Cell and Gene Therapy Answers: Top considerations for ocular cell and gene therapies

Your source for answers to the complex challenges of cell and gene therapy development.

As the field of ocular cell and gene therapies advances, biopharma companies face unique challenges in bringing these innovative treatments from the lab to the clinic. To shed light on the complexities and opportunities in this growing area of therapeutic research, we sat down with Labcorp Study Director and Nonclinical Ocular Toxicology Lead, Peter Sonnentag, to discuss four crucial aspects of ocular cell and gene therapies: the preclinical development process, regulatory requirements for ophthalmic gene therapy programs, the role of immune suppression in both cell and gene therapies and the distinct characteristics that contribute to a successful preclinical program. Here’s what to know. 
 

How is the preclinical development of cell and gene therapies different for ophthalmic indications? 

The first thing to keep in mind is that cell and gene therapies aren't a monolith. I like to think of cell therapies in two main categories: adoptive cell therapies, like CAR T-cells, and then regenerative cell therapies, i.e., those therapies that typically rely on a stem cell-derived cell type that's used to repair or replace damaged cells or even whole tissues. When we're talking about cell therapies in the eye, we're usually referring to those regenerative cell therapies, often attempting to replace RPE or damaged photoreceptors in the eye to combat blinding diseases like age-related macular degeneration. 
When we're talking about gene therapies, there are three main categories we can think about: 

1. Those that modulate expression, e.g., oligonucleotides—usually a small interfering RNA. It's introduced to modulate expression of a protein product to try to treat a disease condition.
2. Gene replacement where viral or non-viral vectors are introduced and transfect host cells, and those host cells use their own machinery to produce a desired protein product that's either missing or nonfunctioning in a disease state.
3. Gene editing. Gene editing is when a viral or non-viral vector introduces an endonuclease to edit a faulty genomic DNA sequence, permanently correcting a gene and downstream protein to address a disease state.

When we're talking about ophthalmic gene therapy, all three of these categories are in play, and researchers are going after diseases like Leber’s congenital amaurosis, retinitis pigmentosa, and AMD, just to name a few, using gene therapy concepts.
So how is the development of an ocular gene therapy or cell therapy different from systemic products? 

Amenable

The eye is unique in that it's susceptible and amenable to cell and gene therapy treatments and it's also observable in a nonclinical space in vivo. 

What do we mean by amenable? There are a number of dose routes that we can use to access different tissues or cell types in the eye, from topical eye drops to intracameral injections to access the front of the eye (lens, cornea), to intravitreal, subretinal, or suprachoroidal injections to access the cells of the retina or choroid in the back of the eye. We can get highly localized concentrations of a therapy to these tissues because of the way the eye is built and because of the compartmentalization of the structures of the eye. 
Along those same lines, the eye is considered immune privileged. It's behind the blood-retinal barrier or blood-aqueous barrier, just like the brain is behind a blood-brain barrier, which really limits systemic exposure from an ocular-administered therapy in many cases. 

On a practical level, the eye is a relatively small organ, allowing for reduced material needs. This is really important when we're talking about very expensive cell and gene therapy products. In some non-pivotal study designs, we can use the fellow eye, so the opposite eye that's undosed, as a control within the same animal.
From a gene therapy standpoint, a lot of genetic diseases in the eye are quite well understood. For a given disease, a vector can be targeted to the appropriate cell type and loaded with a transgene to express a missing or faulty protein characteristic of that disease to hopefully reestablish function. 
In terms of cell therapy in the eye, there are stem cell-derived cell lines that are available and well studied for eye cell types. This includes retinal pigmented epithelial cells and photoreceptor cells. This provides researchers and drug developers endless opportunities for modification and novel delivery of these cells to the eye to treat disease or even injury. 

Observable

The eye is built from different tissue types, many of which are transparent. We can leverage the observability of the cornea, the lens, the aqueous and vitreous to observe the eye and the effects of an experimental therapy. Ophthalmoscopic examination, for example, is a noninvasive way of gathering valuable information that’s very translatable to the clinic. Tools like fundus photography and more advanced techniques like autofluorescence imaging are also useful. Using an imaging technology called optical coherence tomography, we can actually get an in-life cross-section through the back of the eye to look at the different layers of the retina in the same eye over time as a study progresses. So, we can use these tools to monitor treatment-related changes in the eye. We can also use these tools to assess the immune response we might see in response to a cell or gene therapy product administered to the eye. 

We also have functional tests of the visual system to reveal changes that may not be so obvious anatomically. The most important tool in terms of assessing the function of the eye is electroretinography, or ERG. In the simplest terms, ERG is measuring the bioelectrical response of the retina to light stimuli. By varying the stimulus, the color intensity and frequency of light, we can isolate functional changes in the retina to either a cell type or even a geographic area in the eye. For cell or gene therapy animal models of disease, sometimes we can even measure functional rescue of a disease phenotype.

The eye is particularly ripe with opportunity to integrate these anatomical changes we see with functional readouts and histopathology and, even further, to correlate these changes with biodistribution of a vector or a cell and transgene expression as well—looking at things like PCR to look at biodistribution of a vector or a human cell therapy, immunohistochemistry to understand markers or where a transgene might be expressed, etc. This allows investigators to weave together safety, biodistribution and efficacy into a narrative to inform those clinical study designs, which is our ultimate goal in preclinical safety assessment. 

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What do regulators require of a preclinical ophthalmic gene therapy program?

Any gene therapy program needs to show safety and sometimes even efficacy in a relevant animal model to support downstream clinical studies. The majority of ocular gene therapies have therapeutic targets in the retina, and as such, the FDA has a specific guidance for what to consider in a preclinical program for a retinal gene therapy. This is something that we can apply not only to retinal gene therapies, but really across any gene therapy in the eye. 

1. Proof-of-concept studies

One of the things that the FDA recommends is proof-of-concept studies in an animal model that demonstrates the biological response to the gene therapy product similar to what we would expect to see in humans. That response might be targeting in transfection and expression in the relevant cell type all the way up to rescue of a disease phenotype in an animal model of the retinal disease—choosing an animal model that's relevant, that your therapy will work in, to prove the concept is important and something that regulators are going to be looking for. 

2. Biodistribution studies 

They're also going to be looking for biodistribution studies to be conducted. Just like traditional drug development, we want to look at the biodistribution and pharmacokinetics of these therapies. In terms of gene therapies, we want to look at the distribution, persistence and clearance of the vector itself, and sometimes even the express transgene product in vivo as well from the site of administration, usually in the eye, to other ocular target tissues, non-ocular tissues, other vital organs, and systemically, throughout the body. 

3. Toxicology studies should incorporate elements of the planned clinical trial 

Toxicity studies and safety studies need to be included in these programs as well and should incorporate elements of the planned clinical trial. They should mimic the dose range, route of administration, dose schedule, and even the evaluation endpoints on study to the extent feasible in these nonclinical studies as you'd expect to do in the clinical study. These studies should identify potential local ocular and systemic toxicities and tie those to the dose level at which we see those toxicities. Gene therapies often deal with inflammatory reactions or immune-mediated responses to these therapies, and the agency would like to see those responses characterized and assessed as potentially attributed to either the vector or the transgene.

4. Safety in animal model with anatomical relevant eye 

There are a lot of animal models of retinal disorders in rodents, so it's easy to have transgenic or knockout models in rodents that mimic a disease to generate proof-of-concept data. However, since the rodent eye is so much smaller, and the anatomy is quite different from the human eye, the agency does request to see animal models with more “human-like” eyes—rabbits, pigs, non-human primates, for example—to provide applicable safety information.

5. Program should be individualized to the product  

These programs are often bespoke programs, individualized in terms of scope, complexity and overall design. The agency expects these to be designed to the specific therapy and not necessarily be an off-the-shelf solution to filing an IND. The agency also encourages sponsors with these gene therapies to explore opportunities to reduce, refine and replace animals in the preclinical program with sufficient scientific justification. So, it's important that the decisions made in the preclinical design process are put in front of the agency early enough to get buy-in before moving forward with this expensive and time-consuming endeavor. 

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Do animals need to be immunosuppressed for preclinical ophthalmic gene and cell therapy programs?

In terms of immune suppression on cell therapy programs, the answer is, almost unequivocally, yes. Even administering a cell therapy to the immune-privileged eye requires immune suppression of an animal model because we're introducing a human cell type, and it's expected to be immunogenic in these animals. We want that cell type or that graft to not be rejected, and so immune suppression is typically required for these studies. Now in rodents, there are models commercially available that are immune-deficient and can be used for these purposes. These animals require special facilities and husbandry but have established record of use in cell therapy development programs in the eye. 

When it comes to large animals, we don’t typically have access to animals that are bred to be immune deficient, so we rely on chemical immune suppression in these animals. In addition to special facilities and husbandry, they may require monitoring of blood levels to ensure immunosuppressants remain in the therapeutic window. When it comes to gene therapies, typically immune suppression is necessary in these studies, though not to the extent we see with cell therapies, but it's still something we must discuss and consider. With a gene therapy product in the eye, we have to consider the response not only to the vector that's being administered to the eye, but also to the transgene product. So, considering the vector, we often prescreen animals for the presence of neutralizing antibody to the vector that's being administered. For example, if an AAV8 serotype vector is being administered to primates in a study, we prescreen those animals for neutralizing antibody to AAV8 and select specific animals for inclusion on study. A lower or negative neutralizing antibody titer in the animal population can allow for reduced immune responses and better vector transgene expression. When it comes to the transgene, we're typically expressing a human or humanized protein with these gene therapies and that can also sometimes be immunogenic in an animal model. In these cases, we're relying on systemic immune suppression to address the immune response to that protein and to the vector to some extent. In this case, we're usually looking at everything from a lighter immune suppression regimen like methylprednisolone, either orally or intramuscularly, or sometimes a more intensive regimen, closer to those we'd see used in a cell therapy study.

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What are the characteristics of a successful preclinical ophthalmic gene or cell therapy program?

One of the biggest keys to success in any cell or gene therapy program, including ocular programs, is high-quality and well-characterized product. Ensuring a consistent, well-characterized and suitable product for dosing is critical to the success of studies, from early dose ranging studies through IND-enabling studies. 

Next, informed study design is critical to the success of these programs, not only in meeting all of the regulatory expectations in a relevant animal model, but also in taking a carefully considered risk-based approach to safety assessment. Consider what safety liabilities my particular product, my vector, my transgene or cell type have based on existing information and how I can query those specific risks. Also, assess endpoints that are clinically relevant. What endpoints on the nonclinical side of things can we assess that are most relevant to product safety and that are translatable to the clinic? Some endpoints, like histopathology, for example, aren't necessarily translatable to the clinic. So, what surrogate endpoints do we have? If we're concerned with a finding that we have histopathologically in the retina, we might include OCT as well in that study, because OCT is translatable to the clinic. 

Finally, expertise and experience. There's really no substitute for experience in this space. From study direction to technical and scientific staff, everyone involved in these studies needs to be properly trained and experienced, not only in preclinical safety assessment of ophthalmic products, but also in the nuances of cell and gene therapy products in the ocular space.
The path from preclinical development to successful treatment for ocular cell and gene therapies is complex and multifaceted. At Labcorp, our mission to improve health and improve lives remains at the forefront of our work in ocular cell and gene therapies. By investing in our deeply experienced staff, state-of-the-art facilities and commitment to scientific excellence, we continue to support researchers and developers in navigating the challenges and opportunities in this field, with the ultimate goal of bringing transformative treatments for eye health to the patients who need them.

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