Skip to Main Content. Oliver Koch Prof. First published: 5 April About this book This is the first resource to provide researchers in academia and industry with an urgently needed update on drug intervention against trypanosomatides. As such, it covers every aspect of the topic from basic research findings, via current treatments to translational approaches in drug development and includes both human and livestock diseases.
The outstanding editor and contributor team reads like a Who? His research centered on the redox metabolism of pathogens, function and catalytic mechanism of peroxiredoxins, target evaluation, characterization, and drug development against infectious diseases, especially tuberculosis and trypanosomiasis. Oliver Koch studied pharmacy and computer science at the Philipps-University of Marburg, Germany, where he also received his PhD in the field of computer-aided drug design.
Currently, he is junior research group leader at the faculty of chemistry, TU Dortmund, Germany.
His research interests focus on the development and application of computational methods in drug discovery and molecular design. Leopold Flohe studied philosophy, medicine and biochemistry and obtained his MD and the venia legendi for Biochemistry from the University of Tubingen, Germany. Polycistronic transcription relies on post-transcriptional control of gene expression and, consistent with this, a large number of trypanosomal RNA binding proteins have key roles in mRNA maturation, stability and translation control [ 67 ].
The process of translation itself also presents novel targets at the level of the ribosome [ 68 ] and the aminoacyl tRNA synthetases [ 69 ]. There are several examples of trypanosomatid-specific targets that have been investigated. One example involves redox metabolism: trypanosomes have a unique di-thiol, trypanothione.
Trypanosomatid Diseases: Molecular Routes to Drug Discovery
Several enzymes involved in the synthesis and modulation of the trypanothione redox system, including trypanothione reductase TryR [ 70 ] and synthetase TryS [ 71 , 72 ], are essential for parasite survival. A large number of attempts have been made to discover drug-like inhibitors of TryR [ 73 , 74 ]. Multiple series have been identified from several large and medium scale screens of synthetic libraries and natural products, some of which have been used in structure-based drug design. Unfortunately, so far none have delivered compounds suitable for clinical development. A key reason appears to be the large hydrophobic active site of TryR [ 70 ], which is difficult to inhibit with a small drug-like molecule.
Active compounds have also been designed against the companion biosynthetic enzyme TryS [ 75 ]. Much of this effort involved re-purposing compounds developed as antifungals or as cholesterol-lowering agents. Clinical trials tested two CYP51 inhibitors, posaconazole [ 78 ] and fosravuconazole also known as E; a prodrug of ravuconazole Fig.
- Antitrypanosomatid drug discovery: an ongoing challenge and a continuing need.
Although there was initial clearance of parasites with posaconazole and fosravuconazole, disease recurred after treatment ceased, indicating that neither agent is suitable for treatment, at least as a monotherapy. The reasons for these failures are not fully understood, but they highlight the need for animal models Box 2 that can distinguish between compounds that are efficacious in humans and those, such as posaconazole, that are not [ 79 ].
A vinyl sulfone irreversible inhibitor of cruzipain K was advanced to preclinical development [ 80 , 81 ] but abandoned due to poor tolerability even at low dose in primates and dogs. Folate metabolism has also been the subject of extensive drug discovery programmes, in particular the enzymes dihydrofolate reductase and a trypanosome-specific target, pteridine reductase 1 PTR1. Both are thought to be essential, at least in T.
There are similarities between the substrates for these enzymes and inhibitors have been identified which inhibit both enzymes [ 84 ]. Despite extensive work in this area, for reasons that are not fully understood, there is little correlation between activity against the enzyme and activity against the parasite [ 85 ]. As far we understand, no inhibitors for these targets have been progressed to preclinical development.
Trypanosomatids lack purine biosynthesis and take up purines from the host. In leishmaniasis, this dependence on external purine has been targeted with allopurinol.
Trypanosomatid Diseases | Wiley Online Books
Allopurinol is taken up by the parasites and then phosphoribosylated to the corresponding nucleotide, which then acts as a cellular poison [ 86 ]. It is used for the treatment of leishmaniasis in dogs and has been in clinical trials in humans, but has not progressed. To circumvent the challenges of target-based drug discovery, phenotypic approaches have been widely used for most of the neglected disease agents, including for the trypanosomatids [ 87 ].
Here, the key requirements are appropriate chemical libraries for screening [ 5 , 88 ], robust assays and appropriate screening cascades. Many different cellular assays are available for analysis of trypanosome responses to compounds Fig. It is especially important to establish that compounds are effective against the clinically relevant life-cycle stages, which can be problematic for the intracellular stages of Leishmania spp.
Compounds must cross multiple membranes to reach the parasite in cellular assays; three in the case of Leishmania spp. In animal models the situation is more complex still, with additional barriers to cross. Various lifecycle stages can be used for the purpose of hit discovery that range from insect forms to host-stage forms in animal models.
The different technologies that can be used for phenotypic assays depend on the parasite form and stage and have specific advantages and disadvantages. Examples of compounds whose antitrypanosomal activity was detected using insect forms, in vitro host-stage forms and animal models are shown. To identify molecules suitable for drug discovery, it is essential to use an appropriate combination of assays to build confidence in the chemical start points hits.
For example, initial hit finding generally requires a high-throughput assay to access chemical diversity, followed by confirmation by more physiologically relevant, but lower throughput, assays Fig. Additional cellular assays, which are representative of the in vivo situation, are important to support combined pharmacokinetic and pharmacodynamic analyses in animal models.
These provide an indication of the concentration and exposure time of a given compound required to kill the parasites in animals and to predict the likely situation in humans. The best combination and the optimal order of phenotypic assays depends on the parasite in question. For T. A secondary assay is therefore required to select those hits that are cytocidal, either using washout experiments to demonstrate a lack of reversibility [ 29 , 89 , 90 ], or direct cell viability assays [ 91 ]. A further issue for HAT is that compounds need to penetrate the blood-brain-barrier to be active against second stage disease.
Currently there are no reliable in vitro cell-based assays for predicting blood brain barrier penetration. However, the physicochemical properties of compounds which are likely to penetrate this barrier have been analyzed [ 92 — 94 ], which can assist in the selection of compounds for screening. However, as T. Therefore, hits need to be followed up in a cidality assay. There is now also a drive to remove compounds that target CYP51 see above and assays directly assessing activity against CYP51 [ 78 , 96 ] need to be added to the cascade.
For Leishmania spp. Although this eliminates the need for further cidality assays, the hit-rates are low [ 21 ], and throughput can be relatively poor. Furthermore, it is challenging to identify potentially valuable but weak or poorly selective hits. One solution to the low throughput is to use an axenic free growing amastigote assay as the primary screen.
Axenic amastigotes do not occur naturally, so care must be taken in interpreting the data. Such assays also need to be designed to only identify ytocidal compounds to prevent false positives, as we have recently reported [ 99 ]. Hits can then be confirmed in an intracellular assay. For all trypanosomatids, as in other areas of anti-infective drug discovery, it is also critical to measure activity against a panel of clinical isolates before progressing compound series too far, to be sure that activity is not laboratory-strain specific.
For all cell-based assays, replication rate, starting density and rate-of-kill are key factors to correctly interpret compound potency, it is important to define these parameters as clearly as possible before interpreting data on new hits. To date, phenotypic approaches have been more successful in discovering new developable series compared to target-based screens.
In the case of HAT, the two compounds currently in clinical trials, fexinidazole and the oxaborole SCYX, were both derived from phenotypic approaches Fig. Recognizing that nitroheterocycles have anti-trypanosomal activity, DND i sourced and screened a large number and re-discovered fexinidazole, a compound that had been investigated preclinically by Hoechst, but then abandoned [ ].
Nitroheterocycles can be genotoxic, so counter-screening for genotoxicity at an early stage was a key selection criterion [ ]. Fexinidazole, like nifurtimox, is a prodrug that requires activation by a nitroreductase [ ]. Sulfoxide and the sulfone metabolites of fexinidazole, rather than the parent drug, are the active compounds against the intramacrophage form of Leishmania spp. Results of a phase II proof-of-concept clinical trial against visceral leishmaniasis are expected soon.
Unfortunately, the doses used in this trial caused safety and tolerability issues and the trial was stopped. Another nitroheterocycle DNDI-VL showed activity in animal models of leishmaniasis [ ] and was selected for further development from a series of nitroimidazooxazoles being investigated preclinically by DND i. Unfortunately, toxic effects were noted and the progression of the compound has been stopped. A backup for this compound DNDi has now been selected and is in preclinical development Fig.
The antitubercular drug, delamanid, which belongs to the same chemical class, has also been proposed as a possible candidate [ ]. A novel nitroreductase NTR2 has been identified as the activating enzyme for these bicyclic nitroheterocycles in leishmania [ ]. From a library of oxaboroles, the benzoxaborole 6-carboxamides were particularly active against T.
The mode of action of oxaboroles against HAT is still not understood, but may include polypharmacology [ ]. Another oxaborole, DNDi has recently been moved into preclinical development with DND i for visceral leishmaniasis. A series of diamidines showed potent activity against HAT, one of which pafuramidine was taken into clinical trials.
Pafuramidine is a prodrug that is metabolized by the host into the active compound, diamidine DB [ 75 ]. Although the precise mode s of action are unknown, like other diamidines, the drug is selectively concentrated within parasites [ ]. However, clinical trials were unsuccessful and were stopped due to safety concerns [ ].
Sitamaquine, an orally bioavailable 8-aminoquinoline, was discovered by the Walter Read Army Institute of Research and has been progressed into clinical trials by GlaxoSmithKline for visceral leishmaniasis [ , ]. The mechanism of its action is not fully understood [ ].