Low kidney uptake of GLP-1R-targeting, beta cell-specific PET tracer, 18F-labeled [Nle14,Lys40]exendin-4 analog, shows promise for clinical imaging
© The Author(s). 2016
Received: 20 September 2016
Accepted: 29 November 2016
Published: 13 December 2016
Several radiometal-labeled, exendin-based tracers that target glucagon-like peptide-1 receptors (GLP-1R) have been intensively explored for β cell imaging. The main obstacle has been the high uptake of tracer in the kidneys. This study aimed to develop a novel GLP1-R-specific tracer, with fluorine-18 attached to exendin-4, to label β cells for clinical imaging with PET (positron emission tomography). We hypothesized that this tracer would undergo reduced kidney uptake. 18F-labeled [Nle14,Lys40]exendin-4 analog ([18F]exendin-4) was produced via Cu-catalyzed click chemistry. The biodistribution of [18F]exendin-4 was assessed with ex vivo organ γ-counting and in vivo PET imaging. We also tested the in vivo stability of the radiotracer. The localization of 18F radioactivity in rat and human pancreatic tissue sections was investigated with autoradiography. Receptor specificity was assessed with unlabeled exendin-3. Islet labeling was confirmed with immunohistochemistry. The doses of radiation in humans were estimated based on biodistribution results in rats.
[18F]exendin-4 was synthesized with high yield and high specific activity. Results showed specific, sustained [18F]exendin-4 uptake in pancreatic islets. In contrast to previous studies that tested radiometal-labeled exendin-based tracers, we observed rapid renal clearance of [18F]exendin-4.
[18F]exendin-4 showed promise as a tracer for clinical imaging of pancreatic β cells, due to its high specific uptake in native β cells and its concomitant low kidney radioactivity uptake.
KeywordsPancreas β cell Exendin PET 18F
The determination of β cell mass is a challenge, because the target is a diffuse collection of cell clusters, highly dispersed throughout the pancreas. Indeed, the β cell mass constitutes less than 2% of the total pancreas mass, and this mass is likely to decrease during the course of diabetes. The ongoing development of therapeutic approaches to diabetes, whether based on pharmacology or islet transplantation, also calls for the development of methods for longitudinal in vivo assessments of β cell mass and function. There is an unmet need for an imaging agent that offers potential for clinical trial applications.
Glucagon-like peptide-1 (GLP-1) is secreted from the intestine in response to elevated blood glucose levels. GLP-1 binds to specific receptors on pancreatic islets to stimulate insulin secretion. Exendin-4 is a long-acting agonist of the glucagon-like peptide-1 receptor (GLP-1R); it mimics the actions of short-lived GLP-1 (plasma half-life <2 min). Several efforts have been made to label pancreatic β cells with exendin-based tracers that carry radiometals, such as 99mTc and 111In for SPECT imaging [1, 2] and 64Cu and 68Ga for PET imaging . However, high radioactivity concentrations were observed in the kidney. During imaging, this high radioactivity compromises tissue visualization in the kidney region, limits quantitative imaging, and reduces diagnostic accuracy. Because the kidney is a radiosensitive organ, the radiation burden might rapidly become unacceptably high with long-lived radioisotopes . Some success has been achieved in efforts to reduce the renal radioactivity of various radiolabeled peptides. For example, some studies co-administered amifostine, albumin derivatives, poly-glutamic acid, or Gelofusine [5–7] to limit renal uptake; and others used cleavable linkers, to allow excretion of radioactive metabolites into the urine [8, 9].
Most peptides are excreted via the kidneys, but studies have shown that persistent retention is characteristic for radiometal-labeled exendin derivatives . As an alternative, preclinical studies were conducted with 18F-labeled exendin-4; they showed high, sustained tracer uptake in INS-1 tumor cells and xenograft models . However, importantly, after initially high radioactivity levels in the kidneys, clearance from the kidneys was rapid, compared to radiometal-labeled analogs [12, 13]. Wu et al.  observed specific uptake of [18F]TTCO-Cys40-exendin-4 in islets transplanted into NOD/SCID mice but limited kidney retention. These results suggested a need for further development of [18F]exendin-4 analogs for clinical applications.
The present study aimed to develop a 18F-labeled [Nle14,Lys40]exendin-4 tracer ([18F]exendin-4) via copper-catalyzed click chemistry. This tracer could be clinically useful for imaging pancreatic β cells to calculate mass. To date, no optimal tracers are available for this application. The vesicular monoamine transporter type II (VMAT2) has been suggested as a target to calculate β cell mass, but so far issues with this approach remain controversial [15–17].
Precursors and radiochemistry
Aqueous [18F]fluoride was produced by proton bombardment of 18O-enriched water as reported previously . Synthesis started by allowing tosylate 1 to react with a [18F]fluoride-Kryptofix complex in anhydrous DMSO (Fig. 1). After purification by HPLC, the collected fraction was concentrated on an Oasis HLB cartridge (Waters, Milford, MA, USA). After elution of the cartridge with THF, the obtained [18F]fluoro-propargyl-triethylene glycol was allowed to react with peptidyl azide 2 in the presence of pre-activated copper sulfate/sodium ascorbate for 10 min at room temperature. Subsequently, the reaction mixture was diluted in water and purified by HPLC. The collected fraction was retained on a Sep-Pak C8 cartridge (Waters) and formulated in ethanol/saline. Radiochemical analysis of 18F-labeled [Nle14,Lys40]exendin-4 ([18F]exendin-4) was performed with HPLC and thin layer chromatography (TLC). The reaction details and a full characterization of the tracer will be reported elsewhere.
Experimental animals and tissue samples of human pancreas
Sprague–Dawley rats (285 ± 30 g, N = 43) were obtained from the Central Animal Laboratory, University of Turku. Finnish landrace pigs (33 ± 3 kg, N = 3) were obtained from a local farmer. The animal experiments were approved by the animal experiment board of the University of Turku and by the State Provincial Office of Southern Finland (ESAVI/3899/04.10.07/2013, ESAVI/823/04.10.07/2013). Human pancreatic tissue was obtained from one subject (woman, 36 years old), who had undergone a pancreatic resection (pancreatic mucinous neoplasm) at Turku University Hospital (Turku, Finland). Approval for the use of human pancreatic tissue was obtained from the Ethics Committee of the Hospital District of Southwest Finland (ETMK 42/180/2008). The subject provided written informed consent before participating in the study.
Small-animal PET imaging
The in vivo kinetics of [18F]exendin-4 were evaluated with the Inveon Multimodality PET/CT scanner (Siemens, Knoxville, TN, USA). Rats were anesthetized with isoflurane/O2 and injected intravenously (i.v.) with [18F]exendin-4 (radioactivity 23 ± 5 MBq/kg, mass 2 ± 1 μg/kg). PET scans were acquired for up to 6 h (dynamic 0–60 min; static 210–240 min and 330–360 min; N = 2–5 rats/scan). The data were reconstructed with the ordered subset expectation maximization OSEM2D algorithm. Analyses were performed with Inveon Research Workplace 3 software (Siemens), after defining the volumes of interest (VOIs) on selected tissues.
Ex vivo biodistribution
Rats were anesthetized with isoflurane/O2 and injected (i.v.) with [18F]exendin-4 (radioactivity 23 ± 5 MBq/kg, mass 2 ± 1 μg/kg). After injection, animals were allowed to recover from the anesthesia (except animal sacrificed 15 min p.i.). We further investigated tracer distribution in the brain by injecting one rat with a highly radioactive dose (50 MBq) of [18F]exendin-4. Animals were sacrificed at 15, 30, 60, 120, 240, and 360 min (N = 2–11 rats/time point). Blood was extracted, and selected organs were dissected and weighed. Tissue radioactivity was measured with a γ-counter (Wizard, PerkinElmer, Turku, Finland). Tracer specificity was tested by co-injecting radiolabeled exendin-4 with an excess of cold exendin-3 (3 mg/kg) (Peptide Specialty Laboratories GmbH). Measurements were corrected for radionuclide decay, and results were reported as the percentage of the injected dose of radioactivity per gram of tissue (%ID/g).
Ex vivo autoradiography
The kidney, pancreas, and brain were sectioned and exposed to imaging plates (Fuji BAS-TR2025; Fuji Photo Film Co., Tokyo, Japan) for approximately 4 h. Imaging plates were scanned with a BAS-5000 scanner (Fuji Photo Film Co.) and the images were analyzed with AIDA 4.5 software (Raytest, Isotopenmessgeräte, Straubenhardt, Germany). Three pancreatic sections per rat were analyzed (N = 2–8 rats/time point). We determined the mean density values (photostimulated luminescence per square millimeter, PSL/mm2) of ten prominently labeled islets and of the exocrine tissue and corrected for background noise. Finally, the islet-to-exocrine tissue ratio was calculated. This was done in order to be able to demonstrate the accumulation of radioactivity into the islets as a function of time and to be able to compare the islet-to-exocrine tissue ratios between animals of different time points. The pancreatic sections used for autoradiography were then stained with insulin antibody (1’ mouse-anti-insulin, Nordic Biosite, Täby, Sweden, 2’ MAHC1 mouse probe, Biocare Medical, Concord, CA, USA), counterstained with hematoxylin, and imaged with a Slide Scanner (Pannoramic 250 F, 3DHistech, Budapest, Hungary).
The metabolic stability of [18F]exendin-4 was evaluated by radio-HPLC, as reported previously . Briefly, samples were collected at 5, 15, 60, and 120 min post injection (p.i.; N = 1–3 samples/time point). Plasma and urine proteins were precipitated with equivalent volumes of acetonitrile and removed by centrifugation. Samples of the pancreas, kidney, and duodenum (pyloric sphincter) were homogenized in acetonitrile, purified by centrifugation, and the supernatants were analyzed with radio-HPLC.
In addition, the stability of [18F]exendin-4 was determined in pig plasma. Prior to blood sampling, animals were anesthetized (midazolam 1 mg/kg, xylazine 4 mg/kg intramuscular), connected to a respirator, and ventilated mechanically. Anesthesia was maintained with an i.v. infusion of propofol (10–20 mg/kg/h) and fentanyl (4–8 μg/kg/h). Vital signs were monitored throughout the study. Pigs were injected (radioactivity 1.2 ± 0.2 MBq/kg, mass 0.14 ± 0.04 μg/kg) and blood was sampled at 5, 15, 30, 60, and 180 min p.i. (N = 3 samples/time point). Plasma was treated and analyzed as described above.
In vitro [18F]exendin-4 binding in human and rat pancreas and rat brain
We acquired a piece of human pancreas that was frozen immediately after surgery. We dissected a rat pancreas and brain and froze them immediately after dissection. Prior to incubations with ligand, the tissue sections of human and rat pancreas and GLP-1R-rich areas from rat brain  were incubated for 30 min with 1% BSA in PBS (pH 7.4). Then, sections were incubated for 60 min with various concentrations of [18F]exendin-4 (range 2.5–20 nM) in vitro. To investigate tracer specificity, sections were incubated for 30 min with an excess of cold exendin-3 prior to adding [18F]exendin-4. Sections were then washed and exposed to imaging plates, as described above (ex vivo autoradiography). Next, sections were stained and processed, as described above.
Estimation of radiation doses for humans
Absorbed doses were calculated with OLINDA/EXM1.0 software [20, 21], which includes radionuclide information and a selection of human body phantoms. Rat ex vivo biodistributions were estimated by integrating the area under the time–activity curves. The obtained residence times were converted into corresponding human values by multiplying with organ-specific factors that scaled organs to body weights, as follows: (W TB,rat/W Organ,rat) × (W Organ,human/W TB,human), where W TB,rat and W TB,human are the total body weights of rats and humans (70-kg male), respectively; and W Organ,rat and W Organ,human are the organ weights of rats and humans (70-kg male), respectively.
Results are reported as the mean ± SD. The two-tailed, unpaired Student’s t test and one-way ANOVA were used for determining statistical differences between groups (GraphPad Prism 5.01, GraphPad Software, San Diego, CA, USA). Linear regression analysis was used to assess the relationship between the islet-to-exocrine tissue ratio and time. Values of P < 0.05 were considered statistically significant.
The synthesis of [18F]exendin-4 is depicted in Fig. 1. The copper(I)-catalyzed reaction between exendin-4-azide 2 and [18F]fluoro-propargyl-triethylene glycol proceeded rapidly and efficiently under mild reaction conditions. Technical details of the synthesis optimization studies will be reported elsewhere. The absence of major radioactive by-products ensured neat purification using HPLC. Total synthesis time was approximately 1.5 h. The average isolated yields of [18F]exendin-4 after HPLC purification ranged from 14 to 338 MBq (at the end of synthesis), when the starting amount of aqueous [18F]fluoride was 2.5–18 GBq. The specific activity for [18F]exendin-4 was 12–323 MBq/nmol at end of synthesis, with a radiochemical purity exceeding 90%. Identity of [18F]exendin-4 was confirmed by HPLC comparison test with the cold standard.
Ex vivo biodistribution and PET imaging
Analysis of tracer metabolites
In rat plasma, we detected unchanged [18F]exendin-4 and a more polar radioactive metabolite(s). The plasma radioactivity associated with intact [18F]exendin-4 at 60 min p.i. represented 28.3 ± 3.5% of the total radioactivity, and at 120 min p.i., this level decreased to 16%. At 60 min p.i., [18F]exendin-4 represented 20 ± 7.2% of the radioactivity in the pancreas, compared to 38 ± 3.5% of the radioactivity in the duodenum, and negligible levels of intact [18F]exendin-4 in the kidney and urine. Additionally, in pigs, the plasma radioactivity associated with [18F]exendin-4 at 5 min p.i. represented 87 ± 6% of the radioactivity, and at 60 min p.i., this level decreased to 77 ± 3%.
Effects of [18F]exendin-4 administration in pigs
In pigs, administration of [18F]exendin-4 raised the heart rate from 100–110 beats/min to 230–250 beats/min and body temperature from 37 to 41 °C. The blood pressure decreased, and arrhythmia was observed. Similar symptoms and effects of exendin on heart rate and blood pressure have been reported previously .
Intrapancreatic distribution of [18F]exendin-4 in rats
Tracer binding in tissue sections of human pancreas
Human radiation dosimetry estimates for 18F radioactivity extrapolated from the rat ex vivo data
Red bone marrow
Urinary bladder wall
This report described the synthesis of [18F]exendin-4 and its evaluation in rat and human pancreas. In this study, ex vivo γ-counting of rat whole pancreas and autoradiography analyses of pancreatic sections showed specific and sustained uptake of [18F]exendin-4 in the islets. In vitro labeling of human pancreas revealed specific targeting of the islets and co-localization with insulin, detected with immunohistochemistry. Renal clearance of [18F]exendin-4 was rapid compared to radiometal-labeled exendin derivatives [2, 3, 24, 25].
18F-labeled target-specific peptides are ideal in vivo imaging agents, because 18F is readily available from most small medical cyclotrons, and it has a half-life (110 min) that matches the pharmacokinetics of most peptides. Moreover, 18F decays with a 96% positron branching ratio, and it has a short β+ trajectory (E βmax = 635 keV), which enables the acquisition of high-resolution PET images. Previous reports have described various approaches for the radiofluorination of exendin-4 analogs, which utilize maleimide- or aldehyde-containing prosthetic reagents, and chelating agents [13, 26–28]. However, the main challenge is designing an efficient strategy for preparing radiotracers with very high specific activity, which would render in vivo imaging of native β cells. Click chemistry has generated increasing attention due to its rapid and high-yielding transformation, and tolerance to a broad range of functional groups. These properties make click chemistry one of the preferred strategies in developing 18F-labeled radiopharmaceuticals [14, 29, 30]. The original copper-catalyzed variant of the Huisgen 1,3-dipolar cycloaddition of terminal alkynes and azides features very high reaction rates [31, 32] and has prompted us to advance this reaction into the design of a potential clinical grade tracer for native β cell imaging. It was shown previously that the binding affinity (IC50) of native exendin-4 for GLP-1 receptor was not significantly altered by modification/addition of a fluorinated linker [14, 33]. Noteworthy, the different fluorination strategies did not affect the binding affinity of the cold ligand towards the target receptor, compared to native exendin-4.
The GLP-1R is expressed specifically in pancreatic β cells [34, 35]; little to no expression was found on other pancreatic islet cell types . Because GLP-1R expression levels are low, a sensitive radiotracer with high specific activity is required to avoid receptor saturation with unlabeled peptide . The number of GLP-1Rs in native β cells and transplanted islets is restricted compared to insulinomas, which overexpress the receptor. In this study, [18F]exendin-4 was produced with a specific activity as high as 323 MBq/nmol and an injected mass of 2 ± 1 μg/kg. In humans, the pharmacological dose of exenatide (Byetta®) is low (10 μg BID), and in animal studies, exendin-4 was a potent glucose-lowering agent (dose 0.001 to 10 μg) . These properties of high specific activity and low injected radiotracer mass must be considered carefully. With doses of 7–25 μg of [68Ga]NOTA-exendin-4, subjects experienced palpitation, vomiting, and nausea after tracer injection .
In previous studies, the levels of pancreatic uptake of other labeled exendin-based tracers were similar at 60 min p.i. (Fig. 2b). Nevertheless, [18F]exendin-4 accumulation in the pancreas was significantly higher than that of the 64Cu- and 68Ga-labeled counterparts . Brom et al.  showed that the [111In]DTPA-exendin-4 SPECT tracer was taken up by the pancreas at a higher rate than the PET ligands.
Kastin et al.  reported that exendin-4 crossed the blood–brain barrier in rat. In our studies, GLP-1R-rich areas were observed in in vitro studies of rat brain sections (Fig. 7). However, ex vivo autoradiography, tissue γ-counting, and PET imaging did not reveal any specific binding in rat brain. These results indicated that [18F]exendin-4 did not cross the blood–brain barrier.
Our stability study results demonstrated that the native (unmetabolized) tracer was recovered in significantly higher amounts in pig plasma compared to rat plasma. However, in rats [18F]exendin-4 was rapidly cleared from the blood, and low amounts were taken up in the liver. In contrast, when 18F-labeled GLP-1R targeting radiotracers were investigated in xenograft models, substantial liver uptake was observed [11, 13, 14, 28]. On the other hand, Yue et al.  reported moderate liver retention of [18F]FNEM-[Cys40]-exendin-4 in a xenograft model. High liver uptake might restrict the clinical use of these tracers, because the liver is the site of insulinoma metastasis, and it is also the primary site for islet transplantation. Low tracer accumulation in the liver enables the high target-to-background contrast required for high-quality PET imaging.
In this study, PET imaging and organ radioactivity measurements demonstrated high [18F]exendin-4 uptake in the kidneys soon after injection, but thereafter, clearance was rapid and retention decreased (Fig. 3). In contrast, the renal excretion route and proximal tubular reabsorption cause high retention of metal-labeled exendin tracers in the kidney cortex. At the 60-min time point, kidney uptake of [18F]exendin-4 was significantly lower than the uptake of the metal-labeled counterparts. Furthermore, at several hours p.i., high levels of both [111In]DTPA-exendin-4 [2, 41] and [64Cu]NODAGA-exendin-4  had accumulated at in the kidneys, and [111In]DTPA-exendin-4 showed only minor clearance over time. Consequently, repeated scanning in patients could cause radiation doses that exceed the tolerance of this delicate organ [4, 42]. Melis et al.  showed that high radiation doses of [111In]DTPA-Lys40-exendin-4 caused nephrotoxicity in mice. Several approaches have been tested to overcome high uptake of exendin-based tracers in the kidneys, including co-administration of competitive inhibitors of reabsorption [5–7], addition of metabolizable linkers [8, 9], or modifications in the size, charge, or structure of the tracer [44, 45]. Those attempts have been crucial in finding ways to improve the imaging accuracy, decrease the risk of nephrotoxicity, and improve the efficacy of peptide receptor radionuclide therapy.
Absorbed kidney dose and effective dose of 18F-labeled tracer and metal-labeled exendin PET and SPECT tracers
We found that in vivo injections of the β cell-specific PET tracer, [18F]exendin-4, resulted in specific, sustained tracer uptake in rat pancreatic islets. Additionally, we demonstrated specific uptake of [18F]exendin-4 in human pancreatic tissues in vitro. Unlike its metal-labeled counterparts, [18F]exendin-4 renal clearance was rapid. These properties of [18F]exendin-4 showed promise for its future development for use in clinical imaging of β cells.
Bis in die
Bovine serum albumin
Glucagon-like peptide-1 receptor
High-performance liquid chromatography
Positron emission tomography
Thin layer chromatography
Volume of interest
We acknowledge the technical assistance of the MediCity Research Laboratory and the PET laboratory.
Financial support was received from the EC 7th Framework Program BetaImage (FP7/2007–2013, grant agreement 222980) and BetaCure (FP7/2014–2018, grant agreement 602812), Novo Nordisk Foundation, The Finnish Diabetes Research Foundation, The Finnish Cultural Foundation, and Maud Kuistila Memorial Foundation. This study was conducted within the Finnish Centre of Excellence in Cardiovascular and Metabolic Diseases supported by the Academy of Finland, University of Turku, Turku University Hospital, and Åbo Akademi University.
KM contributed to the design of the study, acquired, analyzed, and interpreted the animal studies and studies with human pancreas, and wrote the manuscript. OS, CBY, and PL performed the development and radiochemical synthesis of [18F]exendin-4. PL and CBY performed the analyses of synthesis results. CBY contributed to the design of the study and writing process of the manuscript and performed the metabolite assay and analysis of the results. SK performed the surgeries and collection of samples of human pancreas. MK contributed to the studies performed with pigs. TT performed the dosimetry calculations. OS coordinated the radiochemistry and tracer production. PN and OS contributed to the design and coordination of the study, revised the manuscript, and gave final approval for the published version. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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