Current Pharmaceutical Design - Volume 16, Issue 14, 2010

When I was first confronted with the idea of in vivo imaging of beta cells, I was not sure whether this was meant seriously. For a nuclear medicine physician or a radiologist, imaging of tumors is part of the daily routine. Diabetes imaging seems far from the scope of clinically relevant imaging. If diabetes is diagnosed, it is usually done based on clinical findings such as pathological oral glucose tolerance. So, what would you need imaging for? Interestingly, since nearly two decades, a small group of people has been trying to find a way to measure the pancreatic beta cell mass or to follow the fate of transplanted islets in vivo. The National Institutes of Health have organized the first workshop on imaging of pancreatic beta cells in 1999, and 3 others have followed in 2003, 2006, and 2009 [1, 2]. In the past years, there have also been specific calls in the field of beta cell imaging from the National Institutes of Health, the Juvenile Diabetes Research Foundation, the European Union and several other grant-giving institutions. So, obviously there is a need for in vivo imaging of beta cells although it is not necessarily obvious to nondiabetologists. According to the WHO, there are currently 180 million people suffering from diabetes mellitus world wide (mostly type 2 diabetes (T2D)) with an expected doubling of this number until 2030 [3]. The WHO predicts that in upper- and middle-income countries the number of diabetes-related deaths will increase by 80% between 2006 and 2015. Despite these impressive numbers and although risk factors for the development of diabetes are well-known, the precise molecular mechanisms leading to the decrease in beta-cell mass responsible for the development of impaired glucose tolerance and diabetes still remain to be elucidated. Specifically, we do not know the natural history of betacell mass loss in human diabetes, nor do we have convincing evidence on beta-cell neogenesis and the preserved actual beta-cell mass in patients with impaired glucose tolerance - we are only able to determine the functional beta cell mass by glucose clamping, a rather invasive method. With the advent of innovative treatment regimens in T2D claiming to preserve or even increase the beta cell mass, it becomes clear that we are in need of reliable, sensitive, specific, and non-invasive methods for detection of living pancreatic beta-cells in vivo to validate these claims. Furthermore, such imaging technology might help to enhance our understanding of the pathophysiology of T2D as well as T1D. Apart from direct determination of beta-cell mass, such imaging techniques may also be useful for imaging of the response of beta-cells to conditions leading to beta-cell dysfunction and eventually apoptosis. In his article, Burkhard Goke from the University of Munich, Germany, summarizes the current challenges in diabetes research that could more efficiently be addressed with in vivo imaging methods [4]. Although the search for imaging technology to detect vital islets of Langerhans in vivo has begun early in the 1990ies, no imaging method is in clinical or research routine use to date. Paty et al. have summarized the results of the 2003 NIH workshop on beta cell imaging. Their final statement was that “Early efforts are underway to develop such means (i.e. technology for beta cell imaging) using a variety of techniques including MRI, PET, and optical imaging.”[5]. Obviously, clinical application of the available imaging methods for in vivo detection of beta cells were considered to still be premature. So, has there been progress in the field in the past years? Indeed, excellent imaging methods for in vivo imaging of insulinomas, beta-cell derived tumours, have currently become available. Kauhanen and colleagues give a concise overview over the current state-of-the-art in imaging of insulinomas and beta-cell hyperplasia and they focus on the impressive results that have been obtained with 18F-DOPA PET imaging [6]. However, imaging of tumours (or focal beta cell hyperplasia) is less challenging than imaging of physiological tissue that is not over-expressing the target that is binding the radiolabeled specific ligand.Even worse, in the case of the pancreatic beta cells, the target tissue is spread all over the organ in the tiny islets of Langerhans, contributing to not more than 1-2% of the total pancreatic mass-conditions that are far from optimal for in vivo imaging. In their overview, Brom et al. focus on the technical aspects and challenges of the development of radiotracers for beta cell imaging and give a concise summary of the most important steps that have been taken so far [7]. Although some approaches they describe may hold great promise to reach the ultimate goals of successful in vivo imaging of beta cells, most of the tracers they describe have failed in respect to reaching this aim....

Previously, studies of the endocrine pancreatic β-cell were mainly performed ex vivo by morphological means. This data supported the analyis of pathophysiological changes in the pancreatic islet during insults such as diabetes mellitus. Metabolic testing of the pancreatic islet by assaying hormone parameters such as plasma insulin or C-peptide combined with more or less sophisticated calculations allowed conclusions about states of insulin resistance or secretory failure. It also allowed certain correlations of endocrine function with β-cell mass. Today, with firmer pathophysiological concepts about β-cell failure, modern protocols of islet transplantation, and drugs on the market coming with promises of preservation or even expansion of β-cell mass in diabetes mellitus it has become very attractive to search for tools measuring β-cell mass, if possible even repeatedly in the same organism in vivo. From a clinical point of view, the potential of pancreatic β-cell mass imaging technologies is looked upon with high expectations. Methodologically, the decisive question is whether it is likely that future β-cell imaging will provide significant advantages over the metabolic methods already in hand. With new in vivo tools, studies of β-cell mass and function may offer even new approaches stratifying patients to anti-diabetic therapies.

Persistent hyperinsulinemic hypoglycemia (PHH) is caused by solitary benign insulinoma or hyperplasia of pancreatic beta cells. In infants, PHH is caused by functionally defective hyperplastic beta cells, which are either diffusely or focally distributed in the pancreas. In adults, insulinoma is the most common cause of PHH, but recently, an increasing number of beta-cell hyperplasias has been reported among adults. The cause of adult beta-cell hyperplasia is not known. Whether the increased use of bariatric surgery in the treatment of severe obesity plays a role here is under investigation. Accurate localization of disease focus in both insulinoma and focal beta-cell hyperplasia provides an important support for surgery, especially as the use of laparoscopic surgery has increased. Conventional imaging of these challenging pancreatic lesions has evolved during recent years, but current imaging methods still lack sufficient sensitivity or are invasive. In most pancreatic neuroendocrine tumors (NETs), the usefulness of positron emission tomography (PET) with fluorine-labeled fluorodeoxyglucose ([18F]FDG) for lesion detection is limited because of the low glucose turnover of these tumors. Based on the capacity of pancreatic beta cells to take up and decarboxylate amine precursors, several investigators have studied patients with pancreatic NETs using aminoacid precursors, such as [18F]dihydroxyphenylalanine (DOPA) and [11C]hydroxytryptophan (5-HTP), in an attempt to increase the sensitivity of PET scanning. Another characteristic of NETs is the expression of somatostatin receptors, and thus encouraging studies with somatostatin receptor imaging with [18Ga]-labeled somatostatin analogs have emerged as a new interesting imaging tool for the diagnosis of pancreatic NETs. This article provides an overview of our experiences and the current literature on PET imaging in patients with PHH caused by insulinoma or beta-cell hyperplasia.

The changes in beta-cell mass (BCM) during the course of diabetes are not yet well characterized. A non-invasive method to measure the BCM in vivo would allow us to study the BCM during the onset and progression of the diseases caused by beta-cell dysfunction. PET and SPECT imaging are attractive approaches to determine the BCM because of their high sensitivity and the possibility to quantitatively analyze the images. Several targets and their corresponding radiotracers have been examined for their ability to determine the BCM including radiolabeled antibodies, antibody fragments, peptides and small molecules. Although some of these tracers show promising results, there is still no reliable method to determine the beta-cell mass in vivo. In this review, the targets and the corresponding radiotracers evaluated so far for the determination of the BCM in vivo in humans will be discussed.

In diabetic disease, blood glucose, HbA1c and insulin levels qualify as biomarkers reflecting endocrine pancreas function, but their shortfall in being truly useful predictors or surrogate endpoints of “abnormal processes or disease“ lies in that alteration in their levels are dependent on a variety comorbidities and occur too late in the disease process to be useful sentinels. Non invasive imaging of molecular targets within the beta cell carry the promise of revealing quantitative information about β-cell mass that can, at least theoretically, be used to monitor, in real-time, the natural history of T1DM progression, assess novel therapies designed to drive the proliferation and differentiation of endogenous beta cell progenitors, appraise methods of preserving mature beta cell mass as well as to track the function and viability of transplanted cells and tissues. In this article, we review and deconstruct available information regarding the methodology of making non invasive measurements of VMAT2 in the pancreas and the validity of these measurements to estimate beta cell mass in vivo.

The increasing global incidence of diabetes and advancements in clinical pancreatic islet transplantation for the treatment of Type I diabetes have renewed the interest in understanding the variations of β-cell mass and function relative not only to transplant outcome but also to the onset and progression of diabetes. A deeper comprehension of the molecular and cellular processes involved in pancreatic islet inflammation and cytotoxicity is necessary to further improve efficacy of islet transplantation and to develop new therapies aimed at preserving beta cell function in pathological conditions. Available diagnostic methods based on metabolic response are unsuitable as they lack correlation to islet mass, viability and function. Great emphasis has been placed on developing noninvasive imaging technologies which enable the tracking of both endogenous and transplanted islet mass and potentially function overtime, the characterization of changes in islet vasculature and the degree of T-cell infiltration during insulitis. Among the more relevant modalities are magnetic resonance, positron emitted tomography, single photon emission computed tomography, bioluminescence and fluorescence optical imaging. This review focuses on the most recent advancements in magnetic resonance imaging (MRI) of pancreatic islets. In-vitro approaches aimed at characterizing the potency of isolated islets as well as in-vivo advancements in the assessment of transplanted beta cell mass are presented together with the significant progress made in the in-vivo imaging of the endocrine pancreas and islet vasculature and inflammation. Different experimental approaches are compared via their advantages and limitations with respect to their clinical implementation.

Endocrine beta cells produce and release insulin in order to tightly regulate glucose homeostasis and prevent metabolic pathologies such as Diabetes Mellitus. Optical imaging has contributed greatly to our current understanding of beta cell structure and function. In vitro microscopy of beta cell lines has revealed the localization of molecular components in the cell and more recently their dynamic behavior. In cultured islets, interactions of beta cells with other islet cells and the matrix as well as paracrine and autocrine signaling or reaction to nutrients have been studied. Lastly, microscopy has been performed on tissue sections, visualizing the islets in an environment closer to their natural surroundings. In most efforts to date, the samples have been isolated for investigation and hence have by definition been divorced from their natural environments and deprived of vascularization and innervations. In such a setting the beta cells lack the metabolic information that is primordial to their basic function of maintaining glucose homeostasis. We review optical microscopy; its general principles, its impact in decoding beta cell function and its recent developments towards the more physiologically relevant assessment of beta cell function within the environment of the whole organism. This requires both large imaging depth and fast acquisition times. Only few methods can achieve an adequate compromise. We present extended focus Optical Coherence Microscopy (xfOCM) as a valuable alternative to both confocal microscopy and two photon microscopy (2PM), and discuss its potential in interpreting the mechanisms underlying glucose homeostasis and monitoring impaired islet function.

Systems biology is an emergent field that aims to understand biological systems at system-level. The increasing power of genome sequencing techniques and ranges of other molecular biology techniques is enabling the accumulation of in-depth knowledge of biological systems. This growing information, properly quantified, analysed and presented, will eventually allow the establishment of a system-based cartography of different cellular populations within the organism, and of their interactions at the tissue and organ levels. It will also allow the identification of specific markers of individual cell types. Systems biology approaches to discover diagnostic markers may have an important role in diabetes. There are presently no reliable ways to quantify beta cell mass (BCM) in vivo, which hampers the understanding of the pathogenesis and natural history of diabetes, and the development of novel therapies to preserve BCM. To solve this problem, novel and specific beta cell biomarkers must be identified to enable adequate in vivo imaging by methods such as Positron Emission Tomography (PET). The ideal biomarker should allow measurements by a minimally invasive technology enabling repeated examinations over time, should identify the early stages of decreased BCM, and should provide information on progression of beta cell loss and eventual responses to agents aiming to arrest or revert beta cell loss in diabetes. The present review briefly describes the “state-of-the-art” in the field, and then proposes a step-by-step systems biology approach for the identification and initial testing of novel candidates for beta cell imaging.

Nitric oxide (NO)-sensitive soluble guanylyl cyclase (sGC) is the receptor that catalyzes the formation of the intracellular messenger cyclic guanosine monophosphate (cGMP). Binding of the physiological activator, NO, to the reduced heme moiety of sGC increases the conversion of guanosine triphosphate (GTP) to cyclic GMP (cGMP) and engages crucial effector systems such as protein kinases, phosphodiesterases, and ion channels. The development of compounds that activate sGC independent of NO release has therapeutic implications. Recent studies have demonstrated the potential use of heme-dependent sGC stimulators (e.g. YC-1, BAY 41- 2272, BAY 41-8543, BAY 63-2521, CFM-1571 and A-350619) and heme-independent sGC activators (e.g. BAY 58-2667, HMR-1766, S-3448, A-778935) in the treatment of cardiovascular diseases. Erectile dysfunction (ED) affects millions of men. Phosphodiesterase (PDE)-5 inhibitors, producing an NO-dependent increase in intracellular cGMP concentration, have been a successful approach in the treatment of ED. However, >30% of men with ED do not respond to PDE-5 inhibitor therapy, implying that endogenous NO production may be impaired to such an extent that inhibition of cGMP degradation produces no significant therapeutic advantage. Endogenous NO released from nitrergic nerves in the corpora cavernosa is significantly decreased in various conditions (e.g. diabetes, aging, and hypertension) and has reduced activation of the NO-sGC-cGMP pathway. It is conceivable that sGC stimulators and/or activators may be more effective than PDE5 inhibitors in the treatment of ED in such circumstances by improving NO-sGC-cGMP signaling and erectile function. This novel drug therapy approach for the treatment of ED shows promise.

The occurrence of autoantibodies is a common feature of autoimmune diseases. This review is intended to give an overview of the most important autoantibodies and their role in diagnosis, disease activity and prognosis in rheumatoid arthritis (RA), systemic lupus erythematodes (SLE) and multiple sclerosis (MS). Whereas in RA and SLE these antibodies are meaningful for diagnosis and partially for the prognosis of the disease, the situation is quite different in the case of MS. Up to date, no specific antibody is known to be exclusively present in the serum or cerebrospinal fluid (CSF) of MS-patients compared to the respective fluids of healthy individuals. Nevertheless, there are some antigens that are reported to be bound significantly more often by MS-patients' serum or CSF than by comparable samples of healthy volunteers. In addition to the importance of several autoantibodies for diagnosis of the respective disease, the serum concentration of certain antibodies in RA and SLE is associated with therapy response. Since therapy with biologicals (e.g. TNF- αblockade, B-cell depletion) is expensive, monitoring these autoantibodies seems to be an additional useful tool for early identification of therapy responders or non-responders.

Due to number of problems related with oral, parenteral, rectal and other routes of drug administration, the interest of pharmaceutical scientists has increased towards exploring the possibilities of intranasal delivery of various drugs. Nasal drug delivery system is commonly known for the treatment of local ailments like cold, cough, rhinitis, etc. Efforts have been made to deliver various drugs, especially peptides and proteins, through nasal route for systemic use; utilizing the principles and concepts of various nanoparticulate drug delivery systems using various polymers and absorption promoters. The incorporation of drugs into nanoparticles might be a promising approach, since colloidal formulations have been shown to protect them from the degrading milieu in the nasal cavity and facilitate their transport across the mucosal barriers. The use of nanoparticles for vaccine delivery provides beneficial effect, by achieving good immune responses. This could be due to the fact that small particles can be transported preferentially by the lymphoid tissue of the nasal cavity (NALT). The brain gets benefited through the intranasal delivery as direct olfactory transport bypasses the blood brain barrier and nanoparticles are taken up and conveyed along cell processes of olfactory neurons through the cribriform plate to synaptic junctions with neurons of the olfactory bulb. The intranasal delivery is aimed at optimizing drug bioavailability for systemic drugs, as absorption decreases with increasing molecular weight, and for drugs, which are susceptible to enzymatic degradation such as proteins and polypeptides. This review discusses the potential benefits of using nanoparticles for nasal delivery of drugs and vaccines for brain, systemic and topical delivery. The article aims at giving an insight into nasal cavity, consideration of factors affecting and strategies to improve drug absorption through nasal route, pharmaceutical dosage forms and delivery systems with examples of some patents for intranasal delivery, its advantages and limitations.