC-3 tumor xenografts were visualized with excellent contrast at 1 h and 2 h post-injection (p.i.) with ⁶⁸Ga-ProBOMB2, and very low off-target organ accumulation. ¹⁷⁷Lu-ProBOMB2 enabled clear visualization of PC-3 tumor xenografts by SPECT imaging at 1 h, 4 h, and 24 h p.i. ¹⁷⁷Lu-ProBOMB2 displayed higher tumor uptake than ⁶⁸Ga-ProBOMB2 at 1 h p.i. ¹⁷⁷Lu-ProBOMB2 tumor uptake at 1 h, 4 h, and 24 h p.i. was 14.9±3.1, 4.8±2.1, and 1.7±0.3 %ID/g, respectively. Conclusion: ⁶⁸Ga-ProBOMB2 and ¹⁷⁷Lu-ProBOMB2 are promising radiotracers with limited pancreas uptake, good tumor uptake, and favorable pharmacokinetics for imaging and therapy of GRPR-expressing tumors.

Internal Radiation Therapy

Targeted therapies are applied to increase the efficiency of antitumor treatment by simultaneously decreasing side effects. This can be achieved using carrier molecules which specifically bind to target structures or areas with remodeling activity. These carrier molecules may be coupled to chemotherapeutic drugs or to radioactive isotopes. In most cases, these carrier molecules are antibodies against tumor antigens, peptides, or small molecules which are binders for overexpressed receptors on tumor cells. The paradigm of endoradiotherapy is exemplified by the peptidic tracer DOTATOC which binds to somatostatin receptors and recently also small molecule inhibitors with high affinity for the prostate-specific membrane antigen.

Incorporation of a Hydrophilic Spacer Reduces Hepatic Uptake of HER2-Targeting Affibody-DM1 Drug Conjugates

Affibody molecules are small affinity-engineered scaffold proteins which can be engineered to bind to desired targets. The therapeutic potential of using an affibody molecule targeting HER2, fused to an albumin-binding domain (ABD) and conjugated with the cytotoxic maytansine derivate MC-DM1 (AffiDC), has been validated. Biodistribution studies in mice revealed an elevated hepatic uptake of the AffiDC, but histopathological examination of livers showed no major signs of toxicity. However, previous clinical experience with antibody drug conjugates have revealed a moderate- to high-grade hepatotoxicity in treated patients, which merits efforts to also minimize hepatic uptake of the AffiDCs. In this study, the aim was to reduce the hepatic uptake of AffiDCs and optimize their in vivo targeting properties. We have investigated if incorporation of hydrophilic glutamate-based spacers adjacent to MC-DM1 in the AffiDC, (Z_HER2:2891)₂-ABD-MC-DM1, would counteract the hydrophobic nature of MC-DM1 and, hence, reduce hepatic uptake. Two new AffiDCs including either a triglutamate-spacer-, (Z_HER2:2891)₂-ABD-E₃-MC-DM1, or a hexaglutamate-spacer-, (Z_HER2:2891)₂-ABD-E₆-MC-DM1 next to the site of MC-DM1 conjugation were designed. We radiolabeled the hydrophilized AffiDCs and compared them, both in vitro and in vivo, with the previously investigated (Z_HER2:2891)₂-ABD-MC-DM1 drug conjugate containing no glutamate spacer. All three AffiDCs demonstrated specific binding to HER2 and comparable in vitro cytotoxicity. A comparative biodistribution study of the three radiolabeled AffiDCs showed that the addition of glutamates reduced drug accumulation in the liver while preserving the tumor uptake. These results confirmed the relation between DM1 hydrophobicity and liver accumulation. We believe that the drug development approach described here may also be useful for other affinity protein-based drug conjugates to further improve their in vivo properties and facilitate their clinical translatability.

In Vitro Mouse and Human Serum Stability of a Heterobivalent Dual-Target Probe That Has Strong Affinity to Gastrin-Releasing Peptide and Neuropeptide Y1 Receptors on Tumor Cells

Receptor-targeting radiolabeled molecular probes with high affinity and specificity are useful in studying and monitoring biological processes and responses. Dual- or multiple-targeting probes, using radiolabeled metal chelates conjugated to peptides, have potential advantages over single-targeting probes as they can recognize multiple targets leading to better sensitivity for imaging and radiotherapy when target heterogeneity is present. Two natural hormone peptide receptors, gastrin-releasing peptide (GRP) and Y1, are specifically interesting as their expression is upregulated in most breast and prostate cancers. One of our goals has been to develop a dual-target probe that can bind both GRP and Y1 receptors. Consequently, a heterobivalent dual-target probe, t-BBN/BVD15-DO3A (where a GRP targeting ligand J-G-Abz4-QWAVGHLM-NH2 and Y1 targeting ligand INP-K [ɛ-J-(α-DO3A-ɛ-DGa)-K] YRLRY-NH2 were coupled), that recognizes both GRP and Y1 receptors was synthesized, purified, and characterized in the past. Competitive displacement cell binding assay studies with the probe demonstrated strong affinity (IC50 values given in parentheses) for GRP receptors in T-47D cells (18 ± 0.7 nM) and for Y1 receptors in MCF7 cells (80 ± 11 nM). As a further evaluation of the heterobivalent dual-target probe t-BBN/BVD15-DO3A, the objective of this study was to determine its mouse and human serum stability at 37°C. The in vitro metabolic degradation of the dual-target probe in mouse and human serum was studied by using a 153Gd-labeled t-BBN/BVD15-DO3A and a high-performance liquid chromatography/radioisotope detector analytical method. The half-life (t1/2) of degradation of the dual-target probe in mouse serum was calculated as 7 hours and only ∼20% degradation was seen after 6 hours incubation in human serum. The slow in vitro metabolic degradation of the dual-target probe can be compared with the degradation t1/2 of the corresponding monomeric probes, BVD15-DO3A and AMBA: 15, and ∼40 minutes for BVD15-DO3A and 3.1 and 38.8 hours for AMBA in mouse and human serum, respectively. A possible pathway for in vitro metabolic degradation of the t-BBN/BVD15-DO3A in mouse serum is proposed based on the chromatographic retention times of the intact probe and its degradants.

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.

Interrogating the Role of Receptor-Mediated Mechanisms: Biological Fate of Peptide-Functionalized Radiolabeled Gold Nanoparticles in Tumor Mice

To get a better insight on the transport mechanism of peptide-conjugated nanoparticles to tumors, we performed in vivo biological studies of bombesin (BBN) peptide functionalized gold nanoparticles (AuNPs) in human prostate tumor bearing mice. Initially, we sought to compare AuNPs with thiol derivatives of acyclic and macrocyclic chelators of DTPA and DOTA types. The DTPA derivatives were unable to provide a stable coordination of (67)Ga, and therefore, the functionalization with the BBN analogues was pursued for the DOTA-containing AuNPs. The DOTA-coated AuNPs were functionalized with BBN[7-14] using a unidentate cysteine group or a bidentate thioctic group to attach the peptide. AuNPs functionalized with thioctic-BBN displayed the highest in vitro cellular internalization (≈ 25%, 15 min) in gastrin releasing peptide (GRP) receptor expressing cancer cells. However, these results fail to translate to in vivo tumor uptake. Biodistribution studies following intravenous (IV) and intraperitoneal (IP) administration of nanoconjugates in tumor bearing mice indicated that the presence of BBN influences to some degree the biological profile of the nanoconstructs. For IV administration, the receptor-mediated pathway appears to be outweighed by the EPR effect. By contrast, in IP administration, it is reasoned that the GRPr-mediated mechanism plays a role in pancreas uptake.

Facile rhenium-peptide conjugate synthesis using a one-pot derived Re(CO)3 reagent

We have synthesized two Re(CO)3-modified lysine complexes (1 and 2), where the metal is attached to the amino acid at the Nε position, via a one-pot Schiff base formation reaction. These compounds can be used in the solid phase synthesis of peptides, and to date we have produced four conjugate systems incorporating neurotensin, bombesin, leutenizing hormone releasing hormone, and a nuclear localization sequence. We observed uptake into human umbilical vascular endothelial cells as well as differential uptake depending on peptide sequence identity, as characterized by fluorescence and rhenium elemental analysis.

A New Highly Reactive and Low Lipophilicity Fluorine-18 Labeled Tetrazine Derivative for Pretargeted PET Imaging

A new (18)F-labeled tetrazine derivative was developed aiming at optimal radiochemistry, fast reaction kinetics in inverse electron-demand Diels-Alder cycloaddition (IEDDA), and favorable pharmacokinetics for in vivo bioorthogonal chemistry. The radiolabeling of the tetrazine was achieved in high yield, purity, and specific activity under mild reaction conditions via conjugation with 5-[(18)F]fluoro-5-deoxyribose, providing a glycosylated tetrazine derivative with low lipophilicity. The (18)F-tetrazine showed fast reaction kinetics toward the most commonly used dienophiles in IEDDA reactions. It exhibited excellent chemical and enzymatic stability in mouse plasma and in phosphate-buffered saline (pH 7.41). Biodistribution in mice revealed favorable pharmacokinetics with major elimination via urinary excretion. The results indicate that the glycosylated (18)F-labeled tetrazine is an excellent candidate for in vivo bioorthogonal chemistry applications in pretargeted PET imaging approaches.

GRPR-selective PET imaging of prostate cancer using [(18)F]-lanthionine-bombesin analogs

The gastrin-releasing peptide receptor (GRPR) is overexpressed in a variety of human malignancies, including prostate cancer. Bombesin (BBN) is a 14 amino acids peptide that selectively binds to GRPR. In this study, we developed two novel Al(18)F-labeled lanthionine-stabilized BBN analogs, designated Al(18)F-NOTA-4,7-lanthionine-BBN and Al(18)F-NOTA-2,6-lanthionine-BBN, for positron emission tomography (PET) imaging of GRPR expression using xenograft prostate cancer models. (Methyl)lanthionine-stabilized 4,7-lanthionine-BBN and 2,6-lanthionine-BBN analogs were conjugated with a NOTA chelator and radiolabeled with Al(18)F using the aluminum fluoride strategy. Al(18)F-NOTA-4,7-lanthionine-BBN and Al(18)F-NOTA-2,6-lanthionine-BBN was labeled with Al(18)F with good radiochemical yield and specific activity>30 GBq/μmol for both radiotracers. The logD values measured for Al(18)F-NOTA-4,7-lanthionine-BBN and Al(18)F-NOTA-2,6-lanthionine-BBN were -2.14 ± 0.14 and -2.34 ± 0.15, respectively. In athymic nude PC-3 xenografts, at 120 min post injection (p.i.), the uptake of Al(18)F-NOTA-4,7-lanthionine-BBN and Al(18)F-NOTA-2,6-lanthionine-BBN in prostate cancer (PC-3) mouse models was 0.82 ± 0.23% ID/g and 1.40 ± 0.81% ID/g, respectively. An excess of unlabeled ɛ-aminocaproic acid-BBN(7-14) (300-fold) was co-injected to assess GRPR binding specificity. Tumor uptake of Al(18)F-NOTA-4,7-lanthionine-BBN and Al(18)F-NOTA-2,6-lanthionine-BBN in PC-3 tumors was evaluated by microPET (μPET) imaging at 30, 60 and 120 min p.i. Blocking studies showed decreased uptake in PC-3 bearing mice. Stabilized 4,7-lanthionine-BBN and 2,6-lanthionine-BBN peptides were rapidly and successfully labeled with (18)F. Both tracers may have potential for GRPR-positive tumor imaging.

2-Fluoropyridine prosthetic compounds for the 18F labeling of bombesin analogues

Acetylene-bearing 2-[(18)F]fluoropyridines [(18)F]FPy5yne and PEG-[(18)F]FPyKYNE were prepared via efficient nucleophilic heteroaromatic [(18)F]fluorination of their corresponding 2-trimethylammoniumpyrdinyl precursors. The prosthetic groups were conjugated to azide- and PEG3-modified bombesin(6-14) analogues via copper-catalyzed azide-alkyne cycloaddition couplings to yield mono- and di-mini-PEGylated ligands for PET imaging of the gastrin- releasing peptide receptor. The PEG3- and PEG2/PEG3-bearing (18)F peptides showed decreased lipophilicity relative to an analogous non-mini-PEGylated (18)F peptide. Assessment of water-soluble peptide pharmacokinetics and tumour-targeting capabilities in a mouse model of prostate cancer is currently underway.

Targeting GRPR in urological cancers--from basic research to clinical application

Gastrin releasing peptide (GRP) is a regulatory peptide that acts through its receptor (GRPR) to regulate physiological functions in various organs. GRPR is overexpressed in neoplastic cells of most prostate cancers and some renal cell cancers and in the tumoral vessels of urinary tract cancers. Thus, targeting these tumours with specifically designed GRP analogues has potential clinical application. Potent and specific radioactive, cytotoxic or nonradioactive GRP analogues have been designed and tested in various animal tumour models with the aim of receptor targeting for tumour diagnosis or therapy. All three categories of compound were found suitable for tumour targeting in animal models. The cytotoxic and nonradioactive GRP analogues have not yet shown convincing tumour-reducing effects in human trials; however, the first clinical studies of radioactive GRP analogues--both agonists and antagonists--suggest promising opportunities for both diagnostic tumour imaging and radiotherapy of prostate and other GRPR-expressing cancers.

(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.

Radiolabelled peptides for oncological diagnosis

Radiolabelled receptor-binding peptides targeting receptors (over)expressed on tumour cells are widely under investigation for tumour diagnosis and therapy. The concept of using radiolabelled receptor-binding peptides to target receptor-expressing tissues in vivo has stimulated a large body of research in nuclear medicine. The ¹¹¹In-labelled somatostatin analogue octreotide (OctreoScan™) is the most successful radiopeptide for tumour imaging, and was the first to be approved for diagnostic use. Based on the success of these studies, other receptor-targeting peptides such as cholecystokinin/gastrin analogues, glucagon-like peptide-1, bombesin (BN), chemokine receptor CXCR4 targeting peptides, and RGD peptides are currently under development or undergoing clinical trials. In this review, we discuss some of these peptides and their analogues, with regard to their potential for radionuclide imaging of tumours.

Radiolabeled peptides: valuable tools for the detection and treatment of cancer

Human cancer cells overexpress many peptide receptors as molecular targets. Radiolabeled peptides that bind with high affinity and specificity to the receptors on tumor cells hold great potential for both diagnostic imaging and targeted radionuclide therapy. The advantage of solid-phase peptide synthesis, the availability of different chelating agents and prosthetic groups and bioconjugation techniques permit the facile preparation of a wide variety of peptide-based targeting molecules with diverse biological and tumor targeting properties. Some of these peptides, including somatostatin, bombesin, vasoactive intestinal peptide, gastrin, neurotensin, exendin and RGD are currently under investigation. It is anticipated that in the near future many of these peptides may find applications in nuclear oncology. This article presents recent developments in the field of small peptides, and their applications in the diagnosis and treatment of cancer.

Structural modifications of ⁹⁹mTc-labelled bombesin-like peptides for optimizing pharmacokinetics in prostate tumor targeting

PURPOSE

The main goal of the present study was to investigate the importance of the addition of a positively charged aa in the naturally occurring bombesin (BN) peptide for its utilization as radiodiagnostic agent, taking into consideration the biodistribution profile, the pharmacokinetic characteristics and the tumor targeting ability.

METHODS

Two BN-derivatives of the general structure [M-chelator]-(spacer)-BN(2-14)-NH(2), where M: (99m)Tc or (185/187)Re, chelator: Gly-Gly-Cys-, spacer: -(arginine)(3)-, M-BN-A; spacer: -(ornithine)(3)-, M-BN-O; have been prepared and evaluated as tumor imaging agents.

RESULTS

The peptides under study presented high radiolabelling efficiency (>98%), significant stability in human plasma (>60% intact radiolabelled peptide after 1h incubation) and comparable receptor binding affinity with the standard [(125)I-Tyr(4)]-BN. Their internalization rates in the prostate cancer PC-3 cells differed, although the amount of internalized peptide was the same. The biodistribution and the dynamic γ-camera imaging studies in normal and PC-3 tumor-bearing SCID mice have shown significant tumor uptake, combined with fast blood clearance, through the urinary pathway.

CONCLUSION

The addition of the charged aa spacer in the BN structure was advantageous for biodistribution, pharmacokinetics and tumor targeting ability, because it reduced the upper abdominal radioactivity levels and increased tumor/normal tissue contrast ratios.

Radiopeptide imaging and therapy in the United States

Radiolabeled peptides targeted against receptors on the cell surface have been shown to be remarkably specific and effective in the diagnosis and therapy of malignant disease. Much of the early work in this field took place outside the United States, but in recent years the research effort within the United States has accelerated. Most of the initial studies in the United States focused on somatostatin receptor ligands. (111)In-pentetreotide was approved in 1994 and has been used extensively in the diagnosis and management of a wide variety of neuroendocrine tumors, particularly carcinoid. This work was extended to (99m)Tc-labeled analogs, and the most successful, (99m)Tc-depreotide, was approved in 1999. This agent was found to be accurate in the diagnosis of lung cancer, but it was not particularly successful because it was supplanted by (18)F-FDG imaging with positron tomography. More recently, studies with (68)Ga-labeled somatostatin analogs were initiated in the United States. This effort was delayed relative to that in other parts of the world because of difficulty in obtaining the necessary generators and regulatory uncertainty, both of which are less of a problem currently. Several ligands are being developed to image melanoma through targeting of the melanocyte-stimulating hormone receptor. Other ligands are being developed to use the arginine-glycine-aspartate oligopeptide to target angiogenesis and to use bombesin analogs to target the gastrin-releasing peptide receptor for the diagnosis and potential therapy of prostate cancer. Peptide dimers that target 2 receptors simultaneously are also being constructed, potentially increasing the selectivity of the approach significantly. Radiopeptide therapy has been explored with these ligands, initially with high-dose (111)In-pentetreotide. This step has been followed by U.S. participation in several trials with (90)Y-, (177)Lu-, and (188)Re-labeled analogs. Some of these agents are now available clinically outside the United States, and it is important to design and conduct the appropriate trials so that this therapy can be offered within the United States.

In vitro and in vivo evaluation of a 64Cu-labeled NOTA-Bn-SCN-Aoc-bombesin analogue in gastrin-releasing peptide receptor expressing prostate cancer

INTRODUCTION

Bombesin (BN) is an amphibian peptide that binds to the gastrin-releasing peptide receptor (GRPR). It has been demonstrated that BN analogues can be radiolabeled for potential diagnosis and treatment of GRPR-expressing malignancies. Previous studies have conjugated various chelators to the eight C-terminal amino acids of BN [BN(7-14)] for radiolabeling with 64Cu. Recently, (1,4,7-triazacyclononane-1,4,7-triacetic acid) (NOTA) has been evaluated as the five-coordinate 64Cu complex, with results indicating GRPR-specific tumor uptake. This study aimed to conjugate S-2-(4-isothiocyanatobenzyl)-NOTA (p-SCN-Bn-NOTA) to BN(7-14) such that it could form a six-coordinate complex with 64Cu and to evaluate the resulting peptide.

METHODS

p-SCN-NOTA was conjugated to 8-aminooctanoic acid (Aoc)-BN(7-14) in solution to yield NOTA-Bn-SCN-Aoc-BN(7-14). The unlabeled peptide was evaluated in a cell binding assay using PC-3 prostate cancer cells and 125I-Tyr4-BN to determine the IC50 value. The peptide was radiolabeled with 64Cu and evaluated for internalization into PC-3 cells and for tumor uptake in mice bearing PC-3 xenografts using biodistribution and micro-positron emission tomography imaging studies.

RESULTS

The binding assay demonstrated that NOTA-Bn-SCN-Aoc-BN(7-14) bound with high affinity to GRPR with an IC50 of 1.4 nM. The radiolabeled peptide demonstrated time-dependent internalization into PC-3 cells. In vivo, the peptide demonstrated tumor-specific uptake and imaging that were comparable to those of previously reported 64Cu-labeled BN analogues.

CONCLUSIONS

These studies demonstrate that 64Cu-NOTA-Bn-SCN-Aoc-BN(7-14) binds to GRPR-expressing cells and that it can be used for imaging of GRPR-expressing prostate cancer.

Development of anti-EGF receptor peptidomimetics (AERP) as tumor imaging agent

EGFR is over-expressed in several solid tumors including breast, prostate, pancreas, and lung cancers and is correlated to the metastatic potential of the tumor. Anti-EGFR receptor-binding peptidomimetics (AERP) were examined to assess the small molecule's potential use as tumor-specific imaging agents. The aim of this work was to design and characterize the binding specificity of the radiolabeled peptidomimetics to EGFR over-expressing cell lysate and to A431 xenograft tumors. Our newly designed peptidomimetic, AERP, was conjugated to DTPA and labeled with (99m)Tc. The in vivo tumor accumulation of [(99m)Tc] DTPA-AERP-2 was 1.6±0.1%ID/g and tumor to muscle ratio was 5.5. Our studies suggest that this novel peptidomimetic, AERP-2, warrants further development as an EGFR specific tumor-imaging agent.

In vitro and in vivo evaluation of 64Cu-labeled SarAr-bombesin analogs in gastrin-releasing peptide receptor-expressing prostate cancer

Bombesin is a 14-amino-acid amphibian peptide that binds with high affinity to the gastrin-releasing peptide receptor (GRPR), which is overexpressed on a variety of solid tumors. It has been demonstrated that bombesin analogs can be radiolabeled with a variety of radiometals for potential diagnosis and treatment of GRPR-positive tumors. In this regard, several studies have used different chelators conjugated to the 8 C-terminal amino acids of bombesin(7-14) for radiolabeling with (64)Cu. These analogs have demonstrated GRPR-specific small-animal PET of tumors but have various advantages and disadvantages. The objective of this study was to conjugate the previously described (1-N-(4-aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]-eicosane-1,8-diamine) (SarAr) chelator to bombesin(7-14), radiolabel the conjugate with (64)Cu, and evaluate in vitro and in vivo.

METHODS

SarAr was synthesized as previously published and conjugated to bombesin(7-14) by solid-phase peptide synthesis using standard Fmoc chemistry. Succinic acid (SA), 8-aminooctanoic acid (Aoc), and Gly-Ser-Gly (GSG) were used as linkers between SarAr and bombesin(7-14) to yield the resulting SarAr-SA-Aoc-bombesin(7-14) and SarAr-SA-Aoc-GSG-bombesin(7-14) peptides. The unlabeled peptides were evaluated in a competitive binding assay using PC-3 prostate cancer cells and (125)I-Tyr(4)-bombesin to determine the inhibitory concentration of 50%. The peptides were radiolabeled with (64)Cu and evaluated for internalization into PC-3 cells in vitro and for in vivo tumor uptake in mice bearing PC-3 xenografts using biodistribution and small-animal PET/CT studies.

RESULTS

The competitive binding assay demonstrated that both SarAr-SA-Aoc-bombesin(7-14) and SarAr-SA-Aoc-GSG-bombesin(7-14) bound with high affinity to GRPR with an inhibitory concentration of 50% of 3.5 and 4.5 nM, respectively. Both peptides were radiolabeled with (64)Cu at room temperature without further purification and demonstrated similar internalization into PC-3 cells. In vivo, the radiolabeled peptides demonstrated tumor-specific uptake (13.0 and 8.5 percentage injected dose per gram for (64)Cu-SarAr-SA-Aoc-bombesin(7-14) and (64)Cu-SarAr-SA-Aoc-GSG-bombesin(7-14), respectively, at 1 h) and imaging that was comparable to, or better than, that of the previously reported (64)Cu-labeled bombesin analogs. The (64)Cu-SarAr-SA-Aoc-GSG-bombesin(7-14) had more rapid blood clearance and lower tumor and normal-tissue uptake than (64)Cu-SarAr-SA-Aoc-bombesin(7-14), resulting in similar tumor-to-blood ratios for each analog (15.1 vs. 11.3 for (64)Cu-SarAr-SA-Aoc-bombesin(7-14) and (64)Cu-SarAr-SA-Aoc-GSG-bombesin(7-14), respectively, at 1 h).

CONCLUSION

These studies demonstrate that (64)Cu-SarAr-SA-Aoc-bombesin(7-14) and (64)Cu-SarAr-SA-Aoc-GSG-bombesin(7-14) bound with high affinity to GRPR-expressing cells and that these peptides can be used for PET of GRPR-expressing prostate cancer.

Peptide receptor imaging of prostate cancer with radiolabelled bombesin analogues

Prostate Cancer (PC) is a type of cancer that is often diagnosed at very early stages due to improved detection among man in the Western world. Current imaging techniques are not optimal to determine extent of minimal early stage PC even though this is of great clinical importance. Human PC and high-grade PIN have shown high Gastrin-Releasing Peptide Receptor (GRPR) expression, while normal prostate tissue and BPH revealed to be predominantly GRPR-negative. Radiolabelled Gastrin-Releasing Peptide (GRP) or bombesin (BN) analogues targeting the GRPR can be used as non-invasive tools to diagnose, monitor and potentially treat PC. These BN analogues have already proven to be able to image PC in both tumour-bearing mice and clinical patients showing no important side effects. It's desirable that new peptides require fast-track standardised comparative testing in relevant PC models to select the best performing BN analogues for further evaluation in patients. Although knowledge about GRPR expression and development of new BN analogues can be extended, it is time to study performance of BN analogues for peptide receptor based imaging in patients validating results of PC imaging using histopathology as a golden standard.

Gastrin releasing protein receptor specific gold nanorods: breast and prostate tumor avid nanovectors for molecular imaging

Gastrin releasing protein receptor specific bombesin (BBN) peptide-gold nanoconjugates were successfully synthesized using gold nanorods and dithiolated peptide. The gold nanorod-bombesin (GNR-BBN) conjugates showed extraordinary in vitro stabilities against various biomolecules including NaCl, cysteine, histidine, bovine serum albumin, human serum albumin, and dithiothreitol. Quantitative measurements on the binding affinity (IC(50)) of GNR-BBN conjugates toward prostate and breast tumor cells were evaluated. The IC(50) values establish that GNR-BBN conjugates have strong affinity toward the gastrin releasing peptide receptors on both the tumors. Detailed cellular interaction studies of GNR-BBN conjugates revealed that nanorods internalize via a receptor-mediated endocytosis pathway. The receptor specific interactions of GNR-BBN conjugates provide realistic opportunities in the design and development of in vivo molecular imaging and therapy agents for cancer.

Glycation methods for bombesin analogs containing the (NalphaHis)Ac chelator for 99mTc(CO)3 radiolabeling

The overexpression of peptide receptors in a variety of human carcinomas has generated considerable interest in peptide-based radiopharmaceuticals for peptide receptor imaging and peptide receptor radiotherapy. The gastrin-releasing peptide receptor is overexpressed in human prostate-, breast-, colon- and small cell lung carcinoma cells. We have developed metabolically stable (99m)Tc-radiolabeled bombesin ([Cha(13), Nle(14)]BBS(7-14)) analogs, which bind with high affinity to the gastrin-releasing peptide receptors. However, because of their lipophilicity, they showed unfavorable biodistribution with high hepatic accumulation and hepatobiliary excretion. We now report a study of different glycation methods for [Cha(13), Nle(14)]BBS(7-14) analogs to improve their biodistribution profile. Whereas the glycation using the Maillard reaction was problematic, resulting in low yields, selective introduction of the glycomimetic shikimic acid to the side chain of a Lys residue was possible. A chemoselective ligation of alpha-D-glucose to an amino-oxyacetylated [Cha(13), Nle(14)]BBS(7-14) analog could be achieved, but was complicated by the co-elution of starting peptide and glycopeptide. The best procedure consisted of the [1,3]-cycloaddition of N(3)-beta-D-glucose to a propargylglycine-containing [Cha(13), Nle(14)]BBS(7-14) analog, using a catalytic amount of Cu(I)I. All glycated [Cha(13), Nle(14)]BBS(7-14) analogs showed high affinity for the gastrin-releasing peptide receptor and rapid accumulation into PC-3 tumor cells.

Design, synthesis, and biological evaluation of an antagonist-bombesin analogue as targeting vector

The gastrin releasing peptide receptor (GRP-R) is overexpressed on a number of tumors and cancer cell lines including pancreas, prostate, breast, gastrointestinal, and small cell lung cancer (SCLC). Radiolabeled bombesin (BBN) analogues have exhibited high binding affinity and specificity to the GRP-R. A bombesin analogue with an antagonist targeting vector at the C-terminus, DOTA-aminohexanoyl-[D-Phe(6), Leu-NHCH 2CH 2CH3(13), des Met(14)] BBN[6-14] (1, "Bomproamide"), has been synthesized and displays high binding affinity (IC50 = 1.36 +/- 0.09 nM) against (125)I-Tyr (4)-BBN in in vitro competitive assays using PC-3 cells. Maximum internalization of (111)In-1 reached 14% in PC-3 cells after 45 min of incubation. Rapid (0.25 h PI) and high (12.21 +/- 3.2%ID/g) pancreatic uptake of (111)In-1 was observed in healthy CF-1 mice, and 90% of the activity was blocked by coinjection of 100 mug of BBN. Rapid (0.25 h PI) and high uptake (6.90 +/- 1.06%ID/g) was observed in PC-3 prostate cancer xenografts in SCID mice, as well as visualized clearly in a SPECT/CT study. These results support the use of a bombesin construct with an antagonist C-terminal vector as a candidate of choice for specific in vivo imaging of tumors overexpressing GRP-receptors.

Molecular imaging of metastatic potential

If molecular imaging is to prove clinically useful it will have to surpass current, primarily anatomic techniques in terms of sensitivity and the ability to detect minimal changes in tissue. One of the most important tests for molecular imaging is to determine whether it can image the metastatic potential of tumors. Like all predictive endeavors, the imaging of such "potential" is a daunting task, but one that only molecular imaging--rather than standard, anatomic techniques--is likely to solve. Although difficult, imaging of metastatic potential is also arguably the most important task for molecular imaging of cancer because it is generally the dissemination of malignant tissue, not its prolonged residence in an inopportune site, which kills the patient. Below are examples of uses of molecular imaging of metastases as well as of metastatic potential, the former being a far more developed area of clinical inquiry.

99mTc-labeled bombesin(7-14)NH2 with favorable properties for SPECT imaging of colon cancer

In this report, we present the synthesis and evaluation of the (99m)Tc-labeled beta-Ala-BN(7-14)NH2 (ABN = beta-Ala-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2) as a new radiotracer for tumor imaging in the BALB/c nude mice bearing HT-29 human colon cancer xenografts. The gastrin releasing peptide receptor binding affinity of ABN and HYNIC-ABN (6-hydrazinonicotinamide) was assessed via a competitive displacement of (125)I-[Tyr4]BBN bound to the PC-3 human prostate carcinoma cells. The IC 50 values were calculated to be 24 +/- 2 nM and 38 +/- 1 nM for ABN and HYNIC-ABN, respectively. HYNIC is the bifunctional coupling agent for (99m)Tc-labeling, while tricine and TPPTS (trisodium triphenylphosphine-3,3',3''-trisulfonate) are used as coligands to prepare the ternary ligand complex [(99m)Tc(HYNIC-ABN)(tricine)(TPPTS)] in very high yield and high specific activity. Because of its high hydrophilicity (log P = -2.39 +/- 0.06), [(99m)Tc(HYNIC-ABN)(tricine)(TPPS)] was excreted mainly through the renal route with little radioactivity accumulation in the liver, lungs, stomach, and gastrointestinal tract. The tumor uptake at 30 min postinjection (p.i.) was 1.59 +/- 0.23%ID/g with a steady tumor washout over the 4 h study period. As a result, it had the best T/ B ratios in the blood (2.37 +/- 0.68), liver (1.69 +/- 0.41), and muscle (11.17 +/- 3.32) at 1 h p.i. Most of the injected radioactivity was found in the urine sample at 1 h p.i., and there was no intact [(99m)Tc(HYNIC-ABN)(tricine)(TPPTS)] detectable in the urine, kidney, and liver samples. Its metabolic instability may contribute to its rapid clearance from the liver, lungs, and stomach. Despite the steady radioactivity washout, the tumors could be clearly visualized in planar images of the BALB/c nude mice bearing the HT-29 human colon xenografts at 1 and 4 h p.i. The favorable excretion kinetics from the liver, lungs, stomach, and gastrointestinal tract makes [(99m)Tc(HYNIC-ABN)(tricine)(TPPTS)] a promising SPECT radiotracer for imaging colon cancer.

Peptide-based radiopharmaceuticals and cytotoxic conjugates: potential tools against cancer

A hope for the diagnosis and treatment of cancer is the development of new tumor-specific peptide-based radiopharmaceuticals. The overexpression of many peptide receptors on human tumors makes such receptors an attractive potential target for diagnostic imaging and radiotherapy with specifically designed radiolabeled peptides. The use of solid-phase peptide synthesis, and the availability of a wide range of bifunctional chelating agents for the convenient radiolabeling of bioactive peptides with different radionuclides have produced a wide variety of medicinally useful peptide radiopharmaceuticals. A few of these peptides, such as somatostatin, bombesin, cholecystokinin/gastrin, neurotensin and vasoactive intestinal peptide are currently under investigation for their possible clinical applications in nuclear oncology. This article presents the recent development in radiolabeled small peptides, with major emphasis on somatostatin and bombesin analogs.

DOTA-PESIN, a DOTA-conjugated bombesin derivative designed for the imaging and targeted radionuclide treatment of bombesin receptor-positive tumours

PURPOSE

We aimed at designing and developing a novel bombesin analogue, DOTA-PEG(4)-BN(7-14) (DOTA-PESIN), with the goal of labelling it with (67/68)Ga and (177)Lu for diagnosis and radionuclide therapy of prostate and other human cancers overexpressing bombesin receptors.

METHODS

The 8-amino acid peptide bombesin (7-14) was coupled to the macrocyclic chelator DOTA via the spacer 15-amino-4,7,10,13-tetraoxapentadecanoic acid (PEG(4)). The conjugate was complexed with Ga(III) and Lu(III) salts. The GRP receptor affinity and the bombesin receptor subtype profile were determined in human tumour specimens expressing the three bombesin receptor subtypes. Internalisation and efflux studies were performed with the human GRP receptor cell line PC-3. Xenografted nude mice were used for biodistribution.

RESULTS

[Ga(III)/Lu(III)]-DOTA-PESIN showed good affinity to GRP and neuromedin B receptors but no affinity to BB3. [(67)Ga/(177)Lu]-DOTA-PESIN internalised rapidly into PC-3 cells whereas the efflux from PC-3 cells was relatively slow. In vivo experiments showed a high and specific tumour uptake and good retention of [(67)Ga/(177)Lu]-DOTA-PESIN. [(67)Ga/(177)Lu]-DOTA-PESIN highly accumulated in GRP receptor-expressing mouse pancreas. The uptake specificity was demonstrated by blocking tumour uptake and pancreas uptake. Fast clearance was found from blood and all non-target organs except the kidneys. High tumour-to-normal tissue ratios were achieved, which increased with time. PET imaging with [(68)Ga]-DOTA-PESIN was successful in visualising the tumour at 1 h post injection. Planar scintigraphic imaging showed that the (177)Lu-labelled peptide remained in the tumour even 3 days post injection.

CONCLUSION

The newly designed ligands have high potential with regard to PET and SPECT imaging with (68/67)Ga and targeted radionuclide therapy with (177)Lu.

Targeting prostate cancer with radiolabelled bombesins

The fact that a number of common human tumours, including those of breast and prostate, express increased levels of the gastrin-releasing peptide receptor (GRP-R) means that this receptor is a potential target for peptide receptor mediated scintigraphy and targeted radionuclide therapy. Although clinical application is yet in its infancy, there is a considerable literature on preclinical studies aimed at developing suitable radioligands for potential clinical application. This brief review provides an overview of this research and also describes some of the limited clinical studies that have been published.

MicroPET Imaging of Breast Cancer Using Radiolabeled Bombesin Analogs Targeting the Gastrin-releasing Peptide Receptor

Mammography is a well-established method for detecting primary breast cancer; however, it has some limitations that may be overcome using nuclear imaging methods. Current radiopharmaceuticals have limited sensitivity for detecting small primary lesions and it has been suggested that novel radiopharmaceuticals are necessary for detection of primary breast cancer, as well as for detecting metastases and recurrence, or for monitoring therapy. The gastrin-releasing peptide receptor (GRPR) is a seven-transmembrane G-protein coupled receptor that is overexpressed on primary breast cancer and lymph node metastases. Bombesin (BN) is a tetradecapeptide that binds with high affinity to GRPR and can be radiolabeled with the positron-emitter, copper-64 ((64)Cu) for imaging with positron-emission tomography (PET). The goal of this study was to evaluate BN analogs that could be radiolabeled with (64)Cu for PET imaging of breast cancer. A series of BN analogs containing 4, 5, 6, 8, and 12- carbon linkers were evaluated with regard to their binding and internalization into T-47D human breast cancer cells. The (64)Cu-labeled analogs were then evaluated in mice bearing T-47D xenografts by tissue biodistribution and microPET imaging. These studies showed that all of the analogs had IC(50) values <100 nM and were all internalized into T-47D cells. Biodistribution studies showed that the BN analog with the 8-carbon linker not only had the highest tumor uptake but also had high normal tissue uptake in the liver. The analogs containing the 6- or 8-carbon linkers demonstrated good tumor uptake as determined by microPET imaging. Overall, this study shows the feasibility of using positron-labeled BN analogs for PET detection of GRPR-expressing breast cancer.

Preparation and evaluation of 99mTc-EDDA/HYNIC-[Lys3]-bombesin for imaging gastrin-releasing peptide receptor-positive tumours

BACKGROUND

Bombesin is a peptide that was initially isolated from frog skin and which belongs to a large group of neuropeptides with many biological functions. The human equivalent is gastrin-releasing peptide (GRP), whose receptors are over-expressed in a variety of malignant tumours.

AIM

To prepare a HYNIC-[Lys 3]-bombesin analogue that could be easily labelled with 99mTc from lyophilized kit formulations and to evaluate its potential as an imaging agent for GRP receptor-positive tumours.

METHODS

HYNIC was conjugated to the epsilon-amino group of Lys 3 residue at the N-terminal region of bombesin via succinimidyl-N-Boc-HYNIC at pH 9.0. 99mTc labelling was performed by addition of sodium pertechnetate solution and 0.2 M phosphate buffer pH 7.0 to a lyophilized formulation. Stability studies were carried out by reversed phase HPLC and ITLC-SG analyses in serum and cysteine solutions. In-vitro internalization was tested using human prostate cancer PC-3 cells with blocked and non-blocked receptors. Biodistribution and tumour uptake were determined in PC-3 tumour-bearing nude mice.

RESULTS

99mTc-EDDA/HYNIC-[Lys 3]-bombesin was obtained with radiochemical purities >93% and high specific activity ( approximately 0.1 GBq.nmol). Results of in-vitro studies demonstrated a high stability in serum and cysteine solutions, specific cell receptor binding and rapid internalization. Biodistribution data showed a rapid blood clearance, with predominantly renal excretion and specific binding towards GRP receptor-positive tissues such as pancreas and PC-3 tumours.

CONCLUSION

99mTc-EDDA/HYNIC-[Lys 3]-bombesin obtained from lyophilized kit formulations has promising characteristics for the diagnosis of malignant tumours that over-express the GRP receptor.

Radiolabeled peptide conjugates for targeting of the bombesin receptor superfamily subtypes

Research laboratories around the world are currently focusing their efforts toward the development of radiometallated, site-directed, diagnostic/therapeutic agents based upon small peptides such as octreotide, neurotensin, alpha-melanocyte stimulating hormone, vasointestinal peptide and others. Bombesin (BBN) or derivatives of bombesin are also of significant interest. Bombesin is a 14-amino-acid peptide with very high affinity for the BB2 or gastrin-releasing peptide receptor (GRPr). Over-expression of the GRPr on a variety of human cancers (i.e., breast, prostate, pancreatic, small cell lung, etc.) provides potential efficacy toward development of radiometallated BBN derivatives for targeting and, hence, diagnosis/treatment of these specific diseases. New derivatives are being developed that are also capable of targeting the BB1 and BB3 receptor subtypes that are over-expressed on cancer cells. This review highlights some of the more recent developments toward design of BBN receptor-specific radiopharmaceuticals that have taken place over the past 2 years.