Considerations for clinical trials targeting the myocardial interstitium

Gavin A. Lewis, MBChB;1,2 Susanna Dodd, PhD;3 Josephine H. Naish, PhD;1 Joseph Selvanayagam, DPhil;-4 Marc Dweck, PhD;5 Christopher A Miller, PhD.1,2,6

Word count: 7,492 (body text, references, figure legends)

Affiliations1. Division of Cardiovascular Sciences, School of Medical Sciences, Faculty of Biology,

Medicine and Health, Manchester Academic Health Science Centre, University of Manchester, Oxford Road, Manchester, M13 9PL, UK

2. Manchester University NHS Foundation Trust, Southmoor Road, Wythenshawe, Manchester, M23 9LT, UK

3. Department of Biostatistics, University of Liverpool, Block F, Waterhouse Bld, 1-5 Brownlow Street, Liverpool, L69 3GL, UK

4. Flinders University of South Australia, Flinders Drive, Bedford Park, Adelaide, 5042, Australia

5. Centre for Cardiovascular Science, University of Edinburgh, Little France Crescent, EH16 4SB, UK

6. Wellcome Centre for Cell-Matrix Research, Division of Cell-Matrix Biology & Regenerative Medicine, School of Biology, Faculty of Biology, Medicine & Health, Manchester Academic Health Science Centre, University of Manchester, Oxford Road, Manchester, M13 9PT, UK

Funding: Dr Miller is funded by a Clinician Scientist Award (CS-2015-15-003) from the National Institute for Health Research. Dr Dweck is supported by the BHF (FS/14/78/31020) and is the recipient of the Sir Jules Thorn Award for Biomedical Research 2015 (15/JTA).

DisclosuresThe views expressed in this publication are those of the authors and not necessarily those of the NHS, the National Institute for Health Research or the Department of Health.

Address for correspondenceDr. Christopher A. Miller, Division of Cardiovascular Sciences, School of Medical Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, University of Manchester, Oxford Road, Manchester, M13 9PLTelephone: 0044 161 291 2034. Fax: 0044 161 291 2389Email: Christopher.Miller@manchester.ac.uk

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Abstract

The myocardial interstitium has emerged as a potential therapeutic target, and as a biological

entity to improve risk stratification and better guide existing interventions. Clinical trials

focusing on the myocardial interstitium are required in order to establish causality and

improve patient outcomes. This review will discuss issues around clinical trials targeting the

myocardial interstitium, including antifibrotic therapies, efficacy outcome measurements,

mechanistic outcome measurements and mediation analysis, sample size, trial duration,

considerations for multicentre trials, stratifying trial recruitment according to the interstitium

and approaches to enrich recruitment, using examples of ongoing clinical trials.

Condensed Abstract

The myocardial interstitium has emerged as a potential therapeutic target, and as a biological

entity to improve risk stratification and better guide existing interventions. Clinical trials

focusing on the myocardial interstitium have the potential to improve patient outcomes. This

review will discuss issues around trials targeting the myocardial interstitium, using examples

of ongoing trials.

Main messages

The myocardial interstitium is a potential therapeutic target and a potential risk stratifier

that can guide interventions.

Clinical trials focusing on the myocardial interstitium are required in order to establish

causality and improve patient outcomes.

Cardiovascular magnetic resonance provides quantitative assessment of the myocardial

interstitium, and many other features of cardiovascular structure and function, thus can be

used to stratify recruitment and to evaluate efficacy and mechanism.

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Key words

Myocardial interstitium, myocardial fibrosis, cardiovascular magnetic resonance, clinical

trials

Abbreviations

BNP – brain natriuretic peptide

CMR – cardiovascular magnetic resonance

ECM – extracellular matrix

ECV – extracellular volume

HF – heart failure

HFpEF – heart failure with preserved ejection fraction

LGE – late gadolinium enhancement

LV – left ventricle

RAAS – renin-angiotensin-aldosterone system

RCT – randomised controlled trial

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Introduction

Over the past decade, observational data has demonstrated non-infarct focal and diffuse

myocardial fibrosis to be strongly associated with adverse prognosis across a range of

cardiovascular conditions (1-3).

The fibrotic response to injury and its clinical importance in other organs (cf. liver cirrhosis)

has been widely appreciated for decades (4). The pathophysiological relevance of the

myocardial interstitium in cardiovascular disease has also been recognised for many years (5).

Cardiovascular magnetic resonance (CMR) imaging, particularly the late gadolinium

enhancement (LGE) and extracellular volume (ECV) techniques, has more recently provided

unparalleled non-invasive access to the myocardial interstitium (6-8), which has allowed

observational cohort studies to be conducted at sufficient scale to demonstrate the clinical

implications of myocardial fibrosis. Thus the myocardial interstitium has emerged as a

potential therapeutic target, and as a biological entity to risk stratify patients and better guide

existing interventions.

Why are trials focusing on the myocardial interstitium required?

Clinical trials focusing on the myocardial interstitium are required for two reasons (Central

illustration):

1. To establish causality. Whilst observational data is useful, the association between

myocardial fibrosis and adverse cardiovascular outcomes does not establish causality.

This requires randomised controlled trials (RCT) to show: A) The efficacy of antifibrotic

agents to attenuate fibrosis formation/regress established fibrosis, and determine the

impact this has on other aspects of myocardial structure and function such as contractile

function, remodelling, arrhythmia burden and energetics; B) The clinical effectiveness of

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antifibrotic agents to improve patient survival and reduce hospital admissions; and C) The

clinical effectiveness of risk stratification based on myocardial fibrosis.

2. To improve patient outcomes. The prognosis of many cardiovascular conditions, e.g. heart

failure (HF), remains unacceptably poor. Targeting novel pathophysiological mechanisms,

such as myocardial fibrosis, with new therapeutics agents, and using novel

pathophysiological mechanisms to better target existing interventions, are required.

Is the myocardial interstitium modifiable?

There is extensive preclinical data demonstrating that inhibition of pathways such as the

renin-angiotensin-aldosterone system (RAAS) and transforming growth factor (TGF)-β,

biological therapies such as CCN5 gene transfer, and exercise, significantly attenuate the

formation of non-infarct myocardial fibrosis (e.g. in remote myocardium in models of

myocardial infarction) and regress established myocardial fibrosis (e.g. in models of

hypertension, pressure overload, diabetes) (9-19). In these models, fibrosis inhibition is

associated with improved left ventricular function, reduced incidence of HF and arrhythmias,

and improved survival. Inhibition of factors that detrimentally affect the quality of the

extracellular matrix (ECM), such lysyl oxidase-like 2, an enzyme that catalyses crosslinking

of collagen to form bundles of collagen that are considerably stiffer than individual collagen

fibres, is also associated with improved LV function and reduced incidence of HF

preclinically (20).

Regression of established myocardial fibrosis with pharmacological RAAS inhibition is also

observed in humans. In studies by Izawa et al (in patients with dilated cardiomyopathy) (21),

Brilla et al (22), Schwartzkopff et al (23) and Diez et al (24) (all in hypertensive heart

disease), 6-12 months of RAAS inhibition was associated with significant, albeit modest,

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reductions in histological collagen volume fraction measured from myocardial tissue obtained

at endomyocardial biopsy before and after treatment. Fibrosis regression was associated with

improvements in LV mechanical and microvascular function.

More recently, in studies using CMR ECV to measure ECM volume, Heydari et al

demonstrated a significant reduction in remote myocardial fibrosis formation with omega-3

fatty acid treatment following acute myocardial infarction (25), and Treibel et al showed

aortic valve replacement for aortic stenosis was associated with a significant reduction in

established diffuse myocardial fibrosis, which has been confirmed by Everett et al (26,27)

Antifibrotic drug therapies

In order to investigate the causal role of myocardial fibrosis in cardiovascular disease,

mechanistic trials of haemodynamically neutral therapies that also do not have a direct effect

on cardiomyocyte function are required (28). The beneficial effects of RAAS inhibition is

well recognised to extend beyond their haemodynamic impact, and it is widely hypothesised

that their antifibrotic effect is a key mechanism of their action, however, it is impossible to

separate their haemodynamic and anti-fibrotic effects. In this regard, pirfenidone, an

antifibrotic agent with proven clinical effectiveness in pulmonary fibrosis and which does not

have a haemodynamic effect, holds promise. The PIROUETTE trial (PIRfenidOne in patients

with heart failUre and preserved lEfT venTricular Ejection fraction; NCT02932566) is

evaluating the efficacy and safety of pirfenidone in patients with chronic heart failure and

preserved ejection fraction (HFpEF) and myocardial fibrosis (Figure 1).

A recent review by Li et al has comprehensively summarised existing drug therapies targeting

tissue fibrosis and those in development (29).

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Phase II trial efficacy outcome measurements

ECV is a quantitative measurement of the myocardial interstitial space. Whilst the interstitium

includes other elements, such as capillaries and fluids, the primary interstitial structure is the

ECM, which is predominantly composed of collagen. There is a wealth of data to support

ECV as a robust measure of myocardial fibrosis in non-infarcted myocardium (7,30-35). ECV

is highly reproducible across separate CMR scans and can detect clinical reversal of

myocardial fibrosis (25,30,36-40). Crucially, ECV is clinically meaningful – it strongly

associates with adverse outcome, including death and heart failure admission, in large cohorts

of patients undergoing CMR scanning (2,41,42). As such, ECV is well suited as an outcome

measurement in phase II/experimental medicine trials evaluating the efficacy of interventions

aiming to attenuate myocardial fibrosis formation or regress established myocardial fibrosis.

ECV also allows quantitative measurement of absolute myocardial ECM mass (the product of

LV mass and ECV, which has been referred to as iECV and has also been expressed as a

volume (43)) and myocardial cellular mass (the product of LV mass and (1 – ECV)) (44). In

trials of interventions expected to lead to both cardiomyocyte and myocardial fibrosis

regression, it may be that absolute myocardial ECM mass is a more useful measurement of

fibrosis regression than ECV. For example, Treibel et al found ECV actually increased after

aortic valve replacement (AVR) for aortic stenosis (pre-AVR: 28.2 ± 2.9%; 1 year post-AVR:

29.9 ± 4.0%) (26). However, there was a substantial reduction in total LV mass (88 ± 26g/m2

to 71 ± 19g/m2), and therefore despite the increase in ECV, absolute myocardial ECM mass

reduced by 16%. Interventions that are designed to target one ‘compartment’, for example a

pure anti-fibrotic agent may be better assessed using ECV, in mechanistic studies it is

suggested that all parameters are measured.

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Native myocardial T1 relaxation time also provides an assessment of myocardial fibrosis and

associates with adverse outcomes, and has a significant advantage over ECV in not requiring

gadolinium-based contrast agent administration (45,46). However, native T1 is not specific to

the interstitium, being also determined by cardiomyocyte characteristics, and as a result, the

correlation between native T1 and histological collagen volume fraction is less robust than for

ECV. Furthermore, native T1 is more dependent on field strength, sequence choice and

imaging parameters than ECV.

Whilst LGE is key for selecting patients for trial entry according the presence or absence of

focal non-infarct myocardial fibrosis (see ‘Stratifying trial recruitment according to the

myocardial interstitium’ below), and for excluding patients with confounding conditions (e.g.

amyloidosis), the utility of LGE as an outcome measurement for quantifying changes in non-

infarct myocardial fibrosis is less clear. LGE detection of fibrosis requires spatial

heterogeneity, thus it is not suitable for quantifying non-infarct myocardial fibrosis, indeed it

is not validated as a quantitative metric for this purpose (44,47).

Myocardial mechanical measurements (e.g. strain) are governed by multiple factors in

addition to myocardial fibrosis, such as cardiomyocyte function, myocardial ischaemia, and

loading conditions. Strain measurements therefore do not necessarily correlate with ECV, and

should not be considered surrogates for myocardial fibrosis. The limitations of circulating

biomarkers of collagen metabolism have been reviewed previously (48), but in brief, are not

cardiac specific and are influenced by comorbidities.

Mechanistic outcome measurements and mediation analysis

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Determining the relationships between myocardial fibrosis and other aspects of myocardial

structure and function are important for understanding how myocardial fibrosis exerts a

deleterious effect, the mechanisms by which interventions that target the interstitium may

exert a beneficial effect, and, ultimately, for understanding myocardial pathophysiology in

general.

In preclinical studies, ‘pure’ antifibrotic interventions, i.e. without haemodynamic or direct

cardiomyocyte effects, are associated with improved LV systolic and diastolic function and

reduced incidence of arrhythmias (20,49-51). In the described human studies by Diez et al

(24), Brilla et al (22) and Izawa et al (21), myocardial fibrosis regression was associated with

reduced LV stiffness and improved diastolic function, although this association may be

confounded by the blood pressure lowering effect of the interventions. Human observational

data suggests associations between myocardial fibrosis and mechanical function, capillary

rarefaction and microvascular dysfunction, and arrhythmia (52-54).

Clinical trials of antifibrotic interventions will help establish causal relationships. Mediation

analysis, conducted as part of a trial, allows estimation of the direct and indirect (via a

mediator variable) effects of an intervention on outcome, and therefore may be used to

investigate the mechanistic pathways of trial interventions (Figure 2). For example, mediation

analysis can be used to determine whether a reduction in myocardial fibrosis (the putative

mediator variable) following an intervention causes change in myocardial mechanical

function, LV remodelling or arrhythmia burden (the outcome variables). The mediation

analysis adjusts for baseline covariates that predict both change in myocardial fibrosis and

outcome (e.g. mechanical function) and sensitivity analyses can be conducted to assess the

potential impact of unmeasured confounding between the mediator and outcome.

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A widely held hypothesis is that ECM expansion impairs myocyte capillary blood supply,

which leads to cardiomyocyte energy starvation and impaired energetics. In a substudy of the

PIROUETTE trial, phosphocreatine (PCr) to adenosine triphosphate (ATP) ratio will be

measured using 31phosphorus magnetic resonance spectroscopy in order to determine whether

myocardial fibrosis is leads to impaired energetics, assessed using mediation analysis (Figure

1).

Sample size considerations

A sample size calculation is required in order to determine the number of participants needed

to detect a clinically relevant treatment effect. In a two-arm superiority trial, which aims to

determine whether or not a true difference exists between the control and treatment groups,

the null hypothesis would be that there is no difference between treatments. If the observed

differences between treatment arms are large enough by chance alone, it is possible to reject

the null hypothesis incorrectly (Type I error (α) or a false positive result). Conversely, with

insufficient numbers, it is likely that that the null hypothesis will not be rejected even if it is

false (Type II error (β) or a false negative result). The sample size calculation imposes limits

on the probability of a Type I or Type II error occurring: typically a limit of 5% is imposed

for α and a limit of 10-20% is imposed for β. The power of the study (defined as the

probability that the study will reject the null hypothesis, i.e. conclude that there is a significant

difference between treatments, if such a treatment exists in reality) is equal to 100-β, therefore

the typical power of a study is 80-90%. The restrictions on the false positive rate are more

stringent than the false negative rate because of the relatively less important consequences of

failing to identify the superiority of a new treatment compared with changing practice to an

inferior treatment, if it is wrongfully concluded that the new treatment is superior (55).

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Along with the limits for α and β, the sample size calculation requires estimates of the

minimum clinically important difference (MCID) that the new treatment must demonstrate in

order to justify the use of the treatment in practice, and an estimate of the precision with

which the primary outcome measurement is measured in the population being studied i.e. a

measure of its variability in this population.

The MCID is a clinical decision that is often poorly justified in clinical trials in general (56).

One reason for this is that the magnitude of the change in phase II end points expected to

translate into improved clinical outcomes at phase III is often challenging to determine. As

described by Butler et al (57), while phase II endpoints for lipid and blood pressure

modulating interventions are relatively straightforward, achieving the prescribed phase II

MCID does not necessarily translate into improved patient outcome (58). Clinically

meaningful phase II endpoints for trials in other cardiovascular conditions, such as HF, have

proven even more challenging to identify and apply (57). For example, whilst the prognostic

utility of natriuretic peptides is well established, interventions associated with improvements

in natriuretic peptide levels at phase II have often not translated into improved clinical

outcomes at phase III, indeed natriuretic peptide-guided care has not proven to be clinically

effective (59).

A potential reason for the lack of ‘translatability’ of natriuretic peptides is that they are not

reflective of specific pathophysiological mechanisms, and therefore do not provide feedback

on whether or not an intervention has modulated the mechanism it was designed to target.

Conversely, ECV is directly reflective of the myocardial fibrotic process and thus has the

potential to be a more effective phase II endpoint.

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In a study by Schelbert et al in patients with HFpEF or at risk of HFpEF, a 2% increase in

ECV was associated with a 21% increased risk of clinical adverse outcome (combined end

point of death or hospitalisation for HF) over a median follow-up period of 1.9 years (41).

Thus, this degree of change in ECV, for example, appears to be clinically meaningful.

Regarding estimates of the precision of ECV, natural history studies evaluating longitudinal

changes in ECV, and hence reporting the associated within-patient variability, have been

published in relatively few cardiovascular disease areas. In patients with acute myocardial

infarction, Bulluck et al, who scanned 50 patients at 4 ± 2 days and again at 5 ± 2 months

post-MI, found the standard deviation of within-patient difference in remote myocardial ECV

to be 1.9% (60). Similarly Carberry et al, who scanned 140 patients at 2.3 ± 1.9 days and 6

months post-MI found the standard deviation of within-patient difference in remote

myocardial ECV to be 2.6% (61). In a study by Garg et al, who scanned patients with type II

diabetes before and after 6 months of spironolactone or hydrochlorothiazide, the standard

deviation of within-patient difference in myocardial ECV in the placebo group was 3% (62).

In a study by Everett et al that included a natural history cohort, 61 asymptomatic patients

with aortic stenosis (43% mild, 34% moderate, 23% severe) underwent CMR at baseline and

again 2.1±0.7 years later. The interquartile range of the annualised within-patient difference in

myocardial ECV was -1 to 1%, and the annualised within-patient difference in absolute ECM

volume was 0 to 2.3 ml/m2 (27). Natural history studies in more disease areas are required.

An example sample size calculation for a phase II parallel group, 1:1 randomised placebo-

controlled trial of an antifibrotic drug, using change in ECV as the primary outcome measure,

would be as follows: Using a standard deviation of within-patient differences in ECV from

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baseline of 3% in both groups, 37 patients per group would be required to detect a minimum

difference, between active drug and placebo groups, of 2% in terms of change ECV from

baseline following a period of treatment, with 80% power at a 5% (α) significance level (2-

sided).

Trial duration

The optimal duration for phase II trials investigating antifibrotic interventions seems likely to

depend on whether the aim of the trial is to attenuate myocardial fibrosis formation, for

example fibrosis formation in remote myocardium following myocardial infarction, or to

regress established myocardial fibrosis, for example in aortic stenosis or HFpEF.

Following a myocardial infarction, TGF-β is elevated in remote myocardium from day 1,

peaking at around day 3 to 7 in preclinical models (63). Remote myocardial collagen

accumulation is seen to begin at day 3. In trials demonstrating clinical benefit of RAAS

inhibition in this context, therapy was generally started at days 1-14 post-myocardial

infarction, although there may be greater benefit with initiation earlier in this period (64). The

majority of remodelling, as assessed using LV volumetrics, occurs within the first 3 months

post-myocardial infarction, and early trials of angiotensin converting enzyme inhibitors post-

myocardial infarction found only 4-6 weeks of treatment was associated with significant

reductions in HF and death (65-68). Therefore when designing, for example, a phase II trial

aimed at attenuating remote myocardial fibrosis formation post-myocardial infarction,

initiating treatment early post-myocardial infarction may be associated with maximal benefit,

and a treatment duration of 3 months may be sufficient.

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In the previously described histological studies demonstrating regression of established

myocardial fibrosis with RAAS inhibition in humans, significant fibrosis regression was

observed with 6 months of treatment (Brilla et al (22): 9% relative percentage reduction in

collagen volume fraction), but a larger effect was observed in the studies where treatment

duration was 12 months (Izawa et al (21): approximately 28% reduction; Schwartzkopff et

al(23): 22% reduction; Diez et al (24): 14% reduction). In the aforementioned study by

Treibel et al demonstrating a reduction in absolute myocardial ECM mass measured using

CMR following AVR for aortic stenosis, the follow-up CMR was performed at 12 months

post-AVR (26). Therefore when designing a phase II trial aimed at regressing established

myocardial fibrosis, a treatment duration of 6 – 12 months may be required.

Considerations for multicentre trials

The inclusion of a quantitative imaging biomarker such as ECV in a multicentre trial setting

creates some particular issues with regards to the consistency of results across centres.

Different institutions will typically have access to scanners from different vendors and field

strengths, which may be broadly similar in terms of clinical radiology capability, but which

can have significant differences in terms of T1 quantitation. Prior to commencing a

multicentre trial, acquisition protocols should be standardised for each scanner vendor and

field strength and, as much as possible, harmonised across vendors.

A site qualification process can help to ensure conformance to a consistent protocol across

participating sites. This will generally include an initial technical survey to assess MRI

hardware and software capabilities, followed by a site training session in which the protocol is

implemented at the site, and phantom and possibly also healthy volunteer data are acquired.

T1 phantoms that encompass the range of pre- and post-contrast T1 values expected (such as

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the T1MES phantom (69)) allow some assessment of ECV accuracy and precision. Once a

site has been qualified and entered into the trial, a centralised ongoing site quality assurance

can help to identify and rectify any issues as they arise. Since ECV is calculated as a ratio, it

is inherently more robust to systematic measurement errors than native T1, but regular on-

going acquisition of phantom data can help to detect any drift or step changes that may occur

as a result of software or hardware upgrade or failure.

While some degree of variability in the acquired pre- and post-contrast T1 data may be

inevitable across sites in a multicentre trial, it is possible to avoid additional variability due to

the post processing steps by using a single site for centralised image analysis. Centralised

analysis allows for a defined image analysis pipeline following standard operating procedures

using trained image readers and enables a clear analysis audit trail, but raises additional

considerations around secure data transfer and management of the central analysis site.

Remaining inter-site variability in the assessment of ECV, due to the variability in the

acquired data, may in part be accounted for in the statistical analysis, including stratification

of randomisation by site.

Stratifying trial recruitment according to the myocardial interstitium

Interventions may have more benefit, or may only be beneficial, in groups of patients with

certain pathophysiological characteristics. For example, in the aforementioned studies by

Izawa et al and Diez et al (21,24), significant fibrosis regression with RAAS inhibition was

confined, perhaps intuitively, to patients with a higher burden of fibrosis at baseline.

Similarly, in a subgroup analysis of the Randomized Aldactone Evaluation Study (RALES)

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study, Zannad et al found the prognostic benefit of spironolactone was limited to those

patients with elevated baseline circulating collagen synthesis biomarkers (70).

Predictive enrichment trials select patients according to distinctive individual characteristics

in order to target those patients who are thought more likely to benefit from the intervention

(Table 1). The on-going EVoLVeD (Early Valve Replacement guided by Biomarkers of Left

Ventricular Decompensation in Asymptomatic Patients with Severe Aortic Stenosis;

NCT03094143; Figure 3) and CMR GUIDE (Cardiovascular magnetic resonance-GUIDEd

management of mild to moderate left ventricular systolic dysfunction; NCT01918215; Figure

4) trials are selecting patients according to the presence of focal replacement myocardial

fibrosis, as assessed using CMR LGE. The PIROUETTE trial is selecting patients according

to myocardial fibrosis burden, measured using CMR ECV (Figure 1).

EVoLVeD

Myocardial fibrosis is a key driver of left ventricular decompensation in aortic stenosis and

the transition from hypertrophy to HF (71). Replacement myocardial fibrosis can be imaged

using LGE and observed in a non-ischemic pattern. Recent studies have demonstrated that

non-ischemic LGE progresses rapidly once developed and that it is irreversible following

AVR. This is important because such fibrosis is associated with an adverse prognosis. Indeed

multiple independent studies have confirmed that non-ischemic LGE in patients with aortic

stenosis acts as an objective biomarker of LV decompensation and is a powerful independent

predictor of long-term outcomes (3,72,73). In the study by Musa et al, LGE was a powerful

independent predictor of all-cause (26.4% vs. 12.9%; p<0.001) and cardiovascular mortality

(15.0% vs. 4.8%; p<0.001) in patients with severe AS imaged just prior to AVR (3).

Furthermore, this association appeared dose-dependent: with every 1% increase in LV LGE

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burden, all-cause mortality increased by 11% (hazard ratio (HR) 1.11, 95% confidence

intervals (CI) 1.05-1.17, p<0.001).

This data provides the rationale for the EVOLVED trial. It is a multicentre randomised

controlled trial investigating whether early surgery in asymptomatic patients with severe

aortic stenosis and evidence of replacement myocardial fibrosis (i.e. presence of non-infarct

LGE on CMR) improves clinical outcomes compared to the standard approach of watchful

waiting. Patients undergo a CMR as part of the baseline assessment and those patients with

evidence of replacement myocardial fibrosis are randomised into the full study.

CMR GUIDE

There is clear pre-clinical and clinical evidence that myocardial fibrosis forms the critical

mechanical substrate for ventricular arrhythmias, a common cause of sudden cardiac death

(SCD) in patients with heart disease. As there was previously no direct method to visualise

myocardial fibrosis, LV EF was used as a surrogate for fibrosis burden, and for predicting the

patients most at risk for SCD. Accordingly current international guidelines assign a class I

recommendation for implantable cardioverter defibrillator (ICD) implantation for primary

prevention of SCD in patients with a LVEF ≤ 35% on optimal medical therapy (74).

However, recent studies highlight that these guidelines substantially underestimate the

number of patients who would benefit from ICD implantation (75). The majority of SCD

occurs in patients with mildly impaired or preserved LVEF (76,77), a trend reflected in both

historic and contemporary data, indicating that LVEF is an imperfect surrogate of myocardial

fibrosis and that current risk stratification in patients with a LVEF above 35% are suboptimal.

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The CMR Guide trial is a international, multicentre, combined registry and randomised trial

evaluating the effectiveness of ICD therapy in patients with mild-moderately reduced LV EF

and focal replacement myocardial fibrosis detected on LGE imaging in preventing SCD or

haemodynamically significant ventricular arrhythmia (78). Patients with mild-moderately

reduced LV EF undergo baseline CMR and those that demonstrate LGE are randomised to

implantation of either ICD or implantable loop recorder. Investigators hypothesise that among

patients with mild-moderately reduced LV EF, a CMR-guided management strategy for ICD

implantation based on the presence of focal replacement fibrosis will be superior to a current

standard care. A T1 mapping/ECV substudy will assess the relationship between baseline

ECV and arrhythmic events.

PIROUETTE

Schelbert et al, and others, have shown myocardial fibrosis burden, measured using ECV, is

strongly and independently associated with adverse outcome in patients with heart failure

with preserved ejection fraction (HFpEF) or at risk for HFpEF (41). Whilst myocardial

fibrosis is consistently demonstrated on a group level in patients with HFpEF, it is not

universal, with approximately one-third to one-half of patients having normal measures of

myocardial fibrosis (41,79,80).

In the PIROUETTE trial, eligible patients undergo a baseline CMR scan and only those with

evidence of myocardial fibrosis, as determined by a pre-specified ECV threshold, undergo

randomisation. Those that do not have myocardial fibrosis at baseline enter a registry.

Such personalised medicine approaches, based on individual biological phenotypes, are

consistent with the widely held aim among research funders, deliverers of medical research

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and delivers of healthcare, of providing ‘the right intervention to the right patient at the right

time’. These trials also serve to demonstrate the utility of the deep phenotyping provided by

contemporary cardiovascular imaging.

Yet, these precision medicine approaches come with a cost. In each of the described trials a

substantially greater number of patients are required to undergo baseline CMR than will be

randomised. For example, in CMR GUIDE, it is estimated that, after accounting for 10% drop

out, 949 patients will be required to undergo baseline CMR in order to identify 428 patients

with evidence of myocardial fibrosis i.e. 521 (55%) patients will undergo CMR but will not

be randomised (78). It is important that research funders recognise the extra (short-term)

funding, and time, required to deliver personalised research. Targeting patients based on their

individual pathophysiology intuitively should translate into a higher chance of finding clinical

benefit, hence this approach has the potential for substantial long-term research and clinical

cost savings.

Approaches to enrich recruitment

A cost-effective solution to this problem is to screen patients as having a higher or lower

probability of myocardial fibrosis using cheaper and easily accessible biomarkers. In aortic

stenosis high sensitivity troponin I (hsTnI) has emerged as a sensitive, marker of left

ventricular decompensation and prognostic marker. Patients with a troponin <6ng/L have a

low probability of myocardial fibrosis and a good prognosis (81). In the EVOLVED trial

troponin is therefore being used a screening tool to select patients that should proceed to

CMR imaging. Patients with a normal troponin <6ng/L are considered to have a healthy

myocardium and therefore are kept under routine follow up. Patients with an elevated

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troponin proceed to CMR and if non-infarct LGE is identified are then randomised either to

early valve replacement or the routine approach of watchful waiting.

Similarly, Schelbert et al found myocardial fibrosis, measured using ECV, was strongly

associated with brain natriuretic peptide (BNP) in patients with, and at risk of, HFpEF (41).

Thus BNP could be used to identify patients who are more likely to have an elevated ECV on

CMR.

Conclusions and outlook

Personalised approaches, based on prognostically important pathophysiological mechanisms,

are required in order to improve our rate of positive phase III trials. Myocardial fibrosis, an

example of such a mechanism, has the potential to improve risk stratification, guide care more

precisely and be a therapeutic target. The results of the discussed trials are eagerly awaited.

CMR provides tools to conduct trials that focus on the myocardial interstitium. A single CMR

exam can be used to identify individual patients according to their fibrotic burden, directly

measure the antifibrotic effect of interventions targeting the interstitium rather than non-

mechanism discriminant indirect measurements such as natriuretic peptides and exercise

tolerance, and provide a range of other measurements of myocardial structure and function to

determine the impact of interstitial modulation.

In these regards, CMR has the potential to become a highly valuable resource for evaluating

the efficacy, mechanisms of action and safety of new or repurposed drugs. However, fulfilling

this potential requires standardisation across vendors. Current vendor differences in hardware

and software are undoubtedly holding the CMR field back, to the considerable detriment of

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the CMR field, the pharmaceutical industry, investigators, vendors, and ultimately, our

patients.

Circulating biomarkers that are specific to the structure and function of myocardial

interstitium, which would be more widely applicable, simpler and likely cheaper than CMR,

are highly desirable. Therapeutics that target the myocardial interstitium are also required.

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Figure Legends

Central Illustration – Utility of the myocardial interstitium in clinical trials.

Contemporary clinical trials utilise cardiovascular magnetic resonance (CMR) assessment of

myocardial fibrosis to evaluate the efficacy of antifibrotic interventions, to investigate disease

mechanisms, and for patient selection. ECV – extracellular volume, LV – left ventricle.

Figure 1. PIROUETTE. The PIROUETTE trial identifies patients with heart failure with

preserved ejection fraction and a fibrotic phenotype by measuring myocardial extracellular

volume (ECV) at baseline. Only patients with myocardial fibrosis are randomised. The

primary outcome measure is change in myocardial ECV after 12 months of intervention. A

sub group of patients undergo 31P phosphorous magnetic resonance spectroscopy (31P-MRS)

in order to investigate the relationship between myocardial fibrosis and energetics. BNP –

brain natriuretic peptide, CMR – cardiac magnetic resonance, ECV – extracellular volume,

LVEF – left ventricular ejection fraction, NTproBNP – N-terminal brain natriuretic peptide.

Figure 2. Simple mediation model. Dotted lines indicate indirect effect of intervention on

outcome via mediator; solid line indicates direct effect of intervention on outcome. Interested

readers are directed towards Zhang et al (82).

Figure 3. EVOLVED. The EVOLVED trial is a multicentre randomised controlled trial

investigating whether early surgery in asymptomatic patients with severe aortic stenosis and

evidence of replacement myocardial fibrosis improves clinical outcomes compared to the

standard approach of watchful waiting. Patients are risk stratified according to the presence or

absence of non-infarct late gadolinium enhancement (LGE) on baseline CMR. Patients with

evidence of replacement myocardial fibrosis (i.e. non-infarct LGE) are randomised into the

33

full study. CMR – cardiac magnetic resonance imaging, ECG – electrocardiogram, LVH – left

ventricular hypertrophy.

Figure 4. CMR Guide. The CMR Guide trial is a international, multicentre, combined

registry and randomised trial evaluating the effectiveness of implantable cardioverter

defibrillator (ICD) therapy in patients with mild-moderately reduced left ventricular ejection

fraction (LV EF) and focal replacement myocardial fibrosis detected on late gadolinium

enhancement (LGE) imaging in preventing sudden cardiac death (SCD) or haemodynamically

significant ventricular arrhythmia. Patients are risk stratified according to the presence or

absence of LGE on baseline CMR. Patients with evidence of focal replacement myocardial

fibrosis (i.e. LGE), undergo randomisation. CAD – coronary artery disease, CMR – cardiac

magnetic resonance imaging, eGFR – estimated glomerular filtration rate, HF – heart failure,

ILR – implantable loop recorder, MI – myocardial infarction, MUGA – multigated

acquisition.

34

Tables

Table 1. On-going clinical trials stratifying recruitment according to the myocardial interstitium.

Trial Primary objective Main inclusion criteria

Anticipated number to undergo baseline CMR

Stratifying criteria Number to be randomised

Groups Primary outcome

CMR Guide To assess the effectiveness of ICD therapy in patients with mild to moderate LV dysfunction with myocardial fibrosis in preventing SCD or hemodynamically significant ventricular arrhythmia

ICM or NICMLVEF 36-50%

1055 LGE presence (ischaemic or non-ischaemic)

428 ICD vs. ILR Composite of SCD and haemodynamically significant ventricular arrhythmia

EVoLVeD To determine whether early aortic valve surgery can reduce death and unplanned AS-related hospital admissions in patients with asymptomatic severe AS who have evidence of replacement myocardial fibrosis

Asymptomatic severe aortic stenosis

1000 LGE presence (mid wall fibrosis)

400 Early valve surgery vs. watchful waiting

Composite of all-cause mortality and unplanned AS-related hospital admission

PIROUETTE To evaluate whether pirfenidone leads to regression of myocardial fibrosis in patients with HFpEF and myocardial fibrosis

HFpEF (clinical heart failure, LVEF ≥45%, elevated natriuretic peptides)

200 ECV ≥ 27% 94 Pirfenidone vs. placebo

Change in ECV from baseline to week 52

AF – atrial fibrillation, AS – aortic stenosis, CMR – cardiac magnetic resonance, ECV – extracellular volume, HFpEF – heart failure with

preserved ejection fraction, ICD – implantable cardioverter defibrillator, ICM – ischaemic cardiomyopathy, ILR – implantable loop recorder,

LGE – late gadolinium enhancement, LV – left ventricular, LVEF – left ventricular ejection fraction, NICM – non-ischaemic cardiomyopathy,

SCD – sudden cardiac death.

35