Nitric Oxide (DB00435): A Comprehensive Monograph on its Pharmacology, Clinical Utility, and Therapeutic Frontiers

Executive Summary

Nitric Oxide (NO), a simple diatomic molecule, occupies a unique and paradoxical position in medicine. It is simultaneously a fundamental endogenous signaling molecule essential for vascular homeostasis and a high-risk therapeutic gas requiring complex administration and monitoring. Its primary, evidence-backed clinical role is as a selective pulmonary vasodilator for the treatment of term and near-term neonates with persistent pulmonary hypertension (PPHN) and hypoxic respiratory failure (HRF). In this specific indication, inhaled nitric oxide (iNO) has demonstrably improved oxygenation and reduced the need for invasive extracorporeal membrane oxygenation (ECMO).

The therapeutic action of iNO is rooted in its ability to mimic the endogenous NO pathway, activating soluble guanylate cyclase to produce cyclic guanosine monophosphate (cGMP), which leads to smooth muscle relaxation. The genius of the therapy lies in its pulmonary selectivity; delivered via inhalation, the gas acts locally on the pulmonary vasculature and is then rapidly inactivated upon entering the bloodstream, preventing significant systemic hypotension. This elegant mechanism, however, is also the source of its primary risks. The interaction with hemoglobin that ensures its selectivity can also lead to the formation of methemoglobin, causing methemoglobinemia. Its chemical reactivity with oxygen can produce toxic nitrogen dioxide (NO2​), and its role as a supplement to a physiological pathway means that abrupt withdrawal can trigger severe rebound pulmonary hypertension.

Beyond its narrow FDA-approved indication, iNO is widely used off-label in a variety of critical care settings, including for adult acute respiratory distress syndrome (ARDS) and perioperative cardiac surgery. This broad application, driven by its compelling physiological rationale, exists in tension with a lack of high-level evidence demonstrating improved mortality in these populations. This evidence-practice gap highlights both a significant unmet clinical need and the challenges of translating pathophysiology into proven clinical benefit.

Current and future research is poised to redefine the therapeutic landscape for nitric oxide. Investigations are moving beyond simple inhalation, exploring novel delivery systems such as topical gels and direct intratumoral injections. Entirely new therapeutic frontiers are being explored in oncology, where NO exhibits a dual role as both a potential tumor promoter and a cytotoxic agent, and in infectious diseases, where its antimicrobial properties are being harnessed. This evolution signals a shift from viewing nitric oxide as a single drug for a single condition to a versatile therapeutic platform, with the potential to harness the power of this simple molecule for a diverse range of complex diseases. This report provides an exhaustive analysis of nitric oxide, from its fundamental chemistry to its clinical application and future horizons.

Introduction: From Industrial Gas to Endogenous Signaling Molecule

The story of nitric oxide is a remarkable journey of scientific discovery, transforming a known industrial gas into one of the most fundamental signaling molecules in biology. Its simple structure belies a profound complexity, functioning as both a hazardous chemical and a life-saving therapeutic. Understanding this dual identity is essential to appreciating the nuances of its clinical application, where the goal is to safely harness its physiological power while mitigating its inherent chemical risks.

Physicochemical Profile and Identification

Nitric Oxide is a small molecule with the chemical formula NO and a molecular weight of 30.01 g/mol.[1] Its Chemical Abstracts Service (CAS) Number is 10102-43-9.[1] Under standard conditions, it exists as a colorless gas with a characteristic sharp, sweet odor.[1] A key chemical property with direct clinical relevance is its spontaneous and rapid reaction with oxygen in the air to yield nitrogen dioxide (

NO2​), a toxic, brown irritant gas.[1] This reactivity necessitates that therapeutic nitric oxide be stored and delivered in an oxygen-free environment and that inspired

NO2​ levels be continuously monitored during administration.[3]

Physically, it has a boiling point of −151.7 °C and a melting point of −163.6 °C.[1] It is nonflammable but is classified as a strong oxidizer that enhances the combustion of other substances, contributing to its hazardous material classification.[1] It is only slightly soluble in water.[1]

From a safety perspective, nitric oxide is classified as a toxic gas, poisonous by inhalation (DOT Hazard Class 2.3), and is designated with Globally Harmonized System (GHS) symbols indicating it is an oxidizing agent, a compressed gas, corrosive, and acutely toxic.[1] Regulatory bodies have established strict exposure limits; the National Institute for Occupational Safety and Health (NIOSH) and the Occupational Safety and Health Administration (OSHA) have set a time-weighted average (TWA) exposure limit of 25 ppm (approximately 30 mg/m³).[6] The concentration considered Immediately Dangerous to Life or Health (IDLH) is 100 ppm, a level based on data suggesting that exposure to 100-150 ppm of nitrogen oxides is dangerous for short durations of 30 to 60 minutes.[6] These physicochemical and safety properties are not merely academic; they form the foundation of the complex engineering and stringent protocols required for its safe clinical use. The gaseous state allows for targeted delivery to the lungs, while its inherent reactivity and toxicity demand specialized equipment and constant vigilance.

Table 1: Physicochemical and Safety Properties of Nitric Oxide

Property	Value / Description	Source(s)
Identification
Chemical Name	Nitric Oxide	1
Molecular Formula	NO	1
Molecular Weight	30.01 g/mol	1
CAS Number	10102-43-9	1
DrugBank ID	DB00435	-
Physical Properties
Form	Colorless gas	1
Odor	Sharp, sweet; brown at high concentrations in air	1
Boiling Point	−151.7 °C	1
Melting Point	−163.6 °C	1
Water Solubility	Slightly soluble	1
Hazard Information
GHS Hazard Symbols	GHS03 (Oxidizer), GHS04 (Compressed Gas), GHS05 (Corrosive), GHS06 (Acutely Toxic)	1
DOT Hazard Class	2.3 (Gas poisonous by inhalation)	1
IDLH Concentration	100 ppm	6
OSHA/NIOSH PEL	25 ppm TWA (30 mg/m³)	6

Historical Perspective: The Discovery of Endothelium-Derived Relaxing Factor (EDRF)

The clinical use of nitric oxide is predicated on its role as an endogenously produced signaling molecule. The path to this understanding began long before the molecule itself was identified as a therapeutic agent. In 1846, the chemist Ascanio Sobrero synthesized nitroglycerin and, upon tasting it, noted it caused a profound headache—a symptom soon understood to be the result of cerebral vasodilation.[7] This observation led to the use of nitroglycerin for treating angina and hypertension within two decades, demonstrating that human cells possessed a mechanism to respond to this chemical, or its metabolites.[7]

The critical leap forward occurred in the 1980s. Researchers investigating the effects of molecules like acetylcholine on blood vessels made a pivotal discovery: the vasorelaxant effect occurred only when the delicate inner lining of the blood vessel, the endothelium, was preserved.[7] This led to the hypothesis of a diffusible substance released by the endothelium that acted on the underlying smooth muscle to cause relaxation. This unknown substance was termed "Endothelium-Derived Relaxing Factor" (EDRF).[7]

The search for the chemical identity of EDRF was a major focus of cardiovascular research for nearly a decade. Nitric oxide was a key suspect because it produced similar vasorelaxant effects. Through systematic biochemical and pharmacological comparisons, compelling evidence mounted that nitric oxide was, in fact, the major bioactive component of EDRF.[7] This discovery, which was awarded the Nobel Prize in Physiology or Medicine in 1998, was transformative.[9] It clarified that exogenously administered compounds like nitroglycerin and nitric oxide itself were not acting as foreign substances, but were recruiting the body's own physiological signaling pathways. This foundational concept—that nitric oxide therapy is a form of supplementation for a vital endogenous mediator—underpins its entire clinical application and explains why its effects are so potent and specific.

Comprehensive Pharmacological Profile

The therapeutic utility of nitric oxide is derived from its intricate interactions with cellular signaling pathways. Its pharmacology is characterized by a specific mechanism of action that leads to potent but localized effects, governed by a unique pharmacokinetic profile that ensures its activity is confined primarily to the target organ.

Mechanism of Action: The Soluble Guanylate Cyclase-cGMP Pathway

The primary mechanism of action for nitric oxide, whether produced endogenously or administered as a drug, involves the soluble guanylate cyclase (sGC)-cyclic guanosine monophosphate (cGMP) signaling cascade.[10]

Endogenously, nitric oxide is synthesized in various tissues from the amino acid L-arginine by a family of enzymes known as nitric oxide synthases (NOS).[7] When administered as an inhaled drug (iNO), the gaseous molecule readily diffuses across the alveolar-capillary membrane from the airspaces of the lung into the adjacent pulmonary vascular smooth muscle cells.[8]

Inside the smooth muscle cell, nitric oxide binds to the heme moiety of the enzyme soluble guanylate cyclase (sGC).[10] This binding activates sGC, which then catalyzes the conversion of guanosine triphosphate (GTP) into the second messenger molecule, cyclic guanosine monophosphate (cGMP).[4] The subsequent rise in intracellular cGMP concentration activates cGMP-dependent protein kinases (like protein kinase G, or PKG).[10] These activated kinases phosphorylate several downstream protein targets, including various ion channels in the cell membrane and endoplasmic reticulum. This phosphorylation cascade ultimately leads to a decrease in the intracellular concentration of free calcium ions (

Ca2+), primarily through sequestration into intracellular stores and reduced influx.[10] The reduction in intracellular calcium prevents the activation of the contractile machinery within the smooth muscle cell, resulting in relaxation and, consequently, vasodilation.[10]

This pathway is fundamental not only to understanding nitric oxide's direct effects but also to appreciating its interactions with other drug classes. For instance, phosphodiesterase-5 (PDE5) inhibitors like sildenafil work by blocking the enzyme that degrades cGMP, thereby potentiating the effects of nitric oxide.[13] Conversely, alternative vasodilators like prostacyclin analogues operate through a parallel but distinct pathway involving cyclic adenosine monophosphate (cAMP).[15]

Pharmacodynamic Effects

The activation of the sGC-cGMP pathway by nitric oxide produces several key pharmacodynamic effects that are exploited therapeutically.

Selective Pulmonary Vasodilation

The hallmark of inhaled nitric oxide therapy is its ability to produce selective pulmonary vasodilation.[12] Because the drug is delivered as a gas directly to the alveoli, it preferentially reaches the blood vessels perfusing well-ventilated segments of the lung. This leads to vasodilation in these specific areas, which diverts blood flow from poorly ventilated or collapsed lung regions towards areas where gas exchange is efficient. This process, known as improving ventilation/perfusion (V/Q) matching, reduces the degree of intrapulmonary shunting (blood passing through the lungs without being oxygenated) and leads to a direct improvement in systemic oxygenation.[10]

The selectivity of this effect is not solely due to the mechanism of action, which is common to smooth muscle elsewhere in the body. Rather, it is a direct consequence of the drug's pharmacokinetic properties. Any nitric oxide that diffuses into the bloodstream and escapes the pulmonary circulation is almost instantaneously bound by the heme iron in hemoglobin.[8] This binding inactivates the nitric oxide molecule, preventing it from reaching and acting upon systemic blood vessels. The result is a potent vasodilatory effect localized to the lungs, without the concomitant systemic hypotension that would occur with intravenous administration of a non-selective vasodilator.[10]

Inhibition of Platelet Aggregation

Nitric oxide also exerts effects on platelets. It can diffuse into circulating platelets and activate their internal guanylate cyclase, leading to an increase in platelet cGMP.[10] This, in turn, activates protein kinases that are thought to reduce the binding affinity of fibrinogen to the glycoprotein IIb/IIIa receptor on the platelet surface.[10] As this receptor is critical for platelet cross-linking, its inhibition leads to a partial reduction in platelet aggregation.[10] This anti-platelet effect may contribute to the therapeutic benefits observed in conditions like Acute Respiratory Distress Syndrome (ARDS), where platelet activation in the lung is heightened.[10] However, it also introduces a theoretical risk of increased bleeding, a potential adverse effect that must be considered during therapy.[11]

Immunomodulation

The biological roles of nitric oxide extend to the immune system. Phagocytic cells, including macrophages and neutrophils, generate large amounts of nitric oxide via inducible nitric oxide synthase (iNOS) as part of the innate immune response.[13] This endogenously produced NO is toxic to a range of pathogens, including bacteria and intracellular parasites like

Leishmania and malaria, by mechanisms that include DNA damage and degradation of critical iron-sulfur centers in microbial enzymes.[13]

Furthermore, research suggests that nitric oxide can modulate adaptive immune responses by altering the balance of T helper cells. Specifically, it has been observed to decrease the proliferation of pro-inflammatory T helper 1 (TH1) cells while promoting the activity of T helper 2 (TH2) cells.[10] In this way, nitric oxide may act to inhibit or resolve certain inflammatory responses. This immunomodulatory function forms the basis for ongoing research into its potential use in treating various infectious and inflammatory conditions.[10]

Pharmacokinetics of Inhaled Nitric Oxide (iNO)

The clinical utility and safety profile of inhaled nitric oxide are profoundly shaped by its unique pharmacokinetic properties. The route of administration, rapid systemic absorption, and extremely fast metabolism are key to its therapeutic action.

Table 2: Pharmacokinetic Parameters of Inhaled Nitric Oxide

Parameter	Value / Description	Source(s)
Route of Administration	Inhalation via a calibrated delivery system	10
Onset of Action	Rapid, dose-dependent onset within minutes	10
Absorption	Systemically absorbed after inhalation, crossing the pulmonary capillary bed	10
Distribution	Action is largely confined to the pulmonary vasculature due to rapid inactivation in blood	8
Metabolism	In the bloodstream, NO combines with oxyhemoglobin to form methemoglobin and nitrate. It also reacts with oxygen and water to form nitrogen dioxide and nitrite.	10
Half-Life	2 to 6 seconds in the bloodstream	10
Key Metabolites	Methemoglobin (MetHb), Nitrate (NO3−​), Nitrogen Dioxide (NO2​)	10
Route of Elimination	Approximately 70% of inhaled NO metabolites (primarily nitrate) are eliminated via the kidneys. Clearance corresponds to the glomerular filtration rate.	10

The pharmacokinetic profile is defined by its route of administration via inhalation, which allows for a rapid onset of action, typically within minutes.[10] Upon crossing the alveolar-capillary membrane, nitric oxide is absorbed systemically.[10] However, its fate in the bloodstream is what defines its clinical effect. The molecule has an exceptionally short biological half-life, estimated to be between 2 and 6 seconds.[10]

This fleeting existence is due to its high affinity for the ferrous iron (Fe2+) within the heme group of hemoglobin. Upon entering the blood, nitric oxide rapidly binds to oxyhemoglobin, which has two major consequences. First, the nitric oxide molecule is inactivated, preventing it from exerting vasodilatory effects on the systemic circulation. Second, this reaction oxidizes the hemoglobin's iron to the ferric state (Fe3+), forming methemoglobin (MetHb), a form of hemoglobin that is incapable of transporting oxygen.[10] In parallel, nitric oxide can react with oxygen and water to form other metabolites, including nitrogen dioxide (

NO2​) and nitrite (NO2−​).[10]

Ultimately, the principal end products of inhaled nitric oxide metabolism that are found in the circulation are methemoglobin and nitrate.[10] These metabolites are then cleared from the body. Nitrate is eliminated by the kidneys, with a plasma clearance rate that approximates the glomerular filtration rate.[10] This entire pharmacokinetic journey—from targeted inhalation to rapid inactivation and renal excretion of metabolites—is central to the drug's therapeutic paradigm. It explains both its remarkable pulmonary selectivity and the origin of its most significant toxicities: methemoglobinemia and airway injury from nitrogen dioxide.

Clinical Efficacy and Therapeutic Applications of Inhaled Nitric Oxide (iNO)

The clinical use of inhaled nitric oxide is characterized by a stark contrast between its single, well-established, FDA-approved indication and its widespread off-label use across a spectrum of critical illnesses. This disparity reflects the compelling physiological rationale for its use in any condition involving pulmonary hypertension, juxtaposed with the stringent evidence required to prove a definitive benefit in patient-centered outcomes like mortality.

Primary FDA-Approved Indication: Neonatal PPHN and HRF

The cornerstone of inhaled nitric oxide therapy is its U.S. Food and Drug Administration (FDA) approval for the treatment of term and near-term (gestational age >34 weeks) neonates with hypoxic respiratory failure (HRF) associated with clinical or echocardiographic evidence of persistent pulmonary hypertension of the newborn (PPHN).[16] This indication was first approved in 1999.[20]

PPHN is a critical condition in which an infant’s circulatory system fails to transition from fetal to postnatal circulation. The pulmonary vascular resistance (PVR) remains high, causing blood to be shunted away from the lungs through fetal channels like the foramen ovale and ductus arteriosus, leading to severe hypoxemia.[3] This can be a primary condition or secondary to other neonatal problems such as meconium aspiration syndrome (MAS), sepsis, or respiratory distress syndrome (RDS).[3]

In this context, iNO is used as an adjunctive therapy when infants fail to respond to optimal respiratory support, including mechanical ventilation and oxygenation.[11] Clinical guidelines often recommend initiation based on an elevated Oxygenation Index (OI), a measure of respiratory failure, typically at a threshold greater than 15 to 20.[11] The primary goal of therapy is to improve oxygenation and, critically, to reduce the need for extracorporeal membrane oxygenation (ECMO), a highly invasive, high-risk, and resource-intensive form of life support.[16] Multiple randomized clinical trials have demonstrated that iNO significantly achieves this goal, forming the robust evidence base for its FDA approval.[8]

Off-Label and Investigational Uses in Cardiopulmonary Disease

Despite its narrow approval, the logical application of a selective pulmonary vasodilator has led to extensive off-label use and investigation in other patient populations.

Acute Respiratory Distress Syndrome (ARDS)

iNO is frequently used off-label in both pediatric and adult patients with ARDS, a condition characterized by widespread lung inflammation, edema, and severe hypoxemia.[10] While iNO can often produce a temporary improvement in oxygenation by improving V/Q matching, multiple large clinical trials have failed to demonstrate a benefit in terms of mortality or duration of mechanical ventilation.[4] In fact, some evidence suggests a potential for harm, such as acute renal impairment.[4] The official FDA label for INOmax explicitly notes that clinical studies found it to be ineffective in adult ARDS.[18] Nevertheless, it continues to be employed as a "rescue therapy" in patients with refractory, life-threatening hypoxemia, including in severe cases of ARDS associated with COVID-19, after all other therapeutic options have been exhausted.[10]

Preterm Infants (<34 weeks gestation)

The use of iNO in premature infants is highly controversial. Guidelines from professional bodies, such as the Canadian Paediatric Society, explicitly recommend against the routine use of iNO in preterm infants on respiratory support.[11] This recommendation is based on meta-analyses of clinical trials which have shown that iNO does not reduce the composite outcome of death or bronchopulmonary dysplasia (BPD), a form of chronic lung disease common in premature infants.[11] Furthermore, some studies have suggested a potential for increased risk of severe intraventricular hemorrhage (bleeding in the brain) in this vulnerable population.[11] Despite this, iNO may be considered as a rescue modality in very specific circumstances, such as in preterm infants with early-onset, refractory HRF that is associated with risk factors for PPHN, like prolonged rupture of membranes or oligohydramnios.[3]

Congenital Diaphragmatic Hernia (CDH)

CDH is a birth defect where a hole in the diaphragm allows abdominal organs to move into the chest, leading to lung hypoplasia and pulmonary hypertension. A trial of iNO may be considered for infants with CDH who have persistent HRF despite optimal lung recruitment strategies.[11] However, its use is conditional upon echocardiographic confirmation of supra-systemic pulmonary hypertension (meaning the pressure in the pulmonary artery is higher than in the systemic circulation) and evidence of adequate left ventricular function.[11] If there is no clear clinical or echocardiographic improvement, the therapy should be discontinued promptly.[11]

Perioperative Cardiac Surgery

iNO is used off-label to manage acute pulmonary hypertension that can occur during or after cardiac surgery, particularly following cardiopulmonary bypass.[12] It can help to reduce right ventricular strain and improve hemodynamics in this setting.

Emerging Investigational Uses

The therapeutic potential of nitric oxide continues to be explored in formal clinical trials for a range of conditions, reflecting the ongoing effort to expand its evidence-based applications. Current or recent trials include:

• Oxygen Deficiency (Hypoxia): A completed Phase 1/2 trial investigated the effect of iNO on maximal oxygen consumption during exercise under hypoxic conditions.[25]
• Cardiac Arrest: The iNOCAPA trial, a recruiting Phase 2 study, is evaluating the use of iNO during resuscitation for both pediatric and adult out-of-hospital cardiac arrest.[26]
• Pulmonary Fibrosis: A Phase 2 trial is assessing the efficacy of pulsed inhaled nitric oxide in patients who have developed pulmonary hypertension secondary to pulmonary fibrosis.[27]

This landscape of clinical use illustrates a significant "evidence-practice gap." Clinicians, faced with critically ill patients suffering from pulmonary hypertension from various causes, are often compelled to use iNO based on its powerful and logical mechanism of action. This occurs even when high-level evidence of improved long-term outcomes is absent, unproven, or even negative, as in the case of adult ARDS. This tension drives the continued research and debate surrounding the proper role of this potent but costly therapy.

Table 3: Summary of Clinical Applications for Inhaled Nitric Oxide

Indication	Patient Population	Key Finding / Rationale	Status	Source(s)
PPHN & HRF	Term & Near-Term Neonates (>34 weeks)	Improves oxygenation and reduces the need for ECMO.	FDA Approved	16
Acute Respiratory Distress Syndrome (ARDS)	Adults & Pediatrics	Temporarily improves oxygenation but does not reduce mortality. Potential for renal toxicity.	Off-Label (Rescue Therapy)	4
HRF in Preterm Infants	Neonates <34 weeks	Routine use not recommended; does not reduce death/BPD and may increase risk of IVH.	Off-Label (Rescue in specific cases)	11
Congenital Diaphragmatic Hernia (CDH)	Neonates	May be trialed in cases with supra-systemic PH and adequate LV function.	Off-Label (Conditional)	11
Perioperative Pulmonary Hypertension	Cardiac Surgery Patients	Used to manage acute PH post-cardiopulmonary bypass.	Off-Label	12
Cardiac Arrest	Pediatrics & Adults	Investigating improved outcomes during resuscitation.	Investigational (Phase 2)	26
Pulmonary Hypertension with Fibrosis	Adults	Assessing pulsed iNO for PH secondary to pulmonary fibrosis.	Investigational (Phase 2)	27

Administration, Dosing, and Clinical Management

The therapeutic use of inhaled nitric oxide is not merely a matter of prescribing a drug; it is the implementation of a complex therapeutic system. The narrow therapeutic index and significant potential for toxicity necessitate strict adherence to protocols for dosing, administration, monitoring, and weaning, all of which rely on specialized equipment and highly trained clinical staff.

Dosing and Administration Protocols

For the approved neonatal indication, the standard recommended starting dose of inhaled nitric oxide is 20 parts per million (ppm).[4] This concentration is delivered continuously into the patient's ventilator circuit. It is critical to note that doses higher than 20 ppm are not recommended. Clinical studies have shown that increasing the dose beyond 20 ppm does not typically confer additional therapeutic benefit and may substantially increase the risk of toxicity, particularly methemoglobinemia and the formation of nitrogen dioxide.[10] In certain populations, such as very or extremely preterm infants where iNO is used off-label, clinicians may opt for a lower starting dose of 5 to 10 ppm.[11] Therapy is generally maintained for up to 14 days or until the underlying cause of hypoxemia has resolved and the patient is stable enough to be weaned.[10]

Administration of iNO must be performed using a calibrated, FDA-cleared Nitric Oxide Delivery System (NODS).[4] Commercially available systems include the INOmax® and Genosyl® delivery systems, each designed to be integrated with a mechanical ventilator to deliver a precise and stable concentration of the gas.[4] These systems are sophisticated and require that clinical staff (typically respiratory therapists and critical care nurses) complete a comprehensive training program provided by the manufacturer.[28] Due to the critical nature of the therapy and the risks of system failure, protocols mandate that a backup power supply and an entirely independent reserve nitric oxide delivery system be immediately available at the bedside.[19]

Essential Monitoring and Weaning Strategies

Continuous and vigilant monitoring is an absolute requirement for safe iNO therapy, designed to detect both therapeutic response and emergent toxicity.

Monitoring

Key monitoring parameters include:

• Inspired Nitrogen Dioxide (NO2​): As nitric oxide reacts with oxygen in the ventilator circuit, the toxic byproduct NO2​ is formed. The delivery system continuously monitors the level of NO2​ in the inspired gas. Alarm limits are set to alert clinicians if the concentration exceeds a predefined safety threshold, which is typically kept below 2-3 ppm, and ideally below 0.5 ppm with some systems.[3]
• Methemoglobin (MetHb): The formation of MetHb is a direct consequence of NO interacting with hemoglobin. Blood levels of MetHb must be measured via co-oximetry, typically from an arterial blood gas sample. A baseline level should be checked before starting therapy, with a follow-up measurement within 4 to 8 hours of initiation, and periodically thereafter throughout the course of treatment.[3]
• Oxygenation and Hemodynamics: The patient's response to therapy is monitored continuously. This includes arterial oxygen saturation (SpO2​), partial pressure of arterial oxygen (PaO2​), and hemodynamic parameters such as blood pressure and heart rate to assess for improvement in oxygenation and to detect any adverse cardiovascular effects.[3]

Weaning

Discontinuation of iNO therapy is a high-risk procedure that must be managed carefully. Abrupt cessation of the drug can lead to a rapid and severe increase in pulmonary artery pressure and worsening oxygenation, a phenomenon known as rebound pulmonary hypertension.[17] This can occur even in patients who showed no apparent initial response to the therapy.

To prevent this, weaning must be performed gradually. The process involves down-titrating the iNO dose in a stepwise fashion. For example, the dose may be reduced from 20 ppm to 10 ppm, then to 5 ppm, pausing for several hours at each step to ensure the patient remains stable.[3] Once the dose is low (e.g., 5 ppm), it is typically reduced in smaller increments of 1 ppm at a time until it is discontinued.[3] If at any point during the wean the patient experiences significant desaturation or hemodynamic instability, the iNO dose must be immediately returned to the previous effective level, and the weaning attempt postponed.[19] This meticulous process underscores that iNO therapy is a comprehensive intervention where the drug, the delivery device, and the clinical protocols are inextricably linked to ensure patient safety.

Safety, Toxicology, and Risk Mitigation

While inhaled nitric oxide offers a powerful therapeutic tool, its use is accompanied by significant risks that are direct consequences of its chemical properties and physiological effects. A thorough understanding of its contraindications, warnings, and potential adverse events is paramount for its safe use in the critical care setting.

Contraindications and Critical Warnings

There is one absolute contraindication for the use of inhaled nitric oxide:

• Neonates with Ductal-Dependent Systemic Circulation: iNO is strictly contraindicated in neonates who are dependent on a right-to-left shunt for their systemic blood flow.[17] This occurs in certain complex congenital heart defects (e.g., hypoplastic left heart syndrome). In these infants, the right ventricle pumps blood to the body through a patent ductus arteriosus. By causing pulmonary vasodilation, iNO would lower the pulmonary vascular resistance, potentially reversing the shunt to a left-to-right direction. This would divert blood away from the systemic circulation and into the lungs, leading to profound systemic hypotension and catastrophic circulatory collapse. Echocardiography is essential prior to initiation to rule out such conditions.[3]

In addition to this contraindication, several critical warnings require vigilant management:

• Rebound Pulmonary Hypertension: As previously detailed, abrupt discontinuation of iNO can lead to a sudden and severe worsening of pulmonary hypertension and hypoxemia.[17] This necessitates a gradual and carefully monitored weaning process.
• Methemoglobinemia: The formation of methemoglobin (MetHb) is a dose-dependent risk that can impair the blood's oxygen-carrying capacity, leading to functional hypoxemia.[4] Regular blood monitoring is mandatory.
• Nitrogen Dioxide (NO2​) Toxicity: The formation of NO2​ in the ventilator circuit is unavoidable. NO2​ is a potent pulmonary irritant that can cause direct airway inflammation and lung tissue damage.[1] Continuous monitoring of inspired NO2​ levels is a critical safety feature of all delivery systems.
• Worsening Heart Failure: In patients with pre-existing left ventricular (LV) dysfunction, iNO must be used with extreme caution. By selectively dilating the pulmonary vessels, iNO reduces the resistance against which the right ventricle must pump, thereby increasing pulmonary blood flow and venous return to the left side of the heart. If the left ventricle is already failing and cannot handle this increased preload, it can lead to a rapid increase in left atrial and pulmonary capillary wedge pressures, causing acute pulmonary edema, worsening cardiogenic shock, and potentially cardiac arrest.[17]

Analysis of Major Adverse Events

The safety profile of nitric oxide is a direct reflection of its underlying chemistry and physiology. Each of the major adverse events can be traced back to a fundamental property of the molecule.

• Methemoglobinemia: This occurs when the nitric oxide molecule oxidizes the ferrous iron (Fe2+) in the heme portion of hemoglobin to its ferric state (Fe3+).[10] The resulting MetHb is unable to bind and transport oxygen. While low levels are tolerated, higher levels reduce the effective oxygen-carrying capacity of the blood. Symptoms are related to tissue hypoxia and can include cyanosis (a bluish discoloration of the skin, lips, and nail beds), fatigue, headache, dizziness, and tachycardia.[5] The risk is dose-related, becoming more pronounced at doses above the recommended 20 ppm.[4] Management involves reducing the iNO dose or discontinuing therapy. In severe cases, intravenous administration of antidotes such as methylene blue or vitamin C may be required to reduce the MetHb back to functional hemoglobin.[17]
• Rebound Pulmonary Hypertension: This is a classic pharmacological withdrawal phenomenon. Continuous administration of exogenous iNO can lead to downregulation of the body's endogenous NO production pathways. When the external supply is suddenly removed, the suppressed endogenous system is unable to immediately compensate, resulting in a rapid return of severe pulmonary vasoconstriction.[17] Symptoms include acute hypoxemia, bradycardia, hypotension, and decreased cardiac output.[30] The only effective management is to immediately reinstate the iNO therapy and plan for a more gradual wean at a later time.[19]
• Other Reported Adverse Events: The most frequently reported adverse event in clinical trials is hypotension.[18] Other reported events include atelectasis (lung collapse), hematuria (blood in urine), hyperglycemia, stridor (a high-pitched breathing sound), and an increased risk of hemorrhage, including intracranial, pulmonary, and gastrointestinal bleeding, though a causal link is not always clear.[17]

Management of Overdose and Accidental Staff Exposure

Overdose with inhaled nitric oxide primarily manifests as dangerously elevated methemoglobin levels and signs of acute lung injury from excessive nitrogen dioxide formation.[17] Management consists of immediately discontinuing or reducing the iNO dose and providing aggressive supportive care. This may include specific treatments for severe methemoglobinemia, such as methylene blue infusion or even blood transfusion.[17]

Accidental exposure of hospital staff during cylinder changes or system handling can also occur. Symptoms of acute exposure are typically mild and may include chest discomfort, dizziness, dry throat, and headache.[18] Adherence to safety protocols and proper ventilation in areas where cylinders are handled are key preventative measures. OSHA has established workplace exposure limits for both nitric oxide and nitrogen dioxide to protect healthcare workers.[4]

Significant Drug-Drug Interactions

The potent and specific mechanism of action of nitric oxide creates a potential for clinically significant interactions when it is co-administered with other drugs that affect vascular tone, particularly those acting on related signaling pathways. Understanding these interactions is critical for safe and effective use, especially in the complex polypharmacy environment of the intensive care unit.

Note on Data Integrity: Correction of Nitrous Oxide Conflation

It is imperative to address a common point of confusion found in some databases. Several sources conflate drug interactions for nitric oxide (NO, DrugBank ID DB00435), the vasodilator, with those for nitrous oxide (N2​O, DrugBank ID DB06690), an anesthetic gas.[34] The pharmacology and interaction profiles of these two distinct gases are entirely different. This report will exclusively analyze verified interactions pertaining to nitric oxide (

NO) to ensure clinical accuracy and relevance.

Interaction with Phosphodiesterase-5 (PDE5) Inhibitors (e.g., Sildenafil)

The interaction between inhaled nitric oxide and phosphodiesterase-5 (PDE5) inhibitors, such as sildenafil (Viagra®) and tadalafil (Cialis®), is one of the most clinically significant and mechanistically illustrative.

Mechanism

These two drug classes act on different points of the same critical signaling pathway. Inhaled nitric oxide works by stimulating the enzyme soluble guanylate cyclase (sGC) to increase the production of cyclic guanosine monophosphate (cGMP).[4] PDE5 inhibitors, conversely, work by blocking the action of the phosphodiesterase type 5 enzyme, which is responsible for the

metabolic degradation of cGMP.[13]

When used concurrently, these actions are powerfully synergistic. The increased production of cGMP by iNO is combined with the blocked degradation by the PDE5 inhibitor. This leads to a profound and sustained accumulation of intracellular cGMP, resulting in marked and often excessive smooth muscle relaxation and vasodilation.[14]

Clinical Implications

The clinical context entirely dictates whether this potent interaction is a dangerous contraindication or a therapeutic strategy.

• Outpatient Contraindication: In the outpatient setting, such as for the treatment of erectile dysfunction, the co-administration of PDE5 inhibitors with nitrates (like nitroglycerin, which are nitric oxide donors) is generally considered absolutely contraindicated.[38] The risk of an unpredictable, severe, and potentially life-threatening drop in systemic blood pressure is unacceptably high in an unmonitored environment.[39]
• Critical Care Strategy: In contrast, within the controlled environment of an intensive care unit, this same synergistic effect can be carefully harnessed. For patients with severe, refractory pulmonary hypertension, the combination of inhaled nitric oxide and an oral or intravenous PDE5 inhibitor is used investigationally.[40] The goal is often to achieve a level of pulmonary vasodilation that neither agent can accomplish alone, or to use the PDE5 inhibitor to facilitate weaning from iNO and prevent rebound hypertension.[40] This practice is only considered safe under conditions of continuous, invasive hemodynamic monitoring (e.g., an arterial line for real-time blood pressure) and the immediate availability of vasopressor support to manage any systemic hypotension.[40] The risk is the same, but the ability to anticipate and manage that risk transforms the clinical decision.

Interaction with Prostacyclin Analogues and Other Vasodilators

Nitric oxide can also interact with other classes of vasodilators, most notably prostacyclin analogues.

Mechanism

Prostacyclin analogues, such as epoprostenol, iloprost, and treprostinil, are also potent pulmonary vasodilators. However, they work through a distinct but parallel signaling pathway. They bind to prostacyclin (IP) receptors on the smooth muscle cell surface, which activates adenylyl cyclase to increase the production of a different second messenger, cyclic adenosine monophosphate (cAMP).[15] Like cGMP, cAMP also promotes smooth muscle relaxation. When iNO (acting via cGMP) is combined with a prostacyclin (acting via cAMP), their effects on vasodilation can be additive or synergistic.

Clinical Implications

The combination of iNO and an inhaled or intravenous prostacyclin analogue is sometimes used in patients with severe pulmonary hypertension who do not respond adequately to single-agent therapy.[45] Experimental studies have shown that combining iNO with a prostacyclin analogue like ciloprost can decrease pulmonary pressures more significantly than either drug used alone.[45] However, this combination also carries an increased risk of systemic hypotension due to the additive vasodilatory effects, and requires close hemodynamic monitoring.[46]

Table 4: Clinically Significant Drug Interactions with Inhaled Nitric Oxide

Interacting Drug Class	Example Drug(s)	Mechanism of Interaction	Clinical Effect	Clinical Recommendation / Management	Source(s)
Phosphodiesterase-5 (PDE5) Inhibitors	Sildenafil, Tadalafil	iNO increases cGMP production; PDE5 inhibitors block cGMP degradation.	Synergistic and profound vasodilation, leading to severe hypotension.	Contraindicated in unmonitored settings. May be used together with extreme caution and continuous hemodynamic monitoring in critical care to treat severe PH or facilitate iNO weaning.	14
Prostacyclin Analogues	Epoprostenol, Iloprost, Treprostinil	iNO acts via the cGMP pathway; prostacyclins act via the parallel cAMP pathway.	Additive or synergistic vasodilation.	May be used in combination for severe, refractory PH. Increases the risk of hypotension; requires close hemodynamic monitoring.	15
Nitrate Vasodilators (NO Donors)	Nitroglycerin, Isosorbide Dinitrate	Both agents serve as sources of nitric oxide, leading to an increased NO load.	Additive vasodilatory effects and increased risk of hypotension.	Combination with iNO is not standard practice and may be redundant. Experimental studies suggest iNO is more effective than i.v. nitroglycerin for pulmonary vasodilation.	45

Comparative Analysis of Pulmonary Vasodilator Therapies

Inhaled nitric oxide does not exist in a therapeutic vacuum. It is one of several advanced therapies available for the management of pulmonary hypertension. The choice among these agents depends on a complex interplay of factors including the specific clinical indication (acute vs. chronic), patient characteristics, desired selectivity, institutional capabilities, cost, and risk tolerance. Placing iNO in this comparative landscape is essential for understanding its specific niche in modern medicine.

The primary advantage that has secured iNO's place in critical care, particularly for neonatal PPHN, is its unmatched combination of potent pulmonary selectivity and extremely rapid titratability. While other inhaled agents can also achieve pulmonary selectivity, the fact that iNO is a true gas administered via a precision delivery system allows for near-instantaneous onset of action when therapy is initiated or increased, and a similarly rapid offset when the concentration is reduced or stopped. This level of real-time control is a significant clinical advantage in managing hemodynamically unstable patients, where rapid fluctuations in pulmonary pressure can have immediate life-threatening consequences. This unique pharmacokinetic profile as a titratable gas offers a degree of control that nebulized or systemic alternatives cannot fully replicate, often justifying its substantial cost and complexity in these highly acute scenarios.

Inhaled Prostacyclins (e.g., epoprostenol, iloprost) represent the most direct competitors to iNO for acute pulmonary vasodilation. Like iNO, they can be delivered via inhalation to achieve pulmonary selectivity.[15] Their primary mechanism involves the cAMP pathway, offering an alternative biological target.[15] The most significant advantage of inhaled prostacyclins is their dramatically lower cost compared to iNO.[15] They are also generally easier to administer, as they can be nebulized into a ventilator circuit without the need for the complex, proprietary delivery and monitoring systems required for iNO. Clinical studies have shown that their efficacy is largely comparable to iNO, though with some nuances; some evidence suggests iNO may produce greater improvements in oxygenation (V/Q matching), while inhaled prostacyclins may lead to greater reductions in pulmonary vascular resistance.[15] Their main disadvantages include a longer duration of action compared to the near-instantaneous offset of iNO, and a potential for platelet inhibition.[15]

Phosphodiesterase Inhibitors (e.g., sildenafil), are primarily administered orally or intravenously for the chronic management of pulmonary arterial hypertension.[13] As systemic agents, they lack the pulmonary selectivity of inhaled therapies and carry a risk of systemic hypotension.[15] Their role in the acute setting is typically as an adjunct to inhaled therapies, often to augment their effects or to facilitate weaning from iNO.[40] They offer the advantage of oral administration for long-term treatment, a domain where iNO is not applicable.

Endothelin Receptor Antagonists (e.g., bosentan, ambrisentan) are another class of oral agents used for the long-term management of pulmonary arterial hypertension.[44] They work by blocking the effects of endothelin-1, a potent endogenous vasoconstrictor.[44] Like PDE5 inhibitors, they are systemic therapies intended for chronic use and are not used for acute, titratable management of pulmonary hypertension in the ICU.

For more stable or chronic conditions, or in resource-limited settings, the cost-benefit analysis shifts dramatically away from iNO and in favor of these alternatives. However, for the acute management of a critically ill neonate with PPHN, the precise control afforded by iNO often remains the preferred, albeit expensive, initial strategy.

Table 5: Comparative Overview of Major Pulmonary Vasodilator Therapies

Therapy	Mechanism of Action	Route(s)	Selectivity	Key Advantages	Key Disadvantages / Risks	Primary Clinical Use	Source(s)
Inhaled Nitric Oxide (iNO)	Increases cGMP production	Inhalation (Gas)	Pulmonary	Rapid onset/offset, highly titratable, improves V/Q matching.	High cost, complex delivery system, rebound PH, MetHb, NO2​ toxicity.	Acute (Neonatal PPHN, rescue therapy)	15
Inhaled Prostacyclins	Increases cAMP production	Inhalation (Nebulized)	Pulmonary	Low cost, easier administration, comparable efficacy to iNO.	Less titratable than iNO, rebound PH, platelet inhibition.	Acute (Alternative to iNO)	15
Phosphodiesterase-5 Inhibitors	Blocks cGMP degradation	Oral, IV	Systemic	Oral administration for long-term use, can augment inhaled therapies.	Systemic hypotension, not selective, slower onset.	Chronic (PAH), Acute (Adjunct)	13
Endothelin Receptor Antagonists	Blocks endothelin-1 receptors	Oral	Systemic	Oral administration for long-term use, improves exercise capacity in PAH.	Hepatotoxicity (bosentan), teratogenic, not for acute use.	Chronic (PAH)	44

Regulatory Status and Future Perspectives

The trajectory of nitric oxide as a therapeutic agent is one of evolution, from a single, highly specialized application towards a broad platform with diverse potential. This progression is reflected in its regulatory history, its expanding commercial landscape, and the exciting frontiers of ongoing research.

Regulatory History and Commercial Landscape

Inhaled nitric oxide, under the brand name INOmax®, was first granted approval by the U.S. FDA on December 23, 1999.[21] The indication was, and remains, very specific: for term and near-term (>34 weeks gestation) neonates with hypoxic respiratory failure associated with pulmonary hypertension.[18] This long-standing and narrow approval underscores the high bar for evidence required by regulatory agencies and the historical difficulty in demonstrating a definitive mortality benefit in other patient populations like adult ARDS.

Over time, the commercial landscape has expanded. While INOmax® (sponsored by INO Therapeutics, Inc.) remains a prominent brand, other systems for delivering inhaled nitric oxide have entered the market, including Genosyl®, Ulspira®, and Noxivent®.[28] This indicates a competitive, albeit highly specialized, market for this therapy.

A significant development signaling the expansion of nitric oxide-based therapies occurred in January 2024, with the FDA approval of Zelsuvmi™ (berdazimer sodium).[50] This topical gel is a nitric oxide-releasing agent indicated for the treatment of molluscum contagiosum, a common viral skin infection. The approval of a topical formulation for a dermatological condition represents a major paradigm shift, demonstrating that the therapeutic potential of nitric oxide is not limited to inhalation for cardiopulmonary disease and opening the door for novel delivery mechanisms targeting a wide array of pathologies.

The Therapeutic Frontier: Emerging Research and Novel Applications

The future of nitric oxide therapy is being actively shaped by cutting-edge research that is challenging old paradigms and exploring entirely new applications.

Novel Signaling Paradigms

For decades, the prevailing model assumed that nitric oxide signaled exclusively as a free, highly reactive gas. However, recent research from the Karolinska Institutet and others has challenged this view.[9] A new hypothesis proposes that nitric oxide may bond with freely available heme groups in endothelial cells to form a more stable and mobile compound, NO-ferroheme.[9] This NO-ferroheme complex may be the true signaling entity that travels from the endothelium to the smooth muscle to activate sGC.[9] This fundamental shift in understanding the chemistry of NO signaling could pave the way for the development of entirely new classes of cardiovascular drugs that are more stable and targeted than current NO donors.[9]

Oncology

The role of nitric oxide in cancer is remarkably complex and appears to be bimodal and concentration-dependent.[52] At low concentrations, such as those that might be found in a tumor microenvironment, NO can be pro-tumorigenic, promoting angiogenesis, cell proliferation, and resistance to apoptosis.[54] Conversely, at high concentrations, nitric oxide is cytotoxic and can induce apoptosis in cancer cells and sensitize them to conventional chemotherapy and immunotherapy.[52] This dual role has led to two distinct therapeutic strategies. One involves using NOS inhibitors to block the pro-tumor effects of low-level NO.[54] The other, more active area of clinical research, involves delivering high concentrations of NO to tumors. Clinical trials are currently underway investigating the use of NO donors like nitroglycerin patches to enhance chemotherapy response in non-small cell lung cancer [56] and, more radically, the direct intratumoral injection of ultra-high concentration nitric oxide (UNO) as a novel immunotherapy for solid tumors.[57] Early results from a first-in-human study of UNO showed a promising safety profile and evidence of immune system activation.[57]

Infectious Diseases

Building on the known antimicrobial properties of nitric oxide, which is a key part of the body's innate immune defense [13], researchers are exploring its use as a direct anti-infective agent. A recent randomized clinical study investigated the use of intermittent high-dose (150 ppm) inhaled nitric oxide for the treatment of viral pneumonia, including that caused by COVID-19.[59] The results were promising, showing the therapy was safe and well-tolerated and was associated with faster recovery and reduced duration of oxygen support.[59] This has spurred further investigation into iNO for other severe lung infections, such as those caused by nontuberculous mycobacteria (NTM).[59]

This wave of innovation—from new delivery systems and chemical forms to entirely new therapeutic targets—signals that nitric oxide therapy is evolving from a single, niche drug into a broad and versatile therapeutic platform.

Conclusion and Expert Recommendations

Nitric oxide stands as a testament to the power of translational medicine, bridging the gap from fundamental biochemistry to life-saving clinical intervention. It is a high-risk, high-reward agent whose unique capacity for selective pulmonary vasodilation has firmly established its value in the narrow but critical field of neonatal respiratory care. Its story is one of elegant physiology, where targeted delivery and rapid pharmacokinetics allow clinicians to supplement a vital endogenous pathway with remarkable precision.

However, the therapy is not without substantial challenges. The high cost, the requirement for complex and proprietary delivery systems, and the constant need for vigilant monitoring of toxic byproducts have limited its accessibility and application. Most significantly, the compelling physiological rationale for its use has led to a wide "evidence-practice gap," where its off-label application in conditions like adult ARDS is common despite a consistent failure to demonstrate improvement in patient-centered outcomes such as mortality in high-level clinical trials. This disconnect highlights the crucial distinction between improving a physiological parameter and improving a patient's overall outcome.

The future of nitric oxide therapy is unlikely to be a simple expansion of its current use. Instead, it is evolving into a multifaceted therapeutic platform. The development of novel delivery systems—from topical gels to intratumoral injectors to NO-releasing biomaterials—promises to untether the molecule's potential from the confines of the ventilator circuit. Emerging research in oncology and infectious diseases is opening entirely new therapeutic frontiers, harnessing the cytotoxic and antimicrobial properties of high-concentration NO. Furthermore, fundamental discoveries in its signaling mechanisms, such as the role of NO-ferroheme, may lead to a new generation of more stable and effective cardiovascular drugs.

Based on this comprehensive analysis, the following expert recommendations are proposed:

1. Prioritize Rigorous Clinical Trials for Off-Label Uses: There is an urgent need for well-designed, adequately powered clinical trials to definitively determine the role, if any, of inhaled nitric oxide in common off-label indications like adult ARDS and perioperative cardiac care. These trials must focus on hard, patient-centered endpoints such as mortality, duration of ventilation, and long-term functional outcomes, rather than relying solely on transient improvements in oxygenation. Closing the evidence-practice gap is essential for ensuring patient safety and promoting responsible resource allocation.
2. Invest in Developing Lower-Cost and Simpler Delivery Systems: The high cost and complexity of current iNO delivery systems are major barriers to its wider and more equitable use. Investment in developing safe, reliable, and more affordable methods of generating and delivering medical-grade nitric oxide at the bedside could democratize access to this therapy, particularly for alternative agents like inhaled prostacyclins that have already demonstrated a significant cost advantage.
3. Support Continued Basic and Translational Research: The future potential of nitric oxide therapy lies in innovation. Continued funding and support for basic science research into its complex biological roles—from its signaling chemistry to its bimodal effects in cancer—are critical. Translational research focused on novel delivery technologies and new therapeutic applications in oncology, infectious disease, and chronic cardiovascular conditions should be a high priority, as this is where the next breakthroughs are most likely to emerge.

In conclusion, nitric oxide is a simple molecule with a profoundly complex and evolving story in medicine. While its current role is important but niche, its future potential is vast. By addressing the limitations of current therapy and continuing to explore its fundamental biology, the medical community can hope to fully unlock the therapeutic power of this remarkable gas.

Works cited

1. NITRIC OXIDE | 10102-43-9 - ChemicalBook, accessed July 18, 2025, https://www.chemicalbook.com/ChemicalProductProperty_EN_CB5433122.htm
2. Nitric Oxide - Advanced Specialty Gases, accessed July 18, 2025, https://advancedspecialtygases.com/nitric-oxide/
3. Nitric oxide therapy in the neonate: guideline for the use of inhaled nitric oxide | NHSGGC, accessed July 18, 2025, https://clinicalguidelines.scot.nhs.uk/ggc-paediatric-guidelines/ggc-paediatric-guidelines/neonatology/nitric-oxide-therapy-in-the-neonate-guideline-for-the-use-of-inhaled-nitric-oxide/
4. Nitric Oxide Monograph for Professionals - Drugs.com, accessed July 18, 2025, https://www.drugs.com/monograph/nitric-oxide.html
5. Nitric Oxide - Hazardous Substance Fact Sheet, accessed July 18, 2025, https://www.nj.gov/health/eoh/rtkweb/documents/fs/1357.pdf
6. Nitric oxide - IDLH | NIOSH - CDC, accessed July 18, 2025, https://www.cdc.gov/niosh/idlh/10102439.html
7. Nitric Oxide | Basic & Clinical Pharmacology, 14e - AccessMedicine, accessed July 18, 2025, https://accessmedicine.mhmedical.com/content.aspx?bookid=2249§ionid=175218435
8. Inhaled Nitric Oxide Therapy for Pulmonary Disorders of the Term and Preterm Infant - PMC, accessed July 18, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC5065760/
9. Researchers present novel principle for nitric oxide-mediated ..., accessed July 18, 2025, https://www.sciencedaily.com/releases/2023/09/230914114658.htm
10. Nitric Oxide - StatPearls - NCBI Bookshelf, accessed July 18, 2025, https://www.ncbi.nlm.nih.gov/books/NBK554485/
11. Inhaled nitric oxide use in newborns | Canadian Paediatric Society, accessed July 18, 2025, https://cps.ca/en/documents/position/inhaled-nitric-oxide
12. A Review of Inhaled Nitric Oxide in the Hypoxic Newborn | Respiratory Therapy, accessed July 18, 2025, https://respiratory-therapy.com/public-health/smoking/tobacco/a-review-of-inhaled-nitric-oxide-in-the-hypoxic-newborn/
13. Biological functions of nitric oxide - Wikipedia, accessed July 18, 2025, https://en.wikipedia.org/wiki/Biological_functions_of_nitric_oxide
14. How does Nitroglycerin Interact with Type 5 Phosphodiesterase Inhibitors (PDE5; Avanafil, Sildenafil, Tadalafil, Vardenafil) to Cause Hypotension? - EBM Consult, accessed July 18, 2025, https://www.ebmconsult.com/articles/nitrates-ntg-pde-inhibitors-drug-interaction-mechanism-blood-pressure
15. Alternatives to nitric oxide, accessed July 18, 2025, https://academic.oup.com/bmb/article-pdf/70/1/119/25152100/ldh028.pdf
16. Nitric oxide: Clinical applications in critically ill patients - PMC - PubMed Central, accessed July 18, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC10189363/
17. Nitric Oxide Gas: Respiratory Uses, Warnings, Side Effects, Dosage - MedicineNet, accessed July 18, 2025, https://www.medicinenet.com/nitric_oxide_gas/article.htm
18. This label may not be the latest approved by FDA. For current ..., accessed July 18, 2025, https://www.accessdata.fda.gov/drugsatfda_docs/label/2019/020845s020lbl.pdf
19. Dosing, Administration, & Weaning | INOmax® (nitric oxide) gas, for ..., accessed July 18, 2025, https://www.inomax.com/dosing-admin-weaning/
20. Nitric Oxide Therapy: Uses, How It's Done, Benefits, and Risks - Healthline, accessed July 18, 2025, https://www.healthline.com/health/nitric-oxide-therapy-in-newborns
21. Drug Approval Package: INOmax (Nitric Oxide) NDA# 20-845 - accessdata.fda.gov, accessed July 18, 2025, https://www.accessdata.fda.gov/drugsatfda_docs/nda/99/20845_INOmax.cfm
22. Operating Instructions for Inhaled Nitric Oxide Therapy With Mechanical Ventilation, accessed July 18, 2025, https://www.utmb.edu/policies_and_procedures/19222819
23. Influence of inhaled nitric oxide on bronchopulmonary dysplasia in preterm infants with PPHN or HRF at birth: a propensity score matched study - Frontiers, accessed July 18, 2025, https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2024.1515030/full
24. A Comprehensive Review of Inhaled Nitric Oxide Therapy: Current Trends, Challenges, and Future Directions - PubMed Central, accessed July 18, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC10913844/
25. Oxygen Deficiency Completed Phase 1 / 2 Trials for Nitric Oxide (DB00435) - DrugBank, accessed July 18, 2025, https://go.drugbank.com/indications/DBCOND0081250/clinical_trials/DB00435?phase=1%2C2&status=completed
26. Cardiac Arrest Recruiting Phase 2 Trials for Nitric Oxide (DB00435) | DrugBank Online, accessed July 18, 2025, https://go.drugbank.com/indications/DBCOND0031003/clinical_trials/DB00435?phase=2&status=recruiting
27. Pulmonary Fibrosis Unknown Status Phase 2 Trials for Nitric Oxide (DB00435) - DrugBank, accessed July 18, 2025, https://go.drugbank.com/indications/DBCOND0027985/clinical_trials/DB00435?phase=2&status=unknown_status
28. Nitric Oxide Dosage Guide + Max Dose, Adjustments - Drugs.com, accessed July 18, 2025, https://www.drugs.com/dosage/nitric-oxide.html
29. INOmax, Genosyl - nitric oxide gas (Rx) - Medscape Reference, accessed July 18, 2025, https://reference.medscape.com/drug/inomax-genosyl-nitric-oxide-gas-343457
30. Nitric oxide (inhalation route) - Side effects & uses - Mayo Clinic, accessed July 18, 2025, https://www.mayoclinic.org/drugs-supplements/nitric-oxide-inhalation-route/description/drg-20060881
31. Nitric Oxide Side Effects: Common, Severe, Long Term - Drugs.com, accessed July 18, 2025, https://www.drugs.com/sfx/nitric-oxide-side-effects.html
32. Nitric Oxide | Memorial Sloan Kettering Cancer Center, accessed July 18, 2025, https://www.mskcc.org/cancer-care/patient-education/medications/adult/nitric-oxide
33. www.mayoclinic.org, accessed July 18, 2025, https://www.mayoclinic.org/drugs-supplements/nitric-oxide-inhalation-route/description/drg-20060881#:~:text=Symptoms%20include%3A%20bluish%20lips%20or,blood%20while%20receiving%20this%20medicine.
34. Nitrous oxide: Uses, Interactions, Mechanism of Action | DrugBank Online, accessed July 18, 2025, https://go.drugbank.com/drugs/DB06690
35. Nitrous oxide Interactions Checker - Drugs.com, accessed July 18, 2025, https://www.drugs.com/drug-interactions/nitrous-oxide.html
36. Sildenafil: Uses, Interactions, Mechanism of Action | DrugBank Online, accessed July 18, 2025, https://go.drugbank.com/drugs/DB00203
37. Phosphodiesterase regulation of nitric oxide signaling - Oxford Academic, accessed July 18, 2025, https://academic.oup.com/cardiovascres/article/75/2/303/298750
38. Viagra (sildenafil) and Nitrates: Can you mix them? - Ro, accessed July 18, 2025, https://ro.co/erectile-dysfunction/viagra-nitrates/
39. Viagra and Nitrates: Why These Medications Don't Mix | Good Health by Hims, accessed July 18, 2025, https://www.hims.com/blog/viagra-nitrates-why-these-medications-dont-mix
40. Nitric oxide and sildenafil Interactions - Drugs.com, accessed July 18, 2025, https://www.drugs.com/drug-interactions/nitric-oxide-with-sildenafil-1723-0-2061-0.html?professional=1
41. Nitric Oxide for Pulmonary Hypertension: Uses, Drug Interactions, and Blood Thinners, accessed July 18, 2025, https://www.myphteam.com/resources/nitric-oxide-for-pulmonary-hypertension
42. Combined therapy with zaprinast and inhaled nitric oxide abolishes hypoxic pulmonary hypertension - PubMed, accessed July 18, 2025, https://pubmed.ncbi.nlm.nih.gov/10921573/
43. Prostacyclin in PAH - Ventavis, accessed July 18, 2025, https://www.4ventavis.com/hcp_prostacyclin_in_pah.html
44. Therapy for Pulmonary Arterial Hypertension: Glance on Nitric Oxide Pathway - Frontiers, accessed July 18, 2025, https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2021.767002/full
45. Combination of inhaled nitric oxide with i.v. nitroglycerin or with a prostacyclin analogue in the treatment of experimental pulmonary hypertension - PubMed, accessed July 18, 2025, https://pubmed.ncbi.nlm.nih.gov/8881631/
46. Iloprost: Uses, Interactions, Mechanism of Action | DrugBank Online, accessed July 18, 2025, https://go.drugbank.com/drugs/DB01088
47. Alternatives to nitric oxide - PubMed, accessed July 18, 2025, https://pubmed.ncbi.nlm.nih.gov/15531733/
48. Nitric oxide - brand name list from Drugs.com, accessed July 18, 2025, https://www.drugs.com/ingredient/nitric-oxide.html
49. Nitric oxide Alternatives Compared - Drugs.com, accessed July 18, 2025, https://www.drugs.com/compare/nitric-oxide
50. Zelsuvmi (berdazimer sodium) FDA Approval History - Drugs.com, accessed July 18, 2025, https://www.drugs.com/history/zelsuvmi.html
51. Nitric Oxide Signaling and Regulation in the Cardiovascular System: Recent Advances, accessed July 18, 2025, https://pubmed.ncbi.nlm.nih.gov/38866562/
52. Novel therapeutic applications of nitric oxide donors in cancer: roles in chemo- and immunosensitization to apoptosis and inhibition of metastases - PubMed, accessed July 18, 2025, https://pubmed.ncbi.nlm.nih.gov/18477483/
53. Current Advances of Nitric Oxide in Cancer and Anticancer Therapeutics - PubMed Central, accessed July 18, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC7912608/
54. Targeting Nitric Oxide: Say NO to Metastasis | Clinical Cancer Research - AACR Journals, accessed July 18, 2025, https://aacrjournals.org/clincancerres/article/29/10/1855/726245/Targeting-Nitric-Oxide-Say-NO-to
55. Nitric oxide for cancer therapy - PMC, accessed July 18, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC5137992/
56. Study Details | Nitroglycerin in Non-small Cell Lung Cancer | ClinicalTrials.gov, accessed July 18, 2025, https://clinicaltrials.gov/study/NCT01210378
57. Beyond Cancer™ Presents Positive First-in-Human Clinical Data for Ultra-High Concentration Nitric Oxide (UNO) Therapy in Solid Tumors During the Society for Immunotherapy of Cancer (SITC) 2023 Annual, accessed July 18, 2025, https://beyondcancer.com/news/beyond-cancer-presents-positive-first-in-human-clinical-data-for-ultra-high-concentration-nitric-oxide-uno-therapy-in-solid-tumors-during-the-society-for-immunotherapy-of-cancer-sitc-2023/
58. A Study to Assess the Safety and Efficacy of Nitric Oxide Injection Into Unresectable Solid Primary or Metastatic Tumors | Clinical Research Trial Listing - CenterWatch, accessed July 18, 2025, https://www.centerwatch.com/clinical-trials/listings/NCT05351502/a-study-to-assess-the-safety-and-efficacy-of-nitric-oxide-injection-into-unresectable-solid-primary-or-metastatic-tumors
59. Beyond Air® Publishes Peer-Reviewed Journal Article in Scientific Reports with Clinical Data Showing Nitric Oxide was Safe and Beneficial Adjunct Therapy for Subjects with Viral Pneumonia, accessed July 18, 2025, https://www.beyondair.net/news-events/press-releases/beyond-air-publishes-peer-reviewed-journal-article-in-scientific-reports-with-clinical-data-showing-nitric-oxide-was-safe-and-beneficial-adjunct