Stealth BioTherapeutics Completes Target Enrollment in Global Phase 3 NuPOWER Trial in Patients with Primary Mitochondrial Myopathy

NEEDHAM, Mass.Sept. 11, 2023 /PRNewswire/ — Stealth BioTherapeutics Inc. (the “Company” or “Stealth”), a clinical-stage biotechnology company focused on the discovery, development, and commercialization of novel therapies for diseases involving mitochondrial dysfunction, today announced its achievement of target enrollment in the NuPOWER (SPIMD-301) study in patients with primary mitochondrial myopathy (PMM). The study will continue to enroll through month-end to allow for participation of additional interested patients.

“We are thrilled to progress our PMM development efforts toward pivotal data,” said Chief Executive Officer Reenie McCarthy. “We have partnered closely with the patient community since initiating this program in 2014, and deeply appreciate the over 350 individuals living with PMM and the many dedicated scientists and clinicians who have participated in and helped design and conduct our clinical trials. This important milestone brings us closer to the goal of addressing the unmet need of individuals living with PMM, as we look forward to topline data next year.”

About the NuPOWER Trial
The NuPOWER Phase 3 clinical trial is a 48-week, randomized, double-blind, parallel-group, placebo-controlled trial to assess efficacy and safety of single daily subcutaneous (SC) administration of elamipretide as a treatment for subjects with primary mitochondrial myopathy associated with nuclear DNA mutations (nPMM). The primary efficacy outcome measure is the change from baseline in distance walked on the six-minute walk test, which assesses exercise tolerance, and secondary outcome measures include change from baseline in time to complete the five times sit-to-stand and triple timed up-and-go tests, which assess proximal muscle strength.

The NuPOWER trial design was informed by and enriched based on data from previous clinical trials, with the MMPOWER-3 trial playing the most significant role. Although MMPOWER-3, which enrolled PMM patients with both nuclear and mitochondrial DNA mutations, did not meet its primary endpoint across this heterogenous patient population, a signal was observed in a prespecified subgroup of 56 nPMM patients. For the nPMM subgroup, an improvement was observed (per protocol) in the six-minute walk test, the primary outcome, as well as an exposure response relationship, meaning that the amount of study drug to which nPMM patients were exposed was associated with improvement in the six-minute walk test. These learnings informed the nPMM patient population and increased dose used in the NuPOWER trial. Post-hoc analyses supported further enrichment strategies, including specification of a primary analysis population based on certain nPMM genetic mutations involved in mitochondrial DNA replication, which is crucial for skeletal muscle function.

About nPMM
Primary mitochondrial myopathy is a group of disorders that are associated with changes in nuclear (nPMM) or mitochondrial genetic material (e.g., depletions, deletions, or mutations) affecting predominantly the skeletal muscle system. Symptoms of nPMM include severe muscle weakness, which can impact all muscles in the body, and debilitating fatigue. There are no disease-modifying treatments for nPMM; treatment is aimed at improving or resolving specific symptoms. This program has been granted orphan drug and fast track designation by the FDA and orphan drug designation by the EMA.

About Stealth
The Company is a clinical-stage biotechnology company focused on the discovery, development, and commercialization of novel therapies for diseases involving mitochondrial dysfunction. Mitochondria, found in nearly every cell in the body, are the body’s main source of energy production and are critical for normal organ function. Dysfunctional mitochondria characterize a number of rare genetic diseases and are involved in many common age-related diseases, typically involving organ systems with high energy demands such as the eye, the neuromuscular system, the heart and the brain. The Company is progressing its lead product candidate, elamipretide, into Phase 3 clinical trials in dry age-related macular degeneration and through the pivotal NuPOWER trial in nPMM, as well as continuing regulatory engagement on its Phase 3 data in Barth syndrome, an ultra-rare cardioskeletal disease. The Company is evaluating its second-generation clinical-stage candidate, SBT-272, for ophthalmic and neurological disease indications following promising preclinical data. The Company has a deep pipeline of novel mitochondria-targeted compounds under evaluation as therapeutic product candidates.

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