Interaction of sulfonylurea-conjugated polymer with insulinoma cell line of MIN6 and its effect on insulin secretion

Islet transplantation and encapsulation: an update on recent developments

Human islet transplantation can provide good glycemic control in diabetic recipients without exogenous insulin. However, a major factor limiting its application is the recipient's need to adhere to life-long immunosuppression, something that has serious side effects. Microencapsulating human islets is a strategy that should prevent rejection of the grafted tissue without the need for anti-rejection drugs. Despite promising studies in various animal models, the encapsulated human islets so far have not made an impact in the clinical setting. Many non-immunological and immunological factors such as biocompatibility, reduced immunoprotection, hypoxia, pericapsular fibrotic overgrowth, effects of the encapsulation process and post-transplant inflammation hamper the successful application of this promising technology. In this review, strategies are discussed to overcome the above-mentioned factors and to enhance the survival and function of encapsulated insulin-producing cells, whether in islets or surrogate β-cells. Studies at our center show that barium alginate microcapsules are biocompatible in rodents, but not in humans, raising concerns over the use of rodents to predict outcomes. Studies at our center also show that the encapsulation process had little or no effect on the cellular transcriptome of human islets and on their ability to function either in vitro or in vivo. New approaches incorporating further modifications to the microcapsule surface to prevent fibrotic overgrowth are vital, if encapsulated human islets or β-cell surrogates are to become a viable therapy option for type 1 diabetes in humans.

Hydrotropic polymer micelles containing acrylic acid moieties for oral delivery of paclitaxel

Hydrotropic polymers (HPs) and their micelles have been recently developed as vehicles for delivery of poorly water-soluble drugs, such as paclitaxel (PTX), by oral administration. The release of PTX from HP micelles, however, was slow and it took more than a day for complete release of the loaded PTX. Since the gastrointestinal (GI) transit time is known to be only several hours, pH-sensitive HP micelles were prepared for fast release of the loaded PTX responding to pH changes along the GI tract. Acrylic acid (AA) was introduced, as a release modulator, into HPs by copolymerization with 4-(2-vinylbenzyloxy)-N,N-(diethylnicotinamide) (VBODENA). The AA content was varied from 0% to 50% (in the molar ratio to VBODENA). HPs spontaneously produced micelles in water, and their critical micelle concentrations (CMCs) ranged from 31 microg/mL to 86 microg/mL. Fluorescence probe study using pyrene showed that blank HP micelles possessed a good pH sensitivity, which was clearly observed at relatively high AA contents and pH>6. The pH sensitivity also affected the PTX loading property. Above pH 5, the PTX loading content and loading efficiency in HP micelles were significantly reduced. Although this may be primarily due to the AA moieties, other factors may include PTX degradation and polymer aggregation. The PTX release from HP micelles with more than 20% (mol) AA contents was completed within 12 h in a simulated intestinal fluid (SIF, pH=6.5). The HP micelles without any AA moiety showed very slow release profiles. In the simulated gastric fluid (SGF, pH=1.6), severe degradation of the released PTX was observed. The pH-dependent release of PTX from HP micelles can be used to increase the bioavailability of PTX upon oral delivery.

Biological and Biomaterial Approaches for Improved Islet Transplantation

Islet transplantation may be used to treat type I diabetes. Despite tremendous progress in islet isolation, culture, and preservation, the clinical use of this modality of treatment is limited due to post-transplantation challenges to the islets such as the failure to revascularize and immune destruction of the islet graft. In addition, the need for lifelong strong immunosuppressing agents restricts the use of this option to a limited subset of patients, which is further restricted by the unmet need for large numbers of islets. Inadequate islet supply issues are being addressed by regeneration therapy and xenotransplantation. Various strategies are being tried to prevent beta-cell death, including immunoisolation using semipermeable biocompatible polymeric capsules and induction of immune tolerance. Genetic modification of islets promises to complement all these strategies toward the success of islet transplantation. Furthermore, synergistic application of more than one strategy is required for improving the success of islet transplantation. This review will critically address various insights developed in each individual strategy and for multipronged approaches, which will be helpful in achieving better outcomes.

Improved phenotype of rat islets in a macrocapsule by co-encapsulation with cross-linked Hb

A number of rat islets were co-encapsulated in a diffusion chamber-type device, i.e., macrocapsule, with a thermoreversible polymeric extracellular matrix (ECM) and bioactive ingredient of cross-linked hemoglobin (Hb-C). The ECM was formed from an aqueous solution of N-isopropyl-acrylamide co-polymers with a small amount of acrylic acid, which exhibited unique sol-gel transition in a temperature range of 30-34 degrees C, without noticeable hysteresis. The incorporation of Hb-C in the islet macrocapsule showed a concentration-dependent effect on insulin secretion and viability of the entrapped islets. Insulin secretion stimulation by glucose and cell viability were more than doubled when compared with a control group (without Hb-C), at an optimum Hb-C concentration of 0.25 mM due to its unique oxygen transporting capacity. Furthermore, 0.25 mM Hb-C in the macrocapsule was able to support islet density up to 1000 islets/device in a 154 microl total volume without negative effects on islet functionality and viability. Hb-C incorporation is an effective strategy for a macrocapsule-type biohybrid artificial pancreas for Type-I diabetes treatment, which can be further developed to a rechargeable system by employing the thermoreversible ECM and designing a proper macrocapsule.

Synthesis, bioactivity and specificity of glucagon-like peptide-1 (7-37)/polymer conjugate to isolated rat islets

In order to increase the functionality of islets encapsulated in a biohybrid artificial pancreas (BAP), it was proposed that co-encapsulation with insulinotropic agents would improve insulin secretion from islets. To prevent agents from leaking out, conjugation with high-molecular-weight polymers was inevitable. In this study, synthetic glucagon-like peptide-1 (GLP-1) (7-37) was conjugated to a water-soluble polymer, poly(N-vinyl-2-pyrroridone-co-acrylic acid) (5 mol% acrylic acid, M(w) 445 kDa), via poly(ethylene glycol, M(w) 3.4 kDa) spacer. The chemical conjugation was confirmed by reverse phase-HPLC and the GLP-1 content in the GLP-1/polymer conjugate (VAPG) was determined by UV spectrophotometry at 280 nm (ca. 29 wt/wt%). In a static insulin secretion test, the VAPG increased insulin secretion up to 200% over a control (no stimulation) at high glucose levels, although the insulinotropic activity of VAPG was slightly lower than that of native GLP-1. The bioactivity of VAPG was prolonged for at least 2 weeks, which was examined by co-encapsulation of the conjugate into islet microcapsules. Dose-response curve revealed that the half-maximal effective dose (ED(50)) of VAPG was about 55 nm (25 nm for native GLP-1). By N-terminal analysis using aminopeptidase and RP-HPLC, it was confirmed that the lowered bioactivity of VAPG stemmed from the polymer conjugation to N-terminal histidine moieties, which actively participate in binding to GLP-1 receptors, resulting in only 16% of N-terminal histidine remaining intact after the conjugation reaction. Finally, the specific interaction of the VAPG with isolated rat islets was investigated. Total cellular cyclic AMP levels were measured and confocal microscopy was conducted using GLP-1 and VAPG labeled with fluorescent probes. It was found that VAPG effectively increased the cAMP level in islet cells in a glucose concentration-dependent manner. Moreover, the confocal microscopy study showed that the binding of VAPG occurs at the same location where GLP-1 binds but with less affinity than that of native GLP-1. In summary, a GLP-1/polymer conjugate was synthesized for the first time, and its bioactivity was examined, which must result from its specific interaction with isolated islets.

Long-Term Insulinotropic Activity of Glucagon-Like Peptide-1/Polymer Conjugate on Islet Microcapsules

The biohybrid artificial pancreas (BAP), a promising therapy for type 1 diabetes, faces several obstacles such as the need for a large implantation volume of encapsulated islets because of low functionality. To address such problems, in this study we examined long-term insulinotropic activity of glucagon-like peptide-1 (GLP-1)/polymer conjugate [VAPG: poly(N-vinylpyrrolidone-co-acrylic acid-g-PEG) (VAP)-GLP-1] as well as GLP-1/Zn(2+) crystal by coencapsulation with islets. Microcapsules with VAPG or crystal produced round-shaped beads whereas free GLP-1 showed poor capsule morphology. A perfusion experiment suggested that VAPG showed higher bioactivity than did microcapsules with GLP-1/Zn(2+). In long-term culture (200 mg of glucose/dL [G]), VAPG also enhanced insulinotropic activity over 5 weeks compared with the crystal form of GLP-1. However, maintenance of the high bioactivity of VAPG suddenly declined after week 5, possibly because of degradation, metabolism, and overstimulation. Basal (50 G) and glucose-stimulated (300 G) levels of insulin secretion confirmed a see-saw pattern in which the VAPG gradually decreased insulin secretion from encapsulated islets and then fell below the insulin level secreted from microcapsules containing GLP-1/Zn(2+) crystal. Viability of the microcapsulated islets of each group was not significantly different. Consequently, the coencapsulation of VAPG or GLP-1/Zn(2+) crystal can be a potential approach to reducing BAP volume with further optimization of activity duration.

Incorporation of sulfonylurea into sugar-carrying polymers and their effects on insulin secretion from MIN6 cells in a solution state

A hypoglycemic sulfonylurea (SU) was incorporated into non-reducing glucose-bearing polystyrene (PS) derivative for enhanced interaction with, and insulin secretion from an insulinoma cell line (MIN6). SU derivative was copolymerized with styrene having sugar moiety such as maltose or lactose. About an 80% increase in insulin secretion for poly(N-p-vinylbenzyl-D-maltonamide-co-SU) (p(VMA-co-SU)) was observed when the copolymer was added to the culture medium (SU: 25 mol%). whereas about 50% increase for poly(N-p-vinylbenzyl-D-lactonamide-co-SU) (p(VLA-co-SU)) was observed when compared with unstimulated cells. From the measurement of flow cytometry, the fluorescence intensity of MIN6 cells incubated in culture media containing FITC-labeled SU-incorporated copolymer increased remarkly owing to specific interaction between receptors in the cell membrane and SU ligands in the copolymer. When sugar and/or SU-incorporated copolymer were co-entrapped with the cells in agarose or collagen gel as an extracellular matrix for three-dimensional culture. p(VMA-co-SU) enhanced more insulin secretion from MIN6 cells than other polymers and it was found that collagen gel was more effective in insulin secretion than agarose