Fabrication and Evaluation of Thermoresponsive GPNMB-Hydrogels as an Innovative Osteogenic Therapeutic Strategy

Earlier studies have reported the ability of GPNMB protein (GPNMB) to promote osteoblast differentiation and function. However, the realization of clinical potential of GPNMB in bone regeneration will require suitable delivery systems to overcome challenges pertaining to poor dosing and poor retention at target sites. Distribution of osteogenic therapeutics away from the desired bone regeneration sites has been linked to serious adverse effects.

We developed thermoresponsive GPNMB-hydrogels using PLA-b-PEG-b-PLA copolymer (10-30% w/v) and demonstrated the ability to undergo solution-to-gel transition at physiologically relevant temperatures. The hydrogel formulations were characterized by vial inversion techniques, dynamic light scattering, rheological assessments and bioretention studies. GPNMB loading (1-10 µg/mL) did not interfere with hydrogel's thermo-reversibility and viscoelastic behaviors as obtained from rheological strain and frequency sweep tests.

The in-vitro release of GPNMB reflected a diffusion-controlled kinetic and is supported by hydrogel degradation pattern involving a rapid loss of the PEG units throughout the 8-week period and a delayed degradation of the PLA units. In-vivo long- and short-term safety studies, following GPNMB treatments, showed acceptable serum levels of tissue function and inflammatory markers. There were no detectable signals of ectopic bone formation. Efficacy assessment of GPNMB-hydrogel was based on in-vitro osteoblast differentiation and in-vivo bone regeneration studies in a murine calvaria defect model.

The biofunctional properties of GPNMB-hydrogels were supported by enhancement of bone regeneration. Additional studies are warranted to fully examine the potential of GPNMB-hydrogel in bone regeneration using a disease model of fracture healing.