Insulin particle formation in supersaturated aqueous solutions of poly(ethylene glycol)

Large-scale crystallization as an intermediate processing step in insulin downstream process: explored advantages and identified tool for process intensification

The rising global prevalence of diabetes and increasing demand for insulin, calls for an increase in accessibility and affordability of insulin drugs through efficient and cost-effective manufacturing processes. Often downstream operations become manufacturing bottlenecks while processing a high volume of product. Thus, process integration and intensification play an important role in reducing process steps and time, volume reduction, and lower equipment footprints, which brings additional process efficiencies and lowers the production cost. Manufacturers thrive to optimize existing unit operation to maximize its benefit replacing with simple but different efficient technologies. In this manuscript, the typical property of insulin in forming the pH-dependent zinc-insulin complex is explored. The benefit of zinc chloride precipitation/crystallization has been shown to increase the in-process product purity by reducing the product and process-related impurities. Incorporation of such unit operation in the insulin process has also a clear potential for replacing the high cost involved capture chromatography step. Same time, the reduction in volume of operation, buffer consumption, equipment footprint, and capabilities of product long time storage brings manufacturing flexibility and efficiencies. The data and capabilities of simple operation captured here would be significantly helpful for insulins and other biosimilar manufacturer to make progresses on cost-effective productions.

Insulin Granule-Loaded MicroPlates for Modulating Blood Glucose Levels in Type-1 Diabetes

Type-1 diabetes (T1DM) is a chronic metabolic disorder resulting from the autoimmune destruction of β cells. The current standard of care requires multiple, daily injections of insulin and accurate monitoring of blood glucose levels (BGLs); in some cases, this results in diminished patient compliance and increased risk of hypoglycemia. Herein, we engineered hierarchically structured particles comprising a poly(lactic-co-glycolic) acid (PLGA) prismatic matrix, with a 20 × 20 μm base, encapsulating 200 nm insulin granules. Five configurations of these insulin-microPlates (INS-μPLs) were realized with different heights (5, 10, and 20 μm) and PLGA contents (10, 40, and, 60 mg). After detailed physicochemical and biopharmacological characterizations, the tissue-compliant 10H INS-μPL, realized with 10 mg of PLGA, presented the most effective release profile with ∼50% of the loaded insulin delivered at 4 weeks. In diabetic mice, a single 10H INS-μPL intraperitoneal deposition reduced BGLs to that of healthy mice within 1 h post-implantation (167.4 ± 49.0 vs 140.0 ± 9.2 mg/dL, respectively) and supported normoglycemic conditions for about 2 weeks. Furthermore, following the glucose challenge, diabetic mice implanted with 10H INS-μPL successfully regained glycemic control with a significant reduction in AUC_0-120min (799.9 ± 134.83 vs 2234.60 ± 82.72 mg/dL) and increased insulin levels at 7 days post-implantation (1.14 ± 0.11 vs 0.38 ± 0.02 ng/mL), as compared to untreated diabetic mice. Collectively, these results demonstrate that INS-μPLs are a promising platform for the treatment of T1DM to be further optimized with the integration of smart glucose sensors.

A novel peptide design aids in the expression and its simplified process of manufacturing of Insulin Glargine in Pichia pastoris

Manufacturing of insulin and its analogues relied upon in vitro enzymatic cleavages of its precursor forms (single chain precursor, SCP) at both ends of a connecting peptide (C-peptide) that links the respective B-chain and A-chains to corresponding final forms. We have demonstrated a simplified approach of cleaving P. pastoris expressed SCP, distinctly at one site for conversion to insulin glargine. The design of the precursor was made in such a way that there is no C-peptide in the precursor which needs to be removed in the final product. Instead of traditional both side cleavage of the C-peptide and removing the C-peptide (by trypsin), followed by 2nd enzyme reaction (typically carboxipeptidase B), present work established only one side cleavage of the sequence by only trypsin converts the precursor to final insulin glargine product. The novel design of the precursor helped in producing insulin glargine in a single step with an application of single enzyme brought high degree of process efficiencies. Highly purified product was generated through two reversed phase high pressure chromatographic steps. Purified product was compared with the reference product Lantus®, for various physico-chemical and biological properties. Primary, secondary and tertiary structures as well as biological pharmaco-dynamic effects were found comparable. High cell density fermentation that gave a good yield of the SCP, a single step conversion to insulin glargine, enabled by a unique design of SCP and a distinct purification approach, has led to a simplified and economical manufacturing process of this important drug used to treat diabetes. KEY POINTS: • Novel concept for processing single chain precursor of insulin glargine • Simple and economic process for insulin glargine • Physicochemical characterization and animal Pharmacodynamics show similarity to Lantus.

Quantitative analysis of weakly bound insulin oligomers in solution using polarized multidimensional fluorescence spectroscopy

Being able to measure the size and distribution of oligomers in solution is a critical issue in the manufacture and stability of insulin and other protein formulations. Measuring oligomers reliably can however be complicated, due to their fragile self-assembled structures, which are held together by weak forces. This can cause issues in chromatographic based methods, where dissociation or re-equilibration of oligomer populations can occur e.g. upon dilution in a different eluting buffer, but also for light scattering based methods like dynamic light scattering (DLS) where the size difference involved (often less than a factor 3) does not allow mixtures of oligomers to be resolved. Intrinsic fluorescence offers an attractive alternative as it is non-invasive, sensitive but also because it contains scattered light when implemented via excitation emission matrix (EEM) measurements, that is sensitive to changes in particle size. Here, using insulin at formulation level concentrations, we show for the first time how EEM can both discriminate and quantify the proportion of oligomeric states in solution. This was achieved by using the Rayleigh scatter (RS) band and the fluorescence signal contained in EEM. After validating size changes with DLS, we show in particular how the volume under the RS band correlated linearly with protein/oligomer molecular weight, in agreement with the Debye-Zimm relationship. This was true for the RS data from both EEM and polarized EEM (pEEM) measurements, the latter providing a stronger scatter signal, more sensitive to particle size changes. The fluorescence signal was then used with multivariate curve resolution (MCR) to quantify more precisely the soluble oligomer composition of insulin solutions. In conditions that promoted the formation of mainly one type of oligomer (monomer, dimer, or hexamer), pEEM-MCR helped identify the presence of small amounts of other oligomeric forms, while in conditions that were previously said to favour the insulin tetramer, we show that in the presence of zinc, these insulin samples were instead a heterogenous mixture composed of mostly dimers and hexamers. These MCR results correlated in all cases with the observed discrimination by principal component analysis (PCA), and deviations observed in the RS data. In conclusion, using pEEM scatter and emission components with chemometric data analysis provides a unique analytical method for characterising and monitoring changes in the soluble oligomeric state of proteins.

Thermodynamic phase diagram of amyloid-β (16-22) peptide

The aggregation of monomeric amyloid β protein (Aβ) peptide into oligomers and amyloid fibrils in the mammalian brain is associated with Alzheimer's disease. Insight into the thermodynamic stability of the Aβ peptide in different polymeric states is fundamental to defining and predicting the aggregation process. Experimental determination of Aβ thermodynamic behavior is challenging due to the transient nature of Aβ oligomers and the low peptide solubility. Furthermore, quantitative calculation of a thermodynamic phase diagram for a specific peptide requires extremely long computational times. Here, using a coarse-grained protein model, molecular dynamics (MD) simulations are performed to determine an equilibrium concentration and temperature phase diagram for the amyloidogenic peptide fragment Aβ_16-22 Our results reveal that the only thermodynamically stable phases are the solution phase and the macroscopic fibrillar phase, and that there also exists a hierarchy of metastable phases. The boundary line between the solution phase and fibril phase is found by calculating the temperature-dependent solubility of a macroscopic Aβ_16-22 fibril consisting of an infinite number of β-sheet layers. This in silico determination of an equilibrium (solubility) phase diagram for a real amyloid-forming peptide, Aβ_16-22, over the temperature range of 277-330 K agrees well with fibrillation experiments and transmission electron microscopy (TEM) measurements of the fibril morphologies formed. This in silico approach of predicting peptide solubility is also potentially useful for optimizing biopharmaceutical production and manufacturing nanofiber scaffolds for tissue engineering.

Sealing hyaluronic acid microgels with oppositely-charged polypeptides: A simple strategy for packaging hydrophilic drugs with on-demand release

A simple route to deliver on demand hydrosoluble molecules such as peptides, packaged in biocompatible and biodegradable microgels, is presented. Hyaluronic acid hydrogel particles with a controlled structure are prepared using a microfluidic approach. Their porosity and their rigidity can be tuned by changing the crosslinking density. These negatively-charged polyelectrolytes interact strongly with positively-charged linear peptides such as poly-l-lysine (PLL). Their interactions induce microgel deswelling and inhibit microgel enzymatic degradability by hyaluronidase. While small PLL penetrate the whole volume of the microgel, PLL larger than the mesh size of the network remain confined at its periphery. They make a complexed layer with reduced pore size, which insulates the microgel inner core from the outer medium. Consequently, enzymatic degradation of the matrix is fully inhibited and non-affinity hydrophilic species can be trapped in the core. Indeed, negatively-charged or small neutral peptides, without interactions with the network, usually diffuse freely across the network. By simple addition of large PLL, they are packaged in the core and can be released on demand, upon introduction of an enzyme that degrades selectively the capping agent. Single polyelectrolyte layer appears as a simple generic method to coat hydrogel-based materials of various scales for encapsulation and controlled delivery of hydrosoluble molecules.

Formulation of High-Performance Dry Powder Aerosols for Pulmonary Protein Delivery

PURPOSE

Pulmonary delivery of biologics is of great interest, as it can be used for the local treatment of respiratory diseases or as a route to systemic drug delivery. To reach the full potential of inhaled biologics, a formulation platform capable of producing high performance aerosols without altering protein native structure is required.

METHODS

A formulation strategy using Particle Replication in Non-wetting Templates (PRINT) was developed to produce protein dry powders with precisely engineered particle morphology. Stability of the incorporated proteins was characterized and the aerosol properties of the protein dry powders was evaluated in vitro with an Andersen Cascade Impactor (ACI).

RESULTS

Model proteins bovine serum albumin (BSA) and lysozyme were micromolded into 1 μm cylinders composed of more than 80% protein, by mass. Extensive characterization of the incorporated proteins found no evidence of alteration of native structures. The BSA formulation produced a mass median aerodynamic diameter (MMAD) of 1.77 μm ± 0.06 and a geometric standard deviation (GSD) of 1.51 ± 0.06 while the lysozyme formulation had an MMAD of 1.83 μm ± 0.12 and a GSD of 1.44 ± 0.03.

CONCLUSION

Protein dry powders manufactured with PRINT could enable high-performance delivery of protein therapeutics to the lungs.

Liquid Droplet of Protein-Polyelectrolyte Complex for High-Concentration Formulations

The formulation of high-concentration protein solutions is a challenging issue for achieving subcutaneous administration. Previously, we developed a method of precipitation-redissolution using polyelectrolyte as a precipitant to produce protein solutions at high concentrations. However, the redissolution yield of proteins was insufficient. This study aims to optimize the solution conditions for practical applications by combining IgG and poly-l-(glutamic acid) (polyE). A systematic analysis of solution pH and polyE size conditions revealed that an acidic condition favors precipitation, whereas neutral pH values are more effective for the redissolution. We find that the optimal size for polyE ranged from 15,000 to 50,000. This slight modification in the procedure in comparison with previous studies increased the precipitation and redissolution yields to nearly 100%, without irreversible protein denaturation. The fully reversible IgG-polyE complex formed as a droplet structure, which is similar to a condensate of liquid-liquid phase separation. The droplet structure plays an indispensable role in the salt-induced, redissolved state, which is pertinent to the new application that takes advantage of the methods to produce highly concentrated protein solutions.

Electrically controlled release of insulin using polypyrrole nanoparticles

Conducting polymers present an opportunity for developing programmable, adjustable, spatially, and temporally controllable drug delivery systems. While several small molecule drugs have been released from thin conductive polymeric films successfully, delivering large molecule therapeutics, such as polypeptides and nucleic acids, has remained a significant challenge. Poor drug loading (∼ng cm-2) of thin films coupled with film instability has, in many cases, made conducting polymer films refractory to clinical development. To address these limitations, we have utilized conductive polymer nanoparticulate backbones to controllably release insulin, a high molecular weight, clinically relevant polypeptide. We find that the interaction between insulin and the polymer scaffold can be described by a simple Langmuir-type adsorption model. By modifying the ratio of the amount of nanoparticles to the amount of insulin, we have obtained drug loading percentages estimated to be as high as 51 wt% percent. In vivo experiments in mice confirmed retained bioactivity of the released insulin after electrical stimulation.

Phase diagram of a model of the protein amelogenin

There has been considerable recent interest in the self-assembly and phase behavior of models of colloidal and protein particles with anisotropic interactions. One example of particular interest is amelogenin, an important protein involved in the formation of dental enamel. Amelogenin is primarily hydrophobic with a 25-residue charged C-terminus tail. This protein undergoes a hierarchical assembly process that is crucial to mineral deposition, and experimental work has demonstrated that the deletion of the C-terminus tail prevents this self-assembly. A simplified model of amelogenin has been proposed in which the protein is treated as a hydrophobic sphere, interacting via the Asakura-Oosawa (AO) potential, with a tethered point charge on its surface. In this paper, we examine the effect of the Coulomb interaction between the point charges in altering the phase diagram of the AO model. For the parameter case specific to amelogenin, we find that the previous in vitro experimental and model conditions correspond to the system being near the low-density edge of the metastable region of the phase diagram. Our study illustrates more generally the importance of understanding the phase diagram for proteins, in that the kinetic pathway for self-assembly and the resulting aggregate morphology depends on the location of the initial state in the phase diagram.

Hydrogel Microencapsulated Insulin-Secreting Cells Increase Keratinocyte Migration, Epidermal Thickness, Collagen Fiber Density, and Wound Closure in a Diabetic Mouse Model of Wound Healing

Wound healing is a hierarchical process of intracellular and intercellular signaling. Insulin is a potent chemoattractant and mitogen for cells involved in wound healing. Insulin's potential to promote keratinocyte growth and stimulate collagen synthesis in fibroblasts is well described. However, there currently lacks an appropriate delivery mechanism capable of consistently supplying a wound environment with insulin; current approaches require repeated applications of insulin, which increase the chances of infecting the wound. In this study, we present a novel cell-based therapy that delivers insulin to the wound area in a constant or glucose-dependent manner by encapsulating insulin-secreting cells in nonimmunogenic poly(ethylene glycol) diacrylate (PEGDA) hydrogel microspheres. We evaluated cell viability and insulin secretory characteristics of microencapsulated cells. Glucose stimulation studies verified free diffusion of glucose and insulin through the microspheres, while no statistical difference in insulin secretion was observed between cells in microspheres and cells in monolayers. Scratch assays demonstrated accelerated keratinocyte migration in vitro when treated with microencapsulated cells. In excisional wounds on the dorsa of diabetic mice, microencapsulated RIN-m cells accelerated wound closure by postoperative day 7; a statistically significant increase over AtT-20ins-treated and control groups. Histological results indicated significantly greater epidermal thickness in both microencapsulated RIN-m and AtT-20ins-treated wounds. The results suggest that microencapsulation enables insulin-secreting cells to persist long enough at the wound site for a therapeutic effect and thereby functions as an effective delivery vehicle to accelerate wound healing.

Surface functional modification of self-assembled insulin nanospheres for improving intestinal absorption

In this work we fabricated therapeutic protein drugs such as insulin as free-carrier delivery system to improve their oral absorption efficiency. The formulation involved self-assembly of insulin into nanospheres (INS) by a novel thermal induced phase separation method. In consideration of harsh environment in gastrointestinal tract, surface functional modification of INS with ɛ-poly-L-lysine (EPL) was employed to form a core-shell structure (INS@EPL) and protect them from too fast dissociation before their arriving at target uptake sites. Both INS and INS@EPL were characterized as uniformly spherical particles with mean diameter size of 150-300 nm. The process of transient thermal treatment did not change their biological potency retention significantly. In vitro dissolution studies showed that shell cross-linked of INS with EPL improved the release profiles of insulin from the self-assembled nanospheres at intestinal pH. Confocal microscopy visualization and transport experiments proved the enhanced paracellular permeability of INS@EPL in Caco-2 cells. Compared to that of INS, enteral administration of INS@EPL at 20 IU/kg resulted in more significant hypoglycemic effects in diabetic rats up to 12 h. Accordingly, the results indicated that surface functional modification of self-assembled insulin nanospheres with shell cross-linked polycationic peptide could be a promising candidate for oral therapeutic protein delivery.

Epsilon-poly-L-lysine guided improving pulmonary delivery of supramolecular self-assembled insulin nanospheres

This work presents new spherical nanoparticles that are fabricated from supramolecular self-assembly of therapeutic proteins for inhalation treatment. The formation involved self-assembly of insulin into nanospheres (INS) by a novel thermal induced phase separation method. Surface functional modification of INS with ɛ-poly-L-lysine (EPL), a homopolymerized cationic peptide, was followed to form a core-shell structure (INS@EPL). Both INS and INS@EPL were characterized as spherical particles with mean diameter size of 150-250 nm. The process of transient thermal treatment did not change their biological potency retention significantly. FTIR and CD characterizations indicated that their secondary structures and biological potencies were not changed significantly after self-assembly. The in vivo investigation after pulmonary administration, including lung deposition, alveoli distribution, pharmacological effects and serum pharmacokinetics were investigated. Compared to that of INS, intratracheal administration of INS@EPL offered a pronounced and prolonged lung distribution, as well as pharmacological effects which were indicated by the 23.4% vs 11.7% of relative bioavailability. Accordingly, the work described here demonstrates the possibility of spherical supramolecular self-assembly of therapeutic proteins in nano-scale for pulmonary delivery application.

Protein-poly(amino acid) complex precipitation for high-concentration protein formulation

A method for concentration of protein solutions is required for high-dosage protein formulation. Here, we present a precipitation-redissolution method by poly(amino acid) for proteins, including therapeutic enzymes, antibodies, and hormones. The proteins were fully precipitated by the addition of poly-L-lysine or poly-L-glutamic acid at low ionic strength, after which precipitate was dissolved at physiological ionic strength. The activities and secondary structures of redissolved proteins, especially antibodies, were almost identical to the native state. The precipitation-redissolution method is a simple and rapid technique for concentration of protein formulations.

Thermoresponsive biodegradable PEG-PCL-PEG based injectable hydrogel for pulsatile insulin delivery

An injectable biodegradable hydrogel was prepared for temperature-responsive pulsatile release of insulin. Triblock copolymer of poly(ethylene glycol)-poly(ε-caprolactone)-poly(ethylene glycol) (PEG-PCL-PEG, PECE) was prepared by ring opening bulk copolymerization and characterized using FT-IR, (1) HNMR, and gel permeation chromatography. Aqueous solution of PECE formed an injectable hydrogel, which was solution at room temperature and transformed into gel at 37°C. The temperature-responsive sol-gel transition and crystallinity of PECE hydrogel was studied and compared with pluronic, a well-studied nonbiodegradable injectable hydrogel. In vitro release study revealed that insulin release profile of PECE was similar to pluronic, and its viscosity was 1/30(th) of pluronic sol at 10,000 s(-1) shear rate. Release behavior of insulin from PECE hydrogels followed Fickian diffusion of first order. Insulin retained its secondary structure after release as confirmed by circular dichroism spectrum. A threefold increase in Fickian diffusion coefficient was evidenced when temperature was increased from 34 to 40°C because of crystalline melting of PCL part of PECE. Pulsatile release of insulin showed a correlation coefficient of 0.90 with the change of temperature.

Anisotropy of the Coulomb interaction between folded proteins: consequences for mesoscopic aggregation of lysozyme

Toward quantitative description of protein aggregation, we develop a computationally efficient method to evaluate the potential of mean force between two folded protein molecules that allows for complete sampling of their mutual orientation. Our model is valid at moderate ionic strengths and accounts for the actual charge distribution on the surface of the molecules, the dielectric discontinuity at the protein-solvent interface, and the possibility of protonation or deprotonation of surface residues induced by the electric field due to the other protein molecule. We apply the model to the protein lysozyme, whose solutions exhibit both mesoscopic clusters of protein-rich liquid and liquid-liquid separation; the former requires that protein form complexes with typical lifetimes of approximately milliseconds. We find the electrostatic repulsion is typically lower than the prediction of the Derjaguin-Landau-Verwey-Overbeek theory. The Coulomb interaction in the lowest-energy docking configuration is nonrepulsive, despite the high positive charge on the molecules. Typical docking configurations barely involve protonation or deprotonation of surface residues. The obtained potential of mean force between folded lysozyme molecules is consistent with the location of the liquid-liquid coexistence, but produces dimers that are too short-lived for clusters to exist, suggesting lysozyme undergoes conformational changes during cluster formation.

Phase diagrams and kinetics of phase transitions in protein solutions

The phase behavior of proteins is of interest for fundamental and practical reasons. The nucleation of new phases is one of the last major unresolved problems of nature. The formation of protein condensed phases (crystals, polymers, and other solid aggregates, as well as dense liquids and gels) underlies pathological conditions, plays a crucial role in the biological function of the respective protein, or is an essential part of laboratory and industrial processes. In this review, we focus on phase transitions of proteins in their properly folded state. We first summarize the recently acquired understanding of physical processes underlying the phase diagrams of the protein solutions and the thermodynamics of protein phase transitions. Then we review recent findings on the kinetics of nucleation of dense liquid droplets and crystals. We explore the transition from nucleation to spinodal decomposition for liquid-liquid separation and introduce the new concept of solution-to-crystal spinodal. We review the two-step mechanism of protein crystal nucleation, in which mesoscopic metastable protein clusters serve as precursors to the ordered crystal nuclei. The concepts and mechanisms reviewed here provide powerful tools for control of the nucleation process by varying the solution thermodynamic parameters.

Sustained prolonged topical delivery of bioactive human insulin for potential treatment of cutaneous wounds

Skin damaged by heat, radiation, or chemical exposure is difficult to treat and slow to heal. Indeed full restoration of the tissue is difficult to obtain. Sub-dermal insulin injection was recently shown to stimulate wound healing of the skin by accelerating wound closure, stimulating angiogenesis and inducing a regenerative process of healing. We have developed a topical delivery vehicle that is capable of releasing therapeutic levels of bioactive insulin for several weeks with the potential to stimulate and sustain healing. By encapsulating the crystalline form of insulin within poly(d,l-lactide-co-glycolide) microspheres, we succeeded in stabilizing and then releasing bioactive insulin for up to 25 days. To measure bioactivity we used Rat L6 myofibroblasts, stimulated them with this slow release insulin and determined activation of the receptors on the cell surface by quantifying AKT phosphorylation. There was only a minor and gradual decrease in AKT phosphorylation over time. To determine whether the slow release insulin could stimulate keratinocyte migration, wounding was simulated by scratching confluent cultures of human keratinocytes (HaCaT). Coverage of the scratch "wounds" was significantly faster in the presence of insulin released from microspheres than in the insulin-free control. Extended and sustained topical delivery of active insulin from a stable protein crystal-based reservoir shows promise in promoting tissue healing.

Shape selection in diffusive growth of colloids and nanoparticles

We report numerical investigations of a 3D model of diffusive growth of fine particles, the internal structure of which corresponds to different crystal lattices. A growing cluster (particle) is immersed in and exchanges monomer building blocks with a surrounding medium of diffusing (off-lattice) monomers. On-surface dynamics of the latter is accounted for by allowing, in addition to detachment, monomer motion to the neighboring vacant crystal sites, according to probabilistic rules mimicking local thermalization. The key new feature of our model is the focus on the growth of a single cluster, emerging as a crystalline core, without development of defects that can control large-scale growth modes. This single, defect-free core growth is imposed by the specific dynamic rules assumed. Our results offer a possible explanation of the experimentally observed shape uniformity (i.e., fixed, approximately evenly sized proportions) in the synthesis of uniform colloids and nanoparticles. We demonstrate the basic principles of well-defined particle shape emergence in such growth. Specifically, several shapes are possible for a given crystal structure. The formation of shapes that follow the crystal symmetry and are uniform can be a result of the nonequilibrium nature of the growth process. The shape of a growing particle can be controlled by varying the relative rates of kinetic processes as well as by adjusting the concentration of monomers in the surrounding medium.

Mechanisms of diffusional nucleation of nanocrystals and their self-assembly into uniform colloids

We survey our research on modeling the mechanisms of control of uniformity in the growth of nanosize and colloid size particles. The former are produced as nanocrystals by burst nucleation from solution and the latter are formed by self-assembly (aggregation) of the nanocrystals. In the colloid particle synthesis, the two dynamical processes are coupled, both governed by diffusional transport of the respective building blocks (monomers). The interrelation of the two processes allows for the synthesis of narrow size distribution colloid dispersions, which are of importance in many applications. We first review a mathematical model of diffusive cluster growth by the capture of monomer "singlets." We then analyze burst nucleation of nanoparticles in solution. Finally, we couple it to the secondary process of aggregation of nanoparticles to form colloids and discuss various aspects of the modeling of particle size distribution, as well as other features of the processes considered.

Complexation Hydrogels for Oral Insulin Delivery: Effects of Polymer Dosing on in Vivo Efficacy

Hydrogels comprised of poly(methacrylic acid) grafted with poly(ethylene glycol) (P(MAA-g-EG)) were characterized and examined for their potential as oral insulin carriers. Insulin loaded polymer (ILP) samples were made using two different polymer formulations. The values for the effective molecular weight between crosslinks, M _e , and the network mesh size, xi, were characterized and increased with increasing pH levels for both formulations. Insulin uptake studies indicated a high insulin loading efficiency for all samples tested, however release was dependent on the amount of insulin loaded. The effect of total polymer dosing was investigated by in situ administration in isolated ileal segments in rats. All ILP samples induced a hypoglycemic effect and an increase in insulin levels, proving that insulin was still biologically active. Insulin dosing amounts were varied by (i) maintaining a constant insulin fraction within an ILP sample while changing the amount of ILP and (ii) by varying the insulin fraction while dosing with the same amount of ILP. The total insulin absorption was dependent on both the amount of the polymer present and the concentration of insulin within an ILP sample, with a maximum relative bioavailability of 8.0%.

Uniform particles of pure and silica-coated cholesterol

Uniform crystalline colloidal cholesterol particles of narrow size distribution were obtained by precipitation. The method consisted of adding a miscible non-solvent (water) into cholesterol solutions of different alcohols and acetone, without any additives. The properties of the resulting particles depended in a sensitive way on the concentration of all reactants, temperature, pH, ionic strength, and aging time. The major observed effects were due to the solubility of cholesterol, which was strongly affected by the solvent mixture and temperature. Precipitation in 1-propanol/water system yielded stable dispersions of well-defined particles, which were used to evaluate the effects of different experimental parameters on their properties. Aging of stable dispersions resulted in multi-layered aggregation of the primary platelets, the degree and rate of which process was strongly affected by temperature. Finally, it was shown that the colloidal cholesterol particles could be coated with homogeneous silica layers in order to alter their surface characteristics.

Comparison between Protein−Polyethylene Glycol (PEG) Interactions and the Effect of PEG on Protein−Protein Interactions Using the Liquid−Liquid Phase Transition

Phase transitions of protein aqueous solutions are important for protein crystallization and biomaterials science in general. One source of thermodynamic complexity in protein solutions and their phase transitions is the required presence of additives such as polyethylene glycol (PEG). To investigate the effects of PEG on the thermodynamic behavior of protein solutions, we report measurements on the liquid-liquid phase separation (LLPS) of aqueous bovine serum albumin (BSA) in the presence of relatively small amounts of PEG with an average molecular weight of 1450 g/mol (PEG1450) as a model system. We experimentally characterize two thermodynamically independent properties of the phase boundary: (1) the effect of PEG1450 concentration on the LLPS temperature, (2) BSA/PEG1450 partitioning in the two liquid coexisting phases. We then use a thermodynamic perturbation theory to relate the first property to the effect of PEG concentration on protein-protein interactions and the second property to protein-PEG interactions. As criteria to determine the accuracy of a microscopic model, we examine the model's ability to describe both experimental thermodynamic properties. We believe that the parallel examination of these two properties is a valuable tool for verifying the validity of existing models and for developing more accurate ones. For our system, we have found that a depletion-interaction model satisfactorily explains both protein-PEG interactions and the effect of PEG concentration on protein-protein interactions. Finally, due to the general importance of LLPS, we will experimentally show that protein-PEG-buffer mixtures can exhibit two distinct types of liquid-liquid phase transitions.