ETX is known to be highly toxic, pore-forming toxin (the third most lethal toxin after botulinum and tetanus) and, somewhat surprisingly, there is currently no human vaccine. Consequently, it is likely that any new human vaccine is 5-10 years away.
There is a crude, formaldehyde-inactivated vaccine for sheep, which are routinely inoculated against ETX as lambs and this vaccine has provided effective protection for sheep and goats for the past 50 years. Assuming ETX is the trigger for MS, then it is proposed that the disease would, in the long term, be treated by a vaccine in much in the same way as humans are inoculated against the tetanus toxin or diphtheria and sheep are currently protected from ETX.
As mentioned the current sheep vaccine is crude and not suitable for human use. The current vaccine is based on a formaldehyde-inactivated vaccine for goats and sheep, which are routinely inoculated against ETX as lambs.
Human Vaccine against ETX
To date little effort has been put towards creating a human vaccine against ETX. A research and development company, One Health Ventures, has been working in collaboration with a team at the University of Exeter to develop a better “recombinant” vaccine against ETX for sheep and which could be suitable for humans. This research needs to be taken to the next stage of a human Phase 1 trial.
The toxin is shown below:
The putative, new vaccine involves three mutations to the inactive toxin. To give an analogy – imagine the inactive toxin is a shirt and tie. The toxin is produced in its inactive, less toxic state – the N and C Terminals of the toxin need to be cleaved (stripped off) by trypsin or chymotrypsin in the gut – this is analogous to ripping off the tie. The shirt is then highly toxic.
There are papers indicating that the three mutations in the genetic toxoid candidate are involved in the interaction with both HAVCR1 (16) and MAL. (Morcrette (July 2019) (NPJ Vaccines) “C. Perfringens epsilon toxin vaccine candidate lacking toxicity to cells expressing myelin and lymphocyte protein”) (52). These interactions seem to be important for binding to the cell types affected in MS and insertion into the selected cells to form pores.
The new vaccine candidate involves three mutations as set out above. To extend the analogy, the three mutations are like cutting off the “cuffs”, “collar” and buttons of the shirt. The research to date indicate that by cutting off the cuffs, collar and buttons, the resultant toxin is unable to insert into cell membranes and is no longer binds or is cytotoxic to human MAL. To complete the analogy – the cuffs, collar and buttons are taken off the shirt but the tie is left on – this is the least toxic form of the protein and one that keeps as much of its shape as possible for the immune system to recognise.
The resultant genetically engineered toxoid is the basis of the new vaccine, which should work equally well in animals and humans. To date, experiments on mice, rabbits and sheep have proven to be very successful with each animal producing “neutralising” antibodies to ETX to a level significantly above the minimum required level of “International Units” and without any obvious side-effects, which is promising.
The next stage for this research is to undertake non-clinical research to ensure its safety and efficacy for humans. The next steps would also require significant investment in “Good Manufacturing Practices” to produce the vaccine under the necessary conditions for human use. This would then be followed by a first in human Phase 1 trial within about 18 months. This would be followed by Phase 2 and Phase 3 trials over 2 or 3 years.
The whole process could take 5 years or more before a vaccine could be made widely available to MS patients. The expectation would be that, whilst a vaccine would not stop an initial infection, subsequent infections would be neutralised by the body’s natural response to the vaccine after 2-6 weeks. How long the initial immunity would last for would need to be investigated further.
In the long term, once the above hypothesis is sufficiently accepted, the question arises whether the vaccine should be made available once an ETX infection has occurred or whether the vaccine becomes a prophylactic treatment in the same way as tetanus is currently treated.
OHV is seeking a commercial partner either to:
- licence a novel, proprietary vaccine against epsilon toxin with worldwide opportunities; or
- enter into a joint venture agreement to commercially exploit the vaccine.
The market in India and worldwide
MS is less common in India and in tropical countries – it is more frequently associated with populations living in higher latitudes. However, it is also believed that in MS in India has been under-reported and affects 8.5 people per 100,000 compared with 90 per 100,000 in the US. Globally, it is estimated that approximately 2.5 million people suffer from MS and in India the estimated incidence of disease is 115,000.
Currently it is not possible to treat the cause of MS, and current treatments involve disease modifying drugs (DMDs), which can cost $25,000 to $60,000 per annum in the US and Europe and ameliorate rather than cure the disease.
OHV believes that a vaccine against epsilon toxin could provide a major disruption to the MS DMD industry, which is some $10 billion per annum in the US alone.
The Vaccine Candidate
OHV, in association with Professor Rick Titball and Exeter University in the UK, has developed a “genetic toxoid” of epsilon toxin (Y30A-Y196A-A168F). This means that the protein can be cultured in E. coli and does not require any processing to render it safe to use a vaccine. Y30A-Y196A-A168F is safe when tested towards different cultured cells, and when tested in mice, rabbits and in sheep.
The immunisation of sheep with Y30A-Y196A-A168F and Montanide ISA 61VG adjuvant results in very high levels of neutralising antibodies which persist for at least one year (52). Montanide is not accepted as an adjuvant for human use so the choice of adjuvant will need to be decided. The vaccine candidate has also been shown to have negligible little reactivity with human erythrocytes (red blood cells). Immunisation of sheep with a single dose of vaccine is sufficient to generate high levels of neutralising antibodies.
OHV’s vaccine is covered by two patents and both patents are owned by OHV.
Titball, R.W., Bokori-Brown,M., Naylor, C. Epsilon toxin epitopes from Clostridium perfringens with reduced toxicity. WO2013144636 (A1). Priority date 29/03/2013; (19) and
Titball,R.W. Lewis,N., Bokori-Brown,M., Morcrette,H. Polypeptide and vaccine. Application number 1803401.7. (P3260GB00) Priority date 2/03/18. (https://patentscope.wipo.int/search/en/detail.jsf?docId=WO2019166830&_cid=P20-K0QUC1-54536-1) (49).