Targeting of CCK-2 Receptor–Expressing Tumors Using a Radiolabeled Divalent Gastrin Peptide

Radiolabeled peptides and their expanding role in clinical imaging and targeted cancer therapy

There is an expanding body of evidence showing that synthetic peptides in combination with radioactive isotopes can be utilized for medical purposes. This area is of particular interest in oncology where applications in diagnosis and therapy are at different stages of development. We review the contributions in this area by the group originally founded by Carlo Pedone in Naples many years ago. We highlight the work of this group in the context of other developments in this area, focusing on three biologically relevant receptor systems: somatostatin, gastrin-releasing peptide, and cholecystokinin-2/gastrin receptors. We focus on key milestones, state of the art, and challenges in this area of research as well as the current and future outlook for expanding clinical applications.

Novel small-molecule compound YH7 inhibits the biofilm formation of Staphylococcus aureus in a sarX-dependent manner

The emergence of antibiotic-resistant and biofilm-producing Staphylococcus aureus isolates presents major challenges for treating staphylococcal infections. Biofilm inhibition is an important anti-virulence strategy. In this study, a novel maleimide-diselenide hybrid compound (YH7) was synthesized and demonstrated remarkable antimicrobial activity against methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) in both planktonic cultures and biofilms. The minimum inhibitory concentration (MIC) of YH7 for S. aureus isolates was 16 µg/mL. Quantification of biofilms demonstrated that the sub-MIC (4 µg/mL) of YH7 significantly inhibits biofilm formation in both MSSA and MRSA. Confocal laser scanning microscopy analysis further confirmed the biofilm inhibitory potential of YH7. YH7 also significantly suppressed bacterial adherence to A549 cells. Moreover, YH7 treatment significantly inhibited S. aureus colonization in nasal tissue of mice. Preliminary mechanistic studies revealed that YH7 exerted potent biofilm-suppressing effects by inhibiting polysaccharide intercellular adhesin (PIA) synthesis, rather than suppressing bacterial autolysis. Real-time quantitative PCR data indicated that YH7 downregulated biofilm formation-related genes (clfA, fnbA, icaA, and icaD) and the global regulatory gene sarX, which promotes PIA synthesis. The sarX-dependent antibiofilm potential of YH7 was validated by constructing S. aureus NCTC8325 sarX knockout and complementation strains. Importantly, YH7 demonstrated a low potential to induce drug resistance in S. aureus and exhibited non-toxic to rabbit erythrocytes, A549, and BEAS-2B cells at antibacterial concentrations. In vivo toxicity assays conducted on Galleria mellonella further confirmed that YH7 is biocompatible. Overall, YH7 demonstrated potent antibiofilm activity supports its potential as an antimicrobial agent against S. aureus biofilm-related infections. IMPORTANCE Biofilm-associated infections, characterized by antibiotic resistance and persistence, present a formidable challenge in healthcare. Traditional antibacterial agents prove inadequate against biofilms. In this study, the novel compound YH7 demonstrates potent antibiofilm properties by impeding the adhesion and the polysaccharide intercellular adhesin production of Staphylococcus aureus. Notably, its exceptional efficacy against both methicillin-resistant and methicillin-susceptible strains highlights its broad applicability. This study highlights the potential of YH7 as a novel therapeutic agent to address the pressing issue of biofilm-driven infections.

Development of Highly Potent Clinical Candidates for Theranostic Applications against Cholecystokinin-2 Receptor Positive Cancers

Peptide receptor radionuclide therapy (PRRT) is a promising form of systemic radiation therapy designed to eradicate cancer. Cholecystokinin-2 receptor (CCK2R) is an important molecular target that is highly expressed in a range of cancers. This study describes the synthesis and in vivo characterization of a novel series of 177Lu-labeled peptides ([177Lu]Lu-2b-4b) in comparison with the reference CCK2R-targeting peptide CP04 ([177Lu]Lu-1b). [177Lu]Lu-1b-4b showed high chemical purity (HPLC ≥ 94%), low Log D7.4 (-4.09 to -4.55) with strong binding affinity to CCK2R (KD 0.097-1.61 nM), and relatively high protein binding (55.6-80.2%) and internalization (40-67%). Biodistribution studies of the novel 177Lu-labeled peptides in tumors (AR42J and A431-CCK2R) showed uptake one- to eight-fold greater than the reference compound CP04 at 1, 24, and 48 h. Rapid clearance and high tumor uptake and retention were established for [177Lu]Lu-2b-4b, making these compounds excellent candidates for theranostic applications against CCK2R-expressing tumors.

Effect of N-Terminal Peptide Modifications on In Vitro and In Vivo Properties of 177Lu-Labeled Peptide Analogs Targeting CCK2R

The therapeutic potential of minigastrin (MG) analogs for the treatment of cholecystokinin-2 receptor (CCK2R)-expressing cancers is limited by poor in vivo stability or unfavorable accumulation in non-target tissues. Increased stability against metabolic degradation was achieved by modifying the C-terminal receptor-specific region. This modification led to significantly improved tumor targeting properties. In this study, further N-terminal peptide modifications were investigated. Two novel MG analogs were designed starting from the amino acid sequence of DOTA-MGS5 (DOTA-DGlu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1Nal-NH2). Introduction of a penta-DGlu moiety and replacement of the four N-terminal amino acids by a non-charged hydrophilic linker was investigated. Retained receptor binding was confirmed using two CCK2R-expressing cell lines. The effect on metabolic degradation of the new 177Lu-labeled peptides was studied in human serum in vitro, as well as in BALB/c mice in vivo. The tumor targeting properties of the radiolabeled peptides were assessed using BALB/c nude mice bearing receptor-positive and receptor-negative tumor xenografts. Both novel MG analogs were found to have strong receptor binding, enhanced stability, and high tumor uptake. Replacement of the four N-terminal amino acids by a non-charged hydrophilic linker lowered the absorption in the dose-limiting organs, whereas introduction of the penta-DGlu moiety increased uptake in renal tissue.

Homomultimer strategy for improvement of radiolabeled peptides and antibody fragments in tumor targeting

A homomultimeric radioligand is composed of multiple identical ligands connected to the linker and radionuclide to detect a variety of overexpressed receptors on cancer cells. Multimer strategy holds great potential for introducing new radiotracers based on peptide and monoclonal antibody (mAb) derivatives in molecular imaging and therapy. It offers a reliable procedure for the preparation of biological-based targeting with diverse affinities and pharmacokinetics. In this context, we provide a useful summary and interpretation of the main results by a comprehensive look at multimeric radiopharmaceuticals in nuclear oncology. Therefore, there will be explanations for the strategy mechanisms and the main variables affecting the biodistribution results. The discussion is followed by highlights of recent work in the targeting of various types of receptors. The consequences are expressed based on comparing some parameters between monomer and multimer counterparts in each relevant section.

Update on Preclinical Development and Clinical Translation of Cholecystokinin-2 Receptor Targeting Radiopharmaceuticals

The cholecystokinin-2 receptor (CCK2R) has been a target of interest for molecular imaging and targeted radionuclide therapy for two decades. However, so far CCK2R targeted imaging and therapy has not been introduced in clinical practice. Within this review the recent radiopharmaceutical development of CCK2R targeting compounds and the ongoing clinical trials are presented. Currently, new gastrin derivatives as well as nonpeptidic substances are being developed to improve the properties for clinical use. A team of specialists from the field of radiopharmacy and nuclear medicine reviewed the available literature and summarized their own experiences in the development and clinical testing of CCK2R targeting radiopharmaceuticals. The recent clinical trials with novel radiolabeled minigastrin analogs demonstrate the potential for both applications, imaging as well as targeted radiotherapy, and reinforce the clinical applicability within a theranostic concept. The intense efforts in optimizing CCK2R targeting radiopharmaceuticals has led to new substances for clinical use, as shown in first imaging studies in patients with advanced medullary thyroid cancer. The first clinical results suggest that the wider clinical implication of CCK2R-targeted radiopharmaceuticals is reasonable.

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.

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.

Preclinical SPECT and SPECT-CT in Oncology

Molecular imaging enables both spatial and temporal understanding of the complex biologic systems underlying carcinogenesis and malignant spread. Single-photon emission tomography (SPECT) is a versatile nuclear imaging-based technique with ideal properties to study these processes in vivo in small animal models, as well as to identify potential drug candidates and characterize their antitumor action and potential adverse effects. Small animal SPECT and SPECT-CT (single-photon emission tomography combined with computer tomography) systems continue to evolve, as do the numerous SPECT radiopharmaceutical agents, allowing unprecedented sensitivity and quantitative molecular imaging capabilities. Several of these advances, their specific applications in oncology as well as new areas of exploration are highlighted in this chapter.

Comparative evaluation of dimeric and monomeric forms of ADAPT scaffold protein for targeting of HER2-expressing tumours

ADAPTs are small engineered non-immunoglobulin scaffold proteins, which have demonstrated very promising features as vectors for radionuclide tumour targeting. Radionuclide imaging of human epidermal growth factor 2 (HER2) expression in vivo might be used for stratification of patients for HER2-targeting therapies. ADAPT6, which specifically binds to HER2, has earlier been shown to have very promising features for in vivo targeting of HER2 expressing tumours. In this study we tested the hypothesis that dimerization of ADAPT6 would increase the apparent affinity to HER2 and accordingly improve tumour targeting. To find an optimal molecular design of dimers, a series of ADAPT dimers with different linkers, -SSSG- (DiADAPT6L1), -(SSSG)₂- (DiADAPT6L2), and -(SSSG)₃- (DiADAPT6L3) was evaluated. Dimers in combination with optimal linker lengths demonstrated increased apparent affinity to HER2. The best variants, DiADAPT6L2 and DiADAPT6L3 were site-specifically labelled with ¹¹¹In and ¹²⁵I, and compared with a monomeric ADAPT6 in mice bearing HER2-expressing tumours. Despite higher affinity, both dimers had lower tumour uptake and lower tumour-to-organ ratios compared to the monomer. We conclude that improved affinity of a dimeric form of ADAPT does not compensate the disadvantage of increased size. Therefore, increase of affinity should be obtained by affinity maturation and not by dimerization.

Anti-Leishmanial and Cytotoxic Activities of a Series of Maleimides: Synthesis, Biological Evaluation and Structure-Activity Relationship

In the present study, 45 maleimides have been synthesized and evaluated for anti-leishmanial activities against L. donovani in vitro and cytotoxicity toward THP1 cells. All compounds exhibited obvious anti-leishmanial activities. Among the tested compounds, there were 10 maleimides with superior anti-leishmanial activities to standard drug amphotericin B, and 32 maleimides with superior anti-leishmanial activities to standard drug pentamidine, especially compounds 16 (IC₅₀ < 0.0128 μg/mL) and 42 (IC₅₀ < 0.0128 μg/mL), which showed extraordinary efficacy in an in vitro test and low cytotoxicities (CC₅₀ > 10 μg/mL). The anti-leishmanial activities of 16 and 42 were 10 times better than that of amphotericin B. The structure and activity relationship (SAR) studies revealed that 3,4-non-substituted maleimides displayed the strongest anti-leishmanial activities compared to those for 3-methyl-maleimides and 3,4-dichloro-maleimides. 3,4-dichloro-maleimides were the least cytotoxic compared to 3-methyl-maleimides and 3,4-non-substituted maleimides. The results show that several of the reported compounds are promising leads for potential anti-leishmanial drug development.

Multimerization results in formation of re-bindable metabolites: A proof of concept study with FSC-based minigastrin imaging probes targeting CCK2R expression

Positron emission tomography (PET) with radiolabelled peptide-based tracers has attracted great interest in oncology over the past decades. The success of imaging is closely related to sufficient uptake of the radiotracer in malignant tissue and for this sufficient biological half-life, particularly in the bloodstream, is mandatory. Fast enzymatic degradation during circulation leading to insufficient imaging abilities of peptide-based radioligands remains a major issue. The design of multimeric constructs, bearing multiple targeting moieties, has been widely applied to improve target interaction. This concept may also be applied to prolong the biological half-life of peptide-based radiopharmaceuticals as enzymatic degradation can result in formation of metabolites still capable to interact with the target binding site. In this study we aimed to identify such metabolites and therefore we utilized the siderophore-based bifunctional chelator fusarinine C (FSC) for the design of novel mono- and multimeric constructs, bearing minigastrin (MG) analogues as targeting moieties to address cholecystokinin-2 receptors (CCK2R) which are overexpressed in a variety of cancerous diseases and are well known for fast enzymatic degradation, particularly for truncated des-(Glu)5-MG members, such as MG11. FSC-based imaging probes were radiolabelled with gallium-68 and characterized in vitro (logD, protein binding, affinity and cell-uptake studies, stability and metabolite studies, as well as generation of corresponding metabolites by artificial enzymatic degradation) and in vivo (biodistribution in A431-CCK2R/A431-mock tumour xenografted BALB/c nude mice and stability in blood of living BALB/c mice and analysis of corresponding organ homogenates and urine to identify degradation products). In summary, multimerization was accompanied by partial improvement towards targeting abilities. Identified metabolites formed by artificial enzymatic cleavage of trimeric FSC-MG conjugates in vitro contained intact binding sequences for the receptor. Furthermore, the 68Ga-labelled trimers exhibiting increasing uptake of radioligand in tumour tissue over time and improved in vivo stability in blood samples of living animals of the trimers compared to corresponding mono- and dimers, strongly supporting our hypothesis.

Utilizing Peptide Ligand GPCRs to Image and Treat Pancreatic Cancer

It is estimated that early detection of pancreatic ductal adenocarcinoma (PDAC) could increase long-term patient survival by as much as 30% to 40% (Seufferlein, T. et al., Nat. Rev. Gastroenterol. Hepatol.2016, 13, 74–75). There is an unmet need for reagents that can reliably identify early cancerous or precancerous lesions through various imaging modalities or could be employed to deliver anticancer treatments specifically to tumor cells. However, to date, many PDAC tumor-targeting strategies lack selectivity and are unable to discriminate between tumor and nontumor cells, causing off-target effects or unclear diagnoses. Although a variety of approaches have been taken to identify tumor-targeting reagents that can effectively direct therapeutics or imaging agents to cancer cells (Liu, D. et al., J. Controlled Release2015, 219, 632–643), translating these reagents into clinical practice has been limited, and it remains an area open to new methodologies and reagents (O’Connor, J.P. et al., Nat. Rev. Clin. Oncol. 2017, 14, 169–186). G protein–coupled receptors (GPCRs), which are key target proteins for drug discovery and comprise a large proportion of currently marketed therapeutics, hold significant promise for tumor imaging and targeted treatment, particularly for pancreatic cancer.

Exploiting the Concept of Multivalency with 68Ga- and 89Zr-Labelled Fusarinine C-Minigastrin Bioconjugates for Targeting CCK2R Expression

Cholecystokinin-2 receptors (CCK2R) are overexpressed in a variety of malignant diseases and therefore have gained certain attention for peptide receptor radionuclide imaging. Among extensive approaches to improve pharmacokinetics and metabolic stability of minigastrin (MG) based radioligands, the concept of multivalency for enhanced tumour targeting has not been investigated extensively. We therefore utilized fusarinine C (FSC) as chelating scaffold for novel mono-, di-, and trimeric bioconjugates for targeting CCK2R expression. FSC-based imaging probes were radiolabelled with positron emitting radionuclides (gallium-68 and zirconium-89) and characterized in vitro (log⁡D, IC₅₀, and cell uptake) and in vivo (metabolic stability in BALB/c mice, biodistribution profile, and microPET/CT imaging in A431-CCK2R/A431-mock tumour xenografted BALB/c nude mice). Improved targeting did not fully correlate with the grade of multimerization. The divalent probe showed higher receptor affinity and increased CCK2R mediated cell uptake while the trimer remained comparable to the monomer. In vivo biodistribution studies 1 h after administration of the ⁶⁸Ga-labelled radioligands confirmed this trend, but imaging at late time point (24 h) with ⁸⁹Zr-labelled counterparts showed a clearly enhanced imaging contrast of the trimeric probe compared to the mono- and dimer. Furthermore, in vivo stability studies showed a higher metabolic stability for multimeric probes compared to the monomeric bioconjugate. In summary, we could show that FSC can be utilized as suitable scaffold for novel mono- and multivalent imaging probes for CCK2R-related malignancies with partly improved targeting properties for multivalent conjugates. The increased tumour accumulation of the trimer 24 h postinjection (p.i.) can be explained by slower clearance and increased metabolic stability of multimeric conjugates.

Current Status of Radiopharmaceuticals for the Theranostics of Neuroendocrine Neoplasms

Nuclear medicine plays a pivotal role in the management of patients affected by neuroendocrine neoplasms (NENs). Radiolabeled somatostatin receptor analogs are by far the most advanced radiopharmaceuticals for diagnosis and therapy (radiotheranostics) of NENs. Their clinical success emerged receptor-targeted radiolabeled peptides as an important class of radiopharmaceuticals and it paved the way for the investigation of other radioligand-receptor systems. Besides the somatostatin receptors (sstr), other receptors have also been linked to NENs and quite a number of potential radiolabeled peptides have been derived from them. The Glucagon-Like Peptide-1 Receptor (GLP-1R) is highly expressed in benign insulinomas, the Cholecystokinin 2 (CCK2)/Gastrin receptor is expressed in different NENs, in particular medullary thyroid cancer, and the Glucose-dependent Insulinotropic Polypeptide (GIP) receptor was found to be expressed in gastrointestinal and bronchial NENs, where interestingly, it is present in most of the sstr-negative and GLP-1R-negative NENs. Also in the field of sstr targeting new discoveries brought into light an alternative approach with the use of radiolabeled somatostatin receptor antagonists, instead of the clinically used agonists. The purpose of this review is to present the current status and the most innovative strategies for the diagnosis and treatment (theranostics) of neuroendocrine neoplasms using a cadre of radiolabeled regulatory peptides targeting their receptors.

In vivo inhibition of neutral endopeptidase enhances the diagnostic potential of truncated gastrin (111)In-radioligands

INTRODUCTION

Radiolabeled gastrin analogs represent attractive candidates for diagnosis and therapy of cholecystokinin subtype-2 receptor (CCK2R)-expressing tumors. Radiolabeled des(Glu)5-gastrins show favorably low renal accumulation, but localize poorly in CCK2R-positive lesions. We introduce herein three truncated [DOTA-DGlu(10)]gastrin(10-17) analogs, with oxidation-susceptible Met(15) replaced by: (1), (2), or (3), and study the profile of [(111)In]1/2/3 during in vivo inhibition of neutral endopeptidase (NEP) in comparison to the non-truncated [ ([(111)In]4) reference.

METHODS

Blood samples collected from mice 5 min postinjection (pi) of [(111)In]1/2/3/4 without or with phosphoramidon (PA) coinjection were analyzed by RP-HPLC. Biodistribution was conducted in SCID mice bearing A431-CCK2R(+) or AR42J xenografts 4h after administration of [(111)In]1/2/3/4 without or with PA coinjection.

RESULTS

Firstly, we observed remarkable increases in the amount of radiopeptides detected intact in the blood of PA-treated mice at 5 min pi compared to controls. Secondly, we noted impressive enhancement of [(111)In]1/2/3 localization in AR42J and A431-CCK2R(+) tumors in mice after PA coinjection. Specifically, the uptake of [(111)In]1 at 4h pi increased from 2.6 ± 0.3%ID/g to 13.3 ± 3.5%ID/g in the AR42J tumors and from 4.3 ± 0.6%ID/g to 20.4 ± 3.6%ID/g in the A431-CCK2R(+) xenografts, with comparable improvements noted for [(111)In]2 and [(111)In]3 as well. Thirdly, renal uptake remained favorably low and unaffected by PA (<2.5%ID/g). Conversely, although the stability and tumor targeting of [(111)In]4 improved, its high renal uptake (>85%ID/g) increased even further by PA (>140%ID/g).

CONCLUSIONS

In situ inhibition of NEP represents a promising new tool to enhance the diagnostic efficacy of biodegradable gastrin radioligands in the visualization of CCK2R-positive lesions in man.

Development of a time-resolved fluorescence probe for evaluation of competitive binding to the cholecystokinin 2 receptor

The synthesis, characterization, and use of Eu-DTPA-PEGO-Trp-Nle-Asp-Phe-NH2 (Eu-DTPA-PEGO-CCK4), a luminescent probe targeted to cholecystokinin 2 receptor (CCK2R, aka CCKBR), are described. The probe was prepared by solid phase synthesis. A Kd value of 17±2nM was determined by means of saturation binding assays using HEK-293 cells that overexpress CCK2R. The probe was then used in competitive binding assays against Ac-CCK4 and three new trivalent CCK4 compounds. Repeatable and reproducible binding assay results were obtained. Given its ease of synthesis, purification, receptor binding properties, and utility in competitive binding assays, Eu-DTPA-PEGO-CCK4 could become a standard tool for high-throughput screening of compounds in development targeted to cholecystokinin receptors.

Development of the designed ankyrin repeat protein (DARPin) G3 for HER2 molecular imaging

PURPOSE

Human epidermal growth factor receptor-2 (HER2) overexpression is a predictor of response to anti-HER2 therapy in breast and gastric cancer. Currently, HER2 status is assessed by tumour biopsy, but this may not be representative of the larger tumour mass or other metastatic sites, risking misclassification and selection of suboptimal therapy. The designed ankyrin repeat protein (DARPin) G3 binds HER2 with high affinity at an epitope that does not overlap with trastuzumab and is biologically inert. We hypothesized that radiolabelled DARPin G3 would be capable of selectively imaging HER2-positive tumours, and aimed to identify a suitable format for clinical application.

METHODS

G3 DARPins tagged with hexahistidine (His6) or with histidine glutamate (HE)3 and untagged G3 DARPins were manufactured using a GMP-compatible Pichia pastoris protocol and radiolabelled with (125)I, or with (111)In via DOTA linked to a C-terminal cysteine. BALB/c mice were injected with radiolabelled G3 and tissue biodistribution was evaluated by gamma counting. The lead construct ((HE)3-G3) was assessed in mice bearing HER2-positive human breast tumour (BT474) xenografts.

RESULTS

For both isotopes, (HE)3-G3 had significantly lower liver uptake than His6-G3 and untagged G3 counterparts in non-tumour-bearing mice, and there was no significantly different liver uptake between His6-G3 and untagged G3. (HE)3-G3 was taken forward for evaluation in mice bearing HER2-positive tumour xenografts. The results demonstrated that radioactivity from (111)In-(HE)3-G3 was better maintained in tumours and cleared faster from serum than radioactivity from (125)I-(HE)3-G3, achieving superior tumour-to-blood ratios (343.7 ± 161.3 vs. 22.0 ± 11.3 at 24 h, respectively). On microSPECT/CT, (111)In-labelled and (125)I-labelled (HE)3-G3 could image HER2-positive tumours at 4 h after administration, but there was less normal tissue uptake of radioactivity with (111)In-(HE)3-G3. Preadministration of trastuzumab did not affect the uptake of (HE)3-G3 by HER2-positive tumours.

CONCLUSION

Radiolabelled DARPin (HE)3-G3 is a versatile radioligand with potential to allow the acquisition of whole-body HER2 scans on the day of administration.

Evaluation and comparison of a new DOTA and DTPA-bombesin agonist in vitro and in vivo in low and high GRPR expressing prostate and breast tumor models

We evaluated and compared a new bombesin analog [Tyr-Gly5, Nle(14)]-BBN(6-14) conjugated to DOTA or DTPA and radiolabeled with In-111 in low and high GRPR expressing tumor models. Both peptides were radiolabeled with high radiochemical purity and specific activity. In vitro assays on T-47D, LNCaP and PC-3 cells showed that the affinity of peptides is similar and a higher binding and internalization of DOTA-peptide to PC-3 cells was observed. Both peptides could target PC-3 and LNCaP tumors in vivo and both tumor types could be visualized by microSPECT/CT.

[(68)Ga]FSC-(RGD)3 a trimeric RGD peptide for imaging αvβ3 integrin expression based on a novel siderophore derived chelating scaffold-synthesis and evaluation

Over the last years Gallium-68 ((68)Ga) has received tremendous attention for labeling of radiopharmaceuticals for positron emission tomography (PET). (68)Ga labeling of biomolecules is currently based on bifunctional chelators containing aminocarboxylates (mainly DOTA and NOTA). We have recently shown that cyclic peptide siderophores have very good complexing properties for (68)Ga resulting in high specific activities and excellent metabolic stabilities, in particular triacetylfusarinine-C (TAFC). We postulated, that, starting from its deacetylated form (Fusarinine-C (FSC)) trimeric bioconjugates are directly accessible to develop novel targeting peptide based (68)Ga labeled radiopharmaceuticals. As proof of principle we report on the synthesis and (68)Ga-radiolabeling of a trimeric FSC-RGD conjugate, [(68)Ga]FSC-(RGD)3, targeting αvβ3 integrin, which is highly expressed during tumor-induced angiogenesis. Synthesis of the RGD peptide was carried out applying solid phase peptide synthesis (SPPS), followed by the coupling to the siderophore [Fe]FSC via in situ activation using HATU/HOAt and DIPEA. Subsequent demetalation allowed radiolabeling of FSC-(RGD)3 with (68)Ga. The radiolabeling procedure was optimized regarding peptide amount, reaction time, temperature as well buffer systems. For in vitro evaluation partition coefficient, protein binding, serum stability, αvβ3 integrin binding affinity, and tumor cell uptake were determined. For in vitro tests as well as for the biodistribution studies αvβ3 positive human melanoma M21 and αvβ3 negative M21-L cells were used. [(68)Ga]FSC-(RGD)3 was prepared with high radiochemical yield (>98%). Distribution coefficient was -3.6 revealing a hydrophilic character, and an IC50 value of 1.8±0.6 nM was determined indicating a high binding affinity for αvβ3 integrin. [(68)Ga]FSC-(RGD)3 was stable in PBS (pH7.4), FeCl3- and DTPA-solution as well as in fresh human serum at 37°C for 2hours. Biodistribution assay confirmed the receptor specific uptake found in vitro. Uptake in the αvβ3 positive tumor was 4.3% ID/g 60min p.i. which was 3-fold higher than the monomeric [(68)Ga]NODAGA-RGD. Tumor to blood ratio of approx. 8 and tumor to muscle ratio of approx. 7 were observed. [(68)Ga]FSC-(RGD)3 serves as an example for the feasibility of a novel class of bifunctional chelators based on cyclic peptide siderophores and shows excellent targeting properties for αvβ3 integrin in vivo for imaging tumor-induced neovascularization.

Influence of d-glutamine and d-glutamic acid sequences in optical peptide probes targeted against the cholecystokinin-2/gastrin-receptor on binding affinity, specificity and pharmacokinetic properties

Background

Image-based diagnosis of tumours can be advanced and improved by targeted strategies addressing malignant molecular structures. A promising molecular target is the cholecystokinin-2-receptor (CCK2R) which can be targeted by high-affinity peptides called minigastrins. Here we present how the imaging properties of minigastrins tagged with near-infrared fluorescence (NIRF) dyes can be modulated by the introduction of different spacer sequences. We identify interactions of different probe variants with regard to target affinity, specificity and pharmacokinetic properties to optimize early detection of CCK2R-expressing tumours under clinical conditions.

Methods

Two minigastrin probes with the same near-infrared hemicyanine fluorescence dye (DY-754) for signalling and the same CCK2R-binding peptide A-Y-G-W-M/Nle-N-F-amide but different spacers were designed as follows: ‘QE’ with three alternating d-glutamines and d-glutamic acids and ‘bivQ’ with two minigastrins, each preceded by three d-glutamines. They were tested for affinity and specificity in vitro on CCK2R-expressing and CCK2R-non-expressing cells. In vivo imaging was conducted with subcutaneous tumour-bearing nude mice after i.v. probe injection (54 to 108 nmol/kg) and under competitive conditions with non-fluorescent minigastrin (n = 5/group). We also assessed probe biodistribution as well as NIRF distribution in tumour sections.

Results

Both probes showed high affinity and specificity to CCK2R-expressing cells in vitro. In vivo tumour-to-background contrasts (tumour/background ratios (TBRs) of around 6) enabled identification of CCK2R-expressing tumours by both probes with low accumulation in CCK2R-negative tumours (TBR of around 2). Specificity of the in vivo accumulation, revealed by competition, was higher for QE. Besides renal retention, probe uptake into organs was very low.

Conclusion

The properties of optical minigastrin probes can be specifically modified by the introduction of spacer sequences. A spacer of six hydrophilic amino acids increases affinity. A mix of d-glutamic and d-glutamine acids increased target-to-background contrast. Multimerization could not increase affinity but supposedly lowered stability. The probe QE is a promising candidate for clinical evaluation in terms of diagnosis of CCK2R-expressing tumours.

Bioengineering strategies for designing targeted cancer therapies

The goals of bioengineering strategies for targeted cancer therapies are (1) to deliver a high dose of an anticancer drug directly to a cancer tumor, (2) to enhance drug uptake by malignant cells, and (3) to minimize drug uptake by nonmalignant cells. Effective cancer-targeting therapies will require both passive- and active-targeting strategies and a thorough understanding of physiologic barriers to targeted drug delivery. Designing a targeted therapy includes the selection and optimization of a nanoparticle delivery vehicle for passive accumulation in tumors, a targeting moiety for active receptor-mediated uptake, and stimuli-responsive polymers for control of drug release. The future direction of cancer targeting is a combinatorial approach, in which targeting therapies are designed to use multiple-targeting strategies. The combinatorial approach will enable combination therapy for delivery of multiple drugs and dual ligand targeting to improve targeting specificity. Targeted cancer treatments in development and the new combinatorial approaches show promise for improving targeted anticancer drug delivery and improving treatment outcomes.

rG17PE38, a novel immunotoxin target to gastric cancer with overexpressed CCK-2R

BACKGROUND

Gastrin/cholecystokinin subtype 2 receptor (CCK2R) is overexpressed in several types of tumors. Gastrin-17 (G17) peptide has a high affinity with CCK2R. These characters suggest that G17 may be useful for target cancer therapy.

PURPOSE

Construct a new immunotoxin (IT) targeting of CCK2R overexpressed gastric cancer.

METHODS

Two ITs were generated using forward and reverse G17 peptides fused with PE38. To get a high yield, codon optimized gene and optimized fermentation parameters were used in large-scale protein expression. An immunoaffinity technique was introduced into pseudomonas exotoxin (PE)-derived IT purification procedure. G17 competition, GST pull-down and indirect immunoflourescence assays were carried out to confirm the interaction between rG17 and CCK2R. Then, several cytotoxic assays were carried out on 18 cell lines, and an in vivo antitumor activity experiment was tested in nude mice.

RESULTS

The rG17PE38 showed specific cytotoxicity on three gastric cancer cells, while G17PE38 did not. After optimization, the expression level reached about 40% in medium deprived of NaCl. Next, 15-27.5 mg of pure rG17PE38 per 1 L of cultures was obtained. Results of G17 competition, GST pull-down and indirect immunoflourescence assays demonstrated that rG17 have a specific interact with CCK2R. Purified rG17PE38 showed high cytotoxicity on gastric cancer cell lines with the IC50 value of 0.6-4 ng·mL(-1). Treatment of nude mice inoculated with BGC-823 tumor xenografts with rG17PE38 efficiently inhibited tumor size.

CONCLUSIONS AND DISCUSSION

The present study demonstrates that reversed G17 could be used as target moiety of PE-derived IT and the rG17PE38 could be developed as a new immunotherapy agent. Codon optimized gene could increase the rG17PE38 expression level in E. coli and furthermore NaCl inhibits the rG17PE38 expression in large scale. Meanwhile, our present study inducts an immunoaffinity method in the IT purification procedure, which could purify the PE-derived ITs in native form.

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.

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.

Cholecystokinin and gastrin receptors targeting in gastrointestinal cancer

Cholecystokinin and Gastrin are amongst the first gastrointestinal hormone discovered. In addition to classical actions (contraction of gallbladder, growth and secretion in the stomach and pancreas), these also act as growth stimulants for gastrointestinal malignancies and cell lines. Growth of these tumours is inhibited by antagonists of the cholecystokinin and gastrin receptors. These receptors provides most promising approach in clinical oncology and several specific radiolabelled ligands have been synthesized for specific tumour targeting and therapy of tumours overexpressing these receptors. Therefore, definition of the molecular structure of the receptor involved in the autocrine/paracrine loop may contribute to novel therapies for gastrointestinal cancer. Hence, this review tries to focus on the role and distribution of these hormones and their receptors in gastrointestinal cancer with a brief talk about the clinical trial using available agonist and antagonist in gastrointestinal cancers.

Comparison of the binding and internalization properties of 12 DOTA-coupled and ¹¹¹In-labelled CCK2/gastrin receptor binding peptides: a collaborative project under COST Action BM0607

PURPOSE

Specific overexpression of cholecystokinin 2 (CCK2)/gastrin receptors has been demonstrated in several tumours of neuroendocrine origin. In some of these cancer types, such as medullary thyroid cancer (MTC), a sensitive diagnostic modality is still unavailable and therapeutic options for inoperable lesions are needed. Peptide receptor radionuclide therapy (PRRT) may be a viable therapeutic strategy in the management of these patients. Several CCK2R-targeted radiopharmaceuticals have been described in recent years. As part of the European Union COST Action BM0607 we studied the in vitro and in vivo characteristics of 12 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated CCK2R binding peptides. In the present study, we analysed binding and internalization characteristics. Stability, biodistribution and imaging studies have been performed in parallel by other centres involved in the project.

METHODS

Determination of IC(50) values was performed using autoradiography, with DOTA-peptides displacing (125)I-CCK from receptors on tissue sections from human tumours. Saturation binding and internalization experiments were performed using (111)In-labelled peptides. The rat AR42J cell line and the human A431-CCK2R transfected cell line were utilized for in vitro experiments; dissociation constants (K(d)) and apparent number of binding sites (B(max)) were determined. Internalization was determined in receptor-expressing cells by incubating with tracer amounts of peptide at 37 and 4°C for different times up to 120 min. Surface-bound peptide was then stripped either by acid wash or subsequent incubation with 1 μM unlabelled peptide at 4°C.

RESULTS

All peptides showed high receptor affinity with IC(50) values ranging from 0.2 to 3.4 nM. Saturation experiments also showed high affinity with K(d) values in the 10(-9)-10(-8) M range. B(max) values estimated in A431-CCK2R cells ranged from 0.6 to 2.2 × 10(6) per cell. All peptides showed high levels of internalization when incubated at 37°C.

CONCLUSION

All DOTA-conjugated peptides showed high receptor binding and internalization properties and appear suitable for further characterization, as described in other articles of this issue.

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.

Highly improved metabolic stability and pharmacokinetics of indium-111-DOTA-gastrin conjugates for targeting of the gastrin receptor

The development of metabolically stable radiolabeled gastrin analogues with suitable pharmacokinetics is a topic of recent research activity. These imaging vectors are of interest because the gastrin/CCK2 receptor is highly overexpressed in different tumors such as medullary thyroid cancer, neuroendocrine tumors, and SCLC. The drawback of current targeting agents is either their metabolic instability or their high kidney uptake. We present the synthesis and in vitro and in vivo evaluation of 11 (111)In-labeled DOTA-conjugated peptides that differ by their spacer between the peptide and the chelate. We introduced uncharged but hydrophilic spacers such as oligoethyleneglycol, serine, and glutamine. The affinity of all radiopeptides was high with IC(50) values between 0.5 and 4.8 nM. The improvement of human serum stability is 500-fold within this series of compounds. In addition the kidney uptake could be lowered distinctly and the tumor-to-kidney ratio improved almost 60-fold if compared with radiotracers having charged spacers such as glutamic acid.

Quantitative Accuracy of Low-Count SPECT Imaging in Phantom and In Vivo Mouse Studies

We investigated the accuracy of a single photon emission computed tomography (SPECT) system in quantifying a wide range of radioactivity concentrations using different scan times in both phantom and animal models. A phantom containing various amounts of In-111 or Tc-99m was imaged until the activity had decayed close to background levels. Scans were acquired for different durations, employing different collimator pinhole sizes. VOI analysis was performed to quantify uptake in the images and the values compared to the true activity. The phantom results were then validated in tumour-bearing mice. The use of an appropriate calibration phantom and disabling of a background subtraction feature meant that absolute errors were within 12% of the true activity. Furthermore, a comparison of in vivo imaging and biodistribution studies in mice showed a correlation of 0.99 for activities over the 200 kBq to 5 MBq range. We conclude that the quantitative information provided by the NanoSPECT camera is accurate and allows replacement of dissection studies for assessment of radiotracer biodistribution in mouse models.

Assessing antibody pharmacokinetics in mice with in vivo imaging

Recent advances in small-animal molecular imaging instrumentation combined with well characterized antibody-labeling chemistry have enabled detailed in vivo measurements of antibody distribution in mouse models. This article reviews the strengths and limitations of in vivo antibody imaging methods with a focus on positron emission tomography and single-photon emission computed tomography and a brief discussion of the role of optical imaging in this application. A description of the basic principles behind the imaging techniques is provided along with a discussion of radiolabeling methods relevant to antibodies. Practical considerations of study design and execution are presented through a discussion of sensitivity and resolution tradeoffs for these techniques as defined by modality, signaling probe (isotope or fluorophore) selection, labeling method, and radiation dosimetry. Images and analysis results from a case study are presented with a discussion of output data content and relevant informatics gained with this approach to studying antibody pharmacokinetics.