Assessing The Potential Of Nitric Oxide In The Diabetic Foot

   Researchers have studied nitric oxide (NO) extensively for the past 40 years. However, there has been an increased interest within the past 15 years. In 1998, the Nobel Prize in Physiology and Medicine was awarded to scientists who worked out the signaling mechanisms for NO in the human body.    Nitric oxide is an endogenous gas produced by cells with many diverse physiological effects. The substrate arginine is converted by the enzyme nitric oxide synthase (NOS) to citrulline with the liberation of NO (see “A Closer Look At Nitric Oxide Production” below).1 After NO is released, it has a half-life of seconds with subsequent binding to receptors on or within the cell that causes a second messenger cascade. The result of this cascade depends on the type of NOS. There are many subtypes of NOS including: nNOS, which are found in neurons; iNOS, which are inducible forms found throughout the body; and eNOS, which are found in blood vessel cells. The iNOS and eNOS subtypes are particularly interesting with their implications in the lower extremity, especially when it comes to wound healing.    There are two relevant components of NO on wound healing. The first is the vascular component. Endogenous NO acts upon endothelial cells to cause dilatation of vessels (both arterial and venous). Contrarily, inhibition of NO synthesis causes vasoconstriction and subsequent hypertension.1 Evidence also suggests there is decreased NO activity with an increase in activity levels by vasoconstrictors in the diabetic rat model.2 Studies of humans have also demonstrated decreased NO production in diabetic patients with peripheral neuropathy.3 Further, the vessels of insulin dependent diabetic patients demonstrate less responsiveness to NO and potentially a decrease in availability of NO.4,5 Researchers have also shown that NO increases blood flow to the microcirculation adjacent to wound sites.6    The second component of NO on wound healing is in the inflammatory process. Researchers have measured NO at higher levels in proximity to wound sites.7 Cytokines stimulate macrophages and fibroblasts to produce NO.8,9 Nitric oxide appears to have cytotoxic properties and this suggests some level of antimicrobial activity.10 Also keep in mind that inhibition of NO synthesis retards collagen synthesis and deposition which we know are key components in providing principal strength characteristics of wounds.11    There is growing interest in the clinical use of NO in the lower extremity. Clinically, we know that NO can play a role in two key pathologic components in the diabetic foot: vascular compromise and peripheral neuropathy.    There have been promising results for the use of NO in treating painful peripheral neuropathy. Researchers have demonstrated that monochromatic near-infrared photoenergy (MIRE®, Anodyne® Therapy System) increases sensation and decreases pain.12-14 The proposed principle behind MIRE is that photon energy induces the release of NO, thereby increasing blood flow to vascularly compromised peripheral nerves. Clinicians have also utilized topical NO, in the form of a spray, for the treatment of painful peripheral neuopathy with good success.15

Is There A Role For Topical Nitroglycerin In Diabetic Wounds?

   However, the direct application of NO for the treatment of diabetic wounds has not been well researched. While it is possible to prevent wound occurrence by reversing or retarding the neuropathic process that often leads to wounds, the question remains on whether NO can expedite the healing of existing wounds.    There is anecdotal evidence to support this theory. In regard to isosorbide dinitrate (nitroglycerin), a liberator of NO, podiatric physicians have been using topical nitroglycerin to treat ischemic wounds. There are two published studies demonstrating that topical application of nitroglycerin in the form of a paste may increase perfusion to the foot.16,17 Further, a study conducted by Wheeland, et. al., demonstrated that the application of nitroglycerin led to healing of ischemic digital ulcerations.18 However, these studies were small with non-stringent methodology. No systematic, large-scale study has evaluated the efficacy of topical nitroglycerin application in wound healing in the lower extremity. We have submitted for publication a placebo-controlled, randomized, double-blind study evaluating the ability of topical nitroglycerin to measurably increase perfusion to the foot.19 We examined the use of a nitroglycerin patch on healthy subjects and measured perfusion levels using transcutaneous thermometry and photoplethysmography. Our study revealed that nitroglycerin at a low dose does not measurably increase local perfusion. We did not examine higher doses due to potential side effects including headache and dizziness.    A follow-up study is needed to examine the use of topical nitroglycerin in the lower extremity. Specifically, a placebo-controlled study is required to examine the ability of nitroglycerin to expedite wound healing in the diabetic foot.

In Conclusion

   Some fundamental questions remain regarding the clinical application of NO in wound healing. In our study, we were unable to demonstrate that topical application of nitroglycerin is able to increase perfusion to the foot. However, it is possible that topical application of nitroglycerin can increase local perfusion at higher doses. Assuming that topical nitroglycerin is able to increase local perfusion in the foot, would this increase be able to overcome large vessel compromise upstream?    There are other concerns regarding the application of higher doses of topical nitroglycerin. There is a possibility that they increase the risk for systemic effects. Patients who suffer from chronic wounds often suffer from other disease processes including cardiac disease. There may be a potential interaction with other cardiac drugs and nitroglycerin at higher doses. There may also be unwanted direct effects on the heart. Accordingly, do these risks outweigh the potential benefits of the topical application of nitroglycerin?    Another important question is whether NO has positive effects on wounds in the non-ischemic foot. If a patient has adequate blood flow, can augmentation of NO facilitate wound healing? We know that NO plays a role in inflammation. However, the specifics of this role in the chronic wound environment of a diabetic foot are not clear. Researchers have demonstrated that supplementation with exogenous nitric oxide donors such as molsidomine or supplementation of the substrate L-Arginine promotes wound healing in diabetic rats.20,21 However, there is some evidence to suggest that high levels of NO can be destructive to tissues.7 Some also argue that with the potential harmful effects of NO, future research efforts should develop therapies to inhibit NO production.22 Direct application in a controlled study would better delineate the ability of NO to facilitate healing.    Nitric oxide plays an important role in the vascular system and the inflammatory process. However, we need more data in order to better understand the place of NO in the armamentarium for the treatment of pathologies in the diabetic foot. Dr. Kim is an Assistant Professor at the Arizona Podiatric Medicine Program, Midwestern University College of Health Sciences. He is the author of multiple clinical and basic science publications and has lectured throughout the country. Dr. Steinberg (pictured) is an Assistant Professor in the Department of Surgery at the Georgetown University School of Medicine in Washington, D.C. He is a Fellow of the American College of Foot and Ankle Surgeons.
 

References:

1. Cohen R. The role of nitric oxide and other endothelial-derived vasoactive substances in vascular disease. Progress in Cardiovascular Dis. 1995;38(2):105-128.
2. Bulgrin JP, Shabani M, Debshish C, Smith DJ. Nitric oxide synthesis is suppressed in steroid-impaired and diabetic wounds. Wounds: A Comp of Clin Res and Pract. Mar/Apr 1995;7(2):48-57.
3. Pitei DL, Watkins PJ, Edmonds ME. NO-dependent smooth muscle vasodilation is reduced in NIDDM patients with peripheral sensory neuropathy. Diabet Med. 1997;14:284-290.
4 Johnstone MT, Creager SJ, Scales KM, Cusco JA, Lee BK, Creager MA. Impaired endothelium-dependent vasodilation in patients with insulin–dependent diabetes mellitus. Circ. Dec 1993;88(6):2510-2516.
5. Honing MLH, Morrison PJ, Banga JD, Stroes ESG, Rabelink TJ. Nitric oxide availability in diabetes mellitus. Diab Met Rev. 1998;14:241-249.
6. Veres A, Akbari CM, Primavera J, Donaghue VM, Zacharoulis D, Chrzan JS, DeGirolami U, LoGerfo FW, Freeman R. Endothelial dysfunction and the expression of endothelial nitric oxide synthetase in diabetic neuropathy, vascular disease, and foot ulceration. Diabetes. Mar 1998;47:457-463.
7. Jude EB, Boulton AJM, Ferguson MWJ, Appleton I. The role of nitric oxide synthase isoforms and arginase in the pathogenesis of diabetic foot ulcers: possible modulatory effects by transforming growth factor beta 1. Diabetologia. 1999,42:748-757.
8. Schaeffer MR, Tantry U, Efron P, Ahhrendt, GM, Thorton FJ, Barbul A. Diabetes-impaired healing and reduced wound nitric oxide synthesis: A possible pathophysiologic correlation. Surg. 1997:513-529.
9. Schaeffer MR, Tantry U, Van Wesep RA, Barbul A. Nitric oxide metabolism in wounds. J. of Surg Res. 1997;71:25-37.
10. Hibbs JB, Taintor RR, Vavrin Z, Rachlin EM. Nitric Oxide: a cytotoxic activated macrophage effector molecule. Biochem Biophys Res Commun. 1988;157:87-94.
11. Schaeffer MR, Tantry U, Gross SS, Wasserkrug HL, Barbul A. Nitric oxide regulates wound healing. J. of Surg Res. 1996;63:237-240.
12. Leonard DR, Farooqi MH, Myers S. Restoration of sensation, reduced pain, and improved balance in subjects with diabetic peripheral neuropathy: a double-blind, randomized, placebo-controlled study with monochromatic near-infrared treatment. Diabetes Care. Jan 2004;27(1):168-72.
13. Prendergast JJ, Miranda G, Sanchez M. Improvement of sensory impairment in patients with peripheral neuropathy. Endocr Pract. Jan-Feb 2004;10(1):24-30.
14. Kochman AB, Carnegie DH, Burke TJ. Symptomatic reversal of peripheral neuropathy in patients with diabetes. J Am Pod Med Assoc. Mar 2002;92(3):125-130.
15. Yuen KCJ, Baker NR, Rayman G. Treatment of chronic painful diabetic neuropathy with isosorbide dinitrate spray. Diab Care. Oct 2002;25(10):1699-1703.
16. Francis DR, Hubbard ER, Johnson LE. Nitroglycerin ointment as a vasodilator in the lower extremities. JAPMA. Dec 1983;67(12):874-879.
17. Coakley J. Nitroglycerin ointment for dopamine-induced peripheral ischaemia. The Lancet. Jul/Sep 1983;2:633.
18. Wheeland RG, Gilchrist RW, Young CJ. Treatment of ischemic digital ulcers with nitroglycerin ointment. July 1983;9(7):548-551.
19. Kim PJ, Ballinger LC, Kushner D. Nitroglycerin patch dose of 0.2mg/hr does not measurably increase perfusion to the foot in healthy subjects. Submitted to JAPMA 2005.
20. Witte MB, Kiyama T, Barbul A. Nitric oxide enhances wound healing in diabetes. Br J of Surg. 2002,89:1594-1601.
21. Shiu HP, Most D, Efron DT, Witte MB, Barbul A.. Supplemental L-arginine enhances wound healing in diabetic rats. Wound Repair Regen. May-Jun 2003;11(30):198-203.
22. Hobbs AJ, Higgs A, Salvador Moncada. Inhibition of nitric oxide synthase as a potential therapeutic target. Annu Rev Pharmacol Toxicol. 1999;39:191-220.