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.

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

The structure of glibenclamide in the solid state

The structure of glibenclamide, 5-chloro-N-(2-{4-[(cyclohexylamino)carbonyl] aminosulfonyl}phenyl) ethyl)-2-methoxybenzamide, an important antidiabetic drug, has been studied both in solution and in the solid state by a combination of NMR spectroscopy and theoretical calculations. The possibility that glibenclamide suffers a tautomerization under melting to afford a desmotrope was rejected.

Radionuclide probes for molecular imaging of pancreatic beta-cells

Islet transplantation is a promising treatment option for patients with type 1 diabetes (T1D); however, the fate of the graft over time remains difficult to follow, due to the lack of available tools capable of monitoring graft rejection and inflammation prior to islet graft loss. Due to the challenges imposed by the location of the pancreas and the sparsely dispersed beta-cell population within the pancreas, currently, the clinical verification of beta-cell abnormalities can only be obtained indirectly via metabolic studies, which typically is not possible until after a significant deterioration in islet function has already occurred. The development of non-invasive imaging methods for the assessment of the pancreatic beta-cells, however, offers the potential for the early detection of beta-cell dysfunction prior to the clinical onset of T1D and type 2 diabetes (T2D). Ideal islet imaging agents would have an acceptable residence time in the human body, be capable of providing high-resolution images with minimal uptake in surrounding tissues (e.g., the liver), would not be toxic to islets, and would not require pre-treatment of islets prior to transplantation. A variety of currently available imaging techniques, including magnetic resonance imaging (MRI), bioluminescence imaging (BLI), and nuclear imaging have been tested for the study of beta-cell diseases. In this article, we summarize the recent advances made in nuclear imaging techniques for non-invasive imaging of pancreatic beta-cells. The use of radioactive probes for islet imaging is also discussed.

Synthesis and evaluation of (S)-2-(2-[18F]fluoroethoxy)-4-([3-methyl-1-(2-piperidin-1-yl-phenyl)-butyl-carbamoyl]-methyl)-benzoic acid ([18F]repaglinide): a promising radioligand for quantification of pancreatic beta-cell mass with positron emission tomography (PET)

18F-labeled non-sulfonylurea hypoglycemic agent (S)-2-(2-[(18)F]fluoroethoxy)-4-((3-methyl-1-(2-piperidin-1-yl-phenyl)-butylcarbamoyl)-methyl)-benzoic acid ([(18)F]repaglinide), a derivative of the sulfonylurea-receptor (SUR) ligand repaglinide, was synthesized as a potential tracer for the non-invasive investigation of the sulfonylurea 1 receptor status of pancreatic beta-cells by positron emission tomography (PET) in the context of type 1 and type 2 diabetes. [(18)F]Repaglinide could be obtained in an overall radiochemical yield (RCY) of 20% after 135 min with a radiochemical purity higher than 98% applying the secondary labeling precursor 2-[(18)F]fluoroethyltosylate. Specific activity was in the range of 50-60 GBq/micromol. Labeling was conducted by exchanging the ethoxy-moiety into a 2-[(18)F]fluoroethoxy group. To characterize the properties of fluorinated repaglinide, the affinity of the analogous non-radioactive (19)F-compound for binding to the human SUR1 isoform was assessed. [(19)F]Repaglinide induced a complete monophasic inhibition curve with a Hill coefficient close to 1 (1.03) yielding a dissociation constant (K(D)) of 134 nM. Biological activity was proven via insulin secretion experiments on isolated rat islets and was comparable to that of repaglinide. Finally, biodistribution of [(18)F]repaglinide was investigated in rats by measuring the concentration of the compound in different organs after i.v. injection. Pancreatic tissue displayed a stable accumulation of approximately 0.12% of the injected dose from 10 min to 30 min p.i. 50% of the radioactive tracer could be displaced by additional injection of unlabeled repaglinide, indicating that [(18)F]repaglinide might be suitable for in vivo investigation with PET.

Synthesis and evaluation of fluorine-18 labeled glyburide analogs as beta-cell imaging agents

Glyburide is a prescribed hypoglycemic drug for the treatment of type 2 diabetic patients. We have synthesized two of its analogs, namely N-[4-[beta-(2-(2'-fluoroethoxy)-5-chlorobenzenecarboxamido)ethyl]benzenesulfonyl]-N'-cyclohexylurea (2-fluoroethoxyglyburide, 8b) and N-[4-[beta-(2-(2'-fluoroethoxy)-5-iodobenzenecarboxamido)ethyl]benzenesulfonyl]-N'-cyclohexylurea (2-fluoroethoxy-5-deschloro-5-iodoglyburide, 8a), and their fluorine-18 labeled analogs as beta-cell imaging agents. Both F-18 labeled compound 8a and compound 8b were synthesized by alkylation of the corresponding multistep synthesized hydroxy precursor 4a and 4b with 2-[(18)F]fluoroethyl tosylate in DMSO at 120 degrees C for 20 minutes followed by HPLC purification in an overall radiochemical yield of 5-10% with a synthesis time of 100 minutes from EOB. The octanol/water partition coefficients of compounds 8a and 8b were 141.21 +/- 27.77 (n = 8) and 124.33 +/- 21.61 (n = 8), respectively. Insulin secretion experiments of compounds 8a and 8b on rat islets showed that both compounds have a similar stimulating effect on insulin secretion as that of glyburide. In vitro binding studies showed that approximately 2% of compounds 8a and 8b bound to beta TC3 and Min6 cells and that the binding was saturable. Preliminary biodistribution studies in mice showed that the uptake of both compounds 8a and 8b in liver and small intestine were high, whereas the uptake in other organs studied including pancreas were low. Additionally, the uptake of compound 8b in vivo was nonsaturable. These results tend to suggest that compounds 8a and 8b may not be the ideal beta-cell imaging agents.