Schistosome parasites infect 200 million people, resulting in significant morbidity and more than 200,000 deaths annually. Schistosomiasis control strategies rely almost exclusively on chemotherapy and tens of millions of people are treated with the only available drug, praziquantel (PZQ). There are no new drugs in the clinical pipeline. PZQ cure rates obtained in mass drug administration campaigns are typically less than 50%. Furthermore, with projected levels of PZQ use it is inevitable that PZQ-resistant parasites will evolve. Therefore, it is imperative to identify new drug targets and drugs for schistosomiasis treatment. A number of highly promising compounds with antischistosomal activity have been identified. However, further advancement of these compounds to drugs is impeded because their targets are unknown. In this regard, the benzodiazepine derivative meclonazepam (Ro11-3128) had been identified as a lead candidate. Meclonazepam exhibited potent therapeutic activity against both mature and immature stages of S. mansoni and S. haematobium in humans. Unfortunately, the drug was later discontinued due to its lack of selectivity resulting in unacceptable side effects at therapeutic doses. Meclonazepam is an anxiolytic benzodiazepine that is an allosteric modulator of mammalian gamma-amino butyric acid receptors. No homologues in Schistosoma worms have been identified in genome databases. The closest matches (glutamate-gated transporters) have been investigated and were not inhibited by meclonazepam. This indicates that the target of meclonazepam in schistosomes is not a known benzodiazepine receptor and remains undiscovered. This revised R21 proposal is designed to test the central hypothesis that meclonazepam kills worms through interaction with a worm-specific and unknown target. We propose to identify the target of meclonazepam in Schistosoma parasites using orthogonal and complementary approaches (1) design and synthesis of novel, drug-like meclonazepam-based photoreactive probes, and (2) biophysical changes in proteins bound to inhibitors. We anticipate multiple proteins being identified by these analyses. Proteins will be studied further if (1) they were identified only in species sensitive to meclonazepam and not in those insensitive, (2) they were identified in worms treated with meclonazepam or active analogs and not in worms treated with inactive analogs, and (3) meclonazepam competes with probe binding to the protein. Proteins meeting these strict criteria will be selected for further investigation. Analysis of potential function, recombinant expression and biochemical analysis, and gene silencing will be used to confirm the targets. Identification of the molecular target in the worm is the essential first step needed to develop novel therapeutic targets and produce clinically relevant treatments for schistosomiasis.
The proposed studies focus on schistosomiasis, a significant, unmet medical need in 78 endemic countries, with more than 200,000,000 people infected, 20,000,000 with significant morbidity, and more than 200,000 deaths annually. Current schistosomiasis control strategies rely almost exclusively on chemotherapy, there is only one available drug, and it is inevitable that drug-resistant parasites will evolve. The proposed research will identify the mechanism of action of a promising, but discontinued drug, and lay the groundwork for the discovery and optimization of small molecules for new schistosomiasis treatments.
