Szeto-Schiller Peptide 31
(Elamipretide)

Early 2000s

The Wrong Experiment, The Right Discovery

A Lucky Mistake Changes Everything

In a lab at Weill Cornell Medicine in New York, Dr. Hazel Szeto and her colleague Dr. Peter Schiller in Montreal were trying to build a better painkiller. They were designing tiny chains of amino acids — peptides — that could attach to pain sensors in the brain.

But something strange kept happening. Their peptides weren't just going to pain sensors. They were being pulled deep inside cells, gathering around the mitochondria — the tiny power plants that keep every cell alive. It was like designing a key for one door and finding it opened a completely different room.

Most scientists would have ignored this side effect. Szeto didn't. She knew that broken mitochondria were behind dozens of diseases — heart failure, blindness, muscle wasting, even aging itself. If her peptides could reach mitochondria, maybe they could fix them.

2004-2010

Building the Perfect Molecule

From Accident to Invention

The team had to solve a puzzle. Their first compound, SS-02, worked on mitochondria but still acted like a painkiller — not useful for sick patients who didn't need pain relief. They needed to keep the mitochondria-targeting power while removing the painkiller effect.

Through careful redesign, they created SS-31 — a molecule 2,000 times weaker as a painkiller but with full mitochondria-targeting power. The secret was its shape: alternating water-loving and fat-loving amino acids that let it slip through cell walls without any help.

In 2004, they published their breakthrough in the Journal of Biological Chemistry. The paper showed that SS-31 could find and protect the inner walls of mitochondria, specifically attaching to a fat molecule called cardiolipin — found nowhere else in the body except inside these power plants.

In 2006, Szeto made a bold move. She left her university career and founded a company called Stealth BioTherapeutics to turn SS-31 into a real medicine. The name 'Stealth' reflected how hard it was to get drugs inside mitochondria — like sneaking past two layers of security.

2014-2019

Racing to Save Patients

Hope Takes Shape

In 2014, a desperate community found Stealth BioTherapeutics. Families of boys with Barth syndrome — a rare genetic disease that weakens the heart and muscles — had heard about SS-31. Their sons' mitochondria couldn't make enough cardiolipin, the exact molecule SS-31 was designed to protect.

The timing felt like fate. SS-31 entered clinical trials. One patient, Jacob Wilson, could barely walk a block without gasping for breath when he enrolled. Six months later, he was outpacing his parents on walks. He gained 25 pounds of muscle in a year.

Stealth was growing fast. In February 2019, the company went public on the Nasdaq stock exchange under the ticker symbol 'MITO.' They raised $78 million. The stock opened at $12 per share. Investors believed they were backing the future of medicine.

But storm clouds were already forming.

2019-2025

Everything Falls Apart

Trial Failures, Stock Crash, and Near-Death

December 2019. The results from their biggest trial — MMPOWER-3, testing SS-31 in 218 patients with mitochondrial muscle disease — came back. The drug failed. No improvement over placebo. The stock began its long slide.

Then the heart failure trials disappointed. Then the eye disease trial missed its main goals. One by one, the diseases that SS-31 was supposed to cure showed no clear benefit in the strict world of clinical trials.

By 2022, Stealth's stock had fallen from $12 to fractions of a penny. Nasdaq sent warning letters. In November 2022, the company went private in a deal valuing shares at just 3 cents — a 99.7% loss for investors.

But Stealth refused to die. Hidden in the failed trial data was a clue: patients with a specific type of genetic defect showed real improvement. The drug worked, but only for the right patients.

The Barth syndrome patients told a similar story. The short trial hadn't shown clear results, but patients who stayed on the drug for years kept getting stronger — walking 96 meters farther and gaining 45% more muscle strength by week 168.

2025

Third Time's the Charm

From the Brink to the History Books

Stealth submitted its first FDA application for Barth syndrome. Rejected — the FDA wanted more evidence. They submitted again. In May 2025, after 16 months of review, the FDA rejected them a second time. The advisory committee had voted 10 to 6 in favor, but the FDA said the evidence wasn't strong enough.

Stealth laid off 30% of its staff. CEO Reenie McCarthy told reporters the company was 'hanging on by our fingernails.' Scientists who had spent years on the drug published open letters questioning the FDA's delays.

But the FDA offered one last path: if Stealth could show that muscle strength gains were a meaningful sign of patient benefit, they could try for accelerated approval. In August 2025, Stealth submitted its third application.

On September 19, 2025, the FDA said yes.

FORZINITY — the brand name for elamipretide — became the first FDA-approved treatment for Barth syndrome. More importantly, it became the first drug ever approved that directly targets and repairs mitochondria. After two decades of work, three FDA applications, a stock crash, and near-bankruptcy, the accidental discovery from a painkiller lab had finally become a real medicine.

The story isn't over. Phase 3 trials for age-related blindness are enrolling patients now. A new trial targets the specific patients who benefited in the failed muscle disease study. The little molecule that wasn't supposed to exist is still fighting.