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New therapeutic targets, drug development for RP

Digital EditionOphthalmology Times: May 2024
Volume 49
Issue 5

Simply slowing disease progression is no longer enough.

(Image Credit: AdobeStock/Radomir Jovanovic)

(Image Credit: AdobeStock/Radomir Jovanovic)

Retinitis pigmentosa (RP) is the most common inherited retinal disease. Even though more than 1.5 million people worldwide are affected, there is no cure and treatment options are limited.1

Kevin Yang Wu, MD, DMD, of the Division of Ophthalmology, Department of Surgery, University of Sherbrooke, Quebec, Canada, discussed the gene and cell therapies under development and the novel potential drug targets for RP at the 2024 American Society of Cataract and Refractive Surgery Annual Meeting in Boston, Massachusetts. The current conventional therapies, which only slow the progression of the disease, include retinoids, vitamin A supplements, protection from sunlight, visual aids, and medical and surgical interventions to treat ophthalmic comorbidities.

Gene therapies

Wu pointed out that only individuals with the RPE65 gene mutation, who comprise a small population of patients (ie, 0.3%-1% of all RP cases), are candidates for treatment with voretigene neparvovec Luxturna; Spark Therapeutics), the only available gene therapy. A recent phase 3 clinical trial, in which 31 patients with confirmed biallelic RPE65mutations were treated with reported visual function improvement and no serious adverse events after 1 year and durability of improvement after 3 to 4 years of follow-up.2-4

The types of gene therapies under development include viral vectors, clustered regularly interspaced palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9) gene editing system, and RNA replacement. Viral vectors include adenoviruses, adeno-associated viruses (AAVs), and lentiviruses.5 Of these, AAVs have been receiving the most attention because of their potential and small size that facilitates the efficient targeting of the retinal layers. They also exhibit low immunogenicity, allowing prolonged expression of the target gene after single-dose treatment and safe administration of a second dose in the subretinal space.6 AAVs also are nonpathogenic and have not yet been linked to human diseases.5

Challenges associated with single AAV vector use is their cloning capacity being limited to nonlarge genomes, the procedural invasiveness, and slow onset of transgene expression.6 Overall, novel AAV capsids have overcome these challenges by facilitating intravitreal gene therapy, which has a lower treatment burden than intraretinal injections.7-9

The CRISPR/Cas9 DNA gene editing system is a novel tool that is widely used in gene expression and suppression methods.10,11 The system has achieved higher rates of successful gene editing in vitro; however, a major challenge is the lower performance rates. The success of gene editing, according to Wu, depends on the guide RNA design, cell target type, delivery method of viral or nonviral vectors, and related host factors. Despite this, CRISPR/Cas9 gene editing may be significant for treating inherited retinal diseases, because even with slight gene modification, the phenotype can improve.

Attention has been shifting toward RNA-guided gene editing methods,12 as the Cas13 tool can modify genes during transcriptional activity.13,14 Major drawbacks are the potential for nonspecific collateral RNA cleavage15-17 and the potential neurotoxicity of Cas13 enzymes through the impediment of neuronal development in vitro.18 Overall, CRISPR/cas13 methods are a novel approach for treating retinal diseases, and further studies should assess their potential and safety.19

RNA replacement therapies deplete endogenous mutant RNAs. Noncoding RNAs—of which there are 3—are the key actors in gene expression regulation: micro-RNA (miRNA), small-interfering RNA (siRNA), and short hairpin RNA (shRNA).20 Briefly, miRNA regulates posttranscriptional modifications by binding the binding sites of a target messenger RNA; siRNA, a double-stranded RNA, has a similar mechanism of action to that of miRNA, but its binding capacity is highly specific and can distinguish single nucleotide disparities;21 and shRNA, whose transcripts are structurally similar to those of miRNA, acts on DNA delivery as opposed to RNA effector molecule regulation, as siRNA does.20

Finally, synthetic single-stranded RNAs (antisense oligonucleotides) have therapeutic effects in inherited retinal diseases;20 they bind to RNAs and promote RNA cleavage. A challenge with siRNAs is the susceptibility to degradation by serologic enzymes; in contrast, shRNAs have a sustainable effect and higher potency.22

Stem cell therapies

Cell-based therapies can replace dysfunctional cells with effective stem cells and restore dysfunctional cells by releasing trophic factors. With these therapies, success demands integration over the long term and formation of new synaptic connections with the host.23

Embryonic stem cells are pluripotent stem cells that can self-renew through division and develop into the 3 primary germ cells. A disadvantage is immune system rejection. Induced pluripotent stem cells (iPSCs), an alternative to embryonic stem cells, are generated from a somatic cell line and can differentiate into any somatic cell, which facilitates stem cell extraction without the need for human embryos. Although a dysfunctional retinal pigment epithelium (RPE) affects the photoreceptors with resultant visual loss, replacing the damaged RPE and photoreceptors with healthy pluripotent stem cells can delay disease progression and potentially restore vision.24,25

Several experimental studies have reported improved vision in animal models.26-30 In addition, concerns about using human embryos are eliminated and patients can be matched based on compatible blood types. Drawbacks are epigenetic memory, by which derived cells retain the gene expression of the original cells, possibly affecting senescence and proliferation. Another concern is the ability of iPSCs to proliferate indefinitely and development of teratomas.

Bone marrow stem cell (BMSC) therapies include mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs). BMSCs are mainly looked at when studying RP.31 Their ability to fully differentiate into photoreceptors is under study, but preclinical studies show that BMSCs release antiangiogenic and neurotrophic factors, as well as immunomodulatory proteins such as insulinlike growth factor-1, class II major histocompatibility complex antigens, and Th2-related cytokines.32-34

A study of primitive MSC-derived retinal progenitor cells (RPCs) in mice showed that the cells integrated and countered inflammation.35 Overall, the BMSC advantages include their ability to migrate toward lesion sites, transdifferentiation, and ease of extracting and manipulating HSCs.

Stem cell–derived RPE can be used to provide trophic support for surviving photoreceptors.36 Early-phase clinical trials of encapsulated RPE cells suggested photoreceptor protection in patients with retinal degeneration.37 A study that injected HSCs suggested trophic regenerative effects without electroretinographic changes.38 Recently, conjunctival MSCs have been induced into photoreceptorlike cells in fibrin gel to form 3-dimensional scaffolds.39

This cellular bioengineering is a promising strategy to increase the number of photoreceptors and promote the retinal angiogenesis needed for repair and regeneration.39 In vivo murine studies showed that MSCs transplanted into the vitreous improved the survival and preservation of photoreceptors and delayed retinal degeneration in RP.40

Transplanting fetal RPE cells and MSCs in mice significantly improved the electroretinographic results and increased the survival rate of transplanted cells through increased expression of a protein that activates rhodopsin and rhodopsin levels, as well as a decrease in caspase-3 expression.41 These results suggest that there is greater benefit in coculture transplantation compared with a single-cell transplantation.41

RPCs, multipotent stem cells in the developing neural retina of 16- to 20-week human fetuses,42 can be manipulated in vitro to express photoreceptor markers and differentiate into retinal neuronal cells.43,44 Preclinical studies have shown that RPCs differentiate into rod photoreceptors when transplanted and integrate into the degenerating retina to form synaptic connections and improve visual function.45

A mouse study showed exceptional results in the formation of functional synapses with host cells, potentially stabilizing progressive vision loss.46 The advantages of RPCs include secretion of trophic factors to increase the likelihood of retinal survival and photoreceptor replacement.47

Novel therapeutic targets

Various approaches are under investigation and include optogenetics, a one-for-all therapy that can be used in cases where degradation has occurred, regardless of the specific mutation present, and includes microbial- and animal-derived opsins; neuroprotective agents such as antioxidants, antiapoptotic agents, and neurotropic factors such as ciliary neurotrophic factors, brain-derived neurotropic factors, and fibroblastic growth factors; and exosomes that can be used to deliver biologically active molecules to specific cells and tissues.

“RP may be a difficult condition to understand and treat,” Wu said. “With advances in our understanding of its molecular mechanisms, we can unlock new doors to innovative therapies that will change the way we approach RP and offer a ray of hope for those [with] this disease.”

Kevin Yang Wu, MD, DMD
E: yang.wu@usherbrooke.ca
Wu has no financial interests related to this topic.
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