Eighteen men with the genetic disorder X-linked retinitis pigmentosa were enrolled in a six-month clinical trial, seven of whom saw improvements in their vision for the full duration of the trial.
X-linked retinitis pigmentosa is caused by mutations in the RPGR gene and is the most common cause of blindness in young people. Beginning in early childhood, this disease causes degeneration of photoreceptors in the eye leading to severe sight loss.
The success of a gene therapy in treating another retinal disorder (see BioNews 1036), led the team to believe X-linked retinitis pigmentosa patients could also benefit from a gene therapy approach.
The original method used a viral vector to deliver a healthy copy of the mutated gene, effectively slowing down the degeneration and maintaining the sight of patients with the inherited retinal disorder Leber's Congenital Amaurosis (LCA) .
The RPGR gene presented an additional challenge due to its complicated genetic code that renders it unstable and thus difficult to study. In order to apply the method, the team led by Professor Robert MacLaren of Oxford University had to reprogram the RPGR gene to stabilise it.
In the trial, 18 patients with severe retinal degeneration were treated with increasing doses of the correct version of the RPGR gene.
After one month of treatment seven out of the 12 patients administered one of the top four doses of treatment began to show visual improvements, which lasted for the duration of the trial.
Professor MacLaren said: 'We are delighted with the early results of this clinical trial for a degenerative eye disease. It is becoming more apparent to us that novel genetic therapies, when working, lead to a clear improvement in neuronal function, which holds great hope for a variety of other degenerative conditions that have a genetic basis.'
The trial was not designed to test efficacy of the drug, as the researchers believe appropriate dosage will depend on the severity of degeneration in the patient and the effects of the inflammation experienced.
The trial met its safety requirements and will now begin further testing comparing two doses of the vector therapy with a placebo.
Dr Byron Lam at the Bascom Palmer Eye Institute at the University of Miami, who took part in the international multi-centre study, said: 'This gene therapy study offers hope for patients with this currently untreatable blinding disease.'