Antisense oligonucleotides (ASOs) have shown great promise in targeting disease-causing genes, offering hope for patients suffering from genetic disorders that were previously difficult to address. ASOs bind to specific RNA sequences, preventing the production of harmful proteins or altering gene expressions.
However, any drug developer or sponsor knows that ASOs present a unique set of challenges that must be carefully managed to avoid delays in the development process. Drug developers must be wary of ASOs’ complex toxicity profiles, understand unique pharmacokinetic (PK) characteristics, improve their safety profile, monitor their long-term effects, and navigate tricky regulatory landscapes.
Even the most experienced developers may find all this daunting. The following tips can help ensure a smooth development process toward the safe and effective clinical use of ASO therapeutics.
- Understand the Complex Toxicity Profiles of ASOs
Scientists must develop a firm understanding of all aspects of their ASOs, including the sequence, the chemistry of the backbone, the delivery route, and target tissues. For example, high-exposure tissues vary depending on a local or systemic delivery route, leading to different toxicity profiles. ASOs delivered systemically may result in a range of toxicities consisting of hepatotoxicity, nephrotoxicity, thrombocytopenia, and local/systemic inflammation; ASOs directly delivered to the central nervous system (CNS) can be associated with CNS-specific toxicities.
Following systemic administration, ASOs accumulate in tissues, especially the liver and kidneys. Hepatoxicity is caused by ASOs binding to unintended RNA sequences, disrupting the metabolism, and inducing stress in hepatocytes. As a result, hepatotoxicity is often considered sequence specific. In the kidneys, ASOs can amass in the lysosomes of proximal tubules, which leads to damage and proteinuria. Consequently, nephrotoxicity is mainly regarded as accumulation-related toxicity and primarily sequence unspecific. Other pronounced toxicities include thrombocytopenia and local or systemic inflammation due to inhibition of coagulation and complement activation, respectively. These toxicities are mostly driven by the plasma concentrations exceeding a threshold level. In addition, toxicities associated with CNS local delivery are often observed. Since ASOs are generally charged and too large to cross the blood-brain barrier, intrathecal (IT) or intracerebroventricular (ICV) administration (in rodents) is the most frequently used CNS administration where an immediate high and long-lasting ASO concentration can be achieved in the cerebral spinal fluid and brain, leading to significant pharmacodynamic (PD) effects.
To ensure ASOs’ safety profiles can be well characterized, the following (not limited to) specific biomarkers are often incorporated into toxicology studies for thorough evaluations:
• Liver toxicity (ALT, AST)
• Kidney toxicity (BUN, serum creatinine)
• Inflammation (clinical observations, fever, inflammatory cytokines)
• Coagulation and complement activation (split products Bb and C5a)
It’s recommended that these parameters should be included in early preliminary toxicity studies to establish reliable safety signals, allowing developers to identify risks at an early stage before advancing to clinical trials. - Improve Safety Profiles for ASOs
Chemical modifications can improve ASOs’ stability and enhance their specificity. They can also reduce the minimum therapeutic dose required to achieve PD effects. Lowering the therapeutic dose could reduce the risk of adverse effects, subsequently improving ASOs’ safety margins and making therapies safer for clinical use.
ASOs generally exhibit low immunogenicity, but chemical modifications and altering the sequence and administration route can change how they interact with the immune system. Risk assessment should be carried out early in development, including characterizing anti-drug antibody (ADA) responses. ADA samples are collected during preclinical and clinical trials, and analysis should be guided by PK, PD, and early toxicity profiles. - Consider the Unique PK Profiles of ASOs
The PK profiles of ASOs differ significantly from those of traditional small molecules or biologics. They generally distribute rapidly and have short plasma half-lives of less than an hour, but they can accumulate in tissues, resulting in long tissue half-lives.
To enhance the safety of ASOs, developers should sufficiently characterize PK properties following single and multiple doses early in drug development. When PK properties don’t reflect target tissue distribution, researchers should include PD biomarkers in studies to assess the effect of ASOs accurately. - Monitor Long-Term Effects of ASOs
ASOs are still a relatively young drug type, and only a handful of therapies have been sanctioned by regulatory bodies. This makes it even more crucial to properly understand these treatments’ long-term safety profiles.
Only robust, well-designed studies can fully characterize ASO’s PK, PD, and safety profiles, and monitoring adverse effects throughout preclinical and clinical development is extremely important. This is one of the many areas where working with a trusted lab partner can help, as their expertise and experience can ensure that developers achieve a drug’s maximum potential utility while mitigating any short and/or long-term adverse effects. - Navigate the Regulatory Hurdles of ASOs
Developers can pursue a hybrid regulatory pathway because ASOs share characteristics with small molecules and biologics. Their development follows a similar pathway as small molecules, but additional immunogenicity and target specificity considerations align them with biologics. More regulatory information can be found here.
To mitigate safety risks and ensure a smooth transition from nonclinical development to clinical trials, drug developers should focus on:
• Species selection: Ensuring pharmacological activity in at least one species
• Early PK/PD characterization: Establishing the relationship between dosing, PK profile, and therapeutic effects.
• Biomarker integration: Monitoring common ASO toxicities throughout development.
Regulators have developed tailored frameworks for ASOs to streamline their advancement. There are accelerated pathways for ASOs targeting rare genetic disorders, allowing patients faster access to treatments while developers gather safety data post-approval. This means developers must implement post-marketing surveillance programs that monitor safety and efficacy over long periods.
A Final Word on Developing ASOs
While ASOs offer great hope of treating deadly and difficult-to-address conditions, the challenges associated with their development pose many questions to developers. However, drug developers should know they’re not alone when facing these hurdles. ASO therapies are evolving rapidly, and more challenges will emerge. Collaboration and knowledge-sharing are key to their success. By working with world-class partners, they can ensure that the best therapeutic products reach patients efficiently.
