GPR44 is a pancreatic protein restricted to the human beta cell

One-Step Automatic Radiosynthesis and Evaluation of [18F]TM-30089 as GPR44 Radiotracer

Recently, a G-protein coupled receptor 44 (GPR44) was discovered to play a significant role in the process of inflammation-related diseases, including cancer and diabetes. However, the precise role of GPR44 has yet to be fully elucidated. Currently, there is a strong and urgent need for the development of GPR44 radiotracers as a non-invasive methodology to explore the exact mechanism of GPR44 on inflammation-related diseases and monitor the progress of therapy. TM-30089 is a potent GPR44 antagonist that exhibits a high specificity and selectivity for GPR44. Its structure contains a fluorine nuclide, which could potentially be replaced with 18F. In the present study, we successfully took a highly effective synthesis strategy that pretreated the unprotected carboxylic acid group of the precursor and developed a feasible one-step automatic radiosynthesis strategy for [18F]TM-30089 with a high radiochemical purity and a good radiochemical yield. We further evaluated this radiotracer using mice models implanted with 1.1 B4 cell lines (GPR44-enriched cell lines) and human islets (high GPR44 expression), respectively. The results revealed the persistent and specific uptake of [18F]TM-30089 in GPR44 region, indicating that [18F]TM-30089 is a promising candidate for targeting GPR44. Further evaluation is ongoing.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

[18F]MK-7246 for Positron Emission Tomography Imaging of the Beta-Cell Surface Marker GPR44

The progressive loss of beta-cell mass is a hallmark of diabetes and has been suggested as a complementary approach to studying the progression of diabetes in contrast to the beta-cell function. Non-invasive nuclear medicinal imaging techniques such as Positron Emission Tomography using radiation emitting tracers have thus been suggested as more viable methodologies to visualize and quantify the beta-cell mass with sufficient sensitivity. The transmembrane G protein-coupled receptor GPR44 has been identified as a biomarker for monitoring beta-cell mass. MK-7246 is a GPR44 antagonist that selectively binds to GPR44 with high affinity and good pharmacokinetic properties. Here, we present the synthesis of MK-7246, radiolabeled with the positron emitter fluorine-18 for preclinical evaluation using cell lines, mice, rats and human pancreatic cells. Here, we have described a synthesis and radiolabeling method for producing [¹⁸F]MK-7246 and its precursor compound. Preclinical assessments demonstrated the strong affinity and selectivity of [¹⁸F]MK-7246 towards GPR44. Additionally, [¹⁸F]MK-7246 exhibited excellent metabolic stability, a fast clearance profile from blood and tissues, qualifying it as a promising radioactive probe for GPR44-directed PET imaging.

Emerging diabetes therapies: Bringing back the β-cells

BACKGROUND

Stem cell therapies are finally coming of age as a viable alternative to pancreatic islet transplantation for the treatment of insulin-dependent diabetes. Several clinical trials using human embryonic stem cell (hESC)-derived β-like cells are currently underway, with encouraging preliminary results. Remaining challenges notwithstanding, these strategies are widely expected to reduce our reliance on human isolated islets for transplantation procedures, making cell therapies available to millions of diabetic patients. At the same time, advances in our understanding of pancreatic cell plasticity and the molecular mechanisms behind β-cell replication and regeneration have spawned a multitude of translational efforts aimed at inducing β-cell replenishment in situ through pharmacological means, thus circumventing the need for transplantation.

SCOPE OF REVIEW

We discuss here the current state of the art in hESC transplantation, as well as the parallel quest to discover agents capable of either preserving the residual mass of β-cells or inducing their proliferation, transdifferentiation or differentiation from progenitor cells.

MAJOR CONCLUSIONS

Stem cell-based replacement therapies in the mold of islet transplantation are already around the corner, but a permanent cure for type 1 diabetes will likely require the endogenous regeneration of β-cells aided by interventions to restore the immune balance. The promise of current research avenues and a strong pipeline of clinical trials designed to tackle these challenges bode well for the realization of this goal.

Effects of Arachidonic Acid and Its Metabolites on Functional Beta-Cell Mass

Arachidonic acid (AA) is a polyunsaturated 20-carbon fatty acid present in phospholipids in the plasma membrane. The three primary pathways by which AA is metabolized are mediated by cyclooxygenase (COX) enzymes, lipoxygenase (LOX) enzymes, and cytochrome P450 (CYP) enzymes. These three pathways produce eicosanoids, lipid signaling molecules that play roles in biological processes such as inflammation, pain, and immune function. Eicosanoids have been demonstrated to play a role in inflammatory, renal, and cardiovascular diseases as well type 1 and type 2 diabetes. Alterations in AA release or AA concentrations have been shown to affect insulin secretion from the pancreatic beta cell, leading to interest in the role of AA and its metabolites in the regulation of beta-cell function and maintenance of beta-cell mass. In this review, we discuss the metabolism of AA by COX, LOX, and CYP, the roles of these enzymes and their metabolites in beta-cell mass and function, and the possibility of targeting these pathways as novel therapies for treating diabetes.

Delivery of therapeutic agents and cells to pancreatic islets: Towards a new era in the treatment of diabetes

Pancreatic islet cells, and in particular insulin-producing beta cells, are centrally involved in the pathogenesis of diabetes mellitus. These cells are of paramount importance for the endocrine control of glycemia and glucose metabolism. In Type 1 Diabetes, islet beta cells are lost due to an autoimmune attack. In Type 2 Diabetes, beta cells become dysfunctional and insufficient to counterbalance insulin resistance in peripheral tissues. Therapeutic agents have been developed to support the function of islet cells, as well as to inhibit deleterious immune responses and inflammation. Most of these agents have undesired effects due to systemic administration and off-target effects. Typically, only a small fraction of therapeutic agent reaches the desired niche in the pancreas. Because islets and their beta cells are scattered throughout the pancreas, access to the niche is limited. Targeted delivery to pancreatic islets could dramatically improve the therapeutic effect, lower the dose requirements, and lower the side effects of agents administered systemically. Targeted delivery is especially relevant for those therapeutics for which the manufacturing is difficult and costly, such as cells, exosomes, and microvesicles. Along with therapeutic agents, imaging reagents intended to quantify the beta cell mass could benefit from targeted delivery. Several methods have been developed to improve the delivery of agents to pancreatic islets. Intra-arterial administration in the pancreatic artery is a promising surgical approach, but it has inherent risks. Targeted delivery strategies have been developed based on ligands for cell surface molecules specific to islet cells or inflamed vascular endothelial cells. Delivery methods range from nanocarriers and vectors to deliver pharmacological agents to viral and non-viral vectors for the delivery of genetic constructs. Several strategies demonstrated enhanced therapeutic effects in diabetes with lower amounts of therapeutic agents and lower off-target side effects. Microvesicles, exosomes, polymer-based vectors, and nanocarriers are gaining popularity for targeted delivery. Notably, liposomes, lipid-assisted nanocarriers, and cationic polymers can be bioengineered to be immune-evasive, and their advantages to transport cargos into target cells make them appealing for pancreatic islet-targeted delivery. Viral vectors have become prominent tools for targeted gene delivery. In this review, we discuss the latest strategies for targeted delivery of therapeutic agents and imaging reagents to pancreatic islet cells.

Potential of [11C]UCB-J as a PET tracer for islets of Langerhans

Biomarkers for the measurement of islets of Langerhans could help elucidate the etiology of diabetes. Synaptic vesicle glycoprotein 2 A (SV2A) is a potential marker reported to be localized in the endocrine pancreas. [11C]UCB-J is a novel positron emission tomography (PET) radiotracer that binds to SV2A and was previously evaluated as a synaptic marker in the central nervous system. Here, we evaluated whether [11C]UCB-J could be utilized as a PET tracer for the islets of Langerhans in the pancreas by targeting SV2A. The mRNA transcription of SV2A was evaluated in human isolated islets of Langerhans and exocrine tissue. In vitro autoradiography was performed on pancreas and brain sections from rats and pigs, and consecutive sections were immunostained for insulin. Sprague-Dawley rats were examined with PET-MRI and ex vivo autoradiography at baseline and with administration of levetiracetam (LEV). Similarly, pigs were examined with dynamic PET-CT over the pancreas and brain after administration of [11C]UCB-J at baseline and after pretreatment with LEV. In vivo radioligand binding was assessed using a one-compartment tissue model. The mRNA expression of SV2A was nearly 7 times higher in endocrine tissue than in exocrine tissue (p < 0.01). In vitro autoradiography displayed focal binding of [11C]UCB-J in the pancreas of rats and pigs, but the binding pattern did not overlap with the insulin-positive areas or with ex vivo autoradiography. In rats, pancreas binding was higher than that in negative control tissues but could not be blocked by LEV. In pigs, the pancreas and brain exhibited accumulation of [11C]UCB-J above the negative control tissue spleen. While brain binding could be blocked by pretreatment with LEV, a similar effect was not observed in the pancreas. Transcription data indicate SV2A to be a valid target for imaging islets of Langerhans, but [11C]UCB-J does not appear to have sufficient sensitivity for this application.

The Current State of Beta-Cell-Mass PET Imaging for Diabetes Research and Therapies

Diabetes is a chronic metabolic disease affecting over 400 million people worldwide and one of the leading causes of death, especially in developing nations. The disease is characterized by chronic hyperglycemia, caused by defects in the insulin secretion or action pathway. Current diagnostic methods measure metabolic byproducts of the disease such as glucose level, glycated hemoglobin (HbA1c), insulin or C-peptide levels, which are indicators of the beta-cell function. However, they inaccurately reflect the disease progression and provide poor longitudinal information. Beta-cell mass has been suggested as an alternative approach to study disease progression in correlation to beta-cell function, as it behaves differently in the diabetes physiopathology. Study of the beta-cell mass, however, requires highly invasive and potentially harmful procedures such as pancreatic biopsies, making diagnosis and monitoring of the disease tedious. Nuclear medical imaging techniques using radiation emitting tracers have been suggested as strong non-invasive tools for beta-cell mass. A highly sensitive and high-resolution technique, such as positron emission tomography, provides an ideal solution for the visualization of beta-cell mass, which is particularly essential for better characterization of a disease such as diabetes, and for estimating treatment effects towards regeneration of the beta-cell mass. Development of novel, validated biomarkers that are aimed at beta-cell mass imaging are thus highly necessary and would contribute to invaluable breakthroughs in the field of diabetes research and therapies. This review aims to describe the various biomarkers and radioactive probes currently available for positron emission tomography imaging of beta-cell mass, as well as highlight the need for precise quantification and visualization of the beta-cell mass for designing new therapy strategies and monitoring changes in the beta-cell mass during the progression of diabetes.

PET Imaging of GPR44 by Antagonist [11C]MK-7246 in Pigs

A validated imaging marker for beta-cell mass would improve understanding of diabetes etiology and enable new strategies in therapy development. We previously identified the membrane-spanning protein GPR44 as highly expressed and specific to the beta cells of the pancreas. The selective GPR44 antagonist MK-7246 was radiolabeled with carbon-11 and the resulting positron-emission tomography (PET) tracer [¹¹C]MK-7246 was evaluated in a pig model and in vitro cell lines. The [¹¹C]MK-7246 compound demonstrated mainly hepatobiliary excretion with a clearly defined pancreas, no spillover from adjacent tissues, and pancreatic binding similar in magnitude to the previously evaluated GPR44 radioligand [¹¹C]AZ12204657. The binding could be blocked by preadministration of nonradioactive MK-7246, indicating a receptor-binding mechanism. [¹¹C]MK-7246 showed strong potential as a PET ligand candidate for visualization of beta-cell mass (BCM) and clinical translation of this methodology is ongoing.

Rat prostaglandin EP3 receptor is highly promiscuous and is the sole prostanoid receptor family member that regulates INS-1 (832/3) cell glucose-stimulated insulin secretion

Chronic elevations in fatty acid metabolites termed prostaglandins can be found in circulation and in pancreatic islets from mice or humans with diabetes and have been suggested as contributing to the β‐cell dysfunction of the disease. Two‐series prostaglandins bind to a family of G‐protein‐coupled receptors, each with different biochemical and pharmacological properties. Prostaglandin E receptor (EP) subfamily agonists and antagonists have been shown to influence β‐cell insulin secretion, replication, and/or survival. Here, we define EP3 as the sole prostanoid receptor family member expressed in a rat β‐cell‐derived line that regulates glucose‐stimulated insulin secretion. Several other agonists classically understood as selective for other prostanoid receptor family members also reduce glucose‐stimulated insulin secretion, but these effects are only observed at relatively high concentrations, and, using a well‐characterized EP3‐specific antagonist, are mediated solely by cross‐reactivity with rat EP3. Our findings confirm the critical role of EP3 in regulating β‐cell function, but are also of general interest, as many agonists supposedly selective for other prostanoid receptor family members are also full and efficacious agonists of EP3. Therefore, care must be taken when interpreting experimental results from cells or cell lines that also express EP3.

Ramatroban-Based Analogues Containing Fluorine Group as Potential 18F-Labeled Positron Emission Tomography (PET) G-Protein Coupled Receptor 44 (GPR44) Tracers

Diabetes remains one of the fastest growing chronic diseases and is a leading source of morbidity and accelerated mortality in the world. Loss of beta cell mass (BCM) and decreased sensitivity to insulin underlie diabetes pathogenesis. Yet, the ability to safely and directly assess BCM in individuals with diabetes does not exist. Measures such as blood glucose provide only a crude indirect picture of beta cell health. PET imaging could, in theory, allow for safe, direct, and precise characterization of BCM. However, identification of beta cell-specific radiolabeled tracers remains elusive. G-protein coupled receptor 44 (GPR44) is a transmembrane protein that was characterized in 2012 as highly beta cell-specific within the insulin-positive islets of Langerhans. Accordingly, radiolabeling of existing GPR44 antagonists could be a viable method to accelerate PET tracer development. The present study aims to evaluate and summarize published analogues of the GPR44 antagonist ramatroban to develop 18F-labeled PET tracers for BCM analysis. The 77 corresponding ramatroban analogues containing a fluorine nuclide were characterized for properties including binding affinity, selectivity, and pharmacokinetic and metabolic profile, and 32 compounds with favorable properties were identified. This review illustrates the potential of GPR44 analogues for the development of PET tracers.

News ways of understanding the complex biology of diabetes using PET

The understanding of metabolic disease and diabetes on a molecular level has increased significantly due to the recent advances in molecular biology and biotechnology. However, in vitro studies and animal models do not always translate to the human disease, perhaps illustrated by the failure of many drug candidates in the clinical phase. Non-invasive biomedical imaging techniques such as Positron Emission Tomography (PET) offer tools for direct visualization and quantification of molecular processes in humans. Developments in this area potentially enable longitudinal in vivo studies of receptors and processes involved in diabetes guiding drug development and diagnosis in the near future. This mini-review focuses on describing the overall perspective of how PET can be used to increase our understanding and improve treatment of diabetes. The methodological aspects and future developments and challenges are highlighted.

Novel Strategies to Protect and Visualize Pancreatic β Cells in Diabetes

A common feature in the pathophysiology of different types of diabetes is the reduction of β cell mass and/or impairment of β cell function. Diagnosis and treatment of type 1 and type 2 diabetes is currently hampered by a lack of reliable techniques to restore β cell survival, to improve insulin secretion, and to quantify β cell mass in patients. Current new approaches may allow us to precisely and specifically visualize β cells in vivo and provide viable therapeutic strategies to preserve, recover, and regenerate β cells. In this review, we discuss recent protective approaches for β cells and the advantages and limitations of current imaging probes in the field.

Targeted pancreatic beta cell imaging for early diagnosis

Pancreatic beta cells are important in blood glucose level regulation. As type 1 and 2 diabetes are getting prevalent worldwide, we need to explore new methods for early detection of beta cell-related afflictions. Using bioimaging techniques to measure beta cell mass is crucial because a decrease in beta cell density is seen in diseases such as diabetes and thus can be a new way of diagnosis for such diseases. We also need to appraise beta cell purity in transplanted islets for type 1 diabetes patients. Sufficient amount of functional beta cells must also be determined before being transplanted to the patients. In this review, indirect imaging of beta cells will be discussed. This includes membrane protein on pancreatic beta cells whereby specific probes are designed for different imaging modalities mainly magnetic resonance imaging, positron emission tomography and fluorescence imaging. Direct imaging of insulin is also explored though probes synthesized for such function are relatively fewer. The path for successful pancreatic beta cell imaging is fraught with challenges like non-specific binding, lack of beta cell-restricted targets, the requirement of probes to cross multiple lipid layers to bind to intracellular insulin. Hence, there is an urgent need to develop new imaging techniques and innovative probing constructs in the entire imaging chain of bioengineering to provide early detection of beta cell-related pathology.

Inhibition of the prostaglandin D2-GPR44/DP2 axis improves human islet survival and function

AIMS/HYPOTHESIS

Inflammatory signals and increased prostaglandin synthesis play a role during the development of diabetes. The prostaglandin D₂ (PGD₂) receptor, GPR44/DP2, is highly expressed in human islets and activation of the pathway results in impaired insulin secretion. The role of GPR44 activation on islet function and survival rate during chronic hyperglycaemic conditions is not known. In this study, we investigate GPR44 inhibition by using a selective GPR44 antagonist (AZ8154) in human islets both in vitro and in vivo in diabetic mice transplanted with human islets.

METHODS

Human islets were exposed to PGD₂ or proinflammatory cytokines in vitro to investigate the effect of GPR44 inhibition on islet survival rate. In addition, the molecular mechanisms of GPR44 inhibition were investigated in human islets exposed to high concentrations of glucose (HG) and to IL-1β. For the in vivo part of the study, human islets were transplanted under the kidney capsule of immunodeficient diabetic mice and treated with 6, 60 or 100 mg/kg per day of a GPR44 antagonist starting from the transplantation day until day 4 (short-term study) or day 17 (long-term study) post transplantation. IVGTT was performed on mice at day 10 and day 15 post transplantation. After termination of the study, metabolic variables, circulating human proinflammatory cytokines, and hepatocyte growth factor (HGF) were analysed in the grafted human islets.

RESULTS

PGD₂ or proinflammatory cytokines induced apoptosis in human islets whereas GPR44 inhibition reversed this effect. GPR44 inhibition antagonised the reduction in glucose-stimulated insulin secretion induced by HG and IL-1β in human islets. This was accompanied by activation of the Akt-glycogen synthase kinase 3β signalling pathway together with phosphorylation and inactivation of forkhead box O-1and upregulation of pancreatic and duodenal homeobox-1 and HGF. Administration of the GPR44 antagonist for up to 17 days to diabetic mice transplanted with a marginal number of human islets resulted in reduced fasting blood glucose and lower glucose excursions during IVGTT. Improved glucose regulation was supported by increased human C-peptide levels compared with the vehicle group at day 4 and throughout the treatment period. GPR44 inhibition reduced plasma levels of TNF-α and growth-regulated oncogene-α/chemokine (C-X-C motif) ligand 1 and increased the levels of HGF in human islets.

CONCLUSIONS/INTERPRETATION

Inhibition of GPR44 in human islets has the potential to improve islet function and survival rate under inflammatory and hyperglycaemic stress. This may have implications for better survival rate of islets following transplantation.

A new perspective of probe development for imaging pancreatic beta cell in vivo

Beta cells assume a fundamental role in maintaining blood glucose homeostasis through the secretion of insulin, which is contingent on both beta cell mass and function, in response to elevated blood glucose levels or secretagogues. For this reason, evaluating beta cell mass and function, as well as scrutinizing how they change over time in a diabetic state, are essential prerequisites in elucidating diabetes pathophysiology. Current clinical methods to measure human beta cell mass and/or function are largely lacking, indirect and sub-optimal, highlighting the continued need for noninvasive in vivo beta cell imaging technologies such as optical imaging techniques. While numerous probes have been developed and evaluated for their specificity to beta cells, most of them are more suited to visualize beta cell mass rather than function. In this review, we highlight the distinction between beta cell mass and function, and the importance of developing more probes to measure beta cell function. Additionally, we also explore various existing probes that can be employed to measure beta cell mass and function in vivo, as well as the caveats in probe development for in vivo beta cell imaging.

GPR44 as a Target for Imaging Pancreatic Beta-Cell Mass

PURPOSE OF REVIEW

Quantitative markers for beta-cell mass (BCM) in human pancreas are currently lacking. Medical imaging using positron emission tomography (PET) markers for beta-cell restricted targets may provide an accurate and non-invasive measurement of BCM, to assist diagnosis and treatment of metabolic disease. GPR44 was recently discovered as a putative marker for beta cells and this review summarizes the developments so far.

RECENT FINDINGS

Several small molecule binders targeting GPR44 have been radiolabeled for PET imaging and evaluated in vitro and in small and large animal models. ¹¹C-AZ12204657 and ¹¹C-MK-7246 displayed a dose-dependent and GPR44-mediated binding to beta cells both in vitro and in vivo, with negligible uptake in exocrine pancreas. GPR44 represents an attractive target for visualization of BCM. Further progress in radioligand development including clinical testing is expected to clarify the role of GPR44 as a surrogate marker for BCM in humans.

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.

Synthesis and preclinical evaluation of the CRTH2 antagonist [11C]MK-7246 as a novel PET tracer and potential surrogate marker for pancreatic beta-cell mass

INTRODUCTION

MK-7246 is a potent and selective antagonist for chemoattractant receptor-homologous molecule expressed on T_h2 cells (CRTH2). Within the pancreas CRTH2 is selectively expressed in pancreatic β-cells where it is believed to play a role in insulin release. Reduction in β-cell mass and insufficient insulin secretion in response to elevated blood glucose levels is a hallmark for type 1 and type 2 diabetes. Reported here is the synthesis of [¹¹C]MK-7246 and initial preclinical evaluation towards CRTH2 imaging. The aim is to develop a method to quantify β-cell mass with PET and facilitate non-invasive studies of disease progression in individuals with type 2 diabetes.

METHODS

The precursor N-desmethyl-O-methyl MK-7246 was synthesized in seven steps and subjected to methylation with [¹¹C]methyl iodide followed by hydrolysis to obtain [¹¹C]MK-7246 labelled in the N-methyl position. Preclinical evaluation included in vitro radiography and immune-staining performed in human pancreatic biopsies. Biodistribution studies were performed in rat by PET-MRI and in pig by PET-CT imaging. Saturable tracer binding was examined in pig by scanning before and after administration of MK-7246 (1 mg/kg). Predicted dosimetry of [¹¹C]MK-7246 in human males was estimated based on the biodistribution in rat.

RESULTS

[¹¹C]MK-7246 was obtained with activities sufficient for the current investigations (270 ± 120 MBq) and a radiochemical purity of 93 ± 2%. The tracer displayed focal binding in areas with insulin positive islet of Langerhans in human pancreas sections. Baseline uptake in pig was reduced in tissues with known expression of CRTH2 after administration of MK-7246; pancreas (66% reduction) and spleen (88% reduction). [¹¹C]MK-7246 exhibited a safe human predicted dosimetry profile as extrapolated from the rat biodistribution data.

CONCLUSIONS

Initial preclinical in vitro and in vivo evaluations of [¹¹C]MK-7246 show binding and biodistribution properties suitable for PET imaging of CRTH2. Further studies are warranted to assess its potential in β-cell mass imaging and CRTH2 drug development.

Zinc-Chelating Small Molecules Preferentially Accumulate and Function within Pancreatic β Cells

Diabetes is a hyperglycemic condition characterized by pancreatic β-cell dysfunction and depletion. Whereas methods for monitoring β-cell function in vivo exist, methods to deliver therapeutics to β cells are lacking. We leveraged the rare ability of β cells to concentrate zinc to preferentially trap zinc-binding molecules within β cells, resulting in β-cell-targeted compound delivery. We determined that zinc-rich β cells and islets preferentially accumulated TSQ (6-methoxy-8-p-toluenesulfonamido-quinoline) in a zinc-dependent manner compared with exocrine pancreas. Next, we asked whether appending a zinc-chelating moiety onto a β-cell replication-inducing compound was sufficient to confer preferential β-cell accumulation and activity. Indeed, the hybrid compound preferentially accumulated within rodent and human islets in a zinc-dependent manner and increased the selectivity of replication-promoting activity toward β cells. These data resolve the fundamental question of whether intracellular accumulation of zinc-chelating compounds is influenced by zinc content. Furthermore, application of this principle yielded a proof-of-concept method for β-cell-targeted drug delivery and bioactivity.

Targeting Type 1 Diabetes: Selective Approaches for New Therapies

The clinical onset of type 1 diabetes is characterized by the destruction of the insulin-producing β cells of the pancreas and is caused by autoantigen-induced inflammation (insulitis) of the islets of Langerhans. The current standard of care for type 1 diabetes mellitus patients allows for management of the disease with exogenous insulin, but patients eventually succumb to many chronic complications such as limb amputation, blindness, and kidney failure. New therapeutic approaches now on the horizon are looking beyond glycemic management and are evaluating new strategies from protecting and regenerating endogenous islets to treating the underlying autoimmunity through selective modulation of key immune cell populations. Currently, there are no effective treatments for the autoimmunity that causes the disease, and strategies that aim to delay or prevent the onset of the disease will play an important role in the future of diabetes research. In this review, we summarize many of the key efforts underway that utilize molecular approaches to selectively modulate this disease and look at new therapeutic paradigms that can transform clinical treatment.

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.

The development of a GPR44 targeting radioligand [11C]AZ12204657 for in vivo assessment of beta cell mass

Background

The G-protein-coupled receptor 44 (GPR44) is a beta cell-restricted target that may serve as a marker for beta cell mass (BCM) given the development of a suitable PET ligand.

Methods

The binding characteristics of the selected candidate, AZ12204657, at human GPR44 were determined using in vitro ligand binding assays. AZ12204657 was radiolabeled using ¹¹C- or ³H-labeled methyl iodide ([¹¹C/³H]CH₃I) in one step, and the conversion of [¹¹C/³H]CH₃I to the radiolabeled product [¹¹C/³H]AZ12204657 was quantitative. The specificity of radioligand binding to GPR44 and the selectivity for beta cells were evaluated by in vitro binding studies on pancreatic sections from human and non-human primates as well as on homogenates from endocrine and exocrine pancreatic compartments.

Results

The radiochemical purity of the resulting radioligand [¹¹C]AZ12204657 was > 98%, with high molar activity (MA), 1351 ± 575 GBq/μmol (n = 18). The radiochemical purity of [³H]AZ12204657 was > 99% with MA of 2 GBq/μmol. Pancreatic binding of [¹¹C/³H]AZ12204657 was co-localized with insulin-positive islets of Langerhans in non-diabetic individuals and individuals with type 2 diabetes (T2D). The binding of [¹¹C]AZ12204657 to GPR44 was > 10 times higher in islet homogenates compared to exocrine homogenates. In human islets of Langerhans GPR44 was co-expressed with insulin, but not glucagon as assessed by co-staining and confocal microscopy.

Conclusion

We radiolabeled [¹¹C]AZ12204657, a potential PET radioligand for the beta cell-restricted protein GPR44. In vitro evaluation demonstrated that [³H]AZ12204657 and [¹¹C]AZ12204657 selectively target pancreatic beta cells. [¹¹C]AZ12204657 has promising properties as a marker for human BCM.

Exploring the insulin secretory properties of the PGD2-GPR44/DP2 axis in vitro and in a randomized phase-1 trial of type 2 diabetes patients

Aims/Hypothesis

GPR44 (DP2, PTGDR2, CRTh2) is the receptor for the pro-inflammatory mediator prostaglandin D₂ (PGD₂) and it is enriched in human islets. In rodent islets, PGD₂ is produced in response to glucose, suggesting that the PGD₂-GPR44/DP2 axis may play a role in human islet function during hyperglycemia. Consequently, the aim of this work was to elucidate the insulinotropic role of GPR44 antagonism in vitro in human beta-cells and in type 2 diabetes (T2DM) patients.

Methods

We determined the drive on PGD₂ secretion by glucose and IL-1beta, as well as, the impact on insulin secretion by pharmacological GPR44/DP2 antagonism (AZD1981) in human islets and beta-cells in vitro. To test if metabolic control would be improved by antagonizing a hyperglycemia-driven increased PGD₂ tone, we performed a proof-of-mechanism study in 20 T2DM patients (average 54 years, HbA1c 9.4%, BMI 31.6 kg/m²). The randomized, double-blind, placebo-controlled cross-over study consisted of two three-day treatment periods (AZD1981 or placebo) separated by a three-day wash-out period. Mixed meal tolerance test (MMTT) and intravenous graded glucose infusion (GGI) was performed at start and end of each treatment period. Assessment of AZD1981 pharmacokinetics, glucose, insulin, C-peptide, glucagon, GLP-1, and PGD₂ pathway biomarkers were performed.

Results

We found (1) that PGD₂ is produced in human islet in response to high glucose or IL-1beta, but likely by stellate cells rather than endocrine cells; (2) that PGD₂ suppresses both glucose and GLP-1 induced insulin secretion in vitro; and (3) that the GPR44/DP2 antagonist (AZD1981) in human beta-cells normalizes insulin secretion. However, AZD1981 had no impact on neither glucose nor incretin dependent insulin secretion in humans (GGI AUC _{C-peptide 1-2h} and MMTT AUC _{Glucose 0-4h} LS mean ratios vs placebo of 0.94 (80% CI of 0.90–0.98, p = 0.12) and 0.99 (90% CI of 0.94–1.05, p = 0.45), despite reaching the expected antagonist exposure.

Conclusion/Interpretation

Pharmacological inhibition of the PGD₂-GPR44/DP2 axis has no major impact on the modulation of acute insulin secretion in T2DM patients.

Trial registration

ClinicalTrials.gov NCT02367066.

In Vivo Visualization of β-Cells by Targeting of GPR44

GPR44 expression has recently been described as highly β-cell selective in the human pancreas and constitutes a tentative surrogate imaging biomarker in diabetes. A radiolabeled small-molecule GPR44 antagonist, [¹¹C]AZ12204657, was evaluated for visualization of β-cells in pigs and nonhuman primates by positron emission tomography as well as in immunodeficient mice transplanted with human islets under the kidney capsule. In vitro autoradiography of human and animal pancreatic sections from subjects without and with diabetes, in combination with insulin staining, was performed to assess β-cell selectivity of the radiotracer. Proof of principle of in vivo targeting of human islets by [¹¹C]AZ12204657 was shown in the immunodeficient mouse transplantation model. Furthermore, [¹¹C]AZ12204657 bound by a GPR44-mediated mechanism in pancreatic sections from humans and pigs without diabetes, but not those with diabetes. In vivo [¹¹C]AZ12204657 bound specifically to GPR44 in pancreas and spleen and could be competed away dose-dependently in nondiabetic pigs and nonhuman primates. [¹¹C]AZ12204657 is a first-in-class surrogate imaging biomarker for pancreatic β-cells by targeting the protein GPR44.

Species differences in pancreatic binding of DO3A-VS-Cys40-Exendin4

AIMS

Radiolabeled Exendin-4 has been proposed as suitable imaging marker for pancreatic beta cell mass quantification mediated by Glucagon-like peptide-1 receptor (GLP-1R). However, noticeable species variations in basal pancreatic uptake as well as uptake reduction degree due to selective beta cell ablation were observed.

METHODS

In vitro and ex vivo autoradiography studies of pancreas were performed using [¹⁷⁷Lu]Lu-DO3A-VS-Cys⁴⁰-Exendin4, in order to investigate the mechanism of uptake as well as the islet uptake contrast in mouse, rat, pig, and non-human primate. The autoradiography results were compared to the in vivo pancreatic uptake as assessed by [⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin4 Positron Emission Tomography (PET) in the same species. In vitro, ex vivo, and in vivo data formed the basis for calculating the theoretical in vivo contribution of each pancreatic compartment.

RESULTS

[¹⁷⁷Lu]Lu-DO3A-VS-Cys⁴⁰-Exendin4 displayed the highest islet-to-exocrine pancreas ratio (IPR) in rat (IPR 45) followed by non-human primate and mouse at similar levels (IPR approximately 5) while pigs exhibited negligible IPR (1.1). In vivo pancreas uptake was mainly GLP-1R mediated in all species, but the magnitude of uptake under basal physiology varied significantly in decreasing order: non-human primate, mouse, pig, and rat. The theoretical calculation of islet contribution to the total pancreatic PET signal predicted the in vivo observation of differences in pancreatic uptake of [⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin4.

CONCLUSIONS

IPR as well as the exocrine GLP-1R density is the main determinants of the species variability in pancreatic uptake. Thus, the IPR in human is an important factor for assessing the potential of GLP-1R as an imaging biomarker for pancreatic beta cells.

[11C]5-hydroxy-tryptophan PET for Assessment of Islet Mass During Progression of Type 2 Diabetes

[¹¹C]5-hydroxy-tryptophan ([¹¹C]5-HTP) positron emission tomography of the pancreas has been shown to be a surrogate imaging biomarker of pancreatic islet mass. The change in islet mass in different stages of type 2 diabetes (T2D) as measured by noninvasive imaging is currently unknown. Here, we describe a cross-sectional study where subjects at different stages of T2D development with expected stratification of pancreatic islet mass were examined in relation to individuals without diabetes. The primary outcome was the [¹¹C]5-HTP uptake and retention in pancreas, as a surrogate marker for the endogenous islet mass. We found that metabolic testing indicated a progressive loss of β-cell function, but this was not mirrored by a decrease in [¹¹C]5-HTP tracer accumulation in the pancreas. This provides evidence of retained islet mass despite decreased β-cell function. The results herein indicate that β-cell dedifferentiation, and not necessarily endocrine cell loss, constitutes a major cause of β-cell failure in T2D.

Regulation of pancreatic β-cell function and mass dynamics by prostaglandin signaling

Prostaglandins (PGs) are signaling lipids derived from arachidonic acid (AA), which is metabolized by cyclooxygenase (COX)-1 or 2 and class-specific synthases to generate PGD₂, PGE₂, PGF_2α, PGI₂ (prostacyclin), and thromboxane A₂. PGs signal through G-protein coupled receptors (GPCRs) and are important modulators of an array of physiological functions, including systemic inflammation and insulin secretion from pancreatic islets. The role of PGs in β-cell function has been an active area of interest, beginning in the 1970s. Early studies demonstrated that PGE₂ inhibits glucose-stimulated insulin secretion (GSIS), although more recent studies have questioned this inhibitory action of PGE₂. The PGE₂ receptor EP3 and one of the G-proteins that couples to EP3, Gα_Z, have been identified as negative regulators of β-cell proliferation and survival. Conversely, PGI₂ and its receptor, IP, play a positive role in the β-cell by enhancing GSIS and preserving β-cell mass in response to the β-cell toxin streptozotocin (STZ). In comparison to PGE₂ and PGI₂, little is known about the function of the remaining PGs within islets. In this review, we discuss the roles of PGs, particularly PGE₂ and PGI₂, PG receptors, and downstream signaling events that alter β-cell function and regulation of β-cell mass.

Mass production of functional human pancreatic β-cells: why and how?

Diabetes (either type 1 or type 2) is due to insufficient functional β-cell mass. Research has, therefore, aimed to discover new ways to maintain or increase either β-cell mass or function. For this purpose, rodents have mainly been used as model systems and a large number of discoveries have been made. Meanwhile, although we have learned that rodent models represent powerful systems to model β-cell development, function and destruction, we realize that there are limitations when attempting to transfer the data to what is occurring in humans. Indeed, while human β-cells share many similarities with rodent β-cells, they also differ on a number of important parameters. In this context, developing ways to study human β-cell development, function and death represents an important challenge. This review will describe recent data on the development and use of convenient sources of human β-cells that should be useful tools to discover new ways to modulate functional β-cell mass in humans.