Xanthine oxidase inhibition for hyperuricemic heart failure patients: design and rationale of the EXACT-HF study

Biomarkers of Oxidative Stress Tethered to Cardiovascular Diseases

Cardiovascular disease (CVD) is a broad term that incorporated a group of conditions that affect the blood vessels and the heart. CVD is a foremost cause of fatalities around the world. Multiple pathophysiological mechanisms are involved in CVD; however, oxidative stress plays a vital role in generating reactive oxygen species (ROS). Oxidative stress occurs when the concentration of oxidants exceeds the potency of antioxidants within the body while producing reactive nitrogen species (RNS). ROS generated by oxidative stress disrupts cell signaling, DNA damage, lipids, and proteins, thereby resulting in inflammation and apoptosis. Mitochondria is the primary source of ROS production within cells. Increased ROS production reduces nitric oxide (NO) bioavailability, which elevates vasoconstriction within the arteries and contributes to the development of hypertension. ROS production has also been linked to the development of atherosclerotic plaque. Antioxidants can decrease oxidative stress in the body; however, various therapeutic drugs have been designed to treat oxidative stress damage due to CVD. The present review provides a detailed narrative of the oxidative stress and ROS generation with a primary focus on the oxidative stress biomarker and its association with CVD. We have also discussed the complex relationship between inflammation and endothelial dysfunction in CVD as well as oxidative stress-induced obesity in CVD. Finally, we discussed the role of antioxidants in reducing oxidative stress in CVD.

CoenzymeQ in cellular redox regulation and clinical heart failure

Coenzyme Q (CoQ) is ubiquitously embedded in lipid bilayers of various cellular organelles. As a redox cycler, CoQ shuttles electrons between mitochondrial complexes and extramitochondrial reductases and oxidases. In this way, CoQ is crucial for maintaining the mitochondrial function, ATP synthesis, and redox homeostasis. Cardiomyocytes have a high metabolic rate and rely heavily on mitochondria to provide energy. CoQ levels, in both plasma and the heart, correlate with heart failure in patients, indicating that CoQ is critical for cardiac function. Moreover, CoQ supplementation in clinics showed promising results for treating heart failure. This review provides a comprehensive view of CoQ metabolism and its interaction with redox enzymes and reactive species. We summarize the clinical trials and applications of CoQ in heart failure and discuss the caveats and future directions to improve CoQ therapeutics.

Serum Sulfhydryl Groups, Malondialdehyde, Uric Acid, and Bilirubin as Predictors of Adverse Outcome in Heart Failure Patients due to Ischemic or Nonischemic Cardiomyopathy

Oxidative stress plays a significant role in the pathogenesis of heart failure (HF). The aim of the study was to investigate the prognostic value of oxidation-reduction (redox) markers in patients with HF due to ischemic and nonischemic cardiomyopathy. The study included 707 patients of HF allocated into two groups depending on ethology: ischemic cardiomyopathy (ICM) (n = 435) and nonischemic cardiomyopathy (nICM) (n = 272), who were followed up for one year. The endpoint occurrence (mortality or heart transplantation) in a 1-year follow-up was similar in the ICM and nICM group. The predictive value of endpoint occurrence of oxidative stress biomarkers such as the serum protein sulfhydryl groups (PSH), malondialdehyde (MDA), uric acid (UA), bilirubin, and MDA/PSH ratio and other clinical and laboratory data were assessed in both groups (ICM and nICM) separately using univariate and multivariate Cox regression analyses. In multivariate analysis, the higher concentrations of UA (p = 0.015, HR = 1.024, 95% CI (1.005-1.044)) and MDA (p = 0.004, HR = 2.202, 95% CI (1.296-3.741)) were significantly associated with adverse prognosis in patients with ICM. Contrastingly, in patients with nICM, we observed that higher bilirubin concentration (p = 0.026, HR = 1.034, 95% CI (1.004-1.064)) and MDA/PSH ratio (p = 0.034, HR = 3.360, 95% CI (1.096-10.302)) were significantly associated with increased risk of death or HT. The results showed the association of different oxidative biomarkers on the unfavorable course of heart failure depending on etiology.

Associations of methyl donor and methylation inhibitor levels during anti-oxidant therapy in heart failure

Redox balance and methylation are crucial to homeostasis and are linked by the methionine-homocysteine cycle. We examined whether differences in methylation potential, measured as plasma levels of S-adenosyl methionine (SAM) and S-adenosyl homocysteine (SAH), occur at baseline and during anti-oxidant therapy with the xanthine oxidase inhibitor allopurinol in patients with heart failure with reduced ejection fraction. We analyzed plasma samples collected at baseline and 24 weeks in the Xanthine Oxidase Inhibition for Hyperuricemic Heart Failure Patients (EXACT-HF) study, which randomized patients with heart failure with reduced ejection fraction to allopurinol or placebo. Associations between plasma levels of SAM, SAH, SAM/SAH ratio, and outcomes, including laboratory markers and clinical events, were assessed. Despite randomization, median SAM levels were significantly lower at baseline in the allopurinol group. SAH levels at 24 weeks, and change in SAM from baseline to week 24, were significantly higher in the group of patients randomized to allopurinol compared to the placebo group. A significant correlation was observed between change in SAH levels and change in plasma uric acid (baseline to 24-week changes) in the allopurinol group. There were no significant associations between levels of SAM, SAH, and SAM/SAH ratio and clinical outcomes. Our results demonstrate significant biological variability in SAM and SAH levels at baseline and during treatment with an anti-oxidant and suggest a potential mechanism for the lack of efficacy observed in trials of anti-oxidant therapy. These data also highlight the need to explore personalized therapy for heart failure.

Assessment of Cardiovascular Risk in Older Patients With Gout Initiating Febuxostat Versus Allopurinol: Population-Based Cohort Study

BACKGROUND

Hyperuricemia and gout are associated with an increased risk of cardiovascular disease. Xanthine oxidase inhibitors, allopurinol and febuxostat, are the mainstay of urate-lowering treatment for gout and may have different effects on cardiovascular risk in patients with gout.

METHODS

Using US Medicare claims data (2008-2013), we conducted a cohort study for comparative cardiovascular safety of initiating febuxostat versus allopurinol among patients with gout ≥65 years of age. The primary outcome was a composite end point of hospitalization for myocardial infarction or stroke. Secondary outcomes were individual end points of hospitalization for myocardial infarction, stroke, coronary revascularization, new and recurrent heart failure, and all-cause mortality. We used propensity score matching with a ratio of 1:3 to control for confounding. We estimated incidence rates and hazard ratios for primary and secondary outcomes in the propensity score-matched cohorts of febuxostat and allopurinol initiators.

RESULTS

We included 24 936 febuxostat initiators propensity score-matched to 74 808 allopurinol initiators. The median age was 76 years, 52% were male, and 12% had cardiovascular disease at baseline. The incidence rate per 100 person-years for the primary outcome was 3.43 in febuxostat and 3.36 in allopurinol initiators. The hazard ratio for the primary outcome was 1.01 (95% CI, 0.94-1.08) in the febuxostat group compared with the allopurinol group. Risk of secondary outcomes including all-cause mortality was similar in both groups, except for a modestly decreased risk of heart failure exacerbation (hazard ratio, 0.94; 95% CI, 0.91-0.99) in febuxostat initiators. The hazard ratio for all-cause mortality associated with long-term use of febuxostat (>3 years) was 1.25 (95% CI, 0.56-2.80) versus allopurinol. Subgroup and sensitivity analyses consistently showed similar cardiovascular risk in both groups.

CONCLUSIONS

Among a cohort of 99 744 older Medicare patients with gout, overall there was no difference in the risk of myocardial infarction, stroke, new-onset heart failure, coronary revascularization, or all-cause mortality between patients initiating febuxostat compared with allopurinol. However, there seemed to be a trend toward an increased, albeit not statistically significant, risk for all-cause mortality in patients who used febuxostat for >3 years versus allopurinol for >3 years. The risk of heart failure exacerbation was slightly lower in febuxostat initiators.

Association between smoking and gout: a meta-analysis

Previous studies showed that smoking is linked with the decreased risk of gout, but the results remain controversial. The aim of this meta-analysis was to determine the associations between smoking and the risk of gout. A systematic literature search of PubMed and EMBASE was conducted. Data were extracted by two independent reviewers. The pooled odds ratio (OR) with 95% confidence interval (CI) was used to assess the strength of associations, using random effects and fixed effects models. Five studies with a total of 17,915,507 participants with 16,880 gouts were enrolled. Heterogeneity among the effect sizes of five studies was reported as I² = 87.9%. Our meta-analysis indicated that smoking (ever- vs non-, OR = 0.87, 95% CI = 0.67-1.12) was not associated with the risk of gout in random effects model, whereas there was an association between smoking (ever- vs non-, OR = 0.70, 95% CI = 0.65-0.74) and gout risk in fixed effects model. Our finding indicates that smoking may not be involved in the risk of gout. However, further studies are still needed to confirm our results.

CVD and Oxidative Stress

Nowadays, it is known that oxidative stress plays at least two roles within the cell, the generation of cellular damage and the involvement in several signaling pathways in its balanced normal state. So far, a substantial amount of time and effort has been expended in the search for a clear link between cardiovascular disease (CVD) and the effects of oxidative stress. Here, we present an overview of the different sources and types of reactive oxygen species in CVD, highlight the relationship between CVD and oxidative stress and discuss the most prominent molecules that play an important role in CVD pathophysiology. Details are given regarding common pharmacological treatments used for cardiovascular distress and how some of them are acting upon ROS-related pathways and molecules. Novel therapies, recently proposed ROS biomarkers, as well as future challenges in the field are addressed. It is apparent that the search for a better understanding of how ROS are contributing to the pathophysiology of CVD is far from over, and new approaches and more suitable biomarkers are needed for the latter to be accomplished.

Distinct risk factors of atrial fibrillation in patients with and without coronary artery disease: a cross-sectional analysis of the BOREAS-CAG Registry data

Objective

Although risk factors of atrial fibrillation (AF) in the general population have been characterised, their impacts on patients with specific diseases are unclear. Our aim was to determine whether risk factors of AF are different in patients with and those without coronary artery disease (CAD).

Methods

We enrolled 1871 consecutive patients who underwent coronary angiography for evaluation of symptoms suggestive of CAD in the BOREAS-CAG Registry between August 2014 and January 2015. After exclusion of patients with valvular heart disease or a history of PCI/cardiac surgery, 1150 patients contributed to multivariate logistic regression analysis to identify risk factors of AF. We also retrieved data for 361 consecutive patients with CAD admitted to Sapporo Medical University Hospital between April 2013 and July 2014 and analysed data for 166 patients using the same inclusion and exclusion criteria as those in the BOREAS-CAG Registry.

Results

Unexpectedly, CAD was independently associated with the absence of AF. The patients were then divided into a non-CAD group (n=576) and a CAD group (n=574) for further analysis. The brain natriuretic peptide level showed a strong association with AF regardless of the presence or absence of CAD. In the non-CAD group, lack of statin use was independently associated with AF, whereas high serum uric acid level was an independent explanatory variable of AF in the CAD group. The association of AF with uric acid was confirmed in a separate group of patients (n=166) enrolled in the CAD cohort in Sapporo Medical University Hospital.

Conclusions

Major risk factors of AF are different in patients with CAD and those without CAD. Patients with CAD are more likely to develop AF when the serum uric acid level is high, whereas no statin administration predicts development of AF in patients without CAD.

Review of novel therapeutic targets for improving heart failure treatment based on experimental and clinical studies

Heart failure (HF) is a major public health priority due to its epidemiological transition and the world's aging population. HF is typified by continuous loss of contractile function with reduced, normal, or preserved ejection fraction, elevated vascular resistance, fluid and autonomic imbalance, and ventricular dilatation. Despite considerable advances in the treatment of HF over the past few decades, mortality remains substantial. Pharmacological treatments including β-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone antagonists have been proven to prolong the survival of patients with HF. However, there are still instances where patients remain symptomatic, despite optimal use of existing therapeutic agents. This understanding that patients with chronic HF progress into advanced stages despite receiving optimal treatment has increased the quest for alternatives, exploring the roles of additional pathways that contribute to the development and progression of HF. Several pharmacological targets associated with pathogenesis of HF have been identified and novel therapies have emerged. In this work, we review recent evidence from proposed mechanisms to the outcomes of experimental and clinical studies of the novel pharmacological agents that have emerged for the treatment of HF.

Causal Assessment of Serum Urate Levels in Cardiometabolic Diseases Through a Mendelian Randomization Study

BACKGROUND

Although epidemiological studies have reported positive associations between circulating urate levels and cardiometabolic diseases, causality remains uncertain.

OBJECTIVES

Through a Mendelian randomization approach, we assessed whether serum urate levels are causally relevant in type 2 diabetes mellitus (T2DM), coronary heart disease (CHD), ischemic stroke, and heart failure (HF).

METHODS

This study investigated 28 single nucleotide polymorphisms known to regulate serum urate levels in association with various vascular and nonvascular risk factors to assess pleiotropy. To limit genetic confounding, 14 single nucleotide polymorphisms exclusively associated with serum urate levels were used in a genetic risk score to assess associations with the following cardiometabolic diseases (cases/controls): T2DM (26,488/83,964), CHD (54,501/68,275), ischemic stroke (14,779/67,312), and HF (4,526/18,400). As a positive control, this study also investigated our genetic instrument in 3,151 gout cases and 68,350 controls.

RESULTS

Serum urate levels, increased by 1 SD due to the genetic score, were not associated with T2DM, CHD, ischemic stroke, or HF. These results were in contrast with previous prospective studies that did observe increased risks of these 4 cardiometabolic diseases for an equivalent increase in circulating urate levels. However, a 1 SD increase in serum urate levels due to the genetic score was associated with increased risk of gout (odds ratio: 5.84; 95% confidence interval: 4.56 to 7.49), which was directionally consistent with previous observations.

CONCLUSIONS

Evidence from this study does not support a causal role of circulating serum urate levels in T2DM, CHD, ischemic stroke, or HF. Decreasing serum urate levels may not translate into risk reductions for cardiometabolic conditions.

Prognostic Significance of Hyperuricemia in Patients With Acute Heart Failure

Serum uric acid (UA) is associated with death and hospitalization in chronic heart failure (HF). However, UA in acute HF has not been well studied with respect to its relation to renal dysfunction and vascular congestion. We measured admission serum UA along with baseline variables in 281 patients with acute HF screened from the Loop Diuretics Administration and Acute Heart Failure (Diur-HF) trial. Hyperuricemia was defined as serum UA >7 mg/dl in men and >6 mg/dl in women. Chronic kidney disease (CKD) was defined as an estimated glomerular filtration rate <60 ml/min/1.73 m(2) before hospital admission. Death or HF hospitalization at 6 months was the primary outcome. The mean UA concentration was 6.4 ± 2.5 mg/dl, and 121 patients (43.1%) were classified as hyperuricemic. UA values were significantly increased in patients with CKD compared to patients without CKD (6.8 ± 2.7 vs 6.1 ± 2.1 mg/dl; p = 0.02); however, UA was not associated with the development of acute kidney injury. Patients with hyperuricemia had greater degrees of pulmonary and systemic congestion than normouricemic patients (congestion score 3.5 vs 2.1, p <0.01). Hyperuricemia was associated with higher risk of death or HF rehospitalization (univariate hazard ratio 1.46 [1.02 to 2.10]; p = 0.04, multivariate hazard ratio 1.69 [1.16 to 2.45]; p = 0.005). In conclusion, hospitalized patients with acute HF, elevated UA levels were associated with both CKD and pulmonary congestion. After controlling for potential confounders, hyperuricemia was associated with rehospitalization and death at 6 months.

Relation of serum uric acid to cardiovascular disease

This review summarizes recent published literature on the association between serum uric acid and cardiovascular disease, a relationship which is complex and not fully elucidated. Uric acid may be a marker for risk, a causative agent in cardiovascular disease, or both. Various biologic factors can influence serum uric acid levels, and serum uric acid level itself is closely related to conditions such as hypertension, dyslipidemia, obesity, and impaired glucose metabolism, that contribute to cardiovascular disease pathophysiology. Serum uric acid levels have been found to be associated with adverse outcomes, including mortality, in the general population. In addition, serum uric acid is associated with increased risk for incident coronary heart disease, heart failure, and atrial fibrillation. In the setting of established systolic heart failure, serum uric acid is positively associated with disease severity and mortality risk. Whether targeting treatment based on uric acid levels might affect clinical outcomes is still being studied.

Consensus Meeting on "Uric Acid and Cardiovascular Risk" held at University Magna Graecia, Catanzaro, Italy, May 2014. Publication of the Proceedings as a special issue in the International Journal of Cardiology

Introduction to a special supplement issue on the importance of uric acid in cardiovascular disorders.

Uric acid and xanthine oxidase in heart failure - Emerging data and therapeutic implications

The role of hyperuricaemia as cardiovascular risk factor has exhaustingly been debated for decades. While the association of elevated uric acid (UA) levels with increased mortality risk as convincingly been shown, the question whether UA is independently predictive of just a related effect within a more complex risk factor profile (including metabolic, inflammatory and haemodynamic risk factors) is still a matter of dispute. In heart failure the independent prognostic and functional impact of elevated UA has not only been shown but also the pathophysiologic mechanism(s) and the potential of targeted therapeutic interventions have been investigated in some detail. The emerging picture suggests the increased activity of the enzyme xanthine oxidase (XO) with corresponding increased production of free oxygen radical (ROS) as a main underlying principle with the resulting increase in UA levels being mostly a marker of this up-regulated pathway. While this concept will not diminish the value of UA as a prognostic marker, it provides the basis for a novel metabolic treatment option and the means to identify those patients most eligible for this tailored therapy. This review will summarize the recent evidence on XO as a novel and promising therapeutic target in heart failure.

Innate immunity and the failing heart: the cytokine hypothesis revisited

Elevated levels of inflammatory mediators have been identified in patients with heart failure, including heart failure with reduced and preserved ejection fraction, as well as acute decompensated heart failure. Moreover, experimental studies have shown repeatedly that activation of inflammation in the heart provokes left ventricular remodeling and left ventricular dysfunction. Nonetheless, phase III clinical trials that have attempted to antagonize inflammatory mediators have been negative with respect to the primary end points of the trials, and in some patients, resulted in worsening heart failure or death. The following review will discuss how recent developments in the field of innate immunity have advanced our understanding of the role of inflammation in the pathogenesis of heart failure and will discuss the negative outcomes of the existing clinical trials in light of this new information.

Effects of xanthine oxidase inhibitors on cardiovascular disease in patients with gout: a cohort study

BACKGROUND

Hyperuricemia and gout are associated with an increased risk of cardiovascular disease (CVD). It is unknown whether treating hyperuricemia with xanthine oxidase inhibitors (XOIs), including allopurinol and febuxostat, modifies cardiovascular risks.

METHODS

We used US insurance claims data to conduct a cohort study among gout patients, comparing XOI initiators with non-users with hyperuricemia defined as serum uric acid level ≥6.8 mg/dL. We calculated incidence rates of a composite nonfatal cardiovascular outcome that included myocardial infarction, coronary revascularization, stroke, and heart failure. Propensity score (PS)-matched Cox proportional hazards regression compared the risk of composite cardiovascular endpoint in XOI initiators vs those with untreated hyperuricemia, controlling for baseline confounders. In a subgroup of patients with uric acid levels available, PS-matched Cox regression further adjusted for baseline uric acid levels.

RESULTS

There were 24,108 PS-matched pairs with a mean age of 51 years and 88% male. The incidence rate per 1000 person-years for composite CVD was 24.1 (95% confidence interval [CI] 22.6-26.0) in XOI initiators and 21.4 (95% CI, 19.8-23.2) in the untreated hyperuricemia group. The PS-matched hazard ratio for composite CVD was 1.16 (95% CI, 0.99-1.34) in XOI initiators vs those with untreated hyperuricemia. In subgroup analyses, the PS-matched hazard ratio for composite CVD adjusted for serum uric acid levels was 1.10 (95% CI, 0.74-1.64) among XOI initiators.

CONCLUSIONS

Among patients with gout, initiation of XOI was not associated with an increased or decreased cardiovascular risk compared with those with untreated hyperuricemia. Subgroup analyses adjusting for baseline uric acid levels also showed no association between XOI and cardiovascular risk.

Effects of Xanthine Oxidase Inhibition in Hyperuricemic Heart Failure Patients: The Xanthine Oxidase Inhibition for Hyperuricemic Heart Failure Patients (EXACT-HF) Study

BACKGROUND

Oxidative stress may contribute to heart failure (HF) progression. Inhibiting xanthine oxidase in hyperuricemic HF patients may improve outcomes.

METHODS AND RESULTS

We randomly assigned 253 patients with symptomatic HF, left ventricular ejection fraction ≤40%, and serum uric acid levels ≥9.5 mg/dL to receive allopurinol (target dose, 600 mg daily) or placebo in a double-blind, multicenter trial. The primary composite end point at 24 weeks was based on survival, worsening HF, and patient global assessment. Secondary end points included change in quality of life, submaximal exercise capacity, and left ventricular ejection fraction. Uric acid levels were significantly reduced with allopurinol in comparison with placebo (treatment difference, -4.2 [-4.9, -3.5] mg/dL and -3.5 [-4.2, -2.7] mg/dL at 12 and 24 weeks, respectively, both P<0.0001). At 24 weeks, there was no significant difference in clinical status between the allopurinol- and placebo-treated patients (worsened 45% versus 46%, unchanged 42% versus 34%, improved 13% versus 19%, respectively; P=0.68). At 12 and 24 weeks, there was no significant difference in change in Kansas City Cardiomyopathy Questionnaire scores or 6-minute walk distances between the 2 groups. At 24 weeks, left ventricular ejection fraction did not change in either group or between groups. Rash occurred more frequently with allopurinol (10% versus 2%, P=0.01), but there was no difference in serious adverse event rates between the groups (20% versus 15%, P=0.36).

CONCLUSIONS

In high-risk HF patients with reduced ejection fraction and elevated uric acid levels, xanthine oxidase inhibition with allopurinol failed to improve clinical status, exercise capacity, quality of life, or left ventricular ejection fraction at 24 weeks.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00987415.

Relation of serum uric acid levels and outcomes among patients hospitalized for worsening heart failure with reduced ejection fraction (from the efficacy of vasopressin antagonism in heart failure outcome study with tolvaptan trial)

We investigated the clinical profiles associated with serum uric acid (sUA) levels in a large cohort of patients hospitalized for worsening chronic heart failure with ejection fraction (EF) ≤40%, with specific focus on gender, race, and renal function based interactions. In 3,955 of 4,133 patients (96%) with baseline sUA data, clinical characteristics and outcomes were compared across sUA quartiles. The primary end points were all-cause mortality and a composite of cardiovascular mortality or heart failure hospitalization. Interaction analyses were performed for gender, race, and baseline renal function. Median follow-up was 9.9 months. Mean sUA was 9.1 ± 2.8 mg/dl and was higher in men than in women (9.3 ± 2.7 vs 8.7 ± 3.0 mg/dl, p <0.001) and in blacks than in whites (10.0 ± 2.7 vs 9.0 ± 2.8 mg/dl, p <0.001). Higher sUA was associated with lower systolic blood pressure and EF, higher natriuretic peptides, and more impaired renal function. After accounting for 24 baseline covariates, in patients with enrollment estimated glomerular filtration rate ≥30 ml/min/1.73 m(2), sUA was strongly associated with increased all-cause mortality (hazard ratio 1.44, 95% confidence interval 1.22 to 1.69, p <0.001) and the composite end point (hazard ratio 1.44, 95% confidence interval 1.26 to 1.64, p <0.001). However, in patients with estimated glomerular filtration rate <30 ml/min/1.73 m(2), sUA was not related with either end point (both p >0.4). Adjusted interaction analyses for gender, race, and admission allopurinol use were not significant. In conclusion, sUA is commonly elevated in patients hospitalized for worsening chronic heart failure and reduced EF, especially in men and blacks. The prognostic use of sUA differs by baseline renal function, suggesting different biologic and pathophysiologic significance of sUA among those with and without significant renal dysfunction.

Incorporating common biomarkers into the clinical management of heart failure

Heart failure is a prevalent and costly disease, and its management with polypharmacy is complex. Commonly available biomarkers primarily help to 1) establish or refute the diagnosis of heart failure; 2) help to determine the disease severity; and 3) identify adverse consequences of treatment. Although several of them are commonly ordered (such as electrolytes, renal and liver function), their use is primarily based on broad clinical experience rather than established evidence. The availability of cardiac-specific natriuretic peptide testing has provided an evidence-based breakthrough in our abilities to establish the diagnosis and severity of heart failure, yet the appropriate boundaries to guide management are still in refinement.