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

DGCR2 targeting affibody molecules for delivery of drugs and imaging reagents to human beta cells

A distinctive feature of both type 1 and type 2 diabetes is the waning of insulin-secreting beta cells in the pancreas. New methods for direct and specific targeting of the beta cells could provide platforms for delivery of pharmaceutical reagents. Imaging techniques such as Positron Emission Tomography (PET) rely on the efficient and specific delivery of imaging reagents, and could greatly improve our understanding of diabetes etiology as well as providing biomarkers for viable beta-cell mass in tissue, in both pancreas and in islet grafts.The DiGeorge Syndrome Critical Region Gene 2 (DGCR2) protein has been suggested as a beta-cell specific protein in the pancreas, but so far there has been a lack of available high-affinity binders suitable for targeted drug delivery or molecular imaging. Affibody molecules belong to a class of small affinity proteins with excellent properties for molecular imaging. Here, we further validate the presence of DGCR2 in pancreatic and stem cell (SC)-derived beta cells, and then describe the generation and selection of several Affibody molecules candidates that target human DGCR2. Using an in-house developed directed evolution method, new DGCR2-binding Affibody molecules were generated and evaluated for thermal stability and affinity. The Affibody molecules variants were further developed as targeting agents for delivering imaging reagents to beta cell. The Affibody molecule ZDGCR2:AM106 displayed nanomolar affinity, suitable stability and biodistribution, with negligible toxicity to islets, qualifying it as a suitable lead candidate for further development as a tool for specific delivery of drugs and imaging reagents to beta cells.

PET imaging of GABAA receptors in pancreatic islets by [11C]flumazenil

BACKGROUND

Type 1 diabetes (T1D) is an autoimmune disease characterized by a progressive β-cell destruction. There are no clinically established methods for quantifying endocrine cells of the pancreas and current knowledge is almost exclusively based on autopsy material and functional measurements. Based on the expression of the γ-aminobutyric acid A receptors (GABAARs) in pancreatic islets and the fact that GABAAR agonists are being explored as treatment for T1D, we hypothesized that the positron emission tomography (PET) tracer [11C]flumazenil ([11C]FMZ) could serve as a marker of the endocrine mass of the pancreas. The in vivo uptake of [11C]FMZ in pig pancreas was evaluated by PET/CT, either tracer alone or after blockade of GABAAR by unlabeled flumazenil. The pancreatic binding of [11C]FMZ was investigated in vitro with frozen sections of pig pancreas as well as human organ donors, in addition to isolated mouse and human islets and exocrine preparations. The expression of GABAAR subunits in pig, human and mouse pancreas was explored by immunohistochemistry.

RESULTS

Strong specific in vivo uptake of [11C]FMZ was observed in the pig brain as expected, but in the pancreas the signal was moderate and only partially reduced by blockade. In vitro experiments revealed a positive but weak and variable binding of [11C]FMZ in islets compared to exocrine tissue in the mouse, pig and human pancreas. In pig and mouse pancreatic islets we identified the GABAAR subunits β2 and γ2 but not α2. In the human pancreas from non-diabetic donors, we have identified the α2, β2 (although weak) and γ2 subunits, whereas from a T2D donor the α2 subunit was missing.

CONCLUSIONS

Our findings suggest that [11C]FMZ bind to GABAARs in the islets, but not with a sufficient contrast or magnitude to be implemented as an in vivo PET marker for the endocrine mass of the pancreas. However, GABAARs with different subunits are widely expressed in the endocrine cells within the pancreas in pig, human and mouse. Hence, the GABAAR could still be a potential imaging target for the endocrine cells of the pancreas but would require tracers with higher affinity and selectivity for individual GABAAR subunits.

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.

Serotonin transporter imaging agent as a probe for β-cells of pancreas

OBJECTIVE

Diabetes mellitus (DM) is one of the major diseases in the world. Nuclear medicine imaging may be able to detect functional status of pancreatic β cells in vivo, which might elucidate the pathological mechanisms of diabetes and develop individualized treatment plans. In this study, we evaluated the ability of [125I]ADAM, a serotonin transporter (SERT) imaging agent, as a probe for detecting pancreatic β-cell mass (BCM).

METHODS

In vitro cell studies were evaluated in INS-1 cells (rat islet β cell line). Biodistribution studies were performed in male normal Sprague-Dawley rats and alloxan-induced type 1 diabetes mellitus (T1DM) rats. Distribution and expression of SERT protein in pancreas of rats were also measured by immunofluorescence staining and Western blot.

RESULTS

In vitro cell studies showed that the concentration of [125I]ADAM associated with the INS-1 cells was increased gradually with incubation time, and the SERT specific inhibitor, escitalopram, exhibited the inhibitory effect on this interaction. Biodistribution studies also showed that the uptake of [125I]ADAM in the pancreas of normal rats was decreased in the presence of escitalopram. However, in the T1DM rat model with a significant β cells reduction, the uptake of pancreas was increased when compared with the control. Through immunofluorescence staining and Western blot, it was found that both the endocrine and exocrine cells of the normal pancreas expressed SERT protein, and the level of SERT protein in the exocrine cells was higher than islets. In the diabetic state, the expression of SERT in the exocrine cells was further increased.

CONCLUSIONS

The SERT imaging agent, [125I]ADAM, at the present form will not be suitable for imaging β cells, specifically because there were extraordinarily high non-specific signals contributing from the exocrine cells of pancreas. In addition, we noticed that the level of SERT expression was abnormally elevated in the diabetic state, which might provide an unexpected target for studying the pathological mechanisms of diabetes.

Allo Beta Cell transplantation: specific features, unanswered questions, and immunological challenge

Type 1 diabetes (T1D) presents a persistent medical challenge, demanding innovative strategies for sustained glycemic control and enhanced patient well-being. Beta cells are specialized cells in the pancreas that produce insulin, a hormone that regulates blood sugar levels. When beta cells are damaged or destroyed, insulin production decreases, which leads to T1D. Allo Beta Cell Transplantation has emerged as a promising therapeutic avenue, with the goal of reinstating glucose regulation and insulin production in T1D patients. However, the path to success in this approach is fraught with complex immunological hurdles that demand rigorous exploration and resolution for enduring therapeutic efficacy. This exploration focuses on the distinct immunological characteristics inherent to Allo Beta Cell Transplantation. An understanding of these unique challenges is pivotal for the development of effective therapeutic interventions. The critical role of glucose regulation and insulin in immune activation is emphasized, with an emphasis on the intricate interplay between beta cells and immune cells. The transplantation site, particularly the liver, is examined in depth, highlighting its relevance in the context of complex immunological issues. Scrutiny extends to recipient and donor matching, including the utilization of multiple islet donors, while also considering the potential risk of autoimmune recurrence. Moreover, unanswered questions and persistent gaps in knowledge within the field are identified. These include the absence of robust evidence supporting immunosuppression treatments, the need for reliable methods to assess rejection and treatment protocols, the lack of validated biomarkers for monitoring beta cell loss, and the imperative need for improved beta cell imaging techniques. In addition, attention is drawn to emerging directions and transformative strategies in the field. This encompasses alternative immunosuppressive regimens and calcineurin-free immunoprotocols, as well as a reevaluation of induction therapy and recipient preconditioning methods. Innovative approaches targeting autoimmune recurrence, such as CAR Tregs and TCR Tregs, are explored, along with the potential of stem stealth cells, tissue engineering, and encapsulation to overcome the risk of graft rejection. In summary, this review provides a comprehensive overview of the inherent immunological obstacles associated with Allo Beta Cell Transplantation. It offers valuable insights into emerging strategies and directions that hold great promise for advancing the field and ultimately improving outcomes for individuals living with diabetes.

New Long-Acting [89Zr]Zr-DFO GLP-1 PET Tracers with Increased Molar Activity and Reduced Kidney Accumulation

Positron emission tomography (PET) imaging is used in drug development to noninvasively measure biodistribution and receptor occupancy. Ideally, PET tracers retain target binding and biodistribution properties of the investigated drug. Previously, we developed a zirconium-89 PET tracer based on a long-circulating glucagon-like peptide 1 receptor agonist (GLP-1RA) using desferrioxamine (DFO) as a chelator. Here, we aimed to develop an improved zirconium-89-labeled GLP-1RA with increased molar activity to increase the uptake in low receptor density tissues, such as brain. Furthermore, we aimed at reducing tracer accumulation in the kidneys. Introducing up to four additional Zr-DFOs resulted in higher molar activity and stability, while retaining potency. Branched placement of DFOs was especially beneficial. Tracers with either two or four DFOs had similar biodistribution as the tracer with one DFO in vivo, albeit increased kidney and liver uptake. Reduced kidney accumulation was achieved by introducing an enzymatically cleavable Met-Val-Lys (MVK) linker motif between the chelator and the peptide.

[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.