Biotech · RNA delivery

Sixfold Bioscience

A London company that set out to deliver RNA drugs everywhere except the liver. Nine years in, the science is still preclinical, and the accounts it filed in February show the money almost gone.

Four pages, filed in February

Sixfold Bioscience Limited filed its accounts for the year ended December 31, 2025 on February 19, 2026. Cash at bank and in hand: £53,713. A year earlier it was £203,436, and the year before that £542,200.

The average monthly number of people employed, directors included, was 8. In 2024 it was 23. In 2023 it was 26. Plant and equipment that had cost £166,509 was disposed of during the year.

None of that appears on the company website, which is still up, still describes Mergo, and still says the team is over 20 strong. Both records are public. The distance between them is the story.

The problem is real

RNA drugs work by getting a short strand of nucleic acid inside a cell, where it silences a gene or instructs the cell to make a protein. Getting it there is the hard part. The body clears the standard carriers to the liver, which is why the approved RNA interference medicines treat liver disease.

Sixfold put the number bluntly on its own platform page: of the roughly 500 cell types in the body, one, the liver hepatocyte, has a clinically approved delivery system for RNAi. Everything else is open ground, and a working answer would be worth a great deal to every company with an RNA pipeline.

Sixfold’s bet was that nature already solves this. Most circulating RNA, the company argues, travels without a lipid capsule at all, riding protein shuttles that reach different tissues. Mergo is a chemical tag bolted onto a therapeutic RNA, designed so the tag picks the shuttle and the shuttle picks the destination. Machine learning generates candidate tags, automated chemistry makes them, and pooled animal screens read out where they went.

Two PhD students, and a ring

Anna Perdrix Rosell was in the second year of a cancer biology PhD at the Francis Crick Institute. George Foot was a plant scientist finishing at Cambridge. With Zuzanna Brzosko, a Cambridge neuroscientist, they incorporated the company on March 23, 2017, all three as directors, and spent the first months of 2018 in California on Y Combinator’s Winter 2018 batch.

The first technology was not theirs. In December 2018 the National Cancer Institute published notice in the Federal Register that it was contemplating an exclusive patent license to Sixfold Biosciences Inc. of Walnut, California, covering hexameric RNA nanoparticles, rings with arms that could carry siRNAs, aptamers and imaging agents. Cambridge Judge Business School’s venture page from that era describes exactly that: an RNA nanoring carrying six therapeutics to one cell.

Brzosko resigned as a director on August 31, 2020. By then the ring had become PODS, the Programmable Oligonucleotide Delivery System. By 2022 PODS had become Mergo, and the nanoring was gone from the company’s description of itself.

Grants first, venture money second

UK Research and Innovation records tell the funding story more precisely than the press releases. Gateway to Research lists twelve Innovate UK projects with Sixfold as lead participant, grant offers totalling about £1.73 million, plus £152,129 from an EU project and three UK Future Leaders Fellowships held with Sixfold Bioscience Ltd as lead organisation. Partners include Medicines Discovery Catapult, Pharmidex and the Centre for Process Innovation.

The venture money arrived later and in one piece. A Form D filed on August 6, 2021 records a first sale on February 23, 2021 and $2,266,100 sold to 13 investors. On December 2, 2021 the company announced its seed round, naming Cantos Ventures, Y Combinator, Lombard Street Ventures and Pi Campus. The headline was $10.5 million. The release itself describes that as the “total dilutive and non-dilutive capital raised”, which is to say grants included. Most coverage dropped the qualifier.

What 2025 did

On May 28, 2025 Companies House recorded that the Delaware parent had ceased to be a person with significant control, and the company filed a statement that it believes there is “no registrable person or registrable relevant legal entity in relation to the company”. A Delaware corporation is not a registrable legal entity under the UK rules, so this may be a correction rather than a change of hands. The accounts signed nine months later still say the Delaware company controlled it throughout the year.

That parent had been the funding channel. Its capital contribution reserve in the UK books rose £1.65 million in 2023 and £0.97 million in 2024. In 2025 it fell by £8,282. In August 2024 the company had granted a fixed and floating charge over everything it owned to Rocking Horse Solutions UK Limited, and cleared it in full that October.

The last press release on the company news page is dated June 12, 2025. The company is active and filing on time. It is also, on its own numbers, down to eight people and a very thin cash balance.

What is proven, and what is still claimed

Read plainly: the problem is real, the grant record is verifiable, the patents are granted, and the platform has never been tested in a person or described in a peer-reviewed paper with data. What changed in 2025 was not the science. It was the cash.

What to watch

• The accounts for 2026, due by September 30, 2027.
• Whether the kidney licensing discussions produce a named counterparty.
• Fellowship MR/Y018850/1, the main funded programme left on the record, which runs to April 2028.
• Any filing under CIK 0001877208, where nothing has been lodged since August 2021.

In their words

Sixfold Bioscience funding profile on HVCFHealthcare Venture Capital Fund

Related companies

• Nabla Bio: Also in Drug Discovery and Delivery. A Cambridge company that writes antibodies on a computer and tests them in its own lab, aimed at the cell surface proteins conventional discovery keeps missing
• Arpeggio Bio: Also in Drug Discovery and Delivery. A Boulder, Colorado biotech that read the transcriptome to find drugs for targets long called undruggable
• Algen Biotechnologies: Also in Drug Discovery and Delivery. A CRISPR and machine learning platform for choosing drug targets, spun out of Jennifer Doudna’s Berkeley lab in 2018
• Biocartesian: Also in Drug Discovery and Delivery. A two person, YC-backed lab service that turns tissue and cell samples into what it calls digital molecular maps

Sources

1. Public recordSIXFOLD BIOSCIENCE LTD. financial statements for 2023, 2024 and 2025Companies House
2. Public recordSIXFOLD BIOSCIENCE LTD. overview, officers, filing history, persons with significant controlCompanies House · Sep 22, 2026
3. Public recordMR01 charge 1068 7784 0001, Rocking Horse Solutions UK LimitedCompanies House · Aug 9, 2024
4. Public recordGateway to Research records for SIXFOLD BIOSCIENCE LTD.UK Research and Innovation · Jul 6, 2026
5. Public recordForm D, Sixfold Bioscience Inc., CIK 0001877208SEC EDGAR · Aug 6, 2021
6. Public recordProspective grant of an exclusive patent license, RNA nanoparticlesFederal Register, NCI · Dec 21, 2018
7. Public recordPatents assigned to Sixfold Bioscience Ltd.Justia Patents · Sep 22, 2026
8. IndependentSixfold Bioscience snares U.K. grant to test cancer-fighting siRNA delivery systemFierce Pharma · Oct 1, 2020
9. IndependentSixfold bags VC funding to expand RNA delivery beyond the liverFierce Pharma · Dec 7, 2021
10. IndependentSixfold Bioscience completes $10.5 million seed financingDrug Discovery World · Dec 6, 2021
11. IndependentSixfold Bioscience profile, employee count at Jun 30, 2026Tracxn · Sep 22, 2026
12. InvestorRNA Architects: why we invested in Sixfold BioscienceCantos Ventures · Dec 14, 2021
13. CompanySixfold Bioscience completes $10.5M USD seed financingBusiness Wire · Dec 2, 2021
14. CompanyA revolution in targeted RNA deliveryBiopharma Dealmakers · Jun 1, 2023
15. CompanySixfold Bioscience platform, team and news pagessixfold.bio · Sep 22, 2026