Inhaled Sodium Nitrite Improves Rest and Exercise Hemodynamics in Heart Failure With Preserved Ejection Fraction

Abstract

Rationale

Abnormalities in nitric oxide (NO) signaling play a pivotal role in heart failure with preserved ejection fraction (HFpEF). Intravenous sodium nitrite, which is converted to NO in vivo, improves hemodynamics in HFpEF, but its use is limited by the need for parenteral administration. Nitrite can also be administered using a novel, portable micronebulizer system suitable for chronic use.

Objective

Determine whether inhaled nitrite improves hemodynamics in HFpEF.

Methods and Results

In a double-blind, randomized, placebo-controlled, parallel-group trial, subjects with HFpEF (n=26) underwent cardiac catheterization with simultaneous expired gas analysis at rest and during exercise, prior to and following treatment with inhaled sodium nitrite (90 mg) or placebo. The primary endpoint was the pulmonary capillary wedge pressure (PCWP) during exercise. Prior to study drug, HFpEF subjects displayed an increase in PCWP with exercise from 20±6 to 34±7 mmHg (p<0.0001). Following study drug administration exercise PCWP was substantially improved by nitrite as compared to placebo (baseline-adjusted mean 25±5 vs 31±6 mmHg, ANCOVA p=0.022). Inhaled nitrite reduced resting PCWP (−4±3 vs −1±2 mmHg, p=0.002), improved pulmonary artery compliance (+1.5±1.1 vs +0.6±0.9 ml/mmHg) and decreased mean pulmonary artery pressures at rest (−7±4 vs −3±4 mmHg, p=0.007) and with exercise (−10±6 vs −5±6 mmHg, p=0.05). Nitrite reduced right atrial pressures with no effect on cardiac output or stroke volume.

Conclusions

Acute administration of inhaled sodium nitrite reduces biventricular filling pressures and pulmonary artery pressures at rest and during exercise in HFpEF. Further study is warranted to evaluate chronic effects of inhaled nitrite in HFpEF.

Clinical Trial Registration

This single center randomized clinical trial is registered at clinicaltrials.gov (NCT02262078).

INTRODUCTION

Roughly half of patients with heart failure (HF) have a preserved ejection fraction (HFpEF) and there is no proven effective treatment.^{1, 2} Patients with HFpEF display elevated cardiac filling pressures at rest and on exercise that lead to secondary pulmonary hypertension (PH).^1–5 These hemodynamic abnormalities produce symptoms of dyspnea, are associated with adverse outcomes, and are believed to be related in large part to deficits in nitric oxide (NO) availability.⁶

The inorganic nitrate-nitrite pathway represents a novel NO-providing therapy to improve clinical status in patients with HF and/or PH.⁷ Nitrite is reduced in vivo to NO by a variety of proteins in a reaction that is enhanced with venous hypoxia and acidosis, conditions that develop during exercise. We have recently demonstrated that intravenous nitrite substantially improves exercise hemodynamics and cardiac reserve in patients with HFpEF.⁸ However, like many vasoactive medicines used in patients with HF, the need for parenteral administration represents a major barrier limiting chronic use of this compound.

Effective parenterally-administered medications such as nitrite might also be deliverable as an inhalation, which would be more suitable for chronic ambulatory use in patients with HF.^9–11 Accordingly, we performed a double blind, placebo-controlled trial to evaluate the effects of nebulized inhaled nitrite, administered using a novel hand-held micronebulizer device enabling alveolar drug delivery, on rest and exercise hemodynamics in subjects with invasively-proven HFpEF.

METHODS

This was a double-blind, randomized, placebo controlled, parallel-group trial designed to study the effects of nebulized inhaled sodium nitrite on cardiovascular hemodynamics at rest and during exercise in subjects with HFpEF. Patients referred to the Mayo Clinic cardiac catheterization laboratory for invasive hemodynamic exercise stress testing were enrolled. Written informed consent was provided by all subjects prior to participation in study-related procedures. The trial was approved by the Mayo Clinic Institutional Review Board and was registered (NCT02262078).

Study population

HFpEF was defined by clinical symptoms of chronic HF (dyspnea, fatigue), normal EF (≥50%), and elevated left heart filling pressures (pulmonary capillary wedge pressure, PCWP) at rest (>15mmHg) and/or with exercise (≥25mmHg).^{2–5, 8} Exclusion criteria included significant valvular heart disease (>mild stenosis, >moderate regurgitation), cor pulmonale, significant pulmonary disease, congenital heart disease, glucose 6-phosphate dehydrogenase deficiency, left to right shunt, unstable coronary artery disease, myocardial infarction within 60 days, hypertrophic or infiltrative cardiomyopathy, primary renal or hepatic disease, high output heart failure, or constrictive pericarditis. Patients receiving chronic organic nitrates or phosphodiesterase 5 inhibitors were also excluded.

Study protocol

Subjects were studied on their chronic medications in the postabsorptive state and supine position. Cardiac catheterization was performed with simultaneous expired gas analysis at rest and during supine exercise at 20-Watt (W) workload for 5 minutes as previously described.^{3, 5, 8} After the first exercise phase (prior to any drug administration) and after return to steady-state baseline hemodynamic values, subjects were randomized in a 1:1 fashion using a simple computer-generated randomization program to nebulization of either placebo (inhaled normal saline) or inhaled nebulized sodium nitrite (90 mg, Mast Therapeutics) administered via the Solo-Idehaler device (Aerogen Galway, Ireland/DTF Saint-Etienne, France).

For systemically-acting drugs, a high percentage of particles need to reach the alveoli deep within the periphery of the lung, allowing for fast and efficient absorption via gas exchange. The nebulizer device utilized in this trial achieves robust alveolar delivery of study drug as it aerosolizes to very small particles (≤5 μm) that behave like gas molecules reaching the terminal alveoli, with much higher efficiency than pneumatic nebulizers that aerosolize to larger particles (5–10 μm) which are deposited by inertial impact primarily to the throat and upper airways.^9–11

The nitrite/placebo nebulizations were identical in appearance and were prepared by the research pharmacy, ensuring double-blinding so both the patient and investigator were unaware of nebulizer content. After 5 minutes, hemodynamic measurements were repeated at rest, followed by repeat supine exercise at 20W for 5 minutes, identical to the first phase of the study. Hemodynamic, arterial and venous blood samples and expired gas data were acquired during each stage of the protocol.

Catheterization protocol

Right heart catheterization was performed through a 9-French sheath via the internal jugular vein. Transducers were zeroed at mid-axilla. Pressures in the right atrium (RAP), pulmonary artery (PA), and pulmonary capillary (PCWP) were measured at end expiration (mean of ≥3 beats) using 2-French high fidelity micromanometer-tipped catheters (Millar Instruments, Houston, TX) advanced through the lumen of a 7-French fluid-filled catheter (Arrow) as previously described.^{3, 5, 8} Mean RAP and PCWP were taken at mid A wave. PCWP position was verified by typical waveforms, appearance on fluoroscopy, and direct oximetry (saturation≥94%). Continuously recorded pressure tracings were digitized (240 Hz) and analyzed offline.

Arterial blood pressure (BP) was measured continuously through a 4–6 French radial arterial cannula. Oxygen consumption (VO₂) was measured from expired gas analysis (MedGraphics, St. Paul, MN) taken as the average from the 60 seconds preceding arterial and mixed venous blood sampling. Arterial-venous O₂ content difference (CaO₂ – CvO₂) was measured directly as the difference between systemic arterial and PA O₂ content. Cardiac output (CO) was determined by the direct Fick method (= VO₂/[CaO₂ – CvO₂]). Stroke volume (SV) was determined from the quotient of CO and heart rate (HR). CO and SV were indexed to body surface area. Pulmonary vascular resistance (PVR= [mean PA-PCWP]/CO), PA compliance (PAC= SV/[PA pulse pressure]), and systemic vascular resistance (SVR= [mean BP-RAP]*80/CO) were calculated using standard formulas.⁸

Blood samples

Central venous and arterial blood samples were obtained during each stage to measure methemoglobin level and blood gases. Plasma nitrite concentrations were assessed using a liquid chromatography-fluorometric assay (BASi, West Lafayette, IN, USA) as previously described.⁹

Study endpoints

The primary endpoint of the trial was the PCWP during exercise. Secondary endpoints included changes in resting PCWP as well as rest and exercise changes in RAP, PA pressure, PVR, PAC, systemic BP, HR, SV, CO, VO₂, and CaO₂ – CvO₂. Methemoglobin level (%) was assessed as a safety endpoint.

Statistical analysis

Results are reported as mean (SD), median (IQR) or number (%). Between group differences at individual time points were tested using Student’s t-test, Wilcoxon rank-sum, or Fisher’s exact test. Within-group differences are assessed by paired t test. The effect of nitrite on the primary endpoint of exercise PCWP was assessed by analysis of covariance (ANCOVA), using the initial exercise PCWP measured prior to study drug as the covariate. Between-group differences in rest or exercise hemodynamic responses were compared by unpaired t-test after accounting for respective pre-study drug values. Linear regression was performed to compare hemodynamic responses to exercise before and after study drug, with variables log-transformed as necessary for analysis. All tests were two-sided, with a P-value < 0.05 considered significant. Analyses were performed using JMP 10.0.0 (SAS Institute, Cary, NC, USA).

RESULTS

Of 70 screened patients, 32 were excluded based upon entry criteria (13 lung disease, 9 valvular disease, 5 on organic nitrates, 3 advanced liver/kidney disease, 2 coronary disease) and 12 subjects were excluded because they did not meet the hemodynamic criteria for HFpEF (n=10) or because systemic arterial pressures were too low (n=2). A total of 26 subjects met all entry criteria and were enrolled in the trial between December 2014 and December of 2015. Participants were older aged, obese and displayed typical comorbidities associated with HFpEF including hypertension, diabetes and coronary artery disease (Table 1). On average, subjects displayed normal LV chamber size and mass, mild LV diastolic dysfunction and elevated NT-proBNP levels, with no differences between subjects randomized to placebo or nitrite.

Table 1.

Baseline Characteristics

	Placebo	Nitrite
Age, years	72±10	67±9
Female, n (%)	8 (62)	6 (46)
Body mass index, kg/m2	30.8 (24.3, 36.0)	33.2 (30.3, 38.2)
Comorbidities
Coronary disease, n (%)	7 (54)	7 (54)
Hypertension, n (%)	11 (85)	10 (77)
Diabetes, n (%)	4 (31)	4 (31)
Medications
ACEI or ARB, n (%)	7 (54)	9 (69)
Beta blocker, n (%)	7 (54)	4 (31)
Statin, n (%)	5 (38)	3 (23)
Diuretic, n (%)	8 (62)	5 (38)
Laboratories
Hemoglobin (g/dl)	13.2±2.3	13.0±2.0
Methemoglobin (%)	0.5±4	0.7±0.4
Creatinine (mg/dl)	1.2±0.4	1.2±0.3
NT-proBNP (pg/ml)	977 (196, 3683)	551 (66, 1227)
Echocardiography
LVEDD (mm)	50±6	49±4
LV mass index (g/m2)	84 (73, 130)	85 (75, 95)
LV ejection fraction, (%)	62±6	62±4
LA volume (ml/m2)	40 (31, 53)	33 (27, 40)
Mitral E/A ratio	0.9 (0.6, 1.2)	1.2 (0.7, 1.4)
Mitral e’ velocity (cm/s)	7±2	7±3
E/e’ ratio	12 (7, 17)	13 (9, 17)

ACEI, angiotensin converting enzyme; ARB, angiotensin receptor blocker; LV, left ventricle; EDD, end-diastolic diameter

Rest and exercise hemodynamics before study drug

At rest, HFpEF subjects were hypertensive, with moderate to severely elevated right and left heart filling pressures, mild to moderate pulmonary hypertension, elevated PVR, low PA compliance and normal cardiac index (Table 2). Right and left heart filling pressures increased markedly during exercise (mean RA and PCWP 23 and 34 mmHg, respectively), with secondary elevation in PA pressures. 22 subjects (85%) had both elevated resting and exercise PCWP. 4 subjects (15%; 3 nitrite, 1 placebo) had normal resting PCWP but elevated exercise PCWP. Ventilatory efficiency (V_E/V_CO2 slope) was elevated in HFpEF subjects (38±6), in keeping with advanced disease.

Table 2.

Baseline and Exercise Hemodynamics Prior to Study Drug

	Rest Placebo	Nitrite	20W Exercise Placebo	Nitrite
Vital Signs
Heart rate (bpm)	74±11	70±11	92±11	88±14
Systolic BP (mmHg)	155±25	149±25	177±30	172±35
Mean BP (mmHg)	101±14	97±15	112±19	109±18
Central Pressures
RA (mmHg)	14±7	12±5	25±12	22±7
PA systolic (mmHg)	54±18	45±13	77±21	63±13
PA mean (mmHg)	35±11	27±12	53±14	46±8
PCWP (mmHg)	21±5	19±7	34±8	34±7
Vascular and Ventricular Function
PVR (mmHg/L/min)	3.1±1.7	2.0±1.2	3.0±1.5	1.8±1.2*
PA compliance (ml/mmHg)	2.3±1.0	3.7±1.5*	2.1±0.9	3.1±1.1*
SVR (DSC)	1630±480	1300±470	1090±270	990±380
Integrated Function and Metabolism
VO2 (ml/min)	197±31	265±94*	600±87	686±143
CaO2 – CvO2 (ml/dl)	4.5±0.9	4.5±0.8	9.6±1.7	8.9±1.1
Cardiac index (l/min*m2)	2.3±0.5	2.6±0.8	3.3±0.7	3.5±0.9
SVI (ml/m2)	32±8	39±13	36±9	41±14

Columns show rest-exercise hemodynamics prior to study drug administration in subjects randomized to placebo or nitrite.

^*p<0.05 vs placebo-randomized subjects

BP, blood pressure; RA, right atrium; PA, pulmonary artery; PCWP, pulmonary capillary wedge pressure; PVR/SVR, pulmonary/systemic vascular resistance; DSC, dyne/sec*cm⁵; LVSW, left ventricular stroke work; VO₂, oxygen consumption; CaO₂ – CvO₂, arterial-venous O2 content difference; CO, cardiac output, SVI, stroke volume index

Subjects randomized to placebo tended to display higher PA pressures and PVR at rest and exercise as compared to subjects randomized to nitrite, along with significantly lower PA compliance at rest and on exercise and higher PVR on exercise (Table 2). Resting VO₂ was higher in subjects randomized to nitrite, but this was no longer significant after adjusting for body mass (p=0.5).

Effects of nitrite at rest

There was no effect of inhaled nitrite on heart rate or blood pressure at rest (Table 3). However, as compared to placebo, inhaled nitrite reduced right atrial pressure, PCWP and PA pressures while improving PA compliance (Table 3, Figure 1). Nitrite had no effect on resting pulmonary or systemic vascular resistance, VO₂, CI, CaO₂ – CvO₂, or SVI compared to placebo.

Table 3.

Effects of Nitrite on Resting Hemodynamics

	Placebo	Nitrite	p
Vital Signs
Heart rate (bpm)	−2±4	−1±5	0.7
Systolic BP (mmHg)	−0±7	−3±16	0.6
Mean BP (mmHg)	1±3	0±9	0.8
Central Pressures
RA (mmHg)	−1±1	−2±2	0.05
PA systolic (mmHg)	−5±6	−12±6	0.01
PA mean (mmHg)	−3±4	−7±4	0.007
PCWP (mmHg)	−1±2	−4±3	0.002
Vascular and Ventricular Function
PVR (mmHg/L/min)	−0.4±1.0	−0.6±0.4	0.6
PAC (ml/mmHg)	+0.6±0.9	+1.5±1.1	0.046
SVR (DSC)	+170±330	+130±310	0.8
Integrated Function and Metabolism
VO2 (ml/min)	+13±24	+2±21	0.3
CaO2 – CvO2 (ml/dl)	+0.7±1.2	+0.5±1.0	0.6
Cardiac index (l/min*m2)	−0.2±0.4	−0.2±0.5	0.9
SVI (ml/m2)	−1±6	−2±8	0.7

Table shows baseline-corrected values (resting values after receiving study drug minus resting values prior to study drug administration).

BP, blood pressure; RA, right atrium; PA, pulmonary artery; PCWP, pulmonary capillary wedge pressure; PVR/SVR, pulmonary/systemic vascular resistance; PAC, pulmonary artery compliance; DSC, dyne/sec*cm⁵; LVSW, left ventricular stroke work; VO₂, oxygen consumption; CaO₂ – CvO₂, arterial-venous O2 content difference; CO, cardiac output, SVI, stroke volume index

Figure 1.

Compared to placebo (black), nitrite administration (red) significantly lowered right atrial pressure (RA), pulmonary capillary wedge pressure (PCWP), mean pulmonary artery pressure (mPAP) at rest, while improving pulmonary artery compliance (PAC). Error bars indicate SEM.

Effects of nitrite on exercise hemodynamics

The primary endpoint of exercise PCWP was significantly improved with nitrite compared to placebo (adjusted mean 25±5 vs 31±6 mmHg, ANCOVA p=0.022; Table 4, Figure 2). The reduction in exercise PCWP with nitrite was >2-fold greater than the reduction observed in resting PCWP post nitrite.

Table 4.

Effects of Nitrite on Exercise Hemodynamics

	Placebo (n=13)	Nitrite (n=13)	p
Vital Signs
Heart rate (bpm)	−1±8	−1±10	0.9
Systolic BP (mmHg)	−4±8	−5±9	0.7
Mean BP (mmHg)	−1±3	−4±5	0.07
Central Pressures
RA (mmHg)	−2±3	−5±3	0.02
PA systolic (mmHg)	−7±9	−11±6	0.2
PA mean (mmHg)	−5±6	−10±6	0.05
PCWP (mmHg)	−3±5	−9±6	0.02
Vascular and Ventricular Function
PVR (mmHg/L/min)	−0.4±0.6	−0.3±0.4	0.5
PA compliance (ml/mmHg)	0.4±0.7	0.5±0.7	0.6
SVR (DSC)	−20±90	−40±110	0.6
Integrated Function and Metabolism
VO2 (ml/min)	−3±69	33±64	0.2
CaO2 – CvO2 (ml/dl)	−0.3±0.9	−0.0±0.7	0.4
Cardiac index (l/min*m2)	0.2±0.4	0.1±0.4	0.8
SVI (ml/m2)	2±5	1±7	0.6

Table shows exercise baseline-corrected values (exercise values after receiving study drug minus exercise values prior to study drug administration).

BP, blood pressure; RA, right atrium; PA, pulmonary artery; PCWP, pulmonary capillary wedge pressure; PVR/SVR, pulmonary/systemic vascular resistance; DSC, dyne/sec*cm⁵; LVSW, left ventricular stroke work; VO₂, oxygen consumption; CaO₂ – CvO₂, arterial-venous O2 content difference; CO, cardiac output, SVI, stroke volume index

Figure 2.

[A] Compared to placebo (black), nitrite (red) significantly improved the primary trial endpoint of exercise pulmonary capillary wedge pressure (PCWP), [B] with effects that were observed both at rest (solid line) and during exercise (dashed line). Error bars indicate SEM.

Nitrite had no effect on exercise HR or systolic BP, but it tended to reduce mean BP during exercise more than placebo (p=0.07, Table 4). Nitrite decreased exercise RA and PA mean pressures compared to placebo. While there was no effect of nitrite on exercise PVR or PA compliance, subjects with higher PVR displayed greater PVR reductions with nitrite measured at baseline (r²=0.56, p=0.005) and during exercise (r²=0.68, p<0.001). This relationship was not observed in the placebo group. V_E/V_CO2 slope tended to be lower following nitrite as compared to placebo (+1±4 vs +3±3 compared to pre-study drug, p=0.26). Nitrite had no effect on exercise systemic vascular resistance, VO₂, CaO₂ – CvO₂, CI or SVI.

Safety

No subject developed hypotension, cough, bronchospasm or reported headache or other adverse effects following inhalation of study drug. One participant in the placebo group experienced an episode of vasovagal-type syncope 4 hours after completing the study. One participant in the nitrite group reported a salty taste following the nebulizer treatment. Inhaled nitrite did not alter methemoglobin levels compared to placebo (+0.1±0.2 vs +0.1±0.3%, p=0.8). No subject developed clinically meaningful methemoglobinemia. The highest methemoglobin value observed in any participant (2.0%) was in a subject randomized to nitrite with an elevated resting level (1.6%).

Pharmacokinetics

Plasma nitrite levels were undetectable at baseline and with exercise prior to study drug in all subjects. Nitrite (NO₂) levels increased to 11.1±4.9 μM prior to exercise in subjects receiving active drug (p<0.0001 vs baseline). After the 5 minute exercise period, NO₂ levels decreased to 9.1±3.9 μM (p=0.002 compared to pre-exercise values). The calculated half-life of nitrite in this study was 0.72±0.74 hours. Higher NO₂ levels tended to be associated with greater reductions in exercise PCWP (r= −0.44, p=0.13) and PA systolic pressure (r= −0.46, p=0.11).

DISCUSSION

There are two important new findings presented. We show for the first time that nebulized inhaled inorganic sodium nitrite favorably reduces pulmonary capillary wedge pressure (PCWP), an important, clinically-relevant endpoint, both at rest and particularly during exercise when symptoms develop in people with HFpEF. Second, we show for the first time in a placebo-controlled trial that a nebulized drug formulation can be effectively delivered to achieve pharmacokinetic and pharmacodynamic effects that are similar to intravenous administration in subjects with HFpEF. The beneficial acute hemodynamic effects of inhaled nitrite provide strong rationale to conduct trials testing inhaled nitrite as a chronic therapy in HF. The proof of efficacy using this novel micro-nebulized drug delivery system paves the way for testing other parenterally-administered drugs that may be efficacious for people with acute or chronic HF.

The pathophysiology of HFpEF is complex, being related to left and right ventricular dysfunction, vascular limitations, and impairments in the periphery.^{1, 2, 12} Despite this complexity, elevation in cardiac filling pressures at rest and with exercise is a nearly universal finding.^3–5 Elevated LV filling pressures drive symptoms of dyspnea, they contribute to pulmonary hypertension and development of right ventricular dysfunction,¹³ and they are associated with increased risk of death, even when observed only during exercise.¹⁴ Many patients with HFpEF display high filling pressures during exercise, with normal PCWP at rest,^3–5 and treatments that reduce filling pressures relatively more during exercise might be more effective and better tolerated than standard pharmacologic approaches like diuretics.

Limitations in NO availability are believed to play a key role in driving the elevations in filling pressures and pulmonary hypertension in HFpEF,^{1, 6} and agents targeting the NO/cGMP pathway represent an area of intense interest.¹⁵ One of the oldest approaches to target this pathway is using direct NO donors such as the organic nitrates. This served as the guiding hypothesis for the NEAT-HFpEF trial, where the organic nitrate isosorbide mononitrate was found to decrease rather than increase chronic activity levels in HFpEF, as had been hypothesized.¹⁶ While this result may seem to speak against the importance of NO deficiency in HFpEF, there are number of important caveats. Organic nitrates tonically release NO, so there is not targeted delivery at the time of greatest need (as with exercise). This tonic release may cause venodilation and secondary plasma volume expansion that counteracts any benefits on PCWP reduction (pseudo-tolerance).¹⁷ Pharmacologic tolerance is common with organic nitrates, and organic nitrates are clearly associated with the development of endothelial dysfunction,¹⁸ which is known to be present in HFpEF and associated with greater symptom severity, functional disability, and worse outcomes.^{19, 20}

In contrast to the organic nitrates, there is no tolerance and no development of endothelial dysfunction with inorganic nitrite.^{7, 21} Nitrite and its precursor nitrate were previously considered to be inert byproducts of NO metabolism, but studies over the past decade have shown that nitrite can be reduced back to NO in vivo. This reaction is facilitated in the setting of venous or tissue hypoxia and acidosis, conditions which accompany exercise. As such, nitrite provides a hypoxia-sensitive NO source that is preferentially active at the time of greatest need, allowing for more targeted NO delivery. Nitrite can be administered directly, or it may be derived from orally-administered nitrate that acts as a pro-drug that must undergo reduction by commensal bacteria to nitrite in the mouth prior to swallowing.

Preclinical and clinical studies have shown benefits from inorganic nitrite and nitrate in HFpEF.^{7, 8, 21–25} In human HFpEF, oral nitrate (delivered as beetroot juice) improves exercise capacity, vasodilation and cardiac output reserve when given either as a single dose or as repeated doses over 1 week.^{22, 23} Intravenous nitrite reduces PCWP at rest and during exercise while also improving PA pressures and cardiac output reserve in HFpEF.⁸ Similar benefits have also been reported with intravenous nitrite in patients with HFrEF.²⁶ An important consideration when comparing nitrate and nitrite formulations is that plasma NO₂ levels with nitrate are an order of magnitude lower than intravenously administered nitrite,^{8, 27} and it is not clear that central hemodynamic benefits would be observed with oral nitrate formulations, particularly given the observed relationship between NO₂ levels and PCWP reduction.

Inhaled nitrite has been developed as an alternative drug delivery route with the potential to enable chronic use, circumventing the requirement for intravenous administration.^{7, 9} The current data shows that inhaled nitrite achieves similar plasma NO₂ levels as the intravenous formulation (~10–12 μM), and like parenteral nitrite,⁸ nebulized inhaled nitrite achieves similar pharmacodynamic effects, with reduction in biventricular filling pressures at rest and during exercise, along with decreases in pulmonary artery pressures. In contrast to intravenous nitrite,⁸ inhaled nitrite did not increase stroke volume, stroke work, or cardiac output reserves in the current study. The reasons for this differential effect remain unclear, but may relate to lesser systemic vasodilation, since exercise systemic vascular resistance was not affected by inhaled nitrite but was reduced with intravenous nitrite.

The half-life of inhaled nitrite in this trial was 43 minutes, which is similar to what has been reported in healthy volunteers.⁹ Currently this requires administration every 6 hours during the daytime. However, there is reason to believe that the salutary effects of nitrite may persist even as plasma levels return to baseline, because there are sustained reductions in PA pressure in both humans and mice at trough concentrations.^{28, 29} These sustained effects may be related to longer-lived species such as nitrosothiols and NO-hemoglobin adducts. The ease of use and rapid onset of action with inhaled nitrite suggests that it might be used chronically in a scheduled fashion three times daily, prior to planned physical activity, or even on an as-needed basis for relief of dyspnea. As such, the efficacy of chronic inhaled nitrite in HFpEF is currently being tested in two clinical trials sponsored by the NHLBI (NCT02742129 and NCT02713126).

Nitrite has also been suggested as a treatment for pulmonary arterial hypertension.^{7, 25, 28–30} In the current study, pulmonary arterial compliance was improved with nitrite at rest, consistent with a pulmonary vascular effect. While pulmonary vascular resistance was not improved overall, participants with the most intense PA vasoconstriction (highest PVR) did display greater pulmonary vasodilation with nitrite—an effect that was not observed in subjects receiving placebo. This observation raises the hypothesis that nitrite might be more effective as a pulmonary vasodilator in patients with more advanced pre-capillary causes of pulmonary hypertension, whether it is secondary to chronic HF or primary pulmonary arterial diseases. This question merits further study.

The adult lung contains 300 million alveoli, which serve as the primary sites for gas exchange between the blood and external environment. The lung alveolar-capillary interface thus represents one of the fastest and most efficient sites for absorption of systemically-active drugs.¹¹ However to date, this potential route of administration has been overlooked in HF patients. In order to achieve systemically-relevant biological effects, a high percentage of drug particles need to reach the alveoli.^{10, 11} The efficiency and location of the deposition depends largely on the size of the particles and the velocity of inspiratory flow. Conventional pneumatically powered nebulizers do not produce small enough particle sizes to achieve alveolar deliver, with an overall efficiency of less than 20%. In contrast, the efficiency of the device employed in the current trial is much higher (~75%),⁹ and as shown from the pharmacokinetic data, this enables drug absorption to systemic levels seen with parenteral administration, with very rapid onset of action. These data suggest that other parenterally-active therapies in HF might be administered using this approach, which could greatly expand the armamentarium of medications available for chronic use. These data also suggest one way of speeding the process of drug development for new intravenously-available compounds targeting novel pathways in HF.

Limitations

Patients with conditions that might independently cause symptoms were excluded, including patients with severe valvular or coronary heart disease and those with advanced lung, liver or kidney disease. Thus we cannot determine whether the current results would apply to HFpEF patients with those comorbidities. Hemodynamic effects of nitrite were assessed in the supine position in this trial. While filling pressures are lower when upright as compared to supine, prior studies have shown that the changes in pressures with exercise are similar regardless of body position,³¹ suggesting that the beneficial effects of nitrite may be similar, though we cannot make any conclusions about upright hemodynamics from these data. Cardiac volumes were not assessed during the study, limiting the ability to determine potential effects of nitrite on ventricular diastolic properties. Exercise was performed at low workload (20W) rather than peak. This was done to ensure feasibility and minimize any carryover effect of the initial (pre-study drug) exercise test to the second exercise test following study drug. Thus we cannot make any conclusions about the effect of inhaled nitrite on maximal exercise capacity. However, this level of activity is highly relevant to patients’ activities of daily living. Chronic effects of inhaled nitrite cannot be addressed from this acute study but are the subject of investigation of other trials that are currently underway.

Conclusion

Nebulized inhaled inorganic nitrite favorably attenuates hemodynamic derangements that are present at rest and during exercise in HFpEF. Further study is indicated to determine whether chronic inhaled nitrite administration can improve clinical status in people with HFpEF and other forms of HF and pulmonary hypertension, and whether other parenterally-administered medicines in HF might also be delivered by inhalation to improve clinical status.

Novelty and Significance.

What Is Known?

• Impaired nitric oxide (NO) bioavailability is associated with hemodynamic abnormalities in patients with heart failure (HF) with preserved ejection fraction (HFpEF) that drives symptoms and contributes to increased morbidity and mortality.

• Parenterally-administered inorganic nitrite, which leads to production of NO independent of the traditional NO synthase pathway, has been shown to improve rest and exercise hemodynamics in patients with HFpEF.

• A disadvantage of inorganic nitrite use is the requirement for intravenous administration.

What New Information Does This Article Contribute?

• Inhaled sodium nitrite, delivered via a high efficiency micro-nebulizer device, reduces biventricular filling pressures both at rest and during exercise as compared to placebo in patients with HFpEF.

• Pharmacokinetic and pharmacodynamic effects with inhaled nebulized nitrite were similar to what has been reported with intravenous administration, suggesting that inhaled nitrite may be effective as a chronic therapy.

HFpEF is a common cardiovascular problem without a proven effective treatment. Intravenous sodium nitrite augments NO signaling and has been shown to improve hemodynamic derangements that are associated with symptoms in patients with HFpEF. Currently, use of this compound is limited by the need for parenteral administration. We performed a randomized, double-blind, placebo controlled trial showing that inhaled sodium nitrite significantly attenuates hemodynamic derangements in patients with HFpEF, both at rest and during exercise. This is the first trial to show that a parenterally-active drug can be delivered via an inhaled nebulized route in patients with HFpEF. Inhaled nebulized nitrite shows pharmacokinetics and pharmacodynamic effects similar to the parenteral form. These data have two major implications: (1) that inhaled nitrite may be effective as a chronic therapy for HFpEF; and (2) they open the door for testing inhaled forms of other intravenous drugs to target the pathogenic pathways in heart failure and other cardiovascular diseases.

Nonstandard Abbreviations and Acronyms

HFpEF

Heart failure with preserved ejection fraction

PH

Pulmonary hypertension

NO

Nitric oxide

PCWP

Pulmonary capillary wedge pressure

PA

Pulmonary artery

RAP

Right atrial pressure

VO₂

Oxygen consumption

CaO₂ − CvO₂

arterial-venous oxygen content difference

CO

Cardiac output

SV

Stroke volume

PVR

Pulmonary vascular resistance

SVR

Systemic vascular resistance