A Phase I/IIa Clinical Trial in Duchenne Muscular Dystrophy Using Systemically Delivered Morpholino Antisense Oligomer to Skip Exon 53 (SKIP-NMD)

Contract No.: 305370; EU contribution: € 5,512,424.07; Total costs: € 7,527,838.99; Starting date: 01/11/2012; Duration: 42 months

Background and Objectives

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder that affects 1:5,000¹ live males, caused by mutations in the DMD gene. Approximately 70% of DMD mutations are deletions that lead to the disruption of the transcript reading frame, leading to prematurely truncated dystrophin protein, which results in a progressive and severe muscle weakness, loss of ambulation in the midteens, and death by the third decade, because of cardiac or respiratory complications.^2,3 Currently, glucocorticoids are the only drug that has been shown to slow down the progression of DMD.^4,5 Becker muscular dystrophy (BMD) is a considerably milder allelic form of the disease, caused by mutations that maintain the reading frame, allowing the production of a shorter but functional protein product. ⁶ Exon skipping aims to modulate the pre-mRNA splicing of the dystrophin transcript using antisense oligomers (AONs), thus restoring the reading frame, leading to production of a partially functional BMD-like protein that should ameliorate the DMD phenotype into BMD.

Two different AON chemistries are being studied in DMD patients: the 2′OMethyl backbone (2′OMe) developed by Prosensa Therapeutics and the morpholino chemistry (PMO) developed by Sarepta Therapeutics. Proof of concept of dystrophin restoration by AON-induced exon 51 skipping has been achieved following systemic administration of both 2′OMe and PMO and has reached late-phase clinical trials.^7–12 Both AONs have obtained guidance from the FDA for future fast track approval. 2′OMe AONs targeting exons 44, 45, and 53 have already entered clinical trials, while an exon 45 PMO trial is at the advanced planning stage. A PMO targeting exon 51, developed by members of our SKIP-NMD consortium that are also partners of the UK MDEX consortium (http://www.mdex.org.uk), in collaboration with Sarepta Therapeutics (previously AVI Biopharma), is now in phase III studies. We elected to develop a novel PMO targeting exon 53, as skipping this exon addresses a significant proportion of DMD boys (∼8% of boys carrying deletions). ¹³ Our collaborative project is therefore a phase I/II clinical trial in DMD using a systemically administered morpholino AON, SRP-4053, aimed at restoring dystrophin expression by skipping exon 53 in DMD boys with eligible deletions.

Approach and Methodology

Funded by the European Union FP7 Grant, the SKIP-NMD consortium was assembled to allow synergy between academia, industry, and DMD advocacy groups and to design and perform this new clinical trial. This consortium includes key EU academic opinion leaders in the translational research field of DMD; four industrial companies, including Sarepta Therapeutics (SRPT), who will provide the drug, and three European-based SME/industrial companies (Charles River Laboratories [CRL; Edinburgh, United Kingdom], CRIS [Liège, Belgium], and SYSNAV [Vernon, France]); and six parent organizations.

The SKIP-NMD project milestones are to (1) finalize the lead PMO AON sequence to induce exon 53 skipping (WP1); (2) perform the regulatory preclinical toxicology (WP2); (3) design and perform a randomized control clinical trial at four locations (Newcastle, London [United Kingdom], Paris [France], and Rome [Italy]), to address the safety, biochemical efficacy, and exploratory clinical efficacy of SRP-4053 (WP3 & 4); (4) validate novel outcome measures and assess the role of noninvasive biomarkers in monitoring the response to therapeutic intervention (WP5), and (5) harmonize the approach to ethics procedures across the consortium (WP6).

Main Findings

Work is progressing on schedule and is now beyond the 18-month milestone.

WP1

Eight PMOs were blindly tested on different cell lines, including immortalized myoblasts derived from DMD patients with different exon 53 skippable mutations. All induced specific efficient exon 53 skipping, in a concentration-dependent manner with no detectable toxicity. Low dose–response curves were used to elucidate the optimal PMO for exon 53 skipping. The efficacy of this PMO to restore dystrophin protein was confirmed by Western blot analysis of protein from treated patient cells carrying relevant mutations. This PMO is referred as SRP-4053.

WP2

GLP industry-standard toxicology and safety pharmacology studies were performed by SRPT and CRL in accordance with regulatory requirements to ensure protocol submission for the clinical trial. SRPT developed the manufacturing process and analytical assays to safeguard the compound purity. Large-scale GMP production of SRP-4053 has been assured in order to guarantee the preliminary manufacturing of two of the six batches required for the clinical trial.

WP3

Finalization of the clinical trial design and the standard operative procedures for all the outcomes, including the novel clinical exploratory measures, has involved the formation of the Clinical Trial Management Group led by partner GOSH to oversee and organize the clinical trial. Five meetings attended by all scientific partners, Sarepta, and representatives of advocacy groups facilitated design of the clinical trial protocol. The necessary safety toxicology reports, the related ethics documents, and the full clinical trial application for the SRP-4053 first-in-man clinical trial (4053-101) was submitted for review to the regulatory authorities following the European Medicine Agency Voluntary Harmonization Procedure for multinational trials in June 2014, with subsequent positive opinion. Favorable ethics opinions for the research study were obtained in all participating centers (see www.clinicaltrials.gov).

The study is structured into a dose escalation component followed by a maintenance/treatment part. In the dose escalation study, 12 patients amenable to exon 53 skipping (i.e., deletions of exons such as 42–52, 45–52, 47–52, 48–52, 49–52, 50–52, 52, or 54–58) of ≥6 years of age and above and ambulant (able to walk ≥250 m in 6 min) will be randomized to receive SRP-4053 (n = 8) or placebo (n = 4). Patients will receive a weekly IV infusion of SRP-4053/placebo at escalating dose levels, each for at least 2 weeks: 4 mg/kg/week in weeks 1–2; 10 mg/kg/week in weeks 3–4; 20 mg/kg/week in weeks 5–6; and 30 mg/kg/week beginning on week 7. After this, all (including the placebo-treated boys) will enter the maintenance phase of the study. This will be an open-label treatment with SRP-4053 at the highest tolerated dose as determined by the safety data in part 1. In addition, 12 new DMD patients with mutations amenable to exon 53 skipping will be enrolled, along with up to 24 DMD patients with mutations not amenable to exon 53 skipping, matched for age and motor function. Treated patients will undergo routine safety evaluations.

All subjects will have lower limb MRI scans, including phosphorus MRS, ¹⁴ at least at three time points. Physiotherapy assessment will include validated and widely used outcome measures such as the 6 min walk test ¹⁵ ; the NorthStar ambulatory assessment ¹⁶ ; timed four-step test; newly validated upper limb outcome measures, including tools measuring strength and fatigability, such as the moviplate, myopinch, and handgrip ¹⁷ ; and a functional scale (the performance of upper limb). ¹⁸ As an exploratory endpoint, all patients will wear an accelerometer (actimetry) device, developed by SME SYSNAV and Institut de Myologie, to monitor daily activity for the duration of the study. Quality-of-life data will be collected by means of parental questionnaires. ¹⁹ Finally, blood samples will be collected for exploratory serum biomarker analyses and a comparison will be carried out between the treated and untreated cohorts. ²⁰ The primary biological endpoint will be the quantification of dystrophin detected by published methods.^21–23 We will compare muscle biopsy samples collected at baseline/enrolment and following 48 weeks of PMO treatment.

At the time of writing, the first two patients have been successfully recruited and dosed with SRP-0453. We estimate that this study should be finished by 2Q2016.

WP6

This aims to harmonize the approach to ethics procedures for all the partners involved following highest standard requirements in accordance with EU regulations. An ethics board has been established, chaired by an independent clinician, to promote, support, and assure the highest standard for the process of consenting families in the clinical trial. Child-friendly cartoons to explain the condition have been developed for each center (http://youtu.be/T8GAU90b1Wc), and a train set, which allows exon skipping to be explained in a playful and comprehensible way, has been included in the consent process. The SKIP-NMD website (www.skip-nmd.eu), containing multilingual webpages, is part of this educational plan and has a central role in promoting an educational webinar (www.skip-nmd.eu/patients-area/Webinar).

WP7

We have disseminated SKIP-NMD project results to date via the SKIP-NMD website, 2 publications on dystrophin quantification and biochemical characterization of skippable mutations,^21,22 1 submitted article, ²⁴ and at 11 scientific meetings.

Expected Outcome

RNA therapeutics are rapidly progressing in the field of neuromuscular diseases. A PMO targeting exon 51 has restored robust dystrophin expression in all treated patients to levels expected to produce clinical benefit. ⁷ Indeed, stabilization of clinical course has been demonstrated in treated boys using both PMO and the 2′OMe chemistries targeting exon 51. ⁸

Developing AON targeting novel exons is therefore a logical step forward and our study will not only provide robust and coherent evidence of any efficacy of SRP-0453, but also advance the field of experimental therapies for DMD patients. Successful AON therapies for DMD will contribute significantly to reducing the health burden and improving quality of life for DMD patients.