Heparinoids Could Become Cheap Antidotes for Cobra Venom

Around 1.8 million people across the world are bitten by snakes every year. As many as 138,000 of them die, while a further 400,000 are left with permanent scarring and disability.

The venoms of many cobras damage tissue in ways that existing antivenoms cannot treat. We have found that low-cost, widely available blood-thinning medicines could be repurposed as antidotes to these venoms.

By using CRISPR gene-editing technology, we gained a clearer picture of how the venoms attack cells and discovered that a widely used group of medicines known as heparinoids can shield tissue from venom. Our study is published today in Science Translational Medicine.

Snakebites are a serious problem

Snake venoms contain a wide range of compounds. In broad terms, they attack the heart, nervous system or tissue at the point of exposure, including skin and muscle.

Understandably, much snakebite research has concentrated on the venoms most likely to kill. Consequently, venoms such as cobra venom, which may be less deadly but can still produce lasting harm, have attracted less research attention.

In areas inhabited by cobras, severe bites can have catastrophic consequences, including amputation. These injuries can permanently change lives and remove people’s means of earning a living. The World Health Organization classifies snakebite as a "Category A" neglected tropical disease and aims to halve the burden caused by snakebites by 2030.

At present, antivenoms are the only treatment for snakebites. They are produced by giving non-human animals small doses of venom, then collecting the antibodies generated by their immune response.

Although antivenoms save lives, they come with important limitations. Each product works only against one or several snake species; they are extremely costly, if available at all; they require refrigerated storage; and they need to be given by injection in hospital.

In addition, antivenoms cannot stop local tissue damage. This is largely because their antibody components are too large to travel into peripheral tissue, such as tissue in a limb.

How cobra venom kills cells

Our team, based at the University of Sydney in Australia, the Liverpool School of Tropical Medicine in the United Kingdom and Instituto Clodomiro Picado in Costa Rica, began searching for alternative ways to treat snakebites.

Our first aim was to establish how these venoms function. We focused initially on cobras, which occur throughout Africa and South Asia.

We collected venom from the African spitting cobra, a species known to cause tissue damage, and carried out a whole-genome CRISPR screen.

We used a large population of human cells, disabling a different gene from the full human genome in every cell with CRISPR gene-editing technology. CRISPR uses a specialised enzyme to delete or alter selected sections of a cell’s DNA.

We then exposed the entire cell population to cobra venom and assessed which cells survived and which were killed.

Any surviving cell must have lacked something the venom requires to harm us, allowing us to identify these features quickly.

We discovered that several cobra venoms rely on particular enzymes to kill human cells. These enzymes produce long sugar molecules known as heparan and heparin sulfate.

Heparan sulfate occurs on the surface of human and animal cells. Heparin sulfate is released from cells when the immune system responds to a threat.

The significance of these molecules made intuitive sense. Snake venoms evolved alongside the animals they target, while heparan and heparin have altered very little over evolutionary time. The venoms have therefore commandeered something widespread in animal physiology to inflict harm.

How heparin decoys reduce tissue damage

Heparin has been used as a blood-thinning medicine for nearly 100 years.

We examined the drug in human cells to determine whether filling the system with free heparin could provide decoy targets for the venom. Strikingly, it did: the venoms no longer killed cells, even if heparin was added after venom exposure.

We also assessed heparin against venom from distantly related Asian cobras, where it produced the same protective result. In mice given an artificial "snakebite", we further showed that injecting tinzaparin, a smaller synthetic form of heparin, could lessen tissue damage.

To understand how heparin blocked venom activity, we divided venom into its main components. We found that heparin prevents the action of "cytotoxic three-finger toxins", a leading cause of tissue injury. Previously, no medicines were known to act against these toxins.

The next stage is to examine the effects of heparin in people.

Cheaper, more accessible snakebite treatment

Our aim is to develop a snakebite treatment device containing heparin-like medicines, called heparinoids. It would resemble the EpiPen adrenaline injectors commonly carried by people at risk of serious allergic reactions. Such devices could be supplied to those at high risk of cobra bites.

Heparinoids are already affordable essential medicines used to prevent blood clots. The US Food and Drug Administration has approved them for self-administration by people, which could shorten the otherwise lengthy process of bringing a medicine to market.

They are also stable at room temperature, so heparinoids could be more readily available in remote areas and administered more rapidly in the field.

Other research has likewise demonstrated the value of repurposing medicines to treat snakebites. These drug combinations may signal a new era in snake venom treatment that is not dependent solely on expensive antivenoms.

Previously, our laboratory used CRISPR screening to study box jellyfish venom, and we are now investigating other, more local venoms, from bluebottles to black snakes. This screening approach enables us to reveal a great deal of information about any venom.

It remains early work, but we are finding that many venoms depend on shared targets to bind to cells. All of this research contributes to the wider ambition of creating universal, broad-acting venom antidotes.

Tian Du, PhD candidate in venom genomics, University of Sydney and Greg Neely, Professor of functional genomics, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.