CLINICAL PHARMACOLOGY
Mechanism Of Action
Nitric oxide relaxes vascular
smooth muscle by binding to the heme moiety of cytosolic guanylate cyclase,
activating guanylate cyclase and increasing intracellular levels of cyclic
guanosine 3',5'-monophosphate, which then leads to vasodilation. When inhaled,
nitric oxide selectively dilates the pulmonary vasculature, and because of
efficient scavenging by hemoglobin, has minimal effect on the systemic vasculature.
INOmax appears to increase the partial pressure of
arterial oxygen (PaO2) by dilating pulmonary vessels in better ventilated areas
of the lung, redistributing pulmonary blood flow away from lung regions with
low ventilation/perfusion (V/Q) ratios toward regions with normal ratios.
Pharmacodynamics
Effects On Pulmonary Vascular Tone In PPHN
Persistent pulmonary hypertension of the newborn (PPHN)
occurs as a primary developmental defect or as a condition secondary to other
diseases such as meconium aspiration syndrome (MAS), pneumonia, sepsis, hyaline
membrane disease, congenital diaphragmatic hernia (CDH), and pulmonary
hypoplasia. In these states, pulmonary vascular resistance (PVR) is high, which
results in hypoxemia secondary to right-to-left shunting of blood through the
patent ductus arteriosus and foramen ovale. In neonates with PPHN, INOmax
improves oxygenation (as indicated by significant increases in PaO2).
Pharmacokinetics
The pharmacokinetics of nitric oxide has been studied in
adults.
Absorption And Distribution
Nitric oxide is absorbed systemically after inhalation.
Most of it traverses the pulmonary capillary bed where it combines with
hemoglobin that is 60% to 100% oxygen-saturated. At this level of oxygen
saturation, nitric oxide combines predominantly with oxyhemoglobin to produce
methemoglobin and nitrate. At low oxygen saturation, nitric oxide can combine
with deoxyhemoglobin to transiently form nitrosylhemoglobin, which is converted
to nitrogen oxides and methemoglobin upon exposure to oxygen. Within the
pulmonary system, nitric oxide can combine with oxygen and water to produce
nitrogen dioxide and nitrite, respectively, which interact with oxyhemoglobin
to produce methemoglobin and nitrate. Thus, the end products of nitric oxide
that enter the systemic circulation are predominantly methemoglobin and
nitrate.
Metabolism
Methemoglobin disposition has been investigated as a
function of time and nitric oxide exposure concentration in neonates with
respiratory failure. The methemoglobin (MetHb) concentration-time profiles
during the first 12 hours of exposure to 0, 5, 20, and 80 ppm INOmax are shown
in Figure 1.
Figure 1: Methemoglobin Concentration-Time Profiles
Neonates Inhaling 0, 5, 20 or 80 ppm INOmax
Methemoglobin concentrations
increased during the first 8 hours of nitric oxide exposure. The mean
methemoglobin level remained below 1% in the placebo group and in the 5 ppm and
20 ppm INOmax groups, but reached approximately 5% in the 80 ppm INOmax group.
Methemoglobin levels >7% were attained only in patients receiving 80 ppm,
where they comprised 35% of the group. The average time to reach peak
methemoglobin was 10 ± 9 (SD) hours (median, 8 hours) in these 13 patients, but
one patient did not exceed 7% until 40 hours.
Elimination
Nitrate has been identified as
the predominant nitric oxide metabolite excreted in the urine, accounting for
>70% of the nitric oxide dose inhaled. Nitrate is cleared from the plasma by
the kidney at rates approaching the rate of glomerular filtration.
Clinical Studies
Treatment Of Hypoxic
Respiratory Failure (HRF)
The efficacy of INOmax has been
investigated in term and near-term newborns with hypoxic respiratory failure
resulting from a variety of etiologies. Inhalation of INOmax reduces the
oxygenation index (OI= mean airway pressure in cm H2O Ã fraction of inspired
oxygen concentration [FiO2]Ã 100 divided by systemic arterial
concentration in mm Hg [PaO2]) and increases PaO2 [see Mechanism of Action].
NINOS Study
The Neonatal Inhaled Nitric Oxide Study (NINOS) was a
double-blind, randomized, placebo-controlled, multicenter trial in 235 neonates
with hypoxic respiratory failure. The objective of the study was to determine
whether inhaled nitric oxide would reduce the occurrence of death and/or
initiation of extracorporeal membrane oxygenation (ECMO) in a prospectively
defined cohort of term or near-term neonates with hypoxic respiratory failure
unresponsive to conventional therapy. Hypoxic respiratory failure was caused by
meconium aspiration syndrome (MAS; 49%), pneumonia/sepsis (21%), idiopathic
primary pulmonary hypertension of the newborn (PPHN; 17%), or respiratory
distress syndrome (RDS; 11%). Infants ≤14 days of age (mean, 1.7 days)
with a mean PaO2 of 46 mm Hg and a mean oxygenation index (OI) of 43 cm H2O /
mm Hg were initially randomized to receive 100% O2 with (n=114) or without
(n=121) 20 ppm nitric oxide for up to 14 days. Response to study drug was
defined as a change from baseline in PaO2 30 minutes after starting treatment
(full response = >20 mm Hg, partial = 10–20 mm Hg, no response = <10 mm
Hg). Neonates with a less than full response were evaluated for a response to
80 ppm nitric oxide or control gas. The primary results from the NINOS study
are presented in Table 1.
Table 1: Summary of Clinical Results from NINOS Study
|
Control
(n=121) |
NO
(n=114) |
P value |
Death or ECMO*† |
77 (64%) |
52 (46%) |
0.006 |
Death |
20 (17%) |
16 (14%) |
0.60 |
ECMO |
66 (55%) |
44 (39%) |
0.014 |
* Extracorporeal membrane oxygenation
† Death or need for ECMO was the study's primary end
point |
Although the incidence of death
by 120 days of age was similar in both groups (NO, 14%; control, 17%),
significantly fewer infants in the nitric oxide group required ECMO compared
with controls (39% vs. 55%, p = 0.014). The combined incidence of death and/or
initiation of ECMO showed a significant advantage for the nitric oxide treated
group (46% vs. 64%, p = 0.006). The nitric oxide group also had significantly
greater increases in PaO2 and greater decreases in the OI and the
alveolar-arterial oxygen gradient than the control group (p<0.001 for all
parameters). Significantly more patients had at least a partial response to the
initial administration of study drug in the nitric oxide group (66%) than the
control group (26%, p<0.001). Of the 125 infants who did not respond to 20
ppm nitric oxide or control, similar percentages of NO-treated (18%) and
control (20%) patients had at least a partial response to 80 ppm nitric oxide
for inhalation or control drug, suggesting a lack of additional benefit for the
higher dose of nitric oxide. No infant had study drug discontinued for
toxicity. Inhaled nitric oxide had no detectable effect on mortality. The
adverse events collected in the NINOS trial occurred at similar incidence rates
in both treatment groups [see ADVERSE REACTIONS]. Follow-up exams were
performed at 18–24 months for the infants enrolled in this trial. In the infants
with available follow-up, the two treatment groups were similar with respect to
their mental, motor, audiologic, or neurologic evaluations.
CINRGI Study
This study was a double-blind, randomized,
placebo-controlled, multicenter trial of 186 term and near-term neonates with
pulmonary hypertension and hypoxic respiratory failure. The primary objective
of the study was to determine whether INOmax would reduce the receipt of ECMO
in these patients. Hypoxic respiratory failure was caused by MAS (35%),
idiopathic PPHN (30%), pneumonia/sepsis (24%), or RDS (8%). Patients with a
mean PaO2 of 54 mm Hg and a mean OI of 44 cm H2O / mm Hg were randomly assigned
to receive either 20 ppm INOmax (n=97) or nitrogen gas (placebo; n=89) in
addition to their ventilatory support. Patients who exhibited a PaO2 >60 mm
Hg and a pH < 7.55 were weaned to 5 ppm INOmax or placebo. The primary
results from the CINRGI study are presented in Table 2.
Table 2: Summary of Clinical Results from CINRGI Study
|
Placebo |
INOmax |
P value |
ECMO*† |
51/89 (57%) |
30/97 (31%) |
<0.001 |
Death |
5/89 (6%) |
3/97 (3%) |
0.48 |
* Extracorporeal membrane oxygenation
† ECMO was the primary end point of this study |
Significantly fewer neonates in
the INOmax group required ECMO compared to the control group (31% vs. 57%,
p<0.001). While the number of deaths were similar in both groups (INOmax,
3%; placebo, 6%), the combined incidence of death and/or receipt of ECMO was
decreased in the INOmax group (33% vs. 58%, p<0.001).
In addition, the INOmax group
had significantly improved oxygenation as measured by PaO2, OI, and
alveolar-arterial gradient (p<0.001 for all parameters). Of the 97 patients
treated with INOmax, 2 (2%) were withdrawn from study drug due to methemoglobin
levels >4%. The frequency and number of adverse events reported were similar
in the two study groups [see ADVERSE REACTIONS].
In clinical trials, reduction
in the need for ECMO has not been demonstrated with the use of inhaled nitric
oxide in neonates with congenital diaphragmatic hernia (CDH).
Ineffective In Adult
Respiratory Distress Syndrome (ARDS)
In a randomized, double-blind,
parallel, multicenter study, 385 patients with adult respiratory distress
syndrome (ARDS) associated with pneumonia (46%), surgery (33%), multiple trauma
(26%), aspiration (23%), pulmonary contusion (18%), and other causes, with PaO2/FiO2
<250 mm Hg despite optimal oxygenation and ventilation, received placebo
(n=193) or INOmax (n=192), 5 ppm, for 4 hours to 28 days or until weaned
because of improvements in oxygenation. Despite acute improvements in
oxygenation, there was no effect of INOmax on the primary endpoint of days
alive and off ventilator support. These results were consistent with outcome
data from a smaller dose ranging study of nitric oxide (1.25 to 80 ppm). INOmax
is not indicated for use in ARDS.
Ineffective In Prevention Of Bronchopulmonary Dysplasia
(BPD)
The safety and efficacy of INOmax for the prevention of
chronic lung disease [bronchopulmonary dysplasia, (BPD)] in neonates ≤ 34
weeks gestational age requiring respiratory support has been studied in four
large, multi-center, double-blind, placebo-controlled clinical trials in a
total of 2,600 preterm infants. Of these, 1,290 received placebo, and 1,310
received inhaled nitric oxide at doses ranging from 5-20 ppm, for treatment
periods of 7-24 days duration. The primary endpoint for these studies was alive
and without BPD at 36 weeks postmenstrual age (PMA). The need for supplemental
oxygen at 36 weeks PMA served as a surrogate endpoint for the presence of BPD.
Overall, efficacy for the prevention of bronchopulmonary dysplasia in preterm
infants was not established. There were no meaningful differences between
treatment groups with regard to overall deaths, methemoglobin levels, or
adverse events commonly observed in premature infants, including
intraventricular hemorrhage, patent ductus arteriosus, pulmonary hemorrhage,
and retinopathy of prematurity.
The use of INOmax for prevention of BPD in preterm
neonates ≤ 34 weeks gestational age is not recommended.