Hydroxyguanidines inhibit peroxynitrite-induced oxidation

Prodrug design for the potent cardiovascular agent Nω-hydroxy-L-arginine (NOHA): synthetic approaches and physicochemical characterization

N(ω)-Hydroxy-L-arginine (NOHA)--the physiological nitric oxide precursor--is the intermediate of NO synthase (NOS) catalysis. Besides the important fact of releasing NO mainly at the NOS-side of action, NOHA also represents a potent inhibitor of arginases, making it an ideal therapeutic tool to treat cardiovascular diseases that are associated with endothelial dysfunction. Here, we describe an approach to impart NOHA drug-like properties, particularly by wrapping up the chemically and metabolically instable N-hydroxyguanidine moiety with different prodrug groups. We present synthetic routes that deliver several more or less highly substituted NOHA derivatives in excellent yields. Versatile prodrug strategies were realized, including novel concepts of bioactivation. Prodrug candidates were primarily investigated regarding their hydrolytic and oxidative stabilities. Within the scope of this work, we essentially present the first prodrug approaches for an interesting pharmacophoric moiety, i.e., N-hydroxyguanidine.

Synthesis of mono- and symmetrical di-N-hydroxy- and N-aminoguanidines

Novel mono- and symmetrical di-N-hydroxy- and N-aminoguanidines were readily prepared from the reaction of diverse hydroxylamines or hydrazines with reagent classes di(benzotriazol-1-yl)methanimine 6, (bis-benzotriazol-1-yl-methylene)amines 8a,b, benzotriazole-1-carboxamidines 10a-i, benzotriazole-1-carboximidamides 11a,b, and N'-hydroxy-1H-1,2,3-benzotriazole-1-carboximidamide 18. The preparation is described for a variety of N-hydroxy- and N-aminoguanidines with different substitution patterns in good yields.

Drugs modulating the biological effects of peroxynitrite and related nitrogen species

The term "reactive nitrogen species" includes nitrogen monoxide, commonly called nitric oxide, and some other remarkable chemical entities (peroxynitrite, nitrosoperoxycarbonate, etc.) formed mostly from nitrogen monoxide itself in biological environments. Regardless of the specific mechanisms implicated in their effects, however, it is clear that an integrated pharmacological approach to peroxynitrite and related species is only just beginning to take shape. The array of affected chemical and pathological processes is extremely broad. One of the most conspicuous mechanisms observed thus far has been the scavenging of the peroxynitrite anion by molecules endowed with antioxidant activity. This discovery has in turn lent great significance to several naturally occurring and synthetic antioxidants, which usually protect not only against oxidative reactions, but also from nitrating ones, both in vitro and in vivo. This has proven to be beneficial in different tissues, especially within the central nervous system. Taking these results and those of other biochemical investigations into account, many research lines are currently in progress to establish the true potential of reactive nitrogen species deactivators in the therapy of neurological diseases, ischemia-reperfusion damage, renal failure, and lung injury, among others.

Oxidation of N-hydroxyguanidines by copper(II): model systems for elucidating the physiological chemistry of the nitric oxide biosynthetic intermediate N-hydroxyl-L-arginine

The redox chemistry of models of N-hydroxy-L-arginine, the biosynthetic intermediate in the synthesis of NO by the family of nitric oxide synthase enzymes, has been explored experimentally and theoretically. The oxidation of N-hydroxyguanidine model compounds by Cu(II) was studied as a means of establishing possible metabolic fates and intermediates of this important functional group. These studies indicate than an iminoxyl intermediate is formed and may be an important biological species generated from N-hydroxyguanidines including N-hydroxy-L-arginine.

Is nitric oxide overproduction the target of choice for the management of septic shock?

Sepsis is a heterogeneous class of syndromes caused by a systemic inflammatory response to infection. Septic shock, a severe form of sepsis, is associated with the development of progressive damage in multiple organs, and is a leading cause of patient mortality in intensive care units. Despite important advances in understanding its pathophysiology, therapy remains largely symptomatic and supportive. A decade ago, the overproduction of nitric oxide (NO) had been discovered as a potentially important event in this condition. As a result, great hopes arose that the pharmacological inhibition of NO synthesis could be developed into an efficient, mechanism-based therapeutic approach. Since then, an extraordinary effort by the scientific community has brought a deeper insight regarding the feasibility of this goal. Here we present in summary form the present state of knowledge of the biological chemistry and physiology of NO. We then proceed to a systematic review of experimental and clinical data, indicating an up-regulation of NO production in septic shock; information on the role of NO in septic shock, as provided by experiments in transgenic mice that lack the ability to up-regulate NO production; effects of pharmacological inhibitors of NO production in various experimental models of septic shock; and relevant clinical experience. The accrued evidence suggests that the contribution of NO to the pathophysiology of septic shock is highly heterogeneous and, therefore, difficult to target therapeutically without appropriate monitoring tools, which do not exist at present.

Electrochemical detection of nitroso-arginine as an intermediate between N-hydroxy-arginine and citrulline. An in vitro versus in vivo study using microcarbon electrodes in neuronal nitric oxide synthase and mice brain

The aim of the study was to describe in vivo and in vitro the transformation of N-hydroxy-arginine (NHA) into nitrite and citrulline. The products of NHA oxidation were studied by electrochemical methods. Cyclic voltammetry of NHA on microcarbon electrode showed an oxidation in two steps with one electron and one proton exchanged at each step. The first step gave a radical species NHA(.) with a half-life shorter than 1 micros and the second step gave nitroso-arginine (NA) with a half-life of about 1 s (1.5 s). Coulometric oxidation of NHA gave citrulline and nitrite. Differential pulse voltammetry (DPV) in vivo and in vitro gave a peak in reduction at -1.66 V vs Ag/AgCl for NA. After reductive adsorption of NA on the microelectrode surface in mice brain it gave the two peaks of NHA in oxidation plus another peak identified as nitrite. DPV in native and recombinant rat brain nitric oxide (NO)-synthase gave NA signal permitting K(m) and V(max) determination. All these results showed that NA was synthetized by NO-synthases before the final products, citrulline and nitrite.

Products of the phospholipase A(2) pathway mediate the dihydrorhodamine fluorescence response evoked by endogenous and exogenous peroxynitrite in PC12 cells

A short-term exposure of PC12 cells to tert-butylhydroperoxide promotes a rapid oxidation of dihydrorhodamine sensitive to nitric oxide synthase inhibitors and peroxynitrite scavengers. This response was not directly caused by peroxynitrite, but rather appeared to be mediated by peroxynitrite-dependent activation of phospholipase A(2). The following lines of evidence support this inference: (i) the peroxynitrite-dependent dihydrorhodamine fluorescence response was blunted by low concentrations of two structurally unrelated phospholipase A(2) inhibitors; (ii) under similar conditions, the phospholipase A(2) inhibitors prevented release of arachidonic acid; (iii) low levels of arachidonic acid restored the dihydrorhodamine fluorescence response in nitric oxide synthase- as well as phospholipase A(2)-inhibited cells; (iv) the dihydrorhodamine fluorescence response induced by authentic peroxynitrite was also blunted by phospholipase A(2) inhibitors and restored upon addition of reagent arachidonic acid. We conclude that endogenous, or exogenous, peroxynitrite does not directly oxidize dihydrorhodamine in intact cells. Rather, peroxynitrite appears to act as a signalling molecule promoting release of arachidonic acid, which in turn leads to formation of species causing the dihydrorhodamine fluorescence response.

Nitrogen oxides and hydroxyguanidines: formation of donors of nitric and nitrous oxides and possible relevance to nitrous oxide formation by nitric oxide synthase

The involvement of nitric oxide in numerous biological functions has led to the intense study of nitric oxide (NO) generation by the nitric oxide synthases (NOS) responsible. In addition to NO, nitric oxide synthases produce N(G)-hydroxy-L-arginine, superoxide anion and, indirectly, NOx species such as peroxynitrite and, possibly, nitrous oxide (N2O). Consequently, the interactions of N(G)-hydroxy-L-arginine with NO and other oxides of nitrogen (NOx) are of considerable interest. N(G)-Hydroxy-L-arginine and other monosubstituted hydroxyguanidines react with aqueous aerobic NO, peroxynitrite, and various NOx and nitrosating agents to form compounds that subsequently release NO and N2O. Spectrometric data indicate that the nitrosation product of N(G)-hydroxy-L-arginine is of the same N-nitroso-N-hydroxy/diazeniumdiolate (formerly "NONOate") structure as previously found for the nitrosation products of other model hydroxyguanidines. These decompose in aqueous solution in a pH-dependent manner to yield mainly NO and ureas at low pH, N2O and cyanamides at basic pH, and what appear to be primary nitrosamines/ nitrosoimines. Studies on purified iNOS using a mass spectrometer with a gas-permeable membrane inlet identified both NO and N2O (or 15NO and 15N15NO with 15N-labeled L-arginine as substrate) as products of NOS activity. These experiments suggest that much more NO than N2O is produced under the conditions studied and that N2O formation can be rationalized via the reaction of NOx species with N(G)-hydroxy-L-arginine.