At the current time there is no effective disease modifying therapy for diabetic neuropathy (DN). Previous failed trials of therapy have often targeted individuals with advanced, severe neuropathy. There is a particularly strong incentive to treat neuropathy early in its course while the severity is still mild and to target participants who have impaired glucose tolerance (IGT) or who have well controlled type 2 diabetes mellitus (T2DM). The proposed study design employs a quantifiable early measure of DN, intraepidermal nerve fiber density (IENFD) of the thigh, allowing for accurate assessment of actual nerve fiber density over time, while also incorporating measures of pain and quality of life. Preclinical data supports the use of NR as a potential therapy for diabetic neuropathy. Phase I data indicates safety in humans.

The most common form of diabetes mellitus, T2DM, is projected to affect an estimated 366 million people worldwide by 2030. The lifetime incidence of polyneuropathy is approximately 45% and neuropathy of any type approximately 59% of in patients with T2DM. Studies of nerve conduction tests performed at the time of diabetes mellitus diagnosis demonstrate that neuropathy is already present in patients when the neuropathy is still subclinical. Furthermore, DN leads to severe morbidity, high mortality, major physical disability, poor quality of life, and high cost with estimated total annual costs of $22 billion (www.diabetes.org). Due to the complex structure and anatomy of the peripheral nervous system, DN presents with a very broad spectrum of clinical symptoms and deficits, including severe pain, sensory deficits, foot ulcers and amputations. Despite the high morbidity associated with DN, most randomized clinical trials evaluating therapies for established DN have been disappointing. To date there is no pathogenetic treatment for this condition. Currently, tight glycemic control is the only convincing strategy to prevent or delay the development of DN in patients with type 1 diabetes mellitus. There is less convincing evidence that tight glycemic control improves neuropathy with T2DM.

DN is a diffuse, symmetrical injury to the entire peripheral nervous system. The smallest Aδ thinly-myelinated fibers and the unmyelinated C-fibers are likely the earliest to undergo damage in the natural history of DN. These fibers mediate pain, temperature discrimination, touch perception and autonomic responses, and constitute over 80% of peripheral nerve fibers. When determining the effect of a therapeutic intervention in DN it is important to utilize outcome measures that best identify change in disease. Although both large and small fiber neuropathy occur in T2DM, a small-fiber neuropathy is more common. Importantly, developing appropriate endpoints has been a problem in DN because many of the endpoints have proved too insensitive in clinical trials. For example, there is a need to establish content validity in clinical scales. IENFD is a sensitive and reliable measure in determining change in early DN. Furthermore, IENFD directly correlates with increasing DN severity, and is safe as easy to perform and IENFD currently represents a gold standard in measuring change in small fiber neuropathy. Thus, IENFD was selected as the most appropriate endpoint measure in this clinical trial. The morphometric quantification of IENFD is easily measured from a skin biopsy and this is used as part of routine clinical practice. The skin biopsy is a minimally invasive procedure and less than 1% of participants have mild adverse events such as bleeding, infection, or delayed healing. Inter-observer variability for the assessment of IENFD demonstrates good agreement, especially with assessment at the thigh. Patient refusal rate for this procedure is minimal (less than 1 %).

This is a phase II, single center, randomized, double-blind, placebo-controlled clinical trial to evaluate the effect of NR compared to placebo on measures of small fiber neuropathy in participants with IGT or T2DM and mild DN. Participants with be randomly assigned to either NR 0.5 grams twice a day (total 1 gram/day) or matched placebo in a 1:1 ratio. Participants in the active group will start with a 1 gram/day dose because of the very low risk of adverse events with NR. The 1 gram/day dose will be taken continuously for 6 months. In addition (if they have not already received this information) all participants will be given general diabetic nutritional advice and general advice to exercise for approximately 150 minutes per week. This represents current standard of care information provided to all patients with impaired glucose regulation and DN.

Primary efficacy measure: change in the thigh IENFD at 6 months compared to baseline. Secondary efficacy measure: change in the distal leg IENFD at 6 months compared to baseline.