The target cell's machinery reads the gene and creates the encoded Chop2 protein, which provides light sensitivity to these inner retinal cells, replacing the photosensitivity previously provided by the rods and cones that have become degenerated because of the disease.
In ocular gene therapy, the virus containing the gene of interest is injected into the eye, either subretinally, or intravitreally. Some distinct advantages of gene therapies in the eye are 1) the eye is an enclosed space, so the virus/gene is not distributed widely throughout the body, 2) smaller doses can be administered, 3) while the eye is not completely immune privileged, the immune response within the eye is muted in comparison to systemic administration of a virus.
Potential problems with gene therapies come primarily in the body’s immune response to the virus being used. Immunogenicity can be elicited, which eliminates the virus before it reaches its target and can also cause major side effects.
Early on in gene therapy developments a patient, Jesse Gelsinger, died in a clinical trial and regulatory bodies have since been very cautious with the approach.
Adeno-associated viruses (AAV) are not known to cause any human disease, and illicit a muted immune response. They have been well characterized in human studies. Safety and efficacy has been substantiated in ocular studies. For these reasons, RetroSense will advance with an AAV vector.
Status of DevelopmentChannelrhodopsin-2 is a very well characterized protein. Numerous studies have been published supporting the protein’s efficacy(2, 3, 4) and safety (5, 6) in vision restoration. Published mouse, rat, and marmoset studies support efficacy. Additional published studies support the safety – including neurotoxicity and immunogenicity. In order to enter into human clinical trials, however, RetroSense will be required to perform a battery of safety tests under “Good Laboratory Practices” as established by FDA guidelines.
While RetroSense cannot divulge details on just where they are in the pre-clinical process, the company suggests they could be in human clinical trials in less than two years. Much like the LCA trials recently published, RetroSense expects to see signs of efficacy in a short time frame (as little as two months), with a small number of patients. As drug development goes, this is actually a very short timeline.
Alternative Therapies Under Development for RP and Dry AMD As a means of putting gene therapy into perspective as one of the many approaches that are being developed for the ophthalmologist’s armamentarium for treating RP and dry AMD, I have included the following information about the state of research for other experimental techniques/therapies that may find use in treating these diseases.
For RP:The prime therapies for treating RP are gene therapy, described above; retinal implants; and the use of stem cells. The two latter therapies will be discussed below. Plus, at least one drug therapy is under evaluation, as noted below.
Retinal Implants (7, 8)
This discussion is taken almost entirely from Wikipedia. For more information about retinal implants and their technology, please see the excellent review article by Dr. Raymond Iezzi, recently published in
Review of Ophthalmology (Reference 8).
A retinal implant is a biomedical device meant to partially restore useful vision to people who have lost theirs due to degenerative eye conditions such as retinitis pigmentosa or macular degeneration. Retinal implants are currently being developed by more than fifteen companies and research institutions in more than six countries worldwide.
The various technologies used in the implants consist of : 1) an array of electrodes implanted on the retinal surface; 2) a digital camera worn on the user's body and, 3) a transmitter/image processor that converts the image to electrical signals and beams them to the electrode array in the eye. The technologies, while still rudimentary, allow the user to see a scoreboard type image made up of bright points of light viewed from about arm's length. The developments are currently aimed only at the disabled, however, this technology once perfected, could revolutionize the personal computing and cyborg industries.
There are two types of retinal implants currently showing promise in clinical trials: Epiretinal Implants (on the retinal surface) and Subretinal Implants (within or behind the retina).
Epiretinal Implants sit on top of the retina, directly stimulating ganglia using signals sent from the external camera and power sent from an external transmitter, whereas Subretinal Implants sit under the retina, stimulating bipolar or ganglion cells from underneath. Some subretinal implants use signals and power from external circuitry, while others use only incident light as a power source and effectively replace damaged photoreceptors leaving all other structures within the eye untouched.
Clinical trials with chronic or semi-chronic (one-month) implant durations are currently under way in the United States and Europe. Patients with epiretinal stimulators have the ability to detect light, can ambulate along a white line painted on the floor, have demonstrated object avoidance, and some subjects can identify large letters on a computer screen with high accuracy. In addition, subretinal devices tested in Europe have provided 20/1200 visual acuity in a single subject.
Retinal prosthesis technology has been in development for more than two decades. Early devices are currently in clinical trials and are demonstrating exciting results.
A few of the major efforts in this field include:
● The Boston Retinal Implant Project - external objective (eyeglasses) and subretinal implant
● Retina Implant AG, in Germany - subretinal implant
● Second Sight Medical Products - epiretinal implant
Retinal Implant AGRetina Implant AG presented its latest clinical results at the recent A
merican Association of Ophthalmology Meeting’s Retina Subspecialty Day on October 16th, “Subretinal Implants for Retinitis Pigmentosa”. The presentation discussed findings from Retina Implant's first clinical trial which began in November 2005 and involved implanting 11 patients with a 1500 electrode microchip subretinally. Dr. Walter Wrobel, CEO of the company, explained how the trial was carried out, the subretinal technique to implantation, and the visual results achieved by patients including the ability to recognize foreign objects and read at a basic level.
The company also discussed the technical and clinical results obtained during that first human clinical trial following the results being published in the
Proceedings of the Royal Society B on November 2, 2010. That study titled, "Subretinal electronic chips allow blind patients to read letters and combine them to words," (9) details the visual results achieved during the first clinical trial. Patients in the trial were able to recognize foreign objects and read letters to form words. The study concluded that the implantation of Retina Implant's microchip was successful in restoring useful vision in patients previously blind due to retinitis pigmentosa.
Implant position in the body. (a) The cable from the implanted chip in the eye leads under the temporal muscle to the exit behind the ear, and connects with a wirelessly operated power control unit. (b) Position of the implant under the transparent retina.
Editors Note: The photograph can be enlarged for easier reading by opening it in a new tab or browser window. "The stellar results achieved during our first clinical trial validate our subretinal approach to implantation which we believe is the key to restoring useful vision for patients blinded by retinitis pigmentosa. As we continue on with our second clinical trial, we look forward to expanding on the lessons learned during our first trial by following patients as they return home."
Retina Implant began their second clinical trial earlier this year in Germany with plans to expand the trial to other European countries including the U.K. and Italy. In this clinical trial patients will receive the 1500 electrode implant permanently. Pending positive results from their second clinical trial, Retina Implant intends to submit the results for CE mark approval (for marketing in Europe).
Early Development ProjectOne other interesting project, in the early stages of development, involves research based at Boston College and the University of Massachusetts Medical School, that is attempting to create nano-structured retinal implants. The visual prosthesis is based on a novel, nanostructured, biocompatible energy conversion device whose dimensions enable the density of implanted stimulating electrodes to exceed 100 million/cm2. This constitutes by far the highest pixelation for a retinal implant, tens of thousands of times higher than any competing device, comparable even to the rods and cones in the retina that the implant will replace. Moreover, the novel "nanocoax" architecture of the neuroprosthesis enables unprecedented light-to-energy conversion efficiency. It also establishes a new paradigm for retinal implant technology: ambient light operation, without the need for implanted batteries or wires.
According to the latest information from the inventor of the above nanostructered device, the research team is still awaiting funding to get this concept off of the ground.
Stem CellsIn my recently published
Primer on the Use of Stem Cells in Ophthalmology, I pointed out that at least three of the seven companies targeting the use of stem cells for retinal diseases had mentioned RP as one of their targets. The three specific references were the programs at Scripp’s Research Institute, sponsored by Pfizer Regenerative Medicine; the program at Oregon Health & Science University, sponsored by Stem Cells, Incorporated; and the program at the Fyodorov Eye Microsurgery Center in Moscow, sponsored by Stemedica. (All three programs, by the way, are using adult stem cells.) No further information is available at this time.
Drug TherapyAt least one drug therapy is under evaluation for treating RP.
Neurotech Pharmaceuticals, Inc., has announced that the company's lead product candidate, NT-501 has demonstrated a strong biologic effect in two Phase 2 clinical trials in the treatment for retinitis pigmentosa (RP).
NT-501 is an intraocular implant that consists of human cells that have been genetically modified to secrete ciliary neurotrophic factor (CNTF). CNTF, a growth factor capable of rescuing and protecting dying photoreceptors, is delivered directly to the back of the eye in a controlled, continuous basis by means of the company's proprietary Encapsulated Cell Technology (ECT) platform. Delivery via ECT bypasses the blood-retinal barrier and overcomes a major obstacle in the treatment of retinal disease.
"CNTF has the potential to help people with retinitis pigmentosa and other photoreceptor degenerations," said Dr. Paul Sieving, Director of the National Eye Institute and Principal Investigator of Neurotech's Phase 1 study of NT-501 in RP. "These studies are important as they present an opportunity to move the field forward."
However, it was noted in the news release about the test results that no visual benefit was seen in the treated eye relative to the control eye over this relatively short 12-month time period. So, it appears that photoreceptor preservation occurs, but no regeneration or improvement in sight using this implant – at least to date.
For Dry AMD:The treatment of dry AMD is under study from many fronts, because of it’s presence in so many patients. The main approaches, in addition to gene therapy discussed briefly above, are both externally applied drops and injected drugs, including sustained release types; stem cells; and retinal regeneration using both a laser to stimulate regeneration of retinal pigmented epithelial (RPE) cells, and a combination of laser and stem cells. These will be discussed briefly below.
Therapeutic and Sustained Drug ReleaseI have published two articles in my online Journal on the subject of drug therapies for retinal diseases, including dry AMD, one of which is specific for the treatment of dry AMD. My
AMD Update 6: An Overview of New Treatments for Dry AMD, by Dr. Philip Rosenfeld, addresses the preclinical and Phase I drugs in development for dry AMD; while my writeup on
Iluvien and the Future of Ophthalmic Drug Delivery Systems discusses sustained-release drug delivery systems, a few of which are aimed at treating dry AMD.
The three FDA-approved sustained-release systems are Vitasert (for cytomegalovirus retinitis); Retisert (for uveitis); and Ozurdex (for macula edema following branch or central retinal vein occlusion). Iluvien (for diabetic macula edema) is expected to achieve FDA marketing approval before the end of this year. It is also being tested as a treatment for both dry AMD and geographic atrophy, which occurs during the latter stages of dry AMD.
Several other drugs, mentioned in both of the articles noted above, are under evaluation for treating dry AMD, and at least one, from Neurotech, as noted above in the RP writeup, for treating RP
Stem Cells Again, as noted in my recently published
Primer on the Use of Stem Cells in Ophthalmology, six of the seven companies are targeting the dry stage of AMD with their stem cell programs. Two of the efforts are with human embryonic stem cells, one uses induced pluripotent stem cells, and the remainder are using adult stem cells. It is interesting to note that one program, that sponsored by Stemedica, at the Moscow-based Fyodorov Eye Institute, is attempting to use the injection of stem cells following spot laser damage of the retina.
It should be noted that to date, no FDA-approved human clinical trials using stem cells to treat retinal diseases has begun. However, this is about to change. The first human trials for retinal diseases are scheduled to begin, either in the fourth quarter of this year (for Stargart’s by Advanced Cell Technology) or in the first quarter of next year (for dry AMD by The London Project to Cure Blindness, sponsored by Pfizer Regenerative Medicine).
Retinal RegenerationIn late 2007, I learned of a new, potential laser treatment, under development for treating retinal diseases. It was the Ellex 2RT program, using a specially designed laser to treat retinal pigment epithelial cells, to stimulate the RPE cells to release enzymes that are capable of “cleaning” Bruch’s membrane, thereby rejuvenating the retina by allowing the increased transport of water and chemicals across this important membrane. In doing so, it is hoped that this will alleviate some of the debris (drusen) associated with the early stages of dry AMD. The technology behind this therapy is described in more detail in my first writeup, E
llex 2RT Retina Regeneration Therapy: A First Report.In two later reports, I wrote about the first clinical results with this laser treatment (
Ellex 2RT Retinal Regeneration Laser: An Update – First Clinical Results), which showed improved visual acuity in patients treated with newly diagnosed diabetic maculopathy and/or macula edema; and then earlier this year, following the ARVO meeting, the results of two key pilot studies, one on patients with proliferative diabetic retinopathy, and the other on patients with early (dry) AMD (
Ellex 2RT Updated Clinical Results: ARVO 2010). In the latter study, of interest to this program, after six months, ten of fourteen eyes had improved in visual function (six eyes) or drusen reduction (ten eyes). One-year results were expected to be reported at the AAO Meeting in late October. (As of the date that this report is being written, I have not received any verification that this occurred.)
Other Applications of Gene Therapy in OphthalmologySignificant advancements have been made in understanding the genetic pathogenesis of ocular diseases, and gene replacement and gene silencing are being shown as potentially efficacious therapies. Recent improvements have been made in the safety and specificity of vector-based ocular gene transfer methods. Proof-of-concept for vector-based gene therapies has also been established in several experimental models of human ocular diseases. After nearly two decades of ocular gene therapy research, preliminary successes are now being reported in phase 1 clinical trials for the treatment of Leber congenital amaurosis.
The review article, “Gene Therapy for Ocular Diseases” (
British Journal of Ophthalmology, August 2010) (
BJO), describes current developments and future prospects for ocular gene therapy.
“Novel methods are being developed to enhance the performance and regulation of recombinant adeno-associated virus- and lentivirusmediated ocular gene transfer. Gene therapy prospects have advanced for a variety of retinal disorders, including retinitis pigmentosa, retinoschisis, Stargardt disease and age-related macular degeneration. Advances have also been made using experimental models for non-retinal diseases, such as uveitis and glaucoma. These methodological advancements are critical for the implementation of additional gene-based therapies for human ocular diseases in the near future.”
During my research for this writeup, I discovered a unique program underway at Genzyme, that combines the anti-VEGF properties of a drug like Lucentis (Genentech) with gene thearapy, to produce, hopefully, a longer acting effect of the anti-VEGF factor in treating the wet form of AMD.
As described in
Technology Review (10), Lucentis binds to and neutralizes a wound-healing growth factor known as VEGF. This binding action stalls the excess growth of blood vessels in the eye that characterizes (the wet form of) age-related macular degeneration. Genzyme's gene therapy drug, officially called AAV2-sFLT01, would insinuate itself into the patient's retinal cells to produce the same VEGF-binding protein as Lucentis over far longer periods – (perhaps) up to several years.
A phase 1 clinical trial of Genzyme's gene therapy treatment began at the end of May. Three patients received the treatment, according to Sam Wadsworth, a Genzyme group vice president in charge of gene and cell therapy. Preliminary results should be available in about a year.
Genzyme is collaborating with Applied Genetic Technologies Corp., in developing the gene therapy drug.
The only other ocular disease that I will discuss in detail in this article is Leber’s congenital amaurosis. I will leave the others for another time.
Lebers Congenital Amaurosis (11, BJO))
Leber’s congenital amaurosis (LCA), is a rare form of inherited blindness caused by retinal degenerative disease that strikes in infancy and causes a severe loss of vision. Leber congenital amaurosis (LCA), an autosomal recessive disorder that affects both rods and cones, has been linked to at least 14 genes. Several have been demonstrated as potentially efficacious gene therapy targets. The use of the gene RPE65 in an adeno-associated virus (AAV2/2) vector or carrier has been shown to restore rod photoreceptor function and vision-dependent behavior in some of the patients so treated.
Researchers in the United States (12)(13) and the United Kingdom (14) have injected one eye of LCA patients with a harmless virus carrying a gene coding for an enzyme needed to make a lightsensing pigment. In the first completed trial, the light sensitivity of all 12 partially blind patients improved. Four children gained enough vision to play sports and stop using learning aids at school. (Another team using a similar approach gave full color vision to squirrel monkeys born with red-green colorblindness.(15))
At least three Clinical Trial are currently underway (BJO). In one dose-escalation study involving 12 patients and performed by Malone et al (reference 102 in BJO) safety and efficacy were sustained for at least 2 years post-treatment.
RPE65 gene therapy for LCA has been one of the most successful examples of gene therapy for ocular diseases to date.
In ConclusionTo date, there are no approved treatments for retinitis pigmentosa and/or dry AMD. As shown above, gene therapy holds much promise for RP and, hopefully, for dry AMD as well.
With stem cell therapy about to begin human clinical trials for both Stargardt’s (Advanced Cell Technology) and dry AMD (UCL-London Project to Cure Blindness – Pfizer), and Retinal Regeneration not far behind, it will be interesting to see which new technology/therapy wins the race for treatment of these ocular diseases.
Resources:The RetroSense WebsiteGene Therapy in OphthalmologyUthra S, Kumaramanickavel G. “Gene therapy in ophthalmology”,
Oman J Ophthalmol 2009; 2:108-10.
Gene Therapy for Ocular DiseasesLiu, Melissa M; Tuo, Jinsheng; Chan, Chi-Chao; “Gene therapy for ocular diseases”;
Br J Ophthalmol doi:10.1136/bjo.2010.174912 (BJO)
Gene Therapy Turns Foes into FriendsAbelson, Mark B., Tzekov, Radouil T., Howe, Amanda; “Gene Therapy Turns Foes into Friends”,
Review of Ophthalmology - August 2009
References:1. Evaluation of AAV-Mediated Expression of Chop2-GFP in the Marmoset Retina, Ivanova E, Hwang GS, Pan ZH, Troilo D, Invest Ophthalmol Vis Sci, May 2010.
2. Ectopic expression of a microbial-type rhodopsin restores visual responses in mice with photoreceptor degeneration, Bi A, Cui J, Ma YP, et al, Neuron 50 (1): 23-33, April 2006.
3. Channelrhodopsin-2 gene transduced into retinal ganglion cells restores functional vision in genetically blind rats. Tomita, et al, Exp Eye Res. 2010 Mar;90(3):429-36. Epub 2009 Dec 27.
4. Evaluation of AAV-mediated expression of Chop2-GFP in the marmoset retina, Ivanova, et al, Invest Ophthalmol Vis Sci. 2010 Oct;51(10):5288-96. Epub 2010 May 19.
5. Evaluation of the adeno-associated virus mediated long-term expression of channelrhodopsin-2 in the mouse retina; Ivanova, et al, Molecular Vision 2009; 15:1680-1689
6. Systematic and Local Responses of Channelrhodopsin-2 Gene Therapy; Sugano, et al, ARVO Poster #A615.
7. Retinal Implants, Wikipedia, February 22, 2010
8. An Inside Look at Retinal Prothesis Technology, Iezzi, Raymond, MD, Review of Ophthalmology, April 2010.
9. Subretinal electronic chips allow blind patients to read letters and combine them to words, Zrenner, Eberhart M.D et al, Proceedings of the Royal Society B, November 2, 2010.
10. Gene Therapy for Eye Diseases, Weintraub, Karen, Technology Review, July 15, 2010.
11. Gene Therapy Returns, Science, December 18, 2009.
12. Gene Therapy Restores Vision in Leber Congenital Amaurosis, Children's Hospital of Philadelphia news release October 24, 2009
13. Safety and Efficacy Study in Subjects With Leber Congenital Amaurosis, Clinical Trial, NIH, October 21, 2009.
14. Eye gene therapy boost for young, BBC NEWS, August 24, 2009.
15. Colour blindness corrected by gene therapy, Nature, September 16, 2009.
Editors Note: The introduction to this article is now posted in the Special News items on Gene Therapy Net.
