VP1: Combined Local Delivery of Tacrolimus and Stem Cells in Fibrin Gel is a Viable Potential Treatment for Enhancing Peripheral Nerve Regeneration

INTRODUCTION: The immunosuppressive and neuroregenerative potential of tacrolimus (FK506) may overcome the rejection of stem cells to enhance nerve regeneration. The aim of this study was to determine the feasibility and effectiveness of combining local, sustained delivery of tacrolimus (FK506) with stem cells. MATERIALS AND METHODS: The drug release profile of local FK506 over the course of 35 days was evaluated in phosphate-buffered saline (PBS). FK506 was incorporated into fibrin gel in poly(lactic-co-glycolic) acid (PLGA) microspheres in concentrations of 0.01 ng/mL and 100 ng/mL and incubated in phosphate-buffered saline (PBS) at physiological temperature. Microspheres were prepared and characterized according to validated protocols by Tajdaran et al. (2015). Adipose derived mesenchymal stem cells (MSCs) were cultured in collected PBS to mimic systemic exposure representing released concentrations at day 7, 15 and 28 from 0.01 ng/mL and 100 ng/mL microspheres. MSCs were cultured in the following groups: (i) hydrogel only, (ii) PLGA empty microspheres in hydrogel, (iii) Hydrogel Infused with Stem cells and Tacrolimus (HIST) 0.01 ng/mL, and (iv) HIST 100 ng/mL. Staining against CD90 was conducted to confirm stem cell characterization. MSC viability was evaluated at 24h, 48h, 72h, and seven days using Live/Dead staining and MTS assays. RESULTS: Microspheres containing 0.01 ng/mL showed depletion of FK506 by 13 days. FK506 microspheres containing 100 ng/mL revealed a sustained release up to 35 days. An inflation point was observed at day 15 due to erosion of gel, resulting in an increase of FK506 release. CD90 staining confirmed stem cell characterization. MSCs were viable up to seven days and no significant differences in viability were found between groups. CONCLUSION: Encapsulation of 100 ng/mL FK506 microspheres and MSCs in hydrogel is feasible and has strong potential to enhance survival of transplanted cells, which may ultimately lead to improved nerve regeneration.