Inorganic Nitrate as a Treatment for ANOCA: NO-ANOCA

Females with heart disease often present unique phenotypes but are underrepresented in clinical trials. In females with angina, 40-65% have non-obstructive coronary artery disease (ANOCA), often due to coronary microvascular dysfunction (CMD), which represents ~52% of ANOCA cases. CMD impairs myocardial perfusion (endothelial and/or non-endothelial dysfunction), often detected as reduced coronary flow reserve (CFR). CMD is underdiagnosed despite an elevated risk for major cardiac events and current treatments for it (e.g., beta-blockers, long-acting nitrates) may confer only marginal benefits.

A critical contributor to CMD is impaired nitric oxide (NO) production, essential for vascular function. Oral inorganic nitrate (NO3-) supplementation offers a promising strategy to enhance NO bioavailability through its conversion to nitrite (NO2-) by bacteria in the mouth. NO2- acts as a precursor to NO, releasing it under hypoxic or acidic conditions commonly seen in CMD. This non-enzymatic pathway bypasses the limitations of traditional nitrate therapies, such as systemic hypotension and nitrate tolerance. Moreover, despite established benefits of physical activity ~60% of ANOCA patients experience exercise-induced angina, reduced exercise tolerance, and diminished quality of life (QoL). Recent evidence suggests, however, that exercise can increase endothelial function, endogenous NO production, LVEF, and VO2peak in ANOCA patients. Thus, increased plasma NO2- via inorganic nitrate supplementation may enhance NO delivery, promote physical activity, improve fitness and QoL in this population, which in turn may increase vascular endothelial health.

In the WISE study, >50% of women with angina had minimal or no obstructive CAD (ANOCA), yet experience worse outcomes than asymptomatic women. ANOCA involves diverse mechanisms, including coronary microvascular dysfunction (CMD), endothelial dysfunction, and vasospasm. These heterogeneities hinder responses to traditional treatments, with CMD being the most common and often linked to endothelial dysfunction.

Nitric oxide (NO) is crucial for coronary microvascular function, regulating vasodilation. In CMD, reduced NO bioavailability and endothelial nitric oxide synthase (eNOS) uncoupling impairs vasodilation and increases oxidative stress. Inflammation in CMD further exacerbates microvascular dysfunction.

Our group and others have utilized inorganic nitrate (NO3-) supplementation to increase NO bioavailability in conditions with vascular dysfunction. NO3- decreases blood pressure, arterial stiffness, oxidative stress and inflammation, while improving vascular and cardiac function. NO3- is reduced to NO under slight hypoxic or acidic conditions, common in CMD, bypassing the limitations of long-acting nitrates. ANOCA patients, a population with the high levels of initial dysfunction, may gain the most benefit from exogenous NO3-.

Dr. Allen's lab has shown that acute NO3- supplementation increases NO metabolites and augments post-exercise vascular function. Our preliminary data in postmenopausal females (PMF) also suggest a link between NO3- supplementation and reduced ET-1 and IL-6 levels compared to PL. Supplementation with NO3- may increase the "delivery pool" of NO and improve ET-1 and Il-6 dysregulation, thus augmenting vascular function, leading to improved physical function and quality of life in females with CMD.

Approach:

This will be a single-site, double-blind, randomized crossover pilot study to investigate the effects of twice daily 70 mL of beetroot juice (BTR) (~13 mmol total NO3-/day) versus PL (NO3- -depleted) in eight females. Participants with clinically stable CMD (diagnosis <2 by CMR) will be enrolled. Participants will have angina or equivalent symptoms, and no evidence of obstructive epicardial CAD (stenosis <50%) by invasive catheterization or coronary computer tomography angiography (CCTA) or FFR/CT-FFR > 0.8 within the previous 2 years.

STUDY PROCEDURES:

Participants will undergo eligibility screening and informed consent by the study coordinators. Following baseline testing, subjects will be randomized to receive consume either ∼13 mmol of NO3- (BTR) or NO3- depleted placebo (PL) daily via two (morning and night) 70-mL bottles of juice (Beet It, James White Drinks Ltd., Ipswich, UK) for 14 days. Following a 7 day washout period, participants will consume the alternate treatment (BTR or PL) for an additional 14 days.

Baseline and testing at the end of each treatment allocation will include resting vitals, fasting venous blood draw (for plasma NO3-, NO2-, ET-1, IL-6), vascular function (brachial artery flow-mediated dilation, pulse wave velocity and reflection), exercise economy testing at 2 submaximal cycle workloads, QofL questionnaires (EQ-5D-3L, Duke Activity Status Inventory, Modified Morisky Medicine Scale, Rose Dyspnea Score, and Seattle Angina Q), and stress cardiac MRI (to evaluate myocardial perfusion using dynamic first-pass imaging).