Red blood cells are best known for carrying oxygen. Scarlet Therapeutics is exploring another role: carrying therapeutic proteins through the body, potentially turning these familiar cells into long-acting medicines.
The Bristol biotechnology company emerged from decades of university research into how red blood cells develop and can be engineered. Its platform aims to create cells that contain therapeutic proteins while retaining the natural characteristics that allow red blood cells to circulate.
The ambition is substantial. Instead of repeatedly administering a medicine that may be cleared quickly, engineered cells could potentially deliver therapeutic activity over a longer period. But the concept remains under development, and its success will depend on demonstrating safety, efficacy and practical manufacturing.
A cell with a different job
Scarlet’s approach is built around therapeutic red blood cells, often shortened to tRBCs. Researchers engineer these cells to contain proteins intended to address a disease process. The company says its technology is designed to achieve high levels of therapeutic protein inside the cells.
This matters because red blood cells circulate widely and can survive for an extended period. Keeping a therapeutic protein inside the cell may also help shield it from parts of the immune system. These characteristics could make red blood cells useful carriers for certain treatments.
Scarlet is initially focusing on rare metabolic diseases, including hyperammonemia and hyperoxaluria. In hyperammonemia, the body cannot adequately manage ammonia, which can become toxic. Hyperoxaluria involves excessive oxalate production or accumulation, potentially contributing to kidney stones and kidney damage.
The company’s research seeks to determine whether engineered cells could deliver proteins that help address these underlying biochemical problems. That is a development hypothesis, not an established treatment. The company has not yet demonstrated that its platform can safely and effectively treat patients with these conditions.
Building on Bristol’s blood-cell research
Scarlet was founded in 2022 by blood scientists Professor Ashley Toye and Professor Jan Frayne, drawing on research at the University of Bristol. The company’s scientific roots include work on laboratory-grown red blood cells and the wider effort to understand how cultured cells mature.
Dr Alistair Irvine, Scarlet’s CEO, brings biotechnology and medical-device leadership experience to the company. His role is to guide the transition from a research platform toward product development, manufacturing and clinical evaluation.
The founding science team’s work is central to that transition. Toye’s research focuses on red blood cell biology and synthetic biology approaches. Frayne’s work has examined how human erythroid cells can be generated from different stem-cell sources. Scarlet’s platform builds on that knowledge, while seeking a production route that can support consistent engineered cells.
The company also draws on the RESTORE study, a clinical research effort investigating laboratory-grown red blood cells in human volunteers. RESTORE is not a trial of Scarlet’s therapeutic platform. However, research into how cultured red blood cells mature and circulate can inform the broader scientific questions facing the field.
From preclinical evidence to a development platform
In May 2026, Scarlet reported that its cell-line-derived red blood cells had matured and circulated in vivo, with a half-life comparable to donated red blood cells. The result was presented as a preclinical milestone. It does not establish clinical benefit or regulatory approval.
That announcement accompanied a £3.2 million seed financing round. The company said the funding would support proof-of-concept studies, manufacturing development and regulatory engagement as it advances its initial therapeutic applications.
The work is not limited to a single disease or product. Scarlet describes its platform as having potential across three areas: engineered cells for treatment, manufactured cells for transfusion, and enhanced cells designed for other applications. Its near-term emphasis, however, is on therapeutic candidates for metabolic disease.
Key elements of Scarlet’s platform
– Therapeutic payloads: Red blood cells engineered to carry selected proteins.
– Cell-line production: A platform intended to support more consistent manufacturing.
– Initial disease focus: Rare metabolic conditions, including hyperammonemia and hyperoxaluria.
– Preclinical milestone: Reported circulation of engineered cells in vivo, comparable in half-life to donated cells.
A new frontier in cell engineering
Scarlet’s work has attracted interest beyond its initial disease targets. In September 2026, the company announced an agreement with the U.S. Defense Advanced Research Projects Agency, or DARPA, worth up to $14 million. The programme will explore “smart” red blood cells with sensing and control mechanisms intended to respond to physiological changes.
The effort brings together Scarlet and a multidisciplinary consortium. It is exploratory research, intended to assess feasibility. It does not imply that adaptive engineered cells are ready for clinical use.
Still, the agreement highlights the broader scientific question behind Scarlet’s platform: can a cell that already circulates throughout the body be engineered to perform a carefully controlled therapeutic function?
That question places the company at the intersection of cell biology, synthetic biology and drug development. It also creates demanding requirements. Researchers must show that engineered cells retain appropriate characteristics, carry their payload reliably and do not introduce unacceptable risks. Manufacturing must reproduce those properties at scale.
The evidence still has to lead
Scarlet’s platform is promising, but the distance between a preclinical result and an approved medicine remains considerable. The company must select and validate specific candidates, demonstrate therapeutic effects in relevant studies, establish manufacturing controls and complete the required clinical and regulatory work.
Its latest milestones provide evidence of technical progress, not proof that the proposed treatments will work in people. The company’s next challenge is to turn its engineered-cell concept into a product with a defined clinical benefit.
If that effort succeeds, therapeutic red blood cells could offer a new way to deliver selected medicines. For now, Scarlet Therapeutics is testing the foundations of that possibility, one engineered cell and one development milestone at a time.
