Structure-Bioactivity Relationship for Benzimidazole Thiophene Inhibitors of Polo-Like Kinase 1 (PLK1), a Potential Drug Target in Schistosoma mansoni

Background: Schistosoma flatworm parasites cause schistosomiasis, a chronic and debilitating disease of poverty in developing countries. Praziquantel is employed for treatment and disease control. However, its efficacy spectrum is incomplete (less active or inactive against immature stages of the parasite) and there is a concern of drug resistance. Thus, there is a need to identify new drugs and drug targets. Methodology/Principal Findings: We show that RNA interference (RNAi) of the Schistosoma mansoni ortholog of human polo-like kinase (huPLK)1 elicits a deleterious phenotypic alteration in post-infective larvae (schistosomula or somules). Phenotypic screening and analysis of schistosomula and adult S. mansoni with small molecule inhibitors of huPLK1 identified a number of potent anti-schistosomals. Among these was a GlaxoSmithKline (GSK) benzimidazole thiophene inhibitor that has completed Phase I clinical trials for treatment of solid tumor malignancies. We then obtained GSKs Published Kinase Inhibitor Sets (PKIS) 1 and 2, and phenotypically screened an expanded series of 38 benzimidazole thiophene PLK1 inhibitors. Computational analysis of controls and PLK1 inhibitor-treated populations of somules demonstrated a distinctive phenotype distribution. Using principal component analysis (PCA), the phenotypes exhibited by these populations were mapped, visualized and analyzed through projection to a low-dimensional space. The phenotype distribution was found to have a distinct shape and topology, which could be elicited using cluster analysis. A structure-activity relationship (SAR) was identified for the benzimidazole thiophenes that held for both somules and adult parasites. The most potent inhibitors produced marked phenotypic alterations at 1–2 μM within 1 h. Among these were compounds previously characterized as potent inhibitors of huPLK1 in cell assays. Conclusions/Significance: The reverse genetic and chemical SAR data support a continued investigation of SmPLK1 as a possible drug target and/or the prosecution of the benzimidazole thiophene chemotype as a source of novel anti-schistosomals. Author Summary: Just one drug is available to treat schistosomiasis, a parasitic disease that affects hundreds of millions of people in developing countries. In the search for new drugs and drug targets, therefore, we have been interested in the schistosome version of human polo-like kinase (huPLK)1, an enzyme with critical functions in cell division. We used RNA interference to knock down messenger RNA for the SmPLK1 –the Schistosoma mansoni parasite’s version of huPLK1. This interference caused disruptive changes in the morphology of the immature ‘somule’ stage of the parasite, indicating that SmPLK1 is an important protein for survival. We then purchased, or acquired from GlaxoSmithKline (GSK), various small chemical inhibitors of huPLK1 and tested these against both the somules and adult parasites in culture. Many of these inhibitors caused severe changes in the parasite and, for somules, the differences could be computationally mapped and distinguished from unexposed parasites. For the GSK inhibitors, we observed ‘somule-adult bioactivity clustering,’ that is, chemicals active against the adults were also active against somules. This suggests that certain chemical attributes in the inhibitors are being favoured. Interestingly, many of the GSK inhibitors most active against the parasite are also known to both potently inhibit huPLK1 and kill cancer cells. Overall, our data suggest that SmPLK1 is a possible drug target and that the GSK chemistries could form the basis for developing a new drug to treat schistosomiasis.