Accelerating antischistosomal drug discovery : preclinical studies of antimalarials, synthetic peroxides and praziquantel derivatives

Schistosomiasis is one of the most devastating parasitic diseases in tropical countries and remains a major public health problem, especially in Sub-Saharan Africa. It is caused by blood flukes of the species Schistosoma. Schistosoma  haematobium, S. mansoni and S. japonicum are the main species responsible for the largest number of cases. Chronic schistosomiasis is caused by the immunological response to eggs trapped in tissue and organs. In the case of S. mansoni, eggs get trapped in the gut wall and liver, leading to severe tissue destruction. Treatment with praziquantel (PZQ) is the mainstay of morbidity control. PZQ is safe and efficacious, but it is the only available drug. Moreover, it has a major efficacy limitation due to lacking efficacy on juvenile Schistosoma stages. Hence new backup drugs are strongly required. Antischistosomal drug discovery has been neglected for a long time. As a consequence, the antischistosomal drug pipeline is empty. Technical innovations for antischistosomal drug screening and new promising drug candidates are urgently needed.
My PhD thesis had two major aims. The first was to improve the drug screening process by evaluating new assays and readouts and by refining the screening cascade. The second was to further investigate known lead compounds and to identify entirely new drug candidates and chemical scaffolds.
A literature review conducted at the beginning of my PhD thesis revealed that clinical trials conducted in pediatrics, are mainly concerned with antimalarials and only 3 % are dealing with schistosomiasis. Especially the field of PK trials is completely neglected and the importance of investigating the relationship of infections and PK changes was emphasized.
The common blood-feeding characteristic of schistosomes and Plasmodia has led to studies with antimalarial drugs against schistosomes in recent years. Amongst different chemical structures, mefloquine (MFQ), a 4-quinolinemethanol, and the artemisinins, with their distinct peroxidic scaffold, qualified as leads for antischistosomal drug discovery.
The in vitro and in vivo antischistosomal potential of selected MFQ-related arylmethanols was characterized. Furthermore the role of various iron sources in in vitro drug activity was investigated to get insights into the mode of action (MOA). Pharmacokinetic (PK) and pharmacodynamic (PD) properties of lead candidates were explored in S. mansoni infected and uninfected mice.
The class of 4-quinolinemethanols revealed the best in vitro activity (IC50s < 3.5 µM) against adult schistosomes. A ten-fold increase in activity was observed for the two lead 4-quinolinemethanols (MFQ, WR7930) when incubated in the presence of hemoglobin. High worm burden reductions (83 - 100%) were observed for EP (4-pyridinemethanol) and WR7930 (4-quinolinemethanol) in mice harboring adult S. mansoni or S. haematobium. EP and MFQ were selected for further PK investigations. A HPLC-UV method was successfully validated to measure the two drugs of interest simultaneously within mouse plasma. The migration of schistosomes to the liver after treatment with active drugs, known as hepatic shift, was delayed for both drugs (72 - 168 h). Dramatic changes in the drug disposition of MFQ and EP were triggered by the S. mansoni infection. Increased AUCs and half-lives led to slowed drug clearance.
Driven by these promising antischistosomal properties of antimalarials the MMV Box, containing 200 drug-like and 200 probe-like compounds, was investigated. Two entirely new chemical scaffolds, the diarylureas and the dianilinoquinoxalines, presented excellent in vitro activity (IC50: 0.8 μM) as well as moderate in vivo worm burden reductions (WBR: 40.8 - 52.5 %) following single oral drug administration in S. mansoni infected mice.
To pursue the active peroxidic scaffold further, various promising peroxide classes- ozonides (OZs), 3-alkoxy-1, 2-dioxolanes, tetraoxanes, tricyclic monoperoxides and alphaperoxides- were tested against juvenile and adult S. mansoni stages. Additionally the roles of iron and the peroxidic core in drug activity were evaluated.
Promising in vitro activity on both stages was observed for the alkoxydioxolanes. However, only moderate, non-significant activity was observed in vivo. Iron sources did not alter activity on schistosomes, supporting an iron-independent MOA. Non-peroxidic alkoxydioxolane analogues lacked activity against both parasites, underlining the necessity of a peroxide functional group.
Investigations on the three new peroxidic classes (tetraoxanes, tricyclic monoperoxides and alphaperoxides) aimed to get more insights on the structural needs of peroxidic drugs for antischistosomal activity. High in vitro activitiy was revealed on the schistosomular stage, but decreased susceptibilities were observed on the adult stage. One tetraoxane and one tricyclic monoperoxide presented good in vivo activity against adult Schistosoma infections in mice but lacked efficacy on the juvenile infection in vivo. A non-iron dependent activation was observed for these classes as well. Both lead candidates represent new chemical scaffolds but come along with cytotoxicity limitations. Furthermore both compounds are at the very early stage of drug development, and extensive further research is needed for successful drug development. The most promising peroxidic lead candidate was elucidated amongst the OZs, OZ418, with excellent in vivo activity in both juvenile and adult Schistosoma infections.
Additionally we were interested in the antischistosomal activity of PZQ derivatives resulting from novel organometallic derivatization strategies. However organometallic ferrocenyl PZQ derivatives presented only moderate in vitro effects against adult worms. The derivatization of PZQ with chromium moieties led to promising in vitro results but has so far shown to be inefficacious in vivo.
During the process of evaluating this broad range of chemical scaffolds and lead candidates we worked on the improvement of the screening cascade, identifying new parameters resulting in improved in vitro - in vivo correlation, evaluating cut-offs and their impact on hit decision as well as investigating new readout tools for drug screenings assays. During the work of this thesis an in vitro motility assay based on S. haematobium schistosomula was successfully developed. Colorimetric markers were evaluated as an alternative readout technique, but no promising results were achieved so far.
In conclusion, MFQ and EP are interesting antischistosomal lead candidates, however their extensive half-lives and slow clearance might be an issue regarding the strict safety profile. In general antimalarials presented an excellent starting point for antischistosomal drug discovery. Amongst the peroxidic classes, OZ418 showed great potential for a good lead candidate. Screening the MMV Box elucidated additionally two entirely novel chemical scaffolds. The PK trials emphasized strong impacts of an adult Schistosoma infection on PK parameters and underlined the need for further studies in that field. Finally interesting parameters for the antischistosomal drug-screening cascade were elucidated in the course of this thesis.