Often described as “Goldilocks drugs,” macrocyclic peptides are smaller than biologics like antibodies but larger than small molecules. With their just-right size, macrocyclic peptides can travel into spaces too small for biologics to reach and, thanks to their structural rings of amino acids, can target protein-protein interactions that evade small molecules.
Researchers at the biopharmaceutical company Merck have taken advantage of macrocyclic peptides’ unique properties to develop enlicitide decanoate (enlicitide), an oral macrocyclic peptide drug that lowers “bad” cholesterol by inhibiting proprotein convertase subtilisin/kexin type 9 (PCSK9).
“PCSK9 inhibitors are known to dramatically improve the outcomes for patients who suffer from cardiovascular disease, but the challenge is all the approved therapies are injectables,” said David Thaisrivongs, executive director, head of biocatalysis at Merck. As potentially the first oral PCSK9 inhibitor, enlicitide could be much more accessible to a wide swath of patients.
Merck’s orally delivered macrocyclic peptide drug candidate, enlicitide, inhibits PCSK9, lowering bad cholesterol in people with cardiovascular disease in late-stage clinical trials.
Merck & Co., Inc.
The only problem is that enlicitide is a very complicated drug to make. “This is a monster of a molecule,” said Alison Narayan, a chemist focused on biocatalysis at the University of Michigan.
Recently, researchers at Merck developed a vastly more efficient and sustainable way of synthesizing enlicitide using engineered enzymes to catalyze different reaction steps, a process called biocatalysis.1 The new method reduced the number of synthesis steps by more than half and led to a 39 percent yield of the molecule on a multi-kilogram-scale. The team published their findings in Science.
“They spent years engineering this whole panel of enzymes to be perfect for their synthesis, and that just takes such a commitment and is so cool,” said Narayan, who was not associated with the work. “It's that kind of instant classic that really shows the power of what's possible using biocatalysis…What they have accomplished is so incredibly powerful and impressive.”
Replacing Protecting Groups with Engineered Enzymes
The traditional path to creating peptides involved adding amino acids one by one to a growing peptide chain.
“But to do that chemically, you need to be very careful about the reactivity of each amino acid,” Thaisrivongs explained. To ensure that only the desired part of the amino acid participates in the reaction, chemists add protecting or blocking groups to the amino acid. To continue the reaction of making a peptide chain, they need to remove the protecting group and then add another to the next amino acid in the chain.
“It's a very iterative process,” Thaisrivongs said. “You go around and around and around until you have all of the amino acids assembled in this peptide polymer.” About half of the steps in enlicitide’s original synthesis procedure required protecting groups. But Thaisrivongs and his team at Merck knew that if they wanted to make enlicitide available to as many people who needed it, they had to find a more efficient way to synthesize it. For that, they focused on biocatalysis.
“Biocatalysts can be evolved to be very selective only for the desired reactivity. So, you can perform multiple biocatalytic reactions in the same vessel, and none of the catalysts interfere with any of the other catalysts,” Thaisrivongs explained. By performing multiple reactions in a single pot, the team would save time and resources by not needing to isolate and purify reaction intermediates. They would also avoid using toxic and harsh organic solvents needed for organic chemistry and create much less hazardous chemical waste.
A More Efficient and Sustainable Synthesis
The team at Merck began their biocatalysis research over 25 years ago and has published new synthesis pathways using engineered enzymes for a variety of compounds.2,3 But when it came to applying the approach to synthesizing enlicitide, the main difference was in the type of chemical bond being formed: an amide or peptide bond.
“We had never worked on enzymes to make amide bonds,” Thaisrivongs said. But, he added, “Every living thing, including you and me, have many peptides that are being manufactured from whatever we just ate from lunch, so there's plenty of good natural starting points to search nature for enzymes that at least have the right basic reactivity.”
The team started their new synthesis by working backwards. They first assessed the full enlicitide compound and identified bonds that different enzymes could make between three smaller intermediate molecules. They named these intermediates: the Western, Eastern, and Northern fragments. The team could synthesize the Western and Eastern fragments using traditional chemical synthesis, but the Northern fragment, with its inclusion of three unnatural amino acids and having a macrocyclic structure itself, would require its own biocatalytic synthesis.
“One of the biggest challenges at the beginning of the work when we started it was building the tools to be able to develop this kind of technology…building large libraries of enzymes that we could screen for reactivity to make peptide bonds,” Thaisrivongs said. In the end, they identified and engineered four different microbial-derived enzymes that they could combine into one reaction to create the Northern fragment.
The team then joined the Northern fragment with the Eastern one to create a new intermediate using five engineered enzymes. To add the Western fragment to this intermediate, the team was able to use a chemical approach, and in the same reaction, they identified and engineered an enzyme from a Streptomyces species that closed the final ring to create enlicitide.
“The enthusiasm around this project—the buzz, the hype—has been building for the last two years at conferences as people from Merck’s team have been giving little glimpses into the project, so it was so cool to see this paper come out,” said Narayan. “It's a jaw-dropper.”
One thing the paper lacked, according to Narayan, was details about how the team decided which enzymes to screen and the resulting data on those enzymes. “They basically make it look too easy,” she said. “When they say this enzyme was engineered, there was a whole team of people that were making decisions about how to do that…what to be selecting for, and how to challenge the enzyme to get improvements,” she added. “The discovery and the decision making are all an important part that then the community can learn from.” She hopes that follow-up papers from the Merck team will dive into more of these details. In fact, the researchers just published a new report describing enlicitide’s chemistry and design.4
Overall, Narayan was very impressed by the researchers’ significant amount of work and investment in pursuing a biocatalytic approach to synthesizing enlicitide. “There's nothing easy about what they did, but there's tremendous gain that can come from it,” she said.
Thaisrivongs is very proud of his team and all of the work that went into developing this new enlicitide synthesis approach.
“I was tearing up in my office on Thursday when it came out, to see a decade of work finally roll out the door. But I think the question is why does this matter? We didn't do all of this work just to publish an incredible scientific advancement,” he said. “We're most excited by the opportunity enlicitide has to meaningfully improve the lives of patients around the world.”
- Klapars A, et al. Biocatalytic cascades enable manufacture of the macrocyclic peptide enlicitide. Science. 2026;392:643-647.
- Savile CK, et al. Biocatalytic asymmetric synthesis of chiral amines from ketones applied to sitagliptin manufacture. Science. 2010;329:305-309.
- Huffman MA, et al. Design of an in vitro biocatalytic cascade for the manufacture of islatravir. Science. 2019;366:1255-1259.
- Josien H, et al. Discovery process of enlicitide, a highly engineered macrocyclic peptide therapeutic, through issue-driven fragment-based synthetic assembly and SAR. J Med Chem. 2026.
