Antileishmanial high-throughput drug screening reveals drug candidates with new scaffolds

Differential nuclear staining assay for high-throughput screening to identify cytotoxic compounds

As large quantities of novel synthetic molecules continue to be generated there is a challenge to identify therapeutic agents with cytotoxic activity. Here we introduce a Differential Nuclear Staining (DNS) assay adapted to live-cell imaging for high throughput screening (HTS) that utilizes two fluorescent DNA intercalators, Hoechst 33342 and Propidium iodide (PI). Since Hoechst can readily cross cell membranes to stain DNA of living and dead cells, it was used to label the total number of cells. In contrast, PI only enters cells with compromised plasma membranes, thus selectively labeling dead cells. The DNS assay was successfully validated by utilizing well known cytotoxic agents with fast or slow cytotoxic activities. The assay was found to be suitable for HTS with Z' factors ranging from 0.86 to 0.60 for 96 and 384-well formats, respectively. Furthermore, besides plate-to-plate reproducibility, assay quality performance was evaluated by determining ratios of signal-to-noise and signal-to-background, as well as coefficient of variation, which resulted in adequate values and validated the assay for HTS initiatives. As proof of concept, eighty structurally diverse compounds from a small molecule library were screened in a 96-well plate format using the DNS assay. Using this DNS assay, six hits with cytotoxic properties were identified and all of them were also successfully identified by using the commercially available MTS assay (CellTiter 96® Cell Proliferation Assay). In addition, the DNS and a flow cytometry assay were used to validate the activity of the cytotoxic compounds. The DNS assay was also used to generate dose-response curves and to obtain CC50 values. The results indicate that the DNS assay is reliable and robust and suitable for primary and secondary screens of compounds with potential cytotoxic activity.

A screen against Leishmania intracellular amastigotes: comparison to a promastigote screen and identification of a host cell-specific hit

The ability to screen compounds in a high-throughput manner is essential in the process of small molecule drug discovery. Critical to the success of screening strategies is the proper design of the assay, often implying a compromise between ease/speed and a biologically relevant setting. Leishmaniasis is a major neglected disease with limited therapeutic options. In order to streamline efforts for the design of productive drug screens against Leishmania, we compared the efficiency of two screening methods, one targeting the free living and easily cultured promastigote (insect–infective) stage, the other targeting the clinically relevant but more difficult to culture intra-macrophage amastigote (mammal-infective) stage. Screening of a 909-member library of bioactive compounds against Leishmania donovani revealed 59 hits in the promastigote primary screen and 27 in the intracellular amastigote screen, with 26 hits shared by both screens. This suggested that screening against the promastigote stage, although more suitable for automation, fails to identify all active compounds and leads to numerous false positive hits. Of particular interest was the identification of one compound specific to the infective amastigote stage of the parasite. This compound affects intracellular but not axenic parasites, suggesting a host cell-dependent mechanism of action, opening new avenues for anti-leishmanial chemotherapy.

Leishmaniasis, a disease caused by protozoan parasites of the genus Leishmania, is a poverty-related disease threatening 350 million people throughout the world. Drugs currently available to treat this disease are toxic to the patient and drug-resistant parasites are emerging. New therapeutics are therefore needed. Fortunately, interest in confronting the treatment challenges has grown and new technology has led to an increase in high-throughput screens conducted against Leishmania. In order to gain insight into the most efficient screening strategy, we compared two methods, one targeting the easily cultured insect-infective promastigote stage of the parasite, and the other, targeting the clinically relevant but more difficult to culture intracellular amastigote stage. We show that while a screen against promastigotes is amenable to automation, it fails to recognize all active compounds. These compounds revealed only by an intracellular assay might act on host cell pathways important for parasite development. Targeting such pathways is an emerging strategy in drug discovery against infectious diseases.

3-Trifluoromethylquinoxaline N,N'-dioxides as anti-trypanosomatid agents. Identification of optimal anti-T. cruzi agents and mechanism of action studies

For a fourth approach of quinoxaline N,N'-dioxides as anti-trypanosomatid agents against T. cruzi and Leishmania, we found extremely active derivatives. The present study allows us to state the correct requirements for obtaining optimal in vitro anti-T. cruzi activity. Derivatives possessing electron-withdrawing substituents in the 2-, 3-, 6-, and 7-positions were the most active compounds. With regard to these features and taking into account their mammal cytotoxicity, some trifluoromethylquinoxaline N,N'-dioxides have been proposed as candidates for further clinical studies. Consequently, mutagenicity and in vivo analyses were performed with the most promising derivatives. In addition, with regard to the mechanism of action studies, it was demonstrated that mitochondrial dehydrogenases are involved in the anti-T. cruzi activity of the most active derivatives.

Major prospects for exploring canine vector borne diseases and novel intervention methods using 'omic technologies

Canine vector-borne diseases (CVBDs) are of major socioeconomic importance worldwide. Although many studies have provided insights into CVBDs, there has been limited exploration of fundamental molecular aspects of most pathogens, their vectors, pathogen-host relationships and disease and drug resistance using advanced, 'omic technologies. The aim of the present article is to take a prospective view of the impact that next-generation, 'omics technologies could have, with an emphasis on describing the principles of transcriptomic/genomic sequencing as well as bioinformatic technologies and their implications in both fundamental and applied areas of CVBD research. Tackling key biological questions employing these technologies will provide a 'systems biology' context and could lead to radically new intervention and management strategies against CVBDs.

Visceral leishmaniasis: experimental models for drug discovery

Visceral leishmaniasis (VL) or kala-azar is a chronic protozoan infection in humans associated with significant global morbidity and mortality. The causative agent is a haemoflagellate protozoan Leishmania donovani, an obligate intracellular parasite that resides and multiplies within macrophages of the reticulo-endothelial system. Most of the existing anti-leishmanial drugs have serious side effects that limit their clinical application. As an alternate strategy, vaccination is also under experimental and clinical trials. The in vitro evaluation designed to facilitate rapid testing of a large number of drugs has been focussed on the promastigotes milt little attention on the clinically relevant parasite stage, amastigotes. Screening designed to closely reflect the situation in vivo is currently time consuming, laborious, and expensive, since it requires intracellular amastigotes and animal model. The ability to select transgenic Leishmania expressing reporter proteins, such as the green fluorescent proteins (GFP) or the luciferase opened up new possibilities for the development of drug screening models. Many experimental animal models like rodents, dogs and monkeys have been developed, each with specific features, but none accurately reproduces what happens in humans. Available in vitro and in vivo methodologies for antileishmanial drug screening and their respective advantages and disadvantages are reviewed.