Stealth BioTherapeutics Presents Phase 2 Data From MMPOWER-2 Continuation Trial Supporting Phase 3 Development of Elamipretide in Primary Mitochondrial Myopathy

BOSTON ­- JUNE 29, 2017- Stealth BioTherapeutics (Stealth), a clinical-stage biopharmaceutical company developing therapeutics to treat mitochondrial dysfunction, today announced results from MMPOWER-2, a Phase 2 continuation trial evaluating safety, tolerability and efficacy of treatment with elamipretide for primary mitochondrial myopathy (PMM). Detailed results from the trial were presented today at Mitochondrial Medicine Symposium 2017, the United Mitochondrial Disease Foundation (UMDF) symposium.

“The lives of patients with primary mitochondrial myopathy, for which there are no FDA-approved treatment options, can be significantly impaired by the debilitating muscle weakness and fatigue they experience daily,” said Dr. Amel Karaa, trial investigator, internist and clinical geneticist at Massachusetts General Hospital. “We have seen improvements associated with elamipretide in MMPOWER-2 which merit study in a Phase 3 trial.”

The overall assessment of the top-line MMPOWER-2 results shows benefit across multiple endpoints assessed and is supportive of a Phase 3 trial in this patient population.

The 30 patients enrolled in MMPOWER-2 previously completed MMPOWER, Stealth’s first clinical trial in this patient population, which demonstrated a dose-dependent improvement in distance walked in the six-minute walk test (6MWT) after five days’ treatment with elamipretide. In MMPOWER-2, a longer, four-week treatment period with elamipretide was associated with an average 20 additional meters walked versus placebo during the 6MWT (p=0.08), the primary endpoint. Although the 6MWT efficacy endpoint did not reach significance, a pre-specified analysis showed that patients who were more impaired at baseline (pre-treatment 6MWT less than 450 meters) experienced a greater improvement with elamipretide (24 meters on average) than patients who were less impaired at baseline (pre-treatment 6MWT more than 450 meters; eight meters on average). This finding is consistent with observations from MMPOWER.

MMPOWER-2 was instrumental in identifying additional endpoints for a Phase 3 trial. At four weeks, treatment with elamipretide resulted in statistically significant improvements in Neuro-QoL Fatigue Short Form score (4 units versus placebo; p=0.01), a validated patient-reported scale for fatigue in neurologic disorders, and in the Primary Mitochondrial Myopathy Symptom Assessment (PMMSA, formerly the Mitochondrial Disease Symptom Assessment) Total Fatigue Score (1.7 units; p=0.0006), a proprietary patient-reported outcome tool developed by Stealth specifically for this patient population. Several other PMMSA assessments also showed significant improvements for elamipretide-treated patients, including improvement in the most bothersome symptom reported by each patient (p=0.01). The triple Timed-Up-and-Go test (3TUG; p=0.8), measuring the time it takes to stand from seating, walk six meters, sit and repeat three times, was completed in less than one minute, which may be insufficient to measure endurance-related skeletal muscle weakness and fatigue. The 3TUG test will not be included in Phase 3.

Treatment with elamipretide appeared to be well tolerated, with no serious adverse events. The most common side effect was injection-site reactions (80 percent with elamipretide versus 17 percent with placebo); most were mild redness or itching.

“We are highly encouraged by the MMPOWER-2 trial’s identification of endpoints to measure changes in both skeletal muscle function and quality of life issues, which are so crucial in this patient population,” said Stealth Chief Executive Officer Reenie McCarthy. “These findings confirm the potential of elamipretide for these patients and help us establish and validate critical details for our planned Phase 3 study. We look forward to working closely with U.S. and European regulatory officials to finalize the design of our Phase 3 trial, which will enroll patients with primary mitochondrial myopathy caused by a variety of genetic mutations.”

In March of this year, Stealth initiated RePOWER, a multi-national pre-trial registry of patients with PMM. RePOWER will assess approximately 300 patients, ages 16-65, at a single enrollment visit, where they will complete questionnaires about their current symptoms and quality of life, perform functional assessments and share data from clinical records. Findings from MMPOWER, MMPOWER-2 and RePOWER will together help inform a Phase 3 trial to further evaluate the potential efficacy, safety and tolerability of elamipretide.

For additional information on Stealth’s program in PMM or elamipretide, please refer to Stealth’s website.


MMPOWER-2 is a Phase 2, randomized, double-blind, placebo-controlled crossover study to evaluate the safety, tolerability and efficacy of four weeks’ treatment with once-daily subcutaneous (SC) injections of elamipretide in patients with primary mitochondrial myopathy, or muscle weakness, in patients with genetically confirmed mitochondrial disease previously treated in the MMPOWER study.

Subjects treated in the MMPOWER-2 study were randomized (1:1) to receive either four weeks of treatment with 40 mg elamipretide administered once daily SC in the first treatment period followed by four weeks of treatment with placebo administered once daily SC in the second treatment period, or vice versa. The two treatment periods were separated by a four-week washout period.

The primary efficacy endpoint is change in distance walked in six minutes at the end of each four- week treatment period. Secondary endpoints include safety and tolerability assessments, patient-reported outcomes and global impression scales.

About Stealth

We are a privately held clinical-stage biotechnology company focused on the development of therapeutics for diseases involving mitochondrial dysfunction. We believe there is a strong rationale for our lead product candidate, elamipretide, in indications in these diseases based on encouraging preclinical and early clinical data. We are investigating elamipretide in three primary mitochondrial diseases – primary mitochondrial myopathy (PMM), Barth syndrome and Leber’s hereditary optic neuropathy (LHON) – as well as in heart failure, Fuchs’ corneal dystrophy and dry age-related macular degeneration. We received Fast Track designation for elamipretide for the treatment of PMM from the FDA in December 2015. We are developing our second product candidate, SBT-20, for central nervous system disorders. Our mission is to be the leader in mitochondrial medicine. To learn more information about us and our pipeline, visit


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