Quantification of β-Cell Mass in Intramuscular Islet Grafts Using Radiolabeled Exendin-4

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.

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

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

Non-invasive in vivo imaging of porcine islet xenografts in a preclinical model with [68Ga]Ga-exendin-4

Introduction

Islet xenotransplantation may be a therapeutic option in type 1 diabetes. Recent advances in generating genetically modified source pigs offer advantages as immune suppressants can potentially be eliminated after the transplantation. Therapy monitoring would greatly benefit from noninvasive methods for assessing the viability of transplanted islets. Peptide-based positron emission tomography (PET) targeting the glucagon-like peptide-1 receptor (GLP1R) expression on beta cells may offer a procedure that can directly be translated from an experimental setting to the clinic. The aim of this study was to establish the labeling of the GLP1R ligand [68Ga]Ga-exendin-4, to demonstrate the feasibility of imaging porcine islet xenografts in vivo and to compare signal quality for three different transplantation sites in a mouse model.

Materials and methods

Mice with engrafted neonatal porcine islet cell clusters (NPICCs) under the kidney capsule, into the inguinal fold, or the lower hindlimb muscle were studied. After reaching normoglycemia, the mice were injected with [68Ga]Ga-exendin-4 for PET data acquisition. Subsequent autoradiography (AR) was used for comparing ex vivo data with in vivo uptake.

Results

NPICCs in the lower right hindlimb muscle could be detected in vivo and in AR. Due to the high background in the kidney and urinary bladder, islets could not be detected in the PET data at transplantation sites close to these organs, while AR showed a clear signal for the islets in the inguinal fold.

Discussion

PET with [68Ga]Ga-exendin-4 detects islets transplanted in the hindlimb muscle tissue of mice, offering the potential of longitudinal monitoring of viable porcine islets. Other sites are not suitable for in vivo imaging owing to high activity accumulation of Exendin-4 in kidney and bladder.

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.

Considerations for an Alternative Site of Islet Cell Transplantation

Islet cell transplantation has been limited most by poor graft survival. Optimizing the site of transplantation could improve clinical outcomes by minimizing required donor cells, increasing graft integration, and simplifying the transplantation and monitoring process. In this article, we review the history and significant human and animal data for clinically relevant sites, including the liver, spleen, and kidney subcapsule, and identify promising new sites for further research. While the liver was the first studied site and has been used the most in clinical practice, the majority of transplanted islets become necrotic. We review the potential causes for graft death, including the instant blood-mediated inflammatory reaction, exposure to immunosuppressive agents, and low oxygen tension. Significant research exists on alternative sites for islet cell transplantation, suggesting a promising future for patients undergoing pancreatectomy.

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

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

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.

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012–2017)

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

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.

MECHANISMS IN ENDOCRINOLOGY: Towards the clinical translation of stem cell therapy for type 1 diabetes

Insulin-producing cells derived from human embryonic stem cells (hESCs) or induced pluripotent stem cells (iPSCs) have for long been a promising, but elusive treatment far from clinical translation into type 1 diabetes therapy. However, the field is now on the verge of moving such insulin-producing cells into clinical trials. Although stem cell therapies provide great opportunities, there are also potential risks such as teratoma formation associated with the treatment. Many considerations are needed on how to proceed with clinical translation, including whether to use hESCs or iPSCs, and whether encapsulation of tissue will be needed. This review aims to give an overview of the current knowledge of stem cell therapy outcomes in animal models of type 1 diabetes and a proposed road map towards the clinical setting with special focus on the potential risks and hurdles which needs to be considered. From a clinical point of view, transplantation of insulin-producing cells derived from stem cells must be performed without immune suppression in order to be an attractive treatment option. Although costly and highly labour intensive, patient-derived iPSCs would be the only solution, if not clinically successful encapsulation or tolerance induction protocols are introduced.

Current Status of Radiopharmaceuticals for the Theranostics of Neuroendocrine Neoplasms

Nuclear medicine plays a pivotal role in the management of patients affected by neuroendocrine neoplasms (NENs). Radiolabeled somatostatin receptor analogs are by far the most advanced radiopharmaceuticals for diagnosis and therapy (radiotheranostics) of NENs. Their clinical success emerged receptor-targeted radiolabeled peptides as an important class of radiopharmaceuticals and it paved the way for the investigation of other radioligand-receptor systems. Besides the somatostatin receptors (sstr), other receptors have also been linked to NENs and quite a number of potential radiolabeled peptides have been derived from them. The Glucagon-Like Peptide-1 Receptor (GLP-1R) is highly expressed in benign insulinomas, the Cholecystokinin 2 (CCK2)/Gastrin receptor is expressed in different NENs, in particular medullary thyroid cancer, and the Glucose-dependent Insulinotropic Polypeptide (GIP) receptor was found to be expressed in gastrointestinal and bronchial NENs, where interestingly, it is present in most of the sstr-negative and GLP-1R-negative NENs. Also in the field of sstr targeting new discoveries brought into light an alternative approach with the use of radiolabeled somatostatin receptor antagonists, instead of the clinically used agonists. The purpose of this review is to present the current status and the most innovative strategies for the diagnosis and treatment (theranostics) of neuroendocrine neoplasms using a cadre of radiolabeled regulatory peptides targeting their receptors.