Asked and Answered: Taking Protein Degradation to a New Therapeutic Frontier—Outside the Cell

Effie Tozzo, PhD, is the chief scientific officer of Avilar Therapeutics, a biopharmaceutical company pioneering the discovery and development of extracellular protein degrader therapeutics, a new frontier applying targeted protein degradation as an innovative drug modality for a wide range of diseases. Prior to joining Avilar, Dr. Tozzo was senior vice president of drug development at Cellarity, a company founded by Flagship Pioneering. She has also held R&D leadership positions at Mitobridge/Astellas, Merck, Roche, BMS, Millennium, and Chiron.

Dr. Tozzo recently spoke with Inside Precision Medicine’s editor in chief, Damian Doherty, about Avilar’s unique approach, which harnesses a natural process in the body through which extracellular proteins are degraded. This opens the potential for Avilar to pursue traditionally undruggable targets known to be involved in major diseases.

Q:  How is Avilar evolving the field of targeted protein degradation?

A: Protein degradation has emerged as one of the hottest and rapidly growing new drug modalities with nearly 20 protein degrader therapeutics in clinical trials in just a few short years. Most companies are developing protein degraders for intracellular targets, and these approaches fall primarily into two classes, one called PROTACs “proteolysis-targeting chimeras” and one called molecular glues. The advancement of PROTACs and glues has sparked development of new technologies to harness other natural processes used by our cells for degrading proteins.

At Avilar, we’re moving protein degradation beyond targets inside the cell to degrade extracellular, circulating [proteins] and membrane-bound proteins. Extracellular proteins are known to be implicated in disease pathogenesis in many ways. Some extracellular proteins may become overexpressed, while others exist in aberrant forms or complexes. For example, autoantibodies are circulating proteins produced by an individual’s own immune system that disrupt normal protein function and lead to autoimmune diseases. With nearly 40% of human proteins located outside the cell, and many having a clearly defined role in human disease, we’re tapping into a significant opportunity to develop novel degraders as a new class of medicines for the treatment of multiple serious diseases.

Q:  What was the driver behind the founding of Avilar?

A:  Avilar came out of stealth mode in November 2021 with the announcement of a significant round of financing. Our founding investor, RA Capital, brought many years of deep experience in protein degradation based on previous investments in successful biotech companies developing intracellular degraders. With multiple drug candidates discovered and significant value created with the first-generation protein degrader approaches, RA Capital supported the founding of Avilar to explore the expansion of this new modality to extracellular targets. An experienced team of drug hunters, chemists, and experts in degradation biology was established to assess the opportunities, and this team created the foundations of the Avilar technology platform, including entirely novel and revolutionary chemistry for ASGPR as the first receptor to be harnessed for endocytosis and shuttling of proteins to the endolysosome for degradation. Our mission then and today is to become a pioneer in extracellular protein degradation.

Q:  How does Avilar’s platform for extracellular protein degradation work?

A: There are a few different ways to achieve extracellular protein degradation. Initially, Avilar is developing ATACs (“ASGPR Targeting Chimeras” to degrade pathogenic extracellular proteins whose role is well-validated in disease, yet they are considered undruggable or poorly addressed using current drug modalities. ATACs are heterobifunctional drugs—one part binds to the protein targeted for degradation while the other part is a ligand that binds to ASGPR—a scavenger receptor found on the surface of hepatocytes that mediates the natural process of endocytosis and transport of proteins into the endolysosome for degradation.

Our ATAC degrader platform employs a structure-guided approach to identify high-affinity small molecule ASGPR ligands that are further optimized to achieve favorable drug-like properties. Our ASGPR ligands have superior attributes versus earlier chemistries and enable our ATACs to have a monodentate or monovalent presentation/format, which allows the ATAC molecules to be smaller and effective at lower doses and volumes if administered subcutaneously, and to enable oral degraders. The protein binder on the other end of the ATAC shuttles the pathological proteins that we want to remove from circulation to hepatocytes. We prefer to use either small molecules or peptides as the protein binder to the target of interest but other modalities are also applicable—for example, antibodies, antibody fragments, or nanobodies.

Our linkers are also optimized for specific applications based on desired physicochemical and ADME properties. Linkers of different sizes and designs have an impact on endocytosis and the pharmacokinetics and pharmacodynamics of degradation itself. Using this approach to developing ATACs, we can target and transport pathological proteins to the endolysosome for degradation. Our library of novel and high-affinity ASGPR ligands and optimized linkers allows for modular assembly of ATACs by mixing and matching with a target protein binder.

Q:  What are some of the unique attributes of ATACs that enable them to reach previously undruggable drug targets?

A: Because ATACs can be applied to targets that other drug modalities cannot, they offer the potential to deliver new, meaningful treatments for many poorly served diseases. For example, there are well-validated targets that have been undruggable with existing therapeutic approaches that aim to modulate the functional activity of the protein “by inhibition or activation” In contrast, ATACs simply bind to the protein of interest and deliver it to the endolysosome in hepatocytes for degradation without the requirement for functional activity. This approach also offers the potential to selectively degrade certain protein types or subtypes that drive disease while leaving other closely related but non-pathogenic proteins unaffected. In addition, the small size of ATACs allows them to degrade high-concentration proteins that would be otherwise be infeasible to inhibit using other modalities such as mAbs.

Q:  Avilar recently shared results from preclinical studies. What did those results show?

A: For our proof-of-concept studies and to test the limits of the ATAC platform, the Avilar team designed ATACs to target the extracellular protein immunoglobulin G (IgG), which has a high plasma concentration and long half-life. Our in vitro characterization of the ATAC interactions with ASGPR and IgG revealed potent binding, ASGPR-mediated uptake into hepatocytes, and degradation of IgG via the endolysosomal pathway. These data provided strong mechanistic support for the ATAC approach. In in vivo studies, we achieved robust IgG degradation in both rats and monkeys. Human IgG was robustly depleted from plasma in rat models within 4 hours after the ATAC dose, and pharmacokinetics and liver immunohistochemistry showed ATAC-dependent uptake of IgG into hepatocytes in vivo, subsequent trafficking of IgG to the endolysosome, and degradation of IgG. ATAC-dependent depletion of endogenous IgG was also achieved in monkey studies, with both intravenous and subcutaneous dosing, reaching 35% after a single dose and 85% after repeat dosing. These effects were observed not only with bidentate but also monodentate ATACs.

We believe our in vivo proof-of-concept data with ATACs in a range of preclinical models shows the promise of these novel degrader medicines. These compelling data are now informing the design and development of our internal pipeline of ATACs targeting serious diseases.

Q:  How do you decide which protein targets or therapeutic indications to focus on?

A:  We believe our approach is applicable to a wide array of extracellular proteins (circulating and hepatocyte surface membrane proteins). Our platform includes a proteome mapping system and technical feasibility assessment that aggregates information about extracellular protein targets to help guide decisions about whether and how to pursue a target of interest. The system includes structural and biological information about the protein target, its role in disease, degradation level needed to achieve clinical benefit, as well as medical need and competition. This collective information enables us to query and use this information to prioritize targets, design ligands that bind to the targets of interest, and help guide the development strategy for the resulting ATACs. Overall, strategically we are focused on targets that are biologically de-risked and where there is a clear advantage for using a degrader approach.

Q: Where else can Avilar take targeted protein degradation?

A:  We’re also interested in a diverse range of mechanisms for extracellular protein degradation that use endocytic  receptors different than ASGPR. We know that the scope of the ATAC approach is very broad, but it doesn’t cover the complete set of extracellular targets. For example, ATACs cannot be used to degrade membrane proteins that are not expressed on liver hepatocytes. We see an opportunity to explore other technologies and mechanisms with the potential to exploit different receptors on different cell types. This would allow us to take full advantage of the potential of extracellular degradation to address a wider range of protein targets driving disease.

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