Clinical Trials of Gene Therapy for Leber Congenital Amaurosis

Backgrounder: Gene Transfer Therapy for One Type of Childhood Blindness Studied in Humans

Phase I Clinical Trial: Safety

A phase I clinical trial, funded by the National Eye Institute (NEI) of the National Institutes of Health (NIH), is assessing the safety of a gene-transfer technique in people with Leber congenital amaurosis (LCA). People with LCA are born with severe visual impairment or develop vision loss early in childhood. Previous studies in animal models LCA found this gene-transfer technique safe and effective in restoring visual function. The trial, now underway, is being conducted by investigators at the University of Pennsylvania and at the University of Florida.

In the first phase of the trial investigators evaluate the safety of the treatment. This is done by first administering a single, low dose in adults. A safety monitoring board, composed of medical experts, is providing independent oversight of the trial. Based on the results of the initial treatment group, the investigators may proceed with their plan to administer a higher dose for a second group of patients. If well tolerated, this higher dose will be used in subsequent clinical trials and the effectiveness of the treatment will be evaluated. In subsequent parts of the trial investigators will treat younger patients from age 13-18 and administer a second treatment.

LCA and the Visual Cycle

LCA is the name given to a family of severe retinal degenerative diseases that cause the degeneration and loss of light-sensing rod and cone photoreceptor cells in the retina. Beneath the retina lies a single layer of cells called the retinal pigment epithelium (RPE). These cells help to maintain the health and function of the rods and cones that adjoin the RPE.

NEI and NEI-supported investigators have found that the form of LCA studied in this trial is caused by mutations in the RPE65 gene. Normally, the RPE65 gene produces the RPE65 protein. This protein converts dietary vitamin A into a retina-specific form of vitamin A that is shuttled to the rods, where it helps form rhodopsin. Rhodopsin is the visual pigment that absorbs the light that enters the eye. The RPE protein is critical for regeneration of rhodopsin following exposure to light.

Mutations in the RPE65 gene prevent it from producing the functional protein that is vital to the visual cycle. Without the conversion of vitamin A, the visual cycle is disrupted and vision is suppressed.

Gene Therapy for LCA

Although vision loss is severe in this form of LCA, the structure of the retina, including its connection to the brain, can remain relatively intact for decades before photoreceptor cells degenerate. This feature of the disease presents a window of opportunity for the development of therapies that could overcome or compensate for RPE65 gene defects.

In 1997 NEI investigators found that mutations in the RPE65 genes also caused congenital vision loss in Briard dogs. In 2000 a team of researchers injected a single dose of a gene-transfer treatment containing the functional copies of the RPE65 gene into three Briard dogs. Visual evaluation revealed the dogs had significant recovery of vision, allowing them to track movements and avoid objects when walking. The dogs have retained their vision with no sign of complications. LCA is also associated with rapid involuntary eye movements, called nystagmus. In addition to restoring sight, the gene transfer corrected nystagmus in treated dogs.

Gene Delivery: Vectors

One of the biggest challenges to gene therapy is developing a safe and effective gene delivery system, called a vector, that can target, or transfect, the affected cell type without evoking a damaging immune response. Vectors are created by genetically modifying viruses, which have a natural ability to infect the cells in our body. By removing the virus's own genetic material and inserting the desired therapeutic gene into the virus, scientists can create vectors that act like a fleet of microscopic delivery trucks. Several viruses have been used in vector development but only a few have the ability to transfect cell types in the retina. Each vector has features that make it better or less suited for treating a particular retinal disease.

For this clinical trial, an adeno-associated viral vector, or AAV, is being used. The properties of AAV viruses are well understood. One particular strain of the AAV virus, adeno-associated virus type 2 (AAV-2), is very common; approximately 85 percent of the U.S. population has been exposed to it. AAV-2 is not associated with any human disease. This trial is evaluating the use of a modified adeno-associated viral vector (rAAV2-hRPE65) to deliver the normal RPE65 gene to the retina.

AAV vectors can transfect several retinal cell types. AAV vectors also afford long-lasting expression in the target cell, thereby reducing the need for frequent treatments. For these reasons, AAV-2 vectors are good for treating chronic retinal diseases like LCA that require life-long therapy.

Treatment Procedure

Patients enrolled in this clinical trial will receive a sub-retinal injection of fluid containing the viral vector with the inserted RPE65 gene. The sub-retinal space lies between the retina and the RPE layer of cells. Although the gene-transfer treatment is novel, the use of injections to target retinal diseases is commonly used and associated with infrequent and minor complications that usually can be resolved.

Pre-Clinical Safety Studies

Gene therapy is one of the most heavily monitored experimental modalities in all of medicine. In addition to the U.S. Food and Drug Administration (FDA), the NIH also has an advisory body, called the Recombinant DNA Advisory Committee (RAC), that reviews all gene-based therapies conducted in the United States that are developed with NIH funding. These groups provide rigorous scrutiny of proposed clinical trials to assure that risks to participants are minimized to the fullest extent possible. Central to this concern is the requirement that preclinical safety studies fully characterize potential risks of a novel treatment like gene therapy. Since the successful proof of concept study in Briard dogs in 2000, NIH investigators have been conducting numerous safety studies to gain regulatory approval.

The AAV-2 vector has been tested to determine its distribution in tissues and organ systems beyond the RPE cell layer. Dose-response studies have been performed. Still other studies have examined the short-term and long-term effects of the treatment in animal models. The procedure for injecting the therapeutic genetic material into the sub-retinal space has been studied and its potential complications are well known.

Clinical Trial Safety

During the clinical trial, patients are carefully followed with diagnostic eye examinations to assess visual function to assure the treatment does not affect existing vision. Systemic clinical examinations and blood work are performed to look for evidence of an immune reaction and to monitor the presence of the gene and the vector. Additionally an independent data safety and monitoring board reviews the trial conduct with particular emphasis on safety.

These and other studies and safeguards gave experts with the FDA and RAC the confidence to approve this clinical trial. Patients are carefully informed of all the potential risks and benefits of this novel investigative treatment.

Potential Risks and Benefits

Potential risks include those associated with administering the viral vector and adverse reactions to other study procedures. For example, the viral vector could spread outside the injection site to the bloodstream and to tears; it could interact with other viruses that the patient comes in contact with, forming a new virus that could produce new side effects. The RPE65 genes could damage the DNA in the cells of the patients' retinas. If DNA damage occurred it could put the patient at risk for developing retinal tumors in the future, though this has not occurred in the previous animal studies. A cataract or retinal detachment could occur that could require additional surgery and could lead to worse vision. Some of the other study procedures could pose risks. For example the patient could incur a minor corneal abrasion from the electroretinogram contact lens electrode used in measuring the retina's electrical function.

There are several potential benefits of this clinical trial. It will describe the safety of the method of delivering the viral vector into the retina and the genetic material being used to treat the disease. If successful this trial also will have the benefit of allowing the investigators to progress to phase II trials. In phase II trials the effectiveness of the therapy can be assessed. Overall the phase I trial will provide knowledge about RPE65-associated LCA that will increase our understanding of this severe blinding disease. This increase in scientific knowledge may lead to a safe and effective means to prevent vision loss or restore vision in people with LCA. It may also create a precedent for the development of gene-based therapies for the more than 150 different genes that contain mutations causing retinal degenerative diseases.

Patient Selection Considerations

One of the challenges with this therapy is to determine how late in the disease process one can therapeutically intervene. Mutations in the RPE65 gene cause severe vision loss. However the structure of the retina remains intact for some indeterminate period of time. Eventually the photoreceptor cells begin to die and at some point the disease is no longer treatable with gene transfer. Therefore before a patient can enter a clinical trial in which efficacy is being evaluated, the patient's retinas are assessed to determine if the retinas are still viable.

Using high resolution imaging technology called Optical Coherence Tomography, NEI-supported researchers have studied the structure of the retina of patients with LCA and controls without disease to determine if it is possible to ascertain the health of the retina. Adults with LCA had evidence of thinning in some portion(s) of the retina, indicating that some cell loss had occurred. The study authors found that for the therapy to be successful in adults with LCA, it is necessary to target surviving tissue in the retina.

Another facet of this research program is the evaluation of retinal structure and function in LCA patients with RPE65 mutations. The aim of these studies is to identify additional individuals who would be potential candidates for RPE65 gene transfer based on photoreceptor layer thickness and visual function. In this way, investigators can begin to identify the most appropriate patients for future clinical trials in which the treatment's effectiveness will be evaluated.