Gene Therapy Shows Modest Success for Genetic Blindness

SEATTLE – The latest data from a phase 3 clinical trial shows that gene therapy can counter visual degeneration associated with Leber hereditary optic neuropathy (LHON). The therapy, delivered by intravitreal injection, uses an adeno-associated virus vector to deliver a corrected copy of the mutated ND4 mitochondrial gene.

LHON is a rare, maternally inherited mitochondrial mutation that can cause blindness, most commonly in young men, though it does not happen in all individuals with the mutation. The condition often starts with blindness in one eye, accompanied or followed shortly by blindness in the second eye. Researchers believe that the injected viral vector gets taken up retinal ganglion cells, where the mutated gene interferes with vision. Once synthesized, a mitochondria-targeting sequence facilitates transport of the protein to the mitochondria.

The study protocol called for injection of the therapy into one eye and a placebo into the other, using the patient as his or own placebo control. The results in the treated eye were encouraging, though modest. "This is not hitting it out of the ballpark. But for people whose vision is devastated by this disease, it certainly is a first step," said Nancy J. Newman, MD, during a press conference held March 29 in advance of the 2022 annual meeting of the American Academy of Neurology.

Newman also noted a surprise finding: Visual improvement also occurred in the placebo-control eye. This was noted in previous studies, called RESCUE and REVERSE, and follow-up studies in monkeys found viral vector in the unaffected eye 3-6 months after an injection. "This would imply some kind of transport within retrograde up the opposite optic nerve after crossing in the chiasm to the eye, but this is going to take a fair bit of work to know exactly how that happens," said Newman

Unfortunately, the phase 3 REFLECT study was designed before that process was understood. "This was not a case-control study by person, it was by eye. And that was a mistake, because it turns out there is a does appear to be second eye effects. We do not have naive controls here that did not receive any injection at all in any eye. That's something that we will [do going] forward," said Newman.

Despite the problem with placebo, the results were encouraging. "Those patients who had both eyes injected with the drug did better than in those who had one eye injected with drug and one eye injected with placebo, suggesting some sort of dose effect. There were no adverse events other than what we would expect from injecting [into] eyes. Those treated with the drug had more ocular inflammation, as would also be expected, but all were easily treated with topical medications," said Newman.

What Are the Long-term Effects?

Natalia Rost, MD, who chairs the AAN Science Committee, commented after the presentation: "We're quite impressed with advances in gene therapy. The question is, are there early indications that this improvement in vision will have a lasting effect?"

Newman responded that ongoing data from earlier studies are also encouraging regarding the long-term effect of the treatment. At 4 years, there was a difference of 16.5 Early Treatment of Diabetic Retinopathy Study (ETDRS) letters equivalent between treated patients and natural history controls (P < .01), "which [does] suggest that this effect is maintained," said Newman, who is a professor of ophthalmology and neurology at Emory University, Atlanta.

Rost also wondered if it would be possible to capture patients earlier in their disease process, in the hopes of countering degeneration before it becomes severe enough to impact vision. Newman answered by noting another surprise from the research. Previous studies had shown that intervention while only a single eye is affected had little impact on spread of the condition to the second eye, "which was very disappointing," said Newman. When they stratified patients by time since vision loss, they found that those who received the therapy 6 months or later after vision loss had better responses than those who were treated earlier.

The mechanism of this counter-intuitive finding remains uncertain, "but we do know that acutely in this disease when people are just starting to lose this vision, during the first couple of months, they get swelling of the axons from these retinal ganglion cells. Our hypothesis is that swelling may actually act as a barrier for the drug to get into the retinal ganglion cell bodies themselves and be transfected. So it turns out that earlier may not be better," said Newman.

The study included patients at 13 sites worldwide; 48 were treated bilaterally and 50 treated unilaterally. Just under 80% were male, the mean age was 31.5 years, and the mean duration of vision loss was 8.30 months.

After 1.5 years, the improvement in best-corrected visual acuity between second-affected eyes was stronger in the treatment eye, equivalent to +3 ETDRS letters. The first-affected eye improved by 19 ETDRS letters, and the second-affected eye improved by 16 (P < .0001). Improvement in placebo eyes was +13 ETDRS letters (P < .0001).

Rost has served on a scientific advisory board or data monitoring board for Omniox. Newman has consulted for GenSight, Santhera/Chiesi, and Neurophoenix, and has received research support from GenSight and Santhera/Chiesi.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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Cite this: Jim Kling. Gene Therapy Shows Modest Success for Genetic Blindness - Medscape - Apr 04, 2022.