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For decades, capsid engineering has followed a standard path: screen capsid libraries in vitro and in animal models, identify promising candidates based on their distribution to cells and tissues of interest, and progress the best performing capsids to more advanced studies. This approach, however, carries inherent translational uncertainty, since capsids that perform in rodents or NHPs may not translate to humans.
Our approach is more direct. Instead of selecting capsids that work in animals and hoping that the mechanism translates across species to humans, we start by engineering capsids that bind to human receptors of interest and validate them in humanized models, minimizing the translational guesswork. This strategy has paid off in our efforts to develop new capsids that target the central nervous system (CNS).
Starting with human biology
The foundational validation for this approach was done by our Scientific Founder, Ben Deverman, and his colleagues at the Broad Institute. The researchers used their receptor-targeting platform to engineer TfR1 CapX to bind to human transferrin receptor 1 (hTfR1), with the goal of transporting genetic medicines across the blood-brain barrier (BBB) and achieving broad distribution throughout the CNS.
hTfR1 is a well-characterized receptor that has been investigated by industry and academia for more than 40 years, with one approved therapy and several clinical-stage programs already benefiting from an understanding of its biology. It’s an attractive transport target for therapeutics intended for the CNS because expression of hTfR1 is high throughout the BBB and consistent throughout life as well as across neurological diseases. By targeting hTfR1, Ben and his colleagues leveraged a known and reliable BBB-shuttling mechanism that exists across patient populations and demonstrated that starting with human biology to engineer capsids for human receptors of interest is possible. They published their findings in Science.
Illustration: Human transferrin receptor 1 (TfR1) is well-validated as a valuable brain target on the blood-brain barrier (BBB)
As Ben and his colleagues explain in their study, they screened capsid libraries for their ability to bind to hTfR1 in vitro, identifying a capsid called BI-hTFR1 as the best-performing capsid in terms of gene delivery and transport across human brain endothelial cells. When tested in humanized knock-in mice that express hTfR1, BI-hTFR1 achieved 40 to 50 times greater transduction across different brain regions compared to AAV9, with minimal off-target delivery to other organs.
With Ben, we built on this foundational BI-hTFR1 research to engineer our novel CNS-targeting capsid, TfR1 CapX. Like its predecessor, TfR1 CapX interacts with the apical domain of hTfR1 without interfering with the receptor’s interaction with transferrin, enabling efficient transcytosis across the BBB.
We’ve conducted extensive studies demonstrating that TfR1 CapX has improved CNS selectivity compared to the first-generation BI-hTFR1 capsid. When administering TfR1 CapX to hTfR1 knock-in mice at a dose of 2E12 vg/kg, we observe robust transduction of neurons and astrocytes across different brain regions. TfR1 CapX has also displayed a clean safety profile in hTfR1 knock-in mice at doses as high as 1E14 vg/kg.
Human-centric capsid engineering
What distinguishes TfR1 CapX is that it is specific to human transferrin receptor 1. We characterized amino acid residues in hTfR1 that are necessary and sufficient for TfR1 CapX to bind to it and found that key residues are not present in nonclinical species, including NHPs. Despite a high degree of sequence identity between NHP and human TfR1, a single amino acid change will drive human binding or eliminate it. We understand, at the amino acid level, why TfR1 CapX is specific to humans, and why NHPs are not pharmacologically relevant. This means that traditional techniques to screen in NHPs would not have identified this promising capsid.
Illustration: Though there are similarities between human and NHP TfR1, small sequence differences determine AAV binding and translational relevance
Manufacturing and regulatory readiness
Performance in the lab is only one part of developing genetic medicines. Manufacturing and regulatory considerations are just as critical and should be evaluated before investing time and energy into developing a product.
TfR1 CapX demonstrates manufacturability on par with AAV9 and is compatible with standard production and purification unit operations. A CDMO has manufactured a 50 L batch using a process that is representative of clinical-scale production, supporting its use in GLP safety and biodistribution studies and reducing comparability risk between nonclinical and clinical material. TfR1 CapX has also received positive feedback from the FDA across multiple programs, with a roadmap for preclinical development that doesn’t require NHPs but instead uses transgenic mice that express hTfR1. This means that developers using TfR1 CapX can rapidly and efficiently generate meaningful and rich datasets in a translationally relevant model (the hTfR1 knock-in mice) at higher multiplicities, which can provide a more robust and thorough understanding of product performance.
From mechanism to clinic
TfR1 CapX is on track to enter the clinic later this year, with a second program planned for the first half of 2027, positioning it as a leading IV-administered, BBB-crossing capsid.
As our goal is to enable research that can help bring new genetic medicines to patients, we have recently created Open Aperture, a material transfer agreement (MTA) program that is designed to provide qualified academic researchers with no-cost, non-commercial access to TfR1 CapX.
If you’re an academic investigator working on CNS gene therapy and want to learn more or apply, start here. If you don’t have an academic affiliation (e.g., for-profit, not-for-profit, or foundation), we are happy to discuss alternative access pathways for TfR1 CapX. Please contact us at info@aperturagtx.com.