Innovative Therapies

By Beatrice R Nassanga

TB says: “Worms thanks very much! ”

We say, “Worms watch out!”

Preamble..

Ascaris lumbricoides, Trichuris trichiura, Hookworm and schistosomiasis continue to cause thousands of deaths and thousands of Disability- adjusted life years (DALYS) among the poorest of the poor living in tropical and subtropical countries [1]. They are associated with cough, chest pain and wheezing [1]. Hookworm infections are associated with iron deficiency anaemia[1] . Schistosomiasis is associated with hepatic fibrosis and gastrointestinal bleeding from varices (in case of intestinal schistosomiasis), and hydronephrosis risk for bladder cancer in Schistosoma haematobium chronic cases [1]. 

What antihelminthic treatment is being used currently?

Albendazole (440mg) is the standard treatment used to treat infections due to Ascaris lumbricoides, Trichuris trichuira and Hookworm [1].  

Praziquantel (40mg/kg) is the standard treatment used to treat infections due to Schistosomiasis [1]. Praziquantel is effective against all three major species of Schistosoma (S. mansoni, S. haematobium and S. japonicum), and is the standard treatment against C. sinensis, Opisthorchis viverrini and Opisthorchis felineus, and intestinal flukes[2]. 

Any drawbacks with the current antihelminthic treatment?

Praziquantel is ineffective against the juvenile stage of Schistosoma spp and the tablets are large and bitter due to the failure to separate the inactive (S)-enatiomer from (R)-enatiomer during production [2]. 

Albendazole is effective against Ascaris lumbricoides and Hookworm, but it has been associated with poor cure rates against infections with Trichuris trichiura [2]. 

Any other therapeutic methods that have been explored so far? What is the success rate?

Alternative therapeutic methods have been explored such as ‘Repurposing drugs’. 

The anti-helminthic efficacy of antimalarial drugs has been tested in animal models. Despite Artemisinin-based therapies being effective at the juvenile stages of Schistosoma spp., the overall therapeutic effect was inferior to praziquantel in clinical trials [2]. Mefloquine has excellent antischistosomal properties: (i) it is active against all three major Schistosoma app. and against both the juvenile and adult stages, (ii) it causes severe morphological damage to the fluke and acts independently of the host immune response and, (iii) initial results from clinical trials showed that Mefloquine-artesunate achieved an Egg Reduction Rate (ERR) of 95% against S. haematobium in school aged children [2].  

Efficacy of the anticancer drug, Miltefosine, against Schistosomes and efficacy of Nitazoxanide against Soil Transmitted Helminths (STH) remains controversial [2].

No success stories yet!

We surely need a breakthrough the helminth scourge..

Helminth infections have been associated with modulation of the immune system by inducing a strong Th2 and regulatory response [3]. This immunomodulation shuts down the Th1 response, reducing the immunogenicity of Th1 dependent vaccines such as BCG for Tuberculosis [3]. 

There is a clear and urgent need for a much more effective intervention to interrupt transmission of these helminths and/or prevent infections. 

Mass drug administration alone will not interrupt the infectious life cycle, so other interventions are also needed, either alone or in combination with the mass drug administration. The cost of drug production and delivery to the tropical and sub-tropical countries is high and yet such therapies are unable to interrupt transmission, although they do reduce morbidity.

We propose...

Prophylactic vaccines pose the better solution to preventing helminth infections. This would involve exposing helminth naive individuals to immunodominant worm proteins such that they can mount a memory response which is protective and preventive of future helminth infections. 

Currently, no helminth vaccine exists. 

We would like to propose the development of one such vaccine. This novel vaccine would be an Extracellular vesicle comprised of immunodominant/immunogenic proteins for Ascaris lumbricoides, Hookworm, Trichuris trichiura and Schistosoma mansoni. Research into host-parasite relationships has revealed that parasitic helminthes actively secrete extracellular vesicles, which are taken up by host cells [4]. Extracellular vesicles are the primary mechanisms of protein release from trematodes [4]. Exploring the use of Extracellular vesicles as a means of delivery of helminth-immunodominant proteins into host cells might be a breakthrough in developing of an effective helminth vaccine, as illustrated below. 

Illustration of the Extracellular vesicle vector vaccine construct

Effective helminth control/elimination from the body is accomplished by a strong Th2 response and antibody dependent cellular cytotoxicity (ADCC). There has to be high production of IgE, IgG or IgA in the body which mediate killing of helminthes by binding on Fc receptors on eosinophils, neutrophils, macrophages or platelets causing degranulation and/or complement activation [5]. Use of extracellular vesicles as vectors will induce the Th2 and humoral immune responses that we need to control the helminths.

Usually, in the body, during helminth infection (as illustrated in the figure below), Th2 responses and antibody production develops after egg production and this results in a granulomatous reaction which might render tissues less functional, such as with liver fibrosis [6]. We need to be able to direct the Th2 and antibody responses to the larval stage of the worms, prevent them from maturing and laying eggs. This will prevent tissue damage due to the granulomatous reaction. Therefore in our vaccine construct, we would propose to use immunodominant worm proteins extracted from the larval stages of the different worms.

Figure on the left showing Th2 functions during helminth infections,  Protective immune mechanisms in helminth infection, Nat Rev Immunology. 2007 Dec;7(12):975-987, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2258092/
(AAMs- alternatively activated macrophages)
Figure on the right is an illustration of the helminth stimulating IL-10 production which down modulates response to vaccines such as BCG

However,..
Vaccine design and rolling out the vaccine to be used by a community takes a lot of time and it is expensive (but worth it in the end). It would take a generation to interrupt the transmission of helminthes and/or wipe them out. 

Other therapies need to be explored in order to help the people who are already suffering from helminth infection, to reduce the morbidity in these populations. 

Helminths like Trichuris trichiura depend on microflora (E. coli) of the gut to hatch their eggs, [6], illustrated in the figure below. Drugs blocking microflora - trichuris egg interaction or ovicidal conjugate (consisting of E. coli protein that mediates hatching and the ovoidal component) can be developed. 

For helminths that produce Cathepsin protease (CP), CP inhibitors can be developed to inactivate the action of this enzyme at cleaving IgG and IgE. However, the cost of such therapies needs to be evaluated so that they are accessible to the people who actually need them (the poor).

Illustration of how bacteria such as E. coli enhance hatching of helminth egg (T. muris egg, for example)

REFERENCES

1.         Jürg Utzinger, S.L.B., Stefanie Knopp, Johannes Blum, Andreas L Neumayr, Jennifer Keiser, Christoph F. Hatz, Neglected tropical disease: diagnosis, clinical management, treatment and control. The European Journal of Medical Sciences, 2012(142).

2.         Gordon Panic, U.D., Benjamin Speich, Jennifer Keiser, Repurposing drugs for the treatment and control of helminth infections. International Journal for Parasitology: Drugs and Drug Resistance, 2014. 4: p. 185-200.

3.         Maizels, H.J.M.a.R.M., Helminth infections and Host Immune Regulation. Clinical Microbiology Reviews, 2012. 25(4): p. 585 - 608.

4.         Antonio Marcilla et al., Extracellular Vesicles from Parasitic Helminths Contain Specific Excretory/ Secretory Proteins and Are Internalized in Intestinal Host cells. PLOS ONE, 2012. 7(9).

5.         Chauvin, E.M.a.A., Immunity against Helminths: Interactions with the Host and the Intercurrent Infections. Journal of Biomedicine and Biotechnology, 2010. 2010: p. 9 pages.

6.         K.S.Hayes, A.J.B., M. Goldrick, C. Portsmouth, I.S. Roberts and, R. K Grencis, Exploitation of the Intestinal Microflora by the Parasitic Nematode Trichuris muris. Science, 2010. 328(5984): p. 1391-1394.