In this interview, Dr. Susanne Back explains the growing role of antisense oligonucleotides (ASOs) in neuroscience drug discovery. She discusses their benefits, challenges in administration, in vivo modeling, and methods for assessing bioavailability in CNS disorders like ALS.
Could you please introduce yourself?
I am Susanne Back, Associate Director of Central Nervous System (CNS) Pharmacology at Charles River Labs. I earned my PhD in Pharmacology from the University of Helsinki in Finland and since then have spent the last 15 years developing and working with preclinical models for neurological illnesses such as Parkinson’s disease and amyotrophic lateral sclerosis (ALS). In my current role, I lead and oversee preclincal studies for our clients, employing in vivo models to assist drug development programs in neuroscience, rare diseases, cardiovascular disorders, and oncology.
Can you tell us a bit about what ASOs are, and why they are a trending modality in neuroscience disorders like ALS?
ASOs, or antisense oligonucleotides, are short, synthetic sequences of single-stranded DNA that alter RNA expression or splicing in the process of translation, where genetic code (DNA) is used to produce proteins. ASOs can reduce, restore or modify the expression of a protein. Often, ASOs are described as a gene therapy as they alter translation, but the processes involved in their drug discovery and manfacture are more similar to small molecule drug development, than viral vector-based gene therapies.
There are several reasons why there is increasing interest in ASOs to treat CNS disorders. Firstly there are already examples of ASOs that have been approved by the FDA for the treatment of rare diseases, showing they are safe and effective. Also, their synthetic, 'designed' nature means they can be targeted to a specific RNA transcript related to disease-causing genes. This makes them particularly attractive in diseases where there is a strong genetic link, such as ALS, Huntington's disease, and a wide range of rare and ultra-rare diseases.
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What should be some considerations when moving your ASO program into an in vivo environment in the discovery phase?
When transitioning an ASO program into an in vivo environment, particularly for CNS therapies like ALS, the first thing to consider is the amount of evidence supporting the involvement of a target in the disease.
Strong evidence for ALS often comes from familial forms of the disease, where specific gene mutations— for example in the SOD1, C9orf72, FUS, or TDP-43 genes —are directly linked to the disease. These genetic insights help guide our choice of preclinical models and readouts in the pharmacology phase.
It is also crucial to prioritize the studies that will move the drug discovery program forward as efficiently as possible. ALS is a disease where time is of the essence, so we need to focus on studies that help us avoid unnecessary delays.
For ASO programs, pharmacodynamic readouts—whether the ASO leads to degradation, splicing modulation, or other target effects—are key. However, how we implement these readouts can differ across programs, and early safety assessments, like bio-distribution and immunostimulatory effects, can help streamline the process,
Are there any limitations to animal models?
Yes, there certainly are limitations. While animal models are critical for understanding disease mechanisms and testing therapeutic approaches, they do not always fully mimic the human condition.
For ALS, for example, even the best animal models may not capture the entire complexity of the disease as it occurs in patients. The limitations could be translational, meaning that results from animal studies do not always predict what we will see in human trials. Because of this, there is a growing interest in human-based in vitro models, for example, human iPSC-derived cell models, especially in certain situations where animal models may not be relevant. The challenge is balancing these different models to ensure we get the most accurate and predictive data as we advance ASO therapies, to reduce failure rates and get life-altering therapies to patients faster.
What considerations need to be made in regards to administration of ASOs for neuroscience disorders?
One key difference between small molecule drugs and ASOs is that ASOs do not freely cross the blood brain barrier. This means they cannot be given systemically, for example in a pill format that is taken orally, they need to be directly administered into the CNS. Clinically the most common way of doing this is intrathecal injection, injection into the space around the spinal cord. This method is highly translatable, meaning that in studies in animal models we can use the same method as would be used in the clinic in patients.
Other methods of administration are also under investigation, for example conjugating an ASO to an antibody to enable transport through the blood brain barrier. Disruption of the blood brain barrier with focused ultrasound is also used for gene therapy administration in the clinic, and this is something we can use in animal models as well, in drug discovery and safety studies.
How do you assess the distribution and bioavailability of ASOs in vivo?
There are a few established methods for this. To measure ASO distribution in CNS tissues, we often use tissue dissection followed by techniques like liquid chromatography-mass spectrometry, hybridization ELISA, or qPCR, which allow us to quantify ASO levels in different brain regions.
Another useful approach is microdialysis, a repeated microsampling technique used in animal models, which can enable measurement of ASO concentrations in plasma and cerebrospinal fluid (CSF). However, it is important to remember that ASOs act on intracellular targets so measurement of unbound ASOs in these fluids may not be the most accurate technique. For a more comprehensive view, we can use PET imaging with radio-labeled ASOs to track their distribution in living animals. PET imaging also allows for repeated measurement so tracking of distribution and bioavailability over time.
Could you comment on the half-life of ASOs in the CSF and CNS tissues in animals and ALS patients?
The half-life of ASOs can vary significantly depending on their chemistry and backbone. For example, in studies involving SOD1 ASOs, we have seen knockdown effects lasting up to eight weeks after a single intrathecal injection. In a different case, with the C9orf72 mouse model, knockdown effects were observed for up to 24 weeks after two injections. In general, their therapeutic effects last far longer than small molecules, which is highly beneficial to patients.
About Susanne Back 
Dr. Susanne Back obtained her PhD in pharmacology from the University of Helsinki, where she investigated CNS diseases using rodent models to evaluate novel therapies and biomarkers. She then spent three years as a postdoctoral researcher at the National Institute on Drug Abuse, NIH, developing disease models in rodents and cell-based systems. After working in CNS drug discovery at Orion Pharma, she joined Charles River in 2021. She now leads a team specializing in preclinical in vivo models and pharmacology, supporting drug discovery efforts in CNS and rare diseases.
About Charles River Laboratories
At Charles River, we are passionate about our role in improving the quality of people’s lives. Our dedicated team of preclinical neuroscience CRO scientists want the same thing as you do: to find a cure for the devastating diseases of the central nervous system. From basic research to regulatory approval, we have the leading science, range of services, and collaborative approach you need to discover and develop novel therapies.
We understand the challenges and complexities in the search of potential therapies for neurological disorders. The combination of our comprehensive neuroscience drug discovery services and expertise supports the creation of customizable, innovative and efficient solutions for your research. Our team of neuroscientists continues to establish the most relevant in vitro and in vivo models and assays of acute and chronic neurological diseases to help our partners identify and test new compounds in this challenging field.
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