Near-infrared fluorescent probe for imaging of pancreatic beta cells

Targeting β Cells with Cathelicidin Nanomedicines Improves Insulin Function and Pancreas Regeneration in Type 1 Diabetic Rats

Type 1 diabetes (T1D) is an incurable condition with an increasing incidence worldwide, in which the hallmark is the autoimmune destruction of pancreatic insulin-producing β cells. Cathelicidin-based peptides have been shown to improve β cell function and neogenesis and may thus be relevant while developing T1D therapeutics. In this work, a cathelicidin-derived peptide, LLKKK18, was loaded in poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs), surface-functionalized with exenatide toward a GLP-1 receptor, aiming the β cell-targeted delivery of the peptide. The NPs present a mean size of around 100 nm and showed long-term stability, narrow size distribution, and negative ζ-potential (-10 mV). The LLKKK18 association efficiency and loading were 62 and 2.9%, respectively, presenting slow and sustained in vitro release under simulated physiologic fluids. Glucose-stimulated insulin release in the INS-1E cell line was observed in the presence of the peptide. In addition, NPs showed a strong association with β cells from isolated rat islets. After administration to diabetic rats, NPs induced a significant reduction of the hyperglycemic state, an improvement in the pancreatic insulin content, and glucose tolerance. Also remarkable, a considerable increase in the β cell mass in the pancreas was observed. Overall, this novel and versatile nanomedicine showed glucoregulatory ability and can pave the way for the development of a new generation of therapeutic approaches for T1D treatment.

Multivalent γ-PGA-Exendin-4 conjugates to target pancreatic β-cells

Targeting of glucagon‐like peptide 1 receptor (GLP‐1R), expressed on the surface of pancreatic β‐cells, is of great interest for the development of advanced therapies for diabetes and diagnostics for insulinoma. We report the conjugation of exendin‐4 (Ex‐4), an approved drug to treat type 2 diabetes, to poly‐γ‐glutamic acid (γ‐PGA) to obtain more stable and effective GLP‐1R ligands. Exendin‐4 modified at Lysine‐27 with PEG4‐maleimide was conjugated to γ‐PGA functionalized with furan, in different molar ratios, exploiting a chemoselective Diels‐Alder cycloaddition. The γ‐PGA presenting the highest number of conjugated Ex‐4 molecules (average 120 per polymeric chain) showed a double affinity towards GLP‐1R with respect to exendin per se, paving the way to improved therapeutic and diagnostic applications.

Reagents and models for detecting endogenous GLP1R and GIPR

Glucagon-like peptide-1 receptor (GLP1R) agonists target the GLP1R, whereas dual GLP1R/ gastric inhibitory polypeptide receptor (GIPR) agonists target both the GLP1R and GIPR. Despite the importance of these drug classes for the treatment of diabetes and obesity, still very little is known about the localization of GLP1R and GIPR themselves. Complicating matters is the low abundance of GLP1R and GIPR mRNA/protein, as well as a lack of specific and validated reagents for their detection. Without knowing where GLP1R and GIPR are located, it is difficult to propose mechanisms of action in the various target organs, and whether this is indirect or direct. In the current review, we will explain the steps needed to properly validate reagents for endogenous GLP1R/GIPR detection, describe the available approaches to visualize GLP1R/GIPR, and provide an update on the state-of-art. The overall aim is to provide a reference resource for researchers interested in GLP1R and GIPR signaling.

Optoacoustic Imaging of Glucagon-like Peptide-1 Receptor with a Near-Infrared Exendin-4 Analog

Limitations in current imaging tools have long challenged the imaging of small pancreatic islets in animal models. Here, we report the first development and in vivo validation testing of a broad-spectrum and high-absorbance near-infrared optoacoustic contrast agent, E4x12-Cy7. Our near-infrared tracer is based on the amino acid sequence of exendin-4 and targets the glucagon-like peptide-1 receptor (GLP-1R). Cell assays confirmed that E4x12-Cy7 has a high-binding affinity (dissociation constant, Kd, 4.6 ± 0.8 nM). Using the multispectral optoacoustic tomography, we imaged E4x12-Cy7 and optoacoustically visualized β-cell insulinoma xenografts in vivo for the first time. In the future, similar optoacoustic tracers that are specific for β-cells and combines optoacoustic and fluorescence imaging modalities could prove to be important tools for monitoring the pancreas for the progression of diabetes.

GLP-1 peptide analogs for targeting pancreatic beta cells

Loss or dysfunction of the pancreatic beta cells or insulin receptors leads to diabetes mellitus (DM). This usually occurs over many years; therefore, the development of methods for the timely detection and clinical intervention are vital to prevent the development of this disease. Glucagon-like peptide-1 receptor (GLP-1R) is the receptor of GLP-1, an incretin hormone that causes insulin secretion in a glucose-dependent manner. GLP-1R is highly expressed on the surface of pancreatic beta cells, providing a potential target for bioimaging. In this review, we provide an overview of various strategies, such as the development of GLP-1R agonists (e.g., exendin-4), and GLP-1 sequence modifications for GLP-1R targeting for the diagnosis and treatment of pancreatic beta cell disorders. We also discuss the challenges of targeting pancreatic beta cells and strategies to address such challenges.

Imaging of Insulinoma by Targeting Glucagonlike Peptide-1 Receptor

"Glucagonlike peptide-1 (GLP-1) receptor imaging, using radiolabeled exendin-4, was recently established for detecting insulinoma in patients with hyperinsulinemic hypoglycemia. It has proven to be a sensitive and specific method for preoperative localization of insulinoma. This review introduces the development, clinical research, and perspective of GLP-1 receptor imaging mainly in insulinoma.

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.

Targeted Optical Imaging of the Glucagonlike Peptide 1 Receptor Using Exendin-4-IRDye 800CW

The treatment of choice for insulinomas and focal lesions in congenital hyperinsulinism (CHI) is surgery. However, intraoperative detection can be challenging. This challenge could be overcome with intraoperative fluorescence imaging, which provides real-time lesion detection with a high spatial resolution. Here, a novel method for targeted near-infrared (NIR) fluorescence imaging of glucagonlike peptide 1 receptor (GLP-1R)–positive lesions, using the GLP-1 agonist exendin-4 labeled with IRDye 800CW, was examined in vitro and in vivo. Methods: A competitive binding assay was performed using Chinese hamster lung (CHL) cells transfected with GLP-1R. Tracer biodistribution was determined in BALB/c nude mice bearing subcutaneous CHL-GLP-1R xenografts. In vivo NIR fluorescence imaging of CHL-GLP-1R xenografts was performed. Localization of the tracer in the pancreatic islets of BALB/c nude mice was examined using fluorescence microscopy. Laparoscopic imaging was performed to detect the fluorescent signal of the tracer in the pancreas of mini pigs. Results: Exendin-4-IRDye 800CW binds GLP-1R with a half-maximal inhibitory concentration of 3.96 nM. The tracer accumulates in CHL-GLP-1R xenografts. Subcutaneous CHL-GLP-1R xenografts were visualized using in vivo NIR fluorescence imaging. The tracer accumulates specifically in the pancreatic islets of mice, and a clear fluorescent signal was detected in the pancreas of mini pigs. Conclusion: These data provide the first in vivo evidence of the feasibility of targeted fluorescence imaging of GLP-1R–positive lesions. Intraoperative lesion delineation using exendin-4-IRDye 800CW could benefit open as well as laparoscopic surgical procedures for removal of insulinomas and focal lesions in CHI.

Fluorescence Imaging of Peripheral Nerves by a Nav1.7-Targeted Inhibitor Cystine Knot Peptide

Twenty million Americans suffer from peripheral nerve injury caused by trauma and medical disorders, resulting in a broad spectrum of potentially debilitating side effects. In one out of four cases, patients identify surgery as the root cause of their nerve injury. Particularly during tumor resections or after traumatic injuries, tissue distortion and poor visibility can challenge a surgeon's ability to precisely locate and preserve peripheral nerves. Intuitively, surgical outcomes would improve tremendously if nerves could be highlighted using an exogeneous contrast agent. In clinical practice, however, the current standard of care-visual examination and palpation-remains unchanged. To address this unmet clinical need, we explored the expression of voltage-gated sodium channel Na_v1.7 as an intraoperative marker for the peripheral nervous system. We show that expression of Na_v1.7 is high in peripheral nerves harvested from both human and mouse tissue. We further show that modification of a Na_v1.7-selective peptide, Hsp1a, can serve as a targeted vector for delivering a fluorescent sensor to the peripheral nervous system. Ex vivo, we observe a high signal-to-noise ratio for fluorescently labeled Hsp1a in both histologically prepared and fresh tissue. Using a surgical fluorescent microscope, we show in a simulated clinical scenario that the identification of mouse sciatic nerves is possible, suggesting that fluorescently labeled Hsp1a tracers could be used to discriminate nerves from their surrounding tissues in a routine clinical setting.

Tools for Bioimaging Pancreatic β Cells in Diabetes

When diabetes is diagnosed, the majority of insulin-secreting pancreatic β cells are already dysfunctional or destroyed. This β cell dysfunction/destruction usually takes place over many years, making timely detection and clinical intervention difficult. For this reason, there is immense interest in developing tools to bioimage β cell mass and/or function noninvasively to facilitate early diagnosis of diabetes as well as to assist the development of novel antidiabetic therapies. Recent years have brought significant progress in β cell imaging that is now inching towards clinical applicability. We explore here the need to bioimage human β cells noninvasively in various types of diabetes, and we discuss current and emerging tools for bioimaging β cells. Further developments in this field are expected to facilitate β cell imaging in diabetes.

Molecular imaging of β-cells: diabetes and beyond

Since diabetes is becoming a global epidemic, there is a great need to develop early β-cell specific diagnostic techniques for this disorder. There are two types of diabetes (i.e., type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM)). In T1DM, the destruction of pancreatic β-cells leads to reduced insulin production or even absolute insulin deficiency, which consequently results in hyperglycemia. Actually, a central issue in the pathophysiology of all types of diabetes is the relative reduction of β-cell mass (BCM) and/or impairment of the function of individual β-cells. In the past two decades, scientists have been trying to develop imaging techniques for noninvasive measurement of the viability and mass of pancreatic β-cells. Despite intense scientific efforts, only two tracers for positron emission tomography (PET) and one contrast agent for magnetic resonance (MR) imaging are currently under clinical evaluation. β-cell specific imaging probes may also allow us to precisely and specifically visualize transplanted β-cells and to improve transplantation outcomes, as transplantation of pancreatic islets has shown promise in treating T1DM. In addition, some of these probes can be applied to the preoperative detection of hidden insulinomas as well. In the present review, we primarily summarize potential tracers under development for imaging β-cells with a focus on tracers for PET, SPECT, MRI, and optical imaging. We will discuss the advantages and limitations of the various imaging probes and extend an outlook on future developments in the field.

Preparation of a Trp-BODIPY fluorogenic amino acid to label peptides for enhanced live-cell fluorescence imaging

Fluorescent peptides are valuable tools for live-cell imaging because of the high specificity of peptide sequences for their biomolecular targets. When preparing fluorescent versions of peptides, labels must be introduced at appropriate positions in the sequences to provide suitable reporters while avoiding any impairment of the molecular recognition properties of the peptides. This protocol describes the preparation of the tryptophan (Trp)-based fluorogenic amino acid Fmoc-Trp(C₂-BODIPY)-OH and its incorporation into peptides for live-cell fluorescence imaging-an approach that is applicable to most peptide sequences. Fmoc-Trp(C₂-BODIPY)-OH contains a BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorogenic core, which works as an environmentally sensitive fluorophore, showing high fluorescence in lipophilic conditions. It is attached to Trp via a spacer-free C-C linkage, resulting in a labeled amino acid that can mimic the molecular interactions of Trp, enabling wash-free imaging. This protocol covers the chemical synthesis of the fluorogenic amino acid Fmoc-Trp(C₂-BODIPY)-OH (3-4 d), the preparation of the labeled antimicrobial peptide BODIPY-cPAF26 by solid-phase synthesis (6-7 d) and its spectral and biological characterization as a live-cell imaging probe for different fungal pathogens. As an example, we include a procedure for using BODIPY-cPAF26 for wash-free imaging of fungal pathogens, including real-time visualization of Aspergillus fumigatus (5 d for culturing, 1-2 d for imaging).

Glucagon-Like Peptide-1 and Its Class B G Protein-Coupled Receptors: A Long March to Therapeutic Successes

The glucagon-like peptide (GLP)-1 receptor (GLP-1R) is a class B G protein-coupled receptor (GPCR) that mediates the action of GLP-1, a peptide hormone secreted from three major tissues in humans, enteroendocrine L cells in the distal intestine, α cells in the pancreas, and the central nervous system, which exerts important actions useful in the management of type 2 diabetes mellitus and obesity, including glucose homeostasis and regulation of gastric motility and food intake. Peptidic analogs of GLP-1 have been successfully developed with enhanced bioavailability and pharmacological activity. Physiologic and biochemical studies with truncated, chimeric, and mutated peptides and GLP-1R variants, together with ligand-bound crystal structures of the extracellular domain and the first three-dimensional structures of the 7-helical transmembrane domain of class B GPCRs, have provided the basis for a two-domain-binding mechanism of GLP-1 with its cognate receptor. Although efforts in discovering therapeutically viable nonpeptidic GLP-1R agonists have been hampered, small-molecule modulators offer complementary chemical tools to peptide analogs to investigate ligand-directed biased cellular signaling of GLP-1R. The integrated pharmacological and structural information of different GLP-1 analogs and homologous receptors give new insights into the molecular determinants of GLP-1R ligand selectivity and functional activity, thereby providing novel opportunities in the design and development of more efficacious agents to treat metabolic disorders.

In Vivo Imaging of Transplanted Pancreatic Islets

The beta-cells in the islets of Langerhans in the pancreas secrete insulin and play an important role in glucose homeostasis. Diabetes, characterized by hyperglycemia, results from an absolute or a relative deficiency of the pancreatic beta-cell mass. Islet transplantation has been considered to be a useful therapeutic approach, but it is largely unsuccessful because most of the transplanted islets are lost in the early stage of transplantation. To evaluate the efficacy of intervention methods for the improvement of islet survival, monitoring of the functional islet mass is needed. Various techniques to image and track transplanted islets have been investigated to assess islets after transplantation. In this review, recent progresses in imaging methods to visualize islets are discussed.

Targets and probes for non-invasive imaging of β-cells

β-cells, located in the islets of the pancreas, are responsible for production and secretion of insulin and play a crucial role in blood sugar regulation. Pathologic β-cells often cause serious medical conditions affecting blood glucose level, which severely impact life quality and are life-threatening if untreated. With 347 million patients, diabetes is one of the most prevalent diseases, and will continue to be one of the largest socioeconomic challenges in the future. The diagnosis still relies mainly on indirect methods like blood sugar measurements. A non-invasive diagnostic imaging modality would allow direct evaluation of β-cell mass and would be a huge step towards personalized medicine. Hyperinsulinism is another serious condition caused by β-cells that excessively secrete insulin, like for instance β-cell hyperplasia and insulinomas. Treatment options with drugs are normally not curative, whereas curative procedures usually consist of the resection of affected regions for which, however, an exact localization of the foci is necessary. In this review, we describe potential tracers under development for targeting β-cells with focus on radiotracers for PET and SPECT imaging, which allow the non-invasive visualization of β-cells. We discuss either the advantages or limitations for the various tracers and modalities. This article concludes with an outlook on future developments and discuss the potential of new imaging probes including dual probes that utilize functionalities for both a radioactive and optical moiety as well as for theranostic applications.

Quantitative Impact of Plasma Clearance and Down-regulation on GLP-1 Receptor Molecular Imaging

PURPOSE

Quantitative molecular imaging of beta cell mass (BCM) would enable early detection and treatment monitoring of type 1 diabetes. The glucagon-like peptide-1 (GLP-1) receptor is an attractive target due to its beta cell specificity and cell surface location. We quantitatively investigated the impact of plasma clearance and receptor internalization on targeting efficiency in healthy B6 mice.

PROCEDURES

Four exenatide-based probes were synthesized that varied in molecular weight, binding affinity, and plasma clearance. The GLP-1 receptor internalization rate and in vivo receptor expression were quantified.

RESULTS

Receptor internalization (54,000 receptors/cell in vivo) decreased significantly within minutes, reducing the benefit of a slower-clearing agent. The multimers and albumin binding probes had higher kidney and liver uptake, respectively.

CONCLUSIONS

Slow plasma clearance is beneficial for GLP-1 receptor peptide therapeutics. However, for exendin-based imaging of islets, down-regulation of the GLP-1 receptor and non-specific background uptake result in a higher target-to-background ratio for fast-clearing agents.

An intravital microscopy model to study early pancreatic inflammation in type 1 diabetes in NOD mice

Intravital microscopy (IVM) of the pancreas has been proven to be an invaluable tool in pancreatitis, transplantation and ischemia/reperfusion research. Also in type 1 diabetes (T1D) pancreatic IVM offers unique advantages for the elucidation of the disease process. Female non-obese diabetic (NOD) mice develop T1D spontaneously by 40 weeks of age. Our goal was to establish an IVM-based method to study early pancreatic inflammation in NOD mice, which can be used to screen novel medications to prevent or delay T1D in future studies. This included evaluation of leukocyte-endothelial interactions as well as disturbances of capillary perfusion in the pancreatic microcirculation.

Opening the "White Box" in Tissue Engineering: Visualization of Cell Aggregates in Optically Scattering Scaffolds

The noninvasive and longitudinal imaging of cells or cell aggregates in large optically scattering scaffolds is still a largely unresolved problem in tissue engineering. In this work, we investigated the potential of near-infrared (NIR) photoacoustic (PA) tomography imaging to address this issue. We used clinically relevant sizes of highly light scattering polyethersulfone multibore(®) hollow fiber scaffolds seeded with cells. Since cells have little optical absorption at NIR wavelengths, we studied labeling of cells with absorbers. Four NIR labels were examined for their suitability based on absorption characteristics, resistance to bleaching, and influence on cell viability. On the basis of these criteria, carbon nanoparticles proved most suitable in a variety of cells. For PA imaging, we used a research setup, based on computed tomography geometry. As proof of principle, using this imager we monitored the distribution and clustering of labeled rat insulinoma beta cell aggregates in the scaffolds. This was performed for the duration of 1 week in a nondestructive manner. The results were validated using fluorescence imaging, histology, and light microscopy imaging. Based on our findings, we conclude that PA tomography is a powerful tool for the nondestructive imaging of cells in optically scattering tissue-engineered scaffolds.

Development of a clickable bimodal fluorescent/PET probe for in vivo imaging

Background

Fluorescent imaging agents are becoming evermore important in preclinical and clinical research. They do, however, suffer from poor tissue penetration, which makes optical fluorescence imaging incompatible with whole-body imaging techniques. The design of novel bimodal PET active and fluorescent tracers could therefore combine the benefits of optical imaging with radioactively labeled imaging probes. Herein, we report the synthesis and evaluation of a clickable ¹⁸F-labeled fluorescent dye.

Methods

An azide-modified BODIPY-Fl dye could be successfully radio-labeled with ¹⁸F using an ¹⁸F/¹⁹F exchange reaction of the boron-fluoride core of the BODIPY dye to yield a clickable bimodal PET/fluorescent imaging tool. In vitro as well as in vivo imaging (PET/fluorescence) using a bombesin analog was conducted to study the applicability of the dual-modality imaging probe.

Results

We use the radio-labeled small molecule, ¹⁸F-BODIPY-azide to label site-specifically different targeted peptides, based on a standard modular labeling protocol. Following the synthesis of a bimodal bombesin analog, we determine the peptide tracer’s performance in vitro and in vivo, exploring both the optical as well as PET imaging capabilities.

Conclusion

This versatile methodology has the potential to have a transformational impact on ¹⁸F radiotracer synthesis, opening the door for rapid screening of novel-labeled peptide tracers, both on the cellular (optical) as well as whole-body (PET) level.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0120-4) contains supplementary material, which is available to authorized users.

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.

Combined Optical Coherence and Fluorescence Microscopy to assess dynamics and specificity of pancreatic beta-cell tracers

The identification of a beta-cell tracer is a major quest in diabetes research. However, since MRI, PET and SPECT cannot resolve individual islets, optical techniques are required to assess the specificity of these tracers. We propose to combine Optical Coherence Microscopy (OCM) with fluorescence detection in a single optical platform to facilitate these initial screening steps from cell culture up to living rodents. OCM can image islets and vascularization without any labeling. Thereby, it alleviates the need of both genetically modified mice to detect islets and injection of external dye to reveal vascularization. We characterized Cy5.5-exendin-3, an agonist of glucagon-like peptide 1 receptor (GLP1R), for which other imaging modalities have been used and can serve as a reference. Cultured cells transfected with GLP1R and incubated with Cy5.5-exendin-3 show full tracer internalization. We determined that a dose of 1 μg of Cy5.5-exendin-3 is sufficient to optically detect in vivo the tracer in islets with a high specificity. In a next step, time-lapse OCM imaging was used to monitor the rapid and specific tracer accumulation in murine islets and its persistence over hours. This optical platform represents a versatile toolbox for selecting beta-cell specific markers for diabetes research and future clinical diagnosis.

Dual-purpose linker for alpha helix stabilization and imaging agent conjugation to glucagon-like peptide-1 receptor ligands

Peptides display many characteristics of efficient imaging agents such as rapid targeting, fast background clearance, and low non-specific cellular uptake. However, poor stability, low affinity, and loss of binding after labeling often preclude their use in vivo. Using glucagon-like peptide-1 receptor (GLP-1R) ligands exendin and GLP-1 as a model system, we designed a novel α-helix-stabilizing linker to simultaneously address these limitations. The stabilized and labeled peptides showed an increase in helicity, improved protease resistance, negligible loss or an improvement in binding affinity, and excellent in vivo targeting. The ease of incorporating azidohomoalanine in peptides and efficient reaction with the dialkyne linker enable this technique to potentially be used as a general method for labeling α helices. This strategy should be useful for imaging beta cells in diabetes research and in developing and testing other peptide targeting agents.

Beta cell specific probing with fluorescent exendin-4 is progressively reduced in type 2 diabetic mouse models

Probes based on GLP-1R agonist exendin-4 have shown promise as in vivo β cell tracers. However, questions remain regarding the β cell specificity of exendin-4 probes, and it is unclear if the expression levels of the GLP-1R are affected in a type 2 diabetic state. Using in vivo probing followed by ex vivo imaging we found fluorescent exendin-4 probes to distinctly label the pancreatic islets in mice in a Glp-1r dependent manner. Furthermore, a co-localization study revealed a near 100 percent β cell specificity with less than one percent probing in other analyzed cell types. We then tested if probing was affected in models of type 2 diabetes using the Lepr(db/db) (db/db) and the Diet-Induced Obese (DIO) mouse. Although nearly all β cells continued to be probed, we observed a progressive decline in probing intensity in both models with the most dramatic reduction seen in db/db mice. This was paralleled by a progressive decrease in Glp-1r protein expression levels. These data confirm β cell specificity for exendin-4 based probes in mice. Furthermore, they also suggest that GLP-1R targeting probes may provide a tool to monitor β cell function rather than mass in type 2 diabetic mouse models.

Glucagon-like peptide-1 (GLP-1) analogs: recent advances, new possibilities, and therapeutic implications

Glucagon-like peptide-1 (GLP-1) is an incretin that plays important physiological roles in glucose homeostasis. Produced from intestine upon food intake, it stimulates insulin secretion and keeps pancreatic β-cells healthy and proliferating. Because of these beneficial effects, it has attracted a great deal of attention in the past decade, and an entirely new line of diabetic therapeutics has emerged based on the peptide. In addition to the therapeutic applications, GLP-1 analogs have demonstrated a potential in molecular imaging of pancreatic β-cells; this may be useful in early detection of the disease and evaluation of therapeutic interventions, including islet transplantation. In this Perspective, we focus on GLP-1 analogs for their studies on improvement of biological activities, enhancement of metabolic stability, investigation of receptor interaction, and visualization of the pancreatic islets.

In vivo imaging of GLP-1R with a targeted bimodal PET/fluorescence imaging agent

Accurate visualization and quantification of β-cell mass is critical for the improved understanding, diagnosis, and treatment of both type 1 diabetes (T1D) and insulinoma. Here, we describe the synthesis of a bimodal imaging probe (PET/fluorescence) for imaging GLP-1R expression in the pancreas and in pancreatic islet cell tumors. The conjugation of a bimodal imaging tag containing a near-infrared fluorescent dye, and the copper chelator sarcophagine to the GLP-1R targeting peptide exendin-4 provided the basis for the bimodal imaging probe. Conjugation was performed via a novel sequential one-pot synthetic procedure including (64)Cu radiolabeling and copper-catalyzed click-conjugation. The bimodal imaging agent (64)Cu-E4-Fl was synthesized in good radiochemical yield and specific activity (RCY = 36%, specific activity: 141 μCi/μg, >98% radiochemical purity). The agent showed good performance in vivo and ex vivo, visualizing small xenografts (<2 mm) with PET and pancreatic β-cell mass by phosphor autoradiography. Using the fluorescent properties of the probe, we were able to detect individual pancreatic islets, confirming specific binding to GLP-1R and surpassing the sensitivity of the radioactive label. The use of bimodal PET/fluorescent imaging probes is promising for preoperative imaging and fluorescence-assisted analysis of patient tissues. We believe that our procedure could become relevant as a protocol for the development of bimodal imaging agents.

GLP-1R-targeting magnetic nanoparticles for pancreatic islet imaging

Noninvasive assessment of pancreatic β-cell mass would tremendously aid in managing type 1 diabetes (T1D). Toward this goal, we synthesized an exendin-4 conjugated magnetic iron oxide-based nanoparticle probe targeting glucagon-like peptide 1 receptor (GLP-1R), which is highly expressed on the surface of pancreatic β-cells. In vitro studies in βTC-6, the β-cell line, showed specific accumulation of the targeted probe (termed MN-Ex10-Cy5.5) compared with nontargeted (termed MN-Cy5.5). In vivo magnetic resonance imaging showed a significant transverse relaxation time (T2) shortening in the pancreata of mice injected with the MN-Ex10-Cy5.5 probe compared with control animals injected with the nontargeted probe at 7.5 and 24 h after injection. Furthermore, ΔT2 of the pancreata of prediabetic NOD mice was significantly higher than that of diabetic NOD mice after the injection of MN-Ex10-Cy5.5, indicating the decrease of probe accumulation in these animals due to β-cell loss. Of note, ΔT2 of prediabetic and diabetic NOD mice injected with MN-Cy5.5 was not significantly changed, reflecting the nonspecific mode of accumulation of nontargeted probe. We believe our results point to the potential for using this agent for monitoring the disease development and response of T1D to therapy.

Efficient 18F-Labeling of Synthetic Exendin-4 Analogues for Imaging Beta Cells

A number of exendin derivatives have been developed to target glucagon-like peptide 1 (GLP-1) receptors on beta cells in vivo. Modifications of exendin analogues have been shown to have significant effects on pharmacokinetics and, as such, have been used to develop a variety of therapeutic compounds. Here, we show that an exendin-4, modified at position 12 with a cysteine conjugated to a tetrazine, can be labeled with 18F-trans-cyclooctene and converted into a PET imaging agent at high yields and with good selectivity. The agent accumulates in beta cells in vivo and has sufficiently high accumulation in mouse models of insulinomas to enable in vivo imaging.

Nanoparticle-mediated measurement of target-drug binding in cancer cells

Responses to molecularly targeted therapies can be highly variable and depend on mutations, fluctuations in target protein levels in individual cells, and drug delivery. The ability to rapidly quantitate drug response in cells harvested from patients in a point-of-care setting would have far reaching implications. Capitalizing on recent developments with miniaturized NMR technologies, we have developed a magnetic nanoparticle-based approach to directly measure both target expression and drug binding in scant human cells. The method involves covalent conjugation of the small-molecule drug to a magnetic nanoparticle that is then used as a read-out for target expression and drug-binding affinity. Using poly(ADP-ribose) polymerase (PARP) inhibition as a model system, we developed an approach to distinguish differential expression of PARP in scant cells with excellent correlation to gold standards, the ability to mimic drug pharmacodynamics ex vivo through competitive target-drug binding, and the potential to perform such measurements in clinical samples.

High-resolution magnetic resonance imaging quantitatively detects individual pancreatic islets

OBJECTIVE

We studied whether manganese-enhanced high-field magnetic resonance (MR) imaging (MEHFMRI) could quantitatively detect individual islets in situ and in vivo and evaluate changes in a model of experimental diabetes.

RESEARCH DESIGN AND METHODS

Whole pancreata from untreated (n = 3), MnCl(2) and glucose-injected mice (n = 6), and mice injected with either streptozotocin (STZ; n = 4) or citrate buffer (n = 4) were imaged ex vivo for unambiguous evaluation of islets. Exteriorized pancreata of MnCl(2) and glucose-injected mice (n = 6) were imaged in vivo to directly visualize the gland and minimize movements. In all cases, MR images were acquired in a 14.1 Tesla scanner and correlated with the corresponding (immuno)histological sections.

RESULTS

In ex vivo experiments, MEHFMRI distinguished different pancreatic tissues and evaluated the relative abundance of islets in the pancreata of normoglycemic mice. MEHFMRI also detected a significant decrease in the numerical and volume density of islets in STZ-injected mice. However, in the latter measurements the loss of β-cells was undervalued under the conditions tested. The experiments on the externalized pancreata confirmed that MEHFMRI could visualize native individual islets in living, anesthetized mice.

CONCLUSIONS

Data show that MEHFMRI quantitatively visualizes individual islets in the intact mouse pancreas, both ex vivo and in vivo.

Molecular imaging: a promising tool to monitor islet transplantation

Replacement of insulin production by pancreatic islet transplantation has great potential as a therapy for type 1 diabetes mellitus. At present, the lack of an effective approach to islet grafts assessment limits the success of this treatment. The development of molecular imaging techniques has the potential to fulfill the goal of real-time noninvasive monitoring of the functional status and viability of the islet grafts. We review the application of a variety of imaging modalities for detecting endogenous and transplanted beta-cell mass. The review also explores the various molecular imaging strategies for assessing islet delivery, the metabolic effects on the islet grafts as well as detection of immunorejection. Here, we highlight the use of combined imaging and therapeutic interventions in islet transplantation and the in vivo monitoring of stem cells differentiation into insulin-producing cells.

Experimental approaches for high-resolution in vivo imaging of islet of Langerhans biology

Under physiological conditions and in the pathogenesis of diabetes mellitus systemic influences play a substantial role for function and survival of cells of the islet of Langerhans. Therefore, in vivo studies to understand islet biology are indispensible and imaging techniques are increasingly used for this purpose. Among the diverse imaging modalities currently only laser scanning microscopy (LSM) allows resolution and visualization of individual cells and cellular processes. To overcome limited tissue penetration and working distance of LSM and enable in vivo investigations of islet cell physiology, various experimental approaches have been developed. Especially, the recently developed imaging platforms have significantly improved the possibility to study islets at a cellular level in vivo, and provided novel insight into islet biology in health and disease. The various approaches, their applications, and reported results, as well as their limitations are reviewed in this article.

Accurate measurement of pancreatic islet beta-cell mass using a second-generation fluorescent exendin-4 analog

The hallmark of type 1 diabetes is autoimmune destruction of the insulin-producing β-cells of the pancreatic islets. Autoimmune diabetes has been difficult to study or treat because it is not usually diagnosed until substantial β-cell loss has already occurred. Imaging agents that permit noninvasive visualization of changes in β-cell mass remain a high-priority goal. We report on the development and testing of a near-infrared fluorescent β-cell imaging agent. Based on the amino acid sequence of exendin-4, we created a neopeptide via introduction of an unnatural amino acid at the K(12) position, which could subsequently be conjugated to fluorophores via bioorthogonal copper-catalyzed click-chemistry. Cell assays confirmed that the resulting fluorescent probe (E4(×12)-VT750) had a high binding affinity (~3 nM). Its in vivo properties were evaluated using high-resolution intravital imaging, histology, whole-pancreas visualization, and endoscopic imaging. According to intravital microscopy, the probe rapidly bound to β-cells and, as demonstrated by confocal microscopy, it was internalized. Histology of the whole pancreas showed a close correspondence between fluorescence and insulin staining, and there was an excellent correlation between imaging signals and β-cell mass in mice treated with streptozotocin, a β-cell toxin. Individual islets could also be visualized by endoscopic imaging. In short, E4(×12)-VT750 showed strong and selective binding to glucose-like peptide-1 receptors and permitted accurate measurement of β-cell mass in both diabetic and nondiabetic mice. This near-infrared imaging probe, as well as future radioisotope-labeled versions of it, should prove to be important tools for monitoring diabetes, progression, and treatment in both experimental and clinical contexts.