Positron Emission Tomography to Assess the Outcome of Intraportal Islet Transplantation

Transplantation for type 1 diabetes: radiologist's primer on islet, pancreas and pancreas-kidney transplantation imaging

Whole-organ pancreas, pancreatic-kidney and islet transplantation are surgical therapeutic options for the treatment of type 1 diabetes. They can enable effective glycemic control, improve quality of life and delay/reduce the secondary complications of type 1 diabetes mellitus. Radiologists are integral members of the multidisciplinary transplantation team involved in these procedures, with multimodality imaging serving as the mainstay for early recognition and management of transplant related complications. This review highlights the transplantation procedures available for patients with type 1 Diabetes Mellitus with a focus on the imaging appearance of transplantation-related complications.

Non-invasive quantification of stem cell-derived islet graft size and composition

AIMS/HYPOTHESIS

Stem cell-derived islets (SC-islets) are being used as cell replacement therapy for insulin-dependent diabetes. Non-invasive long-term monitoring methods for SC-islet grafts, which are needed to detect misguided differentiation in vivo and to optimise their therapeutic effectiveness, are lacking. Positron emission tomography (PET) has been used to monitor transplanted primary islets. We therefore aimed to apply PET as a non-invasive monitoring method for SC-islet grafts.

METHODS

We implanted different doses of human SC-islets, SC-islets derived using an older protocol or a state-of-the-art protocol and SC-islets genetically rendered hyper- or hypoactive into mouse calf muscle to yield different kinds of grafts. We followed the grafts with PET using two tracers, glucagon-like peptide 1 receptor-binding [18F]F-dibenzocyclooctyne-exendin-4 ([18F]exendin) and the dopamine precursor 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]FDOPA), for 5 months, followed by histological assessment of graft size and composition. Additionally, we implanted a kidney subcapsular cohort with different SC-islet doses to assess the connection between C-peptide and stem cell-derived beta cell (SC-beta cell) mass.

RESULTS

Small but pure and large but impure grafts were derived from SC-islets. PET imaging allowed detection of SC-islet grafts even <1 mm3 in size, [18F]exendin having a better detection rate than [18F]FDOPA (69% vs 44%, <1 mm3; 96% vs 85%, >1 mm3). Graft volume quantified with [18F]exendin (r2=0.91) and [18F]FDOPA (r2=0.86) strongly correlated with actual graft volume. [18F]exendin PET delineated large cystic structures and its uptake correlated with graft SC-beta cell proportion (r2=0.68). The performance of neither tracer was affected by SC-islet graft hyper- or hypoactivity. C-peptide measurements under fasted or glucose-stimulated conditions did not correlate with SC-islet graft volume or SC-beta cell mass, with C-peptide under hypoglycaemia having a weak correlation with SC-beta cell mass (r2=0.52).

CONCLUSIONS/INTERPRETATION

[18F]exendin and [18F]FDOPA PET enable non-invasive assessment of SC-islet graft size and aspects of graft composition. These methods could be leveraged for optimising SC-islet cell replacement therapy in diabetes.

Functional maturation and longitudinal imaging of intraportal neonatal porcine islet grafts in genetically diabetic pigs

Allogeneic intraportal islet transplantation (ITx) has become an established treatment for patients with poorly controlled type 1 diabetes. However, the loss of viable beta-cell mass after transplantation remains a major challenge. Therefore, noninvasive imaging methods for long-term monitoring of the transplant fate are required. In this study, [68Ga]Ga-DOTA-exendin-4 positron emission tomography/computed tomography (PET/CT) was used for repeated monitoring of allogeneic neonatal porcine islets (NPI) after intraportal transplantation into immunosuppressed genetically diabetic pigs. NPI transplantation (3320-15,000 islet equivalents per kg body weight) led to a reduced need for exogenous insulin therapy and finally normalization of blood glucose levels in 3 out of 4 animals after 5 to 10 weeks. Longitudinal PET/CT measurements revealed a significant increase in standard uptake values in graft-bearing livers. Histologic analysis confirmed the presence of well-engrafted, mature islet clusters in the transplanted livers. Our study presents a novel large animal model for allogeneic intraportal ITx. A relatively small dose of NPIs was sufficient to normalize blood glucose levels in a clinically relevant diabetic pig model. [68Ga]Ga-DOTA-exendin-4 PET/CT proved to be efficacious for longitudinal monitoring of islet transplants. Thus, it could play a crucial role in optimizing ITx as a curative therapy for type 1 diabetes.

Comprehensive review of imaging in pancreas transplantation: a primer for radiologists

Pancreas transplantation is a complex surgical procedure performed to restore normoglycemia in patients with type 1 diabetes and includes whole/segmental organ transplant and islet cell transplantation (ICT). In the United States, simultaneous pancreas-kidney transplant (SPK) is most commonly performed due to the higher occurrence of end-stage renal disease in diabetic patients. Understanding the surgical technique and postoperative anatomy is imperative for effective and accurate surveillance following transplantation. Imaging plays an essential role in patients with pancreatic transplants and is often used to evaluate viability, vascular and parenchymal anatomy, and identify potential complications. Imaging techniques such as ultrasound, color and spectral Doppler, computed tomography (CT), magnetic resonance imaging (MRI), and angiography have a complementary role in the postoperative evaluation following a pancreas transplant. The common complications after a whole organ pancreas transplant include vascular thrombosis, graft rejection, pancreatitis, and infections. Complications can be classified into vascular (partial or complete venous thrombosis, arterial thrombosis, stenosis or pseudoaneurysm), parenchymal (pancreatitis, graft rejection), and bowel-related or miscellaneous causes (bowel obstruction, anastomotic leak, and peripancreatic fluid collections). Islet cell transplantation is an innovative therapy for patients with type 1 diabetes. It involves isolating insulin-producing islet cells from donor pancreas and transplanting into recipients, to provide long-term insulin independence or significantly reduce insulin requirements. In recent years, isolation techniques, immunosuppressive regimens, and post-transplant monitoring advancements have propelled ICT as a viable therapeutic option. This comprehensive review aims to provide insights into the current state-of-the-art imaging techniques discussing both normal and abnormal features following pancreas transplantation.

Imaging in Type 1 Diabetes, Current Perspectives and Directions

Type 1 diabetes (T1D) is characterized by the autoimmune-mediated attack of insulin-producing beta cells in the pancreas, leading to reliance on exogenous insulin to control a patient's blood glucose levels. As progress is being made in understanding the pathophysiology of the disease and how to better develop therapies to treat it, there is an increasing need for monitoring technologies to quantify beta cell mass and function throughout T1D progression and beta cell replacement therapy. Molecular imaging techniques offer a possible solution through both radiologic and non-radiologic means including positron emission tomography, magnetic resonance imaging, electron paramagnetic resonance imaging, and spatial omics. This commentary piece outlines the role of molecular imaging in T1D research and highlights the need for further applications of such methodologies in T1D.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Longitudinal Assessment of 11C-5-Hydroxytryptophan Uptake in Pancreas After Debut of Type 1 Diabetes

The longitudinal alterations of the pancreatic β-cell and islet mass in the progression of type 1 diabetes (T1D) are still poorly understood. The objective of this study was to repeatedly assess the endocrine volume and the morphology of the pancreas for up to 24 months after T1D diagnosis (n = 16), by ¹¹C-5-hydroxytryptophan (¹¹C-5-HTP) positron emission tomography (PET) and MRI. Study participants were examined four times by PET/MRI: at recruitment and then after 6, 12, and 24 months. Clinical examinations and assessment of β-cell function by a mixed-meal tolerance test and fasting blood samples were performed in connection with the imaging examination. Pancreas volume has a tendency to decrease from 50.2 ± 10.3 mL at T1D debut to 42.2 ± 14.6 mL after 24 months (P < 0.098). Pancreas uptake of ¹¹C-5-HTP (e.g., the volume of the endocrine pancreas) did not decrease from T1D diagnosis (0.23 ± 0.10 % of injected dose) to 24-month follow-up, 0.21 ± 0.14% of injected dose, and exhibited low interindividual changes. Pancreas perfusion was unchanged from diagnosis to 24-month follow-up. The pancreas uptake of ¹¹C-5-HTP correlated with the long-term metabolic control as estimated by HbA_1c (P < 0.05). Our findings argue against a major destruction of β-cell or islet mass in the 2-year period after diagnosis of T1D.

Bioluminescence Imaging In Vivo Confirms the Viability of Pancreatic Islets Transplanted into the Greater Omentum

PURPOSE

The liver is the most widely used site for pancreatic islet transplantation. However, several site-specific limitations impair functional success, with instant blood-mediated inflammatory reaction being the most important. The aim of this study was to develop a preclinical model for placement of the islet graft into a highly vascularized omental flap using a fibrin gel. For this purpose, we tested islet viability by bioluminescence imaging (BLI).

PROCEDURES

Pancreatic islets were isolated from luciferase-positive and luciferase-negative rats, mixed at a 1:1 ratio, placed into a plasma-thrombin bioscaffold, and transplanted in standard (10 pancreatic islets/g wt; n = 10) and marginal (4 pancreatic islets/g wt; n = 7) numbers into the omentums of syngeneic diabetic animals. For the control, 4 pancreatic islets/g were transplanted into the liver using the standard procedure (n = 7). Graft viability was tested by bioluminescence at days 14, 30, 60, and 90 post transplant. Glucose levels, intravenous glucose tolerance, and serum C-peptide were assessed regularly.

RESULTS

Nonfasting glucose levels < 10 mmol/l were restored in all animals. While islet viability in the omentum was clearly detected by stable luminescence signals throughout the whole study period, no signals were detected from islets transplanted into the liver. The bioluminescence signals were highly correlated with stimulated C-peptide levels detected at 80 days post transplant. Glucose tolerance did not differ among the 3 groups.

CONCLUSIONS

We successfully tested a preclinical model of islet transplantation into the greater omentum using a biocompatible scaffold made from autologous plasma and human thrombin. Both standard and marginal pancreatic islet numbers in a gel-form bioscaffold placed in the omentum restored glucose homeostasis in recipients with diabetes. Bioluminescence was shown promising as a direct proof of islet viability.

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.

In Vivo Imaging of Pancreatic Islet Grafts in Diabetes Treatment

Transplantation of pancreatic islets has potential to offer life-long blood glucose management in type I diabetes and severe type II diabetes without the need of exogenous insulin administration. However, islet cell therapy suffers from autoimmune and allogeneic rejection as well as non-immune related factors. Non-invasive techniques to monitor and evaluate the fate of cell implants in vivo are essential to understand the underlying causes of graft failure, and hence to improve the precision and efficacy of islet therapy. This review describes how imaging technology has been employed to interrogate the distribution, number or volume, viability, and function of islet implants in vivo. To date, fluorescence imaging, PET, SPECT, BLI, MRI, MPI, and ultrasonography are the many imaging modalities being developed to fulfill this endeavor. We outline here the advantages, limitations, and clinical utility of each particular imaging approach.

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.

[11C]5-Hydroxy-tryptophan model for quantitative assessment of in vivo serotonin biosynthesis, retention and degradation in the endocrine pancreas

[¹¹C]5-Hydroxy-tryptophan ([¹¹C]5-HTP) is a Positron Emission Tomography marker for serotonergic biosynthesis and degradation, with use in imaging of neuroendocrine tumors and recently also the endocrine pancreas in diabetes. In order to further develop [¹¹C]5-HTP as a quantitative in vivo tool for understanding the mechanisms of serotonin signaling in human pancreas, we aimed to develop a kinetic modeling approach sensitive for changes in serotonin biosynthesis, retention and degradation. Cynomolgus monkeys were examined by [¹¹C]5-HTP PET/CT, either at baseline (n=9) or following intravenous pretreatment with 3 mg/kg carbidopa (Dopa Decarboxylase inhibitor, n=3) or 2 mg/kg clorgyline (Monoamine Oxidase-A inhibitor, n=5). The dynamic tissue uptake was analysed by a 2-tissue compartment model including an efflux mechanism from the second tissue compartment (2TC k_loss), which theoretically reproduces the known processing of 5-HTP in neuroendocrine cells. The 2TC k_loss model could accurately describe all three modes of tissue kinetics depending on the pretreatment regiment. Rate constant k₃ (corresponding to DDC activity) and the macro-parameter Flux (K_i) was decreased (P<0.05) by carbidopa pretreatment, while k₂ (corresponding to cellular washout of intact [¹¹C]5-HTP) was increased (P<0.05). The efflux parameter k_loss (corresponding to MAO-A activity) was decreased (P<0.05) by pretreatment of clorgyline, while the macro-parameter Flux/Efflux ratio (K_i/k_loss) was increased (P<0.0001). We present a compartment model analysis method that can quantitatively assess in vivo pharmacological interactions with several of the key enzymatic steps of the serotonergic biosynthesis in pancreas.

Current Progress and Perspective: Clinical Imaging of Islet Transplantation

Islet transplantation has great potential as a cure for type 1 diabetes. At present; the lack of a clinically validated non-invasive imaging method to track islet grafts limits the success of this treatment. Some major clinical imaging modalities and various molecular probes, which have been studied for non-invasive monitoring of transplanted islets, could potentially fulfill the goal of understanding pathophysiology of the functional status and viability of the islet grafts. In this current review, we summarize the recent clinical studies of a variety of imaging modalities and molecular probes for non-invasive imaging of transplanted beta cell mass. This review also includes discussions on in vivo detection of endogenous beta cell mass using clinical imaging modalities and various molecular probes, which will be useful for longitudinally detecting the status of islet transplantation in Type 1 diabetic patients. For the conclusion and perspectives, we highlight the applications of multimodality and novel imaging methods in islet transplantation.

Radiological Advances in Pancreatic Islet Transplantation

Type 1 diabetes mellitus (T1DM) is characterized by hyperglycemia, owing to the loss of pancreatic β cells in response to an autoimmune reaction leading to a state of absolute insulin deficiency. T1DM treatment is shifting from exogenous insulin replacement therapy toward pancreatic β-cell replacement, to restore physiologically responsive insulin secretion to variations in blood glucose levels. β-cell replacement strategies include human whole pancreas transplantation, islet transplantation with cell encapsulation and bioengineered pancreas. Interventional radiology and imaging modalities including positron emission tomography, single-photon emission computed tomography, magnetic resonance imaging, ultrasonography, and molecular imaging are imperative to enable successful β-cell replacement. Herein, the role of radiological modalities in the treatment of T1DM and its prospective use for noninvasive post-transplantation graft monitoring is discussed.

A Trimodal Imaging Platform for Tracking Viable Transplanted Pancreatic Islets In Vivo: F-19 MR, Fluorescence, and Bioluminescence Imaging

PURPOSE

Combining specific and quantitative F-19 magnetic resonance imaging (MRI) with sensitive and convenient optical imaging provides complementary information about the distribution and viability of transplanted pancreatic islet grafts. In this study, pancreatic islets (PIs) were labeled with positively charged multimodal nanoparticles based on poly(lactic-co-glycolic acid) (PLGA-NPs) with encapsulated perfluoro-15-crown-5-ether and the near-infrared fluorescent dye indocyanine green.

PROCEDURES

One thousand and three thousand bioluminescent PIs were transplanted into subcutaneous artificial scaffolds, which served as an alternative transplant site. The grafts were monitored using in vivo F-19 MR, fluorescence, and bioluminescence imaging in healthy rats for 2 weeks.

RESULTS

Transplanted PIs were unambiguously localized in the scaffolds by F-19 MRI throughout the whole experiment. Fluorescence was detected in the first 4 days after transplantation only. Importantly, in vivo bioluminescence correlated with the F-19 MRI signal.

CONCLUSIONS

We developed a trimodal imaging platform for in vivo examination of transplanted PIs. Fluorescence imaging revealed instability of the fluorescent dye and its limited applicability for longitudinal in vivo studies. A correlation between the bioluminescence signal and the F-19 MRI signal indicated the fast clearance of PLGA-NPs from the transplantation site after cell death, which addresses a major issue with intracellular imaging labels. Therefore, the proposed PLGA-NP platform is reliable for reflecting the status of transplanted PIs in vivo.

Evaluation of [68Ga]DO3A-VS-Cys40-Exendin-4 as a PET Probe for Imaging Human Transplanted Islets in the Liver

[⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4, a glucagon-like peptide 1 receptor agonist, was evaluated as a potential PET tracer for the quantitation of human islets transplanted to the liver. The short-lived PET radionuclide ⁶⁸Ga, available on a regular basis from a ⁶⁸Ge/⁶⁸Ga generator, is an attractive choice. Human C-peptide was measured to evaluate human islet function post-transplantation and prior to microPET imaging. [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 was radiosynthesized and evaluated for PET imaging of transplanted human islets in the liver of healthy NOD/SCID mice. The biodistribution of the tracer was evaluated to determine the uptake into various organs, and qPCR of liver samples was conducted to confirm engrafted islet numbers after PET imaging. Measurement of human C-peptide indicated that higher engrafted islet mass resulted in higher human C-peptide levels in post-transplantation. The microPET imaging yielded high resolution images of liver-engrafted islets and also showed significant retention in mouse livers at 8 weeks post-transplantation. Biodistribution studies in mice revealed that liver uptake of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 was approximately 6-fold higher in mice that received 1000 islet equivalent (IEQ) than in non-transplanted mice. qPCR analysis of insulin expression suggested that islet engraftment numbers were close to 1000 IEQ transplanted. In conclusion, human islets transplanted into the livers of mice exhibited significant uptake of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 compared to the livers of untreated mice; and imaging of the mice using PET showed the human islets clearly with high contrast against liver tissue, enabling accurate quantitation of islet mass. Further validation of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 as an islet imaging probe for future clinical application is ongoing.

Pancreatic Blood Flow with Special Emphasis on Blood Perfusion of the Islets of Langerhans

The pancreatic islets are more richly vascularized than the exocrine pancreas, and possess a 5- to 10-fold higher basal and stimulated blood flow, which is separately regulated. This is reflected in the vascular anatomy of the pancreas where islets have separate arterioles. There is also an insulo-acinar portal system, where numerous venules connect each islet to the acinar capillaries. Both islets and acini possess strong metabolic regulation of their blood perfusion. Of particular importance, especially in the islets, is adenosine and ATP/ADP. Basal and stimulated blood flow is modified by local endothelial mediators, the nervous system as well as gastrointestinal hormones. Normally the responses to the nervous system, especially the parasympathetic and sympathetic nerves, are fairly similar in endocrine and exocrine parts. The islets seem to be more sensitive to the effects of endothelial mediators, especially nitric oxide, which is a permissive factor to maintain the high basal islet blood flow. The gastrointestinal hormones with pancreatic effects mainly influence the exocrine pancreatic blood flow, whereas islets are less affected. A notable exception is incretin hormones and adipokines, which preferentially affect islet vasculature. Islet hormones can influence both exocrine and endocrine blood vessels, and these complex effects are discussed. Secondary changes in pancreatic and islet blood flow occur during several conditions. To what extent changes in blood perfusion may affect the pathogenesis of pancreatic diseases is discussed. Both type 2 diabetes mellitus and acute pancreatitis are conditions where we think there is evidence that blood flow may contribute to disease manifestations. © 2019 American Physiological Society. Compr Physiol 9:799-837, 2019.

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 nanobody-based tracer targeting DPP6 for non-invasive imaging of human pancreatic endocrine cells

There are presently no reliable ways to quantify endocrine cell mass (ECM) in vivo, which prevents an accurate understanding of the progressive beta cell loss in diabetes or following islet transplantation. To address this unmet need, we coupled RNA sequencing of human pancreatic islets to a systems biology approach to identify new biomarkers of the endocrine pancreas. Dipeptidyl-Peptidase 6 (DPP6) was identified as a target whose mRNA expression is at least 25-fold higher in human pancreatic islets as compared to surrounding tissues and is not changed by proinflammatory cytokines. At the protein level, DPP6 localizes only in beta and alpha cells within the pancreas. We next generated a high-affinity camelid single-domain antibody (nanobody) targeting human DPP6. The nanobody was radiolabelled and in vivo SPECT/CT imaging and biodistribution studies were performed in immunodeficient mice that were either transplanted with DPP6-expressing Kelly neuroblastoma cells or insulin-producing human EndoC-βH1 cells. The human DPP6-expressing cells were clearly visualized in both models. In conclusion, we have identified a novel beta and alpha cell biomarker and developed a tracer for in vivo imaging of human insulin secreting cells. This provides a useful tool to non-invasively follow up intramuscularly implanted insulin secreting cells.

Human beta cell mass and function in diabetes: Recent advances in knowledge and technologies to understand disease pathogenesis

BACKGROUND

Plasma insulin levels are predominantly the product of the morphological mass of insulin producing beta cells in the pancreatic islets of Langerhans and the functional status of each of these beta cells. Thus, deficiency in either beta cell mass or function, or both, can lead to insufficient levels of insulin, resulting in hyperglycemia and diabetes. Nonetheless, the precise contribution of beta cell mass and function to the pathogenesis of diabetes as well as the underlying mechanisms are still unclear. In the past, this was largely due to the restricted number of technologies suitable for studying the scarcely accessible human beta cells. However, in recent years, a number of new platforms have been established to expand the available techniques and to facilitate deeper insight into the role of human beta cell mass and function as cause for diabetes and as potential treatment targets.

SCOPE OF REVIEW

This review discusses the current knowledge about contribution of human beta cell mass and function to different stages of type 1 and type 2 diabetes pathogenesis. Furthermore, it highlights standard and newly developed technological platforms for the study of human beta cell biology, which can be used to increase our understanding of beta cell mass and function in human glucose homeostasis.

MAJOR CONCLUSIONS

In contrast to early disease models, recent studies suggest that in type 1 and type 2 diabetes impairment of beta cell function is an early feature of disease pathogenesis while a substantial decrease in beta cell mass occurs more closely to clinical manifestation. This suggests that, in addition to beta cell mass replacement for late stage therapies, the development of novel strategies for protection and recovery of beta cell function could be most promising for successful diabetes treatment and prevention. The use of today's developing and wide range of technologies and platforms for the study of human beta cells will allow for a more detailed investigation of the underlying mechanisms and will facilitate development of treatment approaches to specifically target human beta cell mass and function.

[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.

Quantitative and longitudinal imaging of intramuscular transplanted islets of Langerhans with SPECT using [ 123 I]IBZM

A non-invasive imaging method to monitor islet grafts could provide novel and improved insight into the fate of transplanted islets and, potentially, monitor the effect of therapeutic interventions. Therefore, such an imaging method could help improve long-term transplantation outcome. Here, we investigated the use of [ ¹²³ I]IBZM for insulin positive graft volume quantification and longitudinal graft monitoring. SPECT images were acquired 6 weeks after islet transplantation in the calf muscle of rats. For longitudinal graft analysis, rats were monitored by SPECT for 10 weeks. After animals were euthanized, graft containing muscles were dissected for ex vivo analysis and insulin-positive graft volume determination. Six weeks after transplantation, a clear signal was observed in all grafts by SPECT imaging. Moreover, the intensity of the SPECT signal correlated linearly with insulin-positive graft volume, as determined histologically. Longitudinal graft follow-up showed a clear SPECT signal of the transplant from 3 until 10 weeks after transplantation. In this study, we demonstrate for the first time the successful application of a radiotracer, [ ¹²³ I]IBZM, for non-invasive, in vivo graft volume quantification and longitudinal graft monitoring.

Recent progress in the use and tracking of transplanted islets as a personalized treatment for type 1 diabetes

INTRODUCTION

Type 1 diabetes mellitus (T1DM) is an autoimmune disease in which the pancreas produces insufficient amounts of insulin. T1DM patients require exogenous sources of insulin to maintain euglycemia. Transplantation of naked or microencapsulated pancreatic islets represents an alternative paradigm to obtain an autonomous regulation of blood glucose levels in a controlled and personalized fashion. However, once transplanted, the fate of these personalized cellular therapeutics is largely unknown, justifying the development of non-invasive tracking techniques.

AREAS COVERED

In vivo imaging of naked pancreatic islet transplantation, monitoring of microencapsulated islet transplantation, visualizing pancreatic inflammation, imaging of molecular-genetic therapeutics, imaging of beta cell function.

EXPERT COMMENTARY

There are still several hurdles to overcome before (microencapsulated) islet cell transplantation will become a mainstay therapy. Non-invasive imaging methods that can track graft volume, graft rejection, graft function (insulin secretion) microcapsule engraftment, microcapsule rupture, and pancreatic inflammation are currently being developed to design the best experimental transplantation paradigms.

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.