by Stephanie Smelyansky
figures by Jovana Andrejevic

Nature knows to quit when it’s ahead–just take a look at the horseshoe crab. Since its origins 450 million years ago, the animal has remained relatively unchanged. This living fossil continues to trudge through shallow, brackish waters, its large tank-like shell protecting its soft, wriggly underbody, looking for tiny worms and mollusks to scoop into its belly, just like it did millions of years ago. The horseshoe crab isn’t good at doing much, but it’s shockingly good at surviving.

Horseshoe crabs are also pretty good pharmaceutical chemists. The dialogue between nature and synthetic chemistry yields some of the most promising and revolutionary medications. Over its 450-million-year history, the horseshoe crab has been one of the best participants in this conversation.

One crab, lots of molecules

Take, for example, one of the horseshoe crab’s biggest contributions to medical science: its blood. The blood of the Atlantic horseshoe crab possesses a unique molecule that quickly coagulates in the presence of microbial toxins. As a result, the pharmaceutical industry uses horseshoe crab blood to test for microbial contaminants in anything that might end up inside of a person’s body. The mangrove horseshoe crab, which lives on the shores of south Asia, produces tetrodotoxin, a powerful neurological poison that has been used in the clinic to treat pain associated with cancer and heroin withdrawal. 

Horseshoe crabs might seem like medical miracle workers, and indeed, in some ways they are—horseshoe crabs make medically relevant molecules that synthetic chemists couldn’t even imagine. But these miracles wouldn’t be possible without the imagination of the chemists who find, identify, and characterize these molecules, and who realize that with a little tinkering, many of these molecules can be repurposed to push the frontiers of human health. Now, chemists are turning their sights to the horseshoe crab once again, this time in search of a potential cure for cancer: a recent study from researchers at the Queensland University of Technology shows that small modifications to a naturally occurring horseshoe crab protein make it a promising drug lead for treating skin cancer.=

How little proteins act as potent killers

It’s unlikely that cancer is a huge problem for the horseshoe crab, so why should chemists look to the ancient sea dweller for a cancer cure? Well, because even if cancer isn’t a problem, the horseshoe crab is faced with myriads of other medical maladies that it has learned to combat. With a chemist’s eye, one of those protective strategies could potentially be adapted to work as a cancer treatment as well. 

For example, invading microbes are an issue that horseshoe crabs are quite familiar with. Horseshoe crabs have evolved to produce a class of proteins to combat microbial infections called host defense peptides, which are small, positively-charged proteins capable of killing bacteria. While different host defense peptides have different mechanisms of action, many of them kill bacteria by interacting with the bacterium’s negatively charged outer membrane. Think of the bacterium as a water balloon, as shown in Figure 1. The peptide pierces the bacterium’s outer membrane, causing the cell to burst open and die, just like popping a water balloon with a needle. 

Many chemists today are repurposing host defense peptides to target not only microbes, but cancer as well. Like bacteria, certain types of cancer cells have a negatively charged outer membrane. Meanwhile, healthy cells typically have a neutral membrane. Ideally, like two poles of a magnet, these positively charged host defense peptides should only be attracted to and kill cancer cells, while leaving healthy cells alone.

The chemist and the horseshoe crab: an unlikely partnership

Horseshoe crabs secrete a variety of host defense peptides, but one in particular, tachyplesin-I (TI), has captured attention as a promising anticancer drug lead. Early in 2019, researchers in the Peptide Therapeutics and Membrane Biology Research Group at Queensland University of Technology evaluated the ability of TI to kill a variety of cancer cells. The researchers observed that TIis particularly potent towards melanoma, a type of skin cancer. To make the molecule more stable in human blood, the researchers joined the two ends of the molecule like two ends of a string to create a cyclic version of TI called cTI. In their most recent work, the group systematically introduced small changes to the structure of cTI to try to find a more potent cancer drug than TI in its natural form. 

After developing nine synthetic versions of cTI, two of them stood out for their high potency and selectivity for melanoma cells. These analogues were almost 3.5 times more selective for melanoma cells than the original TI molecule. A third analogue was capable of sneaking into cancer cells without popping them, which could be useful for designing a scaffold for drug delivery into melanoma cells. The data suggests that these cTI analogues are promising leads for melanoma drug development, and that cTI in general is a versatile starting point for cancer drug development.

The natural world is full of interesting molecules with potent therapeutic properties beyond TI, ranging from penicillin to morphine to even taxol, a common breast cancer medication. In many ways, evolution has figured out how to make molecules that lab chemists can barely dream of creating. Yet evolution isn’t perfect, and nature doesn’t necessarily have a solution for everything. 

That’s where chemists come in. Many medications were invented in the lab, such as cisplatin, another cancer drug. However, just as many, if not more, medications are modified versions of naturally-occurring molecules, repurposed for new medical applications from their original ecological niche. The horseshoe crab didn’t evolve to treat human illnesses, but it produces a vast array of molecules that a chemist can modify to create new drugs, and TI is just the most recent phenomenon of this sort. 

The horseshoe crab managed to survive for 450 million years on this planet in the face of disease, mass extinctions, and tectonic continental shifts. So, maybe, it’s not that unlikely that medicine should learn a thing or two from the horseshoe crab. 

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Stephanie Smelyansky is a first year PhD student in the Chemistry program at MIT. Follow her on Twitter @ssmelyansky.

Jovana Andrejevic is a fourth-year Applied Physics Ph.D. student in the School of Engineering and Applied Sciences at Harvard University. 

For More Information:

• This article from The Atlantic delves into the blue blood harvest, an annual event in which thousands of horseshoe crabs are captured and their blood is harvested for use in the pharmaceutical industry, and how chemists are trying to design alternatives that don’t harm the animals.
• Carl Zimmer, an acclaimed science journalist, does a good job of explaining how scientists search for interesting compounds in the natural world that can be applied in a therapeutic setting. 
• Professor Donald L. Katz, from the University of Michigan, explains his research on host defense peptides as cancer therapeutics in this video.