Local delivery of FK506 to injured peripheral nerve enhances axon regeneration after surgical nerve repair in rats.

Administration of FK506, an FDA approved immunosuppressant, has been shown to enhance nerve regeneration following peripheral nerve injuries. However, the severe side effects of the systemically delivered FK506 has prevented clinicians from the routine use of the drug. In this study, we analyzed the effectiveness of our fibrin gel-based FK506 delivery system to promote axon regeneration in a rat peripheral nerve transection and immediate surgical repair model. In addition, biodistribution of FK506 from the local delivery system to the surrounding tissues was analyzed in vivo. Rats in the negative control groups either did not receive any delivery system treatment or received fibrin gel with empty microspheres. The experimental groups included rats treated with fibrin gel loaded with solubilized, particulate, and poly(lactic-co-glycolic) acid microspheres-encapsulated FK506. Rats in experimental groups receiving FK506 microspheres and the particulate FK506 regenerated the highest number of motor and sensory neurons. Histomorphometric analysis also demonstrated greater numbers of myelinated axons following particulate FK506 and FK506 microspheres treatment compared to the negative control groups. In biodistribution studies, FK506 was found at the nerve repair site, the sciatic nerve, and spinal cord, with little to no drug detection in other vital organs. Hence, the local application of FK506 via our delivery systems enhanced axon regeneration whilst avoiding the toxicity of systemic FK506. This local delivery strategy represents a new opportunity for clinicians to use for cases of peripheral nerve injuries. STATEMENT OF SIGNIFICANCE: This work for the first time investigated the influence of locally administered FK506 to the site of nerve injury and immediate repair directly on the number of motor and sensory neurons that regenerated their axons. Furthermore, using the immediate nerve repair model, we obtained valuable information about the biodistribution of FK506 within the nervous system following its release from the delivery system implanted at the site of nerve injury and repair. The strategy of local FK506 delivery holds a great promise in the clinical translation, as the localized delivery circumvents the main limitation of the systemic delivery of FK506, that of immunosuppression and toxicity.