Two technetium-99m-labeled cholecystokinin-8 (CCK8) peptides for scintigraphic imaging of CCK receptors

Downregulation of CCKBR Expression Inhibits the Proliferation of Gastric Cancer Cells, Revealing a Potential Target for Immunotoxin Therapy

Background Increased CCKBR expression density or frequency has been reported in many neoplasms. Objective We aimed to investigate whether CCKBR drives the growth of gastric cancer (GC) and its potential as a therapeutic target of immunotoxins. Methods A lentiviral interference system was used to generate CCKBR-knockdown gastric cancer cells. Cell Counting Kit-8 and clonogenic assays were used to evaluate cell proliferation. Wound-healing and cell invasion assays were performed to evaluate cell mobility. Cell cycle was analyzed by flow cytometry. Tumor growth in vivo was investigated using a heterologous tumor transplantation model in nude mice. In addition, we generated the immunotoxin FQ17P and evaluated the combining capacity and tumor cytotoxicity of FQ17P in vitro. Results Stable downregulation of CCKBR expression resulted in reduced proliferation, migration and invasion of BGC-823 and SGC-7901 cells. The impact of CCKBR on gastric cancer cells was further verified through CCKBR overexpression studies. Downregulation of CCKBR expression also inhibited the growth of gastric tumors in vivo. Furthermore, FQ17P killed CCKBR-overexpressing GC cells by specifically binding to CCKBR on the tumor cell surface. Conclusion The CCKBR protein drives the growth, migration, and invasion of gastric cancer cells, and it might be a promising target for immunotoxin therapy based on its aberrant expression, functional binding interactions with gastrin, and subsequent internalization.

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.

Cholecystokinin-2 Receptor Targeting with Radiolabeled Peptides: Current Status and Future Directions

A wide variety of radiolabeled peptide analogs for specific targeting of cholecystokinin- 2 receptors (CCK2R) has been developed in the last decades. Peptide probes based on the natural ligands Minigastrin (MG) and Cholecystokinin (CCK) have a high potential for molecular imaging and targeted radiotherapy of different human tumors, such as Medullary Thyroid Carcinoma (MTC) and Small Cell Lung Cancer (SCLC). MG analogs with high persistent uptake in CCK2R expressing tumors have been preferably used for the development of radiolabeled peptide analogs. The clinical translation of CCK2R targeting has been prevented due to high kidney uptake or low metabolic stability of the different radiopeptides developed. Great efforts in radiopharmaceutical development have been undertaken to overcome these limitations. Various modifications in the linear peptide sequence of MG have been introduced mainly with the aim to reduce kidney retention. Furthermore, improved tumor uptake could be obtained by in situ stabilization of the radiopeptide against enzymatic degradation through coinjection of peptidase inhibitors. Recent developments focusing on the stabilization of the Cterminal receptor binding sequence (Trp-Met-Asp-Phe-NH2) have led to new radiolabeled MG analogs with highly improved tumor uptake and tumor-to-kidney ratio. In this review, all the different aspects in the radiopharmaceutical development of CCK2R targeting peptide probes are covered, giving also an overview on the clinical investigations performed so far. The recent development of radiolabeled MG analogs, which are highly stabilized against enzymatic degradation in vivo, promises to have a high impact on the clinical management of patients with CCK2R expressing tumors in the near future.

Elucidating the Structure-Activity Relationship of the Pentaglutamic Acid Sequence of Minigastrin with Cholecystokinin Receptor Subtype 2

Derivatized minigastrin analogues make up a promising class of candidates for targeting cholecystokinin receptor subtype 2 (CCK2R), which is overexpressed on cancer cells of various neuroendocrine tumors. The pentaglutamic acid sequence of minigastrin influences its biological properties. In particular, it plays a crucial role in the kidney reuptake mechanism. However, the importance of the binding affinity and interaction of this region with the receptor on a molecular level remains unclear. To elucidate its structure-activity relationship with CCK2R, we replaced this sequence with various linkers differing in their amount of anionic charge, structural characteristics, and flexibility. Specifically, a flexible aliphatic linker, a linker with only three d-Glu residues, and a structured linker with four adjacent β³-glutamic acid residues were evaluated and compared to the lead compound PP-F11N (DOTA-[d-Glu^1-6,Nle¹¹]gastrin-13). 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was conjugated to the minigastrin derivatives, which allowed radiolabeling with Lutetium-177. The levels of In vitro internalization into MZ-CRC1 cells and in vivo tumor uptake as well as human blood plasma stability increased in the following order: aliphatic linker < three d-Glu < (β³-Glu)₄ < (d-Glu)₆. The in vitro and in vivo behavior was therefore significantly improved with anionic charges. Computational modeling of a CCK2 receptor-ligand complex revealed ionic interactions between cationic residues (Arg and His) of the receptor and anionic residues of the ligand in the linker.

Tumor targeting with 99m Tc radiolabeled peptides: Clinical application and recent development

Targeting overexpressed receptors on the cancer cells with radiolabeled peptides has become very important in nuclear oncology in the recent years. Peptides are small and have easy preparation and easy radiolabeling protocol with no side-effect and toxicity. These properties made them a valuable tool for tumor targeting. Based on the successful imaging of neuroendocrine tumors with ¹¹¹ In-octreotide, other receptor-targeting peptides such as bombesin (BBN), cholecystokinin/gastrin analogues, neurotensin analogues, glucagon-like peptide-1, and RGD peptides are currently under development or undergoing clinical trials. The most frequently used radionuclides for tumor imaging are ^99m Tc and ¹¹¹ In for single-photon emission computed tomography and ⁶⁸ Ga and ¹⁸ F for positron emission tomography imaging. This review presents some of the ^99m Tc-labeled peptides, with regard to their potential for radionuclide imaging of tumors in clinical and preclinical application.

Structural studies on radiopharmaceutical DOTA-minigastrin analogue (CP04) complexes and their interaction with CCK2 receptor

Background

The cholecystokinin receptor subtype 2 (CCK-2R) is an important target for diagnostic imaging and targeted radionuclide therapy (TRNT) due to its overexpression in certain cancers (e.g., medullary thyroid carcinoma (MTC)), thus matching with a theranostic principle. Several peptide conjugates suitable for the TRNT of MTC have been synthesized, including a very promising minigastrin analogue DOTA-(DGlu)₆-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH₂ (CP04). In this contribution, we wanted to see whether CP04 binding affinity for CCK-2R is sensitive to the type of the complexed radiometal, as well as to get insights into the structure of CP04-CCK2R complex by molecular modeling.

Results

In vitro studies demonstrated that there is no significant difference in CCK-2R binding affinity and specific cellular uptake between the CP04 conjugates complexed with [⁶⁸Ga]Ga³⁺ or [¹⁷⁷Lu]Lu³⁺. In order to investigate the background of this observation, we proposed a binding model of CP04 with CCK-2R based on homology modeling and molecular docking. In this model, the C-terminal part of the molecule enters the cavity formed between the receptor helices, while the N-terminus (including DOTA and the metal) is located at the binding site outlet, exposed in large extent to the solvent. The radiometals do not influence the conformation of the molecule except for the direct neighborhood of the chelating moiety.

Conclusions

The model seems to be in agreement with much of structure-activity relationship (SAR) studies reported for cholecystokinin and for CCK-2R-targeting radiopharmaceuticals. It also explains relative insensitivity of CCK-2R affinity for the change of the metal. The proposed model partially fits the reported site-directed mutagenesis data.

Assessment of cholecystokinin 2 receptor (CCK2R) in neoplastic tissue

The expression of cholecystokinin 2 receptor (CCK2R, CCKBR or gastrin receptor) has been reported on a diverse range of cancers such as colorectal, liver, lung, pancreatic, ovarian, stomach, thyroid and numerous neuroendocrine/carcinoid tumors. Some cancers of the colorectum, lung, pancreas and thyroid have been shown to overexpress CCK2R in relation to normal matched tissues of the same organ. This reported overexpression has led to the development of a number of CCK2R-ligand targeted imaging and therapeutic agents. However, no comprehensive study comparing the expression of CCK2R in multiple cancers to multiple normal tissues has been performed. Herein, we report the immunohistochemical analysis of cancer samples from gastrointestinal stromal tumor (GIST), hepatocellular carcinoma (HCC), non-small cell lung cancer (NSCLC), pancreatic adenocarcinoma, and thyroid cancer against multiple normal tissue samples from esophagus, liver, lung, pancreas, stomach, spleen and thyroid. These results show that CCK2R expression is present in nearly all cancer and normal samples tested and that none of the cancer samples had expression that was statistically greater than that of all of the normal samples.

Development of a Radiolabeled Peptide-Based Probe Targeting MT1-MMP for Breast Cancer Detection

Breast cancer is one of the most frequent and aggressive primary tumors among women of all races. Matrix metalloproteinase (MMPs), a family of zinc- and calcium-dependent secreted or membrane anchored endopeptidases, is overexpressed in varieties of diseases including breast cancer. Therefore, noninvasive visualization and quantification of MMP in vivo are of great interest in basic research and clinical application for breast cancer early diagnosis. Herein, we developed a 99mTc labeled membrane type I matrix metalloproteinase (MT1-MMP) specific binding peptide, [99mTc]-(HYNIC-AF7p)(tricine)(TPPTS), for in vivo detection of MDA-MB-231 breast tumor by single photon emission computed tomography (SPECT). [99mTc]-(HYNIC-AF7p)(tricine)(TPPTS) demonstrated nice biostability and high MT1-MMP binding affinity in vitro and in vivo. Tumor-to-muscle ratio was found to reach to the highest (4.17±0.49) at 2 hour after intravenously administration of [99mTc]-(HYNIC-AF7P)(tricine)(TPPTS) into MDA-MB-231 tumor bearing mice. Overall, [99mTc]-(HYNIC-AF7P)(tricine)(TPPTS) demonstrated great potential for MT1-MMP targeted detection in vivo and it would be a promising molecular imaging probe that are probably beneficial to breast cancer early diagnoses.

Stereochemistry of amino acid spacers determines the pharmacokinetics of (111)In-DOTA-minigastrin analogues for targeting the CCK2/gastrin receptor

The metabolic instability and high kidney retention of minigastrin (MG) analogues hamper their suitability for use in peptide-receptor radionuclide therapy of CCK2/gastrin receptor-expressing tumors. High kidney retention has been related to N-terminal glutamic acids and can be substantially reduced by coinjection of polyglutamic acids or gelofusine. The aim of the present study was to investigate the influence of the stereochemistry of the N-terminal amino acid spacer on the enzymatic stability and pharmacokinetics of (111)In-DOTA-(d-Glu)6-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2 ((111)In-PP11-D) and (111)In-DOTA-(l-Glu)6-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2 ((111)In-PP11-L). Using circular dichroism measurements, we demonstrate the important role of secondary structure on the pharmacokinetics of the two MG analogues. The higher in vitro serum stability together with the improved tumor-to-kidney ratio of the (d-Glu)6 congener indicates that this MG analogue might be a good candidate for further clinical study.

Liposomal doxorubicin doubly functionalized with CCK8 and R8 peptide sequences for selective intracellular drug delivery

A new dual-ligand liposomal doxorubicin delivery system, which couples targeting to enhanced cellular uptake and may lead to a more efficient drug delivery system, is here designed and synthetized. Liposomes based on the composition 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-distearoyl-sn-glycero-3-phosphoethanolamine-Peg2000-R8/(C18)2-L5-SS-CCK8 (87/8/5 mol/mol/mol) were prepared and loaded with doxorubicin. Presence of the two peptides on the external surface is demonstrated by fluorescence resonance energy transfer assay. The combination of the R8 cell-penetrating peptide and of the CCK8 targeting peptide (homing peptide) on the liposome surface is obtained by combining pre-modification and post-modification methods. In the dual-ligand system, the CCK8 peptide is anchored to the liposome surface by using a disulfide bond. This chemical function is inserted in order to promote the selective cleavage of the homing peptide under the reductive conditions expected in proximity of the tumor site, thus allowing targeting and internalization of the liposomal drug.

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.

Radiopharmaceutical development of radiolabelled peptides

Receptor targeting with radiolabelled peptides has become very important in nuclear medicine and oncology in the past few years. The overexpression of many peptide receptors in numerous cancers, compared to their relatively low density in physiological organs, represents the molecular basis for in vivo imaging and targeted radionuclide therapy with radiolabelled peptide-based probes. The prototypes are analogs of somatostatin which are routinely used in the clinic. More recent developments include somatostatin analogs with a broader receptor subtype profile or with antagonistic properties. Many other peptide families such as bombesin, cholecystokinin/gastrin, glucagon-like peptide-1 (GLP-1)/exendin, arginine-glycine-aspartic acid (RGD) etc. have been explored during the last few years and quite a number of potential radiolabelled probes have been derived from them. On the other hand, a variety of strategies and optimized protocols for efficient labelling of peptides with clinically relevant radionuclides such as (99m)Tc, M(3+) radiometals ((111)In, (86/90)Y, (177)Lu, (67/68)Ga), (64/67)Cu, (18)F or radioisotopes of iodine have been developed. The labelling approaches include direct labelling, the use of bifunctional chelators or prosthetic groups. The choice of the labelling approach is driven by the nature and the chemical properties of the radionuclide. Additionally, chemical strategies, including modification of the amino acid sequence and introduction of linkers/spacers with different characteristics, have been explored for the improvement of the overall performance of the radiopeptides, e.g. metabolic stability and pharmacokinetics. Herein, we discuss the development of peptides as radiopharmaceuticals starting from the choice of the labelling method and the conditions to the design and optimization of the peptide probe, as well as some recent developments, focusing on a selected list of peptide families, including somatostatin, bombesin, cholecystokinin/gastrin, GLP-1/exendin and RGD.

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.

Comparative biodistribution of 12 ¹¹¹In-labelled gastrin/CCK2 receptor-targeting peptides

PURPOSE

Cholecystokinin 2 (CCK-2) receptor overexpression has been demonstrated in various tumours such as medullary thyroid carcinomas and small-cell lung cancers. Due to this high expression, CCK-2 receptors might be suitable targets for radionuclide imaging and/or radionuclide therapy. Several CCK-2 receptor-binding radiopeptides have been developed and some have been tested in patients. Here we aimed to compare the in vivo tumour targeting properties of 12 (111)In-labelled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated gastrin/CCK2 receptor-binding peptides.

METHODS

Two CCK8-based peptides and ten gastrin-based peptide analogues were tested. All peptides were conjugated with DOTA and labelled with (111)In. Biodistribution studies were performed in mice with subcutaneous CCK2/gastrin receptor-expressing tumours and with receptor-negative tumours contralaterally. Biodistribution was studied by counting dissected tissues at 1 and 4 h after injection.

RESULTS

Both the CCK analogues displayed relatively low tumour uptake (approximately 2.5%ID/g) as compared to minigastrin analogues. Two linear minigastrin peptides (MG0 and sargastrin) displayed moderate tumour uptake at both 1 and 4 h after injection, but also very high kidney uptake (both higher than 48%ID/g). The linear MG11, lacking the penta-Glu sequence, showed lower tumour uptake and also low kidney uptake. Varying the N-terminal Glu residues in the minigastrin analogues led to improved tumour targeting properties, with PP-F11 displaying the optimal biodistribution. Besides the monomeric linear peptides, a cyclized peptide and a divalent peptide were tested.

CONCLUSION

Based on these studies, optimal peptides for peptide receptor radionuclide targeting of CCK2/gastrin receptor-expressing tumours were the linear minigastrin analogue with six D-Glu residues (PP-F11), the divalent analogue MGD5 and the cyclic peptide cyclo-MG1. These peptides combined high tumour uptake with low kidney retention, and may therefore be good candidates for future clinical studies.

Stabilized (111)in-labeled sCCK8 analogues for targeting CCK2-receptor positive tumors: synthesis and evaluation

Radiolabeled cholecystokinin-8 (CCK8) peptide analogues can be used for peptide receptor radionuclide imaging and therapy for tumors expressing CCK2/gastrin receptors. Earlier findings indicated that sulfated CCK8 (sCCK8, Asp-Tyr(OSO(3)H)-Met-Gly-Trp-Met-Asp-Phe-NH(2)) may have better characteristics for peptide receptor radionuclide therapy (PRRT) than gastrin analogues. However, sCCK8 contains an easily hydrolyzable sulfated tyrosine residue and two methionine residues which are prone to oxidation. Here, we describe the synthesis of stabilized sCCK8 analogues, resistant to hydrolysis and oxidation. Hydrolytic stability was achieved by replacement of the Tyr(OSO(3)H) moiety by a robust isosteric sulfonate, Phe(p-CH(2)SO(3)H). Replacement of methionine by norleucine (Nle) or homopropargylglycine (HPG) avoided undesired oxidation side-reactions. The phenylalanine analogue Phe(p-CH(2)SO(3)H) of l-tyrosine, synthesized by a modification of known synthetic routes, was incorporated in three peptides: sCCK8[Phe(2)(p-CH(2)SO(3)H),Met(3,6)], sCCK8[Phe(2)(p-CH(2)SO(3)H),Nle(3,6)], and sCCK8[Phe(2)(p-CH(2)SO(3)H),HPG(3,6)]. All peptides were N-terminally conjugated with the macrocyclic chelator DOTA (1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) and radiolabeled with In-111. In vitro binding assays on CCK2R-expressing HEK293 cells revealed that all three peptides showed specific binding and receptor-mediated internalization, with binding affinity values (IC(50)) in the nanomolar range. In vitro oxidation studies demonstrated that peptides with Nle or HPG indeed were resistant to oxidation. In vivo targeting studies in mice with AR42J tumors showed that tumor uptake was highest for (111)In-DOTA-sCCK8 and (111)In-DOTA-sCCK8[Phe(2)(p-CH(2)SO(3)H),Nle(3,6)] (4.78 +/- 0.64 and 4.54 +/- 1.15%ID/g, respectively, 2 h p.i.). The peptide with the methionine residues replaced by norleucine ((111)In-DOTA-sCCK8[Phe(2)(p-CH(2)SO(3)H), Nle(3,6)]) showed promising in vivo characteristics and will be further investigated for radionuclide imaging and therapy of CCK2R-expressing tumors.

Radiolabeled CCK/gastrin peptides for imaging and therapy of CCK2 receptor-expressing tumors

Cholecystokinin (CCK) receptors are overexpressed in numerous human cancers, like medullary thyroid carcinomas, small cell lung cancers and stromal ovarian cancers. The specific receptor-binding property of the endogenous ligands for these receptors can be exploited by labeling peptides with a radionuclide and using these as carriers to guide the radioactivity to the tissues that express the receptors. In this way, tumors can be visualized using positron emission tomography and single photon emission computed tomography imaging. A variety of radiolabeled CCK/gastrin-related peptides has been synthesized and characterized for imaging. All peptides have the C-terminal CCK receptor-binding tetrapeptide sequence Trp-Met-Asp-Phe-NH₂ in common or derivatives thereof. This review focuses on the development and application of radiolabeled CCK/gastrin peptides for radionuclide imaging and radionuclide therapy of tumors expressing CCK receptors. We discuss both preclinical studies as well as clinical studies with CCK and gastrin peptides.

Evaluation of Cholecystokinin (CCK-8) Peptide Thermal Stability for Use as Radiopharmaceutical by Means Isothermal and Nonisothermal Approaches

The purpose of this research was to study the thermal stability of cholecystokinin octapeptide (CCK-8) in aqueous solution at pH 12 and ionic strength 0.01 M, which were kept as constants, by using isothermal and nonisothermal methods. The isothermal decomposition of CCK-8 was investigated as a function of temperature (40 degrees C to 70 degrees C). Nonisothermal stability studies were performed using a linear increasing temperature program. Two different nonisothermal studies were carried out at 0.25 degrees K and 0.5 degrees K per hour, and the temperature interval varied from 40 degrees C to 82 degrees C. The degradation of CCK-8 followed first-order kinetics, obeying the Arrhenius equation in the experimental temperature range. This indicated that the degradation mechanism of CCK-8 could be the equal within the temperature range studied. The nonisothermal approach resulted in activation energy (Ea) and shelf-life (t90%) values that agree well with those obtained by the isothermal method. The level of uncertainty in the estimates of t90% and Ea values is determined mainly by the extent of drug degradation and temperature change during the experiment. Therefore, nonisothermal experiments save time, labor and materials (i.e. the amount of drugs necessary to conduct the experiment) compared to the classic isothermal experiments, if they are performed using a suitable experimental design and a precise analytical method.

Coligand effects on the solution stability, biodistribution and metabolism of the (99m)Tc-labeled cyclic RGDfK tetramer

In this study, we present the evaluation of two new ternary ligand (99m)Tc complexes [(99m)Tc(HYNIC tetramer)(tricine)(L)] [L=isonicotinic acid (ISONIC) and 2,5-pyridinedicarboxylic acid (PDA)] as potential radiotracers for tumor imaging. Athymic nude mice bearing MDA-MB-435 human breast cancer xenografts were used to evaluate their biodistribution and metabolic properties. Solution stability data showed that [(99m)Tc(HYNIC tetramer)(tricine)(L)] (L=ISONIC and PDA) had significant decomposition (14% and 35%, respectively) at 6 h in the absence of excess ISONIC or PDA coligand. Biodistribution data clearly showed that [(99m)Tc(HYNIC tetramer)(tricine)(PDA)] had a much lower uptake in most organs of interest than [(99m)Tc(HYNIC tetramer)(tricine)(ISONIC)] during the 2-h study period. Results from metabolism studies revealed that approximately 50% of [(99m)Tc(HYNIC tetramer)(tricine)(ISONIC)] remained intact in fecal samples at 120 min postinjection, whereas only 10% of [(99m)Tc(HYNIC tetramer)(tricine)(PDA)] remained intact in fecal samples. The extent of metabolism correlated well with radiotracer solution stability. The results from this and our previous studies clearly demonstrated that coligands [trisodium triphenylphosphine-3,3',3''-trisulfonate (TPPTS), ISONIC and PDA] have a significant impact on the tumor uptake, excretion kinetics and metabolism of the (99m)Tc-labeled cyclic RGDfK tetramer. Among the three radiotracers evaluated in this tumor-bearing animal model, [(99m)Tc(HYNIC tetramer)(tricine)(TPPTS)] remained the best with respect to blood clearance, tumor uptake and target/background ratios.

Progress in developing cholecystokinin (CCK)/gastrin receptor ligands that have therapeutic potential

Gastrin and cholecystokinin (CCK) are two of the oldest hormones and within the past 15 years there has been an exponential increase in knowledge of their pharmacology, cell biology, receptors (CCK1R and CCK2R), and roles in physiology and pathological conditions. Despite these advances there is no approved disease indication for CCK receptor antagonists and only a minor use of agonists. In this review, the important factors determining this slow therapeutic development are reviewed. To assess this it is necessary to briefly review what is known about the roles of CCK receptors (CCK1R and CCK2R) in normal human physiology, their role in pathologic conditions, the selectivity of available potent CCKR agonists/antagonists as well as to review their use in human conditions to date and the results. Despite extensive studies in animals and in humans, recent studies suggest that monotherapy with CCK1R agonists will not be effective in obesity, nor CCK2R antagonists in panic disorders or CCK2R antagonists to inhibit growth of pancreatic cancer. Areas that require more study include the use of CCK2R agonists for imaging tumors and radiotherapy, CCK2R antagonists in hypergastrinemic states especially with long-term PPI use and for potentiation of analgesia as well as use of CCK1R antagonists for a number of gastrointestinal disorders [motility disorders (irritable bowel syndrome, dyspepsia, and constipation) and pancreatitis (acute and chronic)].

Targeting of a CCK(2) receptor splice variant with (111)In-labelled cholecystokinin-8 (CCK8) and (111)In-labelled minigastrin

PURPOSE

Radiolabelled cholecystokinin (CCK) and gastrin-derived peptides potentially can be used for peptide receptor radionuclide therapy (PRRT). Recently, a splice variant version of the CCK2R has been identified, designated CCK2i4svR. Constitutive expression of this receptor has been demonstrated in human colorectal cancer and in pancreatic cancer, but not in normal tissue. So far, it has never been shown whether radiolabelled peptides can target the CCK2i4svR in vivo. In this paper, we investigated the potential of sulfated (111)In-labelled DOTA-CCK8 (sCCK8), a pan-CCKR-binding peptide, and [(111)In]DOTA-minigastrin (MG0), a CCK2R selective peptide, for the targeting of the CCK2i4svR.

MATERIALS AND METHODS

The receptor binding affinity of [(111)In]DOTA-sCCK8 and [(111)In]DOTA-MG0 for the CCK2R and CCK2i4svR was determined using stably transfected HEK293 cell lines, expressing either CCK2R or CCK2i4svR. Tumour targeting was studied in HEK293-CCK2i4svR tumour-bearing athymic mice.

RESULTS

[(111)In]DOTA-sCCK8 as well as [(111)In]DOTA-MG0 specifically bound both CCK2R and CCK2i4svR with affinities in the low nanomolar range. In vivo experiments revealed that accumulation of both peptides in CCK2i4svR-positive tumours was similar (3.21 +/- 0.77 and 3.01 +/- 0.67%ID/g, sCCK8 and MG0, respectively, 24 h p.i.). Kidney retention of [(111)In]DOTA-MG0 (32.4 +/- 7.5%ID/g, 24 h p.i.) was markedly higher than that of [(111)In]DOTA-sCCK8 (2.75 +/- 0.31%ID/g, 24 h p.i.).

CONCLUSION

We demonstrated that the CCK2i4svR is a potential target for PRRT using a radiolabelled sulfated CCK8 peptide. As this receptor is expressed on colorectal and pancreatic tumours, but not in normal tissue, these tumours are potentially new targets for PRRT with CCK8 and gastrin analogs.

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.

99mTc-labelled HYNIC-minigastrin with reduced kidney uptake for targeting of CCK-2 receptor-positive tumours

PURPOSE

Different attempts have been made to develop a suitable radioligand for targeting CCK-2 receptors in vivo, for staging of medullary thyroid carcinoma (MTC) and other receptor-expressing tumours. After initial successful clinical studies with [DTPA(0),D: Glu(1)]minigastrin (DTPA-MG0) radiolabelled with (111)In and (90)Y, our group developed a (99m)Tc-labelled radioligand, based on HYNIC-MG0. A major drawback observed with these derivatives is their high uptake by the kidneys. In this study we describe the preclinical evaluation of the optimised shortened peptide analogue, [HYNIC(0),D: Glu(1),desGlu(2-6)]minigastrin (HYNIC-MG11).

METHODS

(99m)Tc labelling of HYNIC-MG11 was performed using tricine and EDDA as coligands. Stability experiments were carried out by reversed phase HPLC analysis in PBS, PBS/cysteine and plasma as well as rat liver and kidney homogenates. Receptor binding and cell uptake experiments were performed using AR4-2J rat pancreatic tumour cells. Animal biodistribution was studied in AR4-2J tumour-bearing nude mice.

RESULTS

Radiolabelling was performed at high specific activities and radiochemical purity was >90%. (99m)Tc-EDDA-HYNIC-MG11 showed high affinity for the CCK-2 receptor and cell internalisation comparable to that of (99m)Tc-EDDA-HYNIC-MG0. Despite high stability in solution, a low metabolic stability in rat tissue homogenates was found. In a nude mouse tumour model, very low unspecific retention in most organs, rapid renal excretion with reduced renal retention and high tumour uptake were observed.

CONCLUSION

(99m)Tc-EDDA-HYNIC-MG11 shows advantages over (99m)Tc-EDDA-HYNIC-MG0 in terms of lower kidney retention with unchanged uptake in tumours and CCK-2 receptor-positive tissue. However, the lower metabolic stability and impurities formed in the labelling process still leave room for further improvement.

The [Tc(N)(PNP)]2+ metal fragment labeled cholecystokinin-8 (CCK8) peptide for CCK-2 receptors imaging:in vitro andin vivo studies

The radiolabeling of the natural octapeptide CCK8, derivatized with a cysteine residue (Cys-Gly-CCK8), by using the metal fragment [99mTc(N)(PNP3)]2+ (PNP3 = N,N-bis(dimethoxypropylphosphinoethyl)methoxyethylamine) is reported. The [99mTc(N)(NS-Cys-Gly-CCK8)(PNP3)]+ complex was obtained according to two methods (one-step or two-step procedure) that gave the desired compound in high yield. The complex is stable in aqueous solution and in phosphate buffer. In vitro challenge experiments with an excess of cysteine and glutathione indicate that no transchelation reactions occur, confirming the high thermodynamic stability and kinetic inertness of this compound. Stability studies carried out in human and mouse serum, as well as in mouse liver homogenates, show that the radiolabeled compound remains intact for prolonged incubation at 37 degrees C. Binding properties give Kd (19.0 +/- 4.6 nmol/l) and Bmax (approximately 10(6) sites/cell) values in A431 cells overexpressing the CCK2-R. In vivo evaluation of the compound shows rapid and specific targeting to CCK2-R, a fourfold higher accumulation compared to nonreceptor expressing tumors.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Site-specific 99mTc-labeling of antibody using dihydrazinophthalazine (DHZ) conjugation to Fc region of heavy chain

The development of an antibody labeling method with 99mTc is important for cancer imaging. Most bifunctional chelate methods for 99mTc labeling of antibody incorporate a 99mTc chelator through a linkage to lysine residue. In the present study, a novel site-specific 99mTc labeling method at carbohydrate side chain in the Fc region of 2 antibodies (T101 and rabbit anti-human serum albumin antibody (RPAb)) using dihydrazinophthalazine (DHZ) which has 2 hydrazino groups was developed. The antibodies were oxidized with sodium periodate to produce aldehyde on the Fc region. Then, one hydrazine group of DHZ was conjugated with an aldehyde group of antibody through the formation of a hydrazone. The other hydrazine group was used for labeling with 99mTc. The number of conjugated DHZ was 1.7 per antibody. 99mTc labeling efficiency was 46-85% for T101 and 67-87% for RPAb. Indirect labeling with DHZ conjugated antibodies showed higher stability than direct labeling with reduced antibodies. High immunoreactivities were conserved for both indirectly and directly labeled antibodies. A biodistribution study found high blood activity related to directly labeled T101 at early time point as well as low liver activity due to indirectly labeled T101 at later time point. However, these findings do not affect practical use. No significantly different biodistribution was observed in the other organs. The research concluded that DHZ can be used as a site-specific bifunctional chelating agent for labeling antibody with 99mTc. Moreover, 99mTc labeled antibody via DHZ was found to have excellent chemical and biological properties for nuclear medicine imaging.