Oyster Therapeutics Raises $60M in Series A-2 for Tau Drug

By Olivia Hart

What does the $60 million buy?

Oyster Therapeutics raised $60 million in a Series A-2 financing for a preclinical, oral tau degrader designed to reach the brain. The London-based company also announced its rebrand from Kesmalea Therapeutics, a change it tied to its transition toward clinical development and broader ambitions in neuroscience.

Existing investors Syncona, Oxford Science Enterprises, Mission BioCapital and MGB Ventures participated. The announcement disclosed no valuation, closing date or first-in-human timetable.

CEO Rob Johnson said:

“This financing accelerates our progress towards the clinic and enables us to validate a new approach to targeted protein degradation, or TPD, that solves some of its major limitations through the use of smaller molecules.

Millions of people living with diseases such as Alzheimer’s need new approaches such as TPD to address the root cause of disease.

At Oyster we are defining next-generation TPD medicines that can reliably reach the brain and are orally available to better meet the needs of global patient populations.

We welcome this support from both new and existing investors for our vision of building the world’s leading neuroscience TPD company.”

Johnson cast the round as validation of smaller-molecule TPD. Oyster cautioned that its lead program remains investigational, with safety and effectiveness not established in patients.

Takeaway: The financing buys Oyster development capacity, not proof that its brain-delivery strategy works.

What is Oyster actually funding?

Oyster says it is initially focusing on neurodegenerative diseases involving abnormal tau accumulation. The company says tau maintains healthy neuronal structures, but abnormal forms can accumulate and contribute to tauopathies; Alzheimer’s is among the best-known conditions linked to pathological tau.

Unlike conventional treatments that may inhibit a protein’s activity, Oyster’s targeted protein degraders seek to trigger removal through the body’s natural cellular disposal mechanisms. Its lead program aims to remove harmful tau forms while preserving healthy tau.

Takeaway: In plain terms, Oyster wants to remove tau rather than merely mute it, but no patient data support that aim.

How does SELFTAC address brain delivery?

The blood-brain barrier restricts many therapeutic compounds from entering the central nervous system. Oyster says traditional bifunctional protein degraders may be relatively large, which can limit membrane crossing and brain access.

Its proprietary SELFTAC platform uses a reversible linker to connect two molecular structures, commonly called warheads. One component binds the targeted protein, while the other recruits the cellular machinery responsible for protein degradation.

The reversible linker lets the components exist as smaller molecules before they self-assemble into a functional bifunctional degrader. Oyster believes this could improve central nervous system penetration while preserving the versatility of conventional targeted protein degradation.

Oyster expects its smaller molecular designs to support oral administration. It says oral therapies can simplify administration, permit room-temperature storage and potentially lower manufacturing costs for diseases affecting large populations.

Beyond tau, Oyster plans to generate additional targeted protein degrader candidates with SELFTAC. It says the platform’s modular design supports high-throughput screening of different molecular combinations, intended to accelerate candidate identification and improve drug-discovery efficiency.

Takeaway: SELFTAC is designed to make a bulky degrader easier to deliver, but delivery remains an unproven clinical claim.

What is 4BIO Capital’s case?

4BIO Capital Partner Owen Smith said:

“Alzheimer’s and other tau-driven diseases remain among the hardest problems in medicine and Oyster is taking them on with a clear scientific rationale. Targeted protein degradation has real promise and SELFTAC is an ambitious effort to solve its critical technical challenges so that degraders can reach the brain.

Alongside it, Oyster’s lead program aims to remove disease-associated tau while preserving healthy tau. Much remains to be shown in the clinic, but this is where science meets unmet medical need.

At 4BIO Capital we back teams like this with conviction before every question is answered, helping to build companies with the potential to make a real difference in medicine.”

Smith’s endorsement reflects investor appetite, not clinical validation. His own words acknowledge that much remains to be shown in the clinic.

Takeaway: 4BIO is endorsing the science before the clinic has tested its central claims.

What does the Mass General collaboration add?

Oyster has established a sponsored research agreement with Massachusetts General Hospital. The collaboration provides access to patient-derived neuronal cell lines and specialized methods for generating and detecting oligomeric tau, aggregated forms considered particularly relevant to neurodegenerative disease processes.

Dr. Catarina Silva will supervise the research. She is an investigator at the Mass General Brigham Neuroscience Institute and an Assistant Professor of Neurology at Harvard Medical School.

Oyster says the work will help it evaluate therapeutic candidates in biologically relevant systems and strengthen its understanding of disease-associated tau. The announcement gave no candidate count, assay result or development milestone for the collaboration.

Takeaway: The hospital deal strengthens Oyster’s laboratory toolkit, not the clinical evidence for its lead asset.

What must the $60 million prove?

Oyster says the proceeds will fund continued preclinical development, advancement toward human studies and further investment in SELFTAC. The company will also seek to determine whether its technology can reach the brain and selectively degrade disease-associated proteins.

The broader strategy reflects increasing interest in approaches that directly address proteins implicated in neurodegenerative conditions, according to Oyster. Oyster still must show that its candidates penetrate the brain, spare healthy tau and establish safety and effectiveness in patients.

Takeaway: The next proof points are human-study entry and evidence of brain exposure, selective tau degradation and acceptable safety; until then, the $60 million supports a clinically unvalidated strategy, not evidence of an effective Alzheimer’s treatment.