The World’s Deadliest Mushroom Becomes a Warhead in Cancer Treatment
Amanita phalloides: the world's deadliest mushroom; ADC structure inlaid.
The product information for lab reagents can be dry reading, so it’s eye-catching when one contains something more evocative, like an unexpected origin story.
One way of visualising cells under a microscope is to use an antibody that recognises the component you are trying to see. Antibodies are naturally present in our bodies - they’re molecules our immune systems use to recognise foreign invaders and fight infection. In a microscopy experiment, a good antibody that binds to a molecule that you are interested in is invaluable. But when trying to visualise the fundamental structure of a cell - its cytoskeleton - there is a dye that does a much better job.
Phalloidin dye binds to a protein called actin, a fundamental component of every cell’s cytoskeleton. It’s much smaller than an antibody - just 2 kDa in molecular weight compared to 150 kDa for a regular antibody, meaning that when labelled with a fluorescent tag, it can be used to generate images of actin filaments at a much higher resolution than is possible with an antibody - more dye molecules will fit into a smaller space.
Actin filaments are in a constant flux, being broken down at one end while being formed at the other. Binding of phalloidin stops this flux by binding the filaments at the sites between sub-components, effectively locking them together so they are unable to dissociate. If you think about an amoeba moving, actin filaments dissociate at the back of the amoeba whilst lengthening at the organism’s advancing edge: what phalloidin does is stops the amoeba in its tracks. The actin filaments will lengthen at the front but not shorten at the back, and the whole system will get stuck.
Knowing this, you might not be surprised to learn that phalloidin is a naturally-occurring toxin of the Amanita phalloides mushroom - green death cap mushroom - the most deadly of all known mushrooms. As little as half a death cap mushroom is sufficient to kill an adult human. Even more worrying is that this mushroom is reported to taste pleasant and resembles several edible species, including Caesar's mushroom and the straw mushroom. Mushroom picking unless you're with an expert can be a really bad idea!
Amanita phalloides kills really really slowly, with few symptoms apparent in the first 10 hours. It tends to be after the first 24 hours that one might start to notice anything; by this time, it’s too late for a stomach pump. The victim’s fate has already been sealed.
However, it’s not phalloidin that is responsible for the slow death, as Heinrich Wieland and his team in Munich discovered - phalloidin is much faster-acting - it is another family of toxins present throughout Amanita phalloides that are responsible, the amanitins. The amanitins target an even more fundamental process in the cell: its ability to make new proteins.
Proteins are the tiny machines that ‘do things’ inside cells - they transport substances from one site to another; they communicate instructions to coordinate cellular activities - to move, to survive, or even to die. Proteins are so busy in their multitude of activities that they get worn out and need replacing often. Without the ability to make new proteins, one cell, then two, a whole tissue - eventually the whole organism - will start to shut down.
RNA polymerase II transcribes DNA - the instructions - into the message that enables a cell to start to make any new protein. So what do the amanitins target? Yes, you guessed it - RNA polymerase II. It’s targeted. It’s specific. There’s no messing about and no getting away from it. One cell after another, the amanitins bring the cellular machinery to a grinding halt.
But be rest-assured, there is a happy ending to this story. In the treatment of cancer, the therapy needs to be specific to the cancer cells - any action on normal body cells is what can cause debilitating side effects, such as nausea, vomiting, hair loss, fatigue. And this is why one pharmaceutical company is using amantins as a warhead in their treatment for multiple myeloma.
Heidelberg Pharma develops antibody-drug conjugate therapies for the treatment of cancer. These ADCs use an antibody - we met antibodies for microscopy earlier - which specifically recognises a cancer cell. Attached to this antibody is a payload or warhead - some kind of toxin delivered by the antibody to kill the cancer cell. For Heidelberg Pharma, this toxin is an amanitin which, as we have learned, is very good at its job. A mushroom toxin responsible for 95% of all mushroom-related deaths - a toxin that is very good at killing - is also very good at killing cancer cells.
HDP-101 is currently in Phase I/IIa clinical trials for the treatment of relapsed or refractory multiple myeloma. And the results from the first five cohorts are encouraging, with 3 of the 5 patients in cohort 5 having entered partial remission. This is a very exciting outcome, bringing the possibility of an effective treatment for this aggressive form of myeloma within reach.
Further Reading
Pahl et al., 2018 Amanitins and their development as a payload for antibody-drug conjugates. Drug Discov. Today Technol. 30:85-89.
Wieland, T. 1983 The toxic peptides from Amanita mushrooms. Int. J. Peptide Protein Res. 22(3):257-276.