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\title{Inhaled Nitric Oxide and COVID-19}



\author[1]{Lou Ignarro}%
\affil[1]{UCLA Life Sciences}%


\vspace{-1em}



  
  \date{March 14, 2025}


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Nitric oxide (NO) is a unique signaling molecule in the mammalian
species. NO is produced by a variety of cell types to elicit distinct
physiological actions. In the vascular system, NO is produced by the
endothelium, a single layer of cells forming the inner lining of all
blood vessels. Endothelium-derived NO has several different functions,
one of which is vascular smooth muscle relaxation, resulting in
vasodilation and a consequent decrease in blood pressure and increase in
local blood flow. In the erectile tissue, NO is released as a
neurotransmitter from the nerves innervating the corpus cavernosum
during sexual stimulation, and causes profound smooth muscle relaxation
and increased blood flow to the erectile tissue. This results in
engorgement with blood and consequent penile erection.

The uniqueness of NO as a signaling molecule derives, at least in part,
by the fact that it is a gaseous molecule in its native state. However,
despite being a gas, NO, like oxygen (O\textsubscript{2}), elicits its
pharmacological effects as a solute in aqueous solution. Another unique
characteristic of NO is its fleeting action because of its highly
unstable chemical nature and reactivity. Unlike many other signaling
molecules, NO elicits its wise array of physiological effects by
distinct mechanisms. For example, vascular and nonvascular smooth muscle
relaxation, and inhibition of platelet function are mediated by
intracellular cyclic GMP (cyclic 3', 5'-guanosine monophosphate). NO
elicits many cyclic GMP-independent effects as well. For example, nitric
oxide is a reactive free radical that can covalently modify protein
function. One good example is protein S-nitrosylation, which can result
in both regulatory and aberrant effects. By this and a variety of other
mechanisms, NO also reacts with other molecules, such as reactive oxygen
species, in invading cells such as bacteria, parasites and viruses to
kill them or inhibit their replication or spread.

The first pharmacological action of nitric oxide, demonstrated several
years before it's production in mammals was actually discovered, was
vascular and nonvascular smooth muscle relaxation. One of many examples
of the latter is the smooth muscle enveloping the sinusoidal cavities
within the corpus cavernosum. Another important example is the airway
smooth muscle in the trachea and bronchioles of the lungs. Indeed,
inhalation of NO gas causes bronchodilation and increased delivery of
air into the lungs. However, perhaps more significant than the
bronchodilator effect of inhaled NO is its vasodilator effect. In fact,
advantage was taken of the vasodilator action of NO in the lungs by
Warren Zapol, MD, from the Massachusetts General Hospital in Boston, who
discovered that inhalation of very small amounts of NO gas by newborn
babies with life-threatening, persistent pulmonary hypertension (PPHN)
results in a dramatic and permanent reversal of pulmonary
vasoconstriction. Inhaled NO (INO) literally turned blue babies into
pink babies. Without INO, most babies would have died while others would
have required highly invasive procedures (extracorporeal membrane
oxygenation; ECMO) to oxygenate their lungs, and may not have survived.

Regarding its antiviral action, NO has been shown to increase the
survival rate of mammalian cells infected with SARS-CoV (Severe Acute
Respiratory Syndrome caused by coronavirus). In an \emph{in vitro}study,
NO donors (i.e., S-nitroso-N-acetylpenicillamine) greatly increased the
survival rate of SARS-CoV-infected eukaryotic cells, suggesting direct
antiviral effects of NO (1). In this study, NO significantly inhibited
the replication cycle of SARS CoV in a concentration-dependent manner.
NO also inhibited viral protein and RNA synthesis. Furthermore, NO
generated by inducible nitric oxide synthase inhibited the SARS CoV
replication cycle. The coronavirus responsible for SARS-CoV shares most
of the genome of COVID- 19 indicating potential effectiveness of inhaled
NO therapy in these patients.

In 2004, during the SARS-CoV outbreak in China, the administration of
inhaled NO reversed pulmonary hypertension, improved severe hypoxia and
shortened the length of ventilatory support as compared to matched
control patients with SARS-CoV (2). The mechanism of action was thought
to be pulmonary vasodilation and consequent improved oxygenation in the
blood of the lungs, thereby killing the virus, which does not do well in
a high oxygen environment. In addition, however, I would offer the
opinion that the NO also interacts directly with the virus to kill it
and/or inhibit its replication, as shown in a prior study (1).

Although studies have not yet been reported with COVID-19, NO has been
shown to have an antiviral effect on several DNA and RNA virus families
(3). The NO-mediated S-nitrosylation of viral molecules might be an
intriguing general mechanism for the control of the virus life cycle. In
this regard, it is conceivable that NO could nitrosylate
cysteine-containing enzymes and proteins, including nucleocapsid
proteins and glycoproteins, present in the coronavirus.

In view of the knowledge gained by treating SARS-CoV patients with INO,
it follows that INO might be effective in patients with the current SARS
CoV-2 (COVID-19) infection. Indeed, a clinical trial of inhaled nitric
oxide in patients with moderate to severe COVID-19 with pneumonia and
under assisted ventilatory support recently received IRB (Institutional
Review Board) approval at the Massachusetts General Hospital. Warren
Zapol is director of this project. This trial has now been expanded to
include at least two additional hospitals in the U.S. In the successful
treatment of persistent pulmonary hypertension in newborns, the amount
of NO inhaled is generally one ppm (part per million). In the clinical
trial using COVID-19 patients, the amount of NO will be approximately
100-fold higher, about 100 ppm. This is a safe dose of INO, which could
prove to be effective in killing the virus and allowing recovery of the
patient. The effective use of INO would also lessen the need for oxygen,
ventilators, and beds in the ICU.

One thing I urge everyone to practice during this coronavirus pandemic
is to breathe or inhale through your NOSE and exhale through your mouth.
Swedish investigators at the Karolinska Institute in Stockholm have
shown that the cells and tissues in the nasal sinusoids, but not the
mouth, constantly and continuously produce nitric oxide, which is a gas,
and can be easily detected in the exhaled breath. The physiological
significance of this is that nasally-derived NO, when inhaled through
the nose, improves oxygen delivery into the lungs by causing
bronchodilation. This physiological action of inhaled NO is well-known
by competitive athletes, especially runners. Moreover, when inhaling
through the nose, your nasal nitric oxide is inhaled into your lungs
where it stands a chance of meeting up with the coronavirus particles
and killing them or inhibiting their replication. Inhaling through your
mouth will NOT accomplish this. By the same token, exhaling through your
nose is highly wasteful in that you would be expelling the NO away from
the lungs, where it is needed most.

``INHALE THROUGH YOUR NOSE, AND EXHALE THROUGH YOUR MOUTH!''

\textbf{Key References:}

1. Keyaerts E, Vijgen L, Chen L, Maes P, Hedenstierna G, Van Ranst M.
Inhibition of SARS-coronavirus infection in vitro by
S-nitroso-N-acetylpenicillamine, a nitric oxide donor compound. Int J
Infect Dis. 2004 Jul;8(4):223-6.

2. Chen L, Liu P, Gao H, Sun B, Chao D, Wang F, Zhu Y, Hedenstierna G,
Wang CG. Inhalation of nitric oxide in the treatment of severe acute
respiratory syndrome: a rescue trial in Beijing. Clin Infect Dis. 2004
Nov 15;39(10):1531-5. Epub 2004 Oct 22.

3. Colasanti M,~Persichini T,~Venturini G,~Ascenzi P. S-nitrosylation of
viral proteins: molecular basis for antiviral effect of nitric oxide.
IUBMB Life.~1999 Jul;48(1):25-31.

\textbf{Author contact information:}

Louis J. Ignarro, PhD

Distinguished Professor Emeritus

UCLA School of Medicine

Mailing address:

264 El Camino Drive,

Beverly Hills, CA 90212 USA

Email: lignarro@gmail.com

Telephone: 1-310-995-3500

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