The death-cap looks much like any harmless variety of mushroom and is rumoured to have a pleasant flavour, but as its gruesome name suggests, tasting comes with serious consequences. Known formally as Amanita phalloides, this particular fungus can synthesize various families of toxins, like phallotoxins and amatoxins.

One compound in particular gives the death-cap its reputation: α-Amanitin, which was first isolated just 70 years ago. However, its toxic properties have been known for centuries, with one of the earliest victims on record being the Roman emperor Claudius in 54 CE. And now this same agent is redeeming the mushroom as a potential cancer treatment.

In 2014, Germany-based Heidelberg Pharma developed analogues of α-Amanitin to treat certain cancers and the company is now testing antibody drug conjugates (ADCs) of this toxin. Unfortunately, researchers have depended on extracts from the death-cap mushroom produced through fermentation, a process that only yields milligram amounts.

To study a chemical agent’s toxicity and off-target effects, researchers must be able to work with large, testable quantities of α-Amanitin. This challenge piqued the interest of University of British Columbia Chemistry Professor David Perrin, whose team subsequently developed a synthetic pathway that could produce these much larger volumes. The results of their work were published in the May 2018 issue of the Journal of the American Chemical Society.  

An ADC consists of a monoclonal antibody specific to the antigen associated with the tumor cells, the cytotoxic agent, and a chemical linker. Once the ADC enters the target cells, the cytotoxic agent detaches itself from the linker, which selectively attacks the tumor cells. More specifically, a-Amanitin is a bicyclic octapeptide that inhibits RNA Polymerase II (Pol II), which drives DNA transcription. It stops Pol II from producing malfunctioning proteins in the cell that drive cancer formation.

“Because of its mechanism of action, it can work on rapidly growing cancer cells and dormant cells,” says Dr. Kaveh Matinkhoo, a postdoctoral fellow in Perrin’s lab and the lead author of the study. “The problem is that a lot of chemotherapeutic agents only work on rapidly growing cells, and after chemotherapy is over, the patient is apparently cured but the cancer comes back a few years later.” 

Matinkhoo and his team cite a 2016 Nature study that attests to the need for a faster, synthetic route to a-Amanitin. 

In the Nature study, researchers from Japan injected sublethal doses of α-Amanitin into the body cavities of mice with xenografts — tissues containing human cancer cell lines — that are resistant to common chemotherapeutics. The cancer in the mice did not relapse, and α-Amanitin was also found to inhibit drug-tolerant colonies.

As well, the lack of a synthetic pathway to α-Amanitin had made developing α-Amanitin analogues more difficult and slowed the development of α-Amanitin-wielding ADCs. “To be able to test modified amanitins, one must be able to synthesize the toxin first,” says Matinkhoo. “Then, one has access to numerous derivatives for synthesis and testing.”

ADCs, a relatively new class of drugs, are unique in their ability to distinguish between healthy and cancerous cells. Researchers have been experimenting with various monoclonal antibodies, linkers, and chemical agents to improve selectivity, among other factors. 

Perrin’s group is now looking into α-Amanitin analogues and derivatives that could be used in ADCs, for example.

“The derivatives have a higher chance of reducing the off-target toxicity,” says Matinkhoo. “Other than this feature, they can help reduce or eliminate the metastasis of cancer and lower the required dosage of an amanitin-based drug for chemotherapy.”