Stealth BioTherapeutics Corp.'s experimental therapy elamipretide for a genetic disorder affecting the skeletal muscles failed to improve patients' walking ability and fatigue in a late-stage trial.

Newton, Mass.-based Stealth's stock dropped by 66.1% to $4.72 as of 9:55 a.m. ET on Dec. 20.

The biotechnology company will assess operational resources following these results and provide next steps in January 2020, according to a Dec. 20 press release.

Elamipretide was evaluated in a 32-week phase 3 trial called MMPOWER-3 for primary mitochondrial myopathy, a rare disease characterized by mutations in the mitochondria that affect the skeletal muscle, leading to insufficient energy in the cells. Primary mitochondrial myopathy can also cause muscle fatigue, muscle pain and muscle wasting.

While elamipretide was well-tolerated among the 218 enrolled patients, the drug candidate failed to improve how far patients could walk in a six-minute walk test and fatigue and muscle weakness symptoms.

The company is also testing elamipretide for another rare mitochondrial disorder called Barth syndrome. Stealth intends to meet with the U.S. Food and Drug Administration in early 2020 to determine next steps, as elamipretide has been shown to improve patients' condition in clinical trials for Barth syndrome.