CHICAGO -- One of the first trials aimed at tackling long COVID helped some patients recover from lingering physical and mental fatigue, although the drug developed by Axcella Health Inc failed on the small study's main goal of restoring the normal function of mitochondria - the energy factories of cells.

In the 41-patient pilot study released on Tuesday, for three of 21 patients who received the drug, AXA1125, their physical fatigue scores returned to normal levels after 28 days of treatment, Axcella Chief Medical Officer Margaret Koziel said in a phone interview.

Others who received the drug also reported physical and mental improvements that were deemed to be statistically significant as shown on a scale developed to measure chronic fatigue, according to the preliminary results, and the drug was shown to be safe and well tolerated.

"This trial is suggesting that a drug that's very safe to take and has minimal side effects is causing substantial improvement in people's physical and cognitive experience of fatigue," said Dr. Jason Maley, a consultant for Axcella who runs a long COVID clinic at Beth Israel Deaconess Medical Center in Boston.

The drug, originally developed for fatty liver disease, aims to treat the crushing chronic fatigue reported by more than half of long COVID-19 sufferers by restoring normal function of mitochondria, the minuscule power plants that help cells perform properly.

People in the trial conducted at the University of Oxford received either the Axcella drug or a placebo over a period of 28 days. All were more than 12 weeks post COVID-19 infection and had an abnormal phosphocreatine recovery time, a measure of mitochondrial function.

For the study's primary goal, there was no statistically significant difference between the groups on phosphocreatine recovery time.

The trial also looked at blood tests measuring lactate, a sign of muscle health, as well as patient-reported measures of mental and physical fatigue.

There are currently no approved treatments for chronic fatigue in patients with long COVID-19, a condition estimated to affect more than one hundred million people worldwide.

Axcella Chief Executive Bill Hinshaw said the company is designing new trials and plans to meet with U.S. and U.K. regulators with hopes of seeking an accelerated approval pathway for this enormous unmet need.

Reporting by Julie Steenhuysen, Editing by Bill Berkrot