Prevention of insulin self-association and surface adsorption

Clinical Recommendations for Managing the Impact of Insulin Adsorptive Loss in Hospital and Diabetes Care

BACKGROUND

Adsorption of insulin to infusion sets impacts patient therapeutic outcomes and, unaccounted for, may exacerbate persistent hyperglycemia or result in therapy-induced hypoglycemia. This article aims to provide recommendations for clinicians involved in intensive care and/or outpatient pump therapy contexts.

METHODS

A dynamic adsorption model is used to evaluate time-varying insulin concentration in the infusion set outflow. Hourly and daily percentage insulin loss to adsorption is examined for neonatal, pediatric, and adult intensive care patients, as well as outpatient children and adults weighing 30, 50, and 80 kg. A short review of preconditioning methods is included.

RESULTS

Insulin adsorption in outpatient pump therapy is most pronounced in the first hour, where as much as 80% of the intended insulin dose may be lost to adsorption. Subsequently, insulin adsorptive loss is typically negligible. Overall, extra care should be taken in the first 1-6 h of a new infusion set, particularly in children or teenagers. Typically, insulin adsorption in the adult intensive care unit is negligible unless infused at low flow rates (<2 mL/h). Insulin adsorption is significant in pediatric and neonatal intensive care, resulting in delivery concentrations as low as 5%-50% of that intended. Thus, it is recommended that preconditioning of insulin delivery lines be carried out prior to infusion initiation in this context. However, no preconditioning method completely removes adsorption, and care should still be taken in the first 1-6 h of insulin dosing.

CONCLUSIONS

Recommendations made in this article are dependent on the insulin concentration and flow rate used in each clinical context.

Capacity of Infusion Lines for Insulin Adsorption: Effect of Flow Rate on Total Adsorption

BACKGROUND

Insulin adsorption to clinical materials has been well observed, but not well quantified. Insulin adsorption reduces expected and actual insulin delivery and is unaccounted for in insulin therapy or glycemic control. It may thus contribute to poor control and high glycemic variability. This research quantifies the problem in the context of clinical use.

METHOD

Experimental insulin adsorption data from literature is used to calculate insulin delivery and total insulin adsorption capacities for polyethylene (PE) and polyvinal chloride (PVC) lines at clinically relevant flow rates and concentrations.

RESULTS

Insulin adsorption capacity decreased hyperbolically with flow rate for both PE and PVC, where low flow scenarios result in greater insulin adherence to infusion lines. When the infusion flow rate was halved from 1 to 0.5 mL/h, twice as much insulin adsorbed to the line. Insulin loss to adsorption resulted in up to ~50% of intended insulin not delivered over 24 hours in a low flow and low concentration context.

CONCLUSION

Material capacity for insulin adsorption is not constant, but increases with decreasing flow. Different materials have different adsorption capacities. In low flow and low concentration contexts, such as in neonatal or pediatric intensive care, insulin loss to adsorption represents a significant proportion of daily insulin delivery, which needs to be accounted for.

Conductive polymer nanotube patch for fast and controlled ex vivo transdermal drug delivery

Aim: To uptake and release hydrophilic model drugs and insulin in a novel conductive polymer (CP) nanotube transdermal patch. Materials & methods: The externally controlled transdermal delivery of model drugs and insulin were tested ex vivo and results were compared with CP films. The unique intrinsic properties of CPs provide electrostatic interaction between the model drugs and polymer backbone. Results & discussion: When a pulsed potential was applied, the drug delivery release profile mimics that of injection delivery. With a constant potential applied, the release rate constants of the patch system were up to three-times faster than the control (0 V) and released approximately 80% more drug molecules over 24 h. Conclusion: The CP nanotube transdermal patch represents a new and promising drug method, specifically for hydrophilic molecules, which have been a large obstacle for conventional transdermal drug delivery systems.

Original submitted: 7 March 2013; Revised submitted: 1 August 2013

Pure insulin highly respirable powders for inhalation

The aim of the present research was to investigate the possibility to obtain by spray drying an insulin pulmonary powder respirable and stable at room temperature without the use of excipients. Several insulin spray-dried powders were prepared with or without the addition of excipients (mannitol, bovine serum albumin, aspartic acid) from water dispersions or from acidic aqueous solutions. Each formulation was characterized using laser diffraction, scanning electron microscopy and in vitro aerosol performance with a Turbospin DPI device. Stability was assessed by the quantification of impurities with a molecular mass greater than that of insulin (HMWP) and related proteins (A21+ORP). Insulin powders prepared without excipients from an acid solution showed a shrivelled, raisin-like shape of non-aggregated microparticles and a high respirability (FPF>65%). The optimal result with respect to respirability and stability was reached when the pH of the insulin acetic acid solution to spray dry was adjusted at pH 3.6 with ammonium hydroxide. The median volume diameter of the obtained powder was 4.04 μm, insulin content 95%, emitted dose of 89.5%, MMAD 1.79 μm and fine particle fraction of 83.6%. This powder was stable at room temperature over a period of eighteen months with respect to the content of A21+ORP. As far as the HMWP content was concerned, the powder complied with the specification limits for a period of five months. The insulin acetic powder opens up the possibility of a more effective pulmonary therapy less dependent on refrigerated storage.

Transdermal iontophoresis of insulin. Part 1: A study on the issues associated with the use of platinum electrodes on rat skin

We have studied the issues associated with the use of platinum electrodes for transdermal iontophoretic delivery of peptides, using insulin as a model peptide. Insulin permeation was studied using full-thickness rat skin by varying the donor solution pH as a function of electrode polarity. The stability of insulin under the iontophoretic conditions was studied using TLC, SDS-polyacrylamide gel electrophoresis and HPLC. Large pH shifts were observed during anodal iontophoresis (AI), when the donor solution pH was above the isoelectric point of insulin and in cathodal iontophoresis (CI), when the donor solution pH was below the isoelectric point of insulin. The direction and magnitude of electroosmotic flow was influenced by pH of the donor solution and the electrode polarity. On the other hand, the buffer used to maintain the pH governed the contribution of electrorepulsion to the overall transport of insulin. Electrochemical degradation of insulin was significant during Al at pH 7.4. Among the pH investigated, Al of insulin at pH 3.6 and Cl at pH 8.35 were better, as the pH shift was relatively less and electrochemically more stable during iontophoresis as compared with other pH. In summary, the pH shift caused by platinum electrodes had a significant influence on the permeation and stability of insulin.

Role of a novel excipient poly(ethylene glycol)-b-poly(L-histidine) in retention of physical stability of insulin in aqueous solutions

PURPOSE

This study is to investigate whether poly(ethylene glycol) (PEG)-b-poly(L-histidine) [PEG-polyHis] can reduce aggregation of insulin in aqueous solutions on agitation by forming ionic complexes.

MATERIALS AND METHODS

Insulin aggregation on agitation was monitored spectrophotometrically and by fibrillation studies with a dye Thioflavin T. Pluronic F-127 as a control and PEG-polyHis as a novel multifunctional excipient were added to prevent destabilization of insulin. Conformation of insulin was evaluated in a circular dichroism (CD) study.

RESULTS

Ionic interactions between insulin and PEG-polyHis were induced in the pH range: 5.5-6.5. pH 5.5 was selected for further evaluation based on particle size/zeta potential studies. Ionic complexation with PEG-polyHis is more effective at pH 5.5 in stabilizing insulin (75% of insulin retained versus 0% with no excipient) than Pluronic F-127 (42% retained). PEG-polyHis guards against insulin aggregation in non-complexing pH conditions (pH 7.4), 64% insulin retained versus 58% with F-127 and 0% with no excipient) pointing to the potential role played by PEG in modulation of insulin surface adsorption. Rate of fibrillation was higher for plain insulin compared with addition of PEG-polyHis and Pluronic F-127 at both pH.

CONCLUSIONS

Understanding and manipulation of such polyelectrolyte-protein complexation will likely play a role in protein stabilization.

Application of Micro- and Nano-Electromechanical Devices to Drug Delivery

Micro- and nano-electromechanical systems (MEMS and NEMS)-based drug delivery devices have become commercially-feasible due to converging technologies and regulatory accommodation. The FDA Office of Combination Products coordinates review of innovative medical therapies that join elements from multiple established categories: drugs, devices, and biologics. Combination products constructed using MEMS or NEMS technology offer revolutionary opportunities to address unmet medical needs related to dosing. These products have the potential to completely control drug release, meeting requirements for on-demand pulsatile or adjustable continuous administration for extended periods. MEMS or NEMS technologies, materials science, data management, and biological science have all significantly developed in recent years, providing a multidisciplinary foundation for developing integrated therapeutic systems. If small-scale biosensor and drug reservoir units are combined and implanted, a wireless integrated system can regulate drug release, receive sensor feedback, and transmit updates. For example, an "artificial pancreas" implementation of an integrated therapeutic system would improve diabetes management. The tools of microfabrication technology, information science, and systems biology are being combined to design increasingly sophisticated drug delivery systems that promise to significantly improve medical care.

State of insulin self-association does not affect its absorption from the pulmonary route

This study is designed to compare and contrast the pulmonary absorption profiles of monomeric and hexameric insulin in the presence or absence of ethylene diamine tetraacetic acid (EDTA) or n-tetradecyl-beta-d-maltoside (TDM). The pulmonary absorption of two forms of insulin was studied by monitoring the changes in plasma insulin and glucose levels after intratracheal administration of monomeric or hexameric insulin into anesthetized rodents. EDTA or TDM was added to the formulation in order to evaluate if either of these agents has effects on the rate and extent of pulmonary absorption of monomeric and hexameric insulin. The biochemical changes that may occur after acute administration of TDM-based formulation have also been investigated by estimating lung injury markers in bronchoalveolar lavage fluid. A dose-dependent increase in the plasma insulin and decrease in plasma glucose levels was observed when increasing concentrations of hexameric or monomeric insulin were administered via the pulmonary route. Pulmonary administration of monomeric and hexameric insulin produced comparable absorption profiles in the presence or absence of EDTA or TDM. The bronchoalveolar lavage fluid analysis did not show differences in the levels of injury markers produced in TDM-treated rats and that produced in saline-treated rats, indicating no evidence for adverse effects of TDM in these short-term studies. Overall, in terms of rapidity of action and efficacy to reduce blood sugar, monomeric insulin did not provide advantages over hexameric insulin when administered via the pulmonary route.

Transdermal Iontophoresis of Insulin

The delivery of large peptides through the skin poses a significant challenge, and various strategies are under active investigation for enhancing the transdermal permeation. For large peptides, it is difficult to achieve significant permeation using iontophoresis alone. Hence a combination of fatty acids with iontophoresis was hypothesized to result in higher enhancement than achieved with either of them alone. Saturated fatty acids and cis unsaturated fatty acids were studied in combination with iontophoresis using excised rat skin. The skin was pretreated for 2 h with an ethanolic (EtOH) solution of 5% w/v or v/v fatty acids, namely lauric acid (LA), oleic acid (OA), linoleic acid (LOA) and linolenic acid (LLA), followed by either passive or iontophoretic permeation (0.5 mA/cm2 for 6 h). Fourier transform infrared spectroscopy (FT-IR) was used to investigate the biophysical changes on treatment with fatty acid/EtOH or neat fatty acid, mainly focusing on the infrared region at 2,920, 1,710 and 1,720 cm(-1). Unsaturated fatty acids showed higher enhancement than LA, and the enhancement increased with the number of double bonds. On the other hand, in the presence of iontophoresis, LA/EtOH showed the highest enhancement. Neat LOA did not show any significant difference (p > 0.05) compared to the LOA/EtOH combination. FT-IR studies revealed that fatty acids act by interacting with the skin lipids. All the fatty acids showed synergistic enhancement when combined with iontophoresis. The flux enhancement was highest with LA, which in the presence of iontophoresis showed 20 times enhancement of insulin flux in comparison to passive flux and 9 times enhancement as compared to iontophoresis alone. Flux enhancement of unsaturated fatty acids was in the following decreasing order LOA > OA > LLA.

Evidence for restrictive parameters in formulation of insulin-loaded nanocapsules

Poly(isobutylcyanoacrylate) nanocapsules with an oily core were originally proposed for lipophilic drug encapsulation [Int. J. Pharm. 28 (1986) 125] but insulin, a hydrosoluble protein, has also been successfully encapsulated by Damgé et al. [Diabetes 37 (1988) 246]. The aim of this work was to understand if several parameters were restrictive for the encapsulation of insulin into the oily core of the nanocapsules prepared by interfacial polymerization. The encapsulation efficiency of insulin was not affected by the type of insulin since the peptides adopted the same association state after their addition to the organic phase. Formulation parameters mainly affected the size of the nanocapsules obtained but did not influence the insulin encapsulation efficiency. In contrast, the order of introduction of insulin and of the monomer in the organic phase was shown to control the formation and the characteristics of the nanocapsules. The key parameters, which were found to clearly influence the encapsulation efficiency of insulin, were the pH of the aqueous insulin solution and the origin of the monomer. Both of these parameters can affect the rate of the interfacial polymerization. Consequently, the ability of insulin to be entrapped into the oil containing nanocapsules appeared to be governed more by the rate of the monomer polymerization.

Transdermal iontophoresis of insulin: IV. Influence of chemical enhancers

Transdermal iontophoresis per se may not be able to achieve significant permeation of large peptides like insulin, thereby necessitating the use of combination strategies involving chemical enhancers and iontophoresis. The study investigated effect of pre-treatment with commonly used vehicles such as ethanol (EtOH), propylene glycol (PG), water and their binary combinations, dimethyl acetamide (DMA), 10% dimethyl acetamide in water, ethyl acetate (EtAc) and isopropyl myristate (IPM) on insulin iontophoresis. Solvents, which acted on the lipid bilayer, were able to produce a synergistic enhancement with iontophoresis. The binary solvent systems produced either additive or no effect, when combined with iontophoresis. FT-IR studies showed that EtOH, DMA, EtAc caused lipid extraction and the former two also caused changes in skin proteins, whereas IPM caused increase in lipid fluidity. TGA studies showed that EtOH and PG caused dehydration of skin. Skin barrier property was severely compromised with DMA, followed by EtOH and EtAc, while IPM and PG had relatively minimum skin barrier altering potential. Thus, this study demonstrates the possibility of achieving higher permeation of large peptides like insulin by combining iontophoresis with chemical enhancers that act on the intercellular lipids.

A reversible hydrogel membrane for controlling the delivery of macromolecules

Glucose-sensitive hydrogel membranes have been synthesized and characterized for their rate-of-delivery of macromolecules. The mechanism for changing this rate is based on variable displacement of the affinity interaction between dextran and concanavalin A (con A). Our main objective was to characterize the diffusion of model proteins (insulin, lysozyme, and BSA) through the membrane, in response to changes in environmental glucose concentrations. Membranes were constructed from crosslinked dextrans to which con A was coupled via a spacer arm. Changes in the porosity of the resulting hydrogel in the presence of glucose led to changes in the diffusion rate observed for a range of proteins. Gels of specified thickness were cast around to nylon gauze support (pore size, 0.1 mm) to improve mechanical strength. Diffusion of proteins through the gel membrane was determined using a twin-chamber diffusion cell with the concentrations being continuously monitored using a UV-spectrophotometer. Changes in the transport properties of the membranes in response to glucose were explored and it was found that, while 0.1M D-glucose caused a substantial, but saturateable, increase in the rates of diffusion of both insulin and lysozyme, controls using glycerol or L-glucose (0.1M) had no significant effect. Sequential addition and removal of external glucose in a stepwise manner showed that permeability changes were reversible. As expected, diffusion rates were inversely proportional to membrane thickness. A maximum increase in permeability was observed at pH 7.4 and at 37 degrees C. The results demonstrate that this hydrogel membrane functions as a smart material allowing control of solute delivery in response to specific changes in its external environment.

Transdermal iontophoresis of insulin. II. Physicochemical considerations

Transdermal iontophoresis is one of the potential enhancement strategies for the delivery of large and charged molecules. Insulin, a polypeptide of 6 kDa was used as a model for large peptides to understand the influence of peptide concentration, NaCl concentration, buffer type and its concentration on the transport efficiency of iontophoresis. Maximum enhancement was found at 3 mg/ml (75 IU/ml). The permeation of insulin was found to increase up to 0.05 M NaCl and decreased at higher concentrations of NaCl. The glucose permeation studies showed that permeation of insulin increased in the presence of NaCl due to ion induced convective flow. The flux enhancement of insulin in the presence of phthalate buffer was higher in comparison to citrate buffer, but the enhancement in these two buffers was the same in the presence of 0.05 M NaCl, which was also supported by a similar trend in conductivity values. However, the solution conductivity values did not reflect the influence of co-ions and counter ions on the transport of large peptides across the skin. Overall the findings revealed that the transport efficiency of large peptides like insulin may be improved by the optimisation of competing ions in solution.

Transdermal iontophoresis of insulin. V. Effect of terpenes

To increase the skin permeation of large peptides like insulin, it is necessary to utilize a combination of enhancement strategies. In this regard, this study investigated the effect of terpenes/EtOH combination in comparison to EtOH and neat terpene on transdermal iontophoretic permeation of insulin. Ex-vivo experiments were conducted using full thickness rat skin after pre-treatment for 2 h with 5% of menthol, menthone, cineole and pulegone in EtOH; EtOH alone; neat menthone with and without iontophoresis (0.5 mA/cm(2); 6 h). FT-IR studies were carried out using rat epidermal sheets after pre-treatment with enhancer solution for 2 h and tritiated water permeation studies was used to investigate the alteration in skin barrier property after enhancer or current treatment. The lag time was significantly reduced (P<0.05) with terpene/EtOH pre-treatment in comparison to passive control and EtOH pre-treatment, although there was no significant difference (P>0.05) among the terpenes. Synergistic enhancement in flux was observed with terpene/EtOH, and menthone/EtOH showed highest enhancement among the terpene/EtOH combinations. On the other hand, enhancement with neat menthone was higher than with menthone/EtOH. FT-IR studies showed that terpene/EtOH, EtOH and neat terpene act at the intercellular lipids. The skin barrier property was significantly (P<0.05) compromised with neat menthone treatment. Iontophoresis had a lesser effect on skin barrier property compared to chemical enhancer pre-treatment. Terpene/EtOH caused synergistic enhancement of insulin permeation when combined with iontophoresis and was influenced by the type and concentration of terpene.

Monitoring the in vivo delivery of proteins from carbomer hydrogels by X-ray fluorescence

PURPOSE

To measure the effect of protein size on their disappearance from subcutaneously implanted carbomer hydrogel matrices.

METHODS

A series of different molecular weight (MW) proteins were iodinated, incorporated into Carbopol hydrogels, injected subcutaneously into rats, and monitored using X-ray fluorescence (XRF).

RESULTS

A 10 mg/mL minimum concentration of Carbopol-940 was necessary before protein 150 mg/mL iodinated bovine serum albumin (I-BSA)] retention times increased with increasing hydrogel concentration. The decreasing protein signal was not caused by outward protein diffusion or iodoprotein hydrolysis. As the protein MW increased, protein retention times lengthened [e.g.. 6.2 h for insulin (5.7 kDa) to 13.3 h for thyroglobulin (669 kDa)]. Protein disappearance was monophasic first-order for some proteins and biphasic first-order for others. The disappearance rate constant ranged from 0.093 +/- 0.005 h(-1/2), to 0.187 +/- 0.057 h(-1/2), indicating gel erosion rather than protein diffusion as the rate-limiting mechanism. Entrapped I-BSA in Carbopol-1342 NF. pH 7.4, and Carbopol 2001-ETD, pH 7.4, gel matrices yielded different disappearance rates and profiles than Carbopol-940. The overall 50% disappearance rate of I-BSA was greatest for Carbopol-1342 NF (41 +/- 8 h), followed by Carbopol-2001 ETD (25 +/- 2 h) and Carbopol-940 (10.5 +/- 0.7 h).

CONCLUSION

XRF is a noninvasive technique that can be used to follow the status of macromolecules in vivo.

Feeding liquid, non-ionic surfactant and cyclodextrin affect the properties of insulin-loaded poly(lactide-co-glycolide) microspheres prepared by spray-drying

The potential of spray-drying technique for the encapsulation in poly(lactide-co-glycolide) (PLGA) microspheres of bovine insulin, a poorly stable peptide, has been investigated. Insulin-loaded microspheres were prepared by spray-drying different feeding liquids containing insulin and PLGA, that is a S/O dispersion, a W/O emulsion or an acetic acid solution. In the case of the emulsion, insulin was also co-encapsulated with either non-ionic surfactants such as polysorbate 20 and poloxamer 188, or complexing agents such as HPbetaCD. In the microspheres prepared from the acetic acid solution of insulin and PLGA, HPbetaCD was tested. Microspheres containing surfactants were aggregated, whereas good quality particles displaying a mean diameter in the range 12.1-27.9 microm were produced in the other cases. Insulin was efficiently loaded inside microspheres except for S/O formulation (only 22% of total insulin content was entrapped). The impact of the microencapsulation process on insulin chemical and conformational stability was assessed by HPLC, circular dichroism and turbidimetry studies. Under the adopted manufacture conditions, insulin was encapsulated in the native state and its chemical and conformational stability was preserved along the fabrication process. The formulations containing only insulin displayed low burst effects (6-11%), whereas the addition of surfactants resulted in much higher burst effects (49-54%) and faster release rate. The co-encapsulation of HPbetaCD slowed down the overall release rate and, in the case of microspheres prepared from the emulsion, allowed a constant insulin release up to 45 days. The study of insulin stability along the release phase showed that insulin was released in the intact form and un-released insulin was stable inside all the microsphere formulations. We conclude that insulin can be effectively encapsulated in PLGA microspheres by the spray-drying technique. Additives with complexing properties such as HPbetaCD have demonstrated a potential in optimizing the release rate of insulin when used in microspheres prepared from W/O emulsions.

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.

Effects of low-viscosity sodium hyaluronate preparation on the pulmonary absorption of rh-insulin in rats

PURPOSE

A low-viscosity formulation for pulmonary delivery of rh-insulin as model peptide drugs was developed using a solution of sodium hyaluronate.

METHOD

The effects of different concentrations and pH values of low-viscosity solutions of hyaluronate on the pulmonary absorption of rh-insulin were examined after intratracheal administration in rats. The permeation of fluorescein isothiocyanate (FITC)-dextran (molecular weight 4300; FD-4) and insulin through excised rat trachea in vitro were also examined.

RESULTS

The hyaluronate (2140 kDa) solutions (0.1% and 0.2% w/v) at pH 7.0 significantly enhanced the pharmacological availability (PAB) of insulin compared to the aqueous solution of insulin at pH 7.0. The absorption-enhancing effect at a concentration of 0.1% w/v hyaluronate was greater than that at a concentration of 0.2% w/v hyaluronate. Furthermore, the greatest absorption-enhancing effect was obtained, regardless of the molecular weight of hyaluronate, when the concentration of hyaluronate was adjusted to 0.47 microM. Absorption-enhancing effects were consistent with the effect of a 0.1 w/v hyaluronate preparation at pH 4.0 and 7.0 on the permeation of FITC-dextran and insulin through excised rat trachea in vitro.

CONCLUSION

Low-viscosity hyaluronate preparation was shown to be a useful vehicle for pulmonary delivery of peptide drugs.

Transdermal iontophoretic delivery of bovine insulin and monomeric human insulin analogue

The present study was undertaken to explore the possibility of delivering bovine insulin in streptozocin (STZ)-induced diabetic rats by iontophoresis. Further, the effect of iontophoresis of monomeric human insulin analogue (r-DNA origin) on the plasma glucose level (PGL) of diabetic rats was studied. Iontophoresis of bovine insulin (10-200 IU/ml) was not effective in decreasing the PGL in untreated diabetic rats. Pretreatment of skin with oleic acid or menthol for 3 h followed by iontophoresis of bovine insulin also failed to produce a fall in PGL. Application of a depilatory cream for hair removal (24 h before the experiment), followed by iontophoresis of bovine insulin (10, 30 and 100 IU/ml) produced a concentration-dependent fall in PGL. Further, application of depilatory cream immediately before the experiment produced a substantial fall in PGL both by passive diffusion and iontophoresis. Depilatory cream might have drastically reduced the barrier function of skin such that conventional bovine insulin (dimer and hexamer) penetrates through the intact skin by iontophoresis and even by passive diffusion. Depilatory cream or the active components of depilatory cream may be useful as penetration enhancers for transdermal delivery of drugs especially macromolecules such as insulin. Iontophoresis of monomeric human insulin analogue (B9 Asp, B27 Glu) through intact skin (untreated) produced a significant fall in PGL in diabetic rats. Monomeric human insulin analogues which have low tendency to self aggregation may be promising candidates for the transdermal iontophoretic delivery of insulin.

Characterization of protein release through glucose-sensitive hydrogel membranes

Glucose-sensitive phase-reversible hydrogels have been prepared based on the specific interaction between polymer-bound glucose and concanavalin A (Con-A). The main goal of this study was to characterize the release of model proteins (insulin and lysozyme) through the hydrogel membrane as the free glucose concentration in the environment was changed. The diffusion of the model proteins through the hydrogel membrane was examined using a diffusion cell. Porous poly(hydroxyethyl methacrylate) (PHEMA) membranes were used to sandwich the mixture of glucose-containing polymers and Con-A in between the donor and receptor chambers. The porous PHEMA membranes allowed diffusion of glucose, insulin and lysozyme, while preventing loss of glucose-containing polymers and Con-A in the sol state. The release rate of model proteins through the glucose-sensitive hydrogel membrane was dependent on the concentration of free glucose. The release rate of the proteins did not remain constant, however, due to the change in free glucose concentration resulting from diffusion of glucose from the receptor chamber to the donor chamber. This study demonstrated the possibility that the glucose-sensitive phase-reversible hydrogels can be used to regulate the insulin release as a function of the free glucose concentration in the environment.

Use of poloxamer polymers to stabilize recombinant human growth hormone against various processing stresses

Several processing and shipping stresses were investigated for their effect on the physical stability of recombinant human growth hormone (rhGH). These included exposure to air/water interfaces, adsorption to hydrophobic surfaces, freeze-thaw cycles, and temperature. The interfacially and thermally denatured hormone was evaluated for the presence of insoluble and soluble aggregates by spectrophotometry and by size-exclusion chromatography, respectively. Noncovalent aggregates were generated by either vortexing or multiple passages through a hypodermic needle, processes which exposed the protein to air/water interfaces. Thermal stress also resulted in the generation of aggregates. This aggregation was reduced or eliminated by the use of poloxamer polymers. Under the conditions employed, filtration through some commercially available filters, exposure to hydrophobic beads, or multiple freeze-thaw cycles did not produce any aggregates within the limitations of the analytical procedures used. Based on this study, Poloxamer 407 was found to be the most effective stabilizer for rhGH for protection against interfacial and thermal stress.

Physical stabilization of insulin by glycosylation

The modification of human insulin by the covalent attachment of monosaccharide moieties to insulin amino group(s) alters the aggregation and self-association behavior, improving both the pharmaceutical stability and biological response. The synthesis of p-succinamidophenyl glucopyranoside-insulin conjugate(s) (SAPG-insulin) has resulted in seven possible glucosylated insulin derivatives (three monosubstituted, three disubstituted, and one trisubstituted). These derivatives were isolated and purified using ion exchange chromatography. Characterization of the derivatives includes determining the site and number of sugar groups attached for each individual derivative and an evaluation of biological activity. Nearly all the derivatives retained in vivo biological activity comparable to insulin. In addition, extensive physicochemical characterization of the glucosylated insulin derivatives was undertaken to determine association/aggregation properties using GPC, dynamic light scattering, UV/Vis, and CD spectroscopy. Protein self-association was most suppressed with the disubstituted derivatives, especially those substituted on PheB1, and the trisubstituted derivative. The same general pattern was observed for physical stability of glucosylated insulin derivatives. As the number of glucosyl moieties attached to insulin increased, solution physical stability dramatically improved. Yet, the most significant impact to stability was glycosylation at the PheB1 site.

Kinetics and mechanisms of peptide aggregation. I: Aggregation of a cholecystokinin analogue

Aggregation kinetics for a tetrapeptide analogue of cholecystokinin (A-71623) have been studied by quasi-elastic light scattering. Aggregation kinetics were quantitated with a kinetic model, described herein, which was modified for quasi-elastic light scattering data. The model predicts that the average molecular weight of peptide aggregates increases in a linear fashion with time. Data generated for A-71623 were consistent with the model presented under conditions of varied ethanol concentration at a fixed peptide concentration, as well as varied A-71623 concentration at fixed ethanol concentration. Although not the primary thrust of this study, experimental design permitted some understanding of the effect of environmental changes on the apparent aggregation kinetics of A-71623. These studies suggest A-71623 aggregation may be partially mediated by hydrophobic bonding.

Analysis of proinsulin and its conversion products by reversed-phase high-performance liquid chromatography

Proinsulin is synthesized in the beta-cells of the endocrine pancreas, one of the four cell types found in the islets of Langerhans. Specific enzymatic cleavage of proinsulin results in the formation of equimolar amounts of insulin and C-peptide, via several intermediate split-proinsulin forms. Most mammals produce a single insulin, but in rodents two non-allelic insulin genes are expressed. There is an inverse ratio between the two insulins in rats and mice, the reason for this being unknown. It has been suggested that differences in transcription, translation (biosynthesis) and/or posttranslational processes (enzymatic conversion, intracellular degradation) could be possible explanations. Elevated amounts of proinsulin-immunoreactive material (PIM) have been described to occur in various conditions/diseases, suggesting alterations in beta-cell function, but the composition of the secreted PIM (intact proinsulin or its intermediates) has been incompletely determined. Studies of the biosynthesis of proinsulins and their conversion with the purpose of revealing some of these points depend on accessible reversed-phase high-performance liquid chromatographic (RP-HPLC) analyses capable of separating all the relevant, closely related polypeptides involved. This review will deal with the optimization of the RP-HPLC separations as well as sample preparation and recovery. Applications of the selected methods in the study of proinsulin biosynthesis and its conversion will also be presented.

Differential effects of anionic, cationic, nonionic, and physiologic surfactants on the dissociation, alpha-chymotryptic degradation, and enteral absorption of insulin hexamers

Various surfactants were investigated to compare their effects on insulin dissociation, alpha-chymotryptic degradation, and rat enteral absorption. With a circular dichroism technique, sodium dodecyl sulfate (SDS) at a 5 mM concentration was found to completely dissociate porcine-zinc insulin hexamers (0.5 mg/ml) into monomers. The catalytic activity of alpha-chymotrypsin (0.5 microM) was also abolished by 5 mM SDS. When insulin was injected into the distal jejunum/proximal ileum segment of the rat, 5 mM SDS greatly enhanced its pharmacological availability, from a negligible value to 2.8%. Being a cationic surfactant, hexadecyl trimethylammonium bromide (CTAB) also efficiently dissociated insulin hexamers at concentrations of 1-5 mM. However, extensive charge-charge interaction was observed below a CTAB concentration of 0.6 mM, leading to insulin precipitation at a molar CTAB:insulin ratio of 1:1 to 2:1. An alpha-chymotryptic degradation study also revealed near-complete dissociation of insulin hexamers at 1 mM CTAB. Above 1 mM, however, CTAB acted as an enzyme inhibitor, most likely by means of charge repulsion. Enteral absorption studies showed a much lower pharmacological availability, only 0.29%. Nonionic surfactants such as Tween 80 and polyoxyethylene 9 lauryl ether were ineffective in dissociating insulin hexamers. Tween 80, at 5 mM, neither significantly altered the alpha-chymotryptic degradation pattern nor enhanced the enteral absorption of insulin. The relative effectiveness of different species of bile salts on insulin hexamer dissociation appeared to be similar. Sodium glycocholate at a 30 mM concentration also significantly increased insulin pharmacological availability, to 2.3%. A morphological study did not reveal any significant alteration of the rat intestinal mucosal integrity after exposure to 5 mM SDS for 30 min.2+ transport.

Minimization of shaking-induced formation of insoluble aggregates of insulin by cyclodextrins

Aggregation is known to complicate insulin delivery and the processing and formulation of biotechnology-derived peptide/protein drugs. Shaking-induced formation of insoluble aggregates in bovine insulin and the potential role of cyclodextrins in preventing such aggregation were investigated. Insulin, dissolved in phosphate buffer, pH 7.2, and preserved with 2 mg/ml of phenol was aggregated, in triplicate, by shaking at 450 rpm for 2.5 days on a gyratory shaker. Visible aggregation was quantitated by measuring optical density in the visible range on a spectrophotometer. Solutions were then filtered through a 0.22 mu filter and the amount of insulin remaining in filtrate was determined by HPLC. Aggregation increased at lower concentrations, with solutions turning milky at 0.5 mg/ml; HPLC assay of filtrate indicated a complete loss of insulin. Under the same conditions, except for shaking, control solutions exhibited no insulin loss, excluding absorption as a cause of the insulin loss. The use of cyclodextrins (0.5 mg/ml) to stabilize insulin was investigated. alpha-, beta-, gamma- and hydroxypropyl-beta-cyclodextrin, each at 1.5% level, were used to prevent aggregation. The efficacy of cyclodextrins in preventing aggregation (% insulin aggregated in parentheses), was: hydroxypropyl-beta- (15) approximately beta- (18) > alpha- (54). No protection was observed with gamma-cyclodextrin.

Cyclodextrins as nasal absorption promoters of insulin: mechanistic evaluations

The safety and effectiveness of cyclodextrins (CD) as nasal absorption promoters of peptide-like macromolecules have been investigated. The relative effectiveness of the cyclodextrins in enhancing insulin nasal absorption was found to be in the descending order of dimethyl-beta-cyclodextrin (DM beta CD) greater than alpha-cyclodextrin (alpha-CD) greater than beta-cyclodextrin (beta-CD), hydroxypropyl-beta-cyclodextrin (HP beta CD) greater than gamma-cyclodextrin (gamma-CD). A direct relationship linking absorption promotion to nasal membrane protein release is evident, which in turn correlates well with nasal membrane phospholipid release. The magnitude of the membrane damaging effects determined by the membrane protein or phospholipid release may provide an accurate, simple, and useful marker for predicting safety of the absorption enhancers. In order to estimate further the magnitude of damage and specificity of cyclodextrin derivatives in solubilizing nasal membrane components, the enzymatic activities of membrane-bound 5'-nucleotidase (5'-ND) and intracellular lactate dehydrogenase (LDH) in the perfusates were also measured. HP beta CD at a 5% concentration was found to result in only minimal removal of epithelial membrane proteins as evidenced by a slight increase in 5'-ND and total absence of LDH activity. On the other hand, 5% DM beta CD caused extensive removal of the membrane-bound 5'-ND. Moreover, intracellular LDH activity in the perfusate increased almost linearly with time. The cyclodextrins are also capable of dissociating insulin hexamers into smaller aggregates, and this dissociation depends on cyclodextrin structure and concentration. Enhancement of insulin diffusivity across nasal membrane through dissociation may provide an additional mechanism for cyclodextrin promotion of nasal insulin absorption.

Dissociation of insulin oligomers by bile salt micelles and its effect on alpha-chymotrypsin-mediated proteolytic degradation

Bile salts have been found to be effective absorption promoters of insulin across mucosal barriers, i.e., nasal and gastrointestinal. One of the mechanisms proposed for absorption enhancement is the dissociation of insulin oligomers to monomers, rendering a higher insulin diffusivity. alpha-Chymotryptic degradation and circular dichroism studies were used to characterize such a transition. When zinc insulin (hexamers) and sodium insulin (dimers) were subjected to alpha-chymotryptic degradation, a 3.2-fold difference in the apparent first-order rate constants was observed (zinc insulin being slower than sodium insulin), representing the intrinsic difference in the concentration of total associated species in solution (three times). In the presence of a bile salt, sodium glycocholate (NaGC), the rate of degradation of both zinc and sodium insulin increased in an asymptotic manner. A maximum increase of 5.4-fold was observed for zinc insulin at a 30 mM NaGC concentration and a 2.1-fold increase was noted for sodium insulin at 10 mM NaGC, both values being close to the theoretical numbers of 6- and 2-fold as predicted by the complete dissociation of hexamers and dimers to monomers. The result indicates dissociation of insulin oligomers to monomers by bile salt micelles, probably by hydrophobic micellar incorporation of monomeric units. Circular dichroism studies also revealed progressive attenuation of molecular ellipticities at negative maxima of 276, 222, and 212 nm for zinc insulin solution in the presence of NaGC. Therefore, both alpha-chymotryptic degradation and circular dichroism studies have consistently demonstrated that the bile salts may be capable of dissociating insulin oligomers to monomers, a fact which may play an important role in enhancing insulin bioavailability.

Insulin aggregation in aqueous media and its effect on alpha-chymotrypsin-mediated proteolytic degradation

Self-association of zinc-insulin monomers into dimers and hexamers may lead to enhanced protection of the peptide from proteolytic degradation. The present study has been undertaken to investigate the relationship, if any, between the rate of enzymatic degradation of insulin by a protease, alpha-chymotrypsin, and the extent of insulin aggregation in aqueous solutions. Insulin solutions (0.6 mg/ml) containing varying proportions of dimer and hexamer were obtained by adding ethylene diamine tetraacetic acid (EDTA) within a concentration range of 0.005 to 0.040 mM. As the EDTA concentration was increased above 0.040 mM, a complete dissociation of hexamers to dimers occurred and the rate of enzymatic degradation reached its maximum. The overall first-order rate constants appeared to be linearly related to the square of EDTA concentrations. The apparent first-order rate constants for dimer and hexamer degradation obtained from a linear plot of rate constant versus EDTA squared concentration were found to be 0.02800 +/- 0.00065 and 0.00798 +/- 0.00075 min-1, respectively. Two major insulin degradation products were also detected and the kinetics of product appearance agreed well with the disappearance kinetics of insulin. The results indicated that the degradation of insulin dimers by alpha-chymotrypsin is about 3.5 times faster than the degradation of the hexamer. The second-order dependency of degradation rate on EDTA concentration might be due to the fact that insulin hexamers contain two zinc ions which are sequestered by two EDTA molecules. Chelation of zinc ions by EDTA lead to hexamer deaggregation to dimers as was evidenced from a circular dichroism study.(ABSTRACT TRUNCATED AT 250 WORDS)

Stability of protein pharmaceuticals

Recombinant DNA technology has now made it possible to produce proteins for pharmaceutical applications. Consequently, proteins produced via biotechnology now comprise a significant portion of the drugs currently under development. Isolation, purification, formulation, and delivery of proteins represent significant challenges to pharmaceutical scientists, as proteins possess unique chemical and physical properties. These properties pose difficult stability problems. A summary of both chemical and physical decomposition pathways for proteins is given. Chemical instability can include proteolysis, deamidation, oxidation, racemization, and beta-elimination. Physical instability refers to processes such as aggregation, precipitation, denaturation, and adsorption to surfaces. Current methodology to stabilize proteins is presented, including additives, excipients, chemical modification, and the use of site-directed mutagenesis to produce a more stable protein species.

Self-association and solubility behaviors of a novel anticancer agent, brequinar sodium

To aid in the selection of appropriate excipients to formulate brequinar sodium [6-fluoro-2-(2'-fluoro-1,1'-biphenyl-4-yl)-3-methyl-4-quinolinecarboxyli c acid sodium salt; DuP 785], studies were initiated to characterize thoroughly its solubility behavior. The measured solubilities at RT (approximately 23 degrees C) agreed with the theoretical values in the pH range from 0.5 to 7.2, but became significantly greater than theoretical values at pH values above 7.2. This deviation was likely due to the vertical stacking-type self-association between brequinar molecules in water. The NMR and pH methods determined a critical association concentration of 15 mg/mL. Sodium salicylate, which has been proven to interfere with molecular self-association, reduced drug solubility from 116 to 10 mg/mL. But urea, another deaggregative agent, gave about a twofold increase rather than a decrease in solubility. Addition of sodium chloride caused a 226-fold decrease in solubility. The apparent solubility product did not remain constant but decreased as sodium chloride concentration increased, suggesting that the added salt decreased the degree of self-association between brequinar molecules. Among four surfactants examined (a bile salt with a rigid fused ring versus three ordinary surfactants with a flexible chain structure), only sodium cholate significantly increased the aqueous solubility of brequinar sodium.

Application of scanning electron microscopy and energy-dispersive X-ray analysis to the solution behaviour of Zn-insulin: precipitation phenomena

Scanning electron microscopy (SEM) has been applied in combination with energy dispersive X-ray analysis (EDAX) to identify and analyse particles or particulate matter, occasionally present in clear neutral Zn-insulin solutions. SEM photographs revealed the existence of three different types of precipitate, consisting of particles with a crystalline, amorphous or gel-like nature, respectively. At present, it is not clear which conditions lead specifically to each of these three types of precipitate. The advantages of the EDAX method are shown. The technique enables semi-quantitative analysis to be performed on a single particle as small as 0.2 microns. It was demonstrated with the EDAX method that the particles occasionally found in clear Zn-insulin solutions contain insulin as well as Zn in roughly the same ratio as in the insulin starting material. It is concluded that the EDAX method has great potential in pharmaceutical technology, inter alia for the analysis of emulsion systems (in the frozen state), as well as suspensions and particulate matter in injection fluids. This technique is particularly useful in the latter case, due to its applicability to extremely small sample sizes.

Effect of sodium glycocholate and polyoxyethylene-9-lauryl ether on the hydrolysis of varying concentrations of insulin in the nasal homogenates of the albino rabbit

Protease inhibition has been postulated to be one of the several mechanisms by which penetration enhancers promote the mucosal absorption of peptide and protein drugs. The objective of this study was to determine whether protease inhibition by Na glycocholate and polyoxyethylene-9-lauryl ether, two extensively studied enhancers, led to suppression of insulin proteolysis over a range of insulin concentrations. To this end, the rate of insulin proteolysis in nasal tissue supernatants of the albino rabbit was determined in the presence of 0.1-2% Na glycocholate and polyoxyethylene-9-lauryl ether and at insulin concentrations ranging from 5 to 100 microM. Partly due to self-association, insulin was self-stabilizing against nasal proteolysis as its concentration was raised from 5 to 100 microM. At insulin concentrations lower than 50 microM, both Na glycocholate and polyoxyethylene-9-lauryl ether reduced the rate of insulin proteolysis. By contrast, at 100 microM insulin concentration, both enhancers accelerated insulin proteolysis. Such an effect was attributed to the deaggregation of insulin by the enhancers, increasing the proportion of monomers available for nasal proteolysis. The incorporation of 0.1 mM PCMPS, a potent inhibitor of insulin proteolysis, partly overcame the accelerating effect of Na glycocholate on insulin proteolysis.