Treating cancer is a balancing act. The root cause of the disease is the body’s own mutated cells, which are susceptible to the same toxins as healthy cells. In addition to considering how effectively anticancer drugs inhibit and reverse tumor growth, physicians must weigh the widespread damage that the substances will cause to the rest of the body before administering them. 

That’s why researchers like Hua Wang, a biochemist at the University of Illinois Urbana-Champaign, seek more selective delivery methods for cancer treatments. In a recent study published in the Proceedings of the National Academy of Sciences, Wang and his colleagues reported that they distinguished cancer cells from healthy ones and more specifically targeted cancer stem cells (CSC) (1). 

CSC can confer drug resistance to a tumor and help cancer cells metastasize. Often, the most toxic anticancer drugs are required to kill them and reduce the risk of recurrence and malignancy. To direct these toxic drugs to CSC, the researchers labeled the cells with a chemical tag capable of recruiting and activating a highly toxic anticancer drug. Their work demonstrates that chemistry-based strategies have the potential to enhance the efficacy of anticancer drugs and limit the side effects of even the most potent therapies. 

To provide a unique chemical handle for anticancer drugs, the team decided to use an unnatural, metabolically active sugar that can be converted into a cell surface marker distinct from anything else that the body produces. “A bigger question was how to make it cancer-specific, because sugar can be metabolized by both cancer cells and normal cells,” said Wang. 

To address this challenge, the researchers sought out an enzyme that is overexpressed in CSC. They homed in on ALDH1A1, which plays a role in alcohol metabolism but is also a strong indicator of malignancy for a variety of cancers. ALDH1A1 is one of many oncogenic proteins located intracellularly, beyond the reach of antibody-based strategies that target proteins on the cancer cell surface (2). 

The researchers then designed the unnatural sugar molecule AAMCHO, which has a protective bond that only ALDH1A1 can cleave. This reaction causes the sugar to break down into pieces, including an azide chemical group that serves as the artificial cell surface marker.

The azide is a staple in a collection of rapid, highly specific click chemistry reactions. When it is paired with a ringed cyclooctyne group on another molecule, the two chemical groups click together like a seatbelt. The researchers created another molecule containing both a cyclooctyne and monomethyl auristatin E (MMAE), an anticancer drug so toxic that it cannot be administered without a selective agent like an antibody. If all went according to plan, the MMAE molecule would circulate through the body, unreactive until it encountered an azide on the surface of a CSC. After the azide and cyclooctyne clicked together and bound the MMAE molecule to the surface of the CSC, the cell would engulf the anticancer drug. Once inside the cell, another enzyme would cleave the cyclooctyne group from MMAE, activating its toxic abilities.

We showed really good results that this chemistry we designed, this sugar compound we designed, is really specific to ALDH1A1. 
- Hua Wang, University of Illinois Urbana-Champaign

Wang’s team first evaluated the distribution of their unnatural sugar label in plated tumor cells. They treated the cells first with AAMCHO and then a molecule containing the cyclooctyne group and Cy5, a fluorescent signaling molecule. In this case, when the azide and cyclooctyne reacted, Cy5 entered the cell, allowing the researchers to see where the azides were introduced. They observed strong fluorescent signals in cells expressing high levels of ALDH1A1, confirming that AAMCHO selectively labeled these cells. When they performed this experiment in living mice, they got the same result. 

“We showed really good results that this chemistry we designed, this sugar compound we designed, is really specific to ALDH1A1,” said Wang. “The second challenge, which is more challenging, comes from whether this enzyme is specific to cancer stem cells.” 

CSC express ALDH1A1 at much higher levels than other cells in a tumor, but additional cancer cell types may express the enzyme and end up with azide labels of their own. Still, the platform holds value for targeted cancer therapy. “We can label a good fraction of cancer stem cells, and even if it labels a small fraction of subpopulations of other tumor cells, that is still useful for many drug delivery applications,” Wang said. 

The team demonstrated this by injecting a cancer mouse model with AAMCHO followed by MMAE. Delivering the drug via click chemistry led to greater reductions in tumor size and longer survival times for mice with metastatic, late-stage, and drug resistant tumors compared to administering MMAE alone. It also decreased toxic side effects on the animals, even though the researchers used MMAE at a higher dose with the click chemistry method than when they administered the drug by itself. 

Justin Lathia, a cancer biologist at the Cleveland Clinic Lerner Research Institute, was not involved in the study, but is excited about what the results mean for the future of cancer treatment. He thinks that the combination of selective labeling via unnatural sugars and click chemistry is an innovative complement to research profiling enzyme expression in CSC. “I do not think anyone is using click chemistry in this way,” said Lathia.

For now, Wang is still focused on basic research, but he was also on the advisory board of IRIA Pharma, a startup that his colleagues launched to translate the click chemistry technology to clinical applications. 

Correction: January 12, 2024: An earlier version of the story stated that Hua Wang is on the advisory board of Surio Therapeutics. He was on the advisory board of IRIA Pharma. The text has been updated to correct this error. 

References

1. Yang, B. et al. Leveraging intracellular ALDH1A1 activity for selective cancer stem-like cell labeling and targeted treatment via in vivo click reaction. PNAS  120, e2302342120 (2023).
2. Ponziani, S. et al. Antibody-drug conjugates: the new frontier of chemotherapy. Int J Mol Sci  21, 5510 (2020).