The furoxan system as a useful tool for balancing "hybrids" with mixed alpha 1-antagonist and NO-like vasodilator activities

Recent progress in synthesis and application of furoxan

This review highlights recent developments in the synthesis and application of furoxan. The chemistry of furoxan is relatively underdeveloped compared to that of other heterocycles owing to its difficult synthesis, which is ascribed to the labile nature of this molecule under various reaction conditions. Nevertheless, recent studies have conducted a variety of bond-forming reactions on the furoxan ring via a post-ring introduction of substituents (PRIS) strategy. This strategy enables the synthesis of furoxan molecules of interest more directly than the conventional methods that rely on the pre-installation of substituents on the furoxan ring precursors. In this review, the PRIS strategy for furoxan synthesis is classified and discussed according to the type of bond formed. Additionally, recent progress in the application of furoxan molecules, predominantly facilitated by the development of new synthetic methods, is covered in this review.

NO-HDAC dual inhibitors

HDAC inhibitors and NO donors have both demonstrated independently broad therapeutic potential in a variety of diseases. Borretto et al. presented the topic of NO-HDAC dual inhibitors for the first time in 2013 as an attractive new topic. Here we collected the general structure of all synthesized NO-HDAC dual inhibitors, lead compounds, synthesis methods and biological features of the most potent dual NO-HDAC inhibitor in each category with the intention of assisting in the synthesis and optimization of new drug-like compounds for diverse diseases. Based on studies done so far, NO-HDAC dual inhibitors have displayed satisfactory results against wound healing (3), heart hypertrophy (3), inflammatory, cardiovascular, neuromuscular illnesses (11a-11e) and cancer (6a-6o, 9a-9d, 10a-10d, 16 and 17). NO-HDAC dual inhibitors can have high therapeutic potential for various diseases due to their new properties, NO properties, HDAC inhibitor properties and also due to the effects of NO on HDAC enzymes.

Borylfuroxans: Synthesis and Applications

Herein we report the first synthesis of borylfuroxans via the reaction of sulfonylfuroxans with Lewis base-ligated boranes under radical conditions. As a synthetic application, the transformation of borylfuroxans to a range of 1,2-dioximes and their derivatives is demonstrated.

Synthesis of alkynyl furoxans. Rare carbon-carbon bond-forming reaction on a furoxan ring

A rare carbon-carbon-bond forming reaction on a furoxan ring has been developed. The nucleophilic aromatic substitution (S_NAr) reaction of 4-nitrofuroxans with alkynyl lithium proceeded with high yields, which enabled the first practical synthesis of both alkynyl furoxan regioisomers. Due to the versatility of the alkyne functional group, various derivatizations of the carbon-carbon triple bond in the afforded products were possible. Thus, this developed method is a convergent approach to a wide spectrum of carbon-substituted furoxans.

New praziquantel derivatives containing NO-donor furoxans and related furazans as active agents against Schistosoma mansoni

A series of NO-donor praziquantel hybrid compounds was obtained by combining praziquantel (PZQ) and furoxan moieties in a single entity. NO-donor properties of the furoxan derivatives were evaluated by detecting nitrite after incubation of the products in 7.4 pH buffered solution in the presence of L-cysteine. Structurally-related furazans, devoid of NO release capacity, were also synthesized for control purposes. All products were studied for their ability to inhibit recombinant Schistosoma mansoni thioredoxin glutathione reductase (TGR). Mobility and death of adult Schistosoma mansoni worms cultured in the presence of the products were evaluated versus PZQ. Analysis of the results showed that some products were endowed with both PZQ and NO-dependent antiparasitic properties. Compounds 6, 7, 18, and 24 emerged as the most interesting balanced hybrids, worthy of additional study on PZQ-resistant parasites.

Recent developments in nitric oxide donor drugs

During the 1980s, the free radical, nitric oxide (NO), was discovered to be a crucial signalling molecule, with wide-ranging functions in the cardiovascular, nervous and immune systems. Aside from providing a credible explanation for the actions of organic nitrates and sodium nitroprusside that have long been used in the treatment of angina and hypertensive crises respectively, the discovery generated great hopes for new NO-based treatments for a wide variety of ailments. Decades later, however, we are still awaiting novel licensed agents in this arena, despite an enormous research effort to this end. This review explores some of the most promising recent advances in NO donor drug development and addresses the challenges associated with NO as a therapeutic agent.

Inactivation of parasite cysteine proteinases by the NO-donor 4-(phenylsulfonyl)-3-((2-(dimethylamino)ethyl)thio)-furoxan oxalate

NO-donors block Plasmodium, Trypanosoma, and Leishmania life cycle by inactivating parasite enzymes, e.g., cysteine proteinases. In this study, the inactivation of falcipain, cruzipain, and Leishmania infantum cysteine proteinase by the NO-donor 4-(phenylsulfonyl)-3-((2-(dimethylamino)ethyl)thio)-furoxan oxalate (SNO-102) is reported. SNO-102 inactivates dose- and time-dependently parasite cysteine proteinases; one equivalent of NO, released from SNO-102, inactivates one equivalent of L. infantum cysteine proteinase. With SNO-102 in excess over the parasite cysteine proteinase, the time course of enzyme inhibition corresponds to a pseudo-first-order reaction for more than 90% of its course. The concentration dependence of the pseudo-first-order rate constant is second-order at low SNO-102 concentration but tends to first-order at high NO-donor concentration. This behavior may be explained by a relatively fast pre-equilibrium followed by a limiting pseudo-first order process. Kinetic parameters of L. infantum cysteine proteinase inactivation by SNO-102 are affected by the acidic pK shift of one apparent ionizing group (from pK(unl)=5.8 to pK(lig)=4.7) upon enzyme inhibition. Falcipain, cruzipain and L. infantum cysteine proteinase inactivation is prevented and reversed by dithiothreitol and L-ascorbic acid. Furthermore, the fluorogenic substrate N-alpha-benzyloxycarbonyl-Phe-Arg-(7-amino-4-methylcoumarin) protects parasite cysteine proteinases from inactivation by SNO-102. The absorption spectrum of the inactive S-nitrosylated SNO-102-treated L. infantum cysteine proteinase displays a maximum at about 340 nm. These results indicate that the parasite cysteine proteinase inactivation by SNO-102 occurs via the NO-mediated S-nitrosylation of the Cys25 catalytic residue.

Design and Synthesis of 3‘- and 5‘-O-(3-Benzenesulfonylfuroxan-4-yl)-2‘-deoxyuridines: Biological Evaluation as Hybrid Nitric Oxide Donor−Nucleoside Anticancer Agents

A group of 3'-O- and 5'-O-(3-benzenesulfonylfuroxan-4-yl)-2'-deoxyuridines possessing a variety of substituents (H, Me, I, F, CF(3)) at the C-5 position of the nucleoside moiety were synthesized for evaluation as hybrid anticancer agents that have the ability to simultaneously release cytotoxic nitric oxide (*NO). Incubation of these nitric oxide donor-nucleoside conjugates in the presence of 18 mM L-cysteine released a high percentage of *NO (21-48% at 1 h; 37-86% at 16 h). The release of *NO in the absence of the thiol cofactor was negligible. These hybrid *NO donor-nucleosides exhibited high cellular toxicity (CC(50) = 10(-6)-10(-8) M range) against a battery of tumor cell lines (143B-LTK, 143B, EMT-6, KBALB-STK, and KBALB) and normal human fibroblasts (Hs578Bst). No differences in cytotoxicity between nontransfected (143B, KBALB) and the corresponding transfected (143B-LTK, KBALB-STK) cancer cell lines possessing the herpes simplex virus type 1 (HSV-1) thymidine kinase gene (TK(+)) were observed, indicating that expression of the viral TK enzyme did not provide a gene therapeutic effect.

New potential uroselective NO-donor alpha1-antagonists

A recent uroselective alpha(1)-adrenoceptor antagonist, REC15/2739, has been joined with nitrooxy and furoxan NO-donor moieties to give new NO-donor alpha(1)-antagonists. All the compounds studied proved to be potent and selective ligands of human cloned alpha(1a)-receptor subtype. Derivatives 6 and 7 were able to relax the prostatic portion of rat vas deferens contracted by (-)-noradrenaline because of both their alpha(1A)-antagonist and their NO-donor properties.

Current trends in QSAR on NO donors and inhibitors of nitric oxide synthase (NOS)*

This article evaluates the quantitative structure-activity relationships (QSAR) of nitric oxide (NO) radical donors and nitric oxide synthases (NOS) inhibitors, using the C-QSAR program of Biobyte. Furoxans, triazines, amidoximes, tetrazoles, imidazoles and N(omega)-2-nitroarylamino acid analogues were included in this survey. In nine out of seventeen cases, the clog P plays a significant part in the QSAR of the NO radical donors and of the NOS inhibition. Many of the compounds must be interacting with a hydrophobic space in a non-specific way. In some cases molecular refractivity CMR/MR as well as sterimol parameters (B(1) and L) are important. Electronic effects, with the exception of the Hammett's constant sigma and the Swain-Lupton parameter F, are not found to govern the biological activity. Stereochemical and electronic features are also found to be important. Indicator variables were used after the best model was found to account for the usual structural features.

A new class of ibuprofen derivatives with reduced gastrotoxicity

A new series of nonsteroidal antiinflammatory drugs (NSAIDs) obtained by linking ibuprofen to selected furoxan moieties and to related furazans were synthesized and tested for their antiinflammatory, antiaggregatory, and ulcerogenic properties. All the derivatives are endowed with antiinflammatory activity comparable to that of ibuprofen, but, unlike this drug, they display reduced acute gastrotoxicity. The masking of the ibuprofen-free carboxylic group seems to be principally at the basis of this reduced topical irritant action. The two furoxan derivatives 8 and 9 also trigger potent antiaggregatory effects, principally as a consequence of their NO-donor ability.

New 1,4-dihydropyridines conjugated to furoxanyl moieties, endowed with both nitric oxide-like and calcium channel antagonist vasodilator activities

A series of 4-phenyl-1,4-dihydropyridines substituted at the ortho and meta positions of the phenyl ring with NO-donating furoxan moieties and their non-NO-releasing furazan analogues were synthesized and pharmacologically characterized. The vasodilator activities of these compounds were evaluated on rat aorta and expressed as EC50 values or as EC50iGC values when obtained in the presence of inhibitors of guanylate cyclase methylene blue (MB) and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). Affinities to 1, 4-DHP receptors on Ca2+ channels, expressed as IC50 values, were determined through displacement experiments of [3H]nitrendipine on rat cortex homogenates. A linear correlation between IC50 and EC50 values was found for compounds unable to release NO. EC50calcd values for derivatives containing NO-donor moieties, expression of the Ca2+-blocking component of their vasodilator activity, were interpolated on this linear regression. They showed a good correspondence with EC50iGC values determined in the presence of soluble guanylate cyclase inhibitors. Analysis of EC50iGC/EC50 ratios provided a useful tool to distinguish well-balanced hybrids from derivatives biased toward Ca2+-blocking or NO-dependent vasodilator activity. A detrimental effect on affinity to the 1, 4-DHP receptor, due to substitution at the ortho and meta positions of the 4-phenyl ring, was observed. SAR to explain this effect is proposed.

Pyrazole analogues of prazosin

A series of analogues of prazosin, in which 1-methyl or 1-phenylpyrazole moieties were substituted for the furan ring, were synthesized and studied for their alpha 1-adrenoceptor antagonist activity. The role of the five member heterocyclic substructures in determining the affinity for the alpha 1-receptor is briefly discussed.

Studies on agents with mixed NO-dependent vasodilating and beta-blocking activities

PURPOSE

A series of derivatives having a propranolol-like moiety linked to NO-donor furoxan substructures were synthesized. The main objective of this investigation was to obtain agents with mixed NO-dependent vasodilating and beta-blocking activities.

METHODS

Most of the target compounds were synthesized from the appropriate furoxans bearing XCH2CH2NH2 (X = O, S, SO2) chains at the 4 position of the ring, using Al(C2H5)3 in methylene chloride solution and (+/-)2,3-epoxypropyl 1-naphtyl ether. Two of the final products (X = CONH) were obtained by coupling the appropriate furoxancarboxylic acids with N-[2-hydroxy-3-(1-naphthoxy)propyl]-ethylenediamine. beta 1- and beta 2-blocking activities were examined on isolated guinea pig right atria and on guinea pig trachea respectively. Vasodilating properties were assessed on endothelium denuded strips of rat aorta.

RESULTS

Some derivatives behave as well balanced "hybrids" displaying NO-dependent vasodilating and beta-blocking properties in the same concentration range. Some others display either prevalent beta-blocking or vasodilating activity. Generally speaking hybrid formation lowers the affinity for beta-receptors, in particular for beta 2-type, to give an increase in beta 1/beta 2 selectivity.

CONCLUSIONS

The furoxan system is a flexible tool in designing analogues of propranolol whose NO-donating and beta-blocking properties are modulated over a wide range.

Water soluble furoxan derivatives as NO prodrugs

The synthesis, characterization, NO donor properties, and in vitro vasodilating activity of a series of water soluble furoxans (5-14a,b) are described. All of the compounds released NO when treated with a large excess of cysteine under physiological conditions (pH 7.4; 37 degrees C). The amount of NO produced after 1 h of incubation was evaluated by detecting nitrites, via the Griess reaction. Derivatives 5b, 7b, and 14b were able to release nitric oxide also in the absence of the thiol cofactor. The initial rates of NO release were determined at different concentrations, using a spectrophotometric technique based on the NO-induced oxidation of oxyhemoglobin (HbO2) to methemoglobin (MetHb). The initial rates of NO release were linearly dependent on the concentrations of the single compounds. The vasodilating potency (EC50) of all the derivatives was assessed on rat aortic strips precontracted with noradrenaline. Correlation between potency and initial NO release rate is discussed.