Scientists have extracted mitochondria from a woman’s leg muscles and injected them into the vitreous fluid of her eyes in an attempt to treat severe blindness. The procedure caused no inflammation or side effects — but had a limited effect on restoring responses to light in the eyes, and did not restore the woman’s vision.

Transplanting healthy mitochondria — organelles that produce energy in cells — is being explored as a way to treat diseases that damage these organelles in tissues. The healthy mitochondria can be taken up by the damaged cells and, in some cases, boost cell survival and partially restore tissue functions. Previous trials in humans have transplanted mitochondria into the heart and brain. Now, scientists report the first such transplant in the eyes.

A brain bleed in February had injured the woman’s optic nerve and retina, causing nearly complete blindness, with no pupil responses to light flashed in her eyes. After the mitochondrial injections, her pupils showed a response to light, but the effect was temporary and subsided after about four weeks. The findings were reported in a preprint posted on 10 August, which has not been peer reviewed¹.

The study shows that it was “relatively safe” to inject the person’s own mitochondria into the eyes, says Temurkhan Ayupov, a mitochondria biologist at the Institute of Molecular and Clinical Ophthalmology Basel, in Switzerland. But it does not confirm the therapeutic effect of a mitochondrial transplant, he adds.

The goal was to establish the safety of the approach, says David Putrino, a neuroscientist at the Icahn School of Medicine at Mount Sinai in New York City and a co-author of the preprint. “We can’t prove efficacy at all, nor are we trying to,” he adds, but “we saw a significant effect on physiological changes in the eye”.

Mixed success

Putrino and his colleagues wanted to explore whether injecting mitochondria directly into the eye could help to restore function in retinal ganglion cells — neurons in the inner surface of the retina that send vision signals to the brain.

Studies in mice have shown that delivering mitochondria into the vitreous fluid increased the survival of neurons that had remained viable after optic-nerve injury.

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In May, the researchers obtained an emergency approval from the US Food and Drug Administration (FDA) to attempt the mitochondrial transplant in the woman’s eyes.

“The thing to be most concerned about was triggering an immune reaction where the immune system starts to attack these foreign substances that are being injected into the body,” says Putrino. But tests after the procedures found no signs of inflammation in or damage to either eye.

Although the pupil response to light was transient, it “held at levels that were higher than they were prior to the transplant,” says Putrino. “When we conducted a post-transplant low-vision assessment, it was noted that she was perceiving shapes and shadows in her left eye,” he adds.