Prosensa Completes Enrollment in Natural History Study of Duchenne Muscular Dystrophy

Prosensa Completes Enrollment in Natural History Study of Duchenne Muscular
Dystrophy

Leiden, The Netherlands, June 25, 2014 (GLOBE NEWSWIRE) -- Prosensa Holding
N.V. (NASDAQ: RNA), the Dutch biopharmaceutical company focusing on
RNA-modulating therapeutics for rare diseases with high unmet need, today
announced the successful completion of enrollment in its prospective Natural
History Study designed to increase the understanding of Duchenne muscular
dystrophy (DMD).

The study enrolled in total 269 boys with confirmed DMD between the ages three
and 18  from 10  countries across  16 centers  in North  & South  America  and 
Europe, 80% of whom are ambulatory and 20% non-ambulatory.

The  purpose  of  the  study  is  to  characterize  the  natural  history  and 
progression of DMD, which  will help inform the  design of future studies,  to 
capture  biomarkers  of  safety  and  disease  progression,  and  to   provide 
comparative data for  patients with  rare exon  deletions/mutations for  which 
formal controlled trials are  not feasible. No medication  or device is  being 
tested  in  this  study.  Moreover,  the  study  will  be  important  in   the 
confirmatory program for Prosensa's lead DMD compound drisapersen, for which a
New Drug Application is expected  to be submitted later  this year to the  FDA 
under an Accelerated Approval pathway.

Dr. Brenda Wong of Cincinnati Children's Hospital, who has enrolled patients
in the study said, "While we are pleased by the high level of world-wide
attention and commitment by regulators given to DMD, an existing obstacle for
delivering promising treatments to patients relies on accepted clinical
endpoints for human studies. By collecting and analyzing this Natural History
Study data from hundreds of boys, we are better equipped to expedite the
development of promising therapies and offer them to DMD patients anxiously
awaiting treatment options to slow disease progression and improve the outcome
of this disease."

"We continue to work  diligently to pioneer research  and development in  DMD, 
and completing enrollment in  our Natural History Study  is a major  milestone 
and a critical component  of basic research to  understand the disease,"  said 
Dr. Giles Campion,  Prosensa's Chief Medical  Officer. "This study  will be  a 
crucial resource for  our confirmatory  studies with drisapersen  and for  our 
follow-on exon skipping programs. Outcomes of this study will contribute to  a 
growing body of  scientific knowledge and  are fundamental to  our ability  to 
develop innovative treatment options for boys who suffer from this devastating
disease."

All boys participating in  the study will  be followed for  a period of  three 
years, with site visits and assessments  every 6 months. During these  visits, 
investigators will evaluate the boys on a number of physical tests,  including 
the six minute walk test, climbing stairs, breathing in a tube to measure lung
function and performing upper  limb movements. These  parameters are meant  to 
assess how the disease affects their overall quality of life as the  condition 
evolves over time  in addition  to contributing  to the  understanding of  the 
natural progression of the disease. Furthermore, blood and urine samples  will 
be taken to investigate a panel of biomarkers that may be useful in  following 
disease progression.

The study, which  began in  September 2012 and  is sponsored  by Prosensa,  is 
expected to  complete  in  November  2017. This  autumn,  80  boys  will  have 
completed one year of observation and this interim analysis will be  available 
later this  year.  Further  information  on  the  study  can  be  found  here: 
www.ClinicalTrials.gov- Study ID: NCT01753804

About DMD

Duchenne  Muscular  Dystrophy  (DMD)  is  a  severely  debilitating  childhood 
neuromuscular disease that  affects up to  1 in 3,500  live male births.  This 
rare disease is caused by mutations  in the dystrophin gene, resulting in  the 
absence or defect of the dystrophin protein. Patients suffer from  progressive 
loss of muscle function, often making them wheelchair bound before the age  of 
12. Respiratory and cardiac  muscle can also be  affected by the disease.  Few 
patients survive the age of 30.

About exon skipping

The dystrophin gene is the largest gene  in the body, consisting of 79  exons. 
Exons are small  sequences of genetic  code which lead  to the manufacture  of 
sections of protein. In DMD, when  certain exons are mutated/deleted, the  RNA 
cannot read the genetic  code past the  fault. This prevents  the rest of  the 
exons being read,  resulting in  a non-functional dystrophin  protein and  the 
severe symptoms of DMD.

RNA-based therapeutics, specifically antisense oligonucleotides inducing  exon 
skipping, are currently in development for DMD. This technology uses synthetic
antisense oligonucleotides to  skip an  exon next  to a  deletion and  thereby 
correct the  reading frame,  enabling  the production  of a  novel  dystrophin 
protein. Up to 13%  of boys with DMD  have dystrophin gene  mutation/deletions 
amenable to an exon 51 skip.

About Prosensa Holding N.V.

Prosensa (NASDAQ: RNA) is a Dutch biotechnology company engaged in the
discovery and development of RNA-modulating therapeutics for the treatment of
genetic disorders. Its primary focus is on rare neuromuscular and
neurodegenerative disorders with a large unmet medical need, including
Duchenne muscular dystrophy (DMD), Myotonic dystrophy and Huntington's
disease.

Prosensa's current portfolio includes six compounds for the treatment of DMD,
all of which have received orphan drug status in the United States and the
European Union. The compounds use an innovative technique called exon-skipping
to provide a personalized medicine approach to treat different populations of
DMD patients.www.prosensa.com

Forward Looking Statements

This  press   release  contains   certain  forward-looking   statements.   All 
statements, other than statements of historical facts, contained in this press
release,  including  statements  regarding  the  Company's  strategy,   future 
operations, future  financial  position,  future  revenues,  projected  costs, 
prospects, plans and objectives of management, are forward-looking statements.
The words  "anticipate,"  "believe," "estimate,"  "expect,"  "intend,"  "may," 
"plan," "predict," "project," "target," "potential," "will," "would," "could,"
"should,"  "continue,"  and  similar  expressions  are  intended  to  identify 
forward-looking  statements,  although  not  all  forward-looking   statements 
contain these  identifying words.  Forward-looking  statements in  this  press 
release include statements  around the Company's  exon-skipping drug  pipeline 
and financial  position.  Actual  results may  differ  materially  from  those 
projected or implied in such forward-looking statements. Such  forward-looking 
information involves risks and  uncertainties that could significantly  affect 
expected results. These risks and uncertainties are discussed in the Company's
SEC filings, including, but not limited to, the Company's Form 6-K  containing 
this press release and the Company's Annual Report on Form 20-F. In  addition, 
any forward-looking statements represent its views only as of today and should
not be relied upon as representing its views as of any subsequent date.  While 
the Company may elect to update these forward-looking statements at some point
in the future,  the Company specifically  disclaims any obligation  to do  so, 
even if its views change.

CONTACT: Celia Economides
         Senior Director, IR & Corporate Communications
         c.economides@prosensa.nl

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