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

Trace assay of insulin in a pharmacy drug with a paste electrode

This paper describes the development of a voltammetric assay of insulin using a DNA immobilized onto a carbon nanotube paste electrode (CNPE), the peak potential of which was 0.2 V, vs. Ag/AgCl on the CNPE. The cyclic voltammetry (CV) and square-wave (SW) stripping voltammetry parameters of the optimized conditions were determined. Low analytical working ranges of 10-80 ugL-1 CV and 0.01-0.1 ngL-1 SW were attained. The precision of the insulin concentration of 0.01 ugL-1 was 0.14 (n = 15) RSD using the optimum conditions, in which the detection limit was 0.004 ngL-1 (6.9 × 10-12 M) (S/N = 3) using only an accumulation time of 400 s. The developed method was applied to determine insulin in a pharmacy drug from analytical-grade chemicals (from Aldrich).

Drug Shelf Life and Release Limits Estimation Based on Manufacturing Process Capability

Specification limits are the competence regulatory agencies, whereas the release limit is a manufacturer’s internal specification to be applied at the time of batch release to assure that quality attributes will remain within the specification limits until the expiry time. The aim of this work is to propose a method to set the shelf life from drug manufacture process capacity and degradation rate, using a modified version of the proposed method by Allen et al. (1991) Two different data sets were used to do this. The first data set corresponds to analytical method validation to measure the insulin concentration in order to estimate the specification limits, whereas the latter set gathered information on stability data of six batches of human insulin pharmaceutical preparation. In this context, the six batches were divided into two groups: Group 1 (batches 1, 2, and 4) was used to estimate shelf life; Group 2 (batches 3, 5, and 6) was used to test the estimated lower release limit (LRL). The ASTM E2709-12 approach was applied to verify that the future batches fulfill the release criterium. The procedure has been implemented in R-code.

Behavior of Regular Insulin in a Parenteral Nutrition Admixture: Validation of an LC/MS-MS Assay and the In Vitro Evaluation of Insulin Glycation

Parenteral-nutrition (PN)-induced hyperglycemia increases morbidity and mortality and must be treated with insulin. Unfortunately, the addition of insulin to a ternary PN admixture leads to a rapid decrease in insulin content. Our study’s objective was to determine the mechanistic basis of insulin’s disappearance. The literature data suggested the presence of a glycation reaction; we therefore validated an LC-MS/MS assay for insulin and glycated insulin. In a 24-h stability study, 20 IU/L of insulin was added to a binary PN admixture at pH 3.6 or 6.3. When the samples were diluted before analysis with a near-neutral diluent, insulin was fully stable at pH 3.6, while a loss of around 50% was observed at pH 6.3. Its disappearance was shown to be inversely correlated with the appearance of monoglycated insulin (probably a Schiff base adduct). Monoglycated insulin might also undergo a back-reaction to form insulin after acidic dilution. Furthermore, a second monoglycated insulin species appeared in the PN admixture after more than 24 h at high temperature (40 °C) and a high insulin concentration (1000 IU/L). It was stable at acidic pH and might be an Amadori product. The impact of insulin glycation under non-forced conditions on insulin’s bioactivity requires further investigation.

Structural Changes in Insulin at a Soft Electrochemical Interface

Understanding the interaction of proteins at interfaces, which occurs at or within cell membranes and lipoprotein vesicles, is central to our understanding of protein function. Therefore, new experimental approaches to understand how protein structure is influenced by protein-interface interactions are important. Herein we build on our previous work exploring electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) to investigate changes in protein secondary structure that are modulated by protein-interface interactions. The ITIES provides an experimental framework to drive protein adsorption at an interface, allowing subsequent spectroscopic analysis (e.g., Fourier transform infrared spectroscopy) to monitor changes in protein structure. Here, we reveal that the interaction between insulin and the interface destabilizes native insulin secondary structure, promoting formation of α helix secondary structures. These structural alterations result from protein-interface rather than protein-protein interactions at the interface. Although this is an emerging approach, our results provide a foundation highlighting the value of the ITIES as a tool to study protein structure and interactions at interfaces. Such knowledge may be useful to elucidate protein function within biological systems or to aid sensor development.

A comprehensive stability assessment of insulin degludec using New customized validated RP-HPLC and SEC-HPLC methods in an orthogonal testing protocol

Stress testing of biopharmaceuticals plays an important role in preparation of their stability profiles through investigation of possible degradation pathways and identification of degradation products, so in this study Insulin Degludec which is a new generation ultra-long-acting basal insulin is subjected to stress conditions as different temperatures, different pH, oxidation, mechanical agitation, and repeated freeze and thaw cycles to generate possible degradation products and aggregation that are investigated by two new validated RP-HPLC and SEC-HPLC methods in addition to dynamic light scattering (DLS) and native polyacrylamide gel electrophoresis (Nu-PAGE). SEC-HPLC was used to investigate formation of aggregates whose results were correlated with those obtained from DLS and Nu-PAGE, while RP-HPLC was used to investigate any possible chemical modifications. The Proposed RP-method had limit of detection (LOD) and limit of quantitation (LOQ) of 0.012 mg/mL and 0.045 mg/mL respectively and accuracy of 99.22 ± 1.07 %, while the SEC methods had limit of detection (LOD) and limit of quantitation (LOQ) of 0.012 mg/mL and 0.031 mg/mL, respectively. The degradation pattern due to high temperature effect and oxidation is investigated by LC- tandem mass spectrometry. Results showed that Insulin degludec is highly stable under low temperature, mechanical agitation and repeated freeze and thaw stress conditions but elevated temperature and high acidic condition lead to formation of aggregates and also chemical modifications including deamidation, isomerization and oxidation. Such different chemical degradation pathways are due to presence of variable reactive moieties in Insulin degludec structure. Insulin degludec is highly vulnerable to oxidation at the sulfur containing cysteine residue in B chain in position B7 forming trioxidation derivative when exposed to hydrogen peroxide. Formation of A21-Asp and A18-Asp deamidated variants as well as B3-Asp and B3-isoAsp deamidated variants are prominent degradation pathways at neutral pH but at elevated temperature.

Scaling analysis reveals the mechanism and rates of prion replication in vivo

Prions consist of pathological aggregates of cellular prion protein and have the ability to replicate, causing neurodegenerative diseases, a phenomenon mirrored in many other diseases connected to protein aggregation, including Alzheimer's and Parkinson's diseases. However, despite their key importance in disease, the individual processes governing this formation of pathogenic aggregates, as well as their rates, have remained challenging to elucidate in vivo. Here we bring together a mathematical framework with kinetics of the accumulation of prions in mice and microfluidic measurements of aggregate size to dissect the overall aggregation reaction into its constituent processes and quantify the reaction rates in mice. Taken together, the data show that multiplication of prions in vivo is slower than in in vitro experiments, but efficient when compared with other amyloid systems, and displays scaling behavior characteristic of aggregate fragmentation. These results provide a framework for the determination of the mechanisms of disease-associated aggregation processes within living organisms.

Addition of Regular Insulin to Ternary Parenteral Nutrition: A Stability Study

Background: Parenteral nutrition (PN) is a complex medium in which added insulin can become unstable. The aim of this study is, therefore, to evaluate the stability of insulin in PN and to identify influencing factors. Methods: A total of 20 IU/L of regular insulin was added to PN in either glass or Ethylene Vinyl Acetate (EVA) containers. A 24 h stability study was performed via an electrochemiluminescence immunoassay in different media: A ternary PN admixture, separate compartments of the PN bag and a binary admixture. This study was repeated in the absence of zinc, with the addition of serum albumin or tween and with pH adjustment (3.6 or 6.3). Insulin concentration at t time was expressed as a percentage of the initial insulin concentration. Analysis of covariance (ANCOVA) was applied to determine the factors that influence insulin stability. Results: In all PN admixtures, the insulin concentration ratio decreased, stabilising at a 60% and then plateauing after 6 h. At pH 3.6, the ratio was above 90%, while at pH 6.3 it decreased, except in the amino acid solution. ANCOVA (r² = 0.68, p = 0.01) identified dextrose and pH as significant factors influencing insulin stability. Conclusion: A low pH level seems to stabilise insulin in PN admixtures. The influence of dextrose content suggests that insulin glycation may influence stability.

Release of insulin granules by simultaneous, high-speed correlative SICM-FCM

Exocytosis of peptides and steroids stored in a dense core vesicular (DCV) form is the final step of every secretory pathway, indispensable for the function of nervous, endocrine and immune systems. The lack of live imaging techniques capable of direct, label-free visualisation of DCV release makes many aspects of the exocytotic process inaccessible to investigation. We describe the application of correlative scanning ion conductance and fluorescence confocal microscopy (SICM-FCM) to study the exocytosis of individual granules of insulin from the top, nonadherent, surface of pancreatic β-cells. Using SICM-FCM, we were first to directly follow the topographical changes associated with physiologically induced release of insulin DCVs. This allowed us to report the kinetics of the full fusion of the insulin vesicle as well as the subsequent solubilisation of the released insulin crystal.

A rational route to hybrid aptamer-molecularly imprinted magnetic nanoprobe for recognition of protein biomarkers in human serum

Although antibody has played a great role in highly specific recognition of protein biomarkers, it faces poor stability, reproducibility, high-cost and time-consuming preparation, etc. Here, aptamer and molecularly imprinted polymers (MIPs), both as promising substitutes of antibody, were integrated onto magnetic nanoparticles by Au-S bonds and SiO₂ as imprinted layer for preparing a new nanoprobe. Highly specific and sensitive recognition of different protein biomarkers, such as insulin for diabetes and alpha-fetoprotein (AFP) for hepatic carcinoma, were achieved respectively by the system of combining hybrid aptamer-molecularly imprinted magnetic nanoprobe and mass spectrometry. With the double affinities offered by aptamer-MIPs, insulin can be detected at 0.5 ng mL^-1 in human serum dilution, the equlibrium dissociation constant between nanoprobe and insulin is measured as 23.61 ± 2.27 μM. Likewise, AFP can be sufficiently detected in human saliva dilution from 1000 ng mL^-1 to 20 ng mL^-1, and two patients with hepatic carcinoma are discriminated from healthy person due to the abnormally high expression of AFP in serum.

Nanolipogels as a cell-mimicking platform for controlled release of biomacromolecules

We present studies of protein (insulin) efflux rates from nano-sized core-shell systems with a gelled core and a lipid bilayer (nanolipogels). The efflux control mechanism is the manipulation of mesh size, and we show that diffusion control via crosslinking is the dominant mechanism for efflux control. The concept is inspired by the macromolecular crowding effect in human cells, which may be considered as a physical network of undefined mesh size. Our bio-inspired system is made of chemically crosslinked water-swellable poly(ethylene glycol) diacrylate cores, whose mesh size can be manipulated to yield a quantifiable crowding effect that then leads to predictable release rates for biomacromolecules.

Identification of recombinant human insulin and biosynthetic insulin analogues by multiplexed targeted unlabeled mass spectrometry of proteotypic tryptic peptides

Direct qualitative methods that allow the rapid screening and identification of insulin products during early stages of the drug development process and those already in the market can be of great utility for manufacturers and regulatory agencies and the recent scientific literature describes several methods. Herein, a qualitative proteomic method is presented for the identification of recombinant human insulin and all marketed biosynthetic analogues -insulin lispro, aspart, glulisine, glargine, detemir and degludec- via tryptic digestion and identification of proteotypic peptides for each insulin. Individual insulins were first denatured under reducing conditions and the cysteine residues blocked by iodoacetamide. The proteins were then digested with trypsin and the peptide products separated by reversed phase liquid chromatography on an Ascentis® Express ES-C18 column and detected by positive polarity ESI-MS/MS. The digestion peptides were characterized using a multiplexed MRM approach that monitors the fragmentation of the doubly charged unlabeled precursor ion of each peptide into a collection of signature y and b ions. The MRM transitions for the individual peptides were optimized to allow maximal ionization on a standard triple quadrupole mass spectrometer. All products of the digestion procedure for all insulins were detected with adequate signal intensity except for the C-terminal B30Thr whenever it was present and cleaved and the tryptic B1-3 tripeptide of insulin glulisine. The unique proteotypic peptides identified for each of the insulin analogues coupled with their signature y and b ions permitted the unambiguous verification of all sequence variations and chemical modifications. The elution of the A polypeptide chain for all insulins and the tryptic peptides of the B chain, with the exception of a very few, occurred around the same time point. This underscores the close similarity in the physicochemical properties between the digestion peptides and is consistent with the subtle variations in amino acid sequence among the various insulins. Therefore, the identification and distinction of the different types of insulin based solely on the chromatographic retention time of their respective proteolytic products can be deceptive without proper mass spectrometric analysis and may result in false positives.

Towards Point-of-Care Insulin Detection

Good glucose management through an insulin dose regime based on the metabolism of glucose helps millions of people worldwide manage their diabetes. Since Banting and Best extracted insulin, glucose management has improved due to the introduction of insulin analogues that act from 30 minutes to 28 days, improved insulin dose regimes, and portable glucose meters, with a current focus on alternative sampling sites that are less invasive. However, a piece of the puzzle is still missing-the ability to measure insulin directly in a Point-of-Care device. The ability to measure both glucose and insulin concurrently will enable better glucose control by providing an improved estimate for insulin sensitivity, minimizing variability in control, and maximizing safety from hypoglycaemia. However, direct detection of free insulin has provided a challenge due to the size of the molecule, the low concentration of insulin in blood, and the selectivity against interferants in blood. This review summarizes current insulin detection methods from immunoassays to analytical chemistry, and sensors. We also discuss the challenges and potential of each of the methods towards Point-of-Care insulin detection.

Enhanced intranasal insulin delivery by formulations and tumor protein-derived protein transduction domain as an absorption enhancer

One of the key factors for successful development of an intranasal insulin formulation is an absorption enhancer that would deliver insulin efficiently across nasal membranes without causing damage to mucosa or inducing protein aggregation under physiological conditions. In the present study, a protein transduction domain (PTD1) and its L-form with the double substitution A6L and I8A (PTD4), derived from human translationally controlled tumor protein, were used as absorption enhancers. PTD4 exhibited higher compatibility with insulin in terms of biophysical properties analyzed using μDSC, DLS, and CD. In addition, thermodynamic properties indicated stable complex formation but higher propensity of protein aggregation. Arginine hydrochloride (ArgHCl) was used to suppress protein aggregation and carbohydrates (i.e., mannitol, sucrose, and glycerin) were used as osmolytes in the formulation. The relative bioavailability of insulin co-administered intranasally using PTD4, 16 mg/mL glycerin and 100 mM ArgHCl was 58% and that using PTD4, 1 w/v% sucrose, and 25 mM ArgHCl was 53% of the bioavailability obtained via the subcutaneous route. These values represented a remarkable increase in bioavailability of intranasal insulin, causing a significant decrease in blood glucose levels within one hour. The pharmacokinetic properties of intranasal absorption were dependent on the concentration of carbohydrates used. These results suggest that the newly designed formulations with PTD represent a useful platform for intranasal delivery of insulin and other biomolecules.

Monitoring real-time hormone release kinetics via high-content 3-D imaging of compensatory endocytosis

High-content real-time imaging of hormone secretion in tissues or cell populations is a challenging task, which is unlikely to be resolved directly, despite immense translational value. We approach this problem indirectly, using compensatory endocytosis, a process that closely follows exocytosis in the cell, as a surrogate read-out for secretion. The tissue is immobilized in an open-air perifusion chamber and imaged using a two-photon microscope. A fluorescent polar tracer, perifused through the experimental circuit, gets trapped into the cells via endocytosis, and is quantified using a feature-detection algorithm. The signal of the tracer that accumulates into the endocytotic system reliably reflects stimulated exocytosis, which is demonstrated via co-imaging of the latter using existing reporters. A high signal-to-noise ratio and compatibility with multisensor imaging affords the real-time quantification of the secretion at the tissue/population level, whereas the cumulative nature of the signal allows imprinting of the "secretory history" within each cell. The technology works for several cell types, reflects disease progression and can be used for human tissue.

Insulin Instability in Parenteral Nutrition Admixtures

BACKGROUND

Biosynthetic human recombinant short-acting insulin is added to parenteral nutrition (PN) admixtures to nourish glucose-intolerant patients. Insulin, however, is electrostatically attracted and inactivated by ethyl-vinyl-acetate (EVA) bags and filling system tubes. Our aim was to verify and quantify the presence of insulin in PN with and without intravenous lipid emulsion (ILE), just after addition (T0) until the infusion's end (T24).

METHODS

Four undiluted samples of 12 different PN complete admixtures (6 with ILE and 6 without), each containing 250 g of glucose in a 2000 mL volume, were taken and analyzed at T0 and T24 by an automated electrochemiluminescence immunoassay after the addition of biosynthetic human recombinant short-acting insulin at increasing doses (from 6 to 72 IU/bag) by an automated compounding device. Assay sensitivity was set at 2 μIU/mL. Admixtures with and without ILE were compared in terms of insulin-detected amounts at T0 and T24.

RESULTS

Regardless of the amount initially provided, insulin was missing in PN without ILE. In admixtures with ILE, the greater the insulin and ILE doses initially included, the higher the insulin availability at T0 and T24, both in absolute terms and as a percentage of the initial amount (from 3 to 81% at T0 and from 2.5 to 72.5% at T24). ILE may prevent insulin attraction to plastic surfaces.

CONCLUSIONS

Insulin is recovered in the presence of ILE in PN even though considerable amounts are untraceable. This aspect needs verification. Until then, insulin should safely be injected in a different manner in uncontrolled situations.

Fusion pore in exocytosis: More than an exit gate? A β-cell perspective

Secretory vesicle exocytosis is a fundamental biological event and the process by which hormones (like insulin) are released into the blood. Considerable progress has been made in understanding this precisely orchestrated sequence of events from secretory vesicle docked at the cell membrane, hemifusion, to the opening of a membrane fusion pore. The exact biophysical and physiological regulation of these events implies a close interaction between membrane proteins and lipids in a confined space and constrained geometry to ensure appropriate delivery of cargo. We consider some of the still open questions such as the nature of the initiation of the fusion pore, the structure and the role of the Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor (SNARE) transmembrane domains and their influence on the dynamics and regulation of exocytosis. We discuss how the membrane composition and protein-lipid interactions influence the likelihood of the nascent fusion pore forming. We relate these factors to the hypothesis that fusion pore expansion could be affected in type-2 diabetes via changes in disease-related gene transcription and alterations in the circulating lipid profile. Detailed characterisation of the dynamics of the fusion pore in vitro will contribute to understanding the larger issue of insulin secretory defects in diabetes.

Application of capability indices and control charts in the analytical method control strategy

In this study, we assessed the usefulness of control charts in combination with the process capability indices, C_pm and C_pk , in the control strategy of an analytical method. The traditional X-chart and moving range chart were used to monitor the analytical method over a 2-year period. The results confirmed that the analytical method is in-control and stable. Different criteria were used to establish the specifications limits (i.e. analyst requirements) for fixed method performance (i.e. method requirements). If the specification limits and control limits are equal in breadth, the method can be considered "capable" (C_pm  = 1), but it does not satisfy the minimum method capability requirements proposed by Pearn and Shu (2003). Similar results were obtained using the C_pk index. The method capability was also assessed as a function of method performance for fixed analyst requirements. The results indicate that the method does not meet the requirements of the analytical target approach. A real-example data of a SEC with light-scattering detection method was used as a model whereas previously published data were used to illustrate the applicability of the proposed approach.

Immunoassay quantification of human insulin added to ternary parenteral nutrition containers: comparison of two methods

Adding insulin directly into infusion bags seems to be a useful method for controlling hyperglycemia in patients under ternary parenteral nutrition (TPN). Its efficacy is assessed by glycemic monitoring but few data are available on insulin stability in this situation. Among the various methods for quantifying insulin levels in human serum, the immunoassay ones seemed potentially appropriate for a TPN admixture containing high lipid concentrations. We sought to identify and validate which of two immunoassay methods was the better to quantify human insulin and consequently be adapted to studying its stability in a TPN admixture. Two immunoassay methods to quantify recombinant human insulin were assessed in industrial TPN: an immunoradiometric assay (IRMA) and an immunoelectrochemiluminometric assay (IECMA). Validation trials for both methods were based on the accuracy profile method. Interference with immunometric assays due to the high lipidic content of TPN was eliminated through an improved preparation protocol using a bovine serum albumin (BSA) diluted in phosphate buffer saline (PBS). The relative total error of IECMA varied from 1.74 to 4.52% while it varied from -0.32 to 8.37% with IRMA. Only IECMA provided an accuracy profile with a 95% confidence interval of calculated-tolerance limits falling between the chosen acceptance limits (i.e., total error ≤±10%). IECMA combined with a BSA dilution is a simple and semi-automatic method that provides an accurate quantification of human insulin in a TPN admixture without any interference from lipids.

Silicon nanopore membrane (SNM) for islet encapsulation and immunoisolation under convective transport

Problems associated with islet transplantation for Type 1 Diabetes (T1D) such as shortage of donor cells, use of immunosuppressive drugs remain as major challenges. Immune isolation using encapsulation may circumvent the use of immunosuppressants and prolong the longevity of transplanted islets. The encapsulating membrane must block the passage of host’s immune components while providing sufficient exchange of glucose, insulin and other small molecules. We report the development and characterization of a new generation of semipermeable ultrafiltration membrane, the silicon nanopore membrane (SNM), designed with approximately 7 nm-wide slit-pores to provide middle molecule selectivity by limiting passage of pro-inflammatory cytokines. Moreover, the use of convective transport with a pressure differential across the SNM overcomes the mass transfer limitations associated with diffusion through nanometer-scale pores. The SNM exhibited a hydraulic permeability of 130 ml/hr/m²/mmHg, which is more than 3 fold greater than existing polymer membranes. Analysis of sieving coefficients revealed 80% reduction in cytokines passage through SNM under convective transport. SNM protected encapsulated islets from infiltrating cytokines and retained islet viability over 6 hours and remained responsive to changes in glucose levels unlike non-encapsulated controls. Together, these data demonstrate the novel membrane exhibiting unprecedented hydraulic permeability and immune-protection for islet transplantation therapy.

Highly stable selenadiazole derivatives induce bladder cancer cell apoptosis and inhibit cell migration and invasion through the activation of ROS-mediated signaling pathways

Herein highly stable selenadiazole derivatives were synthesized and found to be able to induce bladder cancer cell apoptosis and inhibit cell migration and invasion through the activation of ROS-mediated pathways.

Designed synthesis of aptamer-immobilized magnetic mesoporous silica/Au nanocomposites for highly selective enrichment and detection of insulin

We designed and synthesized aptamer-immobilized magnetic mesoporous silica/Au nanocomposites (MMANs) for highly selective detection of unlabeled insulin in complex biological media using MALDI-TOF MS. The aptamer was easily anchored onto the gold nanoparticles in the mesochannels of MMANs with high capacity for highly efficient and specific enrichment of insulin. With the benefit from the size-exclusion effect of the mesoporous silica shell with a narrow pore size distribution (∼2.9 nm), insulin could be selectively detected despite interference from seven untargeted proteins with different size dimensions. This method exhibited an excellent response for insulin in the range 2-1000 ng mL(-1). Moreover, good recoveries in the detection of insulin in 20-fold diluted human serum were achieved. We anticipate that this novel method could be extended to other biomarkers of interest and potentially applied in disease diagnostics.

Substrate concentration influences effective radial diffusion coefficient in canine cortical bone

Transport of nutrients and waste across osseous tissue is dependent on the dynamic micro and macrostructure of the tissue; however little quantitative data exists examining how this transport occurs across the entire tissue. Here we investigate in vitro radial diffusion across a section of canine tissue, at dimensions of several hundred microns to millimeters, specifically between several osteons connected through a porous microstructure of Volkmann's canals and canaliculi. The effective diffusion coefficient is measured by a "sample immersion" technique presented here, in which the tissue sample was immersed in solution for 18-30 h, image analysis software was used to quantify the solute concentration profile in the tissue, and the data were fit to a mathematical model of diffusion in the tissue. Measurements of the effective diffusivity of sodium fluorescein using this technique were confirmed using a standard two-chamber diffusion system. As the solute concentration increased, the effective diffusivity decreased, ranging from 1.6 × 10(-7) ± 3.2 × 10(-8) cm(2)/s at 0.3 μM to 1.4 × 10(-8) ± 1.9 × 10(-9) cm(2)/s at 300 μM. The results show that there is no significant difference in mean diffusivity obtained using the two measurement techniques on the same sample, 3.3 × 10(-8) ± 3.3 × 10(-9) cm(2)/s (sample immersion), compared to 4.4 × 10(-8) ± 1.1 × 10(-8) cm(2)/s (diffusion chamber).

Membranes to achieve immunoprotection of transplanted islets

Transplantation of islet or beta cells is seen as the cure for type 1 diabetes since it allows physiological regulation of blood glucose levels without requiring any compliance from the patients. In order to circumvent the use of immunosuppressive drugs (and their side effects), semipermeable membranes have been developed to encapsulate and immunoprotect transplanted cells. This review presents the historical developments of immunoisolation and provides an update on the current research in this field. A particular emphasis is laid on the fabrication, characterization and performance of membranes developed for immunoisolation applications.

Facile Synthesis of Robust Free-Standing TiO2 Nanotubular Membranes for Biofiltration Applications

Robust monodisperse nanoporous membranes have a wide range of biotechnological applications, but are often difficult or costly to fabricate. Here, a simple technique is reported to produce free-standing TiO₂ nanotubular membranes with through-hole morphology. It consists in a 3-step anodization procedure carried out at room temperature on a Ti foil. The first anodization (1 h at 80 V) is used to pattern the surface of the metallic foil. Then, the second anodization (24 h at 80 V) produces the array of TiO₂ nanotubes that will constitute the final membrane. A higher voltage anodization (3-5 minutes at 180 V) is finally applied to detach the TiO₂ nanotubular layer from the underlying Ti foil. In order to completely remove the barrier layer that obstructs some pores of the membrane, the latter is etched 2 minutes in a buffered oxide etch solution. The overall process produces 60 μm-thick TiO₂ nanotubular membranes with tube openings of 110 nm on one side and 73 nm on the other side. The through-hole morphology of these membranes has been verified by performing diffusion experiments with glucose, insulin and immunoglobulin G where in differences in diffusion rate are observed based on molecular weight. Such biocompatible TiO₂ nanotubular membranes, with controlled pore size and morphology, have broad biotechnological and biomedical applications.

Light scattering coupled with reversed phase chromatography to study protein self-association under separating conditions

An on-line method, coupling reversed phase chromatography with static light scattering, was developed to determine the association state of freshly eluted proteins. Under downstream process conditions, human insulin desB30 and human insulin AspB28 were tested at concentrations up to 8.5mg/mL. The refractive index increment (dn/dc) for insulin was found to depend strongly on the solvent used. A refractive index increment of 0.184±0.003mL/g was found in an aqueous buffer, pH 7.4, whereas the value was 0.155±0.003mL/g in 30%, w/w ethanol. The methodology combines on-line SLS and UV measurements with the pre-determined refractive index increment values. The developed on-line method was verified by standard off-line measurements establishing the association state at concentrations between 0.2 and 6.0mg/mL. The equipment was calibrated utilizing insulin under conditions reported to ensure either monomer or hexamer forms. The self-association of human insulin desB30 was found to be strongly suppressed in 30%, w/w ethanol at pH 7.4 in which the monomer predominates. When stabilized by zinc ions in 30%, w/w ethanol at pH 7.4, an average association number of 3.7 was found. These data demonstrate the effect of ethanol to lower strongly the energy advantage by protein self-association. Potassium chloride and/or calcium chloride in the eluents were found to be of no consequence to the association state.

Insulin Release Dynamics from Poly(diethylaminoethyl methacrylate) Hydrogel Systems

Novel glucose-sensitive systems for the release of insulin from poly(diethylaminoethyl methacrylate) (PDEAEM) micro-particles and nanoparticles decorated with glucose oxidase and catalase enzymes have been developed. The effect of polymer composition and loading conditions on the insulin loading efficiency and release was studied. The optimal conditions for loading insulin into PDEAEM microparticles were found to be at a loading pH of 5.6, particle to insulin mass ratio of 7:1, a concentration of 1.0 mg/mL insulin, and a collapsing pH of approximately 9.5. Microparticles exhibited a responsive (pH) or intelligent (glucose) release of insulin from a stimulus. Microparticles that had a nominal crosslinking ratio of 10% released a third of the insulin payload after a single stimulus, compared to nearly 70% for microparticles with a 3% crosslinking ratio. PDEAEM micro particles of 150 µm diameter showed promise as components of a system of automated, intelligent delivery method for insulin to type I diabetics.

Size-Exclusion Chromatography for the Analysis of Protein Biotherapeutics and their Aggregates

In recent years, the use and number of biotherapeutics has increased significantly. For these largely protein-based therapies, the quantitation of aggregates is of particular concern given their potential effect on efficacy and immunogenicity. This need has renewed interest in size-exclusion chromatography (SEC). In the following review we will outline the history and background of SEC for the analysis of proteins. We will also discuss the instrumentation for these analyses, including the use of different types of detectors. Method development for protein analysis by SEC will also be outlined, including the effect of mobile phase and column parameters (column length, pore size). We will also review some of the applications of this mode of separation that are of particular importance to protein biopharmaceutical development and highlight some considerations in their implementation.

Folate-decorated PLGA nanoparticles as a rationally designed vehicle for the oral delivery of insulin

Aims: The present study reports a novel approach for enhancing the oral absorption and hypoglycemic activity of insulin via encapsulation in folate-(FA) coupled polyethylene glycol (PEG)ylated polylactide-co-glycolide (PLGA) nanoparticles (NPs; FA-PEG-PLGA NPs). Materials & methods: Insulin-loaded FA-PEG-PLGA NPs (size ∼260 nm; insulin loading ∼6.5% [w/w]; encapsulation efficiency: 87.0 ± 1.92%) were prepared by double-emulsion solvent evaporation method. The bioavailability and hypoglycemic activity of orally administered FA–insulin NPs were studied in diabetic rats. Results & conclusion: FA-PEG-PLGA NPs (50 U/kg) exhibited a twofold increase in the oral bioavailability (double hypoglycemia) without any hypoglycemic shock as compared to subcutaneously administered standard insulin solution. Insulin NPs maintained a continual blood glucose level for 24 h, which, however, was transient (<8 h) in the case of subcutaneous insulin and associated with severe hypoglycemic shock. Overall, we have developed a patient-compliant, oral nanoformulation of insulin, once-daily administration of which would be sufficient to control diabetes for at least 24 h.

Original submitted 16 November 2011; Revised submitted 2 February 2012; Published online 14 May 2012

Bioavailability and variability of biphasic insulin mixtures

Absorption of subcutaneously administered insulin is associated with considerable variability. Some of this variability was quantitatively explained for both soluble insulin and insulin suspensions in a recent contribution to this journal (Søeborg et al., 2009). In the present article, the absorption kinetics for mixtures of insulins is described. This requires that the bioavailability of the different insulins is considered. A short review of insulin bioavailability and a description of the subcutaneous depot thus precede the presentation of possible mechanisms associated with subcutaneous insulin degradation. Soluble insulins are assumed to be degraded enzymatically in the subcutaneous tissue. Suspended insulin crystals form condensed heaps that are assumed to be degraded from their surface by invading macrophages. It is demonstrated how the shape of the heaps affects the absorption kinetics. Variations in heap formation thus explain some of the additional variability associated with suspended insulins (e.g. NPH insulins) compared to soluble insulins. The heap model also describes how increasing concentrations of suspended insulins lead to decreasing bioavailability and lower values of Cmax. Together, the findings constitute a comprehensive, quantitative description of insulin absorption after subcutaneous administration. The model considers different concentrations and doses of soluble insulin, including rapid acting insulin analogues, insulin suspensions and biphasic insulin mixtures. The results can be used in both the development of novel insulin products and in the planning of the treatment of insulin dependent diabetic patients.

Cationic Nanogels Based On Diethylaminoethyl Methacrylate

The effect of polymer composition and polymerization parameters such as comonomers, crosslinking ratio, and polymerization method, on the surface characteristics, surface chemistry, and swelling response of crosslinked 2-(diethylaminoethyl methacrylate) (DEAEM) and polyethylene glycol monoethyl ether monomethacrylate (PEGMMA) nanogels was studied. A novel inverse-emulsion polymerization method was developed, which formed latex nanoparticles on the order of 100-400 nm. The properties of these nanogels were compared to microparticles synthesized via solution polymerization. The new polymerization method allowed the incorporation of PEG surface tethers of lengths 400 Da up to 2000 Da. Surface tethers successfully decreased the ζ-potential of these nanogels from 70 mV to 30 mV in acidic conditions and from -60 mV to 2 mV in basic media. Nanogels swelled from 100 nm in basic media to 800 nm in acidic media due to the protonation of the tertiary amine on DEAEM.

Elution behavior of insulin on high-performance size exclusion chromatography at neutral pH

The pharmacopoeia protocol for HP-SEC of insulin, using an acidic non-physiological eluent, does not represent insulin's association state in the formulation. This study aimed to evaluate insulin's elution behavior in HP-SEC in a "physiological" (aqueous, neutral pH) eluent, using on-line UV absorption and multi-angle laser light scattering detection. The effect of insulin concentration and association state in the formulation (monitored by circular dichroism) and eluent composition (zinc ion, arginine) on its elution behavior was assessed. We showed that the elution behavior of insulin in "physiological" HP-SEC is affected by both dynamic association-dissociation of insulin molecules and insulin-column interactions. Insulin molecules re-equilibrated in the HP-SEC eluent, making its elution behavior practically insensitive to the association state of insulin in the formulation. Zinc ions in the eluent promoted association of insulin to hexamers, whereas arginine overruled the effect of zinc ions and induced on-column dissociation of insulin to dimers and monomers. Combined results from "physiological" and compendial HP-SEC were shown to provide a better view of the aggregation state of heat-stressed insulin than either of the single methods. The insights obtained with this study are crucial for a proper evaluation of HP-SEC data of insulin.

Polybasic Nanomatrices Prepared By UV-initiated Photopolymerization

A novel method for synthesizing nanoscale polymer networks that swell in acidic media is described here using photoinitiated emulsion polymerization. These nanomatrices consist of a crosslinked core of poly[2-(diethylamino)ethyl methacrylate] surface grafted with poly(ethylene glycol) (PDGP) with an average diameter of 50-150 nm. Control over mesh size, surface charge, encapsulation efficiency and in vitro biocompatibility was obtained by varying crosslinking density. The ability to image nanomatrices in their dry state using conventional scanning electron microscopy was made possible by increasing crosslinking density. Theoretical calculations of matrix mesh sizes were supported by the encapsulation of both insulin and colloidal gold 2-5 nm in diameter. The ability to sequester and control the aggregation of an inorganic phase confirmed their use as a nanocomposite matrix material. These networks could be used as imaging agents, drug delivery devices or as components of sensing devices.

Insulin nanoparticles: stability and aerosolization from pressurized metered dose inhalers

Nanoparticles containing insulin have been produced by emulsification processes followed by freeze-drying. Purified nanoparticles were suspended in hydrofluoroalkane (HFA) 134a, using essential oils (cineole and citral) as suspension stabilizers to form pressurized metered dose inhaler (pMDI) formulations. The retention of insulin integrity after formulation processing was determined using high performance liquid chromatography (HPLC), size exclusion chromatography (SEC), circular dichroism (CD) and fluorescence spectroscopy. The results indicated that the native structure of insulin was retained after formulation processing. Aerosolization properties of the manufactured pMDI formulations were determined using a multi-stage liquid impinger. The results showed that the nanoparticles were suitable for peripheral lung deposition, with a fine particle fraction (FPF(<1.7 microm)) of approximately 45% (w/w). In conclusion, the pMDI formulations with nanoparticles containing insulin developed in this study have the potential to deliver protein therapeutics via inhalation for systemic action.