For the first time, MIT chemists have synthesized a fungal compound known as verticillin A, which was discovered more than 50 years ago and has shown potential as an anticancer agent.
The compound has a complex structure that made it more difficult to synthesize than related compounds, even though it differed by only a couple of atoms.
"We have a much better appreciation for how those subtle structural changes can significantly increase the synthetic challenge," says Mohammad Movassaghi, an MIT professor of chemistry. "Now we have the technology where we can not only access them for the first time, more than 50 years after they were isolated, but also we can make many designed variants, which can enable further detailed studies."
In tests in human cancer cells, a derivative of verticillin A showed particular promise against a type of pediatric brain cancer called diffuse midline glioma. More tests will be needed to evaluate its potential for clinical use, the researchers say.
Movassaghi and Jun Qi, an associate professor of medicine at Dana-Farber Cancer Institute/Boston Children's Cancer and Blood Disorders Center and Harvard Medical School, are the senior authors of the study, which appears today in the Journal of the American Chemical Society . Walker Knauss PhD '24 is the lead author of the paper. Xiuqi Wang, a medicinal chemist and chemical biologist at Dana-Farber, and Mariella Filbin, research director in the Pediatric Neurology-Oncology Program at Dana-Farber/Boston Children's Cancer and Blood Disorders Center, are also authors of the study.
A complex synthesis
Researchers first reported the isolation of verticillin A from fungi, which use it for protection against pathogens, in 1970. Verticillin A and related fungal compounds have drawn interest for their potential anticancer and antimicrobial activity, but their complexity has made them difficult to synthesize.
In 2009, Movassaghi's lab reported the synthesis of (+)-11,11'-dideoxyverticillin A, a fungal compound similar to verticillin A. That molecule has 10 rings and eight stereogenic centers, or carbon atoms that have four different chemical groups attached to them. These groups have to be attached in a way that ensures they have the correct orientation, or stereochemistry, with respect to the rest of the molecule.
Once that synthesis was achieved, however, synthesis of verticillin A remained challenging, even though the only difference between verticillin A and (+)-11,11'-dideoxyverticillin A is the presence of two oxygen atoms.
"Those two oxygens greatly limit the window of opportunity that you have in terms of doing chemical transformations," Movassaghi says. "It makes the compound so much more fragile, so much more sensitive, so that even though we had had years of methodological advances, the compound continued to pose a challenge for us."
Both of the verticillin A compounds consist of two identical fragments that must be joined together to form a molecule called a dimer. To create (+)-11,11'-dideoxyverticillin A, the researchers had performed the dimerization reaction near the end of the synthesis, then added four critical carbon-sulfur bonds.
Yet when trying to synthesize verticillin A, the researchers found that waiting to add those carbon-sulfur bonds at the end did not result in the correct stereochemistry. As a result, the researchers had to rethink their approach and ended up creating a very different synthetic sequence.
"What we learned was the timing of the events is absolutely critical. We had to significantly change the order of the bond-forming events," Movassaghi says.
The verticillin A synthesis begins with an amino acid derivative known as beta-hydroxytryptophan, and then step-by-step, the researchers add a variety of chemical functional groups, including alcohols, ketones, and amides, in a way that ensures the correct stereochemistry.
A functional group containing two carbon-sulfur bonds and a disulfide bond were introduced early on, to help control the stereochemistry of the molecule, but the sensitive disulfides had to be "masked" and protected as a pair of sulfides to prevent them from breakdown under subsequent chemical reactions. The disulfide-containing groups were then regenerated after the dimerization reaction.
"This particular dimerization really stands out in terms of the complexity of the substrates that we're bringing together, which have such a dense array of functional groups and stereochemistry," Movassaghi says.
The overall synthesis requires 16 steps from the beta-hydroxytryptophan starting material to verticillin A.
Killing cancer cells
Once the researchers had successfully completed the synthesis, they were also able to tweak it to generate derivates of verticillin A. Researchers at Dana-Farber then tested these compounds against several types of diffuse midline glioma (DMG), a rare brain tumor that has few treatment options.
The researchers found that the DMG cell lines most susceptible to these compounds were those that have high levels of a protein called EZHIP. This protein, which plays a role in the methylation of DNA, has been previously identified as a potential drug target for DMG.
"Identifying the potential targets of these compounds will play a critical role in further understanding their mechanism of action, and more importantly, will help optimize the compounds from the Movassaghi lab to be more target specific for novel therapy development," Qi says.
The verticillin derivatives appear to interact with EZHIP in a way that increases DNA methylation, which induces the cancer cells to under programmed cell death. The compounds that were most successful at killing these cells were N-sulfonylated (+)-11,11'-dideoxyverticillin A and N-sulfonylated verticillin A. N-sulfonylation - the addition of a functional group containing sulfur and oxygen - makes the molecules more stable.
"The natural product itself is not the most potent, but it's the natural product synthesis that brought us to a point where we can make these derivatives and study them," Movassaghi says.
The Dana-Farber team is now working on further validating the mechanism of action of the verticillin derivatives, and they also hope to begin testing the compounds in animal models of pediatric brain cancers.
"Natural compounds have been valuable resources for drug discovery, and we will fully evaluate the therapeutic potential of these molecules by integrating our expertise in chemistry, chemical biology, cancer biology, and patient care. We have also profiled our lead molecules in more than 800 cancer cell lines, and will be able to understand their functions more broadly in other cancers," Qi says.
The research was funded by the National Institute of General Medical Sciences, the Ependymoma Research Foundation, and the Curing Kids Cancer Foundation.
