Human Insulin: Basic Sciences to Therapeutic Uses

Exosome-Mediated Insulin Delivery for the Potential Treatment of Diabetes Mellitus

Exosomes are extracellular nanovesicles between 30 and 150 nm that serve as essential messengers for different biological signaling and pathological processes. After their discovery, a wide range of applications have been developed, especially in therapeutic drug delivery. In this context, the aim of this work was to test the efficiency of exosome-mediated human insulin delivery using exosomes extracted from three different cell lines: hepatocellular carcinoma (HepG2); primary dermal fibroblasts (HDFa) and pancreatic β cells (RIN-m); all are related to the production and/or the ability to sense insulin and to consequently regulate glucose levels in the extracellular medium. The obtained results revealed that the optimal insulin loading efficiency was achieved by a 200 V electroporation, in comparison with incubation at room temperature. Moreover, the maximum in vitro exosome uptake was reached after incubation for 6 h, which slightly decreased 24 h after adding the exosomes. Glucose quantification assays revealed that exosome-mediated incorporation of insulin presented significant differences in HDFa and HepG2 cells, enhancing the transport in HDFa, in comparison with free human insulin effects in the regulation of extracellular glucose levels. No significant differences were found between the treatments in RIN-m cells. Hence, the results suggest that exosomes could potentially become a valuable tool for stable and biocompatible insulin delivery in diabetes mellitus treatment alternatives.

The role of polysaccharides from natural resources to design oral insulin micro- and nanoparticles intended for the treatment of Diabetes mellitus: A review

Oral administration of insulin (INS) would represent a revolution in the treatment of diabetes, considering that this route mimics the physiological dynamics of endogenous INS. Nano- and microencapsulation exploiting the advantageous polysaccharides properties has been considered an important technological strategy to protect INS against harsh conditions of gastrointestinal tract, in the same time that improve the permeability via transcellular and/or paracellular pathways, safety and in some cases even selectivity for targeting delivery of INS. In fact, some polysaccharides also give to the systems functional properties such as pH-responsiveness, mucoadhesiveness under specific physiological conditions and increased intestinal permeability. In general, all polysaccharides can be functionalized with specific molecules becoming more selective to the cells to which INS is delivered. The present review highlights the advances in the past 10 years on micro- and nanoencapsulation of INS exploiting the unique natural properties of polysaccharides, including chitosan, starch, alginate, pectin, and dextran, among others.

Dual chitosan/albumin-coated alginate/dextran sulfate nanoparticles for enhanced oral delivery of insulin

The potential of nanoparticles (NPs) to overcome the barriers for oral delivery of protein drugs have led to the development of platforms capable of improving their bioavailability. However, despite the progresses in drug delivery technologies, the success of oral delivery of insulin remains elusive and the disclosure of insulin mechanisms of absorption remains to be clarified. To overcome multiple barriers faced by oral insulin and to enhance the insulin permeability across the intestinal epithelium, here insulin-loaded alginate/dextran sulfate (ADS)-NPs were formulated and dual-coated with chitosan (CS) and albumin (ALB). The nanosystem was characterized by its pH-sensitivity and mucoadhesivity, which enabled to prevent 70% of in vitro insulin release in simulated gastric conditions and allowed a sustained insulin release following the passage to simulated intestinal conditions. The pH and time-dependent morphology of the NPs was correlated to the release and permeation profile of insulin. Dual CS/ALB coating of the ADS-NPs demonstrated augmented intestinal interactions with the intestinal cells in comparison to the uncoated-NPs, resulting in a higher permeability of insulin across Caco-2/HT29-MTX/Raji B cell monolayers. The permeability of the insulin-loaded ALB-NPs was reduced after the temperature was decreased and after co-incubation with chlorpromazine, suggesting an active insulin transport by clathrin-mediated endocytosis. Moreover, the permeability inhibition with the pre-treatment with sodium chlorate suggested that the interaction between glycocalix and the NPs was critical for insulin permeation. Overall, the developed nanosystem has clinical potential for the oral delivery of insulin and therapy of type 1 diabetes mellitus.

Impact of the in vitro gastrointestinal passage of biopolymer-based nanoparticles on insulin absorption

Behavior of insulin-loaded biopolymer-based nanoparticles upon passage through the gastrointestinal tract. Intestinal pH triggered insulin release from nanoparticles, allowing its permeability through cell-based engineered intestinal models.

Dissecting stromal-epithelial interactions in a 3D in vitro cellularized intestinal model for permeability studies

Absorption evaluation plays an increasingly important role at the early stage of drug discovery due to its potential to scan the ADME (absorption, distribution, metabolism and excretion) properties of new drug candidates. Therefore, a new three-dimensional (3D) in vitro model replicating the intestinal functioning is herein proposed aiming to dissect the stromal-epithelial interactions and evaluate the permeation of a model drug, insulin. Inspired on the intestinal mucosal architecture, the present model comprises intestinal myofibroblasts (CCD18-Co cells) embedded in Matrigel, onto which epithelial enterocytes (Caco-2 cells) and mucus-producing cells (HT29-MTX cells) were seeded. CCD18-Co myofibroblasts showed to have a central role in the remodeling of the surrounding matrix confirmed by the production of fibronectin. Subsequently, this matrix revealed to be essential to the maintenance of the model architecture by supporting the overlying epithelial cells. In terms of functionality, this model allowed the efficient prediction of insulin permeability in which the presence of mucus, the less tight character between Caco-2 and HT29-MTX epithelial cells and the 3D assembly were critical factors. Concluding, this model constitutes a robust tool in the drug development field with potential to bridge the traditional 2D cell culture models and in vivo animal models.

Chitosan-sodium lauryl sulfate nanoparticles as a carrier system for the in vivo delivery of oral insulin

The present work explores the possibility of formulating an oral insulin delivery system using nanoparticulate complexes made from the interaction between biodegradable, natural polymer called chitosan and anionic surfactant called sodium lauryl sulfate (SLS). The interaction between chitosan and SLS was confirmed by Fourier transform infrared spectroscopy. The nanoparticles were prepared by simple gelation method under aqueous-based conditions. The nanoparticles were stable in simulated gastric fluids and could protect the encapsulated insulin from the GIT enzymes. Additionally, the in vivo results clearly indicated that the insulin-loaded nanoparticles could effectively reduce the blood glucose level in a diabetic rat model. However, additional formulation modifications are required to improve insulin oral bioavailability.

The potential role of self-cleaving purification tags in commercial-scale processes

Purification tags are robust tools that can be used to purify a wide selection of target proteins, which makes them attractive candidates for implementation into platform processes. However, tag removal remains an expensive and significant issue that must be resolved before these tags can become widely used. One alternative is self-cleaving purification tags, which can provide the purity and versatility of conventional tags but eliminate the need for proteolytic tag removal. Many of these self-cleaving tags are based on inteins, but other emerging technologies, such as the FrpC and SrtAc proteins, have also been reported. In this review, we cover affinity and non-chromatographic self-cleaving purification tags and their potential industrial applications.

Transport of Insulin in Modified Valia-Chien Chambers and Caco-2 Cell Monolayers

The transport characteristics of insulin were investigated using two different absorption models. Using the modified Valia-Chien chambers, permeability coefficients of insulin in the duodenum, jejunum, and ileum were 0.71x10(-7), 7.11x10(-7) and 9.45x10(-7) cm/s, respectively. In the Caco-2 cell monolayers, the bidirectional transepithelial fluxes of insulin across Caco-2 cell line showed symmetry. Confocal laser scanning microscopy visualized that FD-4 and FITC-insulin were mainly located in the paracellular route. It is evident that the lower intestine might be an advantageous region, and absorption enhancer that helps open tight junctions between cells should be used for oral delivery of insulin.

Insulin's 85th anniversary--An enduring medical miracle

In 1922, the discovery of insulin led to a revolution in diabetes management. Since then, many improvements have been made to insulin preparations: early preparations of bovine and porcine insulins were purified and their duration of action prolonged, giving rise to the introduction of Neutral Protamine Hagedorn (NPH) insulin and monocomponent insulins. Then, with the advances in genetic engineering in the 1980s, it became possible to produce recombinant human insulin. Nowadays, modern molecular biology techniques enable the production of insulin analogues, which have several advantages over human insulin preparations including a reduced risk of hypoglycaemia. Insulin delivery is still predominantly via subcutaneous injections, but alternative routes of insulin administration are being investigated. Pulmonary delivery has emerged as the most feasible option thus far but oral delivery is an ultimate goal, although basic problems of insulin stability in the gut and absorption from the gastrointestinal tract still need to be resolved. The availability of a true artificial pancreas by means of a closed-loop system, linking continuous glucose monitoring with insulin-pump technology, would also constitute a significant advance, but major technological problems still need to be overcome.

Preparation of insulin loaded PLGA-Hp55 nanoparticles for oral delivery

The aim of the present work was to investigate the preparation of PLGA nanoparticles (PNP) and PLGA-Hp55 nanoparticles (PHNP) as potential drug carriers for oral insulin delivery. The nanoparticles were prepared by a modified emulsion solvent diffusion method in water, and their physicochemical characteristics, drug release in vitro and hypoglycemic effects in diabetic rats were evaluated. The particle sizes of the PNP and PHNP were 150+/-17 and 169+/-16 nm, respectively, and the drug recoveries of the nanoparticles were 50.30+/-3.1 and 65.41+/-2.3%, respectively. The initial release of insulin from the nanoparticles in simulated gastric fluid over 1 h was 50.46+/-6.31 and 19.77+/-3.15%, respectively. The relative bioavailability of PNP and PHNP compared with subcutaneous (s.c.) injection (1 IU/kg) in diabetic rats was 3.68+/-0.29 and 6.27+/-0.42%, respectively. The results show that the use of insulin-loaded PHNP is an effective method of reducing serum glucose levels.

Nanoparticulate delivery system for insulin: design, characterization and in vitro/in vivo bioactivity

Insulin-loaded alginate-dextran nanospheres were prepared by nanoemulsion dispersion followed by triggered in situ gelation. Nanospheres were characterized for mean size and distribution by laser diffraction spectroscopy and for shape by transmission electron microscopy. Insulin encapsulation efficiency and in vitro release were determined by Bradford protein assay and bioactivity determined in vitro using a newly developed Western blot immunoassay and in vivo using Wistar diabetic rats. Nanospheres ranged from 267 nm to 2.76 microm in diameter and demonstrated a unimodal size distribution. Insulin encapsulation efficiency was 82.5%. Alginate-dextran particles suppressed insulin release in acidic media and promoted a sustained release at near neutral conditions. Nanoencapsulated insulin was bioactive, demonstrated through both in vivo and in vitro bioassays.

Alginate microparticles as novel carrier for oral insulin delivery

Alginate microparticles produced by emulsification/internal gelation were investigated as a promising carrier for insulin delivery. The procedure involves the dispersion of alginate solution containing insulin protein, into a water immiscible phase. Gelation is triggered in situ by instantaneous release of ionic calcium from carbonate complex via gentle pH adjustment. Particle size is controlled through the emulsification parameters, yielding insulin-loaded microparticles. Particle recovery was compared using several washing protocols. Recovery strategies are proposed and the influence on particle mean size, morphology, recovery yield (RY), encapsulation efficiency, insulin release profile, and structural integrity of released insulin were evaluated. Spherical micron-sized particles loaded with insulin were produced. The recovery process was optimized, improving yield, and ensuring removal of residual oil from the particle surface. The optimum recovery strategy consisted in successive washing with a mixture of acetone/hexane/isopropanol coupled with centrifugation. This strategy led to small spherical particles with an encapsulation efficiency of 80% and a RY around 70%. In vitro release studies showed that alginate itself was not able to suppress insulin release in acidic media; however, this strategy preserves the secondary structure of insulin. Particles had a mean size lower than the critical diameter necessary to be orally absorbed through the intestinal mucosa followed by their passage to systemic circulation and thus can be considered as a promising technology for insulin delivery.

Synergistic effect of anionic lipid enhancer and electroosmosis for transcutaneous delivery of insulin

A lipid formulation consisting of 1,2-dimyristoyl-sn3-phosphatidylserine (DMPS) in a 0.2% sodium dodecylsulfate (SDS) solution was tested as an in vivo enhancer for the transcutaneous delivery of insulin. The formulation when applied to for 15 min was found to permeabilize porcine epidermis and prolong the permeable state as evidenced by electric resistance measurement. The formulation enhanced the transport of insulin through the epidermis by 40- to 100-fold, as compared to epidermis that was treated with SDS or DMPS alone. Application of electroosmosis across the formulation-treated epidermis enhanced the transport of insulin by an additional 10-fold. Pharmacokinetic studies were carried out in Sprague-Dawley rats. Transcutaneous delivery of insulin with formulation treatment and electroosmosis increased the plasma level of insulin by approximately 10-fold over delivery by formulation treatment alone. With the above protocol plasma insulin concentration remained relatively constant for up to 4h. The synergistic application of anionic lipid formulation and electroosmosis offers a promising non-invasive technique to deliver insulin transcutaneously.

Lipid and Electroosmosis Enhanced Transdermal Delivery of Insulin by Electroporation

Transdermal transport of insulin and extraction of interstitial glucose under anodal iontophoresis (electroosmosis) following electroporation in the presence of 1,2-dimyristoylphophatidylserine (DMPS) was studied. An earlier study showed that DMPS increased the transport of insulin across porcine epidermis under electroporation by approximately fourfold. It was suggested that DMPS increased the lifetime of electropores in the epidermis resulting in an enhanced transport of permeants. When electroosmosis was applied across the epidermis following electroporation with DMPS, the enhancement of insulin transport was approximately 18-fold over electroporation alone. When the same strategy was applied to extract interstitial glucose, the enhancement was approximately 23-fold over electroporation alone. Real-time transdermal insulin transport kinetics was measured using FITC-labeled insulin and a custom-made vertical diffusion apparatus that had a fluorescence cuvette as the receiver compartment. Insulin transport by electroporation alone showed a nonlinear kinetics that is most likely due to the resealing of the electropores with time. The transport kinetics when electroporation was carried out in the presence of DMPS was more linear, confirming earlier studies that suggested the DMPS stabilizes transport paths formed by electroporation. The data suggests that in vivo, noninvasive insulin delivery to therapeutic levels and glucose extraction may be achieved by combining electroporation with anionic lipids and electroosmosis.

'Human' insulin versus animal insulin in people with diabetes mellitus

BACKGROUND

Human insulin was introduced for the routine treatment of diabetes mellitus in the early 1980s without adequate comparison of efficacy to animal insulin preparations. First reports of altered hypoglycaemia awareness after transfer to human insulin made physicians and especially patients uncertain about potential adverse effects of human insulin.

OBJECTIVES

To assess the effects of different insulin species by evaluating their efficacy (in particular glycaemic control) and adverse effects profile (mainly hypoglycaemia).

SEARCH STRATEGY

A highly sensitive search for randomised controlled trials combined with key terms for identifying studies on human versus animal insulin was performed using The Cochrane Library, MEDLINE and EMBASE. We also searched reference lists and databases of ongoing trials. Date of latest search: July 2004.

SELECTION CRITERIA

We included randomised controlled clinical trials with diabetic patients of all ages that compared human to animal (for the most part purified porcine) insulin. Trial duration had to be at least one month in order to achieve reliable results on the main outcome parameter glycated haemoglobin.

DATA COLLECTION AND ANALYSIS

Trial selection as well as evaluation of study quality was performed by two independent reviewers. The quality of reporting of each trial was assessed according to a modification of the quality criteria as specified by Schulz and by Jadad.

MAIN RESULTS

Altogether 2156 participants took part in the 45 randomised controlled studies that were discovered through extensive search efforts. Though many studies had a randomised, double-blind design, most studies were of poor methodological quality. Purified porcine and semi-synthetic insulin were most often investigated. No significant differences in metabolic control or hypoglycaemic episodes between various insulin species could be elucidated. Insulin dose and insulin antibodies did not show relevant dissimilarities.

AUTHORS' CONCLUSIONS

A comparison of the effects of human and animal insulin as well as of the adverse reaction profile did not show clinically relevant differences. Many patient-oriented outcomes like health-related quality of life or diabetes complications and mortality were never investigated in high-quality randomised clinical trials. The story of the introduction of human insulin might be repeated by contemporary launching campaigns to introduce pharmaceutical and technological innovations that are not backed up by sufficient proof of their advantages and safety.

Use of natural plant exudates (Sanguis Draxonis) for sustained oral insulin delivery with dramatic reduction of glycemic effects in diabetic rats

Sanguis Draxonis (SD), a kind of natural plant exudates, has been prescribed for handling diabetic disorders as a Chinese traditional herb. Surprisingly, SD was found to be a good material for oral insulin delivery. The Insulin-loaded Sanguis Draxonis nanocapsules (ISDN) were prepared by deposition technique. The average size and width of distribution of ISDN were 184+/-24 and 110+/-16 nm, respectively. The insulin encapsulation efficiency of ISDN reached up to 69.6+/-5.6%. In vitro the release profile of insulin from ISDN can be well modeled using an exponential function [Y=1-exp(-0.0275t)], showing that there was no initial burst release of insulin. The stability studies indicated that the majority of initial amount of insulin in ISDN was preserved not only after incubation of ISDN in three kinds of proteolytic enzyme solutions at 37 degrees C for 30 min, but also after its storage at 25 degrees C for 6 months. After a single oral administration of ISDN at the dose of 25, 50, and 75 IU/kg in STZ-induced diabetic rats, the blood glucose level was depressed to 60.5+/-2.7%, 52.6+/-2.3% and 47.3+/-3.1% of the initial value at time point 8 h, respectively, and these marked decreases lasted 2-4 days. When 125I-labeled ISDN was administered orally, the distribution sequence of isotope intensity of 125I radioactivity in rat organs was as follows: liver, kidney, heart, spleen and lung. In addition, 125I radioactivity disappeared progressively as a function of time, parallel to the biological effect. In conclusion, the results demonstrate that ISDN elicits a long-term hypoglycemic effect significantly after oral administration in STZ-induced diabetic rats and it can be considered as a stable and effective system for oral insulin delivery.

'Human' insulin versus animal insulin in people with diabetes mellitus

BACKGROUND

Human insulin was introduced for the routine treatment of diabetes mellitus in the early 1980s without adequate comparison of efficacy to animal insulin preparations. First reports of altered hypoglycaemia awareness after transfer to human insulin made physicians and especially patients uncertain about potential adverse effects of human insulin.

OBJECTIVES

To assess the effects of different insulin species by evaluating their efficacy (in particular glycaemic control) and adverse effects profile (mainly hypoglycaemia).

SEARCH STRATEGY

A highly sensitive search for randomised controlled trials combined with key terms for identifying studies on human versus animal insulin was performed using the Cochrane Library (issue 2, 2002), Medline (1966 to May, 2002) and Embase (1974 to February, 2002). We also searched reference lists and databases of ongoing trials. Date of latest search: May 2002.

SELECTION CRITERIA

We included randomised controlled clinical trials with diabetic patients of all ages that compared human to animal (for the most part purified porcine) insulin. Trial duration had to be at least one month in order to achieve reliable results on the main outcome parameter glycated haemoglobin.

DATA COLLECTION AND ANALYSIS

Trial selection as well as evaluation of study quality was performed by two independent reviewers. The quality of reporting of each trial was assessed according to a modification of the quality criteria as specified by Schulz and by Jadad.

MAIN RESULTS

Altogether 2156 participants took part in the 45 randomised controlled studies that were discovered through extensive search efforts. Though many studies had a randomised, double-blind design, most studies were of poor methodological quality. Purified porcine and semi-synthetic insulin were most often investigated. No significant differences in metabolic control or hypoglycaemic episodes between various insulin species could be elucidated. Insulin dose and insulin antibodies did not show relevant dissimilarities.

REVIEWER'S CONCLUSIONS

A comparison of the effects of human and animal insulin as well as of the adverse reaction profile did not show clinically relevant differences. Many patient-oriented outcomes like health-related quality of life or diabetes complications and mortality were never investigated in high-quality randomised clinical trials. The story of the introduction of human insulin might be repeated by contemporary launching campaigns to introduce pharmaceutical and technological innovations that are not backed up by sufficient proof of their advantages and safety.

Mechanisms for absorption enhancement of inhaled insulin by sodium taurocholate

The effects of sodium taurocholate (NaTC) on the absorption of inhaled insulin was investigated using both in vivo and in vitro experiments. The absolute bioavailability of insulin when given as a nebulized solution (0.6 mM) to anesthetized intubated beagle dogs was low (2.6+/-0.3%). However, when NaTC at different concentrations (2-32 mM) were included in the formulations the bioavailability increased and at 32 mM it was about nine times higher (23.2+/-4.4%) than for pure insulin. In a similar concentration interval (20-25 mM) NaTC decreased the transepithelial electrical resistance (TEER) across Caco-2 cell monolayers leading to an increased permeability of insulin. At higher concentrations (above 30 mM) the viability of the Caco-2 cells decreased and the insulin permeability increased dramatically. Furthermore, we show that NaTC in the concentration range 2-15 mM gradually decreases the aggregation state of insulin, i.e., produces mono- or dimeric insulin from hexameric insulin. In conclusion, NaTC increases the bioavailability of nebulized insulin, increases the permeability of insulin across Caco-2 cell monolayers, and decreases the aggregation state of insulin at similar concentrations. We suggest that the main mechanisms behind the absorption enhancement of inhaled insulin by NaTC are the production of insulin monomers and an opening of tight junctions between adjacent airway epithelial cells.

Modulated insulin delivery from glucose-sensitive hydrogel dosage forms

Glucose-sensitive hydrogels that undergo sol-gel phase transition were used to develop modulated insulin delivery systems. Glucose-sensitive hydrogels were prepared by mixing glucose-containing polymers and PEGylated concanavalin A (Con A). Glucose was incorporated into the polymer backbone by copolymerization of allyl glucose with comonomers, such as 3-sulfopropylacrylate, potassium salt (SPAK), N-vinyl pyrrolidone (VP), and acrylamide (AM). Con A grafted with five PEG molecules were used to improve the stability of Con A. Three different types of insulin delivery systems were examined: diffusion-controlled reservoir, diffusion-controlled matrix, and erosion-controlled matrix systems. Insulin release through the glucose-sensitive hydrogel membrane and from the glucose-sensitive hydrogel matrix was dependent on the glucose concentration in the receptor chamber. As the glucose concentration was increased from 1 to 4 mg/ml, the release rate increased. The insulin release rate decreased as the glucose concentration was reduced to 1 mg/ml. Modulated insulin release was achieved using the glucose-sensitive membrane and matrix systems. On the other hand, the glucose-sensitive erodible system did not show modulated release as the glucose concentration was changed between 1 and 4 mg/ml.

Development of two high-performance liquid chromatographic methods for the analysis and characterization of insulin and its degradation products in pharmaceutical preparations

Two high-performance liquid chromatography methods either in reversed-phase or as size exclusion separation mode were developed and validated for the analysis of insulin and its degradation products in pharmaceutical preparations. The results show the reliability of the analytical methods for the intended used. The static and dynamic light scattering were used to characterize the insulin and its derivatives. The absolute molecular weight of human insulin monomer and dimer were 5800 and 12400 Da respectively whereas its z-average root mean square radius were 21.6+/-0.4 and 40.5+/-0.7 nm, respectively. In contrast, the hydrodynamic diameter varied between 2.69 and 5.50 nm, depending of the association behavior of insulin.

Porcine insulin biodegradable polyester microspheres: stability and in vitro release characteristics

The stability of porcine insulin in biodegradable polymers, i.e., poly(DL-lactide-co-glycolide) 50:50 (50:50 DL-PLGA) and poly(L-lactide) (L-PLA) was investigated. Insulin encapsulated microspheres were fabricated from both polymers using double-emulsion-solvent evaporation and emulsion-solvent evaporation techniques and subjected to accelerated stability studies at 40 degrees C and 75% relative humidity. Porcine insulin was found to degrade in all microsphere formulations with an average of < 50% of the initial loading amount remaining intact at the end of 4 weeks. The two major degradation products observed in these formulations were determined to be A-21 desamido insulin and covalent insulin dimer with trace amounts of high molecular weight transformation products. In vitro release studies in phosphate buffered saline at 37 degrees C resulted in very slow and incomplete (< 30% in 30 days) release kinetics for all microsphere formulations. Extraction and analyses of the unreleased insulin within the microspheres revealed that an average of approximately 11% of the encapsulated insulin remained intact. The degradation products observed consisted of approximately 15% of three distinct deamidated hydrolysis products including A-21 desamido insulin, approximately 22% covalent insulin dimer, and trace amounts of high molecular weight transformation products. The degradation of porcine insulin within biodegradable polyester microspheres during stability and release studies can be attributed to the gradual decrease in the pH within the microspheres due to progressive polymer hydrolysis resulting in the production of DL-lactic and glycolic acids. The encapsulation of an acid-base indicator, bromophenol blue, in 50:50 PLGA microspheres (as a probe to estimate pH within the microspheres during accelerated stability studies) indicated that the pH decreased to approximately 3.8 after 3 weeks.

Study of the mechanism of insulin encapsulation in poly(isobutylcyanoacrylate) nanocapsules obtained by interfacial polymerization

In previous studies, insulin-loaded poly(alkylcyanoacrylate) nanocapsules were found to reduce the blood glucose level after oral administration to diabetic rats and dogs. The reduction of the glycemia induced by the nanocapsules was the same regardless of the insulin doses administered, but the effect appeared only after a delay of a few days. The purpose of this study was to investigate the mechanism of insulin encapsulation and the type of interactions that may exist between the polymer forming the nanocapsule wall and the insulin. The results of this study showed, based on the interfacial polymerization of isobutylcyanoacrylate, that the insulin molecule is not chemically modified during the nanoencapsulation process. In addition, no interaction between the poly(isobutylcyanoacrylate) and the insulin could be observed. The observed high encapsulation efficiency of intact insulin may be explained by the fact that the ethanol used in the preparation of the nanocapsules is responsible for the initiation of the interfacial polymerization of isobutylcyanoacrylate instead of the insulin. The zeta potential measurements suggest that insulin is located within the core of the nanocapsules. Thus the biological activity of the nanoencapsulated peptide and the high efficiency of insulin encapsulation achieved with this nanoencapsulation process cannot be explained by a specific interaction of the insulin with the polymer forming the nanocapsule's wall. It may be due, however, to the fact that the encapsulated insulin molecule is chemically intact and located within the oily core of the nanocapsules.