Polymers for sustained macromolecule release: procedures to fabricate reproducible delivery systems and control release kinetics

Laser-Activated Drug Implant for Controlled Release to the Posterior Segment of the Eye

To treat chronic posterior eye diseases, frequent intravitreal injections or sustained-release drug implants are the current standard of care. Sustained-release drug implants often involve burst release of the drugs and the dosage from the implants cannot be controlled after implantation, which may lead to local side effects. The present study attempts to develop a dosage-controllable drug delivery implant that consists of a nanoporous biodegradable PLGA capsule and light-activated liposomes. Controllable drug release from the implant was achieved using a pulsed near-infrared (NIR) laser both in vitro and in vivo. The in vitro drug release kinetics from two different initial dose implants, 1000 and 500 μg, was analyzed by fitting zero-order and first-order kinetics, as well as the Korsmeyer-Peppas and Higuchi models. The 1000 and 500 μg implants fit the first-order and zero-order kinetics model, respectively, the best. The multiple drug releases in the vitreous were determined by an in vivo fluorimeter, which was consistent with the in vitro data. The dose released was also clinically relevant. Histology and optical and ultrasound imaging data showed no abnormality in the eyes received implant treatment, suggesting that the drug delivery system was safe to the retina. This on-demand dose-controllable drug delivery system could be potentially used for long-term posterior eye disease treatment to avoid frequent invasive injections.

Release mechanisms and applications of drug delivery systems for extended-release

INTRODUCTION

Drug delivery systems with extended-release profiles are ideal in improving patient compliance with enhanced efficacy. To develop devices capable of a prolonged delivery kinetics, it is crucial to understand the various underlying mechanisms contributing to extended drug release and the impact thereof on modulating the long-term performance of such systems in a practical application environment.

AREAS COVERED

This review article intends to provide a comprehensive summary of release mechanisms in extended-release drug delivery systems, particularly polymer-based systems; however, other material types will also be mentioned. Selected current research in the delivery of small molecule drugs and macromolecules is highlighted. Emphasis is placed on the combined impact of different release mechanisms and drug properties on the long-term release kinetics in vitro and in vivo.

EXPERT OPINION

The development of drug delivery systems over an extended duration is promising but also challenging when considering the numerous interrelated delivery-related parameters. Achieving a well-controlled extended drug release requires advanced techniques to minimize burst release and lag phase, a better understanding of the dynamic interrelationship between drug properties and release profiles over time, and a thorough elucidation of the impact of multiple in vivo conditions to methodically evaluate the eventual clinical efficacy.

Controlling the movement of molecules

The ability to control the movement of molecules is both fascinating scientifically as well as being critically important to the well-being of our planet and its people. In particular, the sustained release of molecules over prolonged periods at controlled rates has had and will continue to have enormous implications for the delivery of substances in medicine, agriculture, the environment, nutrition, aquaculture, household consumer products, and numerous other areas. This field is advancing at a rapidly accelerating pace. In this article, I largely discuss our own work, starting 45 years ago, in enabling the controlled release of macromolecules from biocompatible polymers. I also discuss the synthesis of novel materials to affect molecular movement and I then examine external approaches for controlling the movement of molecules through materials, using forces such as electric, acoustic, and magnetic fields. I further discuss approaches for controlling molecular movement through physiologic barriers, such as the skin, lung, and intestine. Finally, I outline several future areas of this field, including how it can affect the developing world, the ability to control the movement of molecules into mammalian cells, and the design of intelligent approaches to control molecular delivery.

Superior Glycemic Control With a Glucose-Responsive Insulin Analog: Hepatic and Nonhepatic Impacts

We evaluated the hepatic and nonhepatic responses to glucose-responsive insulin (GRI). Eight dogs received GRI or regular human insulin (HI) in random order. A primed, continuous intravenous infusion of [3-3H]glucose began at -120 min. Basal sampling (-30 to 0 min) was followed by two study periods (150 min each), clamp period 1 (P1) and clamp period 2 (P2). At 0 min, somatostatin and GRI (36 ± 3 pmol/kg/min) or HI (1.8 pmol/kg/min) were infused intravenously; basal glucagon was replaced intraportally. Glucose was infused intravenously to clamp plasma glucose at 80 mg/dL (P1) and 240 mg/dL (P2). Whole-body insulin clearance and insulin concentrations were not different in P1 versus P2 with HI, but whole-body insulin clearance was 23% higher and arterial insulin 16% lower in P1 versus P2 with GRI. Net hepatic glucose output was similar between treatments in P1. In P2, both treatments induced net hepatic glucose uptake (HGU) (HI mean ± SEM 2.1 ± 0.5 vs. 3.3 ± 0.4 GRI mg/kg/min). Nonhepatic glucose uptake in P1 and P2, respectively, differed between treatments (2.6 ± 0.3 and 7.4 ± 0.6 mg/kg/min with HI vs. 2.0 ± 0.2 and 8.1 ± 0.8 mg/kg/min with GRI). Thus, glycemia affected GRI but not HI clearance, with resultant differential effects on HGU and nonHGU. GRI holds promise for decreasing hypoglycemia risk while enhancing glucose uptake under hyperglycemic conditions.

Gelatin device for the delivery of growth factors involved in endochondral ossification

Controlled release drug delivery systems are well established as oral and implantable dosage forms. However, the controlled release paradigm can also be used to present complex soluble signals responsible for cellular organization during development. Endochondral ossification (EO), the developmental process of bone formation from a cartilage matrix is controlled by several soluble signals with distinct functions that vary in structure, molecular weight and stability. This makes delivering them from a single vehicle rather challenging. Herein, a gelatin-based delivery system suitable for the delivery of small molecules as well as recombinant human (rh) proteins (rhWNT3A, rhFGF2, rhVEGF, rhBMP4) is reported. The release behavior and biological activity of the released molecules was validated using analytical and biological assays, including cell reporter systems. The simplicity of fabrication of the gelatin device should foster its adaptation by the diverse scientific community interested in interrogating developmental processes, in vivo.

Biodegradable Hydrogel for Controlled Release of Biologically Active Macromolecules

The preparation, swelling and mechanical behavior and in vitro biodegradation of a novel hydrogel suitable for controlled release of biologically active macromolecules were investigated. The hydrogel was based on the biode gradable polypeptide, poly[ N5-(2-hydroxyethyl)-L-glutamide] with methacryl oylated side chains (mole fraction 0.02-0.10). These were used as a "macro monomer crosslinker" in the crosslinking polymerization of acrylamide. The polymerization can be carried out under conditions that avoid loss of the biolog ical activity of proteins. The network density of the gels was controlled by the number of methacryloylated side chains in the macromonomers and by the amount of diluent during gel formation. It was found that the network density decreases with increasing dilution and decreasing concentration of the double bonds in the macromonomer. The enzymatic degradation of the hydrogels by papain increased with decreasing network density. The release of a model pro tein entrapped in the gel was also influenced by the network density of the gel. All the gels can be fully degraded to soluble products.

A novel approach to administration of peptides in women: Systemic absorption of a GnRH agonist via transvaginal ring delivery system

trans-Epithelial delivery of medication across the vagina has proven successful for administration of small, lipophilic molecules such as sex steroids. However, little information is available regarding the vaginal delivery of larger and more polar molecules that currently require parenteral administration because the vaginal epithelium is perceived as a barrier to absorption of larger molecular weight (MW) molecules. Six healthy women underwent administration of 18 or 36mg of leuprolide, a GnRH agonist and a larger MW peptide, via a novel ethylene vinyl acetate (EVA) ring transvaginal drug delivery system (TVDS). Serum levels rose within 8h following insertion: low dose at 310pg/ml and high dose at 1220pg/ml, i.e. levels typically following parenteral injections of leuprolide. GnRHa biological activity was validated by secretion of gonadotropins and sex steroids. These results demonstrate that the non-keratinized vaginal epithelium permits a rapid absorption of a biologically active peptide and that there is significant potential for a novel TVDS to deliver peptides and possibly other macromolecules therapeutically.

SIGNIFICANCE STATEMENT

Current routes of administration of medications can include oral, subcutaneous, intravenous, intramuscular, transcutaneous, etc. Many of these approaches have limitations, including pain, poor tolerability, lack of adherence, and inadequate delivery. Peptides, in particular, cannot typically be given orally because they are broken down in the intestinal tract before they are absorbed. While the skin is an attractive way to deliver medications, its superb intrinsic barrier function often makes this route untenable at times. The vaginal epithelium, in contrast, is not keratinized and can allow absorption of other molecules. In this study, we demonstrate that a novel transvaginal drug delivery system (TVDS) is capable of delivering peptide therapeutics to women in a non-parenteral fashion as demonstrated by both blood levels and biologic effects of its delivery.

Polymeric nanoparticles and microparticles for the delivery of peptides, biologics, and soluble therapeutics

Biologically derived therapeutics, or biologics, are the most rapidly growing segment of the pharmaceutical marketplace. However, there are still unmet needs in improving the delivery of biologics. Injectable polymeric nanoparticles and microparticles capable of releasing proteins and peptides over time periods as long as weeks or months have been a major focus in the effort to decrease the frequency of administration. These particle systems fit broadly into two categories: those composed of hydrophilic and those composed of hydrophobic polymeric scaffolds. Here we review the factors that contribute to the slow and controlled release from each class of particle, as well as the effects of synthesis parameters and product design on the loading, encapsulation efficiency, biologic integrity, and release profile. Generally, hydrophilic scaffolds are ideal for large proteins while hydrophobic scaffolds are more appropriate for smaller biologics without secondary structure. Here we also introduce a Flash NanoPrecipitation method that has been adopted for encapsulating biologics in nanoparticles (40-200nm) at high loadings (50-75wt.%) and high encapsulation efficiencies. The hydrophilic gel interior and hydrophobic shell provide an opportunity to combine the best of both classes of injectable polymeric depots.

Graphene-based electroresponsive scaffolds as polymeric implants for on-demand drug delivery

Stimuli-responsive biomaterials have attracted significant attention in the field of polymeric implants designed as active scaffolds for on-demand drug delivery. Conventional porous scaffolds suffer from drawbacks such as molecular diffusion and material degradation, allowing in most cases only a zero-order drug release profile. The possibility of using external stimulation to trigger drug release is particularly enticing. In this paper, the fabrication of previously unreported graphene hydrogel hybrid electro-active scaffolds capable of controlled small molecule release is presented. Pristine ball-milled graphene sheets are incorporated into a three dimensional macroporous hydrogel matrix to obtain hybrid gels with enhanced mechanical, electrical, and thermal properties. These electroactive scaffolds demonstrate controlled drug release in a pulsatile fashion upon the ON/OFF application of low electrical voltages, at low graphene concentrations (0.2 mg mL(-1) ) and by maintaining their structural integrity. Moreover, the in vivo performance of these electroactive scaffolds to release drug molecules without any "resistive heating" is demonstrated. In this study, an illustration of how the heat dissipating properties of graphene can provide significant and previously unreported advantages in the design of electroresponsive hydrogels, able to maintain optimal functionality by overcoming adverse effects due to unwanted heating, is offered.

Controlled release of polypeptides and other macromolecules

The use of polymeric matrices for the controlled release of polypeptides and other macromolecular drugs is reviewed. Three principal mechanisms of release include diffusion of the polypeptide through the polymer, erosion of the polymer matrix, and the application of magnetic fields to force more drug out of the matrix. The diffusion controlled systems generally utilize ethylene-vinyl acetate copolymer. The advantage of these systems is facile manipulation of the pore structure to obtain desired release kinetics. Release of many different polypeptides from these systems for periods of months has been demonstrated. Bioerosion provides the advantage that the polymer system does not need to be retrieved. Magnetism provides a mechanism whereby desired increases and decreases in polypeptide release rates can be achieved on demand.

Responsive materials for self-regulated insulin delivery

With diabetes mellitus becoming an important public health concern, insulin-delivery systems are attracting increasing interest from both scientific and technological researchers. This feature article covers the present state-of-the-art glucose-responsive insulin-delivery system (denoted as GRIDS), based on responsive polymer materials, a promising system for self-regulated insulin delivery. Three types of GRIDS are discussed, based on different fundamental mechanisms of glucose-recognition, with: a) glucose enzyme, b) glucose binding protein, and c) synthetic boronic acid as the glucose-sensitive component. At the end, a personal perspective on the major issues yet to be worked out in future research is provided.

Biomaterials and biotechnology: from the discovery of the first angiogenesis inhibitors to the development of controlled drug delivery systems and the foundation of tissue engineering

This paper describes the discovery of the first inhibitors of angiogenesis; the discoveries that led to the development of the first biocompatible controlled release systems for macromolecules, and findings that helped to create the field of tissue engineering. In addition, new paradigms for creating biomaterials, early work on nanotechnology in medicine and intelligent drug delivery systems are discussed.

Electroresponsive polymer-carbon nanotube hydrogel hybrids for pulsatile drug delivery in vivo

Drug release triggered by an external non-invasive stimulus is of great interest for the development of new drug delivery systems. The preparation of an electroresponsive multiwalled carbon nanotube/poly(methylacrylic acid) (MWNT/PMAA)-based hybrid material is reported. The hydrogel hybrids achieve a controlled drug release upon the ON/OFF application of an electric field, giving rise to in vitro and in vivo pulsatile release profiles.

Development and evaluation of carvedilol-loaded transdermal drug delivery system: In-vitro and in-vivo characterization study

CONTEXT

The transdermal drug delivery system was prepared and the bioavailability of the selected drug was enhanced by reducing first-pass metabolism.

OBJECTIVE

The objective of this study was to enhance the bioavailability of carvedilol through transdermal patches.

MATERIALS AND METHODS

To develop a matrix-type transdermal patch containing carvedilol with different ratios of polymer combinations by solvent evaporation technique.

RESULTS AND DISCUSSION

In-vitro permeation studies were performed by Franz diffusion cells. The results followed Higuchi kinetics, and mechanism of release was diffusion mediated. On the basis of the in-vitro and physicochemical parameters of carvedilol patches, the code F-1(PVP: Ethyl Cellulose = 4:1) was chosen for the study of in-vivo, ex-vivo, histocompatibility study, and pharmacological study. The bioavailability studies in rats indicated that the carvedilol-loaded transdermal patches provided steady-state plasma concentration and improved bioavailability of 72% in comparison to oral administration. The ex-vivo permeation study in rat's skin indicated that the flux and permeability co-efficient of optimized F-1 patch was 30.08 ± 0.7 μg/cm(2)/h and 0.416 ± 0.05 μg/cm(2)/h, respectively, which was more as compared to plain carvedilol. The histocompatibility study of the F-1 patch on the rat's skin after 24 h ex-vivo study gave less pathological changes as compared to other. The antihypertensive activity of the patch in comparison with oral administration was studied using N-nitro-L-arginine methyl ester-induced hypertensive rats. It was observed that the optimized patch (F-1) significantly controlled hypertension (p < 0.05).

CONCLUSION

The developed patch increases the efficacy of carvedilol through enhancement of bioavailability for the therapy of hypertension.

Oral drug delivery systems comprising altered geometric configurations for controlled drug delivery

Recent pharmaceutical research has focused on controlled drug delivery having an advantage over conventional methods. Adequate controlled plasma drug levels, reduced side effects as well as improved patient compliance are some of the benefits that these systems may offer. Controlled delivery systems that can provide zero-order drug delivery have the potential for maximizing efficacy while minimizing dose frequency and toxicity. Thus, zero-order drug release is ideal in a large area of drug delivery which has therefore led to the development of various technologies with such drug release patterns. Systems such as multilayered tablets and other geometrically altered devices have been created to perform this function. One of the principles of multilayered tablets involves creating a constant surface area for release. Polymeric materials play an important role in the functioning of these systems. Technologies developed to date include among others: Geomatrix(®) multilayered tablets, which utilizes specific polymers that may act as barriers to control drug release; Procise(®), which has a core with an aperture that can be modified to achieve various types of drug release; core-in-cup tablets, where the core matrix is coated on one surface while the circumference forms a cup around it; donut-shaped devices, which possess a centrally-placed aperture hole and Dome Matrix(®) as well as "release modules assemblage", which can offer alternating drug release patterns. This review discusses the novel altered geometric system technologies that have been developed to provide controlled drug release, also focusing on polymers that have been employed in such developments.

Controlled Release of Fibroblast Growth Factor: Activity in Cell Culture

Basic fibroblast growth factor is a multi-potent cell regulatory factor that stimulates proliferation and angiogenesis. Controlled studies of basic fibroblast growth factor in animals have been hindered by the instability of this protein. In addition, many cells appear to require the continuous addition of basic fibroblast growth factor for optimal growth and function in culture. A system for the sustained delivery of active basic fibroblast growth factor might provide both a means to conduct log-term studies on activity and provide a practical alternative to multiple growth factor additions to cell cultures. Basic fibroblast growth factor was incorporated into standard polymer matrices, but the released growth factor had lost over 99% of its bioactivity. Loss of basic fibroblast growth factor activity was found to result from both physical inactivation and adsorption of the protein to surfaces. These problems were avoided by incorporating the growth factor into calcium cross-linked alginate microspheres containing heparinsepharose beads. Basic fibroblast growth factor was incorporated into these microspheres with 71% efficiency and active growth factor was released with predictable kinetics for up to 7 weeks. Release from these microspheres was controlled by the amount of heparin within the device and could be manipulated by simply altering the heparin content during fabrication. Alginate/heparin-sepharose microspheres were placed into growing cultures of bovine aortic endothelial cells and no cytotoxic effects were observed. Furthermore, microspheres containing growth factor provided long-term stimulation of cell proliferation and maintenance of endothelial cell morphology.

Elaborations on the Higuchi model for drug delivery

The Higuchi model for the rate of drug release from matrix devices where the drug loading exceeds the solubility in the matrix medium, whose 50-year anniversary is celebrated in this issue, has proven to be a robust framework and an invaluable tool in developing a significant part of the modern controlled drug delivery industry. This paper reviews the conceptual and mathematical bases for this model and some consequences of its inherent assumptions. In addition, selected extensions of the model that have proven useful over the years are summarized. These include the effects of external mass transfer resistance and spatial variations of drug loading.

A chemical screen identifies novel compounds that overcome glial-mediated inhibition of neuronal regeneration

A major barrier to regeneration of CNS axons is the presence of growth-inhibitory proteins associated with myelin and the glial scar. To identify chemical compounds with the ability to overcome the inhibition of regeneration, we screened a novel triazine library, based on the ability of compounds to increase neurite outgrowth from cerebellar neurons on inhibitory myelin substrates. The screen produced four "hit compounds," which act with nanomolar potency on several different neuronal types and on several distinct substrates relevant to glial inhibition. Moreover, the compounds selectively overcome inhibition rather than promote growth in general. The compounds do not affect neuronal cAMP levels, PKC activity, or EGFR (epidermal growth factor receptor) activation. Interestingly, one of the compounds alters microtubule dynamics and increases microtubule density in both fibroblasts and neurons. This same compound promotes regeneration of dorsal column axons after acute lesions and potentiates regeneration of optic nerve axons after nerve crush in vivo. These compounds should provide insight into the mechanisms through which glial-derived inhibitors of regeneration act, and could lead to the development of novel therapies for CNS injury.

Poly(lactide-co-glycolide) nanoparticle assembly for highly efficient delivery of potent therapeutic agents from medical devices

Controlled delivery of therapeutic agents from medical devices can improve their safety and effectiveness in vivo, by ameliorating the surrounding tissue responses and thus maintaining the functional integrity of the devices. Previously, we presented a new method for providing simultaneous controlled delivery from medical devices, by surface assembly of biodegradable polymer nanoparticles (NPs) encapsulating fluorescent dyes. Here, we continue our investigation with NPs loaded with therapeutic agents, dexamethasone (DEX) or plasmid DNA, and evaluated the bioactivity of the released molecules with macrophage cells associated with inflammation. Over a period of one week, NPs encapsulating DEX released 24.9+/-0.8ng from the probe surface and was successful at suppressing macrophage cell growth by 40+/-10%. This percentage of suppression corresponded to approximately 100% drug delivery efficiency, in comparison with the unencapsulated drug. DNA NP coatings, in contrast, released approximately 1ng of plasmid DNA and were effective at transfecting macrophage cells to express the luciferase gene at 300+/-200 relative luminescence/mg total protein. This amount of luciferase activity corresponded to 100% gene delivery efficiency. Thus, NP coatings were capable of providing continuous release of bioactive agents in sufficient quantities to induce relevant biological effects in cell culture studies. These coatings also remained intact, even after 14 days of incubation with phosphate buffered saline. Although the maximum loading for NP coatings is inherently lower than the more established matrix coating, our study suggests that the NP coatings are a more versatile and efficient approach toward drug delivery or gene delivery from a medical device surface and are perhaps best suited for continuous release of highly potent therapeutic agents.

Endothelial targeting of semi-permeable polymer nanocarriers for enzyme therapies

The medical utility of proteins, e.g. therapeutic enzymes, is greatly restricted by their labile nature and inadequate delivery. Most therapeutic enzymes do not accumulate in their targets and are inactivated by proteases. Targeting of enzymes encapsulated into substrate-permeable polymer nano-carriers (PNC) impermeable for proteases might overcome these limitations. To test this hypothesis, we designed endothelial targeted PNC loaded with catalase, an H(2)O(2)-detoxifying enzyme, and tested if this approach protects against vascular oxidative stress, a pathological process implicated in ischemia-reperfusion and other disease conditions. Encapsulation of catalase (MW 247 kD), peroxidase (MW 42 kD) and xanthine oxidase (XO, MW 300 kD) into approximately 300 nm diameter PNC composed of co-polymers of polyethylene glycol and poly-lactic/poly-glycolic acid (PEG-PLGA) was in the range approximately 10% for all enzymes. PNC/catalase and PNC/peroxidase were protected from external proteolysis and exerted enzymatic activity on their PNC diffusible substrates, H(2)O(2) and ortho-phenylendiamine, whereas activity of encapsulated XO was negligible due to polymer impermeability to the substrate. PNC targeted to platelet-endothelial cell (EC) adhesion molecule-1 delivered active encapsulated catalase to ECs and protected the endothelium against oxidative stress in cell culture and animal studies. Vascular targeting of PNC-loaded detoxifying enzymes may find wide medical applications including management of oxidative stress and other toxicities.

Activity-dependent reorganization of local circuitry in the developing visceral sensory system

Neural activity during critical periods could fine-tune functional synaptic connections. N-methyl-d-aspartate (NMDA) receptor activation is critically implicated in this process and blockade leads to disruption of normal circuit formation. This phenomenon has been well investigated in several neural systems including the somatosensory system, but not yet evidenced in the visceral sensory system. Ultrastructural analysis of GABAergic synapses and electrophysiological analysis of inhibitory and excitatory postsynaptic currents of the rat caudal nucleus tractus solitarii (NTS) cells revealed that developmental changes in the synaptic organizations were blocked by MK-801, an NMDA receptor antagonist, when administered at postnatal days 5-8, a presumed critical period for the visceral sensory system. Normal synapse reorganization during postnatal development dictates undifferentiated neonatal caudal NTS neurons in terms of synaptic input patterns measured by electron microscopy and electrophysiology into two cell groups: small and large cells under far stronger excitatory and inhibitory influence, respectively. Blockade by MK-801 during the critical period might leave adult neurons wired in the undifferentiated synaptic networks, possibly preventing synapse elimination and subsequent stabilization of the proper wiring.

Presentation of a structurally diverse and commercially available drug data set for correlation and benchmarking studies

A multivariate analysis of drugs on the Swedish market was the basis for the selection of a small, physicochemically diverse set of 24 drug compounds. Factors such as structural diversity, commercial availability, price, and a suitable analytical technique for quantification were considered in the selection. Lipophilicity, pKa, solubility, and permeability across human Caco-2 cell monolayers were measured for the compiled data set. The results show that, by use of a physicochemically diverse data set, experimental responses over a wide range were obtained. The paper also shows how experimental difficulties due to the diversity of the data set can be overcome. We anticipate that this data set can serve as a benchmark set for validation of new experimental techniques or in silico models. It can also be used as a diverse starting data set for the development of new computational models.

Relationship between encapsulated drug particle size and initial release of recombinant human growth hormone from biodegradable microspheres

Protein microencapsulation in biodegradable polymers is a promising route to provide for sustained release. One important characteristic in this regard is the size of the particles encapsulated within the microspheres. In this investigation, we have employed spray-freeze drying to generate particles for encapsulation, and examined the effect of various atomization conditions. Conditions were identified resulting in minimization of the particle size for the therapeutic protein recombinant human growth hormone (rhGH). The polymer employed was poly(lactide-co-glycolide) (PLG). The greatest friability for the powder, and hence smallest particle size (e.g., sub-micron), was achieved as the mass flow ratio of atomization (air to liquid) was increased. Protein powders over a range of particle sizes were encapsulated in biodegradable microspheres using a cryogenic, non-aqueous process. The initial release (both in vitro and in vivo) from these batches was found to decrease with decreasing encapsulated protein particle size; these findings are consistent with the percolation theory. Hence, judicious selection of process variables to reduce the particle size of rhGH is one strategy that can be used to minimize initial release of the microencapsulated protein.

Delivery systems for small molecule drugs, proteins, and DNA: the neuroscience/biomaterial interface

Manipulation of cellular processes in vivo by the delivery of drugs, proteins or DNA is of paramount importance to neuroscience research. Methods for the presentation of these molecules vary widely, including direct injection (either systemic or stereotactic), osmotic pump-mediated chronic delivery, or even implantation of cells engineered to indefinitely secrete a factor of interest. Biomaterial-based delivery systems represent an alternative to more traditional approaches, with the possibility of increased efficacy. Drug-releasing biomaterials, either as injectable microspheres or as three-dimensional implants, can deliver a molecule of interest (including small molecule drugs, biologically active proteins, or DNA) over a more prolonged period of time than by standard bolus injection, avoiding the need for repeated administration. Furthermore, sustained-release systems can maintain therapeutic concentrations at a target site, thus reducing the chance for toxicity. This review summarizes applications of polymer-based delivery of small molecule drugs, proteins, and DNA specifically relevant to neuroscience research. We detail the fabrication procedures for the polymeric systems and their utility in various experimental models. The biomaterial field offers unique experimental tools with downstream clinical application for the study and treatment of neurologic disease.

Uniform Encapsulation of Stable Protein Nanoparticles Produced by Spray Freezing for the Reduction of Burst Release

Stable protein nanostructured particles, produced by spray freezing into liquid (SFL) nitrogen, were encapsulated uniformly into microspheres to reduce the burst release over the first 24 h. The denaturation and aggregation of these bovine serum albumin (BSA) high-surface area particles were minimal due to ultra-rapid freezing and the absence of a liquid-air interface. Upon sonication, these friable highly porous, solid protein particle aggregates broke up into submicron particles. These particles were encapsulated into DL-lactide/glycolide copolymer (PLGA) and poly(lactic acid) (PLA) microspheres by anhydrous solid-in-oil-in-oil (s/o/o) techniques. For 5% loading of protein, the burst release after 24 h was only 2.5-4.1%, that is, values fivefold to tenfold lower than those observed for larger more conventional BSA particles. At a loading of 10%, the burst was only 6 and 13% for PLGA and PLA, respectively, and at 15% loading it was only 12% for PLGA. As shown with confocal and scanning electron microscopy (SEM), the low burst is consistent with a uniform distribution of protein nanoparticles, which were about 100 times smaller than the microspheres. Changes in aggregation and secondary structure, which were monitored by size exclusion chromatography and FTIR, respectively, indicated only slight monomer loss (3.9%) and high structural integrity (38% alpha-helix) in the encapsulated protein.

A novel compression-coated doughnut-shaped tablet design for zero-order sustained release

A novel coated doughnut-shaped tablet is evaluated as to its ability to be manufactured in a reproducible manner, and as to whether it releases model drugs at a zero-order rate. The doughnut-shaped tablets were compressed using specially designed punches, which make automated production feasible. In the preliminary part of the experiment, HPMC K15M mixed with gelatin was found to be the most suitable coating tablet material with respect to its disintegration and adherence properties. The adherence of the coating tablet to ibuprofen cores was not optimal, so different concentrations of gelatin, to act as a plasticiser and enhance adherence, were further investigated. Friability results of the coated doughnut-shaped tablet indicate that coating tablets containing 20% and 30% gelatin had a percentage weight losses of less than 1% after 100 revolutions in a Roche friabilator. For all the concentrations of gelatin, the granule blends had angle of repose values in the range of 22.01-17.8 degrees. The compressibility factor, as measured from the slopes of the natural logarithm of compressional force versus crushing strength, were 121.91 +/- 2.36, 132.64 +/- 3.60, and 88.54 +/- 11.52 for the coating tablet granules containing 10%, 20%, and 30% gelatin in HPMC K15M, respectively. The composition of the coating tablet did not affect the rate of release of both caffeine and ibuprofen from the coated doughnut-shaped tablets. The coatings also adhered to the core tablets for the entire duration of the release of the drugs.

Melted glyceryl palmitostearate (GPS) pellets for protein delivery

Lysozyme was incorporated into glyceryl palmitostearate (GPS) pellets by compression and melting at loadings of 2, 5 and 10% (w/w). Released lysozyme from both compressed and melted pellets showed good retention of enzymatic activity (>80% active). The percentage lysozyme recovered during in vitro release experiments, over 120 h, was significantly lower from the melted pellets (<15%) compared with compressed pellets (71-85%). Scanning electron microscopy suggested this difference in release was due to differences in porosity of the compressed and melted pellets. Inclusion of hydrophilic components, PEG 4000 and Gelucire 50/13, in the melted matrices increased the percentage of lysozyme released in vitro. Lysozyme released from GPS/PEG 4000 matrices showed good retention of enzymatic activity (>88% active) while that from GPS/Gelucire 50/13 showed reduced activity (68 and 51% active). PEG 4000 was not completely miscible with GPS at the concentrations studied and heterogenous systems resulted. At a loading of 20-35% (w/w) PEG 4000 in GPS greater than 80% of the incorporated lysozyme was released, indicating the likely achievement of interconnecting hydrophilic channels throughout the GPS matrix. In conclusion, melted GPS demonstrated potential as a matrix for the controlled release of proteins and release rates could be modified by inclusion of hydrophilic components.

N-methyl-d-aspartate receptor activation exerts a dual control on postnatal development of nucleus tractus solitarii neurons in vivo

We have used a morphological approach to evaluate the role of NMDA receptors (NMDAR) in postnatal development of brainstem neurons in awake rats. Chronic NMDAR blockade was performed by placing drug-impregnated Elvax implants over the brainstem at the fifth postnatal day (P5). Compared with control, NMDAR blockade led to a transient increase in dendritic arbor area and filopodium density until P12 followed by a rapid decline in both parameters. Electron microscopy observations showed that these changes correlated with an increase in synapse density at P14 followed by a decrease in synapse density at P28 if chronic NMDAR blockade was maintained until P21. These results support the hypothesis that synapse formation does not require NMDAR activation. In addition, our data suggest a dual role for NMDAR in controlling the synapse number. Early in development NMDARs may be involved in controlling the rate of synapse elimination. Later on, they may subserve synapse stabilization. The physiological significance of these results is discussed.

Sustained release of ascorbate-2-phosphate and dexamethasone from porous PLGA scaffolds for bone tissue engineering using mesenchymal stem cells

The purpose of this research was to develop porous poly(D,L-lactide-co-glycolide) (PLGA) scaffolds from which ascorbate-2-phosphate (AsAP) and dexamethasone (Dex) are continuously released for a month for osteogenesis of mesenchymal stem cells for bone tissue engineering. Porous PLGA matrices containing AsAP and Dex were prepared by solvent casting/particulate leaching method. In vitro release and water uptake studies were performed in Dulbecco's phosphate buffered saline at 37 degrees C and 15 rpm. Drug loading and release rates were determined by high performance liquid chromatography. Release studies of Dex and AsAP showed that, after an initial burst release lasting 4 and 9 days, respectively, release rates followed zero order kinetics with high correlation coefficients at least until 35 days. Incorporation of AsAP into the scaffolds increased the release rates of Dex and AsAP, and the scaffold water uptake. When mesenchymal stem cells (MSCs) were cultured in the AsAP and Dex containing scaffolds in vitro, the amount of mineralization was significantly higher than in control scaffolds. In conclusion, AsAP and Dex were incorporated into porous PLGA scaffolds and continuously released over a month and osteogenesis of MSCs was increased by culture in these scaffolds.

Altered immune response to liposomal allergens of Aspergillus fumigatus in mice

Aspergillus fumigatus has been implicated as the major pathogenic fungus causing Aspergillus-mediated disorders. It secretes complex glycoprotein antigens and allergens, which induce type I and type III mediated hypersensitivity reactions. The immune response to these allergens/antigens in allergic disorders is characterized by elevated levels of specific IgE, Th2 cytokines and eosinophilia. In the current study, the ability of negatively charged liposomes entrapped with glycoprotein antigens and allergens of A. fumigatus to modulate the immune response was studied. Immune response in mice was evaluated with both free and liposomal formulations. Liposome entrapped glycoprotein antigens/allergens of A. fumigatus elicited a Th1 type response with increased levels of TNF-alpha (5.5-folds), IFN-gamma (four-folds), specific IgG (three-folds) and IgG2a (2.4-folds), low titers of specific IgG1 (2.2-folds decrease) and IgE (three-folds decrease), and decreased peripheral eosinophilia by four-folds in comparison to mice receiving free glycoprotein allergens/antigens of A. fumigatus. Histopathological examination of lung tissue sections clearly indicated reduced eosinophil infiltration in mice immunized with liposomal formulations. These results suggest potential of liposomal formulations for A. fumigatus allergens/antigens for exploration in immunotherapy.

Dexamethasone/PLGA microspheres for continuous delivery of an anti-inflammatory drug for implantable medical devices

The purpose of this research was to develop polylactic-co-glycolic acid (PLGA) microspheres for continuous delivery of dexamethasone for over a 1-month period, in an effort to suppress the acute and chronic inflammatory reactions to implants such as biosensors, which interfere with their functionality. The microspheres were prepared using an oil-in-water emulsion technique. The oil phase was composed of 9:1 dichloromethane to methanol with dissolved PLGA and dexamethasone. Some microspheres were predegraded for 1 or 2 weeks. Ten percent of polyethylene glycol was added to the oil phase in alternative formulations to delay drug release. The in vitro release studies were performed in a constant temperature (37 C) warm room, in phosphate-buffered saline at sink conditions. Drug loading and release rates were determined by HPLC-UV analysis. The standard microsphere systems did not provide the desired release profile since, following an initial burst release, a delay of 2 weeks occurred prior to continuous drug release. Predegraded microspheres started to release dexamethasone immediately but the rate of release decreased after only 2 weeks. A mixed standard and predegraded microsphere system was used to avoid this delay and to provide continuous release of dexamethasone for 1 month.

Preparation and Characterization of Polyurethane Porous Membranes by Particulate-leaching Method

Highly porous polyurethane foams were prepared by the particulate-leaching technique, using NaCl as water-soluble solid phase. Melt compounding and compression moulding were used to prepare polymer/salt composite samples. Foams with controlled porosity, surface/volume ratio and pore dimension distribution were therefore obtained after dissolution of salt in distilled water. Dissolution kinetic was evaluated and modelled as a function of filler composition and its size distribution. Kinetic increased by rising salt concentration and by reducing its average size.

Mechanical properties of the foams were correlated to their morphological structure and to the properties of polymeric component.

Drug release kinetics from polymeric films containing propranolol hydrochloride for transdermal use

Polymeric films containing propranolol hydrochloride (PPN) were formulated and evaluated with a view to select a suitable formulation for the development of transdermal drug delivery systems. Films containing different ratios of ethyl cellulose (EC), poly(vinylpyrrolidone) (PVP), and PPN were prepared by mercury substrate method. In vitro drug release and skin permeation studies were conducted using paddle over disk and modified Franz diffusion cell, respectively. The drug release profiles from the polymeric film indicated that the drug content in the film decreased at an apparent first-order rate, whereas the quantity of drug release was proportional to the square root of time. The release rate of PPN increased linearly with increasing drug concentration and PVP fraction in the film, but was found to be independent of film thickness. The increase in release rate may be due to leaching of hydrophilic fraction of the film former, which resulted in the formation of pores. It was also observed that the release of drug from the films followed the diffusion-controlled model at low drug concentration. A burst effect was observed initially, however, at high drug loading level, which may be due to rapid dissolution of the surface drug followed by the diffusion of the drug through the polymer network in the film. The in vitro skin permeation profiles displayed increased flux values with increase of initial drug concentration in the film, and also with the PVP content. From this study, it is concluded that the films composed of EC/PVP/PPN, 9:1:3, 8:2:2, and 8:2:3, should be selected for the development of transdermal drug delivery systems using a suitable adhesive layer and backing membrane for potential therapeutic applications.

Numerical and experimental investigation of the diffusional release of a dispersed solute from polymeric multilaminate matrices

In the present work the release behavior of special, multilaminate matrix-type polymer systems, is studied both theoretically and experimentally. Two different mathematical models have been employed to describe the release of a dispersed solute from both single- and multilayer matrices. A parameter sensitivity study shows that the incorporation of supersaturated matrices in the formation of multilaminate devices, with a nonuniform initial solute loading, can provide a delivery system with optimized performance compared to monolithic ones. Finally, the findings of this theoretical analysis show good agreement with measurements of the release rates of a model disperse dye from both single- and multilayer matrices.

Protein spray-freeze drying. Effect of atomization conditions on particle size and stability

PURPOSE

To investigate the effect of atomization conditions on particle size and stability of spray-freeze dried protein.

METHODS

Atomization variables were explored for excipient-free (no zinc added) and zinc-complexed bovine serum albumin (BSA). Particle size was measured by laser diffraction light scattering following sonication in organic solvent containing poly(lactide-co-glycolide) (PLG). Powder surface area was determined from the N2 vapor sorption isotherm. Size-exclusion chromatography (SEC) was used to assess decrease in percent protein monomer. Fourier-transform infrared (FTIR) spectroscopy was employed to estimate protein secondary structure. PLG microspheres were made using a non-aqueous, cryogenic process and release of spray-freeze dried BSA was assessed in vitro.

RESULTS

The most significant atomization parameter affecting particle size was the mass flow ratio (mass of atomization N2 relative to that for liquid feed). Particle size was inversely related to specific surface area and the amount of protein aggregates formed. Zinc-complexation reduced the specific surface area and stabilized the protein against aggregation. FTIR data indicated perturbations in secondary structure upon spray-freeze drying for both excipient-free and zinc-complexed protein.

CONCLUSIONS

Upon sonication, spray-freeze dried protein powders exhibited friability, or susceptibility towards disintegration. For excipient-free protein, conditions where the mass flow ratio was > -0.3 yielded sub-micron powders with relatively large specific surface areas. Reduced particle size was also linked to a decrease in the percentage of protein monomer upon drying. This effect was ameliorated by zinc-complexation, via a mechanism involving reduction in specific surface area of the powder rather than stabilization of secondary structure. Reduction of protein particle size was beneficial in reducing the initial release (burst) of the protein encapsulated in PLG microspheres.