Development of a new radioligand for cholecystokinin receptor subtype 2 scintigraphy: from molecular modeling to in vivo evaluation

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.

Preliminary Study of a 1,5-Benzodiazepine-Derivative Labelled with Indium-111 for CCK-2 Receptor Targeting

The cholecystokinin-2 receptor (CCK-2R) is overexpressed in several human cancers but displays limited expression in normal tissues. For this reason, it is a suitable target for developing specific radiotracers. In this study, a nastorazepide-based ligand functionalized with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator (IP-001) was synthesized and labelled with indium-111. The radiolabeling process yielded >95% with a molar activity of 10 MBq/nmol and a radiochemical purity of >98%. Stability studies have shown a remarkable resistance to degradation (>93%) within 120 h of incubation in human blood. The in vitro uptake of [¹¹¹In]In-IP-001 was assessed for up to 24 h on a high CCK-2R-expressing tumor cell line (A549) showing maximal accumulation after 4 h of incubation. Biodistribution and single photon emission tomography (SPECT)/CT imaging were evaluated on BALB/c nude mice bearing A549 xenograft tumors. Implanted tumors could be clearly visualized after only 4 h post injection (2.36 ± 0.26% ID/cc), although a high amount of radiotracer was also found in the liver, kidneys, and spleen (8.25 ± 2.21%, 6.99 ± 0.97%, and 3.88 ± 0.36% ID/cc, respectively). Clearance was slow by both hepatobiliary and renal excretion. Tumor retention persisted for up to 24 h, with the tumor to organs ratio increasing over-time and ending with a tumor uptake (1.52 ± 0.71% ID/cc) comparable to liver and kidneys.

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.

Optical and relaxometric properties of monometallic (EuIII, TbIII, GdIII) and heterobimetallic (ReI/GdIII) systems based on a functionalized bipyridine-containing acyclic ligand

The photophysical and relaxometric properties, in aqueous solution, of Ln-BPMNTA complexes and a derived Re^I/Gd^III dinuclear complex are reported in this paper.

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.

High-denticity ligands based on picolinic acid for 111In radiochemistry

Four new acyclic ligands, Bn-H3nonapa (3), H3nonapa (4), p-NO2-Bn-H3nonapa (10), and Bn-H3trenpa (7), were synthesized and studied with nonradioactive In3+ and with radioactive 111In3+. The coordination of these ligands to In3+ was confirmed by high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy. Radiolabeling experiments were performed with 111In3+; these demonstrated H3nonapa (4) to be the best indium ligand of those studied herein, achieving radiochemical yields of ∼97% in 10 min at ambient temperature, and stability to transchelation in mouse serum of 44.5% ± 25.9% after 24 h. Although the radiolabeling kinetics of H3nonapa (4) were excellent, serum stability results were inferior to the previously studied ligands DOTA, DTPA, and H4octapa, suggesting that the presented ligands may find their optimum radiopharmaceutical applications with isotopes other than 111In. Owing to the high denticity of these ligands (9–10 coordinate), they may realize their potential with large ion isotopes such as 177Lu, 86/90Y, and 225Ac.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

A new sesquiterpene from the mangrove endophytic fungus Aspergillus terreus (No. GX7-3B)

A new compound 1 (named botryosphaerin F), along with other three known compounds 2 (13,14,15,16-tetranorlabd-7-ene-19,6b:12,17-diolide), 3 (botryosphaerin B) and 4 (LL-Z1271β), has been isolated from the mangrove fungus Aspergillus terreus (No. GX7-3B). The structure of the new compound was established by analysis of spectroscopic data. The hypothetical biogenic relationship of four sesquiterpene analogues was also described in this paper. Furthermore, in the cytotoxicity assays, compound 1 showed potent inhibiting activity towards MCF-7 and HL-60 cancer cell lines with 50% inhibition of cell growth (IC50) values of 4.49 and 3.43 μM, respectively, and compound 4 exhibited promising activity against HL-60 cell line with an IC50 value of 0.6 μM.

Optical imaging of CCK₂/gastrin receptor-positive tumors with a minigastrin near-infrared probe

PURPOSE

A variety of tumors in different organs with good accessibility to near-infrared light express the cholecystokinin-2 (CCK₂)/gastrin receptor. Therefore, the applicability of fluorescence optical imaging was assessed using a novel peptide probe.

MATERIALS AND METHODS

This study was approved by the regional animal committee. Our optical peptide probe (DY-minigastrin) was synthesized by coupling a hemicyanine dye to a gastrin derivative peptide (minigastrin). In vitro CCK₂/gastrin receptor identification was performed in receptor-positive HT-29 and negative A-375 cells using flow cytometry, laser scanning microscopy, and macroscopic near-infrared fluorescent (NIRF) imaging. For in vivo studies, tumor cells were implanted into mice, and DY-minigastrin in presence or absence of nonlabeled minigastrin (control of signaling specificity) was applied intravenously. Fluorescence signals in tumors and organs were recorded and statistically analyzed.

RESULTS

Flow cytometry, laser scanning microscopy, and in vitro macroscopic imaging of cell pellets revealed a distinct accumulation of our minigastrin probe in HT-29 cells, showing distinct probe internalization. In vivo NIRF whole-body animal imaging, again, demonstrated a clear depiction of HT-29 tumors, which was reversed by blocking with nonlabeled minigastrin. Semi-quantitative fluorescence analysis and histologic observations were in agreement with these observations. A distinct probe organ distribution was observed.

CONCLUSIONS

Our observations indicate that DY-minigastrin-based NIRF optical imaging of CCK₂/gastrin receptor protein is feasible. Because of its widespread occurrence in different tumor types, endoscopic, laparoscopic, and tomographic receptor imaging could be accomplished in the near future.