Visualizing pancreatic β-cell mass with [¹¹C]DTBZ

Abstract

β-Cell mass (BCM) influences the total amount of insulin secreted, varies by individual and by the degree of insulin resistance, and is affected by physiologic and pathologic conditions. The islets of Langerhans, however, appear to have a reserve capacity of insulin secretion and, overall, assessments of insulin and blood glucose levels remain poor measures of BCM, β-cell function and progression of diabetes. Thus, novel noninvasive determinations of BCM are needed to provide a quantitative endpoint for novel therapies of diabetes, islet regeneration and transplantation. Built on previous gene expression studies, we tested the hypothesis that the targeting of vesicular monoamine transporter 2 (VMAT2), which is expressed by β cells, with [¹¹C]dihydrotetrabenazine ([¹¹C]DTBZ), a radioligand specific for VMAT2, and the use of positron emission tomography (PET) can provide a measure of BCM. In this report, we demonstrate decreased radioligand uptake within the pancreas of Lewis rats with streptozotocin-induced diabetes relative to their euglycemic historical controls. These studies suggest that quantitation of VMAT2 expression in β cells with the use of [¹¹C]DTBZ and PET represents a method for noninvasive longitudinal estimates of changes in BCM that may be useful in the study and treatment of diabetes.

Introduction

β-Cell mass (BCM) in the pancreas is a key factor in determining how much insulin can be secreted for the maintenance of normal blood glucose concentrations. At present, information regarding BCM is inferred from blood measurements of stimulated insulin production. Such measurements, however, are insensitive to certain types of β-cell dysfunction due to metabolic stress, as well as to changes in BCM that occur early in diabetic disease and affect the reserve capacity of β-cell function. Histological assessment of BCM in humans has been limited to autopsy studies [1], as the pancreas is not an ideal organ for biopsy. Noninvasive BCM measurements, based on targets other than insulin, have the potential to provide real-time information on the progression and treatment of diabetes.

Type 1 diabetes (T1D) is a result of the autoimmune destruction of the insulin-producing β cells of the islets of Langerhans—the endocrine component of the pancreas [2]. This disease has an insipid beginning and may take years before it can be recognized as clinical hyperglycemia. While it is traditionally thought that the majority of BCM is destroyed at the time of presentation with diabetes, several recent studies have suggested that there may be significant residual insulin-secretory capacity on diagnosis [3]. Moreover, there is a long preclinical period during which an immunologic assault is believed to occur on the islets of Langerhans and that hyperglycemia only develops when a critical mass of β cells is lost and insulin requirement increases.

The natural history of T1D is progression to complete elimination of insulin-secretory capacity and dependence on exogenous insulin for survival. However, it has not been possible to accurately determine the BCM that is present in individuals with diabetes and, therefore, conclusions about the natural history, as well as the effects, of new treatments on this process are based on indirect evidence. Similarly, a number of abnormalities in the insulin-producing capacity of the pancreas have been described for patients with Type 2 diabetes (T2D) [4], but there is currently no method of measuring BCM that differentiates functional versus anatomical defects in insulin secretion in this form of the disease.

A variety of experimental treatments have been developed to treat T1D, including immunotherapy, stem cell therapy and islet transplantation. The treatment of T2D has been largely empirical due to the lack of understanding of the basic mechanisms that are at work in the disease. An understanding of how BCM changes during the various phases of diabetes may provide important information on the development of new therapies for intervention strategies in both T1D and T2D.

Progress towards imaging the diseases of the endocrine pancreas has been described in several studies. Clark et al. [5] demonstrated that the body of the pancreas can be imaged with fluorine-18 4-fluorobenzyltrozamicol, a radioligand that binds to specific neuroreceptors (vesicular acetylcholine transporters) that are present on presynaptic vesicles in neurons innervating the pancreas. Similarly, taking advantage of bicarbonate and/or organic anion transporters expressed by pancreatic acinar cells, [¹¹C]acetate has been used to visualize the exocrine pancreas [6], [7]. In addition, animal studies show that 2-[¹⁸F]fluoro-2-deoxy-d-glucose may be useful in imaging recently transplanted islets [8]. Markmann et al. [9] recently reported that transplanted cadaveric islets, 14 months posttransplantation, induce peri-islet cell mass fat deposits that are visible by chemical shift gradient-echo magnetic resonance imaging (MRI). A possible problem with this approach is that peri-islet steatosis is likely to persist, at least for a few days, following islet allograft rejection, and the method is not suitable for imaging islets in situ.

Other previous attempts to image β cells and T1D-related pathology include studies by Moore et al. [10], [11], [12]. Using a β-cell-specific anti-IC2 monoclonal antibody (mAb) modified with a radioisotope chelator, normal and diabetic rodent pancreata were imaged ex vivo. Radioimmunoscintigraphy showed major differences in the pancreatic uptake of mAb between normal and diabetic rodents [10], but it was unclear if the method was suitable for in vivo imaging. Radioimmunoscintigraphy with antiganglioside mAbs has been less promising [13].

In other studies, the uptake 6-deoxy-6-[¹²⁵I]iodo-d-glucose by pancreata from normal versus streptozotocin (STZ)-injected rats has been compared. Although islets and acinar tissues showed differential uptake of the radioligand and although β-cell-depleted pancreata showed decreased uptake, the clinical utility of this approach is unclear because of the broad specificity of radioligand binding and high uptake in the liver [14]. The pancreatic uptake of a tracer [2-(14)C]alloxan has been studied in normal and STZ-treated rodents. The preferential uptake of radiotracers in a normal pancreas versus a diabetic pancreas has been demonstrated. Alloxan, however, is a well-known diabetogenic agent itself; thus, the clinical utility of this approach remains unproven [15]. Dithizone and sulfonylurea receptor ligands (e.g., ³H glibenclamide) have been studied as possible imaging agents [16], but some show broad tissue distributions of uptake contraindicating feasibility [17], [18], [19], [20].

The use of MRI has been explored in experimental insulitis. Moore et al., using superparamagnetic-particle-labeled T cells (via a CLIO-Tat peptide or major histocompatibility complex tetramer peptide complexes), were able to clearly demonstrate the presence of infiltrating T cells during the evolution of β-cell destruction [10], [11]. MRI has also been used to visualize peri-islet vascular leakage due to insulitis using superparamagnetic nanobeads [21].

Despite different embryological origins, β cells of the endocrine pancreas and neurons share expression of a large number of gene products and display many functional similarities. Previous studies, at both protein and nucleic acid levels, have shown the underlying physiochemical basis for this functional similarity [22], [23], [24]. Our gene expression mapping studies have led us to focus on one such shared gene product, vesicular monoamine transporter 2 (VMAT2; also known as SLC18A2), which is expressed by β cells but is absent from the exocrine pancreas and in a variety of abdominal organs [25]. A specific ligand for VMAT2, dihydrotetrabenazine (DTBZ), is already in clinical use for the positron emission tomography (PET) imaging of central nervous system (CNS) disorders [26]. We studied the binding of [³H]DTBZ to total membrane fractions prepared from purified human islets and purified exocrine pancreas tissues. We found that [³H]DTBZ specifically bound to islet membranes but not to membranes from the exocrine pancreas. Immunohistochemistry further showed that anti-VMAT2 and insulin immunoreactivity colocalized in islet β cells [20], [27], [28], [29], [30]. In this study, we tested whether [¹¹C]DTBZ could be used to image the endocrine pancreas in vivo and whether PET imaging with this radioligand could discriminate euglycemic rats from rats with diabetes induced by STZ.