To date, it is known that NO exerts crucial effects in cancer therapy, antibacterial infections, wound healing, host defense and immune response and so on. In this minireview article, we manage to outline the recent achievements of macromolecular NO donors and special attention is paid to the chemical design strategies.

Although it was not difficult to appreciate that the fabrication of macromolecular NO donors primarily inherits the design concepts of small molecule NO releasers, the incorporation of small molecule NO donors into polymeric scaffolds results in the formation of polymeric NO donors can not only improve the storage and release performance of NO but also optimize the pharmacokinetics.

The following sections are categorized into three parts according to the chemical structures of NO-releasing moieties, which encompasses N -diazeniumdiolates NONOates , S -nitrosothiols SNO , and other NO donors Figure 1. Finally, a conclusion and outlook section is given.

The intention is not to provide an exhaustive literature survey but to showcase many possibilities to construct macromolecular NO donors. The rational design and functional applications of small molecule-based NO donors are now available Zhou et al.

Also, the direct mixing small molecule NO donors and polymeric matrices with the formation of NO-doping polymers through non-covalent interactions does not fall within the scope either Mowery et al.

Without a doubt, this emerging field contains numerous possibilities and the further development of macromolecular NO donors will be further advanced by the cooperation of polymer chemists, materials scientists, biologists, and so on. Figure 1. Fabrication of macromolecular NO donors through the incorporation of small molecule NO donors into polymer scaffolds and their biomedical applications.

The excellent contributions of Drago and coworkers Drago and Paulik, ; Drago and Karstetter, made it possible to prepare and purify NONOate derivatives, which are now of extensive use in investigating the physiological function of NO. These compounds are typically synthesized by the reaction of secondary amines with NO under high pressure e.

NONOate derivatives are relatively stable at basic pH but they spontaneously release 2 moles of NO per mole of donors in the presence of protons. The NO release profiles follow first-order kinetics and the half-lives of NONOates can span from several seconds to hours depending on the chemical structures of the secondary amines Miller and Megson, Interestingly, after NO release, NONOate derivatives were transformed into the original amines without the generation of other metabolites.

To minimize the uncontrolled release and optimize the pharmacokinetics of NONOate-based small molecule donors, researchers have functionalized amine-containing polymeric matrices to obtain NONOate-based macromolecular NO donors Sadrearhami et al.

In the seminal contributions, NONOate-based macromolecular NO donors with varying chain topologies were prepared Smith et al. They ingeniously demonstrated that the NO-releasing kinetics could be remarkably changed, highly dependent on the spatial location and localized microenvironment of the NONOate moieties.

Inspired by these seminal works, polymeric scaffolds containing amine residues such as hyperbranched poly ethylene imine PEI have been widely used for the fabrication of macromolecular NO donors Kang et al. In a recent study, propylene oxide was used to functionalized with hyperbranched PEI with the formation of hydroxyl moieties and the residual amine groups was treated with NO.

In vivo study on mice revealed that the resulting macromolecular NO donors exhibited accelerated wound healing performance in full thickness excisional cutaneous wound model Zhang et al. The PEI-derived NONOate-based NO donors can also be used as the stabilizing agents of inorganic nanoparticles.

As a result, hybrid nanomaterials could be achieved by taking advantages of the physiochemical properties of inorganic nanoparticles and the therapeutic effect of NO Yu et al. In this context, silica nanoparticles were coated with PEI and then treated the surface-bound PEI with NO with the formation of NONOate residues Jeong et al.

These hybrid nanoparticles displayed sustained and prolonged NO release behavior that can be used to treat bacterial keratitis. The selective reaction between amine residues and NO renders it possible to develop versatile macromolecular NO donors.

Besides PEI, methacrylate monomers consisting of linear and cyclic pendant secondary amines in the side chains which were initially protected by tert -butyloxycarbonyl Boc group were devised and synthesized Parzuchowski et al.

After polymerization and deprotection protocols, the deprotected amine residues were further decorated with NO, generating NO-releasing polymers. In addition to linear chain polymers, cross-linked polymers can readily be synthesized by prior cross-linking of the amine-containing linear precursors.

This design concept was further expanded and NONOate-containing block copolymers were synthesized, which self-assembled into micellar nanoparticles in aqueous solution Jo et al. Note that the formation of nanoassemblies can not only shield the labile NONOate moieties from protons in water that led to burst NO release but also render the localized delivery of NO possible by taking advantage of enhanced permeability and retention EPR effect of micellar nanoparticles.

The Schoenfisch group Stasko and Schoenfisch, ; Lu et al. In an early example, they systematically investigated the effects of the dendritic generations and amine types on the storage and release of NO.

Thorough investigations demonstrated that, as compared to the small molecule counterparts, the secondary amine-containing dendrimers showed a unique dendritic effect and exhibited a significantly longer NO release period Stasko and Schoenfisch, After elucidating the correlations between the chemical structures and NO-releasing performance, they examined the antibacterial effect of these NO-releasing dendrimers.

Although these dendrimers cannot efficiently penetrate bacterial biofilms, the incorporation of NO can drastically boost the anti-biofilm activity Sun et al. Notably, one of the most important goals is to explore the potentials of NO-releasing macromolecules in biological systems. Thus, the biocompatibility of polymer scaffolds is of crucial importance.

In this regard, biocompatible polysaccharides e. For example, the amine residues of oligochitosan were first modified with 2-methyl aziridine through ring-opening reaction and the newly formed amine groups were further treated with NO gas Lu et al.

The NO loading capability, maximum NO flux, and half-lives of the resulting NO-releasing oligochitosan were highly dependent on the molar ratio of 2-methyl aziridine to the amine residues.

Cell viability studies revealed that the NO-releasing oligochitosan had minimal toxicity to normal L mouse fibroblast but could efficiently eradicate bacterial biofilm. Detailed antibacterial studies revealed that the water solubility, appropriate molecular weights, and ionic characteristics of the NO-releasing chitosan synergistically contributed to the biofilm killing.

Recently, mono-substituted and multi-substituted β-cyclodextrin β-CD was engineered as NO-releasing materials as well, exhibiting cooperative antibacterial activity by means of NO and antibiotics loaded within the cavity of β-CD via host-guest interaction Jin et al.

Although many of the NONOate-based polymeric NO donors were obtained by the functionalization of secondary amine-containing polymers, aliphatic primary amines can also be transformed into NONOate derivatives as well.

Recently, a statistical ternary copolymer containing primary amine residues was synthesized Namivandi-Zangeneh et al. The primary amine moieties were converted into NONOates in the presence of NO gas under high pressure.

The resulting polymers exerted synergistic antibacterial effects by taking advantages of NO-mediated eradication of biofilm and cationic polymer-assisted membrane disruption of bacteria.

As mentioned above, although amine-containing polymers can be functionalized with NO gas to form NONOate-based macromolecular NO donors and the half-lives and pharmacokinetics could be effectively altered, the spontaneous NO release from these NONOate-based polymeric NO donors cannot be eliminated.

It will be more promising to develop polymeric NO donors with on-demand release behavior that could avoid premature NO leakage. In this context, the terminal oxygens of NONOate derivatives were protected by glycosidase-responsive galactose moieties Zhao et al.

As a result, the spontaneous NO release was remarkably inhibited and controlled NO release could be achieved by incubating the NO donors with glycosidase. After attaching the enzyme-responsive NO donors to chitosan backbones through copper I -catalyzed azide-alkyne cycloaddition CuAAC reaction, the resulting macromolecular NO donors inherited glycosidase-enzyme characteristics.

This newly designed NO-releasing material cannot only inhibit platelet adhesion and prolong partial thromboplastin time but also show increased angiogenesis in a diabetic mouse model. Indeed, beside glycosidase, NONOate derivatives can be selectively caged by many other functional groups and selective uncaging reactions could be actuated by light irradiation, glutathione GSH , and other enzymes such as esterase, nitroreductase, and DT-diaphorase Makings and Tsien, ; Sharma and Chakrapani, The introduction of protected group chemistry opens a new avenue to devise stable NONOate donors, which should be more advantageous in biomedical application due to the possibility to minimize premature NO release and accomplish on-demand NO release at regions of interest.

In comparison with exogenous NONOate-based donors, SNOs have been recognized as endogenous transports of NO and S -nitrosoglutathione GSNO and S -nitrosocysteine CysNO have been identified in biological systems.

SNO derivatives were generally synthesized in aqueous solution by the modification of thiols in the presence of nitrosating agents such as nitrogen dioxide NO 2 , dinitrogen tetroxide N 2 O 4 , dinitrogen trioxide N 2 O 3 , and nitrite NO 2 -.

The nitrosation reactions can also be implemented in an organic solvent using tert -butyl nitrite as the nitrosating agent. Because of the low bond energy, the NO release from SNO derivatives can be readily activated by light irradiation, heat, metal ions [e.

Akin to that of polymeric NONOate donors derived from amine-containing polymers, macromolecular SNO-based donors could be prepared from polymer precursors having thiol residues Coneski et al.

It is known that reversible addition-fragmentation chain transfer RAFT polymerization could be used for the synthesis of polymers with varying compositions and chain topologies.

The as-prepared polymers with RAFT agents at the chain terminals could be converted into free thiol groups. As such, it would be straightforward to synthesize SNO-terminated polymers via the combination of RAFT polymerization, RAFT agent removal, and nitrosation modification.

For example, the benzodithioate terminal of a diblock copolymer synthesized by RAFT polymerization was successively transformed into a free thiol and SNO motif in the presence of hydrazine and nitrite, respectively Yu et al. The NO release kinetics can be modulated by the solution pH and the formation of micellar nanoparticles at basic condition can markedly slow down the NO release rate Hu et al.

As mentioned above, amine-containing polymers can be successfully transformed into NONOates. Notably, they can also be engineered as SNO-type NO donors via an S -nitroso- N -acetylpenicillamine SNAP derivation approach. The SNAP approach provides a robust procedure to functionalize both natural and synthetic peptides bearing amine residues e.

For instance, a natural protein, fibrin, was decorated with SNAP with the formation of NO-releasing peptide and the resulting peptide can remarkably inhibit bacterial adhesion compared with natural fibrin without NO-releasing capability Vanwagner et al.

Besides peptides, natural polysaccharides and oligosaccharides could also be modified with the formation of SNO-type NO donors. For example, the amine residues of chitosan can be easily transformed into thiols with Traut's reagent i.

The NO release could be activated by endogenous ascorbic acid, eliciting a 4-log reduction in the viability of Pseudomonas aeruginosa Lu et al. As a representative example of oligosaccharide, β-CD has been extensively investigated in host-guest chemistry Hu and Liu, Because the hydroxyl groups in β-CD could be selectively functionalized, β-CD with one and seven SNO moieties were synthesized Piras et al.

Moreover, the mono-substituted β-CD-SNO can still be used as a host molecule to include specific guest molecules such as tamoxifen citrate and N -desmethyltamoxifen hydrochloride, which may show synergistic therapeutic performance.

Apart from NONOates and SNOs, other potential NO donors could be incorporated into polymer scaffolds such as organic nitrates and nitrobenzene derivatives. It is known that NO has a high affinity to many transition metal ions and transition ion-containing molecules can thus be used for NO storage Wang et al.

Although metal nitrosyls have been clinically prescribed, the covalent attachment of metal nitrosyls to macromolecular scaffolds has rarely been investigated, possibly due to the difficulties in chemical modification of metal nitrosyls.

To obtain organic nitrates, halogenated alkyl precursors were typically treated with silver nitrate AgNO 3. Recently, 2- nitrooxy acetic acid was conjugated to hyaluronic acid via esterification reaction and photothermal agent e.

The resulting multifunctional nanoparticles exhibited hyaluronidase HAase -mediated size shrinkage and near-infrared NIR light-triggered NO release, exerting a synergistic effect on cancer therapy Hu et al. Intriguingly, besides NIR light, organic nitrates also responded to endogenous reducing agents such as GSH Duong et al.

On the other hand, nitrobenzene derivatives have been developed as photoresponsive NO donors, exhibiting good stability without light irradiation yet photo-triggered NO release upon light exposure.

In this aspect, the Sortino group and other researchers have focused on 4-nitro trifluoromethyl aniline derivatives and demonstrated these photoresponsive NO-releasing molecules having board biomedical applications Caruso et al.

Photo-mediated NO release from 4-nitro trifluoromethyl aniline was ascribed to the presence of trifluoromethyl group in the ortho position that forced the nitro group in a twisted geometry.

Recently, 4-nitro trifluoromethyl aniline and lectin-binding D -mannopyranoside derivatives were attached to an alternative copolymer of poly styrene- alt -maleic acid Yang et al.

Notably, the introduction of bulky groups to nitrobenzene derivatives in a proximal position renders them responsive to light with the capability of releasing NO, which has proved to be a general and efficient method toward nitrobenzene-based NO donors Suzuki et al.

In addition, furoxan derivatives that can selectively release NO trigged by thiol-containing molecules were also integrated into the polymeric system to fabricate polymeric NO donors Wang et al. Significantly, compared with labile NONOates and SNOs, furoxan derivatives were compatible with typical CuAAC reaction conditions, providing many possibilities to construct NO-releasing polymers.

In this minireview, we have summarized the recent achievements of polymeric NO-releasing materials in terms of chemical design strategies and biomedical applications.

These polymeric NO donors can be roughly divided into three categories according to the chemical structures and display promising applications in anticancer, antibacterial, wound healing and so on Table 1.

Only selective literature reports are discussed, but it is not difficult to observe that this emerging field is now receiving increasing interest, particularly for exploring the biomedical applications of these NO-releasing macromolecules.

Despite tremendous achievements, there remain some challenges to be resolved in future studies. Table 1. Summary of the properties of NO donors and working mechanisms in biomedical applications. First, because the unstable nature of many NO donors such as NONOates and SNOs, the preparation of polymeric NO donors was dominantly achieved through a postmodification approach.

Although the stability of NO-releasing precursors could be efficiently elevated and the release durations could be modulated by the polymeric scaffolds, the uncontrolled release nature cannot be eliminated.

To minimize the side effects of NO, polymeric NO donors with controlled release performance are more appealing when considered their biomedical applications.

To this end, it is of crucial importance to screen novel NO donors with sufficient stability at physiological conditions and triggered NO release under specific stimuli such as non-invasive light and endogenous stimuli e. The development of organic nitrate- and nitrobenzene-based NO donors can significantly increase the stability of NO donors, whereas many of the nitrobenzene-based NO donors were only responsive to ultraviolet UV light with poor tissue penetration.

Second, the developed polymeric NO donors through the postmodification approach rendered it difficult to tune the self-assembly behavior of polymeric NO donors due to the non-specific modification.

To date, the self-assembly behavior of polymeric NO donors was far less explored and only a few examples of micelle-based NO carriers have been reported. Recent results suggested that the NO-releasing moieties could be introduced into the preformed nanostructures by the combination of polymerization-induced self-assembly PISA and postmodification approach Sadrearhami et al.

Indeed, the spatial location of NO donors within the assemblies greatly affected the NO-releasing kinetics. Given the shape-dependent interactions between cells and nanoassemblies, it is expected that other self-assembled morphologies nanorods, vesicles, etc.

Softcover ISBN : Due: 22 March eBook ISBN : Published: 07 March Series ISSN : Series E-ISSN : Edition Number : 1. Number of Pages : XI, Topics : Human Physiology , Neurosciences , Physiology , Immunology , Cancer Research. Policies and ethics. Skip to main content.

Editors: Arunabha Ray 0 , Kavita Gulati 1. Arunabha Ray Department of Pharmacology, Hamdard Institute of Medical Sciences and Research, Hamdard University, New Delhi, India View editor publications.

View editor publications. Discusses the latest findings on Nitric Oxide NO research Includes NO in physiology, pathophysiology and therapeutics Written by world renowned experts in the field. Sections Table of contents About this book Keywords Editors and Affiliations About the editors Bibliographic Information Publish with us.

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Front Matter Pages ii-xi. Translating Nitric Oxide Research to Therapeutics: A Critical Appraisal Arunabha Ray, Kavita Gulati, Sana Rehman Pages Nitric Oxide and Cardiovascular Health Yuan Li, Ashok K.

Srivastava, Madhu B. Anand-Srivastava Pages Nitric Oxide and Cardiovascular Diseases: Cardioprotection, Complications and Therapeutics Gaurav Kumar, Sanjay Kumar Dey, Suman Kundu Pages Nitric Oxide and the Heart Autonomic Nervous System Bruno Buchholz, Verena B. Franco-Riveros, Nahuel Méndez Diodati, Ricardo J.

Gelpi Pages Exercise Induced NO Modulation in Prevention and Treatment of Cardiovascular Diseases Ivan M. Srejovic, Vladimir I. Zivkovic, Tamara R. Nikolic Turnic, Aleksandra B. They are highly reactive in the body, and they are important for several bodily processes. However, free radicals are unstable, and at high levels they can build up and create a state called oxidative stress.

During oxidative stress, there are not enough antioxidants to control the free radicals. This imbalance can lead to cell damage and ultimately to diseases like cancer.

Taking L-arginine at a controlled level for a short time has been proven to be safe for most people, including pregnant people and children.

If you are breastfeeding, have recently had a heart attack, or have liver or kidney disease, L-arginine supplements may not be safe for you.

Talk to your healthcare provider before taking this supplement. Also, people with a genetic condition called guanidinoacetate methyltransferase deficiency GAMT may not be able to process arginine. They should not take L-arginine supplements.

L-arginine supplements can interact with several medications and other supplements. Ask your healthcare provider about taking it with:. Dietary supplements are not regulated by the U. Food and Drug Administration FDA the way medicines are regulated.

For this reason, supplements may contain unknown added ingredients, and they may contain less of the active ingredient than they advertise.

Make sure to look for a seal by a third-party quality testing organization on the bottle. These organizations test to make sure the product has the correct type and amount of ingredients listed and does not contain a harmful level of contaminants.

Credible third-party testing organizations include:. In the case of nitric oxide supplementation, adding nitrate-rich foods to your diet may give you the benefits you want without the risks that come with taking dietary supplements.

Because nitric oxide is a highly reactive compound, it works quickly to affect many systems in the body. This can lead to several benefits when nitric oxide is balanced out by antioxidants. However, you can take too much L-arginine which could lead to unhealthy levels of nitric oxide in your body.

Once you have reached the oxidative stress state, your risk for developing diseases and other health problems increases. Taking L-arginine supplements to increase nitric oxide levels can lead to side effects. Side effects of L-arginine may include:. Certain nitric oxide supplements also may trigger herpes outbreaks.

Research has also shown a link between high levels of nitric oxide and skin diseases like psoriasis, eczema, and skin cancer.

Taking smaller doses or dividing up the doses of nitric oxide supplements may reduce the risk of side effects. Nitric oxide is an important chemical messenger in the body.

It can promote blood flow, help nerve cells communicate, and may improve immunity. Taking L-arginine supplements or eating a nitrate-rich diet may be beneficial for people with a variety of health problems, including high blood pressure, peripheral artery disease, and erectile dysfunction.

However, nitric oxide is a free radical that can lead to cell and tissue damage when taken in excess. Research about the exact benefits of nitric oxide supplementation with L-arginine is mixed and incomplete.

Finally, in addition to causing side effects, L-arginine supplements can be dangerous for certain populations. Talk to your doctor about the possible benefits and risks of nitric oxide for your body.

National Library of Medicine: PubChem. Nitric oxide. Encyclopaedia Britannica. Kiani AK, Bonetti G, Medori MC, et al. Dietary supplements for improving nitric-oxide synthesis. J Prev Med Hyg. Zamora R, Vodovotz Y, Billiar TR. Inducible nitric oxide synthase and inflammatory diseases.

Mol Med. Karwowska M, Kononiuk A. Antioxidants Basel. McNeal CJ, Meininger CJ, Reddy D, et al. Safety and effectiveness of arginine in adults.

J Nutr. Assmann TS, Brondani LA, Bouças AP, et al.

Additionally, L-citrulline has been indicated to be a secondary NO donor in the NOS-dependent pathway, since it can be converted to L-arginine. Nitrate and nitrite are the main substrates to produce NO via the NOS-independent pathway. These anions can be reduced in vivo to NO and other bioactive nitrogen oxides.

Other molecules, such as the dietary supplement glycine propionyl-L-carnitine GPLC , have also been suggested to increase levels of NO, although the physiological mechanisms remain to be elucidated.

The interest in all these molecules has increased in many fields of research. In relation with exercise physiology, it has been suggested that an increase in NO production may enhance oxygen and nutrient delivery to active muscles, thus improving tolerance to physical exercise and recovery mechanisms.

Several studies using NO donors have assessed this hypothesis in a healthy, trained population. After polymerization and deprotection protocols, the deprotected amine residues were further decorated with NO, generating NO-releasing polymers.

In addition to linear chain polymers, cross-linked polymers can readily be synthesized by prior cross-linking of the amine-containing linear precursors.

This design concept was further expanded and NONOate-containing block copolymers were synthesized, which self-assembled into micellar nanoparticles in aqueous solution Jo et al.

Note that the formation of nanoassemblies can not only shield the labile NONOate moieties from protons in water that led to burst NO release but also render the localized delivery of NO possible by taking advantage of enhanced permeability and retention EPR effect of micellar nanoparticles.

The Schoenfisch group Stasko and Schoenfisch, ; Lu et al. In an early example, they systematically investigated the effects of the dendritic generations and amine types on the storage and release of NO.

Thorough investigations demonstrated that, as compared to the small molecule counterparts, the secondary amine-containing dendrimers showed a unique dendritic effect and exhibited a significantly longer NO release period Stasko and Schoenfisch, After elucidating the correlations between the chemical structures and NO-releasing performance, they examined the antibacterial effect of these NO-releasing dendrimers.

Although these dendrimers cannot efficiently penetrate bacterial biofilms, the incorporation of NO can drastically boost the anti-biofilm activity Sun et al. Notably, one of the most important goals is to explore the potentials of NO-releasing macromolecules in biological systems.

Thus, the biocompatibility of polymer scaffolds is of crucial importance. In this regard, biocompatible polysaccharides e. For example, the amine residues of oligochitosan were first modified with 2-methyl aziridine through ring-opening reaction and the newly formed amine groups were further treated with NO gas Lu et al.

The NO loading capability, maximum NO flux, and half-lives of the resulting NO-releasing oligochitosan were highly dependent on the molar ratio of 2-methyl aziridine to the amine residues. Cell viability studies revealed that the NO-releasing oligochitosan had minimal toxicity to normal L mouse fibroblast but could efficiently eradicate bacterial biofilm.

Detailed antibacterial studies revealed that the water solubility, appropriate molecular weights, and ionic characteristics of the NO-releasing chitosan synergistically contributed to the biofilm killing.

Recently, mono-substituted and multi-substituted β-cyclodextrin β-CD was engineered as NO-releasing materials as well, exhibiting cooperative antibacterial activity by means of NO and antibiotics loaded within the cavity of β-CD via host-guest interaction Jin et al. Although many of the NONOate-based polymeric NO donors were obtained by the functionalization of secondary amine-containing polymers, aliphatic primary amines can also be transformed into NONOate derivatives as well.

Recently, a statistical ternary copolymer containing primary amine residues was synthesized Namivandi-Zangeneh et al. The primary amine moieties were converted into NONOates in the presence of NO gas under high pressure.

The resulting polymers exerted synergistic antibacterial effects by taking advantages of NO-mediated eradication of biofilm and cationic polymer-assisted membrane disruption of bacteria.

As mentioned above, although amine-containing polymers can be functionalized with NO gas to form NONOate-based macromolecular NO donors and the half-lives and pharmacokinetics could be effectively altered, the spontaneous NO release from these NONOate-based polymeric NO donors cannot be eliminated.

It will be more promising to develop polymeric NO donors with on-demand release behavior that could avoid premature NO leakage. In this context, the terminal oxygens of NONOate derivatives were protected by glycosidase-responsive galactose moieties Zhao et al.

As a result, the spontaneous NO release was remarkably inhibited and controlled NO release could be achieved by incubating the NO donors with glycosidase. After attaching the enzyme-responsive NO donors to chitosan backbones through copper I -catalyzed azide-alkyne cycloaddition CuAAC reaction, the resulting macromolecular NO donors inherited glycosidase-enzyme characteristics.

This newly designed NO-releasing material cannot only inhibit platelet adhesion and prolong partial thromboplastin time but also show increased angiogenesis in a diabetic mouse model. Indeed, beside glycosidase, NONOate derivatives can be selectively caged by many other functional groups and selective uncaging reactions could be actuated by light irradiation, glutathione GSH , and other enzymes such as esterase, nitroreductase, and DT-diaphorase Makings and Tsien, ; Sharma and Chakrapani, The introduction of protected group chemistry opens a new avenue to devise stable NONOate donors, which should be more advantageous in biomedical application due to the possibility to minimize premature NO release and accomplish on-demand NO release at regions of interest.

In comparison with exogenous NONOate-based donors, SNOs have been recognized as endogenous transports of NO and S -nitrosoglutathione GSNO and S -nitrosocysteine CysNO have been identified in biological systems. SNO derivatives were generally synthesized in aqueous solution by the modification of thiols in the presence of nitrosating agents such as nitrogen dioxide NO 2 , dinitrogen tetroxide N 2 O 4 , dinitrogen trioxide N 2 O 3 , and nitrite NO 2 -.

The nitrosation reactions can also be implemented in an organic solvent using tert -butyl nitrite as the nitrosating agent.

Because of the low bond energy, the NO release from SNO derivatives can be readily activated by light irradiation, heat, metal ions [e. Akin to that of polymeric NONOate donors derived from amine-containing polymers, macromolecular SNO-based donors could be prepared from polymer precursors having thiol residues Coneski et al.

It is known that reversible addition-fragmentation chain transfer RAFT polymerization could be used for the synthesis of polymers with varying compositions and chain topologies. The as-prepared polymers with RAFT agents at the chain terminals could be converted into free thiol groups.

As such, it would be straightforward to synthesize SNO-terminated polymers via the combination of RAFT polymerization, RAFT agent removal, and nitrosation modification. For example, the benzodithioate terminal of a diblock copolymer synthesized by RAFT polymerization was successively transformed into a free thiol and SNO motif in the presence of hydrazine and nitrite, respectively Yu et al.

The NO release kinetics can be modulated by the solution pH and the formation of micellar nanoparticles at basic condition can markedly slow down the NO release rate Hu et al. As mentioned above, amine-containing polymers can be successfully transformed into NONOates. Notably, they can also be engineered as SNO-type NO donors via an S -nitroso- N -acetylpenicillamine SNAP derivation approach.

The SNAP approach provides a robust procedure to functionalize both natural and synthetic peptides bearing amine residues e. For instance, a natural protein, fibrin, was decorated with SNAP with the formation of NO-releasing peptide and the resulting peptide can remarkably inhibit bacterial adhesion compared with natural fibrin without NO-releasing capability Vanwagner et al.

Besides peptides, natural polysaccharides and oligosaccharides could also be modified with the formation of SNO-type NO donors. For example, the amine residues of chitosan can be easily transformed into thiols with Traut's reagent i.

The NO release could be activated by endogenous ascorbic acid, eliciting a 4-log reduction in the viability of Pseudomonas aeruginosa Lu et al.

As a representative example of oligosaccharide, β-CD has been extensively investigated in host-guest chemistry Hu and Liu, Because the hydroxyl groups in β-CD could be selectively functionalized, β-CD with one and seven SNO moieties were synthesized Piras et al.

Moreover, the mono-substituted β-CD-SNO can still be used as a host molecule to include specific guest molecules such as tamoxifen citrate and N -desmethyltamoxifen hydrochloride, which may show synergistic therapeutic performance.

Apart from NONOates and SNOs, other potential NO donors could be incorporated into polymer scaffolds such as organic nitrates and nitrobenzene derivatives. It is known that NO has a high affinity to many transition metal ions and transition ion-containing molecules can thus be used for NO storage Wang et al.

Although metal nitrosyls have been clinically prescribed, the covalent attachment of metal nitrosyls to macromolecular scaffolds has rarely been investigated, possibly due to the difficulties in chemical modification of metal nitrosyls.

To obtain organic nitrates, halogenated alkyl precursors were typically treated with silver nitrate AgNO 3. Recently, 2- nitrooxy acetic acid was conjugated to hyaluronic acid via esterification reaction and photothermal agent e.

The resulting multifunctional nanoparticles exhibited hyaluronidase HAase -mediated size shrinkage and near-infrared NIR light-triggered NO release, exerting a synergistic effect on cancer therapy Hu et al.

Intriguingly, besides NIR light, organic nitrates also responded to endogenous reducing agents such as GSH Duong et al. On the other hand, nitrobenzene derivatives have been developed as photoresponsive NO donors, exhibiting good stability without light irradiation yet photo-triggered NO release upon light exposure.

In this aspect, the Sortino group and other researchers have focused on 4-nitro trifluoromethyl aniline derivatives and demonstrated these photoresponsive NO-releasing molecules having board biomedical applications Caruso et al. Photo-mediated NO release from 4-nitro trifluoromethyl aniline was ascribed to the presence of trifluoromethyl group in the ortho position that forced the nitro group in a twisted geometry.

Recently, 4-nitro trifluoromethyl aniline and lectin-binding D -mannopyranoside derivatives were attached to an alternative copolymer of poly styrene- alt -maleic acid Yang et al. Notably, the introduction of bulky groups to nitrobenzene derivatives in a proximal position renders them responsive to light with the capability of releasing NO, which has proved to be a general and efficient method toward nitrobenzene-based NO donors Suzuki et al.

In addition, furoxan derivatives that can selectively release NO trigged by thiol-containing molecules were also integrated into the polymeric system to fabricate polymeric NO donors Wang et al. Significantly, compared with labile NONOates and SNOs, furoxan derivatives were compatible with typical CuAAC reaction conditions, providing many possibilities to construct NO-releasing polymers.

In this minireview, we have summarized the recent achievements of polymeric NO-releasing materials in terms of chemical design strategies and biomedical applications.

These polymeric NO donors can be roughly divided into three categories according to the chemical structures and display promising applications in anticancer, antibacterial, wound healing and so on Table 1. Only selective literature reports are discussed, but it is not difficult to observe that this emerging field is now receiving increasing interest, particularly for exploring the biomedical applications of these NO-releasing macromolecules.

Despite tremendous achievements, there remain some challenges to be resolved in future studies. Table 1. Summary of the properties of NO donors and working mechanisms in biomedical applications.

First, because the unstable nature of many NO donors such as NONOates and SNOs, the preparation of polymeric NO donors was dominantly achieved through a postmodification approach. Although the stability of NO-releasing precursors could be efficiently elevated and the release durations could be modulated by the polymeric scaffolds, the uncontrolled release nature cannot be eliminated.

To minimize the side effects of NO, polymeric NO donors with controlled release performance are more appealing when considered their biomedical applications.

To this end, it is of crucial importance to screen novel NO donors with sufficient stability at physiological conditions and triggered NO release under specific stimuli such as non-invasive light and endogenous stimuli e.

The development of organic nitrate- and nitrobenzene-based NO donors can significantly increase the stability of NO donors, whereas many of the nitrobenzene-based NO donors were only responsive to ultraviolet UV light with poor tissue penetration. Second, the developed polymeric NO donors through the postmodification approach rendered it difficult to tune the self-assembly behavior of polymeric NO donors due to the non-specific modification.

To date, the self-assembly behavior of polymeric NO donors was far less explored and only a few examples of micelle-based NO carriers have been reported. Recent results suggested that the NO-releasing moieties could be introduced into the preformed nanostructures by the combination of polymerization-induced self-assembly PISA and postmodification approach Sadrearhami et al.

Indeed, the spatial location of NO donors within the assemblies greatly affected the NO-releasing kinetics. Given the shape-dependent interactions between cells and nanoassemblies, it is expected that other self-assembled morphologies nanorods, vesicles, etc.

likely altered the extracellular and intracellular NO delivery performance Hu et al. Therefore, the self-assembly behavior of polymeric NO donors should be investigated. In this context, the stability of polymeric NO donors should be a prerequisite since the spontaneous NO release will change the chemical compositions and in turn affect the self-assembly behavior.

On the other hand, besides the postmodification method, new synthetic strategies such as direct polymerization of NO-releasing monomers appears to be an option, possibly generating well-defined block copolymers capable of self-assembling into various nanostructures Wang et al.

Finally, small molecule-based NO donors such as nitroglycerin and sodium nitroprusside have been widely used in clinical trials Nichols et al. Preliminary results of polymeric NO donors have revealed promising perspective in biomedical applications.

Besides NO itself, it can also be delivered with other therapeutic agents to reverse multidrug resistance and thus improve therapeutic outcomes. However, it appears that the macromolecular NO donors cannot easily bypass the long-standing difficulties of other polymeric nanomedicines including relatively low delivery efficiency, undesirable biodistributions, and systemic side effects.

On the other hand, it is well-documented that the biological functions of NO are highly concentration dependent. Besides NO donors that can selectively release NO in the pathological environment, the development of NO-responsive polymers that can efficiently scavenge endogenous NO may open new avenues for specific disease therapy Hu et al.

Although many efforts have to be devoted to resolving the current problems of polymeric NO donors as well as other macromolecular nanomedicines, we believe that the continuous development of nanotechnology, polymer chemistry, and biology will make polymeric NO donors and other nanomedicines smarter that could finally conquer the difficulties.

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The financial support from Natural Science Foundation of China NNSFC project , , and the Fundamental Research Funds for the Central Universities WK is gratefully acknowledged. Carpenter, A. Nitric oxide release: PART II. Therapeutic applications. doi: PubMed Abstract CrossRef Full Text Google Scholar.

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Projects Nitrate and nitrite as a substrate for NO generation Airway nitric oxide NO and the gastrointestinal system NO as a marker for inflammation Antibacterial effects of nitrite.