Prosensa Regains Rights to Drisapersen From GSK and Retains Rights to All Other Programs for the Treatment of Duchenne Muscular Dystrophy (DMD)

LEIDEN, Netherlands and LONDON, Jan. 13, 2014 (GLOBE   NEWSWIRE) — Prosensa Holding N.V. (Nasdaq:RNA) and GlaxoSmithKline (GSK)   today announced that Prosensa has regained all rights from GSK to drisapersen   and will retain rights to all other programs for the treatment of Duchenne   Muscular Dystrophy (DMD). This transfer of rights represents the termination   of the collaboration agreement between GSK and Prosensa executed in 2009.

Prosensa will now have the full, unencumbered rights to continue   the development of drisapersen as well as each of its DMD programs.

The parties have agreed that Prosensa is well suited to continue   the development of all of the DMD programs. Prosensa and GSK have also agreed   to make certain data from the drisapersen studies available in due course to   the scientific community for the purpose of furthering the general   understanding of DMD.

“We are fully committed to our mission of developing   innovative, RNA-based therapeutics to address unmet medical needs for   patients with rare genetic disorders,” said Hans Schikan, Prosensa’s   Chief Executive Officer. “Prosensa is now in a favorable strategic   position to advance the DMD portfolio, which includes drisapersen and five   additional compounds, three of which are currently in clinical development.   We will continue to work closely with patient groups, investigators, academia   and regulators to ensure that we do everything we can to bring treatments to   boys affected by DMD.”

“We have completed our review and believe that further   analysis of drisapersen data may benefit from being viewed in the context of   the body of information which has been developed by Prosensa through their   work on additional exon-skipping programs. We greatly valued the scientific   expertise that Prosensa provided since our collaboration started in   2009.” said Dr Carlo Russo, Senior Vice President, Head of GSK Rare   Diseases Research & Development. “We worked closely with Prosensa   with the aim of developing new medicines to help boys with DMD and thanks to   the dedication of the boys and their families, we have gathered the largest   clinical dataset related this condition. The value of this data to the   scientific community’s understanding of DMD should not be underestimated, and   we would like to thank everyone who participated in the studies for their   contributions and patience as we worked through our analyses.”

Prosensa’s Schikan, added “GSK has been a valuable   development partner, and we are grateful for the commitment made by the   company over the last four years to develop a disease modifying therapy for   this devastating disease. Together we have been able to progress the largest   global clinical program in DMD, and we have made tremendous progress in   understanding the natural history and epidemiology of this rare and   devastating disease.  We look forward to updating the community with our   plans as soon we are in a position to do so.”

About drisapersen and the   clinical development program

Drisapersen, (previously GSK2402968/PRO051), an antisense   oligonucleotide which induces exon skipping of exon 51, is currently in-late   stage development for DMD. In 2009 GSK obtained an exclusive worldwide   license from Prosensa to develop and commercialize drisapersen. Drisapersen   has orphan drug status in the EU, US, Japan and Australia. In June 2013,   drisapersen was granted Breakthrough Therapy designation by the US Food and   Drug Administration.

The overall drisapersen clinical program comprises three   double-blind, placebo-controlled studies (DMD114117, DMD114876 and DMD114044)   and two long term open-label extension studies (DMD114673 and DMD114349).

For more information regarding the ongoing clinical studies   involving drisapersen visit www.clinicaltrials.gov.

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 use a wheelchair   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, myotonic dystrophy and Huntington’s disease.

www.prosensa.com

GSK – one   of the world’s leading research-based pharmaceutical and healthcare companies   – is committed to improving the quality of human life by enabling people to   do more, feel better and live longer. For further information please   visit www.gsk.com.

Prosensa 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 our 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 our exon -skipping drug candidates and our   collaboration with GlaxoSmithKline (GSK). 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 certain sections of the Company’s Registration Statement on Form   F-1. In addition, any forward-looking statements represent our views only as   of today and should not be relied upon as representing our views as of any   subsequent date. While we may elect to update these forward-looking   statements at some point in the future, we specifically disclaim any   obligation to do so, even if our views change.

GSK Cautionary statement   regarding forward-looking statements

GSK cautions investors that any forward-looking statements or   projections made by GSK, including those made in this announcement, are   subject to risks and uncertainties that may cause actual results to differ   materially from those projected. Factors that may affect GSK’ s operations   are described under Item 3.D ‘Risk factors’ in the company’s Annual Report on   Form 20-F for 2012.

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