Anti-trypanosomatid drug discovery: progress and challenges

Abstract

Leishmaniasis (visceral and cutaneous), Chagas disease and human African trypanosomiasis cause substantial death and morbidity, particularly in low- and middle-income countries. Although the situation has improved for human African trypanosomiasis, there remains an urgent need for new medicines to treat leishmaniasis and Chagas disease; the clinical development pipeline is particularly sparse for Chagas disease. In this Review, we describe recent advances in our understanding of the biology of the causative pathogens, particularly from the drug discovery perspective, and we explore the progress that has been made in the development of new drug candidates and the identification of promising molecular targets. We also explore the challenges in developing new clinical candidates and discuss potential solutions to overcome such hurdles.

Fig. 1. Potential effects of parasite persisters on drug treatment.

Persisters are phenotypically defined as a transient subpopulation that is less susceptible to drug treatment, possibly owing to slow growth and changed metabolism. Following drug treatment, multiple outcomes have been observed. Some compounds induce reversible growth arrest (for example, GNF7686 (ref.), which is active against T. cruzi), and parasites start dividing rapidly upon removal of the drug (top panel). Other compounds kill most of the intracellular parasites, with only a very small number surviving treatment (persisters; middle panel). The surviving parasites have been shown to be in a state of spontaneous and reversible growth arrest, which is likely a key factor in their ability to survive drug treatment. The ideal outcome of drug treatment is that all parasites are killed, which can be achieved by treating parasites for extended durations and/or with high concentrations of drugs^,, or potentially with compounds that target mechanisms essential for the survival of persisters (bottom panel).

Fig. 2. Efficacy studies.

Whole-body images of mice (ventral (V) and dorsal (D)) infected with bioluminescent Trypanosoma cruzi, taken with an IVIS system. a | Untreated mice. b | Mice treated with benznidazole (100 mg kg⁻¹) from day 74 (D74) to D93, followed by immunosuppression with cyclophosphamide (200 mg kg⁻¹; D113, D118 and D128). c | Mice treated with posaconazole (20 mg kg⁻¹), formulated in Noxafil, dosed with the same procedure as in panel b, including immunosuppression. d | As per panel c, except posaconazole was formulated in HPMC-SV. e | Ex vivo imaging of the mouse organs, following euthanasia at the end of the procedure. Adapted from ref., CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/).

Fig. 3. New anti-trypanosomatid drugs and targets.

There has been great progress in drug discovery and development for kinetoplastid diseases, and shown are some examples of new chemical entities in development, including their mode of action if known. Fexinidazole, which is a general toxin, has been registered for the treatment of human African trypanosomiasis caused by Trypanosoma brucei gambiense and is in clinical trials for treatment of human African trypanosomiasis caused by Trypanosoma brucei rhodesiense. Acoziborole is an orally active oxaborole, now in phase IIb/III clinical studies. This compound has been shown to inhibit T. brucei cleavage and polyadenylation specificity factor 3 (CPSF3), which is an endonuclease involved in mRNA maturation. The vast majority of drug discovery related to leishmaniasis has been focused on visceral leishmaniasis, and less on cutaneous leishmaniasis, mucocutaneous leishmaniasis or post-kala-azar dermal leishmaniasis. This includes GSK3186899/DDD853651, which has been shown to inhibit cdc2-related kinase 12 (CRK12), as well as two compounds that have been shown to be proteasome inhibitors, one from the University of Dundee–GlaxoSmithKline collaboration (GSK3494245/DDD01305143) and one from Novartis (LXE408). These compounds inhibit the ‘chymotrypsin’-like active site on the proteasome β5 subunit. Other compounds in clinical development include the oxaborole DNDI-6148, which, like acoziborole, targets CPSF3, and the nitroimidazole DNDI-0690, the mechanism of action of which is yet to be determined.

Fig. 4. Representative screening cascades for drugs to treat visceral leishmaniasis and Chagas disease.

a | Key assays for a phenotypic hit discovery programme are shown. For visceral leishmaniasis, the main purpose is to quickly identify compounds that are active in intracellular models, as such compounds have a high chance of demonstrating proof of concept in animal models (provided they have appropriate pharmacokinetics). High-throughput screening for visceral leishmaniasis is typically performed with axenically grown parasites, in particular axenic amastigotes. Initial screening can also be conducted in intracellular models, if high-throughput assays are available, or for smaller compound libraries. Following axenic assays, compounds that are non-selective regarding human cells are removed, and compounds of interest progress to intracellular assays. At this point, any compounds with suitable activity should be validated through structure and purity determination and/or resynthesis. Confirmed active compounds are next subjected to analysis of known modes of action, in particular to identify compounds that have a mode of action that is already being tested in the clinic. For Chagas disease, the cascade needs to quickly remove compounds with undesirable modes of action and identify compounds that can achieve complete cure. Hits are usually identified in high-throughput intracellular systems, as there are no suitable axenic models. Typically, a large fraction of hits act through undesirable modes of action such as CYP51, and screening to remove these is done early in the cascade. Remaining compounds of interest should at this stage be validated for structure and purity. For Chagas disease, it is thought that compounds that can kill all parasites have the highest chance of success in the clinic. To assess ability to achieve sterile cure, compound washout and parasite outgrowth assays are applied. For both diseases, validated hits next progress to hit-to-lead chemistry, with the main aim to achieve proof-of-concept efficacy in a suitable animal model of disease. b | For key compounds in each series, a full biological profile should be determined. This includes determination of potency against multiple relevant strains and host cells, determination of the rate of kill of compounds and profiling against different life stages. In addition, to understand potential for future combination treatments, key series can be profiled in combination experiments.