Synthesis and evaluation of (S)-2-(2-[18F]fluoroethoxy)-4-([3-methyl-1-(2-piperidin-1-yl-phenyl)-butyl-carbamoyl]-methyl)-benzoic acid ([18F]repaglinide): a promising radioligand for quantification of pancreatic beta-cell mass with positron emission tomography (PET)

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.

Beta Cell Imaging-From Pre-Clinical Validation to First in Man Testing

There are presently no reliable ways to quantify human pancreatic beta cell mass (BCM) in vivo, which prevents an accurate understanding of the progressive beta cell loss in diabetes or following islet transplantation. Furthermore, the lack of beta cell imaging hampers the evaluation of the impact of new drugs aiming to prevent beta cell loss or to restore BCM in diabetes. We presently discuss the potential value of BCM determination as a cornerstone for individualized therapies in diabetes, describe the presently available probes for human BCM evaluation, and discuss our approach for the discovery of novel beta cell biomarkers, based on the determination of specific splice variants present in human beta cells. This has already led to the identification of DPP6 and FXYD2ga as two promising targets for human BCM imaging, and is followed by a discussion of potential safety issues, the role for radiochemistry in the improvement of BCM imaging, and concludes with an overview of the different steps from pre-clinical validation to a first-in-man trial for novel tracers.

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.

A pilot study of pancreatic islet amyloid PET imaging with [18F]FDDNP

OBJECTIVES

Pancreatic islet amyloid deposition occurs before β-cell damage in type 2 diabetes mellitus patients. The islet and Alzheimer's disease β-amyloid shares similar secondary structures. The Alzheimer's disease β-amyloid targeting tracer [F]FDDNP could be used to image pancreatic islet amyloid with PET.

PATIENTS AND METHODS

Consecutive pancreatic tissue sections from a 69-year-old male type 2 diabetes mellitus patient were stained by hematoxylin and eosin, anti-amylin antibody, Congo Red, periodic acid-Schiff, and [F]FDDNP reference compound, respectively. The pancreatic tissue sections were also incubated with [F]FDDNP with and without its reference compound for autoradiography. Subsequently, we performed control [F]FDDNP pancreatic PET/CT imaging in four healthy individuals. The mean standardized uptake values of [F]FDDNP uptake in the pancreatic head, neck, body, and tail, blood pool, liver, and vertebral bone from 5 to 120 min after injection were determined.

RESULTS

Islet amyloid was observed in all four standard staining methods in the pancreas tissue. Similar islet amyloid distribution and phenotypes were observed clearly in the [F]FDDNP reference compound-stained pancreas tissue. [F]FDDNP was intensively accumulated in the same pancreatic tissue in autoradiography, which was largely blocked by its reference compound. In the PET/CT scans of control human participants, the mean standardized uptake values in pancreas decreased to the blood pool level in 30 min and all parts of the pancreas had similar [F]FDDNP uptake. The pancreas could be distinguished clearly from the liver at all-time points.

CONCLUSION

These results suggested that [F]FDDNP is a potential tracer for pancreatic islet amyloid PET imaging.

Synthesis and Characterization of a Promising Novel FFAR1/GPR40 Targeting Fluorescent Probe for β-Cell Imaging

Diabetes affects an increasing number of patients worldwide and is responsible for a significant rise in healthcare expenses. Imaging of β-cells bears the potential to contribute to an improved understanding, diagnosis, and development of new treatment options for diabetes. Here, we describe the first small molecule fluorescent probe targeting the free fatty acid receptor 1 (FFAR1/GPR40). This receptor is highly expressed on β-cells, and was up to now unexplored for imaging purposes. We designed a novel probe by facile modification of the selective and potent FFAR1 agonist TAK-875. Effective and specific binding of the probe was demonstrated using FFAR1 overexpressing cells. We also successfully labeled FFAR1 on MIN6 and INS1E cells, two widely used β-cell models, by applying an effective amplification protocol. Finally, we showed that the probe is capable of inducing insulin secretion in a glucose-dependent manner, thus demonstrating that functional activity of the probe was maintained. These results suggest that our probe represents a first important step to successful β-cell imaging by targeting FFAR1. The developed probe may prove to be particularly useful for in vitro and ex vivo studies of diabetic cellular and animal models to gain new insights into disease pathogenesis.

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

The synthesis of [18F]pitavastatin as a tracer for hOATP using the Suzuki coupling

Fluorine-18 labeled radiotracers, such as [¹⁸F]fluorodeoxyglucose, can be used as practical diagnostic agents in positron emission tomography (PET).

Beta-cell imaging: call for evidence-based and scientific approach

INTRODUCTION

Advances in positron emission tomography (PET) imaging have provided opportunities to develop radiotracers specific for imaging insulin-producing pancreatic β-cells. However, a host of lingering questions should be addressed before these radiotracers are advocated for noninvasive quantification of β-cell mass (BCM) in vivo in the native pancreas.

METHOD

We provide an overview of tetrabenazine-based PET tracers developed to image and quantify BCM and discuss several theoretical, technical, and biological limitations of applying these tracers in clinical practice.

DISCUSSION

VMAT2, a transporter protein expressed on pancreatic β-cells, has been advocated as a promising target for PET imaging tracers, such as dihydrotetrabenazine. However, the lack of radiotracer specificity for these proteins hampers their clinical application. Another important argument against their use is a striking discrepancy between radiotracer uptake and BCM in subjects with type I diabetes mellitus and healthy controls. Additionally, technical issues, such as the finite spatial resolution of PET, partial volume effects, and movement of the pancreas during respiration, impede PET imaging as a viable option for BCM quantification in the foreseeable future.

CONCLUSION

The assertion that BCM can be accurately quantified by tetrabenazine derived β-cell-specific radiotracers as density per unit volume of pancreatic tissue is not justifiable at this time. The fallacy of these claims can be explained by technical as well as biological facts that have been disregarded and ignored in the literature.

Imaging beta-cell mass and function in situ and in vivo

Glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells is critical to the maintenance of blood glucose homeostasis in animals. Both decrease in pancreatic beta-cell mass and defects in beta-cell function contribute to the onset of diabetes, although the underlying mechanisms remain largely unknown. Molecular imaging techniques can help beta-cell study in a number of ways. High-resolution fluorescence imaging techniques provide novel insights into the fundamental mechanisms underlying GSIS in isolated beta-cells or in situ in pancreatic islets, and dynamic changes of beta-cell mass and function can be noninvasively monitored in vivo by imaging techniques such as positron emission tomography and single-photon emission computed tomography. All these techniques will contribute to the better understanding of the progression of diabetes and the search for the optimized therapeutic measures that reverse deficits in beta-cell mass and function.

Imaging of β-cell mass and insulitis in insulin-dependent (Type 1) diabetes mellitus

Insulin-dependent (type 1) diabetes mellitus is a metabolic disease with a complex multifactorial etiology and a poorly understood pathogenesis. Genetic and environmental factors cause an autoimmune reaction against pancreatic β-cells, called insulitis, confirmed in pancreatic samples obtained at autopsy. The possibility to noninvasively quantify β-cell mass in vivo would provide important biological insights and facilitate aspects of diagnosis and therapy, including follow-up of islet cell transplantation. Moreover, the availability of a noninvasive tool to quantify the extent and severity of pancreatic insulitis could be useful for understanding the natural history of human insulin-dependent (type 1) diabetes mellitus, to early diagnose children at risk to develop overt diabetes, and to select patients to be treated with immunotherapies aimed at blocking the insulitis and monitoring the efficacy of these therapies. In this review, we outline the imaging techniques currently available for in vivo, noninvasive detection of β-cell mass and insulitis. These imaging techniques include magnetic resonance imaging, ultrasound, computed tomography, bioluminescence and fluorescence imaging, and the nuclear medicine techniques positron emission tomography and single-photon emission computed tomography. Several approaches and radiopharmaceuticals for imaging β-cells and lymphocytic insulitis are reviewed in detail.

Imaging of the β-cells of the islets of Langerhans

The major aim of this paper is to review the present status of the techniques for the non-invasive imaging and quantification of insulin-producing pancreatic islet β-cells. Emphasis is placed on both the expansion of prior work already considered in a prior review and novel achievements. Thus, the use of d-mannoheptulose analogs, hypoglycemic sulfonylureas and glinides, neural imaging agents, neuro-hormonal receptor ligands and nanoparticles is first dealt with. Thereafter, consideration is given on optical imaging technologies, the identification of new β-cells specific binding and target proteins, the functional imaging of islets transplanted into the eye anterior chamber and in vivo manganese-enhanced magnetic resonance imaging.

Obstacles on the way to the clinical visualisation of beta cells: looking for the Aeneas of molecular imaging to navigate between Scylla and Charybdis

For more than a decade, researchers have been trying to develop non-invasive imaging techniques for the in vivo measurement of viable pancreatic beta cells. However, in spite of intense research efforts, only one tracer for positron emission tomography (PET) imaging is currently under clinical evaluation. To many diabetologists it may remain unclear why the imaging world struggles to develop an effective method for non-invasive beta cell imaging (BCI), which could be useful for both research and clinical purposes. Here, we provide a concise overview of the obstacles and challenges encountered on the way to such BCI, in both native and transplanted islets. We discuss the major difficulties posed by the anatomical and cell biological features of pancreatic islets, as well as the chemical and physical limits of the main imaging modalities, with special focus on PET, SPECT and MRI. We conclude by indicating new avenues for future research in the field, based on several remarkable recent results.

Sulfonylurea receptor as a target for molecular imaging of pancreas beta cells with (99m)Tc-DTPA-glipizide

OBJECTIVE

This study was aimed to assess pancreas beta cell activity using (99m)Tc-diethyleneaminepentaacetic acid-glipizide (DTPA-GLP), a sulfonylurea receptor agent. The effect of DTPA-GLP on the blood glucose level in rats was also evaluated.

METHODS

DTPA dianhydride was conjugated with GLP in the presence of sodium amide, yielding 60%. Biodistribution and planar images were obtained at 30-120 min after injection of (99m)Tc-DTPA-GLP (1 mg/rat, 0.74 and 11.1 MBq per rat, respectively) in normal female Fischer 344 rats. The control group was given (99m)Tc-DTPA. To demonstrate pancreas beta cell uptake of (99m)Tc-DTPA-GLP via a receptor-mediated process, a group of rats was pretreated with streptozotocin (a beta cell toxin, 55 mg/kg, i.v.) and the images were acquired at immediately-65 min on day 5 post-treatment. The effect on the glucose levels after a single administration (ip) of DTPA-GLP was compared to glipizide (GLP) for up to 6 h.

RESULTS

The structure of DTPA-GLP was confirmed by NMR, mass spectrometry and HPLC. Radiochemical purity assessed by ITLC was >96%. (99m)Tc-DTPA-GLP showed increased pancreas-to-muscle ratios, whereas (99m)Tc-DTPA showed decreased ratios at various time points. Pancreas could be visualized with (99m)Tc-DTPA-GLP in normal rat, however, (99m)Tc-DTPA has poor uptake suggesting the specificity of (99m)Tc-DTPA-GLP. Pancreas beta cell uptake could be blocked by pre-treatment with streptozotocin. DTPA-GLP showed an equal or better response in lowering the glucose levels compared to the existing GLP drug.

CONCLUSIONS

It is feasible to use (99m)Tc-DTPA-GLP to assess pancreas beta cell receptor recognition. (99m)Tc-DTPA-GLP may be helpful in evaluating patients with diabetes, pancreatitis and pancreatic tumors.

Striated muscle as implantation site for transplanted pancreatic islets

Islet transplantation is an attractive treatment for selected patients with brittle type 1 diabetes. In the clinical setting, intraportal transplantation predominates. However, due to extensive early islet cell death, the quantity of islets needed to restore glucose homeostasis requires in general a minimum of two donors. Moreover, the deterioration of islet function over time results in few insulin-independent patients after five-year followup. Specific obstacles to the success of islet transplantation include site-specific concerns for the liver such as the instant blood mediated inflammatory reaction, islet lipotoxicity, low oxygen tension, and poor revascularization, impediments that have led to the developing interest for alternative implantation sites over recent years. Within preclinical settings, several alternative sites have now been investigated and proven favorable in various aspects. Muscle is considered a very promising site and has physiologically properties and technical advantages that could make it optimal for islet transplantation.

Imaging the islet graft by positron emission tomography

Clinical islet transplantation is being investigated as a permanent cure for type 1 diabetes mellitus (T1DM). Currently, intraportal infusion of islets is the favoured procedure, but several novel implantation sites have been suggested. Noninvasive longitudinal methodologies are an increasingly important tool for assessing the fate of transplanted islets, their mass, function and early signs of rejection. This article reviews the approaches available for islet graft imaging by positron emission tomography and progress in the field, as well as future challenges and opportunities.

In vivo imaging of transplanted islets with 64Cu-DO3A-VS-Cys40-Exendin-4 by targeting GLP-1 receptor

Glucagon-like peptide 1 receptor (GLP-1R) is highly expressed in pancreatic islets, especially on β-cells. Therefore, a properly labeled ligand that binds to GLP-1R could be used for in vivo pancreatic islet imaging. Because native GLP-1 is degraded rapidly by dipeptidyl peptidase-IV (DPP-IV), a more stable agonist of GLP-1 such as Exendin-4 is a preferred imaging agent. In this study, DO3A-VS-Cys(40)-Exendin-4 was prepared through the conjugation of DO3A-VS with Cys(40)-Exendin-4. The in vitro binding affinity of DO3A-VS-Cys(40)-Exendin-4 was evaluated in INS-1 cells, which overexpress GLP-1R. After (64)Cu labeling, biodistribution studies and microPET imaging of (64)Cu-DO3A-VS-Cys(40)-Exendin-4 were performed on both subcutaneous INS-1 tumors and islet transplantation models. The subcutaneous INS-1 tumor was clearly visualized with microPET imaging after the injection of (64)Cu-DO3A-VS-Cys(40)-Exendin-4. GLP-1R positive organs, such as pancreas and lung, showed high uptake. Tumor uptake was saturable, reduced dramatically by a 20-fold excess of unlabeled Exendin-4. In the intraportal islet transplantation models, (64)Cu-DO3A-VS-Cys(40)-Exendin-4 demonstrated almost two times higher uptake compared with normal mice. (64)Cu-DO3A-VS-Cys(40)-Exendin-4 demonstrated persistent and specific uptake in the mouse pancreas, the subcutaneous insulinoma mouse model, and the intraportal human islet transplantation mouse model. This novel PET probe may be suitable for in vivo pancreatic islets imaging in the human.

Pancreatic beta cell mass PET imaging and quantification with [11C]DTBZ and [18F]FP-(+)-DTBZ in rodent models of diabetes

PURPOSE

The aim of this study is to compare the utility of two positron emission tomography (PET) imaging ligands ((+)-[(11)C]dihydrotetrabenazine ([(11)C]DTBZ) and the fluoropropyl analog ([(18)F]FP-(+)-DTBZ)) that target islet β-cell vesicular monoamine transporter type II to measure pancreatic β-cell mass (BCM).

PROCEDURES

[(11)C]DTBZ or [(18)F]FP-(+)-DTBZ was injected, and serial PET images were acquired in rat models of diabetes (streptozotocin-treated and Zucker diabetic fatty) and β-cell compensation (Zucker fatty). Radiotracer standardized uptake values (SUV) were correlated to pancreas insulin content measured biochemically and histomorphometrically.

RESULTS

On a group level, a positive correlation of [(11)C]DTBZ pancreatic SUV with pancreas insulin content and BCM was observed. In the STZ diabetic model, both [(18)F]FP-(+)-DTBZ and [(11)C]DTBZ correlated positively with BCM, although only ∼25% of uptake could be attributed to β-cell uptake. [(18)F]FP-(+)-DTBZ displacement studies indicate that there is a substantial fraction of specific binding that is not to pancreatic islet β cells.

CONCLUSIONS

PET imaging with [(18)F]FP-(+)-DTBZ provides a noninvasive means to quantify insulin-positive BCM and may prove valuable as a diagnostic tool in assessing treatments to maintain or restore BCM.

Radionuclide probes for molecular imaging of pancreatic beta-cells

Islet transplantation is a promising treatment option for patients with type 1 diabetes (T1D); however, the fate of the graft over time remains difficult to follow, due to the lack of available tools capable of monitoring graft rejection and inflammation prior to islet graft loss. Due to the challenges imposed by the location of the pancreas and the sparsely dispersed beta-cell population within the pancreas, currently, the clinical verification of beta-cell abnormalities can only be obtained indirectly via metabolic studies, which typically is not possible until after a significant deterioration in islet function has already occurred. The development of non-invasive imaging methods for the assessment of the pancreatic beta-cells, however, offers the potential for the early detection of beta-cell dysfunction prior to the clinical onset of T1D and type 2 diabetes (T2D). Ideal islet imaging agents would have an acceptable residence time in the human body, be capable of providing high-resolution images with minimal uptake in surrounding tissues (e.g., the liver), would not be toxic to islets, and would not require pre-treatment of islets prior to transplantation. A variety of currently available imaging techniques, including magnetic resonance imaging (MRI), bioluminescence imaging (BLI), and nuclear imaging have been tested for the study of beta-cell diseases. In this article, we summarize the recent advances made in nuclear imaging techniques for non-invasive imaging of pancreatic beta-cells. The use of radioactive probes for islet imaging is also discussed.

In vitro phage display in a rat beta cell line: a simple approach for the generation of a single-chain antibody targeting a novel beta cell-specific epitope

AIMS/HYPOTHESIS

The aim of the present study was to evaluate in vitro phage display in a beta cell line as a novel strategy for the isolation of beta cell-specific agents/biomarkers.

METHODS

A single-chain antibody (SCA) library was pre-incubated with AR42J cells in order to eliminate SCAs with exocrine binding properties. It was then panned against INS-1 cells to select beta cell-targeted antibodies.

RESULTS

By these means, we isolated a novel antibody, SCA B5, that binds rapidly (6.0 min) and with a 450-fold higher specificity to beta cells relative to exocrine cells. We estimated for SCA B5 a binding affinity in the low micromol/l range and 858 binding sites per beta cell. Confocal microscopy showed binding to the beta cell surface and confirmed subsequent internalisation. Moreover, staining of rat and human pancreatic tissue sections with SCA B5 suggests that the target epitope is presented in pancreatic beta cells of different origins. Infrared imaging revealed that labelling of beta cells with tracer SCA B5 is strictly dependent on beta cell mass. With competition assays we excluded insulin, glutamate decarboxylase, C-peptide and islet amyloid polypeptide as SCA B5 targets. In accordance with these predictions, SCA B5 homed in vivo highly selectively to normal beta cells and dysfunctional beta cells of diabetic rats. Moreover, accumulation of radioactively labelled SCA B5 in the pancreas was reduced by 80% after pre-injection with unlabelled SCA B5, thereby confirming the specific uptake in the pancreas.

CONCLUSIONS/INTERPRETATION

We report a simple strategy for the generation of an SCA targeting a novel beta cell-specific epitope.

Evidence for modulation of opioidergic activity in central vestibular processing: A [(18)F] diprenorphine PET study

Animal and functional imaging studies had identified cortical structures such as the parieto-insular vestibular cortex, the retro-insular cortex, or the anterior cingulate cortex belonging to a vestibular cortical network. Basic animal studies revealed that endorphins might be important transmitters involved in cerebral vestibular processing. The aim of the present study was therefore to analyse whether the opioid system is involved in vestibular neurotransmission of humans or not. Changes in opioid receptor availability during caloric air stimulation of the right ear were studied with [(18)F] Fluoroethyl-diprenorphine ([(18)F]FEDPN) PET scans in 10 right-handed healthy volunteers and compared to a control condition. Decrease in receptor availability to [(18)F]FEDPN during vestibular stimulation in comparison to the control condition was significant at the right posterior insular cortex and the postcentral region indicating more endogenous opioidergic binding in these regions during stimulation. These data give evidence that the opioidergic system plays a role in the right hemispheric dominance of the vestibular cortical system in right-handers.

(18)F-AV-133: A Selective VMAT2-binding Radiopharmaceutical for PET Imaging of Dopaminergic Neurons

The early detection and monitoring of neurodegenerative diseases, including Parkinson disease, Alzheimer disease, dementia with Lewy bodies and other dementias, and movement disorders, represent a significant unmet medical need. Tools for accurate and early differential diagnosis are necessary to determine the appropriate treatment for patients and to minimize inappropriate use of potentially harmful treatments. Such diagnostic imaging tools are expected to permit monitoring of disease progression and will thus accelerate testing and development of disease-modifying drugs. The new imaging tests may be useful as prognostic tools by identifying humans with neurodegenerative diseases before the clinical manifestations become evident.

Approaches for imaging islets: recent advances and future prospects

The establishment of improved technologies for imaging of the pancreas is a key element in addressing several aspects of diabetes pathogenesis. In this respect, the development of a protocol that allows for non-invasive scoring of human islets, or islet beta-cells, is of particular importance. The development of such a technology would have profound impact on both clinical and experimental medicine, ranging from early diagnosis of diabetes to the evaluation of therapeutic regimes. Another important task is the development of modalities for high-resolution imaging of experimental animal models for diabetes. Rodent models for diabetes research have for decades been instrumental to the diabetes research community. The ability to image, and to accurately quantify, key players of diabetogenic processes with molecular specificity will be of great importance for elucidating mechanistic aspects of the disease. This chapter aims to overview current progress within these research areas.

Generation of novel single-chain antibodies by phage-display technology to direct imaging agents highly selective to pancreatic beta- or alpha-cells in vivo

OBJECTIVE

Noninvasive determination of pancreatic beta-cell mass in vivo has been hampered by the lack of suitable beta-cell-specific imaging agents. This report outlines an approach for the development of novel ligands homing selectively to islet cells in vivo.

RESEARCH DESIGN AND METHODS

To generate agents specifically binding to pancreatic islets, a phage library was screened for single-chain antibodies (SCAs) on rat islets using two different approaches. 1) The library was injected into rats in vivo, and islets were isolated after a circulation time of 5 min. 2) Pancreatic islets were directly isolated, and the library was panned in the islets in vitro. Subsequently, the identified SCAs were extensively characterized in vitro and in vivo.

RESULTS

We report the generation of SCAs that bind highly selective to either beta- or alpha-cells. These SCAs are internalized by target cells, disappear rapidly from the vasculature, and exert no toxicity in vivo. Specific binding to beta- or alpha-cells was detected in cell lines in vitro, in rats in vivo, and in human tissue in situ. Electron microscopy demonstrated binding of SCAs to the endoplasmatic reticulum and the secretory granules. Finally, in a biodistribution study the labeling intensity derived from [(125)I]-labeled SCAs after intravenous administration in rats strongly predicted the beta-cell mass and was inversely related to the glucose excursions during an intraperitoneal glucose tolerance test.

CONCLUSIONS

Our data provide strong evidence that the presented SCAs are highly specific for pancreatic beta-cells and enable imaging and quantification in vivo.

Noninvasive imaging of pancreatic beta cells

Noninvasive imaging and quantification of pancreatic, insulin-producing beta cells has been considered a high-priority field of investigation for the past decade. In the first review on this issue, attention was already paid to various agents for labeling beta cells, including 6-(125)I-D-glucose, (65)Zn, (3)H-glibenclamide, (3)H-mitiglinide, an (125)I-labeled mouse monoclonal antibody against beta-cell surface ganglioside(s), D-(U-(14)C)-glucose and 2-deoxy-2-(18)F-D-glucose to label glycogen accumulated in beta cells in response to sustained hyperglycemia, and, last but not least, an analog of D-mannoheptulose. This Review discusses these methods and further contributions. For instance, emphasis is placed on labeling beta cells with (11)C-dihydrotetrabenazine, which is the most advanced method at present. Attention is also drawn to the latest approaches for noninvasive imaging and functional characterization of pancreatic beta cells. None of these procedures is used in clinical practice yet.

Advances in beta-cell imaging

Diabetes mellitus results in impaired insulin production by pancreatic beta-cells due to their death and/or dysfunction. There is a growing unmet need among diabetes researches and clinicians to assess the level of surviving beta-cells non-invasively. This review will focus on employment of state-of-the-art in vivo imaging methods to estimate and evaluate beta-cell mass in animal models of diabetes.

Efforts to develop methods for in vivo evaluation of the native beta-cell mass

Visualization and quantification of the native beta-cell mass in vivo in humans appear to be important in the study of the natural course of diabetes, and in ongoing trials aimed at preserving beta-cell mass in patients with diabetes. This cannot be done by biopsy sampling, and therefore there is a great need for development of a non-invasive method. This article discusses the principle theoretical requirements for reaching this goal. In addition, it provides an overview of tracer probes, which have been examined as potential beta-cell mass imaging agents in the past. Finally, some future perspectives are discussed.

Opioid receptor PET reveals the psychobiologic correlates of reward processing

Little is known about the neurobiologic correlates of human personality. On the basis of the key role of the central opioidergic system in addiction and substance abuse, we investigated the relationship between certain personality traits that are supposed to be relevant in addiction and the opioid receptor status in healthy subjects.

METHODS

We investigated 23 healthy male volunteers who were extensively clinically tested to exclude substance abuse. All of the subjects underwent 1 PET scan with the subtype-nonselective opioidergic radioligand 18F-fluoroethyl-diprenorphine under resting conditions without sensory or cognitive stimulation. Subsequently, the subjects were psychologically tested for the personality traits novelty seeking, harm avoidance, reward dependence, and persistence, according to Cloninger's biosocial model of personality. The binding potential (BP) as a parameter of regional cerebral opioid receptor availability was computed by means of the modified Logan plot using the occipital cortex as a reference region. Further imaging data analysis was performed using statistical parametric mapping; after stereotactic normalization, the correlations were calculated between the regional BP and the psychologic scores on a voxel-by-voxel basis.

RESULTS

The correlation analysis between personality dimensions and opioid receptor availability showed a significant (P < 0.001) positive correlation between the scores of reward dependence and the BP of the bilateral ventral striatum with nucleus accumbens (z scores, 4.52 and 4.33, respectively). The additionally performed region-of-interest-based correlation analysis yielded correlation coefficients of r = 0.84 and r = 0.81 for the left and right ventral striata, respectively. No further significant correlations were detectable between the other personality dimensions and cerebral opioid receptor binding.

CONCLUSION

In healthy subjects, personality traits, which might be predisposing for addictive behavior, are correlated to the opioidergic neurotransmission in core structures of the human reward system.

In vivo imaging of beta-cell mass in rats using 18F-FP-(+)-DTBZ: a potential PET ligand for studying diabetes mellitus

Recent studies on gene expression of beta-cell mass (BCM) in the pancreas showed that vesicular monoamine transporter 2 (VMAT2) is highly expressed in the BCM (mainly in the islets of Langerhans). Imaging pancreatic BCM may provide an important tool for understanding the relationship between loss of insulin-secreting beta-cells and onset of diabetes mellitus. In this article, 9-fluoropropyl-(+)-dihydrotetrabenazine (FP-(+)-DTBZ), which is a VMAT2 imaging agent, was evaluated as a PET agent for estimating BCM in vivo.

METHODS

Organ biodistribution after an intravenous injection of (18)F-FP-(+)-DTBZ (active isomer) and (18)F-FP-(-)-DTBZ (inactive isomer) was evaluated in normal rats. The specificity of uptake of (18)F-FP-(+)-DTBZ was assessed by a pretreatment (3.8 mg of (+)-DTBZ per kilogram and 3.5 mg of FP-(+)-DTBZ per kilogram, intravenously, 5 min prior) or coadministration (2 mg of (+)-DTBZ per kilogram). PET studies were performed in normal rats.

RESULTS

The in vivo biodistribution of (18)F-FP-(+)-DTBZ in rats showed the highest uptake in the pancreas (5% dose/g at 30 min after injection), whereas (18)F-FP-(-)-DTBZ showed a very low pancreas uptake. Rats pretreated with FP-(+)-DTBZ displayed a 78% blockade of pancreas uptake. PET studies in normal rats demonstrated an avid pancreas uptake of (18)F-FP-(+)-DTBZ.

CONCLUSION

The preliminary data obtained with (18)F-FP-(+)-DTBZ suggest that this fluorinated derivative of DTBZ shows good pancreas specificity and has the potential to be useful for quantitative measurement of VMAT2 binding sites reflecting BCM in the pancreas.

Radioiodinated naphthylalanine derivatives targeting pancreatic beta cells in normal and nonobese diabetic mice

An imaging method capable of using a signal from pancreatic beta cells to determine their mass would be of immense value in monitoring the progression of diabetes as well as response to treatment. Somatostatin receptors (SSTRs) are expressed on beta cells and are a potential target for imaging. The main objective of this study was to investigate whether pancreatic beta cells are a target for radiolabeled naphthylalanine derivatives. The molecules were subjected to in vitro and ex vivo evaluations. Pancreatic uptake of radioactivity was lower in nonobese diabetic (NOD) mice than normal mice at all time points investigated (P < .05) and correlated with the number of islets in tissue sections of both control and NOD mice. Immunohistochemical and confocal fluorescent microscopic studies showed colocalization of insulin and the conjugate radioligand in the pancreas. The results demonstrated that pancreatic uptake is receptor-mediated, and that beta cells are the primary target.

Advances in molecular imaging of pancreatic beta cells

The development of non-invasive imaging methods for early diagnosis of beta cell associated metabolic diseases, including type 1 and type 2 diabetes (T1D and T2D), has recently drawn interest from the molecular imaging community and clinical investigators. Due to the challenges imposed by the location of the pancreas, the sparsely dispersed beta cell population within the pancreas, and the poor understanding of the pathogenesis of the diseases, clinical diagnosis of beta cell abnormalities is still limited. Current diagnostic methods are invasive, often inaccurate, and usually performed post-onset of the disease. Advances in imaging techniques for probing beta cell mass and function are needed to address this critical health care problem. A variety of imaging techniques have been tested for the assessment of pancreatic beta cell islets. Here we discuss current advances in magnetic resonance imaging (MRI), bioluminescence imaging (BLI), and nuclear imaging for the study of beta cell diseases. Spurred by early successes in nuclear imaging techniques for beta cells, especially positron emission tomography (PET), the need for beta cell specific ligands has expanded. Progress for obtaining such ligands is presented. We report our preliminary efforts of developing such a peptidic ligand for PET imaging of pancreatic beta cells.

Synthesis and evaluation of radioiodinated substituted -naphthylalanine as a potential probe for pancreatic -cells imaging

A non-invasive imaging technique capable of relating a signal from the beta-cells to their mass will be of immense value in understanding the progression of diabetes. Several molecular markers have indeed been identified and investigations are ongoing aimed at accomplishing the said goal. These include pancreatic islet antigen (IC-2), somatostatin receptors (SSTRs), and sulfonylurea receptors (SURs) on the pancreatic beta-cells. Therefore investigations exploiting the potential application of the radiolabeled ligands for these receptors for beta-cell imaging are receiving intensive research attention. Radioiodinated peptidomimetic based on beta-naphthylalanine and n-hexanediamine has been synthesized. The molecule was subjected to in vitro and in vivo evaluation. Radioligand binding studies on CHO cell line expressing the SSTR2 showed very low affinity. Nonetheless, biodistribution in normal mice showed significant uptake in the pancreas. There was partial blockage of the pancreatic uptake when excess of the peptidomimetic was coinjected. The result implies that the pancreatic uptake was receptor mediated but may not involve the SSTR2 and therefore warrants further investigation.

Functional MR microimaging of pancreatic beta-cell activation

The increasing incidence of diabetes and the need to further understand its cellular basis has resulted in the development of new diagnostic and therapeutic techniques. Nonetheless, the quest to noninvasively ascertain beta-cell mass and function has not been achieved. Manganese (Mn)-enhanced MRI is presented here as a tool to image beta-cell functionality in cell culture and isolated islets. Similar to calcium, extracellular Mn was taken up by glucose-activated beta-cells resulting in 200% increase in MRI contrast enhancement, versus nonactivated cells. Similarly, glucose-activated islets showed an increase in MRI contrast up to 45%. Although glucose-stimulated Ca influx was depressed in the presence of 100 microM Mn, no significant effect was seen at lower Mn concentrations. Moreover, islets exposed to Mn showed normal glucose sensitivity and insulin secretion. These results demonstrate a link between image contrast enhancement and beta-cell activation in vitro, and provide the basis for future noninvasive in vivo imaging of islet functionality and beta-cell mass.

Synthesis and in vitro evaluation of (S)-2-([11C]methoxy)-4-[3-methyl-1-(2-piperidine-1-yl-phenyl)-butyl-carbamoyl]-benzoic acid ([11C]methoxy-repaglinide): a potential beta-cell imaging agent

The 11C-labeled sulfonylurea receptor 1 (SUR1) ligand (S)-2-([11C]methoxy)-4-[3-methyl-1-(2-piperidine-1-yl-phenyl)-butyl-carbamoyl]-benzoic acid ([11C]methoxy-repaglinide) was synthesized in an overall radiochemical yield of 35% after 55 min with a radiochemical purity higher than 99%. This compound is considered for the noninvasive investigation of the SUR1 receptor status of pancreatic beta-cells by positron emission tomography (PET) in the context of type 1 and type 2 diabetes. The specific activity was 40-70 GBq/micromol. In vitro testing of the nonradioactive methoxy-repaglinide was performed to characterize the affinity for binding to the human SUR1 isoform. Methoxy-repaglinide induced a complete monophasic inhibition curve with a Hill coefficient close to 1 (1.03) yielding a dissociation constant (KD) of 83 nM and an IC50 of 163 nM. Insulin secretion experiments on isolated rat islets were performed to prove biological activity, which was determined to be in the same range as that of original repaglinide.

Update on PET radiopharmaceuticals: life beyond fluorodeoxyglucose

Twenty-eight years after its inception, 2-[18F]FDG- is still the most widely used radiopharmaceutical for PET studies, but numerous more specific radiotracers have been developed and applied in neuroscience and oncology. The advances in radiotracer chemistry, especially the nucleophilic substitution reaction, have played the pivotal role in synthesizing various no-carrier-added 18F-labeled radiotracers for PET studies of various receptor systems. This article lists some of the radiotracers that are available for PET studies in neuroscience and oncology. The prospects for developing other new radiotracers for imaging other organ diseases also seem to be promising.