Schistosomiasis Mansoni: Novel Chemotherapy Using a Cysteine Protease Inhibitor

Abstract

Background

Schistosomiasis is a chronic, debilitating parasitic disease infecting more than 200 million people and is second only to malaria in terms of public health importance. Due to the lack of a vaccine, patient therapy is heavily reliant on chemotherapy with praziquantel as the World Health Organization–recommended drug, but concerns over drug resistance encourage the search for new drug leads.

Methods and Findings

The efficacy of the vinyl sulfone cysteine protease inhibitor K11777 was tested in the murine model of schistosomiasis mansoni. Disease parameters measured were worm and egg burdens, and organ pathology including hepato- and splenomegaly, presence of parasite egg–induced granulomas in the liver, and levels of circulating alanine aminotransferase activity as a marker of hepatocellular function. K11777 (25 mg/kg twice daily [BID]), administered intraperitoneally at the time of parasite migration through the skin and lungs (days 1–14 postinfection [p.i.]), resulted in parasitologic cure (elimination of parasite eggs) in five of seven cases and a resolution of other disease parameters. K11777 (50 mg/kg BID), administered at the commencement of egg-laying by mature parasites (days 30–37 p.i.), reduced worm and egg burdens, and ameliorated organ pathology. Using protease class-specific substrates and active-site labeling, one molecular target of K11777 was identified as the gut-associated cathepsin B1 cysteine protease, although other cysteine protease targets are not excluded. In rodents, dogs, and primates, K11777 is nonmutagenic with satisfactory safety and pharmacokinetic profiles.

Conclusions

The significant reduction in parasite burden and pathology by this vinyl sulfone cysteine protease inhibitor validates schistosome cysteine proteases as drug targets and offers the potential of a new direction for chemotherapy of human schistosomiasis.

Abstract

A significant reduction in parasite burden and pathology by a vinyl sulfone cysteine protease inhibitor suggests a new direction for chemotherapy of human schistosomiasis.

Editors' Summary

Background.

Schistosomiasis, a disease caused by a type of parasitic flatworm that lives in the blood, infects around 200 million people worldwide. The disease is a serious problem in sub-Saharan Africa, South America, China, and southeast Asia. Although this disease can kill, it is better known as a lifelong chronic infection with debilitating symptoms mainly due to an immune reaction raised against parasite eggs trapped in the liver, spleen, and gut. The worm's life cycle is complicated and involves a free-swimming form that emerges from certain types of snails that live in lakes and ponds. This can penetrate the skin of people in contact with the water. After a period spent in the skin and around the lungs, the parasites move to veins around the gut, and develop into adult worms that mate and lay eggs. These eggs eventually return to the water through the person's feces or urine. A particular group of proteins called cysteine proteases are thought to be very important in the biology of these worms, especially in their function as digestive enzymes in the parasite's gut. These proteases could represent an exciting opportunity for development of new drugs to treat schistosomiasis. The researchers are looking at whether it is possible to block the activity of cysteine proteases and, as a result, kill the worms or prevent them from developing and thriving.

Why Was This Study Done?

At the moment there is only one drug, praziquantel, in common use for treatment of schistosomiasis; it is cheap and effective. However many organizations are worried about relying on a single drug to treat a serious disease which affects so many people worldwide. The research group here has been looking at molecules that block cysteine protease activity, to see if any of these could be good drug candidates for schistosomiasis. One molecule they have been looking at goes by the name of K11777, which is under evaluation as a drug candidate for another parasitic infection (Chagas' disease). Here, the researchers wanted to find out whether K11777 had any activity against schistosome worms.

What Did the Researchers Do and Find?

In this study, the researchers deliberately infected laboratory mice with the schistosome parasite. These mice were then either injected with K11777 solution twice daily, or with equivalent volumes of water as a comparison. The researchers examined the effects of injecting K11777 either “early” in infection (using a 14 day course, starting 1 day after infection with the parasite) or “late” in the worms' development (using an 8 day treatment course starting 30 days after infection). The outcomes used as measures of success of treatment with K11777 included the number of worms recovered from mice after euthanasia, the number of worm eggs counted in the liver; the extent of the damage to the liver; and finally, the researchers also looked at activity levels of cysteine proteases in the worms themselves, in particular, those proteases associated with the parasite gut.

The results of the early-treatment experiment showed a substantial decrease in worm numbers and egg production. In five of the seven mice treated, eggs were eliminated entirely. Also, there was little measurable liver damage. For the late-treatment experiment, decreased burdens of worms and eggs in the livers of K11777 treated mice were also found, and there was less damage to the livers. Those worms surviving treatment and removed from mice also had much less activity of gut cysteine proteases suggesting that K11777 exerts its effects by targeting worm cysteine proteases.

What Do These Findings Mean?

These experiments show that K11777 is a potent antischistosomal agent in mice. It might therefore be a good ‘candidate' molecule for developing future treatments for human schistosomiasis. However, before that stage can be reached, it would be important to carry out clinical trials to test whether K11777 is both safe and effective in schistosomiasis patients. Full details as to which worm cysteine protease(s) is the critical target of K11777 would also need to be worked out, and more information would be needed as to whether the dosing plan used in this study (twice-daily injections for a week to 14 days) can be decreased.

Additional Information.

Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040014.

• World Health Organization pages about schistosomiasis including links to details on further research into the disease

• Information from the US Centers for Disease Control for patients and health professionals about schistosomiasis

• Wikipedia pages on schistosomiasis (Wikipedia is an internet encyclopedia anyone can edit)

• PLoS Neglected Tropical Diseases is a new journal from the Public Library of Science that is devoted to publishing research on the world's most neglected tropical diseases, including schistosomiasis