Local Delivery of Neurotrophin-3 and Anti-NogoA Promotes Repair After Spinal Cord Injury.

Tissue and functional repair after spinal cord injury (SCI) continue to elude researchers. Neurotrophin-3 (NT-3) and anti-NogoA have been shown to promote axonal regeneration in animal models of SCI; however, localized and sustained delivery to the central nervous system (CNS) remains a critical challenge for these and other macromolecular therapeutics. An injectable drug delivery system (DDS) has previously been developed, which can provide safe local delivery to the spinal cord. This DDS, composed of poly(lactic-co-glycolic acid) (PLGA) nanoparticles (nps) dispersed in a hyaluronan methylcellulose hydrogel, was adapted for the tunable bioactive delivery of NT-3 and anti-NogoA. Furthermore, the combined delivery of NT-3 and anti-NogoA from the DDS in an impact/compression model of SCI increases axon density and improves locomotor function. The benefits of this np/hydrogel DDS observed for NT-3 and anti-NogoA demonstrate the utility of the DDS as a local delivery strategy for protein therapeutics to the CNS.

Encapsulation-free controlled release: Electrostatic adsorption eliminates the need for protein encapsulation in PLGA nanoparticles.

Encapsulation of therapeutic molecules within polymer particles is a well-established method for achieving controlled release, yet challenges such as low loading, poor encapsulation efficiency, and loss of protein activity limit clinical translation. Despite this, the paradigm for the use of polymer particles in drug delivery has remained essentially unchanged for several decades. By taking advantage of the adsorption of protein therapeutics to poly(lactic-co-glycolic acid) (PLGA) nanoparticles, we demonstrate controlled release without encapsulation. In fact, we obtain identical, burst-free, extended-release profiles for three different protein therapeutics with and without encapsulation in PLGA nanoparticles embedded within a hydrogel. Using both positively and negatively charged proteins, we show that short-range electrostatic interactions between the proteins and the PLGA nanoparticles are the underlying mechanism for controlled release. Moreover, we demonstrate tunable release by modifying nanoparticle concentration, nanoparticle size, or environmental pH. These new insights obviate the need for encapsulation and offer promising, translatable strategies for a more effective delivery of therapeutic biomolecules.

Controlled release of bioactive PDGF-AA from a hydrogel/nanoparticle composite.

Polymer excipients, such as low molar mass poly(ethylene glycol) (PEG), have shown contradictory effects on protein stability when co-encapsulated in polymeric nanoparticles. To gain further insight into these effects, platelet-derived growth factor (PDGF-AA) was encapsulated in polymeric nanoparticles with vs. without PEG. PDGF-AA is a particularly compelling protein, as it has been demonstrated to promote cell survival and induce the oligodendrocyte differentiation of neural stem/progenitor cells (NSPCs) both in vitro and in vivo. Here we show, for the first time, the controlled release of bioactive PDGF-AA from an injectable nanoparticle/hydrogel drug delivery system (DDS). PDGF-AA was encapsulated, with high efficiency, in poly(lactide-co-glycolide) nanoparticles, and its release from the drug delivery system was followed over 21 d. Interestingly, the co-encapsulation of low molecular weight poly(ethylene glycol) increased the PDGF-AA loading but, unexpectedly, accelerated the aggregation of PDGF-AA, resulting in reduced activity and detection by enzyme-linked immunosorbent assay (ELISA). In the absence of PEG, released PDGF-AA remained bioactive as demonstrated with NSPC oligodendrocyte differentiation, similar to positive controls, and significantly different from untreated controls. This work presents a novel delivery method for differentiation factors, such as PDGF-AA, and provides insights into the contradictory effects reported in the literature of excipients, such as PEG, on the loading and release of proteins from polymeric nanoparticles. STATEMENT OF SIGNIFICANCE: Previously, the polymer poly(ethylene glycol) (PEG) has been used in many biomaterials applications, from surface coatings to the encapsulation of proteins. In this work, we demonstrate that, unexpectedly, low molecular weight PEG has a deleterious effect on the release of the encapsulated protein platelet-derived growth factor AA (PDGF-AA). We also demonstrate release of bioactive PDGF-AA (in the absence of PEG). Specifically, we demonstrate the differentiation of neural stem and progenitor cells to oligodendrocytes, similar to what is observed with the addition of fresh PDGFAA. A differentiated oligodendrocyte population is a key strategy in central nervous system regeneration. This work is the first demonstration of controlled PDGF-AA release, and also brings new insights to the broader field of protein encapsulation.

Sustained delivery of bioactive neurotrophin-3 to the injured spinal cord.

Spinal cord injury is a debilitating condition that currently lacks effective clinical treatment. Neurotrophin-3 (NT-3) has been demonstrated in experimental animal models to induce axonal regeneration and functional improvements, yet its local delivery remains challenging. For ultimate clinical translation, a drug delivery system is required for localized, sustained, and minimally invasive release. Here, an injectable composite drug delivery system (DDS) composed of biodegradable polymeric nanoparticles dispersed in a hyaluronan/methyl cellulose hydrogel was injected into the intrathecal space to achieve acute local delivery to the spinal cord after a thoracic clip compression injury. NT-3 was encapsulated in the DDS and released in vitro for up to 50 d. With a single injection of the DDS into the intrathecal space of the injured spinal cord, NT-3 diffused ventrally through the cord and was detectable in the spinal cord for at least 28 d therein. Delivery of NT-3 resulted in significant axon growth with no effect on the astroglial response to injury in comparison with vehicle and injury controls. NT-3 treatment promoted functional improvements at 21 d according to the Basso Beattie Bresnahan locomotor scale in comparison with the DDS alone. The sustained delivery of bioactive NT-3 to the injured spinal cord achieved in this study demonstrates the promise of this DDS for central nervous system repair.

Cell and biomolecule delivery for tissue repair and regeneration in the central nervous system.

Tissue engineering frequently involves cells and scaffolds to replace damaged or diseased tissue. It originated, in part, as a means of effecting the delivery of biomolecules such as insulin or neurotrophic factors, given that cells are constitutive producers of such therapeutic agents. Thus cell delivery is intrinsic to tissue engineering. Controlled release of biomolecules is also an important tool for enabling cell delivery since the biomolecules can enable cell engraftment, modulate inflammatory response or otherwise benefit the behavior of the delivered cells. We describe advances in cell and biomolecule delivery for tissue regeneration, with emphasis on the central nervous system (CNS). In the first section, the focus is on encapsulated cell therapy. In the second section, the focus is on biomolecule delivery in polymeric nano/microspheres and hydrogels for the nerve regeneration and endogenous cell stimulation. In the third section, the focus is on combination strategies of neural stem/progenitor cell or mesenchymal stem cell and biomolecule delivery for tissue regeneration and repair. In each section, the challenges and potential solutions associated with delivery to the CNS are highlighted.