Strain Differences Determine the Suitability of Animal Models for Noninvasive In Vivo Beta Cell Mass Determination with Radiolabeled Exendin

Conjugation to a cell-penetrating peptide drives the tumour accumulation of the GLP1R antagonist exendin(9-39)

PURPOSE

Exendin, an analogue of the glucagon-like peptide 1 (GLP1), is an excellent tracer for molecular imaging of pancreatic beta cells and beta cell-derived tumours. The commonly used form, exendin-4, activates the GLP1 receptor and causes internalisation of the peptide-receptor complex. As a consequence, injection of exendin-4 can lead to adverse effects such as nausea, vomiting and hypoglycaemia and thus requires close monitoring during application. By comparison, the antagonist exendin(9-39) does not activate the receptor, but its lack of internalisation has precluded its use as a tracer. Improving the cellular uptake of exendin(9-39) could turn it into a useful alternative tracer with less side-effects than exendin-4.

METHODS

We conjugated exendin-4 and exendin(9-39) to the well-known cell-penetrating peptide (CPP) penetratin. We evaluated cell binding and internalisation of the radiolabelled peptides in vitro and their biodistribution in vivo.

RESULTS

Exendin-4 showed internalisation irrespective of the presence of the CPP, whereas for exendin(9-39) only the penetratin conjugate internalised. Conjugation to the CPP also enhanced the in vivo tumour uptake and retention of exendin(9-39).

CONCLUSION

We demonstrate that penetratin robustly improves internalisation and tumour retention of exendin(9-39), opening new avenues for antagonist-based in vivo imaging of GLP1R.

68Ga-labelled-exendin-4: New GLP1R targeting agents for imaging pancreatic β-cell and insulinoma

OBJECTIVE

Glucagon-like peptide-1 receptor (GLP1R) specifically expressed on the surface of pancreatic β-cells and insulinoma, is a potential biomarker for imaging β-cell mass (BCM). In this study, two new ⁶⁸Ga-labelled GLP1R targeting agents were prepared and their biological properties for imaging BCM and insulinoma were evaluated.

METHODS

[⁶⁸Ga]Ga-HBED-CC-MAL-Cys³⁹-exendin-4 ([⁶⁸Ga]Ga-4) and its dimer ([⁶⁸Ga]Ga-5) were synthesized from corresponding precursors. Cell uptake studies were evaluated in INS-1 cells. Biodistribution and microPET studies were performed in male normal Sprague-Dawley rats, diabetic rats and insulinoma xenograft NOD/SCID mice.

RESULTS

[⁶⁸Ga]Ga-4 and [⁶⁸Ga]Ga-5 were efficiently radiolabelled by a simple one-step reaction without purification leading to high radiochemical yields and radiochemical purities (both >95%, decay corrected, n = 6, molar activity 15 GBq/μmol). They both showed excellent stability (~95%) in phosphate-buffered saline, pH 7.4, and in rat serum (~90%) for 2 h. Biodistribution studies and small animal PET/CT imaging showed that [⁶⁸Ga]Ga-4 displayed specific uptake in rat pancreas and mouse insulinoma, and a reduced uptake in the pancreas of diabetic rat was observed (~62% reduction). Notably, it exhibited a rapid time-to-peak pancreatic uptake (0.96 ± 0.19%ID/g in 15 min) and fast clearance from the kidney (42% clearance in 30 min). Results suggested a favorable in vivo kinetics for human imaging studies.

CONCLUSIONS

[⁶⁸Ga]Ga-4 targeting GLP1R of pancreatic β-cells may be a potentially useful PET agent and a suitable candidate for further structural modification studies. This agent has demonstrated several advantages, rapid time-to-peak pancreatic uptake and faster clearance from the kidney, factors may enhance diagnosis of diabetes and insulinoma.

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.

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.

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.

Investigation of the preservation effect of canagliflozin on pancreatic beta cell mass using SPECT/CT imaging with 111In-labeled exendin-4

Radiolabeled exendin derivatives are promising for non-invasive quantification of pancreatic beta cell mass (BCM); longitudinal observation of BCM for evaluation of therapeutic effects has not been achieved. The aim of this study is to demonstrate the usefulness of our developing method using [Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4 to detect longitudinal changes in BCM. We performed a longitudinal study with obese type 2 diabetes model (db/db) mice administered canagliflozin, which is reported to preserve BCM. Six-week-old mice were assigned to a canagliflozin-administered group or a control group. Blood glucose levels of the canagliflozin group were significantly lower than those of the control group. Plasma insulin levels, insulin secretion during OGTT and insulin content in the pancreas were preserved in the canagliflozin group in comparison with those in the control group. According to SPECT/CT imaging analysis using [Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4, pancreatic uptake was significantly decreased in the control group, whereas there was no significant change in the canagliflozin group. After nine weeks, both pancreatic uptake and BCM of the canagliflozin group were significantly higher than those of the control group, and a correlation between them was observed. In conclusion, our imaging method confirmed the BCM-preservation effect of canagliflozin, and demonstrated its potential for longitudinal evaluation of BCM.

Blocking of Glucagonlike Peptide-1 Receptors in the Exocrine Pancreas Improves Specificity for β-Cells in a Mouse Model of Type 1 Diabetes

The diabetes community has long desired an imaging agent to quantify the number of insulin-secreting β-cells, beyond just functional equivalents (insulin secretion), to help diagnose and monitor early stages of both type 1 and type 2 diabetes mellitus. Loss in the number of β-cells can be masked by a compensatory increase in function of the remaining cells. Since β-cells form only about 1% of the pancreas and decrease as the disease progresses, only a few imaging agents, such as exendin, have demonstrated clinical potential to detect a drop in the already scarce signal. However, clinical translation of imaging with exendin has been hampered by pancreatic uptake that is higher than expected in subjects with long-term diabetes who lack β-cells. Exendin binds glucagonlike peptide-1 receptor (GLP-1R), previously thought to be expressed only on β-cells, but recent studies report low levels of GLP-1R on exocrine cells, complicating β-cell mass quantification. Methods: Here, we used a GLP-1R knockout mouse model to demonstrate that exocrine binding of exendin is exclusively via GLP-1R (∼1,000/cell) and not any other receptor. We then used lipophilic Cy-7 exendin to selectively preblock exocrine GLP-1R in healthy and streptozotocin-induced diabetic mice. Results: Sufficient receptors remain on β-cells for subsequent labeling with a fluorescent- or ¹¹¹In-exendin. Conclusion: Selective GLP-1R blocking, which improves contrast between healthy and diabetic pancreata and provides a potential avenue for achieving the long-standing goal of imaging β-cell mass in the clinic.

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.

Squalene-PEG-Exendin as High-Affinity Constructs for Pancreatic Beta-Cells

Novel drug delivery systems targeting native, transplanted, or cancerous beta-cells are of utmost importance. Herein, we present new exendin-4 derivatives with modified unnatural amino acids at strategic positions within the polypeptide sequence. The modified peptides allowed modular orthogonal chemical modifications to attach imaging agents and amphiphilic squalene-PEG groups. The resulting conjugates, SQ-PEG-ExC₁-Cy5 and SQ-PEG-ExC₄₀-Cy5 fluorescence probes, display low nanomolar affinity to GLP-1R in fluorescence-based binding assays with EC₅₀ at 1.1 ± 0.2 and 0.8 ± 0.2 nM, respectively. Naturally expressing GLP-1R MIN6 cells and recombinantly transfected CHL-GLP-1R positive cells were specifically targeted by all of the new beta-cell probes in vitro. Specific islet targeting was observed after i.v. injection of SQ-PEG-ExC₁-Cy5 with SQ-PEG in normoglycemic mice ex vivo. Semiquantitative biodistribution analysis by epifluorescence indicated prolonged blood half-life (3.8 h) for the amphiphilic Ex conjugate. Liver and pancreas were identified as main biodistribution organs for SQ-PEG-ExC₁-Cy5.

Ablation of somatostatin cells leads to impaired pancreatic islet function and neonatal death in rodents

The somatostatin (SST)-secreting cells were mainly distributed in the pancreatic islets, brain, stomach and intestine in mammals and have many physiological functions. In particular, the SST-secreting δ cell is the third most common cell type in the islets of Langerhans. Recent studies have suggested that dysregulation of paracrine interaction between the pancreatic δ cells and β cells results in impaired glucose homeostasis and contributes to diabetes development. However, direct evidence of the functional importance of SST cells in glucose homeostasis control is still lacking. In the present study, we specifically ablated SST-secreting cells by crossing Sst-cre transgenic mice with R26 ^DTA mice (Sst ^Cre R26 ^DTA ). The Sst ^Cre R26 ^DTA mice exhibited neonatal death. The life spans of these mice with severe hypoglycemia were extended by glucose supplementation. Moreover, we observed that SST cells deficiency led to increased insulin content and excessive insulin release, which might contribute to the observed hypoglycemia. Unexpectedly, although SST is critical for the regulation of insulin content, factors other than SST that are produced by pancreatic δ cells via their endogenous corticotropin-releasing hormone receptor 2 (CRHR2) activity play the main roles in maintaining normal insulin release, as well as neonatal glucose homeostasis in the resting state. Taken together, our results identified that the SST cells in neonatal mouse played critical role in control of insulin release and normal islet function. Moreover, we provided direct in vivo evidence of the functional importance of the SST cells, which are essential for neonatal survival and the maintenance of glucose homeostasis.

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.

Improved Quantification of the Beta Cell Mass after Pancreas Visualization with 99mTc-demobesin-4 and Beta Cell Imaging with 111In-exendin-3 in Rodents

OBJECTIVE

Accurate assessment of the ¹¹¹In-exendin-3 uptake within the pancreas requires exact delineation of the pancreas, which is highly challenging by MRI and CT in rodents. In this study, the pancreatic tracer ^99mTc-demobesin-4 was evaluated for accurate delineation of the pancreas to be able to accurately quantify ¹¹¹In-exendin-3 uptake within the pancreas.

METHODS

Healthy and alloxan-induced diabetic Brown Norway rats were injected with the pancreatic tracer ^99mTc-demobesin-4 ([^99mTc-N₄-Pro¹,Tyr⁴,Nle¹⁴]bombesin) and the beta cell tracer ¹¹¹In-exendin-3 ([¹¹¹In-DTPA-Lys⁴⁰]exendin-3). After dual isotope acquisition of SPECT images, ^99mTc-demobesin-4 was used to define a volume of interest for the pancreas in SPECT images subsequently the ¹¹¹In-exendin-3 uptake within this region was quantified. Furthermore, biodistribution and autoradiography were performed in order to gain insight in the distribution of both tracers in the animals.

RESULTS

^99mTc-demobesin-4 showed high accumulation in the pancreas. The uptake was highly homogeneous throughout the pancreas, independent of diabetic status, as demonstrated by autoradiography, whereas ¹¹¹In-exendin-3 only accumulates in the islets of Langerhans. Quantification of both ex vivo and in vivo SPECT images resulted in an excellent linear correlation between the pancreatic uptake, determined with ex vivo counting and ¹¹¹In-exendin-3 uptake, determined from the quantitative analysis of the SPECT images (Pearson r = 0.97, Pearson r = 0.92).

CONCLUSION

^99mTc-demobesin-4 shows high accumulation in the pancreas of rats. It is a suitable tracer for accurate delineation of the pancreas and can be conveniently used for simultaneous acquisition with ¹¹¹In labeled exendin-3. This method provides a straightforward, reliable, and objective method for preclinical beta cell mass (BCM) quantification with ¹¹¹In-exendin-3.