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

https://doi.org/10.1016/j.nucmedbio.2003.12.003Get rights and content

Abstract

Glyburide is a prescribed hypoglycemic drug for the treatment of type 2 diabetic patients. We have synthesized two of its analogs, namely N-{4-[β-(2-(2′-fluoroethoxy)-5-chlorobenzenecarboxamido)ethyl]benzenesulfonyl}-N′-cyclohexylurea (2-fluoroethoxyglyburide, 8b) and N-{4-[β-(2-(2′-fluoroethoxy)-5-iodobenzenecarboxamido)ethyl]benzenesulfonyl}-N′-cyclohexylurea (2-fluoroethoxy-5-deschloro-5-iodoglyburide, 8a), and their fluorine-18 labeled analogs as β-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-[18F]fluoroethyl tosylate in DMSO at 120°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 ∼2% of compounds 8a and 8b bound to βTC3 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 β-cell imaging agents.

Introduction

Diabetes mellitus is a major public health problem, affecting more than 5% of the world's population and 25% of persons over 60 years of age. Diabetes mellitus comprises a heterogeneous group of disorders characterized by high blood glucose levels. Two major types of diabetes mellitus have been defined: type 1 and type 2. Although hyperglycemia is the common denominator of both type 1 and type 2 diabetes, the etiology and syndromes are distinctly different. Type 1 accounts for about 10% of all diabetes; it is a chronic autoimmune disease characterized by the selective destruction of insulin-producing β-cells of the pancreas, leading to a near total deficiency in insulin secretion. When autoimmune destruction affects more than 90% of the β-cells mass, the resulting insulin deficiency culminates into the development of overt hyperglycemia. In contrast, type 2 is the most common form of diabetes, accounting for more than 90% of cases; it is a chronic, progressive metabolic disorder of carbohydrate and lipid metabolism. Type 2 diabetes is caused by two physiological defects: resistance to the action of insulin, combined with a deficiency in insulin secretion [1], [2]. There are two types of drug therapy for type 2 diabetes mellitus: 1) oral agents (sulfonylureas, biguanides, thiazolidinediones, α-glucosidase inhibitors, meglitinide analogs); and 2) parenteral agents (insulin, insulin analogs, amylin agonists, glucagon-like peptide, glucagon antagonists) [3]. Fewer than half of all diabetics receive treatment, and of these only a very small proportion achieve a level of glucose control that is sufficient to avoid the morbidity associated with the disease. To date, there have been no reported techniques to image the endocrine pancreas. The basis of our inability to image the endocrine pancreas has been due to the unavailability of a marker specific for islet β-cells. In the context of type 1 diabetes mellitus, the chronic and progressive loss of β-cells resulting from autoimmune destruction has led to concerted efforts to prevent further loss of β-cells by autoantigen-specific immunotherapy of prediabetic patients [4]. In addition, a number of novel strategies for therapy of diabetes mellitus are based on replication of β-cells and islet transplantation. Since we have an ongoing islet transplantation program [5], [6], it is of considerable importance to have a reliable noninvasive method to monitor the progressive loss of β-cells mass during the silent phase of prediabetes or transplanted islet mass.

Positron emission tomography (PET) coupled with appropriate radiotracers has the unique capability of noninvasively measuring biochemical and metabolic processes. Sulfonylureas are antidiabetic agents that block pancreatic ATP-sensitive potassium channels, located at the insulin-producing β cells of the islets of Langerhans, either directly or via a plasma membrane-associated protein, resulting in an increase of intracellular calcium ion and consequent insulin secretion [7], [8]. Therefore, if sulfonylureas were labeled with a positron emitter, they may serve as β-cell imaging agents. Tolbutamide and glyburide are hypoglycemic drugs that bind to sulfonylurea receptors (SUR) [9], [10], [11] in HIT-β cells with a wide ranges of affinities (Ki= 25-55 μmol/L [12] and 0.7-7 nmol/L [13] for tolbutamide and glyburide, respectively). The uptake of [3H]glyburide has been shown to be proportional to the number of β-cells and is saturable [14]. The Bmax of SUR in the pancreatic β-cells and mouse islets are 1,400-1,600 fmol/mg protein [15]. In addition, the fluoro analog of tolbutamide, 1-[(p-fluorobenzenesulfonyl)]-3-butylurea, was reported to have a similar hypoglycemic potency as tolbutamide [16]. Therefore, we have synthesized 1-(4-(2-[18F]fluoroethoxy)-benzenesulfonyl)-3-butylurea [17], 1-[(p-[18F]fluorobenzenesulfonyl)]-3-butylurea (p-desmethyl-p[18F]fluorotolbutamide) and N-{4-[β-(2-(2′- [18F] fluoroethoxy-5-chlorobenzenecarboxamido)ethyl]benzenesulfonyl}-N′-cyclohexylurea ([18F]fluoroethoxyglyburide, 8b) as potential β-cell imaging agents [18]. We report herein the synthesis of another glyburide analog, N-{4-[β-(2-(2′-fluoroethoxy)-5-iodobenzenecarboxamido)ethyl]benzenesulfonyl}-N′-cyclohexylurea (2-fluoroethoxy-5-deschloro-5-iodoglyburide, 8a) and the evaluation of these two glyburide analogs (8a and 8b) (Fig. 1) as potential β-cell imaging agents. Part of this study has previously appeared in abstract form [19].

Section snippets

Methods and materials

Ethylene glycol di-p-tosylate, 5-chlorosalicyclic acid, 5-iodosalicyclic acid, 1-bromo-2-fluoroethane, ethyl chloroformate, 4-(2-aminoethyl)benzenesulfonamide, cyclohexyl isocyanate, copper(I) chloride and borontrifluoride etherate were purchased from Aldrich Chemical Company (Milwaukee, WI) and used without further purification. C18 Sep-Pak cartridges were obtained from Waters Chromatography Division, Millipore Corporation (Milford, MA). Radioactivity was determined using a calibrated ion

Results and discussion

To date, there have been no reported techniques to image the endocrine pancreas. Recently, several radioactive compounds, such as 65Zn [22], [3H]mitiglinide [23], a 125I-labeled mouse monoclonal antibody directed against pancreatic β-cell surface ganglioside(s) [24], and a 111In labeled monoclonal antibody specific for mouse pancreatic β-cells [25] have been used to visualize β-cells in vitro or in vivo, but none of them was successful or feasible for clinical application in humans. Glyburide,

Acknowledgements

The authors thank Mr. Harry J. White for providing [18F]fluoride. This work was partially supported by a pilot grant from the Society of Nuclear Medicine (C-Y S) and Juvenile Diabetes Foundation International (Ali Naji).

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