Development of a new thiol site-specific prosthetic group and its conjugation with [Cys(40)]-exendin-4 for in vivo targeting of insulinomas

Comparison of Exendin-4 and Its Single Amino Acid Substitutions as Parent Peptides for GLP-1 Receptor Imaging Probes

Glucagon-like peptide-1 receptor (GLP-1R) is an emerging critical target for the diagnosis and treatment of various diseases. Radiolabeled exendin-4 (Ex-4), a GLP-1R agonist, has been widely used as an imaging probe. However, its potential to induce hypoglycemia, especially in patients with insulinoma, limits its applicability. This study evaluated whether Ex-D3, a Glu3Asp substitution of Ex-4 with a higher internalization rate, could enhance the imaging efficacy of Ex-4 while reducing its hypoglycemic effects. We synthesized derivatives with an additional C-terminal Cys (Ex-D3-C40) for site-specific 125I labeling. Surface plasmon resonance analysis revealed that C-terminus modification did not significantly alter the binding affinity of Ex-D3-C40 to GLP-1R. In vivo studies in mice demonstrated that Ex-D3-C40 induced weaker hypoglycemic effects than Ex-4-C40. Biodistribution studies showed that 125I-labeled Ex-D3 ([125I]I-Ex-D3) achieved significantly higher pancreatic accumulation and higher pancreas-to-blood and pancreas-to-muscle ratios than [125I]I-Ex-4. Ex vivo autoradiography confirmed the binding specificity of [125I]I-Ex-D3 to GLP-1R-expressing pancreatic β-cells. These findings indicate that Ex-D3 is a promising parent peptide for the development of superior GLP-1R imaging probes with reduced hypoglycemic risk, highlighting the importance of considering pharmacological effects in designing molecular imaging probes.

Synthesis and Application of 1-[18F]Fluoro-4-isothiocyanatobenzene for Radiofluorination of Peptides in Aqueous Medium

Conjugation of radiofluorinated prosthetic groups to primary amines of peptides in an aqueous medium is still considerably challenging. Herein, we report a one-pot cascade synthesis of 1-[18F]fluoro-4-isothiocyanatobenzene ([18F]2d), an isothiocyanate-functionalized prosthetic group for radiolabeling of various peptides in aqueous medium. The developed compound [18F]2d was synthesized in >99% radiochemical purity with 22.9 ± 3.8% (n = 12) decay-corrected yield having molar activity of 0.65 ± 0.19 (n = 12) GBq/μmol. Various clinically important peptides including prostate-specific membrane antigen vector, octreotide acetate, biotin analogue, Arg-Gly-Asp analogue, and bradykinin were successfully conjugated with [18F]2d in an aqueous medium in a good to moderate radiochemical yield. The overall synthesis of [18F]2d and its conjugation with a peptide take around 155 min, including purification.

Efficient Radiolabeling of Proteins and Antibodies via Maleamate-Cysteine Bioconjugation

The study introduces a novel maleamate-based prosthetic group specifically designed for efficient, site-specific radioiodination of biomolecules that contain or are modified with cysteine residues. This strategy is a compelling alternative to the conventional maleimide-based approach, demonstrating outstanding attributes such as high radiochemical yield, rapid reaction kinetics, applicability in aqueous media at neutral pH, and exceptional stability under a competitive environment.

From molecules to medicine: thiol selective bioconjugation in synthesis of diagnostic and therapeutic radiopharmaceuticals

Radiolabeling of biomolecules and cells with radiolabeled prosthetic groups has significant implications for nuclear medicine, imaging, and radiotherapy. Achieving site-specific and controlled incorporation of radiolabeled prostheses under mild reaction conditions is crucial for minimizing the impact on the bioactivity of the radiolabeled compounds. The targeting of natural and abundant amino acids during radiolabeling of biomolecules often results in nonspecific and uncontrolled modifications. Cysteine is distinguished by its low natural abundance and unique nucleophilicity. It is therefore an optimal target for site-selective and site-specific radiolabeling of biomolecules under controlled parameters. This review extensively discusses thiol-specific radiolabeled prosthetic groups and provides a critical analysis and comprehensive study of the synthesis of these groups, their in vitro and in vivo stability profiles, reaction kinetics, stability of resulting adducts, and overall impact on the targeting ability of radiolabeled biomolecules. The insights presented here aim to facilitate the development of highly efficient radiopharmaceuticals, initially in preclinical settings and ultimately in clinical applications.

Theranostic in GLP-1R molecular imaging: challenges and emerging opportunities

Theranostic in nuclear medicine combines diagnostic imaging and internal irradiation therapy using different therapeutic nuclear probes for visual diagnosis and precise treatment. GLP-1R is a popular receptor target in endocrine diseases, non-alcoholic steatohepatitis, tumors, and other areas. Likewise, it has also made breakthroughs in the development of molecular imaging. It was recognized that GLP-1R imaging originated from the study of insulinoma and afterwards was expanded in application including islet transplantation, pancreatic β-cell mass measurement, and ATP-dependent potassium channel-related endocrine diseases. Fortunately, GLP-1R molecular imaging has been involved in ischemic cardiomyocytes and neurodegenerative diseases. These signs illustrate the power of GLP-1R molecular imaging in the development of medicine. However, it is still limited to imaging diagnosis research in the current molecular imaging environment. The lack of molecular-targeted therapeutics related report hinders its radiology theranostic. In this article, the current research status, challenges, and emerging opportunities for GLP-1R molecular imaging are discussed in order to open a new path for theranostics and to promote the evolution of molecular medicine.

Distinctive detection of insulinoma using [18F]FB(ePEG12)12-exendin-4 PET/CT

Specifying the exact localization of insulinoma remains challenging due to the lack of insulinoma-specific imaging methods. Recently, glucagon-like peptide-1 receptor (GLP-1R)-targeted imaging, especially positron emission tomography (PET), has emerged. Although various radiolabeled GLP-1R agonist exendin-4-based probes with chemical modifications for PET imaging have been investigated, an optimal candidate probe and its scanning protocol remain a necessity. Thus, we investigated the utility of a novel exendin-4-based probe conjugated with polyethylene glycol (PEG) for [¹⁸F]FB(ePEG12)12-exendin-4 PET imaging for insulinoma detection. We utilized [¹⁸F]FB(ePEG12)12-exendin-4 PET/CT to visualize mouse tumor models, which were generated using rat insulinoma cell xenografts. The probe demonstrated high uptake value on the tumor as 37.1 ± 0.4%ID/g, with rapid kidney clearance. Additionally, we used Pdx1-Cre;Trp53^R172H;Rb^f/f mice, which developed endogenous insulinoma and glucagonoma, since they enabled differential imaging evaluation of our probe in functional pancreatic neuroendocrine neoplasms. In this model, our [¹⁸F]FB(ePEG12)12-exendin-4 PET/CT yielded favorable sensitivity and specificity for insulinoma detection. Sensitivity: 30-min post-injection 66.7%, 60-min post-injection 83.3%, combined 100% and specificity: 30-min post-injection 100%, 60-min post-injection 100%, combined 100%, which was corroborated by the results of in vitro time-based analysis of internalized probe accumulation. Accordingly, [¹⁸F]FB(ePEG12)12-exendin-4 is a promising PET imaging probe for visualizing insulinoma.

GLP-1 receptor agonists (GLP-1RAs): cardiovascular actions and therapeutic potential

Type 2 diabetes mellitus (T2DM) is closely associated with cardiovascular diseases (CVD), including atherosclerosis, hypertension and heart failure. Some anti-diabetic medications are linked with an increased risk of weight gain or hypoglycemia which may reduce the efficacy of the intended anti-hyperglycemic effects of these therapies. The recently developed receptor agonists for glucagon-like peptide-1 (GLP-1RAs), stimulate insulin secretion and reduce glycated hemoglobin levels without having side effects such as weight gain and hypoglycemia. In addition, GLP1-RAs demonstrate numerous cardiovascular protective effects in subjects with or without diabetes. There have been several cardiovascular outcomes trials (CVOTs) involving GLP-1RAs, which have supported the overall cardiovascular benefits of these drugs. GLP1-RAs lower plasma lipid levels and lower blood pressure (BP), both of which contribute to a reduction of atherosclerosis and reduced CVD. GLP-1R is expressed in multiple cardiovascular cell types such as monocyte/macrophages, smooth muscle cells, endothelial cells, and cardiomyocytes. Recent studies have indicated that the protective properties against endothelial dysfunction, anti-inflammatory effects on macrophages and the anti-proliferative action on smooth muscle cells may contribute to atheroprotection through GLP-1R signaling. In the present review, we describe the cardiovascular effects and underlying molecular mechanisms of action of GLP-1RAs in CVOTs, animal models and cultured cells, and address how these findings have transformed our understanding of the pharmacotherapy of T2DM and the prevention of CVD.

Non-invasive Beta-cell Imaging: Visualization, Quantification, and Beyond

Pancreatic beta (β)-cell dysfunction and reduced mass play a central role in the development and progression of diabetes mellitus. Conventional histological β-cell mass (BCM) analysis is invasive and limited to cross-sectional observations in a restricted sampling area. However, the non-invasive evaluation of BCM remains elusive, and practical in vivo and clinical techniques for β-cell-specific imaging are yet to be established. The lack of such techniques hampers a deeper understanding of the pathophysiological role of BCM in diabetes, the implementation of personalized BCM-based diabetes management, and the development of antidiabetic therapies targeting BCM preservation and restoration. Nuclear medical techniques have recently triggered a major leap in this field. In particular, radioisotope-labeled probes using exendin peptides that include glucagon-like peptide-1 receptor (GLP-1R) agonist and antagonist have been employed in positron emission tomography and single-photon emission computed tomography. These probes have demonstrated high specificity to β cells and provide clear images accurately showing uptake in the pancreas and transplanted islets in preclinical in vivo and clinical studies. One of these probes, ¹¹¹indium-labeled exendin-4 derivative ([Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4), has captured the longitudinal changes in BCM during the development and progression of diabetes and under antidiabetic therapies in various mouse models of type 1 and type 2 diabetes mellitus. GLP-1R-targeted imaging is therefore a promising tool for non-invasive BCM evaluation. This review focuses on recent advances in non-invasive in vivo β-cell imaging for BCM evaluation in the field of diabetes; in particular, the exendin-based GLP-1R-targeted nuclear medicine techniques.

Exendin-4 analogs in insulinoma theranostics

Insulinomas, neuroendocrine tumors arising from pancreatic beta cells, often show overexpression of the glucagon‐like peptide‐1 receptor. Therefore, imaging with glucagon‐like peptide analog exendin‐4 can be used for diagnosis and preoperative localization. This review presents an overview of the development and clinical implementation of exendin‐based tracers for nuclear imaging, and the potential use of exendin‐4 based tracers for optical imaging and therapeutic applications such as peptide receptor radionuclide therapy or targeted photodynamic therapy.

Synthesis and evaluation of 18F-PTTCO-Cys40-Exendin-4 for PET imaging of ectopic insulinomas in rodents

A major limitation in the development of radiolabeled Exendin-4 analogues (short half-life isotopes) is an inability to efficiently and rapidly separate final products from precursors. This is important as lack of purity in the final product decreases probe efficiency. The purpose of this study was to develop a method to prepare the high-purity imaging reagent [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4. To accomplish this, magnetic TCO-beads were incubated with the crude product to remove unlabeled Exendin-4. In rodents pre-treatment with purified [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 (~1.85 MBq) allowed precise microPET imaging of ectopic insulinomas. Moreover, analogue uptake was successfully blocked by administering non-labelled "cold" Exendin-4. Biodistribution data revealed that [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 accumulated specifically in GLP-1R-enriched insulinomas in mice, confirming results obtained using miroPET. Investigation of [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 as a tracer to image portal vein-transplanted pancreatic islets is proceeding in animals.

Advances in GLP-1 receptor targeting radiolabeled agent development and prospective of theranostics

In the light of theranostics/radiotheranostics and prospective of personalized medicine in diabetes and oncology, this review presents prior and current advances in the development of radiolabeled imaging and radiotherapeutic exendin-based agents targeting glucagon-like peptide-1 receptor. The review covers chemistry, preclinical, and clinical evaluation. Such critical aspects as structure-activity-relationship, stability, physiological potency, kidney uptake, and dosimetry are discussed.

18 F-Labeling of Sensitive Biomolecules for Positron Emission Tomography

Positron emission tomography (PET) imaging study of fluorine-18 labeled biomolecules is an emerging and rapidly growing area for preclinical and clinical research. The present review focuses on recent advances in radiochemical methods for incorporating fluorine-18 into biomolecules via "direct" or "indirect" bioconjugation. Recently developed prosthetic groups and pre-targeting strategies, as well as representative examples in 18 F-labeling of biomolecules in PET imaging research studies are highlighted.

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.

Design and Synthesis of an Easily Obtainable Maleimide Reagent N-[2-(4-[18F]fluoro-N-methylbenzenesulfonamido)ethyl]maleimide ([18F]FBSEM) to Radiolabel Thiols in Proteins

An easily obtainable thiol-selective labeling reagent [¹⁸F]FBSEM (N-[2-(4-[¹⁸F]fluoro-N-methylbenzenesulfonamido)ethyl]maleimide) was developed. The advantage of the design is that the precursor and [¹⁸F]FBSEM have the same backbone and backbone construction is not required; in contrast, known thiol-specific labeling reagents do require backbone construction, and this is thought to be the cause of their complicated synthesis. [¹⁸F]FBSEM was successfully obtained in higher yield (25%) and in a simpler way (two fluorination and deprotection steps in 65 min) than the widely used [¹⁸F]FBEM (N-[2-(4-[¹⁸F]fluorobenzamide)ethyl]maleimide). The labeling efficacy of [¹⁸F]FBSEM was confirmed by conjugation with glutathione. [¹⁸F]FBSEM is a promising labeling agent for proteins.

Low kidney uptake of GLP-1R-targeting, beta cell-specific PET tracer, 18F-labeled [Nle14,Lys40]exendin-4 analog, shows promise for clinical imaging

Background

Several radiometal-labeled, exendin-based tracers that target glucagon-like peptide-1 receptors (GLP-1R) have been intensively explored for β cell imaging. The main obstacle has been the high uptake of tracer in the kidneys. This study aimed to develop a novel GLP1-R-specific tracer, with fluorine-18 attached to exendin-4, to label β cells for clinical imaging with PET (positron emission tomography). We hypothesized that this tracer would undergo reduced kidney uptake. ¹⁸F-labeled [Nle¹⁴,Lys⁴⁰]exendin-4 analog ([¹⁸F]exendin-4) was produced via Cu-catalyzed click chemistry. The biodistribution of [¹⁸F]exendin-4 was assessed with ex vivo organ γ-counting and in vivo PET imaging. We also tested the in vivo stability of the radiotracer. The localization of ¹⁸F radioactivity in rat and human pancreatic tissue sections was investigated with autoradiography. Receptor specificity was assessed with unlabeled exendin-3. Islet labeling was confirmed with immunohistochemistry. The doses of radiation in humans were estimated based on biodistribution results in rats.

Results

[¹⁸F]exendin-4 was synthesized with high yield and high specific activity. Results showed specific, sustained [¹⁸F]exendin-4 uptake in pancreatic islets. In contrast to previous studies that tested radiometal-labeled exendin-based tracers, we observed rapid renal clearance of [¹⁸F]exendin-4.

Conclusions

[¹⁸F]exendin-4 showed promise as a tracer for clinical imaging of pancreatic β cells, due to its high specific uptake in native β cells and its concomitant low kidney radioactivity uptake.

Advancement in treatment and diagnosis of pancreatic cancer with radiopharmaceuticals

Pancreatic cancer (PC) is a major health problem. Conventional imaging modalities show limited accuracy for reliable assessment of the tumor. Recent researches suggest that molecular imaging techniques with tracers provide more biologically relevant information and are benefit for the diagnosis of the cancer. In addition, radiopharmaceuticals also play more important roles in treatment of the disease. This review summaries the advancement of the radiolabeled compounds in the theranostics of PC.

Development of 68Ga- and 89Zr-Labeled Exendin-4 as Potential Radiotracers for the Imaging of Insulinomas by PET

Clinical studies have demonstrated the potential of radiometallated exendin-4 derivatives for the imaging of glucagonlike peptide-1 receptor-overexpressing insulinomas. Recently investigated exendin-4 derivatives were radiolabeled with the SPECT isotopes 99mTc or 111In. Despite promising results, the low spatial resolution associated with SPECT and the occasional need to perform imaging several days after injection for the demarcation of insulinomas from the kidneys represent current limitations. The aim of this work was the development of exendin-4 derivatives for the imaging of insulinomas by high-resolution PET at early or late time points after injection of the radiotracer.

METHODS

An exendin-4 derivative conjugated to desferrioxamine (DFO) was used for radiolabeling with the PET isotopes 68Ga and 89Zr. Both radiotracers were evaluated in vitro with RIN-m5F cells for their cell internalization properties as well as affinities and specificities toward the glucagonlike peptide-1 receptor. Serum stabilities of the radiopeptides were assessed in blood serum, and their distribution coefficient was determined by the shake-flask method. Biodistribution experiments were performed with nude mice bearing RIN-m5F xenografts. For all experiments, clinically evaluated [Lys40-(AHX-DTPA-111In)NH2]exendin-4 was used as a reference compound.

RESULTS

[Lys40-(AHX-DFO)NH2]exendin-4 was labeled with 89Zr and 68Ga in high radiochemical yield and purity. In vitro experiments showed favorable cell uptake and receptor affinity for [Lys40-(AHX-DFO-68Ga)NH2]exendin-4, and [Lys40-(AHX-DFO-89Zr)NH2]exendin-4 and [Lys40-(AHX-DTPA-111In)NH2]exendin-4 performed similarly well. In biodistribution experiments, [Lys40-(AHX-DFO-68Ga)NH2]exendin-4 exhibited a significantly enhanced tumor uptake 1 h after injection in comparison to the other 2 radiotracers. Tumor uptake of [Lys40-(AHX-DFO-89Zr)NH2]exendin-4 was comparable to that of [Lys40-(AHX-DTPA-111In)NH2]exendin-4 at 1-48 h after injection. All compounds showed a fast blood clearance and low accumulation in receptor-negative organs and tissue with the exception of the kidneys, a known characteristic for exendin-4-based radiotracers.

CONCLUSION

68Ga- and 89Zr-radiolabeled [Lys40-(AHX-DFO)NH2]exendin-4 exhibit characteristics comparable or superior to the clinically tested reference compound [Lys40-(AHX-DTPA-111In)NH2]exendin-4 and, thus, represent potential new tracers for the imaging of insulinomas by PET.

GLP-1 Receptor Mediated Targeting of a Fluorescent Zn(2+) Sensor to Beta Cell Surface for Imaging Insulin/Zn(2+) Release

The pancreatic islet beta cell plays an essential role in maintaining the normal blood glucose level by releasing insulin. Loss of functional beta cell mass leads to diabetes—a disease affecting ∼9% of the population worldwide. There has been great interest and intense effort in developing imaging probes for monitoring islet beta cells, and glucagon-like peptide-1 receptor (GLP-1R) has emerged as a valuable biomarker for targeting beta cells. However, efforts thus far in GLP-1R mediated beta cell labeling and imaging has largely, if not exclusively, focused on developing imaging probes for monitoring beta cell mass, and few studies have investigated imaging beta cell function (insulin release) through GLP-1R. We now report the design and synthesis of a bioconjugate, ZIMIR-Ex4(9-39), that consists of a fluorescent Zn(2+) sensor and a truncated exendin 4 peptide for imaging insulin/Zn(2+) release in islet beta cells. In vitro, the conjugate bound to Zn(2+) with high affinity and displayed a robust fluorescence enhancement upon Zn(2+) chelation. When added to beta cells at submicromolar concentration, ZIMIR-Ex4(9-39) rapidly labeled cell surface in minutes to report the dynamics of insulin/Zn(2+) release with high spatiotemporal resolution. Future explorations of this approach may lead to probes for tracking beta cell function using different imaging modalities.

Preliminary evaluation of [18F]AlF-NOTA-MAL-Cys39-exendin-4 in insulinoma with PET

BACKGROUND

High expression of glucagon-like peptide-1 receptor (GLP-1R) in insulinoma supplies a potential drug target for tumor imaging. Exendin-4 can specifically bind to GLP-1R as an agonist and its analogs are extensively used in receptor imaging studies.

PURPOSE

A new GLP-1R imaging agent, [(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4, was designed and prepared for insulinoma imaging.

METHODS

Cys(39)-exendin-4 was conjugated with NOTA-MAL, then the compound was radiolabeled with [(18)F]AlF complex to obtained [(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4. The tumor-targeting characters of the tracer were evaluated in INS-1 cells and BALB/c nude mice models.

RESULTS

[(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4 can be efficiently produced with a yield of 17.5 ± 3.2% (non-decay corrected) and radiochemical purity of >95%. The IC50 value of displacement [(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4 with Cys(39)-exendin-4 was 13.52 ± 1.36 nM. PET images showed excellent tumor visualization with high uptake (9.15 ± 1.6%ID/g at 30 min and 7.74 ± 0.87%ID/g at 60 min). The tumor to muscle, pancreas and liver ratios were 63.25, 3.85 and 7.29 at 60 min after injection. GLP-1R binding specificity was demonstrated by co-injection with an excess of unlabeled Cys(39)-exendin-4 and the tumor uptake was found to be reduced significantly.

CONCLUSION

[(18)F]AlF-NOTA-MAL-Cys(39)-exendin-4 shows favorable characteristics for insulinoma imaging and may be translated to clinical studies.

Insulinoma imaging with glucagon-like peptide-1 receptor targeting probe (18)F-FBEM-Cys (39)-exendin-4

PURPOSE

Glucagon-like peptide-1 receptor (GLP-1R) is a specific target for insulinomas imaging since it is overexpressed in the tumor. Exendin-4 exhibits high affinity for the GLP-1R. In this study, a novel (18)F-labeled exendin-4 analog, (18)F-FBEM-Cys(39)-exendin-4, was synthesized and its potentials for GLP-1R imaging were also evaluated.

METHODS

(18)F-FBEM was synthesized by coupling (18)F-fluorobenzoic acid ((18)F-FBA) with N-(2-aminoethyl) maleimide, and the reaction conditions were optimized. Cys(39)-exendin-4 was then conjugated with (18)F-FBEM to obtain (18)F-FBEM-Cys(39)-exendin-4. The GLP-1R targeting potential and pharmacokinetic profile of the tracer were analyzed in INS-1 insulinoma and MDA-MB-435 breast tumor model, respectively.

RESULTS

Under the optimal conditions, the yield of radiolabeled (18)F-FBEM was 49.1 ± 2.0 % (based on (18)F-FBA, non-decay corrected). The yield of (18)F-FBEM-Cys(39)-exendin-4 was 35.1 ± 2.6 % (based on the starting (18)F-FBEM, non-decay corrected). The radiochemical purity of (18)F-FBEM-Cys(39)-exendin-4 is >95 %, and the specific activity was at least 35 GBq/μmol. The GLP-1R-positive INS-1 insulinoma xenograft was clearly visible with good contrast to background, whereas GLP-1R-negative MDA-MB435 breast tumor was barely visible. Low levels of radioactivity were also detected at pancreas and lungs due to few GLP-1R expressions. GLP-1R binding specificity was demonstrated by reduced INS-1 tumor uptake of the tracer after coinjection with an excess of unlabeled Cys(39)-exendin-4 at 1 h postinjection.

CONCLUSION

The thiol-reactive reagent, (18)F-FBEM, was prepared with high yield and successfully conjugated to Cys(39)-exendin-4. Favorable preclinical data showing specific and effective tumor targeting by (18)F-FBEM-Cys(39)-exendin-4 suggest that the tracer may be a potential probe for insulinomas imaging.

(64)Cu labeled sarcophagine exendin-4 for microPET imaging of glucagon like peptide-1 receptor expression

The Glucagon-like peptide 1 receptor (GLP-1R) has become an important target for imaging due to its elevated expression profile in pancreatic islets, insulinoma, and the cardiovascular system. Because native GLP-1 is degraded rapidly by dipeptidyl peptidase-IV (DPP-IV), several studies have conjugated different chelators to a more stable analog of GLP-1 (such as exendin-4) as PET or SPECT imaging agents with various advantages and disadvantages. Based on the recently developed Sarcophagin chelator, here, we describe the construction of GLP-1R targeted PET probes containing monomeric and dimeric exendin-4 subunit. The in vitro binding affinity of BarMalSar-exendin-4 and Mal2Sar-(exendin-4)2 was evaluated in INS-1 cells, which over-express GLP-1R. Mal2Sar-(exendin-4)2 demonstrated around 3 times higher binding affinity compared with BaMalSar-exendin-4. After (64)Cu labeling, microPET imaging of (64)Cu-BaMalSar-exendin-4 and (64)Cu-Mal2Sar-(exendin-4)2 were performed on subcutaneous INS-1 tumors, which were clearly visualized with both probes. The tumor uptake of (64)Cu-Mal2Sar-(exendin-4)2 was significantly higher than that of (64)Cu-BaMaSarl-exendin-4, which could be caused by polyvalency effect. The receptor specificity of these probes was confirmed by effective blocking of the uptake in both tumor and normal positive organs with 20-fold excess of unlabeled exendin-4. In conclusion, sarcophagine cage conjugated exendin-4 demonstrated persistent and specific uptake in INS-1 insulinoma model. Dimerization of exendin-4 could successfully lead to increased tumor uptake in vivo. Both (64)Cu-BaMalSar-exendin-4 and (64)Cu-Mal2Sar-(exendin-4)2 hold a great potential for GLP-1R targeted imaging.

One-pot two-step radiosynthesis of a new (18)F-labeled thiol reactive prosthetic group and its conjugate for insulinoma imaging

N-(2-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)ethyl)-6-fluoronicotinamide ([¹⁸F]FNEM), a novel prosthetic agent that is thiol-specific, was synthesized using a one-pot two-step strategy: (1) ¹⁸F incorporation by a nucleophilic displacement of trimethylammonium substrate under mild conditions; (2) amidation of the resulting 6-[¹⁸F]fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester with N-(2-aminoethyl)maleimide trifluoroacetate salt. The radiosynthesis of the maleimide tracer was completed in 75 min from [¹⁸F]fluoride with 26 ± 5% decay uncorrected radiochemical yield, and specific activity of 19–88 GBq/μmol (decay uncorrected). The in vitro cell uptake, in vivo biodistribution, and positron emission tomography (PET) imaging properties of its conjugation product with [Cys⁴⁰]-exendin-4 were described. [¹⁸F]FNEM-Cys⁴⁰-exendin-4 showed specific targeting of glucagon-like peptide 1 receptor (GLP-1R) positive insulinomas and comparable imaging results to our recently reported [¹⁸F]FPenM-Cys⁴⁰-exendin-4.