Collateral sprouting of human epidermal nerve fibers following intracutaneous axotomy

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Abstract
Abstract Despite the clinical need, there are no therapeutic compounds available to promote peripheral nerve regeneration. In part, this may be due to a lack of sensitive measures of nerve growth. Here, we describe a novel approach of measuring collateral sprouting of epidermal nerve fibers (ENF) in human subjects and describe the effect of the neuroimmunophilin ligand timcodar dimesylate on collateral nerve sprouting. The objective of this study was to describe a model of intracutaneous axotomy and evaluate the ability of timcodar dimesylate to accelerate human cutaneous nerve regeneration through collateral sprouting. Subjects were randomized to receive placebo, 12.5 or 50 mg/day timcodar dimesylate in a prospective, two‐center, double‐blind, placebo‐controlled trial. A 3‐mm distal thigh punch skin biopsy was performed at baseline, and a 4‐mm overlapping concentric biopsy was taken after 56 days of treatment. Biopsies were processed to visualize ENF, and the collateral sprouting distance (CSD) was measured. Sixty‐two subjects completed the trial, and the CSD was measurable in 52. The CSD (mean ± SEM) was 474.5 µm ± 38.3, 473.4 µm ± 28.4, and 450.8 µm ± 26.5 for the placebo, low and high dose groups, respectively (p = 0.84). The baseline ENF density was associated with the CSD (p = 0.02). Collateral sprouting was efficiently measured using an intracutaneous axotomy model and suggests a collateral sprouting rate of 8.5 µm/day in healthy subjects. The model was consistent across treatment groups and had a low coefficient of variation. Timcodar dimesylate treatment was safe over an 8‐week period but did not improve collateral sprouting among healthy subjects.