Simultaneous multi-parametric analysis of Leishmania and of its hosting mammal cells: A high content imaging-based method enabling sound drug discovery process

Repurposing of a library for high-content screening of inhibitors against Echinococcus granulosus

BACKGROUND

Cystic echinococcosis (CE) is a zoonotic disease caused by the larval stage of the dog tapeworm Echinococcus granulosus sensu lato (E. granulosus), with a worldwide distribution. The current treatment strategy for CE is insufficient. Limited drug screening models severely hamper the discovery of effective anti-echinococcosis drugs.

METHODS

In the present study, using high-content screening technology, we developed a novel high-throughput screening (HTS) assay by counting the ratio of propidium iodide-stained dead protoscoleces (PSCs) to the total number of PSCs. In vitro and ex vivo cyst viability assays were utilized to determine the effect of drugs on cyst viability.

RESULTS

Using the newly established HTS assay, we screened approximately 12,000 clinical-stage or The Food and Drug Administration (FDA)-approved small molecules from the Repurposing, Focused Rescue, and Accelerated Medchem (ReFRAME) library, as well as the LOPAC1280 and SelleckChem libraries, as a strategic approach to facilitate the drug discovery process. Initial screening yielded 173 compounds with anti-echinococcal properties, 52 of which demonstrated dose-response efficacy against E. granulosus PSCs in vitro. Notably, two agents, omaveloxolone and niclosamide, showed complete inhibition upon further validation in cyst and microcyst viability assays in vitro after incubation for 3 days, and in an ex vivo cyst viability assay using cysts isolated from the livers of mice infected with E. granulosus, as determined by morphological assessment.

CONCLUSIONS

Through the development of a novel HTS assay and by repurposing libraries, we identified omaveloxolone and niclosamide as potent inhibitors against E. granulosus. These compounds show promise as potential anti-echinococcal drugs, and our strategic approach has the potential to promote drug discovery for parasitic infections.

Assay development in leishmaniasis drug discovery: a comprehensive review

INTRODUCTION

Cutaneous, muco-cutaneous and visceral leishmaniasis occur due to an infection with the protozoan parasite Leishmania. The current therapeutic options are limited mainly due to extensive toxicity, emerging resistance and variation in efficacy based on species and strain of the Leishmania parasite. There exists a high unmet medical need to identify new chemical starting points for drug discovery to tackle the disease.

AREAS COVERED

The authors have highlighted the recent progress, limitations and successes achieved in assay development for leishmaniasis drug discovery.

EXPERT OPINION

It is true that sophisticated and robust phenotypic in vitro assays have been developed during the last decade, however limitations and challenges remain with respect to variation in activity reported between different research groups and success in translating in vitro outcomes in vivo. The variability is not only due to strain and species differences but also a lack of well-defined criteria and assay conditions, e.g. culture media, host cell type, assay formats, parasite form used, multiplicity of infection and incubation periods. Thus, there is an urgent need for more physiologically relevant assays that encompass multi-species phenotypic approaches to identify new chemical starting points for leishmaniasis drug discovery.

Design and Application of a High-Throughput, High-Content Screening System for Natural Product Inhibitors of the Human Parasite Trichomonas vaginalis

It is estimated that Trichomonas vaginalis affects an astonishing 3.9% of the world's population, and while many of those infected are asymptomatic, progression of the disease can lead to serious health problems. Currently, the nitroimidazoles constitute the only drug class approved to treat trichomoniasis in the United States, which makes the spread of drug resistance a realistic concern. We developed a new image-based, high-throughput, and high-content assay for testing natural products (purified compounds and extracts) for antitrichomonal activity. Applying this assay system to a library of fungal natural product extracts led to the identification of three general classes of natural product inhibitors that exhibited moderate to strong activities against T. vaginalis: anthraquinones, xanthone-anthraquinone heterodimers, and decalin-linked tetramic-acid-containing metabolites. The tetramate natural products emerged as the most promising candidate molecules with pyrrolocin A (51) exhibiting potent activity against the parasite (EC50 = 60 nM), yet this metabolite showed limited toxicity to mammalian cell lines (selectivity index values of 100 and 167 versus 3T3 fibroblast and Ect1 normal cervical cells, respectively). The imaging-based assay system is a powerful tool for the bioassay-guided purification of single-component antitrichomonal biomolecules from complex natural product mixtures.

Meta-analysis and discussion on challenges to translate Leishmania drug resistance phenotyping into the clinic

Antimicrobial resistance (AMR) threatens the prevention and treatment of infections caused by a large range of microorganisms. Leishmania is not an exception and treatment failure due to drug-resistant organisms is increasingly reported. Currently, no molecular methods and marker are validated to track drug-resistant organism and antimicrobial susceptibility tests are roughly not amenable to a clinical setting. Taking these facts into account, it is essential to reflect on ways to translate basic knowledge into methodologies aimed to diagnose leishmania drug resistance. As a matter of fact, a meta-analysis of the literature discloses the reliability of the promastigotes antimicrobial susceptibility tests (AST) to predict intracellular amastigotes susceptibility status. Promastigote cultures that are easy to perform, typically inexpensive and amenable to standardization should represent a candidate to diagnose resistance. Using AST performed on promastigote, we propose a way to improve leishmania drug resistance diagnosis in the framework of guidance and guideline of the bacterial drug resistance diagnosis. In this review, we highlight challenges that remained and discuss the definition of clinical breakpoints, including the epidemiological cutoff (ECOFF), to track drug-resistant isolates. Our analysis paves the ways to standardize and analyze anti-leishmania susceptibility tests output in order to guide the characterization of drug-resistant isolates, the clinical decision during treatment and the search for new molecular markers.

Discovery of novel hit compounds with broad activity against visceral and cutaneous Leishmania species by comparative phenotypic screening

The limited success of recent phenotypic anti-leishmanial drug screening campaigns calls for new screening strategies for the discovery of clinically relevant hits. Here we present such a novel strategy based on physiologically relevant, ex vivo biology. We established high content phenotypic assays that combine primary murine macrophages and lesion-derived, virulent L. donovani and L. amazonensis amastigotes, which we applied to validate previously identified, anti-leishmanial hit compounds referred to as ‘GSK Leish-Box’. Together with secondary screens using cultured promastigotes, our pipeline distinguished stage- and/or species-specific compounds, including 20 hits with broad activity at 10 µM against intracellular amastigotes of both viscerotropic and dermotropic Leishmania. Even though the GSK Leish-Box hits were identified by phenotypic screening using THP-1 macrophage-like cells hosting culture-derived L. donovani LdBob parasites, our ex vivo assays only validated anti-leishmanial activity at 10 µM on intra-macrophagic L. donovani for 23 out of the 188 GSK Leish-Box hits. In conclusion, our comparative approach allowed the identification of hits with broad anti-leishmanial activity that represent interesting novel candidates to be tested in animal models. Physiologically more relevant screening approaches such as described here may reduce the very high attrition rate observed during pre-clinical and clinical phases of the drug development process.

Arginase activity in pathogenic and non-pathogenic species of Leishmania parasites

Proliferation of Leishmania (L.) parasites depends on polyamine availability, which can be generated by the L-arginine catabolism and the enzymatic activity of arginase (ARG) of the parasites and of the mammalian hosts. In the present study, we characterized and compared the arginase (arg) genes from pathogenic L. major and L. tropica and from non-pathogenic L. tarentolae. We quantified the level of the ARG activity in promastigotes and macrophages infected with pathogenic L. major and L. tropica and non-pathogenic L. tarentolae amastigotes. The ARG's amino acid sequences of the pathogenic and non-pathogenic Leishmania demonstrated virtually 98.6% and 88% identities with the reference L. major Friedlin ARG. Higher ARG activity was observed in all pathogenic promastigotes as compared to non-pathogenic L. tarentolae. In vitro infection of human macrophage cell line (THP1) with pathogenic and non-pathogenic Leishmania spp. resulted in increased ARG activities in the infected macrophages. The ARG activities present in vivo were assessed in susceptible BALB/c and resistant C57BL/6 mice infected with L. major, L. tropica and L. tarentolae. We demonstrated that during the development of the infection, ARG is induced in both strains of mice infected with pathogenic Leishmania. However, in L. major infected BALB/c mice, the induction of ARG and parasite load increased simultaneously according to the time course of infection, whereas in C57BL/6 mice, the enzyme is upregulated solely during the period of footpad swelling. In L. tropica infected mice, the footpads' swellings were slow to develop and demonstrated minimal cutaneous pathology and ARG activity. In contrast, ARG activity was undetectable in mice inoculated with the non-pathogenic L. tarentolae. Our data suggest that infection by Leishmania parasites can increase ARG activity of the host and provides essential polyamines for parasite salvage and its replication. Moreover, the ARG of Leishmania is vital for parasite proliferation and required for infection in mice. ARG activity can be used as one of the main marker of the disease severity.

Chemotherapy of leishmaniasis: present challenges

Cutaneous and visceral leishmaniasis are amongst the most devastating infectious diseases of our time, affecting millions of people worldwide. The treatment of these serious diseases rely on a few chemotherapeutic agents, most of which are of parenteral use and induce severe side-effects. Furthermore, rates of treatment failure are high and have been linked to drug resistance in some areas. Here, we reviewed data on current chemotherapy practice in leishmaniasis. Drug resistance and mechanisms of resistance are described as well as the prospects for applying drug combinations for leishmaniasis chemotherapy. It is clear that efforts for discovering new drugs applicable to leishmaniasis chemotherapy are essential. The main aspects on the various steps of drug discovery in the field are discussed.

High-content assay for measuring intracellular growth of Leishmania in human macrophages

Leishmania species are sandfly-transmitted protozoan parasites that cause a spectrum of diseases, ranging from localized skin lesions to fatal visceral disease, in more than 12 million people worldwide. These parasites primarily target macrophages in their mammalian hosts and proliferate as non-motile amastigotes in the phagolysosomal compartment of these cells. High-throughput screens for measuring Leishmania growth within this intracellular niche are needed to identify host and parasite factors that are required for virulence and to identify new drug candidates. Here we describe the development of a new high-content imaging method for quantifying the intracellular growth of Leishmania mexicana parasites in THP-1 macrophages. Wild-type parasites were pre-stained with the fluorescent dye CellTracker(™) Orange CMRA and used to infect THP-1 macrophages in 384-well plates. Infected and uninfected macrophages were subsequently stained with CellTracker Green CMFDA, allowing accurate quantitation of the number of parasites per macrophage using separate detector channels. We validated this method for use in high-content drug screening by examining the dose dependence of known anti-leishmanial drugs on intracellular growth. Unlike previous protocols, this method does not require the generation of transgenic fluorescent or bioluminescent parasite lines and can be readily adapted for screening different Leishmania species, strains, or mutant lines in a wide range of phagocytic host cell types.