Antisense oligonucleotides (ASOs) are short, synthetic strands of genetic material designed to bind to RNA and block harmful genes from producing disease-causing proteins. This targeted approach allows researchers to tackle conditions such as Alzheimer’s disease and Parkinson’s disease at their genetic root, offering patients a precise treatment option.
These neurodegenerative disorders manifest in the brain. However, when scientists try to deliver ASOs there, they encounter a significant hurdle: the blood-brain barrier. This barrier protects the brain from pathogens and other harmful substances, but it complicates drug delivery. While some molecules can pass through the blood-brain barrier using specific transporter proteins, many therapies, including ASOs, struggle to penetrate it effectively, requiring alternative delivery methods.
Joe Lewcock, the Chief Scientific Officer at Denali Therapeutics, explained that healthcare professionals currently deliver ASOs to patients with neurodegenerative diseases through invasive techniques such as intrathecal delivery, which involves injecting the therapy directly into the cerebrospinal fluid surrounding the brain and spinal cord. This method can be uncomfortable and results in high drug concentrations at the injection site with limited dispersal to targeted brain regions.
To address these challenges, researchers at Denali Therapeutics developed a novel delivery system known as the oligonucleotide transport vehicle (OTV) that effectively travels to the brain via the bloodstream. Their recent study, published in Science Translational Medicine, demonstrated the proof of concept that the OTV binds to transferrin receptor 1 (TfR1), a protein that sits at the blood-brain barrier and facilitates iron transport into the brain through the brain’s vast capillary network (1).
What this means is that long after the OTV molecule is cleared from circulation the ASO continues to function, meaning dosing can be less frequent.
- Joe Lewcock, Denali Therapeutics
The researchers tested how efficiently the OTV delivered ASOs across the blood-brain barrier in mice compared to ASOs without a delivery vehicle or ASOs attached to a non-TfR1-binding antibody. They found that the OTV delivered the most ASO to the mouse brain.
Next, the team administered the OTV intravenously into a mouse model expressing human TfR1, as well as macaques. They saw that the OTV successfully delivered ASOs across the blood-brain barrier in both the mice and macaques, leading to significant knockdown of the target Malat1 RNA in various brain regions and spinal cord tissues. They also noted that they had accomplished this effect with minimal peripheral toxicity.
Imaging of mouse and macaque brains revealed that the OTV provided a more uniform distribution of ASOs throughout the brain than intrathecal delivery. This delivery method enhanced therapeutic potential by targeting the brain broadly — including the cortex and the hippocampus — which is important for treating Alzheimer's disease and other disorders that affect many brain regions. It also avoided the dose-limiting toxicities associated with traditional drug delivery methods, such as hind limb paralysis from high lumbar concentrations.
“Part of the prolonged knockdown with OTV comes from the fact that once you deliver the ASO to cells, it is not quickly degraded and instead stays within cells for an extended time period and continues to knock down expression of target genes,” explained Lewcock. “What this means is that long after the OTV molecule is cleared from circulation, the ASO continues to function, meaning dosing can be less frequent.” The OTV's ability to retain activity after clearance is a significant advantage compared to many antibody-based or small molecule drugs, which lose effectiveness once cleared by the body.
Lewcock is enthusiastic about the adaptability of the OTV system, noting that researchers could easily integrate it with various ASO targets, making it a versatile tool for addressing different neurological conditions. He highlighted the potential for the OTV to maintain a prolonged knockdown effect of the target molecule, which is crucial for effective treatment, because ASOs delivered via OTV had a longer half-life in the blood circulation compared to unbound ASOs. The team anticipates filing an Investigational New Drug application and beginning clinical trials within the next 12 to 18 months. “There’s growing excitement about these ASOs being used more broadly,” said Lewcock. “Our hope is to really be able to transform this class of therapeutics into something that can be delivered via IV instead of intrathecally.”
Elizabeth Rhea, a neurobiologist at the University of Washington, noted that while using the transferrin receptor as an entry point for ASOs is not new, Denali Therapeutics' approach of coupling ASOs with transport vehicles to cross the blood-brain barrier represents a significant advance. She is excited about the broader implications of this technology, not only for central nervous system disorders but also for peripheral health issues influenced by the brain such as obesity and aging. However, she cautioned that the delivery system's lack of specificity to the brain or particular cell types within the brain poses a limitation.
Both Lewcock and Rhea emphasized that further research is needed to explore the long-term effects and potential side effects of the OTV. While the current study is limited to animal models, the next crucial step will be conducting human trials to validate the safety and effectiveness of this promising new delivery method.
Rhea added, "As we're learning more about how these drugs are structured and how they act, then we can develop better tools to really target what we want to target.”
Reference
1. Barker, S.J. et al. Targeting the transferrin receptor to transport antisense oligonucleotides across the mammalian blood-brain barrier. Sci Transl Med 16, eadi2245 (2024).
