Prosensa Completes Enrollment in Natural History Study of Duchenne Muscular Dystrophy

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| Source: Prosensa Holding N.V.

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.

Celia Economides
Senior Director, IR & Corporate Communications