Why did Forzinity receive accelerated approval?

It received accelerated approval because FDA considered improved knee-extensor strength reasonably likely to predict a practical benefit, such as standing more easily or walking farther. The signal came from a small open-label extension, not a successful randomized comparison. FDA therefore required another randomized, placebo-controlled trial to verify that the strength change translates into meaningful function.

Did the TAZPOWER trial meet its main outcomes?

No. The randomized phase did not show superiority to placebo on six-minute walking distance or the Barth syndrome fatigue score. Knee-strength gains appeared later during an open-label extension, when everyone knew they were receiving elamipretide and no concurrent placebo group remained. FDA used that strength measure for accelerated approval while requiring confirmation.

Does Forzinity approval cover compounded SS-31?

No. FDA-approved versions of this molecule exist, but the compounded formulations offered here are not FDA-approved. Forzinity is a specific manufactured elamipretide injection with a narrow Barth syndrome indication for people weighing at least 30 kilograms. Its label, clinical evidence and approval do not transfer to a compounded SS-31 prescription.

What happens next for Forzinity?

A confirmatory trial must test whether the knee-strength change predicts real functional benefit. FDA lists the final report as due March 31, 2030. As of the writing date, the U.S. registry listed the trial as not yet recruiting, while the European registry showed it authorized on July 29, 2026 with recruitment pending.

Forzinity Approval: One Year Later

The Forzinity approval arrived on September 19, 2025. FDA granted accelerated approval to elamipretide for improving muscle strength in adults and children with Barth syndrome who weigh at least 30 kilograms. It was the first treatment for this rare genetic condition, but it was also a deliberately narrow decision. It did not approve every product called SS-31, and it did not settle whether the drug produces broader benefits outside Barth syndrome.

One year later, the approval remains conditional on another trial. That is the part of the anniversary worth understanding.

September 19, 2025: the Forzinity approval letter

The date is exact. FDA's September 19, 2025 approval letter says the decision took effect that day for Forzinity, Stealth BioTherapeutics' manufactured elamipretide injection. The agency's announcement called it the first treatment for Barth syndrome.

Barth syndrome is an inherited disorder of mitochondria, the parts of cells that make usable energy. It can involve heart disease, muscle weakness, low stamina and serious illness beginning in infancy. The label covers adults and children who weigh at least 30 kilograms, about 66 pounds, and names one purpose: improving muscle strength.

Elamipretide is also known as SS-31. It is a peptide four amino acids long that binds cardiolipin, a fat in the inner mitochondrial membrane. The SS-31 peptide guide explains that biology; this anniversary is about how one specific product crossed the regulatory line.

What the world looked like the day before

On September 18, 2025, there was no approved treatment for Barth syndrome. The path to the next day's decision had not been straight.

In October 2021, FDA sent a refusal-to-file letter, meaning the agency found the first application too incomplete to begin a full review. Stealth reported in an SEC filing that FDA pointed to the negative randomized part of TAZPOWER and did not accept its open-label extension as an adequate controlled trial.

That trial enrolled just 12 males with genetically confirmed Barth syndrome. During the blinded crossover phase, when each person received elamipretide and placebo at different times, it missed both primary endpoints, the outcomes chosen in advance as the trial's main tests: walking distance in six minutes and a fatigue score. The peer-reviewed TAZPOWER paper says so plainly.

The later extension looked more encouraging, but everyone knew they were receiving the drug and there was no simultaneous placebo group. Ten people entered; eight reached week 168. Their walking distance, knee strength and some heart measures improved from the extension baseline, according to the 2024 follow-up paper. Those observations mattered, but they carried more uncertainty than a randomized comparison.

FDA accepted a second application for review in 2024. That October, an advisory committee voted 10–6 that there was at least some evidence elamipretide worked in Barth syndrome. FDA then issued a complete response letter on May 15, 2025, meaning the application could not be approved as submitted. The agency's integrated review describes concerns about evidence and a manufacturing-facility inspection, followed by an August 15 resubmission. So the public record shows two major regulatory stops, but only the 2025 one was a complete response letter.

Families, clinicians and the Barth Syndrome Foundation kept the condition visible through petitions, meetings and congressional outreach. The foundation's advocacy timeline records that public campaign. It did not replace the evidence review, but it explains why this small patient community was present throughout it.

What the Forzinity approval changed

Accelerated approval is a pathway that can allow a drug for a serious condition to reach patients based on a measure considered reasonably likely to predict benefit. The measure here was knee-extensor strength, or the force of the muscles that straighten the leg, measured with a handheld force gauge.

FDA's trial snapshot is unusually candid. The randomized phase did not show Forzinity was superior to placebo on the main outcomes, and it did not show a significant knee-strength difference. The knee-strength increases appeared during the longer open-label period. FDA considered that intermediate measure reasonably likely to predict practical benefits such as standing more easily or walking farther.

The approval therefore created access to a specific manufactured product for a specific group. It also created an obligation. The approval letter requires a randomized, double-blind, placebo-controlled study in people age 5 and older with Barth syndrome. That confirmatory trial, a study run after approval to verify patient benefit, must test whether the strength measurement translates into how people actually function.

The year since approval

As of September 9, 2026, the day this article was written, FDA still lists Forzinity among ongoing accelerated approvals, with the confirmatory report due March 31, 2030. The current DailyMed label still carries the same narrow indication and a Revised: 9/2025 line.

The confirmatory study is called 4TAZPower, or SPIBA-401. Its ClinicalTrials.gov record, last updated May 6, 2026, listed it as not yet recruiting. A fresher European registry entry shows a July 29, 2026 authorization decision and says recruitment is pending across four countries. Taken together, the public registries showed trial setup, not enrollment underway, before the anniversary.

The planned study runs 72 weeks and includes males age 5 and older with genetically confirmed Barth syndrome. It will compare elamipretide with placebo and measure everyday function alongside muscle strength. The distinction between a trial plan and a personal treatment plan matters; the SS-31 protocol explainer keeps those two things separate.

What the approval did not change

Forzinity's decision did not establish elamipretide as a general treatment for energy, exercise performance, healthy aging, heart failure or eye disease. Those questions have separate studies, including several trials that missed their main outcomes. The SS-31 benefits evidence covers that wider record without borrowing certainty from Barth syndrome.

SS-31 through Promise is dispensed as a compounded medication, which is different from an FDA-approved product: the formulation offered here is not FDA-approved. An FDA-approved elamipretide product exists for one rare genetic condition, and that approval does not extend to the compounded formulation prescribed here.

FDA status is one fact in a medical decision, not a marketing gate. A licensed provider may still prescribe a compounded formulation when medically appropriate; that decision is between the patient and doctor.

What this history means for prescribed SS-31

The useful line is accountability. A branded product with a narrow label, a compounded prescription and a gray-market vial sold without a prescription are not interchangeable. They come with different evidence, manufacturing records and clinical oversight.

At Promise, a licensed provider reviews every request for SS-31 and prescribes only when medically appropriate. Not everyone qualifies. If a prescription is written, a licensed U.S. compounding pharmacy prepares and dispenses it, and the reviewing provider remains part of the care path.