Ocular conditions can arise from aging, environmental factors, or genetic mutations. The latter require complex treatments such as antisense oligonucleotides (ASOs), which are becoming critical tools to battle genetic and multifactorial ophthalmic diseases.
Researchers have found much potential in ASOs. They act at the RNA level, altering gene expression regardless of the function of the proteins encoded by their target. This enables them to focus on disease-causing genes without affecting other cellular processes.
ASOs can potentially treat conditions such as retinitis pigmentosa, macular degeneration, and glaucoma. As with most cutting-edge medical frontiers, challenges must be overcome to make the most of them, and there are several ways developers and sponsors can ensure they’re unlocking the potential of ASOs.
1. Master ASOs’ Mechanisms of Action
ASOs utilize different mechanisms to address genetic targets in ophthalmic conditions. Each method can be used to target a variation of conditions. These mechanisms include:
RNA degradation: ASOs bind to specific RNA to destroy it and prevent the production of disease-causing proteins.
Splicing modulation: ASOs are used to correct splicing defects or to skip faulty exons in genes associated with genetic disorders.
Translational inhibition: ASOs block the process of translating RNA into protein, thus preventing protein production.
Non-coding RNA targeting: ASOs target the parts of the genetic code that don’t make proteins but play a vital role in regulating gene activity.
Allele-specific targeting: ASOs also target mutant alleles of genes while sparing the wild-type allele.
2. Address the Challenges of ASO Delivery
The eye’s unique anatomical and physiological properties present challenges in delivering ASOs. Parts of the eye, including the cornea, conjunctiva, blood-aqueous barrier, sclera, and blood-retinal barrier, can limit the penetration of ASOs into target tissues. This makes the delivery method crucial in determining the safety and effectiveness of ASOs.
Intravitreal injection allows direct access to the retina and delivers high local concentrations, but it is an invasive procedure and comes with risks, including potentially causing a significant immune response. Topical delivery methods, like eye drops, offer patient-friendly administration but suffer from poor bioavailability due to limited penetration and tear dilution.
New methods include nanoparticle-based delivery and sustained-release formulations. Both approaches offer promise to improve efficacy and safety. The optimal delivery method depends on ASO properties (size, charge, formulation), the specific ocular condition, and the patient population.
3. Ensure Optimal ADME and Bioanalysis of ASOs
The anatomy of the eye also affects the absorption, distribution, metabolism, elimination (ADME), and bioanalysis of ASOs. The cornea, for example, makes absorption more difficult due to its multi-layered structure, which includes the epithelium, stroma, and endothelium.
ASOs are often delivered via intravitreal injection to pass the corneal barrier, allowing direct access to the vitreous humor and distribution to the retina. During this procedure, ASOs distribute a gel-like substance that fills the eye. The treatment then diffuses to adjacent structures, including the retina and retinal pigment epithelium (RPE). Distribution is influenced by the size and charge of the ASOs and their formulation. Like the cornea, the retina has several layers, including the RPE, photoreceptors, and inner retinal layers. ASOs must penetrate these layers to reach target cells. The RPE is the most significant barrier to cross, and the efficacy of ASOs is often heavily influenced by their ability to pass this hurdle. Distribution can also be uneven, depending on the ASO’s binding affinity to the various retinal cell types.
Enzymes in the vitreous humor and within retinal cells can lead to the metabolism of ASOs, reducing their efficacy. Nuclease metabolizes ASOs, so they are not expected to be metabolized by cytochrome P450 (CYP) enzymes and thus have a low risk of drug–drug interaction.
The eye has several ways to eliminate substances, including tear, aqueous humor, and lymphatic drainage. The half-life of ASOs can be limited by their removal from the vitreous humor through these pathways.
Bioanalysis of ASO levels in ocular tissues requires a range of approaches, including:
- Liquid chromatography-mass spectrometry
- Liquid chromatography-high-resolution mass spectrometry
- Quantitative polymerase chain reaction
- Liquid chromatography-fluorescence detection
- Ligand binding assays
4. Navigate the Rapidly Evolving Regulation of ASOs
Limited regulatory guidance exists on oligonucleotide therapies, so developers face challenges navigating the approval process. However, U.S. regulators recently released a draft of a non-clinical safety assessment for this type of drug, and a handful of white papers have assessed preclinical ADME evaluation of oligo treatments.
ASOs are well suited to eye treatment as the eye is an immune-privileged closed compartment, which should limit off-target effects. However, it’s important to follow regulatory guidance to assess immunotoxicity. Many oligonucleotides engage the innate immune system. Stand-alone, dedicated immunotoxicity assessments are not usually warranted, but evaluation of the effects on the immune system in general toxicity testing can help understand observations in these studies.
Oligos can also provoke the production of antidrug antibodies, so developers should consult ICH requirements and guidance from U.S. regulatory bodies to determine whether a non-clinical assessment for this issue is warranted.
A Final Word on ASOs’ Potential in Eye Care
ASOs hold great potential in eye care. Their targeted, gene-based approach can counter previously untreatable conditions, and although few have been approved to date, many new drugs are on the horizon. However, challenges in bioanalysis, delivery, and regulatory navigation remain significant. By partnering with an experienced and trusted laboratory, drug developers can streamline the path from discovery to clinic—ensuring development is not only scientifically rigorous, but also efficient and cost-effective.
