A cure for neglected tropical diseases

How many tropical diseases do you know of? Malaria, dengue and sleeping sickness immediately come to mind. Maybe leprosy, if you think hard enough. But, many of us may not have heard of Cutaneous Leishmaniasis, a less dangerous but a much more prevalent cousin of kala azar (black fever). Cutaneous Leishmaniasis (CL) is caused by the protozoan Leishmania, which are transmitted by the bite of infected female sandflies.

Dr Shailza Singh and her team from the National Centre for Cell Science (NCCS), Pune, have been studying this disease extensively for years. In a recent collaborative study with Dr Sudipta Basu and his team from the Indian Institute of Science Education and Research (IISER), Pune, the researchers have discovered a new lead compound to help combat this neglected tropical disease. Often called white leprosy, CL is known to cause skin lesions and ulcers on exposed parts of the body, while often leaving behind life-long scars and severe disabilities. According to statistics released by the World Health Organisation, around 0.9 to 1.3 million new cases of Leishmaniasis surface every year, leading to up to 20,000 to 30,000 deaths annually. Conventional methods of treating CL include administration of antimony-based compounds or oral, topical or liposomal preparations of anti-fungal compounds. However, these approaches have increasingly been proven ineffective, insufficient or too expensive to use.

The problem of causative protozoans becoming resistant to most anti-leishmanial drugs further exacerbates the situation. Thus, it has become paramount to identify new compounds that could be used to treat leishmaniasis. Several natural compounds such as alkaloids, phenolic compounds, terpenes and saponins are known to have anti-leishmanial properties. Recent studies have found a class of anti-fungal compounds called ‘coumarins’ that  possess anti-protozoal properties.

Testing therapeutic efficacy
In order to take this lead forward, researchers from NCCS, comprehensively surveyed several coumarin derivatives for their therapeutic efficacy against leishmaniasis, and in this pursuit, also collaborated with the team at IISER Pune. “Our goal was to explore the possibility of using coumarin derivatives as anti-leishmanial agents. Towards this, we have designed a set of coumarin derivatives using computer aided drug designing, which could serve as probable drug candidates for the treatment of leishmaniasis,” explains Dr Shailza.

The research team’s initial screening of coumarin derivatives led to the identification of some 1000-odd compounds, which were ultimately narrowed down to five promising drug candidates, based on various judging criteria like their three-dimensional shape, size, and chemical properties. After this, the scientists wanted to test these five drug candidates for their biological activity and compatibility. They performed various microbiological experiments in this regard, and arrived at one of the compounds which showed the best anti-leishmanial properties, nicknamed ‘C2’ (compound 2). C2 was shown to cause a reduction in size and mobility of the parasitic protozoan cells, which are known to cause leishmaniasis. In addition, C2 was also shown to attenuate these protozoal cells’ ability to infect macrophages, a specialised infection-fighting cell found in the immune system of higher animals.

Further, in order to check the effect of C2 on lesions manifested during Leishmaniasis, the scientists took experimental mice infected with cutaneous leishmaniasis lesions, and then treated them with oral doses of C2. To their surprise, the leishmanial lesions in mice reduced in size by almost 50%. It was thus evident that C2 exhibited the highest anti-leishmanial properties amongst the shortlisted drug candidates both in vitro and in vivo. In order to further enhance the solubility of C2 inside the body, the scientists designed a tiny nanometre scale drug carrier made up of lipids. This lipid-based drug carrier ensures that the drug is released in a much more sustained and prolonged fashion inside the body.

Effective strategy
In further experiments with protozoan cell cultures of Leishmania, the scientists used these lipid-based drug carriers to deliver C2 inside the Leishmania cells. Their results successfully demonstrated that a sustained release of C2 within these Leishmania cells triggered them to commit cellular suicide. Such instances of cellular suicide are generally characterised by an observable decrease in the integrity of mitochondrial membranes and can be quantified using mitochondrial dyes.

This cellular suicide in Leishmania cells can be viewed as an instance of programmed cell death triggered by toxic external factors — the compound C2 in this case. Such a targeted killing of protozoal cells could be a very effective strategy of combating the spread of leishmaniasis infections within the host. Backed by these conclusive results, the researchers now feel that C2 could serve as a promising drug candidate, which, with further modification and development, can be converted into a potent anti-leishmanial compound.

(The author is with Gubbi Labs, a Bengaluru-based research collective)