For thousands of years, animal venoms have been used in traditional medicine. Today , many of their compounds, specifically from snakes, are being examined for their potential in therapy. Here, we explore the various drugs developed from snake venoms and future trends in venom therapy.
by Vanessa Lunardi
Most medicines, as we know them, are synthesised in labs or manufacturing plants, and sold at local pharmacies as seemingly identical bottled remedies or strips of tablets. However, some of the earliest sources of drugs are much more exotic than pharma labs. Dating as far back as the fourth century BCE, Aristotle’s Historia Animalium describes how venom can be used in the production of anti-venom. In the first century CE, theriac, a concoction containing snake venom alongside many other ingredients, was the mainstay of Galenic medicine and continued to be used until the 18th century as a remedy for almost every human disease.1 During the height of the Roman empire, historians have found evidence of venom being incorporated into medicines used to treat fever, leprosy, smallpox, and wounds.
Although most of the primitive knowledge and superstitious basis revolving around the use of animal venom in medicine have been falsified, some of which have been shown to be placebos (if not harmful), interest in the medical uses of animal venom, particularly that of snakes, persists to this day. Through intensive modern biochemical studies of venoms, several proven medications derived from venom are available for prescription today. By harnessing the analgesic activity of cobra venom, scientists have developed drugs for asthma, hypertension, leprosy, and other venom-based ointments and cremes.2 However, despite the long-standing interest and extensive history of venom handling, only a few toxins have found real application. In fact, there are only 11 venom-derived drugs approved by the U.S. Food and Drug Administration (FDA), six of which are derived from snakes.3
In this article, we discuss how venoms are used in medicine, what distinguishes snake venom from other animal venoms as a tool for biodiscovery, the obstacles faced in venom-derived drug discovery, and the current trends and prospects in snake venom therapy.
Venom in Medicine
With around 150,000 venomous animal species in the world, each of which is capable of producing venoms that contain up to 100 different molecules, animal venoms are highly rich and complex cocktails of biologically active molecules with a variety of molecular targets and functions.4 Venoms are generally classified by origin (i.e. venoms from scorpions, snakes, spiders, etc.), or by the effects on the body (i.e. cytotoxic, hemotoxic, neurotoxic, etc.), and are usually composed of various enzymes, non-enzymatic proteins, and peptides.
Since one of the primary objectives of venoms is to kill or immobilise prey or predators, their toxins generally cause potent dysfunction of the cardiovascular, muscular, and nervous systems by acting on various ion channels. For instance, snake neurotoxins, which target ligand-gated channels, specifically nicotinic acetylcholine receptors, can block the flow of acetylcholine, leading to numbness and muscle paralysis.2 The paralysing effects of snake venoms generally begin on the muscles around the eyes, manifesting as fixed dilated pupils, reduced eye movements and droopy eyelids, followed by other signs such as increased difficulty in talking, swallowing, and ultimately, breathing.5
Interestingly, the insidious effects of these animal toxins also make them excellent resources for designing new cosmeceuticals, diagnostic tools, and most importantly, therapeutic agents. Currently, components of animal venom, specifically that of snakes, are being used as powerful medicines and pharmacological research tools. Snake venom-derived drugs such as Aggrastat, Captopril, and Eptifibatide have been developed to prevent blood clots and early myocardial infarction, treat hypertension, and reduce the risk of heart attacks respectively. While there are many other venomous organisms including but not limited to amphibians, cone snails, scorpions, and spiders that demonstrate potential for biotechnological or pharmacological applications, snake venom has inspired most approved drugs. Therefore, many may wonder what distinguishes snake venom from that of other animals, especially in its importance for biodiscovery.6
Snake Venom
While most early studies of snake venoms primarily focused on understanding the effect of snakebites on humans, the components of snake venoms have long been known and harnessed as medical tools for thousands of years in alternative medicine like Ayurveda, a traditional medicine system from India, and homoeopathy, a system built on the principle that “like cures like”. In Ayurveda, cobra venom was used to treat arthritis, inflammation, and joint pain. Cobra venom has also been used by the Chinese to treat opium addiction and by the Indians, who combined it with opium to treat pain.
Today, with advancements in modern biotechnology, scientists have been able to gain a better understanding of the composition of snake venoms, how they differ from other organisms, and how their pharmacological effects can be exploited for developing new drugs.6 Compared to venoms from other animals, snake venoms have a distinct complexity. Other animal venoms typically exert pharmacological effects through disulphide-bridged peptides, whereas snake venoms consist of a more diverse array of larger proteins and peptides that induce a broader range of pharmacological and toxicological effects. There are three main types of pharmacological effects of snake venoms: cytotoxic, hemotoxic, and neurotoxic. These effects arise as a result of the lone or combined action of major toxins such as phospholipases A2 (PLA2s), snake venom metalloproteinases (SVMPs), snake venom serine proteases (SVSPs), and three-finger peptides (3FTX).
For example, PLA2s and 3FTX can act as antagonists of ion channels and nicotinic or muscarinic receptors on pre- or post-synaptic junctions, which can induce severe neurotoxicity, resulting in paralysis and respiratory failure. Some SVSPs and SVMPs can also induce cardiovascular and haemostatic effects such as coagulopathy, haemorrhage, and hypotension. Interestingly, some of these side effects have been found to be therapeutically beneficial in treating several diseases.
Besides their complexity, snake venoms have also yielded a greater number of drugs compared to other animal venoms due to their greater abundance. The amount of venom that can be extracted from snakes is greater in contrast to the minute amounts produced by other organisms such as scorpions and snails. The first animal venom-derived drug approved by the FDA, Captopril, is based on the bradykinin-stimulating peptide of the Brazilian arrowhead viper Bothrops jararaca. Captopril acts as a potent inhibitor of angiotensin-converting enzyme (ACE) and is used to treat congestive heart failure and hypertension.7
The development of Captopril has since sparked great interest in the immense potential of snake venoms and other animal venoms for drug development. In 2001, an anti-platelet drug derived from the venom of the southeastern pygmy rattlesnake Sistrurus miliarius barbouri was developed to prevent acute cardiac ischemia.8 A group of snake α-neurotoxins named waglerins from the viper Tropidolaemus wagleri has also been used to develop anti-wrinkle cosmetics, now commercialised as Syn-AKE.9 In addition, batroxobin, a serine protease purified from Bothrops atrox and Bothrops moojeni, has been found to exhibit thrombin-like activity and hence developed into “Reptilase” to stop bleeding and “Defibrase” to break up blood clots. Another innovative way in which batroxobin has been used is in a system called “Vivostat” for on-site preparation of a fibrin glue that can be used on patients during surgery.10
Besides medicines, snake toxins have also been applied as diagnostic tools. Based on the fraction of two snake venoms from the Australian Eastern brown snake Pseudonaja textilis (Textarin) and the saw-scaled viper Echis carinatus (Ecarin), the Textarin: Ecarin ratio test can be used as a confirmatory test for Lupus anticoagulants, one of the clinical manifestations of Antiphospholipid Syndrome.7,11 “Pefakit”, a V-activating serine protease from the venom of Russel’s viper Daboia russelii, is also widely used in clotting assays to diagnose resistance to activated protein C.12 Activated protein C resistance is the most frequent hereditary defect associated with deep vein thrombosis.13
Evidently, snake venoms demonstrate valuable therapeutic and diagnostic applicability. Given the variability of toxin composition in snake venoms, which is affected by age, diet, gender, location, and season, among other factors, their diverse toxic effects can be exploited to home in on a wide range of challenging drug targets. Nevertheless, there remains a large gap between the number of compounds with potential pharmacological properties that are derived from animal poisons and venoms, and those that have been approved.
Obstacles in Developing Animal Toxin-Based Drugs
With as many as 100 to 500 pharmacologically active compounds in venoms, there are about 10 to 50 million natural compounds that can be used for drug discovery. However, less than 0.01 per cent of these compounds have been identified and characterised. A large proportion of toxins act on unknown receptors and most known toxins have not been described completely due to difficulties in obtaining reliable sources of venoms, hurdles in purifying and characterising toxins in detail, inadequate screening tests, and the limited number of academic or industrial groups researching animal venom-based drugs. Our limited understanding of these unknown elements poses issues in formulation, mechanism of action, selectivity, and stability among others, thereby contributing to the frequent failure of venom components to meet the requirements of potential therapeutic application.
Some compounds lack the ability to cross key barriers in humans, including the blood-brain barrier, which may interfere with their delivery. Animal venom components are also susceptible to blood proteases and may be immunogenic, which can lead to their biopharmaceutical degradation in vivo. Besides, they are also relatively large in size and have other physicochemical properties that may be incompatible with drug action. Considering all these obstacles, conducting preclinical studies of toxin-based drugs that meet the requirements of regulatory agencies worldwide is not only complicated but also costly and time-consuming.14 They must pass through an extensive range of in vitro and in vivo tests to validate their carcinogenicity, pharmacology, and safety among other factors.15
In addition, although randomised clinical trials are the gold standard to assess specific drug-related issues, such as efficacy and safety, performing clinical trials for toxin-based drugs for special populations such as children, pregnant women, and the elderly is rare in fear of possible adverse effects, hence further complicating the approval of such new drugs. Besides, new therapeutic drugs must achieve very high standards to be accepted as they must compete with older and well-known drugs on the market, which may be more effective and cheaper. There are also instances when the toxin’s target is less relevant than previously thought for the manifestation of a particular disease, consequently lowering its efficacy. If the target is expressed in different cells or if the toxin binds promiscuously to other targets, unexpected and unwanted effects could arise in vivo, thereby causing adverse effects and dose-limiting toxicity.14
In such cases, drug recalls or withdrawals from the market may occur. In 2006, a mimetic peptide isolated from Naja spp. cobra venom, Ximelagatran, which was initially approved for thrombin inhibition, was discontinued due to hepatotoxic potential.16 A phase III study of agkisacutacin, derived from Deinagkistrodon acutus venom for perioperative bleeding, was ceased due to anaphylactic reactions.17 Therefore, it is crucial to gain a complete understanding of the mechanisms of toxicity of toxin-based drugs, as well as their efficacy, safety, and side effects.
Current Trends and Prospects in Snake Venom Therapy
Although the quest for toxin-based drugs has proved to be challenging, recent developments in genomics, proteomics and bioactivity assays, as well as in the understanding of human physiology in health and disease, are enhancing the quality and speed of research into snake venoms. In the last decade, deciphering the toxin composition of venoms in great detail has become common practice, leading to the emergence of the new field of “venomics”, which refers to the proteomic characterisation of venom proteomes.2
Currently, there are several snake toxin-based drugs in clinical development that show promising therapeutic potential for a variety of diseases. Derived from the Chinese cobra Naja naja, Receptin is being investigated to treat multiple sclerosis and adrenomyeloneuropathy, while Pepteron is being studied to treat amyotrophic lateral sclerosis, herpes simplex keratitis, and human immunodeficiency virus respectively. According to a phase 1 clinical trial for multiple sclerosis, Receptin is proven to be safe and effective, and Pepteron has been shown to be effective in a preclinical study against the human immunodeficiency virus, as it appears to inhibit viral replication.18 Novel therapeutics for the treatment of solid cancers and pain are also being developed from the crotoxin of the pit-viper Crotalus durissus terrificus.
In addition, snake venom-derived toxins have been widely investigated for potential therapeutic applications in neurodegenerative diseases. Since some natural peptides in snake venoms can regulate glutamate release, modify neurotransmitter levels, block ion channel activation, decrease the number of protein aggregates, and increase the levels of neuroprotective factors, there is growing interest in snake venom as a potential therapeutic tool to slow or even halt neurodegeneration.19
While neurodegenerative diseases share the common hallmark of the accumulation of aggregated proteins, there are several features unique to each type which are often used to treat symptoms of the disease. For instance, cholinergic deficits, which occur in the progression of Alzheimer’s disease, are believed to contribute to widespread cognitive dysfunction and decline. Therefore, most commonly prescribed treatments for Alzheimer’s disease are acetylcholinesterase (AChE) inhibitors such as donepezil and galantamine. One of the less toxic proteins found in Dendroaspis angusticeps venom, the fasciculins, are known to inhibit AChE activity, potentiate acetylcholine action, and produce generalised muscle fasciculation, suggesting that these toxins could be used to relieve acetylcholine deficits in disorders such as Alzheimer’s disease.20 Alternately, efforts have been made to determine the structure of the fasciculin-AChE complex and inspire the design of novel molecules with AChE inhibitory activity.
Snake venom has also been studied to treat Parkinson’s disease. A peptide isolated from the venom of Bothrops atrox was found to display neuroprotective activity and induce neuritogenesis when tested using cells treated with the dopaminergic neurotoxin MPP+. Sequenced as Glutamic acid–Valine–Tryptophan, the peptide can decrease cell death and exert protective effects by reducing the activity of apoptotic proteases, caspase-9 and caspase-3, which are responsible for inducing cell death. According to one study, neurites outgrowth was observed in this Parkinson’s cellular model after treatment with the peptide, demonstrating neurotrophic effects.21
Conclusion
While many of these studies are still preliminary and limited, they demonstrate the importance and the great potential of animal venoms as prospective pharmacological tools. Through further studies and improvements in bioanalysis, clinical trials, and our understanding of the diverse biological functions of snake venoms, animal toxins might offer new and effective therapeutic options, facilitating the development of disease-modifying drugs to treat presently incurable diseases in the near future. [APBN]