The immunosuppressant FK506 increases functional recovery and nerve regeneration following peripheral nerve injury.

Regeneration of peripheral nerve fibers over long distances often requires extended periods of convalescence. Loss to society can be measured in terms of increased health care costs, decreased productivity and, in the case of job-related injuries, larger workers' compensation claims. The availability of drugs to increase axonal regeneration would be beneficial not only to patients but also to society in general by decreasing these costs. In the present paper, we present our initial studies on the regenerative effects of the new immunosuppressive agent FK506. Rats given a sciatic nerve crush (axotomy) received daily subcutaneous injections of FK506 (1.0 mg/kg); axotomized control animals received saline. Clinical signs of recovery in the hind feet were manifested two days earlier in FK506-treated than in saline-treated animals; movement in the toes, and walking on the hind feet and toes were observed at 16 and 17 days, respectively, in saline-treated rats and at 14 and 15 days, respectively, in FK506-treated rats. Measurement of interdigit distances in the hind feet at 18 days following axotomy showed a return toward normal position of the toes (increased interdigit distances) during walking in FK506-treated rats. Light and electron microscopy performed at 18 days following axotomy confirmed the clinical appearance of increased functional recovery in FK506-treated rats. Distal to the crush site, the sciatic nerve and its terminal branches from FK506-treated animals contained more myelinated fibers compared to saline-treated animals; in the soleus nerve, the numbers of myelinated axons was increased 2.75-fold. Taken together, the present results suggest that FK506 enhances recovery of function in the rat by increasing the rate of axonal regeneration following a sciatic nerve crush.