Synthesis and biological activity of polyethylene glycol-mouse nerve growth factor conjugate

Transport of epidermal growth factor in the stroke-injured brain

Stroke is a neurological disorder that currently has no cure. Intrathecal delivery of growth factors, specifically recombinant human epidermal growth factor (rhEGF), stimulates endogenous neural precursor cells in the subventricular zone (SVZ) and promotes tissue regeneration in animal models of stroke. In this model, rhEGF is delivered with an invasive minipump/catheter system, which causes trauma to the brain. A less invasive strategy is to deliver rhEGF from the brain cortex; however, this requires the protein to diffuse through the brain, from the site of injection to the SVZ. Although this method of delivery has great potential, diffusion is limited by rapid removal from the extracellular space and hence for successful translation into the clinic strategies are needed to increase the diffusion distance. Using integrative optical imaging we investigate diffusion of rhEGF vs. poly(ethylene glycol)-modified rhEGF (PEG-rhEGF) in brain slices of both uninjured and stroke-injured animals. For the first time, we quantitatively show that PEG modification reduces the rate of growth factor elimination by over an order of magnitude. For rhEGF this corresponds to a two to threefold increase in predicted brain penetration distance, which we confirm with in vivo data.

Protective effect of PEGylation against poly(amidoamine) dendrimer-induced hemolysis of human red blood cells

Poly(amidoamine) (PAMAM) dendrimers are widely used in medical applications. However, dendrimers bearing positively charged surface groups are prone to destabilize cell membrane and cause cell lysis. The lytic effect of dendrimers on red blood cells (RBCs) namely hemolysis is extremely dangerous when administered in vivo. To diminish the hematologic toxicity, we modified PAMAM dendrimers with poly(ethylene glycol) (PEG) of three molecular weights (2k, 5k, and 20k). The protective effect of PEGylation against PAMAM dendrimer-induced hemolysis was studied. RBCs morphology and surface structure were analyzed by optical microscopy (OM) and atomic force microscopy (AFM). The results indicated that PAMAM and PEG-2k modified dendrimers induced hemolysis at 0.1 and 0.5 mg/mL respectively, whereas PEG-5k and PEG-20k modified dendrimers showed no significant difference in hemolysis compared with control even at 5 mg/mL. OM and AFM investigation indicated PAMAM and PEG-2k modified dendrimers caused RBCs aggregation and lysis. However, no changes were observed in the overall shape of RBCs treated with PEG-5k and PEG-20k modified dendrimers. The surface roughness of RBCs treated with PEGylated dendrimers were far lower than that of RBCs treated with PAMAM dendrimers. This study demonstrated that hemocompatibility of PAMAM dendrimers could be greatly enhanced by PEGylation.

PEGylation of brain-derived neurotrophic factor for preserved biological activity and enhanced spinal cord distribution

Brain-derived neurotrophic factor (BDNF) was covalently attached to polyethylene glycol (PEG) in order to enhance delivery to the spinal cord via the cerebrospinal fluid (intrathecal administration). By varying reaction conditions, mixtures of BDNF covalently attached to one (primary), two (secondary), three (tertiary), or more (higher order) PEG molecules were produced. The biological activity of each resulting conjugate mixture was assessed with the goal of identifying a relationship between the number of PEG molecules attached to BDNF and biological activity. A high degree of in vitro biological activity was maintained in mixtures enriched in primary and secondary conjugate products, while a substantial reduction in biological activity was observed in mixtures with tertiary and higher order conjugates. When a biologically active mixture of PEG-BDNF was administered intrathecally, it displayed a significantly improved half-life in the cerebrospinal fluid and an enhanced penetration into spinal cord tissue relative to native BDNF. Results from these studies suggest a PEGylation strategy that preserves the biological activity of the protein while also improving the half-life of the protein in vivo. Furthermore, PEGylation may be a promising approach for enhancing intrathecal delivery of therapeutic proteins with potential for treating disease and injury in the spinal cord.

The decrease of PAMAM dendrimer-induced cytotoxicity by PEGylation via attenuation of oxidative stress

Due to their unique structure, poly(amidoamine) (PAMAM) dendrimers have been widely used in medical applications. However, PAMAM dendrimers bearing amino terminals show certain cytotoxicity. In order to improve their biocompatibility, we modified Generation-5 PAMAM dendrimers by conjugating them with poly(ethylene glycol) (PEG) of two different molecular weights and different number of chains. The IC(50) values of PEGylated dendrimers were 12-105 fold higher than those of PAMAM dendrimers. To investigate the influence of PEGylation on PAMAM-induced cytotoxicity, the intracellular responses, reactive oxygen species (ROS) content, mitochondrial membrane potential (MMP), and apoptosis were examined. The results indicated that conjugation with PEG could effectively reduce the PAMAM-induced cell apoptosis by attenuating the ROS production and inhibiting PAMAM-induced MMP collapse. Meanwhile, dendrimers conjugated with less PEG of lower molecular weight did not significantly change the endocytic properties. Dendrimers conjugated with more PEG of higher molecular weight were much less cytotoxic. This study provided a novel insight into the effects of PEGylation on the decrease of cytotoxicity at the molecular level.

Controlled release of stromal cell-derived factor-1 alpha in situ increases c-kit+ cell homing to the infarcted heart

Stromal-derived factor 1alpha (SDF-1alpha) is a key stem cell homing factor that is crucial for mobilization of stem cells from bone marrow to peripheral blood and subsequent engraftment to the tissue of diseased organs. It has been reported that SDF-1alpha is transiently over-expressed in ischemic myocardium. Therefore, there may be a limited time window after acute myocardial infarction (AMI) during which stem cells are recruited to injured myocardium for repair. This study aimed at investigating whether controlled release of SDF-1alpha via a novel conjugated poly(ethylene glycol) (PEG) (PEGylated) fibrin patch at the infarct site would increase the rate of stem cell recruitment and offer potential therapeutic benefits. Recombinant mouse SDF-1alpha was covalently bound to the PEGylated fibrinogen as evidenced by immunoprecipitation and western blotting. The PEGylated fibrinogen, bound with recombinant mouse SDF-1alpha, was mixed with thrombin to form the PEGylated fibrin patch. The release kinetics of SDF-1alpha were detected in vitro using enzyme-linked immunosorbent assay. Using a mouse AMI model produced by a ligature on the left anterior descending coronary artery, a PEGylated fibrin patch bound with SDF-1alpha (100 ng) was placed on the surface of the infarct area of the left ventricle. Infarct size, left ventricular (LV) function, and the percentage of sca-1(+)/c-kit(+) cells within the infarct area were measured at days 7, 14, and 28 after AMI. In vitro results showed that SDF-1alpha was successfully bound to the PEGylated fibrin patch and can be released from the patch constantly for up to 10 days. Two weeks after infarction, the myocardial recruitment of c-kit(+) cells was significantly higher in the group treated with the SDF-1alpha PEGylated fibrin patch (n = 9) than in the AMI control group (n = 10) (p < 0.05; 11.20 +/- 1.71% vs. 4.22 +/- 0.96%, respectively). At day 28 post-AMI, unlike the control group, the group with the SDF-1alpha-releasing patch maintained stable release of SDF-1alpha concurrent with additional stem cell homing. Moreover, LV function was significantly better than in the control group. These data demonstrate that the PEGylated fibrin patch based SDF-1alpha delivery can improve the rate of c-kit(+) cell homing and improve LV function in hearts with postinfarction LV remodeling.

Nanosystems for simultaneous imaging and drug delivery to T cells

The T-cell response defines the pathogenesis of many common chronic disease states, including diabetes, rheumatoid arthritis, and transplant rejection. Therefore, a diagnostic strategy that visualizes this response can potentially lead to early therapeutic intervention, avoiding catastrophic organ failure or prolonged sickness. In addition, the means to deliver a drug dose to those cells in situ with the same specificity used to image those cells would provide for a powerful therapeutic alternative for many disease states involving T cells. In this report, we review emerging nanosystems that can be used for simultaneous tracking and drug delivery to those cells. Because of their versatility, these systems--which combine specific receptor targeting with an imaging agent and drug delivery--are suited to both basic science and applications, from developing therapeutic strategies for autoimmune and alloimmune diseases, to noninvasive tracking of pathogenic T-cell migration.

A nanoscopic multivalent antigen-presenting carrier for sensitive detection and drug delivery to T cells

Both monoclonal T cell-specific antibodies and multivalent major histocompatibility complex proteins are used as diagnostic reagents for T cell-mediated diseases. However, their widespread use as vehicles for drug delivery has been hindered by the lack of versatile methods that couple the targeting potential of these reagents with drugs of clinical relevance. To address this problem, we engineered a multivalent nanoscopic drug carrier that flexibly tethers to a variety of T-cell antigens. Our carriers bound their target T cells specifically and with enhanced sensitivities as compared with free antigen. Additionally, they consistently inhibited the proliferation of the target T cells in vitro and in vivo, whereas drug-free constructs elicited strong stimulation of the target populations. As a result of the flexibility of incorporating multivalent antigen and drug, these carriers have wide potential use as sensitive T-cell detection reagents as well as promising immunostimulatory or immunosuppressive tools.

Characterization of EGF coupling to aminated silicone rubber surfaces

Tethering of growth factors to biomaterial substrates via a polyethylene glycol (PEG) spacer has been established as a means of controlling dosage and conformation of the protein at the material surface, while retaining biological activity. However, the extent of modification through a comparison of bound versus unbound protein has not generally been characterized. In this work, covalent tethering of epidermal growth factor (EGF) to allylamine plasma modified polydimethylsiloxane (PDMS) substrates is characterized to determine the nature of the bound growth factor and to optimize the conditions for the reaction. Tethering is achieved via conjugation of EGF with homobifunctional N-hydroxysuccinimide (NHS) ester of PEG-butanoic acid (SBA2-PEG) in solution, followed by exposure of the pegylated EGF to the aminated surfaces (solution first reaction). SDS-PAGE analysis indicates that a low ratio of EGF:PEG is required to maximize the yield of the EGF-PEG reaction; a relatively short reaction time is needed to limit hydrolysis of the NHS ester. With increasing amounts of PEG and a higher reaction time, a higher fraction of the EGF can be covalently tethered to the surfaces, as shown by binding of 125I-labeled EGF and subsequent washing with sodium dodecyl sulfate (SDS) to remove adsorbed protein. However, even under the optimal reaction conditions established by the SDS-PAGE analysis, higher molecular weight EGF-PEG complexes are observed by SDS-PAGE and matrix-assisted laser desorption/ionization (MALDI). The presence of these complexes, as well as unreacted growth factor, can lead to a surface of heterogeneous composition. While these surfaces were found to have biological activity, stimulating the adhesion and growth of corneal epithelial cells versus PDMS controls, further optimization of reaction conditions, including the use of a homobifunctional PEG linker and possibly separation of reaction species are required to achieve a uniformly active and well-defined biomaterial surface.

Covalently immobilized gradients of bFGF on hydrogel scaffolds for directed cell migration

Basic fibroblast growth factor (bFGF) was immobilized to hydrogel scaffolds with retention of mitogenic and chemotactic activity. The bFGF was functionalized in order to incorporate it covalently within polyethylene glycol (PEG) hydrogel scaffolds by reaction with acryloyl-PEG-NHS. Hydrogels were formed by exposing aqueous solutions of PEG diacrylate, acryloyl-PEG-RGDS, and acryloyl-PEG-bFGF to long-wavelength ultraviolet light in the presence of a photoinitiator. These bFGF-modified hydrogels with RGD adhesion sites were evaluated for their effect on vascular smooth muscle cell (SMC) behavior, increasing SMC proliferation by approximately 41% and migration by approximately 15%. A covalently immobilized bFGF gradient was formed using a gradient maker to pour the hydrogel precursor solutions and then photopolymerizing to lock in the concentration gradient. Silver staining was used to detect the bFGF gradient, which increased linearly along the hydrogel's length. Cells were observed to align on hydrogels modified with a bFGF gradient in the direction of increasing tethered bFGF concentration as early as 24 h after seeding. SMCs also migrated differentially, up the concentration gradient, on bFGF-gradient hydrogels compared to control hydrogels with and without a constant bFGF concentration. These hydrogel scaffolds may be useful for studying protein gradient effects on cell behavior and for directing cell migration in tissue-engineering applications.

Multiphoton microscopy guides neurotrophin modification with poly(ethylene glycol) to enhance interstitial diffusion

Brain-derived neurotrophic factor (BDNF) is a promising therapeutic agent for the treatment of neurodegenerative diseases. However, the limited distribution of this molecule after administration into the brain tissue considerably hampers its efficacy. Here, we show how multiphoton microscopy of fluorescently tagged BDNF in brain-tissue slices provides a useful and rapid screening method for examining the diffusion of large molecules in tissues, and for studying the effects of chemical modifications-for example, conjugating with polyethylene glycol (PEG)-on the diffusion constant. This single variable, obtained by monitoring short-term diffusion in real time, can be effectively used for rational drug design. In this study on fluorescently tagged BDNF and BDNF-PEG, we identify slow diffusion as a major contributing factor to the limited penetration of BDNF, and demonstrate how chemical modification can be used to overcome this barrier.

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.

Effects of PEG conjugation on insulin properties

The goal of this research was to determine whether the site-specific attachment of poly(ethylene glycol) to insulin could enhance the physical and pharmacological properties of insulin without negatively affecting its biological activity or immunological properties. Electrophilically activated derivatives of low-molecular-weight monomethoxypoly(ethylene glycol) (mPEG) were chemically coupled to insulin via its amino groups at positions phenylalanine-B1 or lysine-B29, with an amide bond being formed between the polymer and protein. The site-specific attachment of mPEG to insulin did not substantially alter insulin's secondary/tertiary structure, self-association behavior, or potency in vivo. However, mPEG attachment did significantly enhance insulin's resistance to aggregation. In addition, the pegylation of insulin almost completely eliminates the resultant conjugate's immunogenicity, allergenicity, and antigenicity. Finally, the conjugates were observed to remain in the systemic circulation for longer periods of time than unmodified insulin after subcutaneous administration.

Poly(ethylene glycol) block copolymers

The ubiquitous use of poly(ethylene glycol) in the biomaterials field has also boosted the research activity in the chemical derivatization of this polymer. We focused our interest on the preparation of tailor-made poly(ethylene glycol)-based structures and on the study of structure-activity relationships for its functionalization, as preliminary steps for the preparation of smart functional materials. More specifically, amphiphilic and cationic block copolymers were prepared for prospective use in the preparation of self-assembled carriers, and Michael-type addition of thiols onto acrylates was studied as a model for end-group reaction leading to hydrogel formation.

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.

Pegylated brain-derived neurotrophic factor shows improved distribution into the spinal cord and stimulates locomotor activity and morphological changes after injury

The neurotrophin brain-derived neurotrophic factor (BDNF) shows promise for the treatment of central nervous system (CNS) trauma and disease. Effective delivery methods are required, however, for BDNF to be useful as a therapeutic agent. To this end, we examined the penetration of intrathecally infused N-terminal pegylated BDNF (peg-BDNF) compared to similar infusion of native BDNF after spinal cord injury (SCI). Pegylation dramatically improved delivery of BDNF to the spinal cord and induced the expression of Fos in spinal cord neurons. To test whether enhanced delivery would improve the modest effects on behavioral recovery and axonal outgrowth observed with native BDNF infusion, we assessed the efficacy of 2-week 25 microg/day peg-BDNF treatment, beginning 12-24 h (early) or 15 days (delayed) after midthoracic spinal contusion. Similar to native BDNF, early treatment with peg-BDNF accelerated the recovery of stepping in the open-field and acutely stimulated locomotor central pattern generator activity, as seen by the activation of hindlimb airstepping during either period of administration. The infusion of peg-BDNF, regardless of the timing of delivery, was related to enhanced sprouting of putative cholinergic fibers, like that observed after high dose native BDNF treatment. Despite improved delivery, however, neither axonal responses nor the extent of locomotor recovery were enhanced compared to native BDNF treatment. This suggests that alternative strategies, such as neurotrophin treatment in conjunction with cell transplantation techniques, or treatment nearer the cell bodies of target neurons might be employed in an attempt to effect significant repair after SCI.

Growth factor delivery for tissue engineering

A tissue-engineered implant is a biologic-biomaterial combination in which some component of tissue has been combined with a biomaterial to create a device for the restoration or modification of tissue or organ function. Specific growth factors, released from a delivery device or from co-transplanted cells, would aid in the induction of host parenchymal cell infiltration and improve engraftment of co-delivered cells for more efficient tissue regeneration or ameliorate disease states. The characteristic properties of growth factors are described to provide a biological basis for their use in tissue engineered devices. The principles of polymeric device development for therapeutic growth factor delivery in the context of tissue engineering are outlined. A review of experimental evidence illustrates examples of growth factor delivery from devices such as microparticles, scaffolds, and encapsulated cells, for their use in the application areas of musculoskeletal tissue, neural tissue, and hepatic tissue.

Growth factor delivery for tissue engineering

A tissue-engineered implant is a biologic-biomaterial combination in which some component of tissue has been combined with a biomaterial to create a device for the restoration or modification of tissue or organ function. Specific growth factors, released from a delivery device or from co-transplanted cells, would aid in the induction of host parenchymal cell infiltration and improve engraftment of co-delivered cells for more efficient tissue regeneration or ameliorate disease states. The characteristic properties of growth factors are described to provide a biological basis for their use in tissue engineered devices. The principles of polymeric device development for therapeutic growth factor delivery in the context of tissue engineering are outlined. A review of experimental evidence illustrates examples of growth factor delivery from devices such as microparticles, scaffolds, and encapsulated cells, for their use in the application areas of musculoskeletal tissue, neural tissue, and hepatic tissue.