Researchers from the International Centre for Genetic Engineering and Biotechnology (ICGEB), Delhi have found a novel route to discover new drug targets and potential drugs for parasites that cause several diseases such as Loa loa nematode (roundworm) and Schistosoma mansoni platyhelminths (flatworm). The results were published in the journal PLOS Neglected Tropical Diseases.

Both these parasites cause a major health burden particularly in African countries. There are limited treatment options and there is the usual threat of drug resistance. There is little interest in developing drugs for these diseases by pharmaceutical companies; they are hence called neglected tropical diseases.

Instead of blindly screening molecules, which takes a long time and is expensive, a team led by Dr. Amit Sharma from the Molecular Medicine Group at ICGEB looked at Aminoacyl-tRNA synthetases (aaRSs) of the two parasites.

The aaRSs are vital enzymes that decode genetic information and enable protein translation. “The reason why we chose the tRNA synthetase enzyme family is because it is conserved (genomic similarity) in malaria and other parasites including L. loa and S. mansoni,” says Dr. Sharma, the corresponding author of the paper. The novel approach of looking at the conserved region of the parasites is direct, quicker and cheaper.

The aaRSs enzyme family has 20 members and each one of the enzymes contributes to protein synthesis. Even if one of the 20 enzymes is missing then protein synthesis cannot happen. “We have elaborated all the critical aaRs enzymes that contribute to protein synthesis,” he says.

In a next step, the team picked up one of the enzymes and validated it as a drugable target. For that purpose, the enzymes were recombinantly produced and their activities were studied. “Cladosporin, a very potent compound that targets the malaria parasite in both blood and liver stages, seems to inhibit the enzymes of the L. loa and S. mansoni with high potency,” Dr. Sharma says.

The researchers studied the crystal structure of the enzyme with cladosporin. This revealed how tightly the drug binds within the active site of the enzyme. The researchers could understand the active sites of the enzyme and how the drug inhibits their enzyme activity.

“Once we have solved the crystal structure of the enzyme we were able to make an atomic map of all the interacting atoms of both the drug and enzyme. The crystal structure helps us to identify the drug pockets in the enzyme where the drug binds,” Dr. Sharma says.

So once the genome sequence of any parasite is known then it becomes possible to look at the same interacting atoms in the highly conserved tRNA synthetase enzyme family and assess whether the drug will be able to inhibit the enzyme activity. “In the days before the genome sequence was available this approach could not have been used,” he says.

Cladosporin promise

Cladosporin is a very potent compound that targets the malaria parasite. In this study the researchers have shown that the drug compound is a “very potent inhibitor” of essential enzymes in L. loa and S. mansoni as well. The proof-of-concept data shows that it is possible to use the compound to target the tRNA synthetase enzyme family of other parasitic worm diseases.

Since bioavailability of cladosporin compound is a major issue, derivatives of the compound have to be developed for use as an effective drug. “Drug discovery for malaria is being done based on derivatives of this compound. So these derivates can be tested on other parasites too. It will save a lot of time and resources,” he says.

Published in The Hindu on December 6, 2016