The future of click chemistry

It has been almost a quarter-century since K. Barry Sharpless, M.G. Finn, and Hartmuth C. Kolb published the paper that some refer to as the click manifesto. In it, the researchers presented a vision for synthetic chemistry that prioritizes quick and easy access to functional molecules.

Today, click reactions can be found nearly everywhere organic bonds come in handy. They even garnered Sharpless, Carolyn Bertozzi, and Morten Meldal the Nobel Prize in Chemistry in 2022.

“It’s been fun to watch things progress over the years” as more reactions, tools, and applications grow out of click chemistry, Jennifer Prescher says. As a graduate student, Prescher worked on bioorthogonal click chemistry with Bertozzi in the early days of the field. She now runs a lab at the University of California, Irvine, researching chemical probes for cellular communication.

With about 1,700 new publications on the subject added to the SciFinder database each year for over a decade, that progress seems set to continue.

Synthesis made simple

The authors of the manifesto envisioned a future in which the grand challenge of synthetic chemistry would be figuring out not how to make a molecule but what molecule to make and what it would be good for, says Finn, currently a professor of chemistry at the Georgia Institute of Technology. The key to achieving that future: reactions of exceptional speed, ease, selectivity, and reliability.

The functional groups involved in a click reaction are effectively molecular soulmates. They will find each other no matter what the conditions are. And they have eyes only for each other. There are no side reactions and ideally no purification needed to get a pristine end product. The reaction is as close to perfect as possible.

“It’s a never-ending game trying to find the best reactivity,” says Qinheng Zheng, who did his PhD research in Sharpless’s lab. He currently works on cancer drug discovery as a postdoctoral researcher in Kevan Shokat’s lab at the University of California, San Francisco (UCSF).

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) has been the most-used click reaction for more than 20 years because the reactive components are very easy to install. But the click label has also been applied to sulfur fluoride exchange (SuFEx), thiol-ene reactions, Diels-Alder cycloadditions, and several other useful bond-making methods.

“Useful” is central to the whole operation. The power of click reactions is that they’re accessible to practically anyone—including biologists—who wants to be able to link two molecules together, for any reason, says John Moses, whose group at Cold Spring Harbor Laboratory (CSHL) focuses on bringing click chemistry to biomedical research. “There are unlimited applications with it.”

Alkyne-azide cycloaddition and other click reactions have been used to stitch together antibody-drug conjugates and proteolysis-targeting chimeras (PROTACs) for cancer treatment. Click also provides an easy way to quickly attach radioisotopes or other agents for tumor imaging. The San Francisco–based biotechnology firm Shasqi, one of several companies for which Bertozzi is a scientific adviser, uses a click reaction between a tetrazine and a trans-cyclooctene to deliver drugs directly to tumors. Shasqui’s first cancer therapy candidate entered human clinical trials in 2020. A few small-molecule drug candidates assembled using CuAAC chemistry, including the antibiotic solithromycin, have also made it to clinical trials, but none have yet been approved by the US Food and Drug Administration.

Welcome to ‘screeniversity’

One of the original uses that Finn, Sharpless, and Kolb envisioned for click chemistry was to accelerate small-molecule drug discovery. The idea is to create combinatorial libraries of molecular fragments that can be combined via click reactions to rapidly generate many variations on a theme. The click linkages themselves—for example, triazoles created by alkyne-azide cycloaddition—can also be useful medicinal motifs.

“Even for the experienced organic chemist, it’s not possible to synthesize hundreds of compounds individually” at the bench, UCSF’s Zheng says. But with the power of click chemistry and high-throughput automation, a researcher can carry out thousands of reactions at once, know exactly what the product of each ought to be, and efficiently screen the products—without needing to purify them first—for pharmaceutical promise.

Zheng will be establishing his own lab later this year. He plans to use high-throughput tools and a touch of machine learning to search for new cancer treatments that can be assembled through multistep, click-based syntheses.

At CSHL, Moses and his group have their own multiclick strategy, which they call “diversity-oriented clicking,” for making potential drug molecules. This approach combines the parts needed for classic cycloadditions and conjugate additions with SuFExable groups on a single “hub” to create easily diversifiable cores. In 2022, Moses, Sharpless, and coworkers published a paper in which they described using the technique to search for molecules that inhibit human neutrophil elastase, a protein associated with conditions such as cystic fibrosis.

But being able to quickly click a bunch of molecules together is less important than quickly determining which ones are useful, and for what. “We have to explore and sample,” Moses says.

The ideal scenario, he says, is that anyone who makes new molecules should be able to rapidly screen them for multiple applications—anticancer, antibiotics, antifungal, anti-Alzheimer’s disease—you name it—to maximize the chance of discovering something useful. Moses calls this “screeniversity,” a term first coined by Sharpless. This democratized approach to screening and discovery is ambitious and potentially expensive, Moses says, but “what comes out at the end of it is going to be far more valuable than the cost.”

Future perfect

Twenty years ago, click’s function-forward philosophy flipped the script on how to think about synthetic chemistry. Researchers now say that some future developments in the field might involve flipping the script on click’s founding principles.

For example, irreversibility—a one-way trip with a large thermodynamic driving force at the wheel—has long been one of the hallmarks of a click reaction. But Moses and Prescher both say that future applications may benefit from less permanent attachments. The ability to link and unlink molecules on demand would be a powerful tool for things such as delivering drugs or influencing how molecules move around within cells. “Being able to control things at the level of atoms and bonds, that’s a chemical biologist’s dream,” Prescher says.

Another rule that’s not set in stone is the idea that a click reaction must work exactly the same in every circumstance. Chemists know that functional groups like to react in specific and reproducible ways under certain conditions, Finn says. For example, SuFEx is sensitive to the local concentration of protons. Chemists can leverage that information to design molecules that will click together under specific conditions—such as protein pockets with lots of available protons. The concept bears resemblance to in situ click chemistry; though it has been around for many years, Finn and Bertozzi say they think it is due for a renaissance.

It’s difficult to cast predictions about the future of click chemistry, because the possibilities are near limitless by design. It seems safe to say, though, that click will remain rooted in practical, efficient, and widely applicable chemistry that scientists can use to make new discoveries. As Finn says: “Every time a new click reaction happens, a whole new world opens up.”