Prosensa Initiates First Patient Dosing In Phase I/II Trial Of PRO053

Leiden, The Netherlands, Sept. 6, 2013 (GLOBE NEWSWIRE) -- Prosensa Holding N.V. (NASDAQ: RNA), the Dutchbiopharmaceutical company focusing on RNA-modulating therapeuticsfor rare diseases with high unmet need, today announced the dosingof the first patient in its Phase I/II clinical trial of PRO053, anexon-skipping compound for the treatment of Duchenne musculardystrophy (DMD).

"We are pleased that our fourth exon-skippingcompound, PRO053, has advanced into clinical trials." GilesCampion, Prosensa's Chief Medical Officer, said. "Bringing new drugcandidates, such as PRO053, into the clinic exemplifies how we canleverage our scientific and regulatory expertise gleaned from theexperience in advancing the investigational development compound,drisapersen, through the clinic. Most importantly, a broaderclinical portfolio may offer additional patients with subtypes ofDMD a much-needed potential treatment option."

PRO053, the company's fourth drug developmentcandidate, induces exon 53 skipping and may be applicable toapproximately 8% of all DMD patients.

This is an open-label study designed to assessthe safety, efficacy, tolerability and pharmacokinetics of multipledoses of PRO053 in a subset of patients with DMD. The primaryoutcome measure of the study will be change from baseline in thesix-minute walk test (6MWT) after 48 weeks of treatment. A numberof secondary outcome measures will also be assessed, includingsafety, muscle function and strength, and functional outcome basedon a questionnaire. The initial dose-finding portion of the studywill be conducted at several clinical trial sites in Europe. Thestudy may then be extended to additional territories in and outsideof Europe.

Prosensa's current portfolio includes sixcompounds for the treatment of DMD, all of which have receivedorphan drug status in the United States and the European Union. Thecompounds use an innovative technique called exon-skipping toprovide a personalized medicine approach to treat differentpopulations of DMD patients. The company's lead investigationaldrug candidate, drisapersen, which was exclusively licensed toGlaxoSmithKline, is in Phase III clinical trials.

Notes to editors

About Prosensa Holding N.V.

Prosensa (NASDAQ: RNA) is a Dutch biotechnology company engagedin the discovery and development of RNA-modulating therapeutics forthe treatment of genetic disorders. Its primary focus is on rareneuromuscular and neurodegenerative disorders with a large unmetmedical need, including Duchenne muscular dystrophy, myotonicdystrophy and Huntington's disease.

www.prosensa.com

About DMD

Duchenne muscular dystrophy (DMD) is a severely debilitatingchildhood neuromuscular disease that affects up to 1 in 3,500 boys.This rare disease is caused by mutations in the dystrophin gene,resulting in the absence or defect of the dystrophin protein. As aresult, patients suffer from progressive loss of muscle strength,often rendering them wheelchair-bound before the age of 12. Fewpatients survive the age of 30.

About exon skipping and RNA modulation

The dystrophin gene is the largest gene in the body, consistingof 79 exons. Exons are small sequences of genetic code which, viaan intermediate step involving RNA, lead to the assembly ofsections of protein. In DMD, when certain exons aremutated/deleted, the RNA cannot read past the fault. This preventsthe remainder of the exons from being read, resulting in anon-functional dystrophin protein and the severe symptoms of DMD.RNA-based therapeutics, specifically antisense oligonucleotidesinducing exon skipping, are currently in development for DMD. Theseantisense oligonucleotides skip an exon next to a defective exonand thereby correct the reading frame, enabling the production of anovel, largely functional dystrophin protein. Prosensa's exonskipping technology was licensed from Leiden University MedicalCenter.

Forward Looking Statements

This press release contains certainforward-looking statements.  All statements, other thanstatements of historical facts, contained in this press release,including statements regarding our strategy, future operations,future financial position, future revenues, projected costs,prospects, plans and objectives of management, are forward-lookingstatements. The words "anticipate," "believe," "estimate,""expect," "intend," "may," "plan," "predict," "project," "target,""potential," "will," "would," "could," "should," "continue," andsimilar expressions are intended to identify forward-lookingstatements, although not all forward-looking statements containthese identifying words. Forward-looking statements in this pressrelease include statements around our exon 53-skipping drugcandidate, PRO053 and development plans around this program. Actualresults may differ materially from those projected or implied insuch forward-looking statements.  Such forward-lookinginformation involves risks and uncertainties that couldsignificantly affect expected results.  These risks anduncertainties are discussed in the Company's SEC filings,including, but not limited to, the Company's Form 6-K containingthis press release and certain sections of the Company'sRegistration Statement on Form F-1. In addition, anyforward-looking statements represent our views only as of today andshould not be relied upon as representing our views as of anysubsequent date. While we may elect to update these forward-lookingstatements at some point in the future, we specifically disclaimany obligation to do so, even if our views change.
CONTACT: Prosensa Holding N.V.         Celia Economides, Director IR & Corporate Communications         Phone: +1 917 975 1983         Email: c.economides@prosensa.nl

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