Workshop report

Measuring clinical effectiveness of medicinal products for the treatment ofDuchenne muscular dystrophy

Stephen Lynn a, Annemieke Aartsma-Rus a,b, Kate Bushby a, Pat Furlong c, Nathalie Goemans d,Annamaria De Luca e, Anna Mayhew a, Craig McDonald f, Eugenio Mercuri g,

Francesco Muntoni h, Marita Pohlschmidt i, Jan Verschuuren b, Thomas Voit j, Elizabeth Vroom k,Dominic J. Wells l, Volker Straub a,*

a Newcastle University, Newcastle upon Tyne, UKb Leiden University Medical Centre, Leiden, The Netherlandsc Parent Project Muscular Dystrophy, Hackensack, NJ, USA

d University of Leuven, Leuven, Belgiume University of Bari, Bari, Italy

f University of California Davis, Davis, CA, USAg Catholic University Rome, Rome, Italy

h University College London, London, UKi Muscular Dystrophy Campaign, London, UK

j University Pierre et Marie Curie Paris 6, Institut de Myologie, UPMC-INSERM UMR 974, CNRS FRE 3617, GH Pitié-Salpêtrière, Paris, Francek Duchenne Parent Project, Amsterdam, The Netherlands

l Royal Veterinary College, London, UK

Received 4 August 2014

1. Introduction

In June 2014 TREAT-NMD organised a workshop at TheWellcome Trust head office in London to discuss issues suchas natural history, outcome measures, efficacy measurements,animal models and clinical study design as part of thecoordinated response to the public consultation by theEuropean Medicines Agency (EMA) on the draft guidelinesfor clinical investigation of medicinal products in Duchenneand Becker muscular dystrophy (DMD/BMD). The workshopattracted over 60 participants from 9 countries representingpatients, academics, and industry. It was also attended byexperts involved with the production of the draft EMAguidelines.

The discovery of the genetic cause of Duchenne musculardystrophy (DMD) almost 30 years ago raised hope that atreatment for one of the most common and devastating geneticdiseases could soon be developed. Although to date no drug hasbeen licenced for DMD, it is nevertheless promising thatmultiple therapeutic approaches have made it into the clinicaltrial phase for DMD and other rare inherited neuromusculardiseases. Over the past years international collaborationsinvolving patient organisations, academics and industry have

contributed to the development of patient registries, standardsof care, clinical outcome measures, biomarkers and animproved understanding of the pathogenesis and natural historyof DMD under the auspices of the TREAT-NMD network. Inresponse to these advances regulatory agencies are in theprocess of developing guidelines for the conduct of clinicaltrials in DMD and its milder allelic form Becker musculardystrophy (BMD). The publication of a draft guideline on theclinical investigation of medicinal products for the treatment ofDMD and BMD by the European Medicines Agency (EMA)prompted the TREAT-NMD network to host a workshop jointlywith COST Action BM1207 to discuss the guideline amongvarious stakeholder representatives and come to a consolidatedresponse to the public consultation. This report summarises themain topics of the workshop discussions.

2. Background to the Workshop

The development of medicinal products intended to treatrare diseases is a research area that has attracted a lot ofattention over the past few years. For the stakeholders in thefield, including the regulatory agencies, translational researchand drug development for rare diseases is still fairly newterritory that holds plenty of challenges. While there has been alot of progress over the past decades in diagnostic approachesfor rare genetic diseases, developing therapies for clinicalapplication in patients has lagged behind. Some of the majorstumbling blocks for conducting clinical trials in rare diseaseswere and continue to be the lack of standardised patient cohorts,

* Corresponding author. The Harold Macmillan Chair of Medicine, MRCCentre for Neuromuscular Diseases at Newcastle, Institute of GeneticMedicine, Newcastle University, Central Parkway, Newcastle upon TyneNE1 3BZ, UK. Tel.: +44(0)191 241 8762/8655; fax: +44(0)191 241 8770.

E-mail address: volker.straub@ncl.ac.uk (V. Straub).

http://dx.doi.org/10.1016/j.nmd.2014.09.0030960-8966/© 2014 Elsevier B.V. All rights reserved.

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

Available online at www.sciencedirect.com

ScienceDirect

Neuromuscular Disorders ■■ (2014) ■■–■■www.elsevier.com/locate/nmd

limited knowledge about the natural history of disease, lack ofvalidated outcome measures, the dearth of validated biomarkersand the difficulty to recruit patients.

The missing trial culture in the neuromuscular field wasaddressed by the European Union (EU) funding a translationalresearch network for rare inherited neuromuscular diseases(NMD) under its Sixth Framework Programme for Research(FP6). TREAT-NMD was established in 2007 as a networkof excellence with the aim to promote and develop trialreadiness and therapy delivery (http://www.treat-nmd.eu). Thisconsensus-building collaborative model has resulted in thereshaping of the neuromuscular research environment withan emphasis on Duchenne muscular dystrophy (DMD). DMDis an X-linked recessive disease caused by mutations inthe DMD gene leading to absence of functional dystrophinprotein. Normally, dystrophin stabilises muscle fibres duringcontraction. Lacking dystrophin, patients accumulate muscledamage, eventually leading to progressive muscle wastingand weakness. Patients lose the ability to walk independentlyaround 12 years of age and normally die between the secondand third decades due to cardiac or respiratory failure, but deathduring teenage years can occur. Recent implementation ofstandards of care is allowing patients to walk a few years longerand to potentially survive into the 4th decade, but the severemuscle weakness leading to paralysis still dominates theclinical picture in the advanced stages. Slowing down orstopping the progression of the disease is consideredmeaningful to patients because it would preserve their qualityof life, delaying by months or years the next loss of function.Becker muscular dystrophy (BMD) is also caused by mutationsin the DMD gene, but here mutations allow the expression ofpartially functional dystrophin proteins. Consequently, patientswith BMD show a less severe but highly variable clinicalphenotype. This variation is thought to be due to the effect thatthe mutation in the DMD gene has on its reading frame [1].

There is currently no therapy for DMD, but many therapeuticapproaches are in preclinical and clinical development.Because DMD is a rare disease, and a significant part of thetherapeutic approaches in clinical trials is mutation specific,clinical trials generally involve multiple trial sites andmultiple countries. Good translation from preclinical toclinical efforts and conducting multinational clinical trialsrequire standardisation of preclinical and clinical tests andinfrastructure. TREAT-NMD aimed to address this and TREAT-NMD activities have contributed to the publication of standardoperating procedures (SOPs) for the assessment of animalmodels [2–4], the establishment of patient registries [5], theformation of a care and trial site registry [6] and the collectionof biomaterial for biomarker discovery. Through its website andthe close collaboration with patient organisations around theworld and the Centre for Disease Control, TREAT-NMD hasbeen instrumental in developing, publishing and disseminatingbest-practice guidelines for clinical care and management ofindividuals with DMD [7–9]. The family friendly version of theguidelines has been translated into 30 different languages.

Over the past years TREAT-NMD, together with patientgroups, other EU-funded projects and cross-Atlantic

collaborations, has built a comprehensive infrastructurededicated to support the launch and conduct of academic andindustry led clinical trials in DMD. As part of this role TREAT-NMD has also facilitated several workshops on regulatoryissues around clinical trials in DMD. A first workshop washosted by the European Medicines Agency (EMA) on the 25thSeptember 2009, to discuss the development of antisenseoligonucleotide therapies for DMD, and a workshop report wassubsequently published in 2010 [10]. The workshop includeddiscussions about outcome measures for clinical trials in DMDand over the past few years several trials have been conductedthat used the 6 minute walk test (6MWT) as a primary endpoint.Based on these trials and a number of natural history studies,substantial data on the 6MWT and other outcome measureshave been collected since this EMA workshop (Table 1, Fig. 1).When the EMA released the draft guideline on medicinalproducts for the treatment of DMD and BMD in the springof 2013 for public consultation (http://www.treat-nmd.eu/industry/regulatory-affairs/dmd-workshop-2013/), TREAT-NMD together with several patient groups, the COST ActionBM1207 (http://www.exonskipping.eu) and other stakeholdersprepared a submission to the consultation, which also includeda workshop held on the 21st June 2013 in London to discussthe issues raised in the draft guideline. The workshopbrought together academics, patient representatives, industryrepresentatives and individual experts who provide advice toEMA committees. This meeting allowed TREAT-NMD tosubmit a consolidated response to the draft guideline on behalfof the NMD field in August 2013 (http://www.treat-nmd.eu/industry/regulatory-affairs/dmd-workshop-2013/).

Based on the main themes of the draft guidelines, theLondon workshop focussed on four topics: (1) What isclinically meaningful and how do we measure this? (2) Methodsof efficacy measurement – strength versus function. (3) Animalmodels and biochemical outcome measures. (4) Strategy anddesign of clinical studies – extrapolating to other stages of thedisease.

3. Report of specific topics

3.1. What is clinically meaningful and how to measure this?

In a disease that shows progressive loss of muscle strengthand function, preservation of ambulation appears to be asensible and meaningful objective for clinical trials. The 6minute walk test (6MWT), a validated outcome measure thathad already been used in a number of trials for differentdiseases [11], had therefore been suggested as a primaryendpoint for clinical trials in DMD. Although the test wasinitially developed for non-neuromuscular diseases, it wasmodified specifically for DMD and has been used in severalnatural history studies and interventional clinical trials inDMD[12,13]. The 6MWT has been validated in DMD as aglobal and integrated measure of multiple systems involved inwalking and as a measure of disease progression [14]. In DMDthe 6MWT correlates with measures of gait pathomechanicsand disease progression [12,13], skeletal muscle strength asmeasured by quantitative knee extension [14], biomechanical

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

2 S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

Table 1Decision framework for inclusion of clinical outcome measures in Phase II/III trials. The table shows the application of outcome measures relevant for DMD inrelation to age and validation status (✓). To be validated (?).

Outcome measure Griffithslocomotor

BayleyIII grossmotor

North StarAmb. Ass.(NSAA)

Timedfunctiontests

6MWT StrengthMMT

Strengthquant.

Pulmonaryfunctiontests

PerfUpperLimb(PUL)

PROs-PODCI

PROs-PROM

Myotools

Clinicalsubgroups

0–8 years 1–42 mo. 3.5 yearsuntilnon-amb

4 yearsuntilnon-amb.

5 yearsuntilnon-amb.

4 years tograde 2−

LE: 5–12UE: 5–20+

7–20+years

7–20+years

3–21years

7–20+years

5+ years

Conceptualframeworkfits DMD

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Reliability ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Validationwith othermeasures

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Inprogress

✓

Normativeranges

✓ ✓ ✓ ✓ ✓ ✓ Inprogress

✓ Inprogress

✓ Inprogress

✓

Ongoingnaturalhistorystudies

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Multicentrestudies

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Responsivenessto treatment

? ? ✓ ✓ ✓ ✓

or (-)✓

or (-)✓

If age ≥ 10? ? ? In

progressClinical

meaningfulness

? ? ✓ ✓ ✓ ✓ ? ✓ ✓ ✓ ✓ Inprogress

Fig. 1. Schematic natural history of DMD.

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

3S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

efficiency [14], endurance as measured by 10 minutecontinuous step activity [15], and gross motor skills [16–18]. Inthe first workshop session chaired by Jan Verschuuren, CraigMcDonald presented data on the 6MWT that had been collectedin DMD patient cohorts from the Cooperative InternationalNeuromuscular Research Group (CINRG) network over thepast few years. He reported that the minimal clinicallyimportant difference (MCID) for the 6MWT in DMD boysbased on statistical distribution properties was shown to beapproximately 30 metres [14]. Recent data confirmed that a30 metre decrement in the 6MWT from mean baseline predictsthe likelihood of experiencing 10% deterioration in ambulatoryfunction over the next 12 months [19]. Eugenio Mercuripresented data from a study in 113 Italian DMD patients thatsuggested if the distance walked in 6 minutes is at least330 metres at baseline, the risk of losing ambulation within 2years is significantly reduced [20]. Furthermore, it wasdemonstrated that for every 30 metre incremental decrease inthe baseline 6MWT, the percentage of patients who remainambulatory over the following 2 years decreases substantially.Craig McDonald showed there was a strong correlationbetween the 6MWT and the global Pediatric Outcomes DataCollection Instrument (PODCI; adjusted R squared = 0.83), ahealth-related quality of life measure of functional ability inchildren with orthopaedic limitations that focuses on globalquality of life (transfers and basic mobility, sports and physicalfunctioning, pain and comfort, and upper limb function) [21].He showed that at most levels of function typically seen inambulatory DMD subjects, the threshold change in the 6MWT,which would correspond to clinically meaningful changes inthe PODCI transfer and basic mobility scale, would be around28–36 metres depending on the level of functional disability.He also showed that the PODCI was more responsive and moreclosely associated with clinical measures of disease progressionin DMD than the Pediatric Quality of Life Inventory (PedsQL)and that the generic PedsQL did not correlate particularly wellwith disease progression in DMD [21,22]. The fact that a30 metre difference has a significant impact on the likelihood oflosing ambulation and correlates with the PODCI scalesupports the notion that preservation of a mean of 30 metres inthe 6MWT over a 48 week period is meaningful in terms ofactivities of daily living for boys with DMD, and slowing oflonger-term disease progression. A number of studies haveidentified no evidence of concern for either a learning effector risk of placebo response with the 6MWT which measuresdisease progression in DMD. Further longitudinal dataconfirm the 6MWT to be a more sensitive measure of diseaseprogression than quantitative strength and timed function tests[14,19,21].

Another widely used instrument to acquire informationon DMD disease progression is the North Star AmbulatoryAssessment (NSAA). This is a 17-function disease andstage specific rating scale with a clearly defined conceptualframework for DMD [23]. Reliability and validity have beenestablished using traditional and modern psychometric methods[17,24,25] and the ordinal scale has recently been transformedinto a 100 point linearised scale [26]. A 10 point change

has clinical meaning (e.g. 90–80 = can no longer hop,50–40 = inability to rise independently from the floor, and21–11 = loss of ability to stand still and upright). For the NSAAa minimal important difference (MID), calculated as 1⁄2 SD hasbeen demonstrated to be slightly less than 10 points on thetransformed NSAA scale [26]. The linearised NSAA scale hasbeen shown to detect a difference in the clinical efficacy oftwo commonly used steroid regimes (daily versus intermittentsteroids). The NSAA is being increasingly used as a secondaryendpoint in clinical trials and change scores are clearlymeaningful to boys and their families as they relate to activitiesof daily living.

Eugenio Mercuri presented data concerning the usefulnessof the Griffiths Locomotor Scale (0–8 years), Bayley III MotorScale (1–42 months), and the NSAA in DMD patients 3.5 yearsand older. These scales show promise for clinical trials ininfants and toddlers with DMD.

Anna Mayhew informed workshop participants thatmeasures for non-ambulant patients had been developed as wellwith the aim of providing data on clinically meaningful changein this population. One of those measures is the Performance ofUpper Limb (PUL), which was specifically designed to assessupper limb function and the associated range of motion inDMD [27]. Developing this clinician-reported outcomeassessment tool was a collaboration between patients, theiradvocacy groups and a multi-disciplinary international team ofclinical experts. This collaboration established clinicalmeaningfulness and relevance of individual PUL items toactivities of daily living. The PUL is now recognised as a scalewith robust internal reliability, validity, and hierarchicalscalability that are capable of measuring hand performance,related to activities of daily living, even in the oldest andweakest patients [28]. Long-term functional ability in non-ambulant patients with DMD can also be measured with thevalidated Egen Klassifikation scale (EK), a disease specificmeasure to assess and evaluate the overall physical ability in thelater stages of DMD, not solely as an arm-function measure, butreflecting most of the physical/functional changes [29]. Theinitiatives around outcome measures in DMD over the past fewyears have ensured a clear link between measured performanceand meaningful activities.

3.2. Methods of efficacy measurement – strengthversus function

The draft guidelines from the EMA suggested that ademonstrated treatment effect on muscle strength shouldalways be translated into parameters of motor function, or viceversa and therefore proposed co-primary endpoints to bepre-specified from the domains of motor functioning andmuscle strength for interventional clinical trials in DMD. Thisproposition was discussed in the session chaired by FrancescoMuntoni. Craig McDonald provided data that showed that therelationship between quantitative knee extension strength andwalking function (walking velocity or 6MWT) is not linear butlogarithmic [14,30]. Thus, depending on the stage of the diseasethere is dissociation between strength and function. Kneeextension has been shown to be the lower extremity strength

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

4 S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

most closely associated with ambulatory function. Early in theambulatory phase (4–7 years) large decrements in strength areassociated with little change in function [14,19]. After the ageof 7–8 years a small change in strength is associated with alarge change in function [19]. For example, later in theambulatory phase very large changes in ambulatory functionmeasured by 10 metre walk/run or the 6MWT occur with verysmall changes in knee extension strength. Thus a key issue forchoice of primary endpoint is the lack of a linear correlationbetween muscle strength and function in ambulant DMDpatients, which would cause concern in the use of the 6MWTand muscle strength as co-primary endpoints. Strength may bean appropriate endpoint for therapeutics leading to short-termbenefit in terms of increased force production of fibres.Strength on the other hand did not appear to be an appropriateendpoint for clinical trials of therapeutics that stabilisefunctional loss without changing strength (e.g. dystrophinrestoration). It was argued that for many therapeutics it wouldbe better not to dilute the effect of the most sensitive andmeaningful primary endpoint (function as measured by the6MWT) by adding a pre-specified co-primary endpoint that wasnot as sensitive in specific stages of disease or for specificmechanisms (strength). Most importantly statistical power willbe compromised by the use of co-primary endpointsparticularly when one (function) is more responsive to therapyand more meaningful to patients than the other (strength). Itwas concluded that while some therapeutics may be bestassessed by measures of function without need for additionalstrength measures, clinical trials of therapeutics that lead tostrength and/or lean muscle tissue increments as primaryoutcomes should also include functional measuresas key secondary endpoints to demonstrate the clinicalmeaningfulness of a given change in strength or functionalmuscle tissue by imaging.

Clinical trials using ambulation as primary outcomemeasure are compromised by loss of ambulation which canoccur rapidly in DMD, i.e. over a trial period of 12 monthsin patients with a 6MWT distance of up to 250 metres atinclusion and future treatment indications will target alsonon-ambulant patients. Therefore objective measures of forceand function of the upper extremity, which will be preservedacross the period when ambulation is lost and long termthereafter such as the PUL, are of interest. Thomas Voitpresented a validated set of tools which measure distal arm andhand function in a 30 second timed test (MOVIPLATE) andgrip force (MYOGRIP) as well as pinch force (MYOPINCH)which can be applied to DMD patients until the second andthird decades [31]. Longitudinal studies show a measurabledecline of grip and pinch force in non-ambulant DMD patientsover a 12 month interval which is correlated to functional scalessuch as MFM or the Cochin scale, and also to lung vitalcapacity.

3.3. Animal models and biochemical outcome measures

In the draft guidelines of the EMA it was pointed out that theproposed mechanism of action of any new product should bedescribed and discussed in relation to possible testing in

available animal models. Both the mdx mouse and the goldenretriever muscular dystrophy (GRMD) dog were mentioned aspotential animal models for dystrophin deficiency, but it wasremarked that the mdx mouse is considered a poor model of theDMD phenotype, while the predictive value of results in theGRMD dog is still unknown. In the workshop session on animalmodels and biomarkers chaired by Annemieke Aartsma-Rus,Annamaria De Luca and Dominic Wells informed participantsabout the standard operating procedures for the assessmentof therapeutics in the mdx mouse and the GRMD dogthat were developed by TREAT-NMD (http://www.treat-nmd.eu/research/preclinical/dmd-sops/) and emphasised theimportance of controlled and standardised experimentalsettings to minimise the known limitations of both models andenhance predictability of the data [3,4]. Annamaria De Lucaalso underlined that both animal models greatly contributedto understand pathological events consequent to dystrophinabsence and to advance in designing promising therapies.Dominic Wells further showed that in mdx studies done by himand Annemieke Aartsma-Rus, low levels of dystrophin restoredmuscle function but not muscle strength [32,33].

Allison Morgan (Prosensa Therapeutics, The Netherlands)and Annemieke Aartsma-Rus explained that potentialbiomarkers need to demonstrate robustness and be validatedtechnically (sensitivity and reproducibility) and functionallyin well-defined patient cohorts. To allow use as surrogateendpoints the biomarkers should correlate to defined clinicaloutcome measures or be predictive of clinical benefit in thefuture. In many of the clinical studies to date the re-expressionof dystrophin has been suggested as an appropriate biomarkerfor both drug effect (pharmacodynamics biomarker) as wellas the restoration of muscle fibre integrity. In the 2009workshop with EMA, specific limitations on the techniquesused to reliably measure dystrophin production werehighlighted. Allison Morgan briefly presented the outcomeof an international effort to improve the standard operativeprocedures related to dystrophin production.

The assumption that dystrophin restoration should beaccompanied by improved muscle function is based on themilder BMD patients and on studies in the mdx and GRMDanimal models. The negative outcome of a Phase III study inwhich increased production of dystrophin did not show clearclinical benefit challenges this assumption. It should also beconsidered that dystrophin restoration is likely to have adifferent effect depending on the stage of progression of thedisease and the extent of muscle fibre loss before treatment wasinitiated.

Participants were informed about past and current effortsto identify serum biomarkers for DMD and ongoing work innatural history studies and clinical trials [34]. This workshould contribute to the identification of suitable biomarkersthrough the application of advanced omics technologies.Several publications have reported the discovery of candidatebiomarkers, with many identified in animal models or singlepatient studies. One candidate biomarker for DMD is serummatrix metalloproteinase-9 (MMP9) [35], which is involved inthe breakdown of extracellular matrix, notably collagen IV and

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

5S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

V. Levels of serum MMP9 have been found higher in the mdxmouse, DMD and BMD patients compared to age-matchedcontrols, and levels increase with disease progression in DMDpatients. Currently, MMP9 is an exploratory marker in exonskipping trials using antisense oligonucleotides.

Muscle-specific miRNAs (myomiRs) found in serum arealso possible candidate biomarkers and increased levels havebeen reported in the mdx mouse and DMD patients [36–39].The studies in the mdx mouse have shown that serum levelsof myomiRs normalise after exon skipping. To date, onlysmall patient cohorts have been studied for these candidatebiomarkers and further validation is needed. Future work onserum biomarkers will involve a number of stakeholdermeetings to address candidate biomarker discovery andvalidation, as well as similar efforts that are ongoing for othersurrogate markers, such as imaging. Indeed an additional toolthat is increasingly used to establish quantitative and objectiveoutcome measures in DMD and other muscle diseases ismagnetic resonance imaging and spectroscopy (MRI/MRS) andfirst promising results have now been published [40–44]. Again,a number of ongoing natural history studies and interventionaltrials have now included MRI and MRS as exploratoryendpoints.

3.4. Strategy and design of clinical studies – extrapolating toother stages of the disease

Despite the fact that DMD is one of the most commonneuromuscular diseases, patient numbers are limited andmutation- or stage-specific inclusion criteria for clinical trialsfurther narrow down the number of eligible patients. Becauseof these limitations, randomised controlled trials will rather bethe exception than the rule in rare diseases. Thomas Voit andNathalie Goemans facilitated the discussion in the final sessionof the workshop. They emphasised the need for flexible trialdesign, which takes into account the rapidly evolving field andemerging data. The development of new therapeutic candidatesalso requires that trials consider other stages of the disease,such as the non-ambulant and very young DMD patients(<5 years). These populations bring additional challenges todesigning pivotal trials based on the reduced amount of muscletissue in the non-ambulant patients and regulatory concerns inrecruiting very young patients into trials without the availabilityof sufficient safety data in this population.

Natural history studies are ongoing in both young and non-ambulant DMD patients, but more data are needed, and newoutcome measures are required to address the needs of thesepopulations, along with validated biomarkers. Given therelatively slow, progressive nature of the disease, pivotal studiesmay need to be extended over more years to confirm therapeuticvalue and reduce adverse events. However, extending trialsup to 5 years would be seen as a barrier to bringing drugs tomarket by industry. Principally regulators agree that long-termmonitoring would potentially provide the information that isrequired before market authorisation could be granted. Theextrapolation of data to other groups and stages of the diseasewould be required, as data are limited due to the smallpopulation size, and could form part of the development phase.

This could then be supplemented with conditional approval anda monitoring programme to eventually collect and provide therequired data for regulatory submission. Regulatory agencies,such as the EMA, are willing to provide advice on the use ofextrapolation in future clinical studies. The field has noticedthat competing clinical trials quickly reduce the number ofeligible patients further. With a decreasing patient populationavailable for new studies, it is necessary to look at other studydesigns besides the more traditional full pivotal programme.This would include smaller unpowered studies and adaptiveseamless Phase II/III studies with short and long-termfollow-up as well as extrapolation of data from other studies.Some studies may have to rely on patients at different stagesof the disease (ambulant and non-ambulant) and so theuse of personalised endpoints will be required. The patientcommunity believes it is essential to underline the high medicalneed as well as the risk of doing nothing as there are fewoptions for patients with rare, progressive, debilitating, andlife-limiting conditions, such as DMD. Extrapolation of datafrom other groups of patients should be considered as animportant opportunity for the community to help support theongoing discussions on adaptive approaches and benefit–riskconsiderations in clinical research [45].

An additional challenge with trial design in small patientpopulations is the use of small placebo controlled groups anddata from contemporaneous controls, who have received similarstandards of care as those subjects in the trial. A number ofnatural history studies and placebo arms from previous trialshave shown a decline in the 6MWT over time. However, due tothe heterogeneity in the DMD population there is variabilityacross the various studies to date. There are currently a numberof natural history studies ongoing. It is hoped data from thesestudies collected over the coming years will be collated andused as reference information for future studies, particularlythose where the study population would be too small for aproperly powered placebo-controlled trial.

Extrapolating to young patients is an importantconsideration as the ultimate goal of many therapeuticapproaches is dystrophin restoration, which relies on themuscle quality at treatment initiation. As such restoringdystrophin in the very young, where muscle quality is stillrelatively good, would be expected to give the best opportunityto prevent muscle damage and slow down disease progression.However, there are some feasibility issues with conductingtrials in young children due to compliance with outcomemeasures and the age at diagnosis. In addition, there canbe confounding issues, such as the age at which corticosteroidsare started and the potential impact of variable delayedpsychomotor development on an outcome measure.

Extrapolating to non-ambulant patients opens up another setof issues. Maintenance of upper limb function, cardiac andrespiratory function is important for all patients, but a primaryconcern for non-ambulant patients. For this patient populationoutcome measures are in development using DMD specificassessment tools for upper limb function and patient-reported outcome measures (Table 1). The use of multipleassessment tools will be essential in this population as the

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

6 S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

individual response to treatment will vary due to theheterogeneity of affected muscles and it is anticipated that anobservable treatment effect on muscle function will take longerthan in younger patients. Notably, there is evidence thatsuggests that certain mutations lead to a slightly differentdisease progression [46], implying that when a potentialtreatment is tested in patients with a certain mutation, oneshould probably not use natural history data from patients withanother DMD mutation as a reference. Therefore, personalnatural history data may help to show an effect once the patientis recruited into a trial.

Given the natural history of the disease it will be importantto extend treatment studies to both young infants/childrenand to older, non-ambulant patients. Fig. 1 shows a conceptualframework for natural history across the lifespan of DMD anddemonstrates the changing natural history due to interventionsuch as corticosteroids, provision of non-invasive ventilation,and cardiac management with pharmacologic afterloadreduction and management of heart failure. Fig. 2 shows aschematic conceptual framework for more slowly progressiveBMD, highlighting the fundamental differences in bothfunctional abilities and survival between BMD and DMD (themean age of survival in BMD is 67 years). There are ongoingstudies to adequately capture the natural history in these diseasestates and they will help to inform future studies and theuse of appropriate assessment tools. A decision framework forinclusion of clinical outcome measures in Phase II/III trials waspresented (Table 1) to illustrate the multiple factors to considerwhen choosing the most optimal endpoints. However, it is stillnot known which will be the most sensitive parameters to usethat will indicate a response to a specific treatment, but giventhe progress over the last few years it would be desirable to

reassess progress on a regular (annual) basis on the utility of thetools currently in development.

4. Conclusions

Important messages from the DMD workshop that TREAT-NMD and EMA co-organised in London in 2009 [10] were thatnatural history studies and outcome measures should be revisedand expanded to include both younger and non-ambulantpatients with DMD and those interventional randomisedplacebo-controlled trials should show the functional relevanceof the proposed outcome measures and of increased dystrophinlevels. At the current TREAT-NMD workshop in London itbecame apparent that over the past 4 years the neuromuscularcommunity has put a lot of effort into the collection of naturalhistory data and the prospective evaluation of an extendedpanel of outcome measures, including also very young andnon-ambulant DMD patients. In addition, various studies havelooked more closely at clinical, muscle biopsy and biomarkerdata from patients with BMD. Networks like CINRG andTREAT-NMD in close collaboration with patient advocacygroups and pharmaceutical companies have worked together toachieve trial readiness for patients with DMD and have helpedto launch a number of large interventional industry sponsoredtrials.

Shortly after the workshop in London it was announced byGlaxoSmithKline (GSK) and Prosensa that their Phase IIIDMD clinical study of drisapersen, an investigational antisenseoligonucleotide for exon skipping of exon 51, did not meet theprimary endpoint of a statistically significant improvement inthe 6MWT compared to placebo. There was also no treatmentdifference in key secondary assessments of motor function,

Fig. 2. Schematic natural history of BMD.

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

7S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

including the 10 metre walk/run test, the 4-stair climb and theNorth Star Ambulatory Assessment. This news was of coursea big disappointment for the entire patient and clinicalcommunity. The results came somewhat unexpected, as earlierPhase II clinical trials with the same compound had shownpromising results. The different results between the earlierstudies and the Phase III trial may be explained by a moresevere and more heterogeneous cohort in the Phase III trial(and more heterogeneous standards of care). Exon skippingdystrophin restoration therapy appears to have greater impacton measures of ambulatory capacity and gross motor measuresin younger, more functional patients with more preservedmuscle fibres. In addition, precipitous declines in selectedpatients with greater disease severity produce variability inclinical measures and negate those benefits seen in less severepatients. Subsequent analyses of patients in extension protocolsacross the entire drisapersen development programme seem toindicate that more severe patients demonstrate benefits at twoyears’ follow-up, and that in the future, longer duration studiesmay be necessary to demonstrate benefit in more severelyaffected patients with DMD. Although biopsy and other moredetailed data from the study have not yet been published, thetrial showed how challenging it is to recruit a homogenousand comparable patient cohort in a rare disease sub-population.Shortly after the GSK announcement Sarepta Therapeuticsreported that the U.S. Food and Drug Administration(FDA) considered its application for accelerated approvalof eteplirsen, another investigational antisense oligonucleotidefor exon skipping of exon 51 in DMD patients, to be premature.The FDA decision also illustrates that more data needto be collected that reliably show a positive correlationbetween dystrophin restoration (which was conclusivelydemonstrated in the eteplirsen treated patients) and clinicalbenefit.

The neuromuscular community has been very successfulover the past few years in collecting clinical, genetic, musclebiopsy, serum biomarker and imaging data to monitor skeletalmuscle pathology in DMD and increasingly also in BMDpatients (Fig. 2). Additional data have been collected oncardiac, respiratory, gastrointestinal, behavioural and otherorgan functions. Both DMD and BMD are complex multi-system diseases that have only fairly recently attracted theattention of pharmaceutical companies. Despite the fact thatthere is still no licensed drug with a specific indication forDMD and BMD and that the results from recent interventionaltrials have been somewhat disappointing, the improvedunderstanding of the natural history of dystrophin deficiencyand the wealth of the recently collected outcome measure dataforms a very good foundation to inform new trials and drugdevelopment programmes. Linking positive findings of clinicaltrials to novel biomarkers such as those derived from omicstechnologies and MRI/MRS should both reduce the future needfor invasive monitoring and also allow investigators to assessresponse in smaller cohorts of DMD patients, which will beessential when experimental approaches such as exon skippingmove to rarer exons for which large studies will never befeasible.

5. List of participants

Annemieke Aartsma-Rus (Leiden, The Netherlands)Pavel Balabanov (London, UK)Ralph Bax (London, UK)Enrico Bertini (Rome, Italy)Filippo Buccella (Parent Project Onlus representative)Didier Caizergues (Paris, France)Annamaria De Luca (Bari, Italy)Michelle Eagle (Newcastle, UK)Andre Elferink (London, UK)Juliet Ellis (London, UK)Philippa Farrant (Duchenne Family Support Group

representative)Julaine Florence (St. Louis, USA)Keith Foster (Reading, UK)Pat Furlong (PPMD representative)Nathalie Goemans (Leuven, Belgium)Michela Guglieri (Newcastle, UK)Joseph Irwin (Lakeside Regulatory Consulting Services)John Johnston (London, UK)Stephen Lynn (Newcastle, UK)Craig McDonald (Davis, USA)Anna Mayhew (Newcastle, UK)Eugenio Mercuri (Rome, Italy)Allison Morgan (Leiden, The Netherlands)Francesco Muntoni (London, UK)Erik Niks (Leiden, The Netherlands)Marita Pohlschmidt (MDC representative)Diana Riberio (Action Duchenne representative)Valeria Ricotti (London, UK)Agata Robertson (Newcastle, UK)Laurent Servais (Paris, France)Volker Straub (Newcastle, UK)Jan Verschuuren (Leiden, The Netherlands)Thomas Voit (Paris, France)Elizabeth Vroom (DPP representative)Dominic Wells (London, UK)

Acknowledgements

We would like to express our thanks to the MuscularDystrophy Campaign, the Parent Project Muscular Dystrophy,the United Parent Projects Muscular Dystrophy, the AFMTelethon and the BMBS COST Action BM1207 for kindlyfunding the workshop, which was held at the WellcomeTrust in London. Thanks also go to the secretariat of theTREAT-NMD Alliance at Newcastle University for organisingthe meeting.

References

[1] Hoffman EP. Genotype/phenotype correlations in Duchenne/Beckerdystrophy. Mol Cell Biol Hum Dis Ser 1993;3:12–36.

[2] Willmann R, De Luca A, Benatar M, et al. Enhancing translation:guidelines for standard pre-clinical experiments in mdx mice.Neuromuscul Disord 2012;22:43–9.

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

8 S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

[3] Nagaraju K, Willmann R. Developing standard procedures for murine andcanine efficacy studies of DMD therapeutics: report of two expertworkshops on “Pre-clinical testing for Duchenne dystrophy”: WashingtonDC, October 27th–28th 2007 and Zurich, June 30th–July 1st 2008.Neuromuscul Disord 2009;19:502–6.

[4] Willmann R, Possekel S, Dubach-Powell J, Meier T, Ruegg MA.Mammalian animal models for Duchenne muscular dystrophy.Neuromuscul Disord 2009;19:241–9.

[5] Bladen CL, Rafferty K, Straub V, et al. The TREAT-NMD Duchennemuscular dystrophy registries: conception, design, and utilization byindustry and academia. Hum Mutat 2013;34:1449–57.

[6] Rodger S, Lochmüller H, Tassoni A, et al. The TREAT-NMD care andtrial site registry: an online registry to facilitate clinical research forneuromuscular diseases. Orphanet J Rare Dis 2013;8:171.

[7] Bushby K, Finkel R, Birnkrant DJ, et al. Diagnosis and managementof Duchenne muscular dystrophy, part 2: implementation ofmultidisciplinary care. Lancet Neurol 2010;9:177–89.

[8] Bushby K, Finkel R, Birnkrant DJ, et al. Diagnosis and management ofDuchenne muscular dystrophy, part 1: diagnosis, and pharmacologicaland psychosocial management. Lancet Neurol 2010;9:77–93.

[9] Sejerson T, Bushby K. Standards of care for Duchenne musculardystrophy: brief TREAT-NMD recommendations. Adv Exp Med Biol2009;652:13–21.

[10] Muntoni F, Meeting Steering Committee, TREAT-NMD Network. Thedevelopment of antisense oligonucleotide therapies for Duchennemuscular dystrophy: report on a TREAT-NMD workshop hosted bythe European Medicines Agency (EMA), on September 25th 2009.Neuromuscul Disord 2010;20:355–62.

[11] ATS statement: guidelines for the six-minute walk test. Am J Respir CritCare Med 2002;166:111–17.

[12] McDonald CM, Henricson EK, Han JJ, et al. The 6-minute walk test as anew outcome measure in Duchenne muscular dystrophy. Muscle Nerve2010;41:500–10.

[13] McDonald CM, Henricson EK, Han JJ, et al. The 6-minute walk test inDuchenne/Becker muscular dystrophy: longitudinal observations. MuscleNerve 2010;42:966–74.

[14] McDonald CM, Henricson EK, Abresch RT, et al. The 6-minute walk testand other clinical endpoints in duchenne muscular dystrophy: reliability,concurrent validity, and minimal clinically important differences from amulticenter study. Muscle Nerve 2013;48:357–68.

[15] McDonald CM, Widman LM, Walsh DD, Walsh SA, Abresch RT. Use ofstep activity monitoring for continuous physical activity assessment inboys with Duchenne muscular dystrophy. Arch Phys Med Rehabil2005;86:802–8.

[16] Goemans N, van den Hauwe M, Wilson R, van Impe A, Klingels K, BuyseG. Ambulatory capacity and disease progression as measured by the6-minute-walk-distance in Duchenne muscular dystrophy subjects ondaily corticosteroids. Neuromuscul Disord 2013;23:618–23.

[17] Mazzone E, Martinelli D, Berardinelli A, et al. North Star AmbulatoryAssessment, 6-minute walk test and timed items in ambulantboys with Duchenne muscular dystrophy. Neuromuscul Disord2010;20:712–16.

[18] Ricotti V, Ridout DA, Scott E, et al. Long-term benefits andadverse effects of intermittent versus daily glucocorticoids in boys withDuchenne muscular dystrophy. J Neurol Neurosurg Psychiatry2013;84:698–705.

[19] McDonald CM, Henricson EK, Abresch RT, et al. The 6-minute walk testand other endpoints in Duchenne muscular dystrophy: longitudinalnatural history observations over 48 weeks from a multicenter study.Muscle Nerve 2013;48:343–56.

[20] Mazzone ES, Pane M, Sormani MP, et al. 24 month longitudinal datain ambulant boys with Duchenne muscular dystrophy. PLoS ONE2013;8:e52512.

[21] Henricson E, Abresch R, Han JJ, et al. The 6-minute walk testand person-reported outcomes in boys with duchenne muscular dystrophyand typically developing controls: longitudinal comparisons andclinically-meaningful changes over one year. PLoS Currents 2013;5.doi:10.1371/currents.md.9e17658b007eb79fcd6f723089f79e06.

[22] McDonald CM, McDonald DA, Bagley A, et al. Relationship betweenclinical outcome measures and parent proxy reports of health-relatedquality of life in ambulatory children with Duchenne muscular dystrophy.J Child Neurol 2010;25:1130–44.

[23] Scott E, Eagle M, Mayhew A, et al. Development of a functionalassessment scale for ambulatory boys with Duchenne musculardystrophy. Physiother Res Int 2012;17:101–9.

[24] Mazzone ES, Messina S, Vasco G, et al. Reliability of the North StarAmbulatory Assessment in a multicentric setting. Neuromuscul Disord2009;19:458–61.

[25] Mayhew A, Cano S, Scott E, Eagle M, Bushby K, Muntoni F. Onbehalf of The North Star Clinical Network for PaediatricNeuromuscular Disease. Moving towards meaningful measurement:Rasch Analysis of North Star Ambulatory Assessment in Duchennemuscular dystrophy. Developmental Medicine & Child Neurology2011;53:535–42.

[26] Mayhew AG, Stefan J, Cano SJ, et al. The North Star Clinical Networkfor Neuromuscular Disease. Detecting meaningful change using theNorth Star Ambulatory Assessment in Duchenne muscular dystrophy.Developmental Medicine & Child Neurology 2013;55:1046–52.

[27] Mayhew A, Mazzone ES, Eagle M, et al. Development of the Performanceof the Upper Limb module for Duchenne muscular dystrophy. Dev MedChild Neurol 2013;55:1038–45.

[28] Pane M, Mazzone ES, Fanelli L, et al. Reliability of the Performance ofUpper Limb assessment in Duchenne muscular dystrophy. NeuromusculDisord 2014;24:201–6.

[29] Lyager S, Steffensen B, Juhl B. Indicators of need for mechanicalventilation in Duchenne muscular dystrophy and spinal muscular atrophy.Chest 1995;108:779–85.

[30] Abresch RT, Carter GT, Han JJ, McDonald CM. Exercise inneuromuscular diseases. Phys Med Rehabil Clin N Am 2012;23:653–73.

[31] Servais L, Deconinck N, Moraux A, et al. Innovative methods to assessupper limb strength and function in non-ambulant Duchenne patients.Neuromuscul Disord 2013;23:139–48.

[32] van Putten M, Hulsker M, Young C, et al. The effects of low levels ofdystrophin on mouse muscle function and pathology. PLoS ONE2012;7:e31937.

[33] Sharp PS, Bye-a-Jee H, Wells DJ. Physiological characterisationof muscle strength with variable levels of dystrophin restorationin mdx mice following local antisense therapy. Molecular Therapy2011;19:165–71.

[34] Aartsma-Rus A, Ferlini A, Vroom E. Biomarkers and surrogateendpoints in Duchenne: meeting report. Neuromuscul Disord2014;24:743–5.

[35] Nadarajah VD, van Putten M, Chaouch A, et al. Serum matrixmetalloproteinase-9 (MMP-9) as a biomarker for monitoring diseaseprogression in Duchenne muscular dystrophy (DMD). NeuromuscularDisord 2011;21:569–78.

[36] Hu J, Kong M, Ye Y, Hong S, Cheng L, Jiang L. Serum miR-206 and othermuscle-specific microRNAs as non-invasive biomarkers for Duchennemuscular dystrophy. J Neurochem 2014;129:877–83.

[37] Mizuno H, Nakamura A, Aoki Y, et al. Identification of muscle-specificmicroRNAs in serum of muscular dystrophy animal models: promisingnovel blood-based markers for muscular dystrophy. PLoS ONE2011;6:e18388.

[38] Vignier N, Amor F, Fogel P, et al. Distinctive serum miRNA profilein mouse models of striated muscular pathologies. PLoS ONE2013;8:e55281.

[39] Zaharieva IT, Calissano M, Scoto M, et al. Dystromirs as serumbiomarkers for monitoring the disease severity in Duchenne muscularDystrophy. PLoS ONE 2013;8:e80263.

[40] Hollingsworth KG, Garrood P, Eagle M, Bushby K, Straub V. Magneticresonance imaging in Duchenne muscular dystrophy: longitudinalassessment of natural history over 18 months. Muscle Nerve2013;48:586–8.

[41] Finanger EL, Russman B, Forbes SC, Rooney WD, Walter GA,Vandenborne K. Use of skeletal muscle MRI in diagnosis and monitoring

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

9S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■

disease progression in Duchenne muscular dystrophy. Phys Med RehabilClin N Am 2012;23:1–10, ix.

[42] Fischmann A, Hafner P, Gloor M, et al. Quantitative MRI and loss of freeambulation in Duchenne muscular dystrophy. J Neurol 2013;260:969–74.

[43] Forbes SC, Lott DJ, Finkel RS, et al. MRI/MRS evaluation of a femalecarrier of Duchenne muscular dystrophy. Neuromuscul Disord2012;22(Suppl. 2):S111–21.

[44] Willcocks RJ, Arpan IA, Forbes SC, et al. Longitudinal measurements ofMRI-T2 in boys with Duchenne muscular dystrophy: effects of age anddisease progression. Neuromuscul Disord 2014;24:393–401.

[45] Franson TR, Peay H Benefit-risk assessments in rare disorders. ParentProject Muscular Dystrophy. <http://bit.ly/1o8Ggna>.

[46] Pane M, Mazzone ES, Sormani MP, et al. 6 Minute walk test in DuchenneMD patients with different mutations: 12 month changes. PLoS ONE2014;9(1):e83400.

ARTICLE IN PRESS

Please cite this article in press as: Stephen Lynn, et al., Measuring clinical effectiveness of medicinal products for the treatment of Duchenne muscular dystrophy, NeuromuscularDisorders (2014), doi: 10.1016/j.nmd.2014.09.003

10 S. Lynn et al. /Neuromuscular Disorders ■■ (2014) ■■–■■