What are haematological malignancies?
Haematological malignancies are cancers that originate in blood-forming tissues such as the bone marrow. They are the fifth-most-common type of cancer in the UK, with one person diagnosed every 20 minutes. Examples include acute myeloid leukaemia (AML), acute lymphoblastic leukaemia (ALL), multiple myeloma, myeloproliferative neoplasms, and lymphomas. Of these, AML and ALL are responsible for the majority of the caseload, accounting for approximately 3,000 deaths every year. A large proportion of these patients are aged 80 years and older and also represent the highest-risk population, with just 10% responding to chemotherapy. Unfortunately, other treatment options offer little to improve overall survival and in many cases are more aggressive, with adverse side-effects. As such, there remains a pressing need to develop more effective and better-tolerated therapies to treat AML and ALL.
Snakes and snake venom
There are approximately 3,600 species of snake, of which 600 are venomous, and only 200 species pose a significant threat to humans. Snakes are found on every continent and land mass except for Antarctica, Ireland, Greenland, Iceland and New Zealand. In Ireland, legends hold that the country did once have snakes, but the Catholic Saint Patrick chased the snakes of Ireland into the sea, banishing them.
Snakes have considerable diversity in both their size and the toxicity of their venom. Highly venomous snakes produce venom as a defence against predators and to immobilise prey. The venom is secreted, synthesised and stored in the venom glands, located on either side of the head and protected by a muscular sheath. Snake venom is only toxic once it enters the blood system, which is why snakes have to bite other animals, injecting their venom. The toxicity of a snake’s venom can be measured and expressed by a murine LD50 test (lethal dose 50%): the lower the value, the more toxic the snake’s venom.
Snake venom in medicine
Interestingly, snake venom has been used for medicinal purposes throughout history, with the first venom-based medicine dating back to 380 BC, in Ancient Greece. Accounts of various ‘snake oil’ remedies were also apparent in 18th-century Europe and 19th-century USA. Derived primarily from boiled, skinned rattlesnakes and pit vipers, ‘viper oil’ became a commonly recommended remedy for the treatment of various skin diseases and rheumatism. These remedies largely today fall under the umbrella term of ‘snake oil’ – pseudo-medical remedies promoted as cures for all diseases despite little scientific validation of their efficacy.
Today however, there are many approved venom-derived medicines, such as captopril. Developed from a peptide found in Bothrops jararaca (pit viper) venom, captopril is an angiotensin-converting enzyme inhibitor used to treat hypertension and congestive heart failure.
Snake venom is currently being investigated as a potential anti-envenomation therapy. According to the World Health Organization (WHO), there are approximately 5.4 million snake bites every year, with 2.3 million cases of envenomation reported annually. Approximately 100,000 deaths result worldwide. As such, snake venoms are being extensively researched and used to develop novel, anti-envenomation therapies, often by hyper-immunising donor animals to non-lethal doses of one or more snake venoms.
Although snake venom is a rich source of natural, bio-active compounds that can be used to synthesise anti-envenomation therapies, many of these compounds are now being shown to have anticancer properties.
Snake venom as an anticancer agent
Snake venom is a complex mixture of proteins, peptides, enzymes and nucleotides. Purification and characterisation of some of these specific compounds has led to studies highlighting a capacity for snake venom toxins and compounds to interfere with key tumorigenesis processes such as, cancer cell invasion and metastasis. Some of the most exciting and efficacious compounds with anticancer capabilities identified so far include L-amino acid oxidase’s (LAAOs), snake venom metalloproteinases (SVMPs), disintegrins, C-type lectins and phospholipase A2 (PLA2) enzymes. Many of their mechanisms of action on cancer cells have also been described, with some listed below:
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reactive oxygen-species dependent DNA damage (e.g. LAAOs and PLA2s)
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blockade of extracellular matrix-integrin signalling (SVMPs, C-type lectins and disintegrins)
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inhibition of cancer cell proliferation, migration and invasion (LAAOs, PLA2s, C-type lectins, SVMPs and disintegrins)
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induction of apoptosis through the extrinsic or intrinsic pathways (LAAO and PLA2s).
Snake venom in haematology – what we know so far?
Many snake-venom toxins and compounds have been shown to possess selective toxicity and anticancer activity in breast, cervical and other cancer cell lines. In vitro studies have also identified novel snake-venom toxins and compounds effective at killing blood cancer cells. One of these exciting compounds is an LAAO isolated from Micrurus mipartitus snake venom shown to evoke morphological changes in the cell nucleus of T cell ALL cells (Jurkat cell line), also causing significant reduction in mitochondrial membrane potential. The study recognised that the LAAO induced apoptotic cell death via a H2O2-mediated signalling pathway, also observing an upregulation of the tumour suppressor gene, p53. The authors proposed the potential development of M. mipartitus compounds as a potential therapy for T-ALL.
Another example of a novel, snake venom compound identified as being cytotoxic towards blood cancer cells includes an LAOO purified from Calloselasma rhodostoma snake venom. Shown to have an enhanced cytotoxicity towards myeloproliferative-neoplasm cell lines positive for the JAK2V617F mutation, the study compared the cytotoxicity of the isolated LAAO with etoposide, a standard of care chemotherapy. The LAAO was shown to be more effective at eliciting cytotoxicity on the two cell lines, and was later shown to operate by provoking activation of the extrinsic apoptosis pathway in JAKV617F-mutation-positive cells.
LAAOs are not the only snake venom component shown to be effective at interacting with and killing blood cancer cells. Benati and colleagues used a novel snake-venom-derived PLA2 enzyme isolated from B. moojeni snake venom to demonstrate selective cytotoxicity of the PLA2 towards HL-60, promyelocytic leukaemia cells and chronic myeloid leukaemia cells (K562-S and K562-R cell lines). The enzyme, MjTX-I, was shown to be cytotoxic towards leukaemic cancer cells, yet non-cytotoxic towards normal peripheral blood mononuclear cells. This raised the question of whether these snake-venom-derived enzymes can selectively target cancer cells. Interestingly, research in our laboratory group using normal bone-marrow stem cells has demonstrated selective cytotoxicity of crude snake venom from Crotalus vegrandis towards ALL cells, similar to the work by Benati and colleagues. A possible cancer-cell-specific, selective mechanism of action appears to be in play.
Snake-venom PLA2 enzymes have also emerged as a viable compound of interest for treating blood cancers, as they possess anticoagulant properties via mechanisms dependent and independent of their main function of glycerophospholipid hydrolysis. This holds many possible benefits for patients, particularly in the context of cancer-associated and chemotherapeutic-induced thrombosis, which are leading causes of death in cancer patients. Studies are now taking place to elucidate pathways and sites anticoagulant PLA2 enzymes may be operating through, potentially leading to the development of novel, anticoagulant therapeutics that can be used to treat cardiovascular diseases but also potentially be used as a complementary therapy in cancer patients who are being treated with thrombosis-associated therapies such as, chemotherapy. Ultimately then, it is hoped that by combining the potential of PLA2 enzymes to induce cancer-cell specific cytotoxicity, with their capacity to also induce anticoagulation, could one day lead to a novel blood-cancer therapeutic.
Where to next?
More snake-venom toxins effective at killing leukaemic cells are being discovered. One example is the recently discovered SVMP, Nasulysin-1, isolated from Porthidium nasutum snake venom and shown to induce apoptosis in the leukaemic cell lines (Jurkat and K562 cells) via caspase-3 and apoptosis inducing factor (AIF)-dependent mechanisms. For now, characterising their mechanisms of action in blood cancer cell lines as well as identifying other novel, effective compounds appears to be the main focus for research. As such, use of snake venom as an anti-cancer therapy for diseases like AML and ALL remains at an early stage, but this is a rapidly developing area with new papers published regularly. The next step will be to take Nasulyn-1, and the other snake-venom components highlighted in this article, forward into in vivo studies to further test their therapeutic potential. There is therefore hope that one day a ‘captopril’ will emerge as a future treatment for blood cancers.
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