deficiency increases susceptibility to infection, and systemic infection is associated with an increase in circulating nitrate and nitrite compounds.4,5 The gaseous physiochemical nature and small molecular radi- us of NO allows it to readily cross cellular membranes at sites where it is released and spreads within contiguous tissues via a high-to-low concentration diffusion gradient.5,10 The biologic effects produced by NO are both concentration-dependent and duration-dependent, with higher concentrations providing more sudden antimicrobial activity (eg, neutrophil "burst"), and lower concentrations providing signaling and modulation of specific biologic activities mediated by a variety of cell types (to be discussed later).4-6,10,11 Essentially every cell type is capable of NO expression, including neutrophils, macrophages, keratinocytes, dendritic cells, melanocytes, broblasts, endo- thelial cells, and adipocytes.5,8,11 As NO is highly reactive and its half-life is very short, both the concentration of NO that is released and the duration of tissue exposure are important factors that impact on its biologic effects.5,10,11 Synthesis of NO occurs via the enzymatic activity of one of three isoforms of NO synthase (NOS).4,5,10,11 As noted above, NO may be constitutively expressed at low levels (less than 1 μM) to function as a signaling molecule that provides immunoregulatory and antimicrobial activities; low uxes of NO enhance multiple cellular immunologic activities (ie, proliferation, differentiation, apoptosis), adhesion factor expression, and extracellular matrix synthesis and deposition.5,11,13 Two endogenous forms of NOS are constitutively expressed, tightly regulated, and characteristically produce NO that is released in short bursts at low ux levels. They are neuronal NOS (NOS1; nNOS) and endothelial NOS (NOS3; eNOS).5,11,13 A third isoform of NOS, referred to as inducible NOS (NOS2; iNOS), is stimulated by speci c microbi- al products (ie, polysaccharides, endotoxins, cytokines), which “induce” the rapid production and more sustained release of high quantities of NO (greater than 1 μM).5,11,13 The multiple intermediate derivatives of NO that are present in high concentrations after the release of a large NO ux innately inhibit infection via a variety of modes of action that are cytotoxic to microbial cells, such as nitration, nitrosation, and oxidation (to be discussed later).4-6,10-13 What Are the Basic Challenges Related to the Development of Topical Nitric Oxide-Based Formualtions? The many biologic properties of NO provide a basis for research of potential drug delivery platforms that must prove to provide the necessary stability and release characteristics that harness NO into an effective and safe topical therapeutic agent. As NO is a diatomic gas that is highly reactive chemically and very short-lived after its release into tissues (t 1⁄2 less than 1 second), a major challenge has been the development of formulations that are stable when stored and effectively deliver NO to the target site of disease after topical application.2,3,5,6,10,11,14 However, it is important to rst understand the biologic properties of NO and its derivative byproducts.The ultimate goal is to effectively translate these biologic properties into pharmacologic modes of action when delivered to the skin. As the activity of NO is both concentration-dependent and duration-dependent, differ- ent delivery systems may potentially in uence the therapeutic properties of a given formulation depending on its individual pharmacologic and pharmacokinetic profiles.2,3,5,6,10-12 What Properties Support the Consideration of Topically Applied Nitric Oxide as a Therapeutic Agent in Dermatology? As many dermatologic diseases have been associated with alterations in NO activity, or may involve pathways of in amma- tion that can be therapeutically regulated by NO, the evaluation of topical NO formulations as potential therapeutic agents in dermatology is both a rational and important endeavor.5,11,14,15 Although the term "oxidative stress" is often perceived as having a negative connotation, in fact, normal physiology within host tissues incorporates a variety of reduction/oxidation-based (re-dox) molecules, which are naturally produced to serve as vital components of inherent immunity and tissue repair.4,5 NO and its derivative reactive nitrogen molecules (RNMs) are among a group of natural redox intermediates that provide “double duty” along with endogenously produced reactive oxygen molecules (ROMs), such as peroxide and superoxide.5Thus, NO participates directly in the eradication of microbial pathogens and also con- tributes to downstream signaling pathways that serve to further modulate immunologic and tissue restorative responses.4-6,11,15 It is important for the reader to understand that the array of bio- logic effects and chemical reactivities associated with NO and RNMs in host tissues is highly detailed and complex.4-6,10 The fol- lowing serves to summarize the major biologic activities of NO and its derivative byproducts that appear to correlate with poten- tial therapeutic applications in dermatology. Antimicrobial Properties Innate antimicrobial activity against a vast array of microbial pathogens has been well described as a major biologic func- tion of NO.3-6,8,9,11,15-19 Importantly, RNMs and ROMs produced by autoxidation of NO induce activity against various micro- bial targets through a variety of cytotoxic modes of action which are summarized in Table 1.3-6,10,11,13,15 The diversity of these mechanisms decreases the capability of microbial pathogens to develop resistance to the antimicrobial effects of NO, especially in the presence of high concentrations of NO and RNMs.5,15,16 Studies completed with a variety of NO-based delivery platforms have demonstrated antimicrobial activity against several bacteria and other microbial organisms.3-5,11,15-21 These include Staphylococcus aureus (including methicillin-resistant strains [MRSA]), Group B Streptococcus spp, Propionibacterium acnes,
