Controlled release of proteins to tissue transplants for the treatment of neurodegenerative disorders

Progress in brain targeting drug delivery system by nasal route

The blood-brain barrier (BBB) restricts the transport of potential therapeutic moieties to the brain. Direct targeting the brain via olfactory and trigeminal neural pathways by passing the BBB has gained an important consideration for delivery of wide range of therapeutics to brain. Intranasal route of transportation directly delivers the drugs to brain without systemic absorption, thus avoiding the side effects and enhancing the efficacy of neurotherapeutics. Over the last several decades, different drug delivery systems (DDSs) have been studied for targeting the brain by the nasal route. Novel DDSs such as nanoparticles (NPs), liposomes and polymeric micelles have gained potential as useful tools for targeting the brain without toxicity in nasal mucosa and central nervous system (CNS). Complex geometry of the nasal cavity presented a big challenge to effective delivery of drugs beyond the nasal valve. Recently, pharmaceutical firms utilized latest and emerging nasal drug delivery technologies to overcome these barriers. This review aims to describe the latest development of brain targeted DDSs via nasal administration.

CHEMICAL COMPOUNDS STUDIED IN THIS ARTICLE

Carbopol 934p (PubChem CID: 6581) Carboxy methylcellulose (PubChem CID: 24748) Penetratin (PubChem CID: 101111470) Poly lactic-co-glycolic acid (PubChem CID: 23111554) Tween 80 (PubChem CID: 5284448).

Mathematical modelling of glycosaminoglycan production by stem cell aggregates incorporated with growth factor-releasing polymer microspheres

Systems composed of high density cells incorporated with growth factor-releasing polymer microspheres have recently been shown to promote chondrogenic differentiation and cartilage formation. Within these systems, the effects of spatial and temporal patterning of growth factor release on hyaline cartilage-specific extracellular matrix production have been examined. However, at present, it is unclear which microsphere densities and growth factor delivery profiles are optimal for inducing human mesenchymal stem cell differentiation and glycosaminoglycan production. A mathematical model to describe glycosaminoglycan production as a function of initial microsphere loading and microsphere degradation rate over a period of 3 weeks is presented. Based on predictions generated by this model, it may be feasible to design a bioactive microsphere system with specific spatiotemporal growth factor presentation characteristics to promote glycosaminoglycan production at controllable rates. Copyright © 2014 John Wiley & Sons, Ltd.

Hydrological tracers using nanobiotechnology: proof of concept

In order to answer questions that involve differentiating among multiple and potentially interacting hydrological flowpaths, it would be ideal to use multiple tracers with identical transport properties that can nonetheless be distinguished from each other. This paper describes the development and proof of concept of a new kind of engineered tracer system that allows a large number of individual tracers to be simultaneously distinguished from one another. This new tracer is composed of polylactic acid (PLA) microspheres into which short strands of synthetic DNA and paramagnetic iron oxide nanoparticles are incorporated. The synthetic DNA serves as the "label" or "tag" in our tracers that allow us to distinguish one tracer from another, and paramagnetic iron oxide nanoparticles are included in the tracer to facilitate magnetic concentration of the tracers in potentially dilute water samples. Some potential advantages of this tracer concept include: virtually limitless uniquely labeled tracers, highly sensitive detection, and relatively moderate expense. Three proof-of-concept experiments at scales ranging from orders of 10 cm to 100 m demonstrated the use of the tracer system.

Salicylic acid-derived poly(anhydride-ester) electrospun fibers designed for regenerating the peripheral nervous system

Continuous biomaterial advances and the regenerating potential of the adult human peripheral nervous system offer great promise for restoring full function to innervated tissue following traumatic injury via synthetic nerve guidance conduits (NGCs). To most effectively facilitate nerve regeneration, a tissue engineering scaffold within a conduit must be similar to the linear microenvironment of the healthy nerve. To mimic the native nerve structure, aligned poly(lactic-co-glycolic acid)/bioactive polyanhydride fibrous substrates were fabricated through optimized electrospinning parameters with diameters of 600 ± 200 nm. Scanning electron microscopy images show fibers with a high degree of alignment. Schwann cells and dissociated rat dorsal root ganglia demonstrated elongated and healthy proliferation in a direction parallel to orientated electrospun fibers with significantly longer Schwann cell process length and neurite outgrowth when compared to randomly orientated fibers. Results suggest that an aligned polyanhydride fiber mat holds tremendous promise as a supplement scaffold for the interior of a degradable polymer NGC. Bioactive salicylic acid-based polyanhydride fibers are not limited to nerve regeneration and offer exciting promise for a wide variety of biomedical applications.

The administration of BDNF and GDNF to the brain via PLGA microparticles patterned within a degradable PEG-based hydrogel: Protein distribution and the glial response

Tailored delivery of neurotrophic factors (NFs) is a critical challenge that continues to inhibit strategies for guidance of axonal growth in vivo. Of particular importance is the ability to recreate innervation of distant brain regions by transplant tissue, for instance rebuilding the nigrostriatal track, one focus in Parkinson's disease research. Many strategies have utilized polymer drug delivery to target NF release in space and time, but combinatorial approaches are needed to deliver multiple NFs at relevant therapeutic times and locations without toxic side effects. Here we engineered a paradigm of PLGA microparticles entrapped within a degradable PEG-based hydrogel device to locally release two different types of NFs with two different release profiles. Hydrogel/microparticle devices were developed and analyzed for their ability to release GDNF in the caudal area of the brain, near the substantia nigra, or BDNF in the rostral area, near the striatum. The devices delivered their respective NFs in a region localized to within 100 μm of the bridge, but not exclusively to the targeted rostral or caudal ends. BDNF was slowly released over a 56-day period, whereas a bolus of GDNF was released around 28 days. The timed delivery of NFs from implanted devices significantly reduced the microglial response relative to sham surgeries. Given the coordinated drug delivery ability and reduced localized inflammatory response, this multifaceted PEG hydrogel/PLGA microparticle strategy may be a useful tool for further development in combining tissue engineering and drug delivery, and recreating the nigrostriatal track.

New biotechnological and nanomedicine strategies for treatment of lysosomal storage disorders

This review discusses the multiple bio- and nanotechnological strategies developed in the last few decades for treatment of a group of fatal genetic diseases termed lysosomal storage disorders. Some basic foundation on the biomedical causes and social and clinical relevance of these diseases is provided. Several treatment modalities, from those currently available to novel therapeutic approaches under development, are also discussed; these include gene and cell therapies, substrate reduction therapy, chemical chaperones, enzyme replacement therapy, multifunctional chimeras, targeting strategies, and drug carrier approaches.

Mechanistic examination of protein release from polymer nanofibers

Therapeutic proteins have emerged as a significant class of pharmaceutical agents over the past several decades. The potency, rapid elimination, and systemic side effects have prompted the need of spatiotemporally controlled release for proteins maybe more than any other active therapeutic molecules. This work examines the release of two model protein compounds, bovine serum albumin (BSA) and an anti-integrin antibody (AI), from electrospun polycaprolactone (PCL) nanofiber mats. The anti-integrin antibody was chosen as a model of antibody therapy; in particular, anti-integrin antibodies are a promising class of therapeutic molecules for cancer and angiogenic diseases. The release kinetics were studied experimentally and interpreted in the framework of a recently published theory of desorption-limited drug release from nondegrading--or very slowly degrading--fibers. The results are consistent with a protein release mechanism dominated by desorption from the polymer surface, while the polycaprolactone nanofibers are not degrading at an appreciable rate.

Recombinant human NGF-loaded microspheres promote survival of basal forebrain cholinergic neurons and improve memory impairments of spatial learning in the rat model of Alzheimer's disease with fimbria-fornix lesion

Neurotrophic factors are used for the experimental treatment of neurological disorders, such as Alzheimer's disease. However, delivery of the neurotrophic factors into the brain remains a big challenge. Recombinant human nerve growth factor (NGF)-loaded microspheres were fabricated and characterized in vitro and in vivo in our previous study. The present study was to assess the therapeutic benefit of rhNGF-loaded microspheres in treating the rat model of Alzheimer's disease with fimbria-fornix lesion. Recombinant human NGF-loaded microspheres were implanted into the basal forebrain of the rats with fimbria-fornix lesion. Four weeks after implantation in the basal forebrain, immunohistochemical analysis showed that rhNGF-loaded microspheres had a significant effect on the survival of axotomized cholinergic neurons in the medial septum (MS) and vertical diagonal branch (VDB) (p<0.05). Y-maze tests showed rhNGF-loaded microspheres can significantly improve the ability of spatial learning and memory of the rats with fimbria-fornix lesion (p<0.05). These results indicate that rhNGF-loaded microspheres are an effective means for the treatment of Alzheimer's disease.

Spatiotemporal control over growth factor signaling for therapeutic neovascularization

Many of the qualitative roles of growth factors involved in neovascularization have been delineated, but it is unclear yet from an engineering perspective how to use these factors as therapies. We propose that an approach that integrates quantitative spatiotemporal measurements of growth factor signaling using 3-D in vitro and in vivo models, mathematic modeling of factor tissue distribution, and new delivery technologies may provide an opportunity to engineer neovascularization on demand.

Approaches to neural tissue engineering using scaffolds for drug delivery

This review seeks to give an overview of the current approaches to drug delivery from scaffolds for neural tissue engineering applications. The challenges presented by attempting to replicate the three types of nervous tissue (brain, spinal cord, and peripheral nerve) are summarized. Potential scaffold materials (both synthetic and natural) and target drugs are discussed with the benefits and drawbacks given. Finally, common methods of drug delivery, including degradable/diffusion-based delivery systems, affinity-based delivery systems, immobilized drug delivery systems, and electrically controlled drug delivery systems, are examined and critiqued. Based on the current body of work, suggestions for future directions of research in the field of neural tissue engineering are presented.

Stability and release of antiviral drugs from ethylene vinyl acetate (EVA) copolymer

The use of polymer based drug delivery systems in dentistry is a relatively new area of research with the exception of the inhibition of secondary caries by the release of fluoride ions from polyalkenoate cements and their predecessors silicate cements. The present study was to test on orally biocompatible material, ethylene vinyl acetate copolymer (EVA), for release of antiviral drugs at oral therapeutic levels over extended periods of time. We also determined their stability during film casting and release. Materials studied include gancyclovir (GCY), acyclovir (ACY), dichloromethane (DCM), and ethylene vinyl acetate (EVA). The square films (3 x 3 x 0.1 cm) were prepared from the dry sheet obtained by solvent evaporation of polymer casting solutions. These solutions were made of EVA and the drug (40:1) in 70 ml of dichloromethane at 38 degrees C. Then drug release characteristics from the drug loaded films were examined at 37 degrees C for a minimum of 14 days in 10 ml medium (ddwater) replaced daily. Kinetics of drug release were followed by spectral measurements using previously determined lambda(max) values (GCY = 250 nm; ACY = 253 nm). A minimum of three samples was tested and reproducible results were obtained. Drug stability (ACY) during film casting and its release was determined using 1H NMR spectrometer (Bruker DRX-500 and 400). Rate of drug release was determined from the part of the curve (rate vs. time) after the onset of the "burst." Although GCY has a larger molecular weight (255) than ACY (225), GCY exhibited about three times higher rate of release than ACY. This difference in rate values may be explained due to its relatively greater solubility in EVA, facilitating faster diffusion of the molecules through the channels present in EVA. This is consistent with the observation that the rate at which drug molecules diffuse through the channels of the polymer, can be increased by decreasing the molecular weight. In the case of ACY, the molecules may be undergoing molecular associations, perhaps dimerization or trimerization in addition to its lower solubility in EVA. The diffusion of ACY tends to be slower under these circumstances compared to GCY resulting in lower rate value than in the case of GCY. Biological studies revealed that ACY exhibited a remarkable decrease in a number of viral organisms present in virus infected cell culture system using real-time polymerase chain reaction (RT-PCR). NMR analysis indicates that the chemical structure of the drug remains stable during film casting process and release.

Impact of cell type and density on nerve growth factor distribution and bioactivity in 3-dimensional collagen gel cultures

Local delivery of protein agents is potentially important in many tissue engineering systems. In this report, we evaluate an experimental system for measuring the rate of nerve growth factor (NGF) transport and biological activity within a 3-dimensional, tissue-like environment. Fetal brain cells or PC12 cells were suspended throughout collagen gel cultures; controlled-release matrices were used to control the spatial and temporal pattern of NGF release. Experimentally measured concentration profiles were compared to profiles predicted by a mathematical model encompassing diffusion and first-order elimination. Our results suggest that NGF moves through gels by diffusion while being eliminated at a rate that depends on cell density. Since diffusion and elimination also govern protein transport in brain tissue, the collagen gel serves as a model system that replicates the main features of transport in the brain and, therefore, can be used to identify new strategies that enhance NGF distribution in the central nervous system. As an example of the utility of this biophysical model, we demonstrate that implantation of multiple controlled-release matrices can broaden NGF distribution in gel cultures; this broadening was accompanied by a significant increase in cellular biological activity. This approach may be useful in customizing NGF distribution throughout degenerating or damaged central nervous system tissue while minimizing toxicity to surrounding healthy tissue.

Treatment of focal cerebral ischemia with liposomal nerve growth factor

Liposomal nerve growth factor (NGF) was used for the treatment of focal cerebral ischemia in a rat model. Positive charge inducing agents of sphingosine (SP) and stearylamine (S) were formulated in the liposomal NGF. Dose-response of intraventricular injection of liposomal NGF showed significant reduction in infarct volume at the dose of 5 and 10 microg/rat of NGF. The liposomal NGF formulated with SP or S demonstrated similar results in the reduction of total infarct volume in rats. When we increased the molar ratio of SP and S from 0.15 to 0.3, the infarct volume from rats showed a similar value as that of the control treated with NGF solution. Liposomal NGF was given prior to the development of ischemia. We found that NGF was effective in prevention of neuronal death. The NGF concentrations in brain for liposomal NGF were maintained in a level significantly higher than those for NGF solution. This was attributed to the positively charged liposomal NGF bound effectively in brain ventricle and caused longer retention time than free NGF for localization in brain. Therefore, the effect of liposomal NGF on reduction of infarct volume was significant. We assumed that the transportation of NGF might go through the cerebrospinal fluid pathway throughout the ventricular system and subarachnoid system to cerebral cortex to produce a therapeutic effect on ischemia.

Incorporation of protein-eluting microspheres into biodegradable nerve guidance channels for controlled release

Nerve guidance channels (NGCs) promote axonal regeneration after transection injury of the peripheral nerve or spinal cord, yet this regeneration is limited. To enhance regeneration further, we hypothesize that localized delivery of therapeutic molecules combined with the NGC is required. In an attempt to achieve such an NGC, we designed and synthesized a novel NGC in which protein-encapsulated microspheres were stably incorporated into the tube wall. Specifically, poly(lactide-co-glycolide) (PLGA 50/50) microspheres were physically entrapped in the annulus between two concentric tubes, consisting of a chitosan inner tube and a chitin outer tube. Taking advantage of the extensive shrinking that the outer chitin tube undergoes with drying, >15 mg of microspheres were loaded within the tube walls. Using BSA-encapsulated microspheres as the model drug delivery system, BSA was released from microsphere loaded tubes (MLTs) for 84 days, and from freely suspended PLGA microspheres for 70 days. An initial burst release was observed for both MLTs and free microspheres, followed by a degradation-controlled release profile that resulted in a higher release rate from MLTs initially, which was then attenuated likely due to the buffering effect of chitin and chitosan tubes. Epidermal growth factor (EGF), co-encapsulated with BSA in PLGA 50/50 microspheres in MLTs, was released for 56 days with a similar profile to that of BSA. Released EGF was found to be bioactive for at least 14 days as assessed by a neurosphere forming bioassay.

Drug release from cast films of ethylene vinyl acetate (EVA) copolymer: Stability of drugs by 1H NMR and solid state 13C CP/MAS NMR

The study utilizes an oral biocompatible material based on ethylene vinyl acetate copolymer (EVA) designed to release drugs in vitro at therapeutic levels over several days. We examined the drug stability during film casting process using proton and solid state NMR techniques. The drug-loaded EVA films were prepared from the dry sheet obtained by solvent (dichloromethane) evaporation of polymer casting solutions. Drugs tested include chlorhexidine diacetate (CDA), doxycycline hydrochloride (DOH), tetracycline hydrochloride (TTH) and nystatin (NST). Drug release from the films was examined for at least 14 days in 10 ml ddH2O (NST in water/ethanol (4:1)) which was replaced daily. Changes in optical density were followed spectraphotometrically. Effect of temperature on rate measurements was studied and the energies of activation (E*) were calculated using Arrhenius plots. Effect of EVA copolymer composition on CDA release rate was also investigated. The enhanced rates with temperature increase may be attributed to the formation of channels with increased geometry in the polymer. The highest E* observed for CDA compared to DOH and TTH may be related to their average molecular weights. Spectral analyses for CDA and NST revealed that the chemical and physical structures of the drugs remained unaffected during the film casting process.

Sustained delivery and expression of plasmid DNA based on biodegradable polyester, poly(d,l-lactide-co-4-hydroxy-l-proline)

Gene expression mediated by a non-viral vector usually lasts only a few days. The objective of this study was to synthesize and characterize a non-toxic, polymeric gene carrier, poly(D,L-lactide-co-4-hydroxy-L-proline) (PLHP) for sustained gene delivery. The copolymer was synthesized by ring-opening polymerization of D,L-lactide (DLLA) with N-cbz-4-hydroxy-L-proline (HP) in the presence of stannous octoate (Sn(Oct)(2)). The resulting copolymer was characterized by (1)H nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). Degradation of PLHP was examined by monitoring the medium pH change and molecular weight (MW) of the remaining polymer. It showed a rapid initial degradation and followed by a slower degradation for about 30 days at 37 degrees C. The cytotoxicity of copolymer was significantly lower than polyethylenimine (PEI) and poly-L-lysine hydrochloride (PLL) by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The plasmid DNA (pDNA)-loaded microspheres based on the copolymer were prepared by a water-oil-water (w/o/w) solvent evaporation emulsion method. The release profile of pDNA from PLHP microspheres showed an initial burst release, and then a slower and continuous release for about 18 days at 37 degrees C. Gene transfer efficiency of PLHP/pDNA delivery system showed a sustained activity (over a week) when compared with PEI and PLL, and can be further improved by the addition of cationic liposomes. The results suggest that PLHP is a promising candidate for long-term gene delivery with good biocompatibility and biodegradability.

A polymeric device for delivery of anti-microbial and anti-fungal drugs in the oral environment: effect of temperature and medium on the rate of drug release

OBJECTIVES

The use of drug delivery systems in dentistry is a relatively new area of research with the exception of fluoride ion release from polyalkenoate cements and their predecessor silicate cements. The present study is based on the use of a bio-compatible material ethylene vinyl acetate copolymer (EVA) that enables constant release of drugs of therapeutic levels over extended periods of time at doses suitable for the treatment of oral conditions.

METHODS

Polymer casting solutions were made by dissolving EVA and the drug in the ratio of 40:1 in 70 ml of dichloromethane at 38 degrees C for 6 h. Thin square films of 3 x 3 cm2 with a thickness of 1 mm were cut from the dry sheet obtained by solvent evaporation technique. Drug loaded samples were extracted for a minimum of 14 days in 10 ml medium (double distilled water or water/ethanol (4:1)) which was replaced daily. Spectral measurements were made to follow changes in optical densities (OD) during release kinetics. Effect of temperature (24 and 37 degrees C) on the rate of drug release was studied and the energies of activation (DeltaE not equal ) were calculated using Arrehenius equation for the diffusion (translocation) of molecules of tetracycline hydrochloride (TTH), doxycycline hydrochloride (DOH), and chlorhexidine diacetate (CDA) in water as extracting medium. Effect of extracting medium (water and water/ethanol (4:1)) was also investigated on the rate of drug release measurements at 24 degrees C.

RESULTS

Analysis of variance of the data revealed that significantly enhanced rates were observed at the higher temperature (37 degrees C) and when extracting medium was changed to water/ethanol (4:1) for TTH, DOH and CDA (p<0.0015). The enhanced rate values seem to be due to the formation of channels in the polymer. The largest activation energy (21.83 kcal mol(-1)) observed for CDA was interpreted as due to the highest average molecular weight (626) compared to TTH (481) and DOH (481).Significance. These in vitro rate of drug release measurements will provide a basis for establishing a novel approach (treatment modality) for sustained intra-oral drug delivery over extended time periods using laboratory methods and materials that are readily available to dentists.

Diffusion of nerve growth factor in rat striatum as determined by multiphoton microscopy

Neurotrophins such as nerve growth factor (NGF) may be useful for treating diseases in the central nervous system; our ability to harness the potential therapeutic benefit of NGF is directly related to our understanding of the fate of exogenously supplied factors in brain tissue. We utilized multiphoton microscopy to quantify the dynamic behavior of NGF in coronal, 400- micro m thick, fresh rat brain tissue slices. We administered a solution containing bioactive rhodamine nerve growth factor conjugate via pressure injection and monitored the dispersion in the striatal region of the coronal slices. Multiphoton microscopy facilitated repeated imaging deep ( approximately 200 micro m) into tissue slices with minimal photodamage of tissue and photobleaching of label. The pressure injection paradigm approximated diffusion from a point source, and we therefore used the corresponding solution to the diffusion equation to estimate an apparent diffusion coefficient in brain tissue (D(b)(34 degrees C)) of 2.75 +/- 0.24 x 10(-7) cm(2)/s (average +/- SE). In contrast, we determined a corresponding free diffusion coefficient in buffered solution (D(f)(34 degrees C)) of 12.6 +/- 0.9 x 10(-7) cm(2)/s using multiphoton fluorescence photobleaching recovery. The tortuosity, defined as the square root of the ratio of D(f) to D(b), was 2.14 and moderate in magnitude.

Polyanhydrides: an overview

Polyanhydrides have been considered to be useful biomaterials as carriers of drugs to various organs of the human body such as brain, bone, blood vessels, and eyes. They can be prepared easily from available, low cost resources and can be manipulated to meet desirable characteristics. Polyanhydrides are biocompatible and degrade in vivo into non-toxic diacid counterparts that are eliminated from the body as metabolites. Owing to their usefulness, this review focuses on the development, synthesis methods, structures and characterization of polyanhydrides, which will provide an overview for the researchers in the field. Their in vitro and in vivo degradability, toxicity, biocompatibility and applications are discussed in the subsequent chapters of this special issue on polyanhydrides and poly(ortho esters).

Pharmacokinetics of the carmustine implant

Controlled release delivery of carmustine from biodegradable polymer wafers was approved as an adjunct to surgical resection in the treatment of recurrent glioblastoma multiforme after it was shown in clinical trials to be well tolerated and effective. Given the localised nature of the drug in the brain tissue, no direct pharmacokinetic measurements have been made in humans after implantation of a carmustine wafer. However, drug distribution and clearance have been extensively studied in both rodent and non-human primate brains at various times after implantation. In addition, studies to characterise the degradation of the polymer matrix, the release kinetics of carmustine and the metabolic fate of the drug and polymer degradation products have been conducted both in vitro and in vivo. GLIADEL wafers have been shown to release carmustine in vivo over a period of approximately 5 days; when in continuous contact with interstitial fluid, wafers should degrade completely over a period of 6 to 8 weeks. Metabolic elimination studies of the polymer degradation products have demonstrated that sebacic acid monomers are excreted from the body in the form of expired CO(2), whereas 1,3-bis-(p-carboxyphenoxy)propane monomers are excreted primarily through the urine. Carmustine degradation products are also excreted primarily through the urine. Pharmacokinetic studies in animals and associated modelling have demonstrated the capability of this modality to produce high dose-delivery (millimolar concentrations) within millimetres of the polymer implant, with a limited penetration distance of carmustine from the site of delivery. The limited spread of drug is presumably due to the high transcapillary permeability of this lipophilic molecule. However, the presence of significant convective flows due to postsurgical oedema may augment the diffusive transport of drug in the hours immediately after wafer implantation, leading to a larger short-term spread of drug. Additionally, in non-human primates, the presence of significant doses in more distant regions of the brain (centimetres away from the implant) has been shown to persist over the course of a week. The drug in this region was presumed to be transported from the implant site by either cerebral blood flow or cerebrospinal fluid flow, suggesting that although drug is able to penetrate the blood-brain barrier at the site of delivery, it may re-enter within the confines of the brain tissue.

Building drug delivery into tissue engineering

The creation of efficient methods for manufacturing biotechnology drugs--many of which influence fundamental but complex cell behaviours, such as proliferation, migration and differentiation--is creating new opportunities for tissue repair. Many agents are potent and multifunctional; that is, they produce different effects within different tissues. Therefore, control of tissue concentration and spatial localization of delivery is essential for safety and effectiveness. Synthetic systems that can control agent delivery are particularly promising as materials for enhancing tissue regeneration. This review discusses the state of the art in controlled-release and microfluidic drug delivery technologies, and outlines their potential applications for tissue engineering.

Delivery of neurotrophic factors to the central nervous system: pharmacokinetic considerations

Neurotrophic factors are proteins with considerable potential in the treatment of central nervous system (CNS) diseases and traumatic injuries. However, a significant challenge to their clinical use is the difficulty associated with delivering these proteins to the CNS. Neurotrophic factors are hydrophilic, typically basic, monomeric or dimeric proteins, mostly in the size range of 5 to 30 kDa. Neurotrophic factors potently support the development, growth and survival of neurons, eliciting biological effects at concentrations in the nanomolar to femtomolar range. They are not orally bioavailable and the blood-brain and blood-cerebrospinal fluid barriers severely limit their ability to enter into and act on sites in the CNS following parenteral systemic routes of administration. Most neurotrophic factors have short in vivo half-lives and poor pharmacokinetic profiles. Their access to the CNS is restricted by rapid enzymatic inactivation, multiple clearance processes, potential immunogenicity and sequestration by binding proteins and other components of the blood and peripheral tissues. The development of targeted drug delivery strategies for neurotrophic factors will probably determine their clinical effectiveness for CNS conditions. Achieving significant CNS target site concentrations while limiting systemic exposure and distribution to peripheral sites of action will lessen unwanted pleiotropic effects and toxicity. Local introduction of neurotrophic factors into the CNS intraparenchymally by direct injection/infusion or by implantation of delivery vectors such as polymer matrices or genetically modified cells yields the highest degree of targeting, but is limited by diffusion restrictions and invasiveness. Delivery of neurotrophic factors into the cerebrospinal fluid (CSF) following intracerebroventricular or intrathecal administration is less invasive and allows access to a much wider area of the CNS through CSF circulation pathways. However, diffusional and cellular barriers to penetration into surrounding CNS tissue and significant clearance of CSF into the venous and lymphatic circulation are also limiting. Unconventional delivery strategies such as intranasal administration may offer some degree of CNS targeting with minimal invasiveness. This review presents a summary of the neurotrophic factors and their indications for CNS disorders, their physicochemical characteristics and the different approaches that have been attempted or suggested for their delivery to the CNS. Future directions for further research such as the potential for CNS disease treatment utilising combinations of neurotrophic factors, displacement strategies, small molecule mimetics, chimaeric molecules and gene therapy are also discussed.

Transplantation of brain cells assembled around a programmable synthetic microenvironment

Cell therapy is a promising method for treatment of hematopoietic disorders, neurodegenerative diseases, diabetes, and tissue loss due to trauma. Some of the major barriers to cell therapy have been partially addressed, including identification of cell populations, in vitro cell proliferation, and strategies for immunosuppression. An unsolved problem is recapitulation of the unique combinations of matrix, growth factor, and cell adhesion cues that distinguish each stem cell microenvironment, and that are critically important for control of progenitor cell differentiation and histogenesis. Here we describe an approach in which cells, synthetic matrix elements, and controlled-release technology are assembled and programmed, before transplantation, to mimic the chemical and physical microenvironment of developing tissue. We demonstrate this approach in animals using a transplantation system that allows control of fetal brain cell survival and differentiation by pre-assembly of neo-tissues containing cells and nerve growth factor (NGF)-releasing synthetic particles.

Controlled DNA delivery systems

PURPOSE

Genes are of increasing interest as pharmaceuticals, but current methods for long-term gene delivery are inadequate. Controlled release systems using biocompatible and/or biodegradable polymers offer many advantages over conventional gene delivery approaches. We have characterized systems for controlled delivery of DNA from implantable polymer matrices (EVAc: poly (ethylene-co-vinyl acetate)) and injectable microspheres (PLGA and PLA: poly (D, L-lactide-co-glycolide) copolymer and poly (L-lactide), respectively).

METHODS

Herring sperm DNA and bacteria phage lambda DNA were encapsulated as a model system. Released DNA concentration was determined by fluoroassays. Agarose electrophoresis was used to determine the dependence of release rate on DNA size. The Green Fluorescent Protein (GFP) gene was used to determine the integrity and functionality of released DNA.

RESULTS

Both small and large DNA molecules (herring sperm DNA, 0.1-0.6 kb; GFP, 1.9 kb; lambda DNA, 48.5 kb) were successfully encapsulated and released from EVAc matrices, and PLGA or PLA microspheres. The release from DNA-EVAc systems was diffusion-controlled. When co-encapsulated in the same matrix, the larger lambda DNA was released more slowly than herring sperm; the rate of release scaled with the DNA diffusion coefficient in water. The chemical and biological integrity of released DNA was not changed.

CONCLUSIONS

These low cost, and adjustable, controlled DNA delivery systems, using FDA-approved biocompatible/biodegradable and implantable/injectable materials, could be useful for in vivo gene delivery, such as DNA vaccination and gene therapy.

Millimeter-scale positioning of a nerve-growth-factor source and biological activity in the brain

Toxicity prevents the systemic administration of many therapeutic proteins, and attempts at protein targeting via the circulatory system (i.e., "magic bullets") have failed in all but a few special cases. Direct administration at the target site is a logical alternative, particularly in the central nervous system, but the limits of direct administration have not been defined clearly. Nerve growth factor (NGF) enhances survival of cholinergic neurons and, therefore, has generated considerable interest for the treatment of Alzheimer's disease. We tested the effectiveness of local delivery by implanting small polymer pellets that slowly released NGF into the central nervous system of adult rats at controlled distances from a target site containing transplanted fetal cholinergic cells. NGF-releasing implants placed within 1-2 mm of the treatment site enhanced the biological function of cellular targets, whereas identical implants placed approximately 3 mm from the target site of treatment produced no beneficial effect. Effective NGF therapy required millimeter-scale positioning of the NGF source, and efficacy correlated with the spatial distribution of NGF concentration in the tissue; this result suggests that NGF must be delivered within several millimeters of the target to be effective in treating Alzheimer's disease. Because the human brain is divided into functional regions that are typically several centimeters in diameter and often irregular in shape, new methods for sculpting larger-scale drug fields are needed. We illustrate a concept, called pharmacotectonics, in which drug-delivery systems are arranged spatially in tissues to shape concentration fields for potent agents.

Intracranial delivery of recombinant nerve growth factor: release kinetics and protein distribution for three delivery systems

PURPOSE

Three different polymeric delivery systems, composed of either poly(ethylene-co-vinyl acetate) (EVAc) or poly(lactide-co-glycolide) (PLGA), were used to administer recombinant human nerve growth factor (rhNGF) intracranially in rats.

METHODS

The delivery systems were characterized with respect to release kinetics, both in the brain and in well-stirred buffer solutions.

RESULTS

During incubation in buffered saline, the delivery systems released rhNGF in distinct patterns: sustained (EVAc), immediate (PLGA1) and delayed (PLGA2). One 10-mg delivery system was implanted in each rat and an ELISA technique was used to determine the amount of rhNGF in 1-mm coronal brain slices produced immediately after removal of the delivery system. High levels of rhNGF (as high as 60,000 ng in a brain slice of approximately 50 microliters) were recovered from the brain tissue at 1, 2, and 4 weeks after implantation. With all three delivery systems, the amount of rhNGF in each brain slice decreased exponentially with distance from the implant site: the distance over which concentration decreased by 10-fold was 2-3 mm for all delivery systems. When rhNGF release was moderate (10 to 200 ng rhNGF/day), the total amount of rhNGF in the brain increased linearly with release rate, suggesting an overall rate of rhNGF elimination of 0.4 hr-1 or a half-life of 1.7 hr. With higher release rates (500 to 50,000 ng rhNGF/day), total amounts of rhNGF in the brain were considerably higher than anticipated based on this rate of elimination.

CONCLUSIONS

Polymeric controlled release can provide high, localized doses of rhNGF in the brain. All of the experimental data were consistent with penetration of rhNGF through the brain tissue with a diffusion coefficient approximately 8 x 10(-7) cm2/s, which is approximately 50% of the diffusion coefficient in water.

Materials for protein delivery in tissue engineering

The ability of protein agents to modulate cellular behaviors, such as motility, proliferation, adhesion and function, is the subject of intense research; new therapies involving proteins will likely result. Unfortunately, many proteins have short half-lives and the potential for toxicity after systemic delivery, so traditional routes of administration are not appropriate. Alternate methods for sustained delivery of these agents to the desired cells and tissues in biologically active conformations and concentrations are necessary. Techniques similar to those long used in the controlled delivery of drugs have been used to administer certain growth factors to cells and tissues; although clinical success has been limited to date, studies in animal models suggest the potential for tremendous advances in the near future. This review outlines the basic technology of controlled protein delivery using polymeric materials, and discusses some of the techniques under investigation for the efficient administration of proteins in tissue engineering.

Localized delivery of proteins in the brain: can transport be customized?

Certain central nervous system (CNS) diseases are characterized by the degeneration of specific cell populations. One strategy for treating neurodegenerative diseases is long-term, controlled delivery of proteins such as epidermal growth factor (EGF) and nerve growth factor (NGF). Since proteins permeate through brain capillaries very slowly, local administration using polymeric implants, continuous infusion pumps, or transplanted, protein-secreting cells may be required to achieve therapeutic concentrations in the tissue. The efficiency of local distribution, and hence effectiveness of local therapy, depends on the rate of protein migration through tissue. The rate of dispersion of molecules in a quiescent, isotropic medium can be characterized by the molecular diffusion coefficient, D, which can be measured by techniques such as quantitative autoradiography, iontophoresis, and fluorescence photobleaching recovery (FPR). These methods are reviewed, with an emphasis on their application to measurement of D for proteins in the brain. Biophysical techniques yield quantitative descriptions of local protein distribution and may enable discrimination of mechanisms of protein transport in the brain. This capability suggests a new paradigm for design of protein therapies, in which proteins and delivery systems are collectively customized to provide sustained protein availability over predetermined volumes of tissue.

Polymeric controlled delivery for immunization

Current vaccine technology has limitations; for example, most vaccines require repeat administration for long-term protection, and immunity at mucosal surfaces is difficult to achieve. In animal models, polymeric controlled-release systems provide long-lasting systemic or mucosal immunoprotection, often after a single administration. Polymeric devices that deliver a controlled amount of antibody can provide passive immunity against genital herpes infections in mice; orally administered polymeric-microsphere-based vaccines produce enhanced immune responses in rodents and primates. These new delivery technologies have many desirable features, and so their use in humans could have a substantial impact on worldwide public health.