Controlling the dose-dependent, synergistic and temporal effects of NGF and GDNF by encapsulation in PLGA microparticles for use in nerve guidance conduits for the repair of large peripheral nerve defects

Neurotrophic factor delivery via biodegradable nerve guidance conduits may serve as a promising treatment for the repair of large peripheral nerve defects. However, a platform for controlled delivery is required because of their short in vivo half-life and their potential to impede axonal regeneration when used in supraphysiological doses. In this study, we investigated the dose-dependent, synergistic and temporal effects of NGF and GDNF on neurite outgrowth, adult dorsal root ganglia axonal outgrowth, Schwann cell migration and cytokine production in vitro. Using the optimal dose and combination of NGF and GDNF, we developed a PLGA microparticle-based delivery platform to control their delivery. The dose-dependent effects of both NGF and GDNF individually were found to be non-linear with a saturation point. However, the synergistic effect between NGF and GDNF was found to outweigh their dose-dependent effects in terms of enhancing Schwann cell migration and axonal outgrowth while allowing a 100-fold reduction in dose. Moreover, a temporal profile that mimics the physiological flux of NGF and GDNF in response to injury, compared to one that resembles an early burst release delivery profile, was found to enhance their bioactivity. The optimized NGF- and GDNF-loaded microparticles were then incorporated into a guidance conduit, and their capacity to enhance nerve regeneration across a 15 mm sciatic nerve defect in rats was demonstrated. Enhanced nerve regeneration was seen in comparison to non-treated defects and very encouragingly, to a similar level compared to the clinical gold standard of autograft. Taken together, we suggest that this delivery platform might have significant potential in the field of peripheral nerve repair; allowing spatial and temporal control over the delivery of potent neurotrophic factors to enhance the regenerative capacity of biomaterials-based nerve guidance conduits.