Prosensa Update: Regulatory Discussions Ongoing & Re-dosing on Track

Dear Patient Group Representative,

We trust this letter finds you well. As announced in a press release and during our quarterly update for the financial community on May 20, we are encouraged by our ongoing interactions with the regulatory authorities, in both the United States and Europe, and remain on track to communicating a potential regulatory path forward for drisapersen by the end of June.

We also remain on track to re-dosing the first group of boys in the 3rd quarter of this year. Re-dosing is taking a staged approach, under new or existing treatment protocols or via expanded access programs. In a first wave, we will initiate re-dosing at sites in both North America and Europe, and we are actively working with these sites and investigators to execute these plans.

We held an investigator briefing on May 14 to review the planned re-dosing protocol with US and Canadian sites. Eligible boys are those who completed DEMAND V (DMD114876), those that are currently in the DMD115501 protocol and US/Canadian boys who participated in the DEMAND IV study (DMD114349). Subjects will initially be dosed in the clinic for the first 4 weeks, but home dosing is intended, subject to site requirements. The protocol has been revised with the aim of minimizing the burden of participating in clinical trials for patients, while still collecting valuable scientific data. To this end, no biopsies will be taken under this treatment protocol and home dosing will be implemented where possible.

More detailed information about the re-dosing protocol for the first group of boys in Europe will follow. We have added a dedicated page to the Prosensa website at www.prosensa.eu/patients-and-family/drisapersen-re-dosing to enable easier tracking of the latest information regarding drisapersen re-dosing. Families and patients that have previously participated in drisapersen clinical trials are encouraged to remain in contact with the investigator at their site.

On April 30, during the American Academy of Neurology (AAN) Annual Meeting, 48 week follow up data from DEMAND IV (DMD114349) involving 113 patients were presented by Dr. Nathalie Goemans, Head of the Neuromuscular Reference Center for Children at the University HospitalsLeuven (UHL) in Belgium and one of the key investigators in the drisapersen clinical program. This presentation included data from 69 patients with 96 weeks of drisapersen treatment. The data are supportive of the hypothesis that treating earlier in the disease and treating for a longer durationconfers a treatment benefit for boys with DMD. Full information regarding the efficacy and safety details presented are contained in the poster which was presented at the AAN Annual Meeting and can be accessed here.

Prosensa’s next most advanced compound for DMD, PRO044, has completed a phase l/ll study in Europe, and an extension study is planned for the second half of this year. PROO45 and PRO053 are currently in phase I/ll clinical trials and we expect confirmatory studies for these products to start in the first half of 2015. PRO052 and PROO55 are in advanced preclinical development. PROSPECT is in early development and applies multiple exon skipping focused on the exon 10-30 region.

We continue to expand our knowledge of the disease via our Research and Development pipeline but also through our comprehensive Natural History study which commenced enrollment last summer. We are pleased to announce that the 250 patient Natural History study has almost completed enrollment, with 247 patients enrolled to date. This study aims to characterize the natural history and progression of DMD to help inform the design of future studies, to capture biomarkers of safety and disease progression and to provide comparative data for the development of rare exons for which formal controlled trials are not feasible.

Prosensa is dedicated to enabling long term patient access to drisapersen and our follow-on candidates as novel treatments for DMD. We are determined to accomplish this as soon as possible.

We would like to express our gratitude for the support we are receiving from patient groups worldwide and encourage you to remain in close contact.

With kind regards,

Giles Campion, MD — Chief Medical Officer and SVP Research & Development

Claire Leyten, PharmD — Manager Patient Group Relations and primary point of contact for enquiries from patients and their families (patientinfo@prosensa.nl)