Constructive Bio's Syn61 strain, an E. coli with 61 codons instead of 64, ferments glucagon like peptide 1 (GLP 1) drugs in standard bioreactors, aiming to replace a chemical process that scales poorly and generates large volumes of toxic solvent waste.
A UK synthetic biology company is pitching a manufacturing shortcut for the drugs behind Ozempic and Wegovy: brew the peptides inside a strain of E. coli whose genome has been rewritten at the codon level, in place of the toxic, hard-to-scale chemistry that dominates the field today.
Constructive Bio, founded in 2022 and backed by a $58 million Series A, is positioning its recoded E. coli strain, called Syn61, as a fermentation route to therapeutic peptides containing non-canonical amino acids, the modified building blocks that give glucagon-like peptide-1 (GLP-1) drugs their metabolic staying power. The pitch is straightforward in outline: turn a notoriously wasteful chemical process into a standard microbial fermentation, and let industrial bioreactors do the work. The honest reading is more cautious. Syn61's underlying biology is peer-reviewed and the strain is real, but the GLP-1 manufacturing case still rests on the company's own early fermentation data rather than independent third-party scale-up results.
The mechanism is the part most worth slowing down for. E. coli reads DNA in three-letter chunks called codons, 64 in total, which together encode the 20 standard amino acids that make up natural proteins. Codon recoding is a synthetic-biology maneuver that deletes three of those codons from the entire genome, leaving the cell alive but with 61 working instructions instead of 64. The freed codons can then be reassigned to encode non-canonical amino acids: artificial building blocks, beyond the standard 20, that chemists can bolt onto a peptide to extend its half-life, sharpen its receptor binding, or otherwise tune how it behaves in the body. Syn61, [first reported in a 2019 Nature paper by Julius Fredens and colleagues](https://www.nature.com/articles/s41586-019-1192-5), is the foundational demonstration that an organism can survive and function with a fully recoded genome. Constructive Bio is now trying to turn that demonstration into a peptide factory.
GLP-1 agonists are the obvious first proving ground. The drug class behind Ozempic and Wegovy has moved from a niche diabetes therapy into widespread clinical use over the last decade, and supply has lagged demand at every step. Most GLP-1 peptides are made today by solid-phase peptide synthesis, a stepwise chemical assembly line that builds a short protein one amino acid at a time inside a reactor. The process works. It also requires large volumes of toxic solvents and is governed by a relatively fixed ceiling on batch size, which is one of the structural reasons list prices for the drugs remain high. That is the gap Constructive Bio is targeting. "We believe recombinant production can deliver these therapeutic peptides at scale," Rob Salmon, the company's head of bioprocess, told GEN, framing the claim as "can," not "does."
What the company says it has is a strain that fits the existing industrial fermentation playbook. E. coli is already the workhorse of biologic drug manufacturing; the same stainless-steel bioreactors used to make insulin and monoclonal antibodies could, in principle, host a recoded strain the same way. Constructive Bio says Syn61 has been taken through industrial fermentations and produced "promising titers", meaning encouraging yields per liter of broth, and is preparing to present those numbers at the Bioprocessing Summit in Boston. The company is also targeting a scale of thousands of liters per batch, an industrial footprint that would matter only if the per-liter economics can compete with the per-gram economics of solid-phase synthesis. That comparison is not yet public.
The evidence gap is real and cuts both ways. The 2019 Nature paper established that Syn61 grows, that the recoded genome is stable, and that the freed codons can be reassigned; the strain has been replicated and is shared openly as Addgene bacterial strain #174513. Constructive Bio's own publication list extends the platform into non-canonical amino acid backbone chemistry and, in a 2022 Nature Chemistry paper, into genetically programmed cell-based synthesis of non-natural peptide and depsipeptide macrocycles. The leap from "we can grow the strain and incorporate unusual building blocks" to "we can outcompete solid-phase synthesis on cost, waste, and scale for GLP-1 peptides" is a manufacturing claim rather than a biological one, and the only numbers behind it so far come from the company itself. Independent bioprocess data, ideally a published life-cycle assessment or a third-party peptide-fermentation benchmark, has not appeared.
Biology-first peptide manufacturing is also not brand new. Codexis and its enzyme-based routes, along with academic groups working on recombinant peptide expression, have already tried to displace parts of the solid-phase workflow. The specific wager Constructive Bio is making is that a fully recoded genome buys something those older approaches could not: open-ended access to non-canonical amino acid chemistry at manufacturing scale. If that bet pays, the practical effect would be a wider supply-chain menu for a drug class under genuine demand pressure, not necessarily lower prices or a sustainability win. The Bioprocessing Summit numbers, and whatever follows them, will start to answer whether the toolkit is expanding or whether the highest-volume peptides will actually move to a new chemistry.