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Local FK506 Drug Delivery Enhances Nerve Regeneration Through Fresh Nerve Allografts
Kevin J. Zuo, MD, MASc1,2, Golsa Shafa, BSc1,2, Katelyn Chan, B.Eng BioSci1,2, Jennifer Zhang, MD, PhD2, Kasra Tajdaran, MASc, PhD2, Tessa Gordon, PhD1,2, Gregory Borschel, MD, FACS1,2.
1University of Toronto, Toronto, ON, Canada, 2The Hospital for Sick Children, Toronto, ON, Canada.

PURPOSE Peripheral nerve allografts are a strategy of nerve gap reconstruction with advantages including native nerve microstructure, donor Schwann cells that maintain a pro-regenerative milieu, and limitless supply with no donor site morbidity. Despite good outcomes in animal experiments and clinical series, the clinical use of unprocessed fresh nerve allografts is limited due to the requirement for transient systemic immunosuppression with its associated adverse effects. To circumvent the systemic effects of FK506 (an FDA-approved immunosuppressant), our laboratory developed a local FK506 drug delivery system that provides sustained, targeted drug release over 28 days. The objective of this study was to investigate if local FK506 drug delivery enhances nerve regeneration in a rodent model of nerve gap defect reconstruction using unprocessed fresh nerve allografts. METHODS In male Lewis rats, a 10 mm hindlimb common peroneal nerve gap defect was reconstructed with either 20 mm long nerve isografts from donor Lewis rats or 20 mm long fresh nerve allografts from donor ACI rats. Rats with nerve allograft reconstruction received either systemic FK506 (2 mg/kg/day intraperitoneal injections), local FK506 (420 g FK506 encapsulated in poly(lactic-co-glycolic acid) microspheres suspended in a fibrin hydrogel and applied once directly to the site of nerve injury), or no treatment. After 4 weeks, nerve regeneration was evaluated using (1) retrograde labeling to enumerate motor and sensory neurons that regenerated axons through the graft, (2) quantitative histomorphometry of the midgraft and distal common peroneal nerve, and (3) serum cytokine profile. RESULTS Rats with untreated fresh nerve allografts demonstrated very poor nerve regeneration compared to rats with nerve isografts or rats with nerve allografts treated with FK506 (p<0.001). Rats with nerve allografts treated with local FK506 demonstrated significantly better motor and sensory neuron regeneration than rats with untreated nerve allografts (p<0.001) and not significantly different from rats with nerve isografts or rats with nerve allografts treated with systemic FK506 (p>0.05). Histomorphometric analysis of the midgraft and distal common peroneal nerve revealed that rats with local FK506-treated nerve allografts had comparable numbers of myelinated axons as rats with nerve isografts and rats with nerve allografts treated with systemic FK506 (p>0.05). Compared to rats with untreated nerve allografts, serum concentrations of the pro-inflammatory cytokine IL-12 were significantly lower 1 week after surgery in rats with nerve allografts treated with local FK506 (p<0.05) or systemic FK506 (p<0.001); however, unlike rats treated with systemic FK506, rats treated with local FK506 had undetectable serum levels of FK506. CONCLUSIONS A local FK506 drug delivery system enhances motor and sensory nerve regeneration through fresh nerve allografts comparable to nerve allografts treated with systemic immunosuppression and nerve isografts. These results provide data for future investigations to characterize the neurotrophic and immunosuppressive effects of local FK506 using an immunodeficient transgenic rat, as well as to evaluate functional recovery. Local FK506 drug delivery may have direct clinical application in allotransplantation of peripheral nerve and vascularized composite allografts.


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