In vitro and in vivo pharmacodynamics of three novel antileishmanial lead series

Improving reproducibility and translational potential of mouse models: lessons from studying leishmaniasis

Leishmaniasis is a complex disease caused by protozoan parasites of the genus Leishmania, which are transmitted by phlebotomine sand flies. The clinical manifestations of leishmaniasis are diverse, ranging from self-healing cutaneous lesions to fatal systemic disease. Mouse models are instrumental in advancing our understanding of the immune system against infections, yet their limitations in translating findings to humans are increasingly highlighted. The success rate of translating data from mice to humans remains low, largely due to the complexity of diseases and the numerous factors that influence the disease outcomes. Therefore, for the effective translation of data from murine models of leishmaniasis, it is essential to align experimental conditions with those relevant to human infection. Factors such as parasite characteristics, vector-derived components, host status, and environmental conditions must be carefully considered and adapted to enhance the translational relevance of mouse data. These parameters are potentially modifiable and should be carefully integrated into the design and interpretation of experimental procedures in Leishmania studies. In the current paper, we review the challenges and perspective of using mouse as a model for leishmaniasis. We have particularly emphasized the non-genetic factors that influence experiments and focused on strategies to improve translational value of studies on leishmaniasis using mouse models.

Investigating the C2 Modulation of the Imidazo[1,2-a]pyrazine-Based Hit Compound CTN1122: Synthesis, in vitro Antileishmanial Activity, Cytotoxicity and Casein Kinase 1 Inhibition

Our research group previously discovered CTN1122, an imidazo[1,2-a]pyrazine compound with promising antileishmanial activity against intramacrophage amastigotes of Leishmania major and L. donovani strains. CTN1122 effectively targets Leishmania casein kinase 1 (L-CK1.2) and exhibits a favorable safety profile. To further explore its chemical space, we developed a convergent strategy to modify the C2 position of the imidazo[1,2-a]pyrazine core using Suzuki-Miyaura coupling of the corresponding triflate intermediate. Among 15 newly synthesized analogs, seven derivatives featuring variously substituted phenyl rings at C2 demonstrated L-CK1.2 inhibition within micromolar to submicromolar ranges and antileishmanial activity in vitro with low cytotoxicity in macrophages. Compounds 7 d and 7 l were particularly potent, with IC50 values of 1.25 μM and 0.92 μM against L. major, and 1.44 μM and 2.34 μM against L. donovani, respectively. They showed IC50 L-CK1.2=0.30 μM and 0.57 μM with enhanced selectivity indices (SI=3.8 and 1.6) over the human CK1ϵ ortholog. Additionally, four C2 analogs and two C5 isomers exhibited notable antiparasitic effects without strongly inhibiting L-CK1.2, indicating a possible alternative mechanism of action. Compound 7 k displayed the highest general activity, with IC50 values of 0.31 μM on L. major and 0.27 μM on L. donovani, coupled with favorable selectivity indexes.

Synthesis and in vitro antitrypanosomatid activity of novel 5-nitroindole-rhodanine conjugates

AIM

Trypanosomatid diseases, leishmaniasis and trypanosomiasis are vector-borne parasitic diseases that can cause death and catastrophic economic losses for millions of people. The growing resistance of trypanosomatid parasites to current treatments highlights the urgent need for new therapeutic agents. This study explored 5-nitroindole-rhodanine conjugates to identify promising new compounds with the potential for future development as antitrypanosomatid treatments.

MATERIALS AND METHODS

The conjugates were synthesized in a multi-step process and evaluated in vitro for antileishmanial activity against Leishmania (L.) donovani and L. major strains. Cytotoxicity was assessed on Vero and THP-1 cells. Due to the taxonomic relation to Trypanosoma spp. the compounds were also screened for in vitro activity against species that cause zoonotic trypanosomiasis.

RESULTS AND CONCLUSION

Several hits were found with leishmanicidal activity against both L. donovani and L. major strains. Of these, 3d was identified as a potential early lead that exhibited nanomolar cidal activity against L. major, and greater selectivity than the reference drug amphotericin B. However, the compounds did not have similar activity levels against Trypanosoma spp. Hence, these compounds should be further investigated for their mechanism of action and in vivo antileishmanial activity to determine their potential as a leishmaniasis treatment.

N-Pyrazolyl- and N-Triazolylamines and -Ureas as Antileishmanial and Antitrypanosomal Drugs

Three types of modifications of antileishmanial pyrazole lead compounds 7 and 8 were conducted to expand understanding of the relationships between structural features and antileishmanial/antitrypanosomal activity: (1) the pyrazole core was retained or replaced by a 1,2,4-triazole ring; (2) various aryl moieties including 2-fluorophenyl, pyridin-3-yl and pyrazin-2-yl rings were attached at 3-position of the core azole; (3) either arylmethylamino or ureido substituents were introduced at 5-position of the azole core. The synthesis followed established routes starting with esters 9 or 15 and anhydride 21. The synthesized 3-arylpyrazoles and 3-aryl-1,2,4-triazoles had only very low antileishmanial activity. The 2-fluorophenyl-substituted pyrazole 18c revealed the highest antileishmanial activity of this series of compounds, but its IC50 value (20 μM) still indicates low activity. However, low micromolar antitrypanosomal activity was detected for the pyridin-3-yl-substituted pyrazoles 12b (IC50=4.7 μM) and 14a (IC50=2.1 μM). Their IC50 values are comparable with the IC50 values of the reference compounds benznidazole and nifurtimox. Whereas only low unspecific cytotoxicity at the primary peritoneal mouse macrophages (PMM) was detected, considerable cytotoxicity at MRC-5 human fibroblast cells was found for both pyrazoles 12b an 14a. The activity of pyrazole 12b against T. cruzi is 4-fold higher than its unspecific MRC-5 cytotoxicity.

In Vivo Bioluminescence Imaging Reveals Differences in Leishmania infantum Parasite Killing Kinetics by Antileishmanial Reference Drugs

The bioluminescent Leishmania infantum BALB/c mouse model was used to evaluate the parasiticidal drug action kinetics of the reference drugs miltefosine, paromomycin, sodium stibogluconate, and liposomal amphotericin B. Infected mice were treated for 5 days starting from 7 days post-infection, and parasite burdens were monitored over time via bioluminescence imaging (BLI). Using nonlinear regression analyses of the BLI signal, the parasite elimination half-life (t1/2) in the liver, bone marrow, and whole body was determined and compared for the different treatment regimens. Significant differences in parasiticidal kinetics were recorded. A single intravenous dose of 0.5 mg/kg liposomal amphotericin B was the fastest acting with a t1/2 of less than 1 day. Intraperitoneal injection of paromomycin at 320 mg/kg for 5 days proved to be the slowest with a t1/2 of about 5 days in the liver and 16 days in the bone marrow. To conclude, evaluation of the cidal kinetics of the different antileishmanial reference drugs revealed striking differences in their parasite elimination half-lives. This BLI approach also enables an in-depth pharmacodynamic comparison between novel drug leads and may constitute an essential tool for the design of potential drug combinations.

Practical Synthesis of 6-Amino-1-hydroxy-2,1-benzoxaborolane: A Key Intermediate of DNDI-6148

Visceral leishmaniasis (VL), a parasitic, poverty-linked, neglected disease, is endemic across multiple regions of the world and fatal if untreated. There is an urgent need for a better and more affordable treatment for VL. DNDI-6148 is a promising drug candidate being evaluated for the treatment of VL; however, the current process for producing the key intermediate of DNDI-6148, 6-amino-1-hydroxy-2,1-benzoxaborolane, is expensive and difficult to scale up. Herein, we describe two practical approaches to synthesizing 6-amino-1-hydroxy-2,1-benzoxaborolane from inexpensive and readily available raw materials. Starting with 4-tolunitrile, the first approach is a five-step sequence involving a Hofmann rearrangement, resulting in an overall yield of 40%. The second approach utilizes 2-methyl-5-nitroaniline as the starting material and features borylation of aniline and continuous flow hydrogenation as the key steps, with an overall yield of 46%. Both routes bypass the nitration of 1-hydroxy-2,1-benzoxaborolane, which is challenging and expensive to scale. In particular, the second approach is more practical and scalable because of the mild operating conditions and facile isolation process.

A QSAR Study for Antileishmanial 2-Phenyl-2,3-dihydrobenzofurans †

Leishmaniasis, a parasitic disease that represents a threat to the life of millions of people around the globe, is currently lacking effective treatments. We have previously reported on the antileishmanial activity of a series of synthetic 2-phenyl-2,3-dihydrobenzofurans and some qualitative structure-activity relationships within this set of neolignan analogues. Therefore, in the present study, various quantitative structure-activity relationship (QSAR) models were created to explain and predict the antileishmanial activity of these compounds. Comparing the performance of QSAR models based on molecular descriptors and multiple linear regression, random forest, and support vector regression with models based on 3D molecular structures and their interaction fields (MIFs) with partial least squares regression, it turned out that the latter (i.e., 3D-QSAR models) were clearly superior to the former. MIF analysis for the best-performing and statistically most robust 3D-QSAR model revealed the most important structural features required for antileishmanial activity. Thus, this model can guide decision-making during further development by predicting the activity of potentially new leishmanicidal dihydrobenzofurans before synthesis.

Further Investigations of Nitroheterocyclic Compounds as Potential Antikinetoplastid Drug Candidates

Due to the lack of specific vaccines, management of the trypanosomatid-caused neglected tropical diseases (sleeping sickness, Chagas disease and leishmaniasis) relies exclusively on pharmacological treatments. Current drugs against them are scarce, old and exhibit disadvantages, such as adverse effects, parenteral administration, chemical instability and high costs which are often unaffordable for endemic low-income countries. Discoveries of new pharmacological entities for the treatment of these diseases are scarce, since most of the big pharmaceutical companies find this market unattractive. In order to fill the pipeline of compounds and replace existing ones, highly translatable drug screening platforms have been developed in the last two decades. Thousands of molecules have been tested, including nitroheterocyclic compounds, such as benznidazole and nifurtimox, which had already provided potent and effective effects against Chagas disease. More recently, fexinidazole has been added as a new drug against African trypanosomiasis. Despite the success of nitroheterocycles, they had been discarded from drug discovery campaigns due to their mutagenic potential, but now they represent a promising source of inspiration for oral drugs that can replace those currently on the market. The examples provided by the trypanocidal activity of fexinidazole and the promising efficacy of the derivative DNDi-0690 against leishmaniasis seem to open a new window of opportunity for these compounds that were discovered in the 1960s. In this review, we show the current uses of nitroheterocycles and the novel derived molecules that are being synthesized against these neglected diseases.

Discovery of pyrazolopyrrolidinones as potent, broad-spectrum inhibitors of Leishmania infection

Introduction

Leishmaniasis is a parasitic disease that affects more than 1 million people worldwide annually, predominantly in resource-limited settings. The challenge in compound development is to exhibit potent activity against the intracellular stage of the parasite (the stage present in the mammalian host) without harming the infected host cells. We have identified a compound series (pyrazolopyrrolidinones) active against the intracellular parasites of Leishmania donovani and L. major; the causative agents of visceral and cutaneous leishmaniasis in the Old World, respectively.

Methods

In this study, we performed medicinal chemistry on a newly discovered antileishmanial chemotype, with over 100 analogs tested. Studies included assessments of antileishmanial potency, toxicity towards host cells, and in vitro ADME screening of key drug properties.

Results and discussion

Members of the series showed high potency against the deadliest form, visceral leishmaniasis (approximate EC₅₀ ≥ 0.01 μM without harming the host macrophage up to 10.0 μM). In comparison, the most efficient monotherapy treatment for visceral leishmaniasis is amphotericin B, which presents similar activity in the same assay (EC₅₀ = 0.2 μM) while being cytotoxic to the host cell at 5.0 μM. Continued development of this compound series with the Discovery Partnership with Academia (DPAc) program at the GlaxoSmithKline Diseases of the Developing World (GSK DDW) laboratories found that the compounds passed all of GSK's criteria to be defined as a potential lead drug series for leishmaniasis.

Conclusion

Here, we describe preliminary structure-activity relationships for antileishmanial pyrazolopyrrolidinones, and our progress towards the identification of candidates for future in vivo assays in models of visceral and cutaneous leishmaniasis.

Anti-trypanosomatid drug discovery: progress and challenges

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.

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.

DNDI-6148: A Novel Benzoxaborole Preclinical Candidate for the Treatment of Visceral Leishmaniasis

Visceral leishmaniasis (VL) is a parasitic disease endemic across multiple regions of the world and is fatal if untreated. Current therapies are unsuitable, and there is an urgent need for safe, short-course, and low-cost oral treatments to combat this neglected disease. The benzoxaborole chemotype has previously delivered clinical candidates for the treatment of other parasitic diseases. Here, we describe the development and optimization of this series, leading to the identification of compounds with potent in vitro and in vivo antileishmanial activity. The lead compound (DNDI-6148) combines impressive in vivo efficacy (>98% reduction in parasite burden) with pharmaceutical properties suitable for onward development and an acceptable safety profile. Detailed mode of action studies confirm that DNDI-6148 acts principally through the inhibition of Leishmania cleavage and polyadenylation specificity factor (CPSF3) endonuclease. As a result of these studies and its promising profile, DNDI-6148 has been declared a preclinical candidate for the treatment of VL.

Discovery of Leishmania donovani topoisomerase IB selective inhibitors by targeting protein-protein interactions between the large and small subunits

Visceral leishmaniasis (VL) is a fatal infectious disease caused by viscerotropic parasitic species of Leishmania. Current treatment options are often ineffective and toxic, and more importantly, there are no clinically validated drug targets available to develop next generation therapeutics against VL. Topoisomerase IB (TopIB) is an essential enzyme for Leishmania survival. The enzyme is organized as a bi-subunit that is distinct from the monomeric topoisomerase I of human. Based on this unique feature, we synthesized peptides composed of partial amino acid sequences of small subunit of Leishmania donovani (Ld) TopIB to confirm a decrease in catalytic activity by interfering the interaction between the two subunits. One of the synthetic peptides, covering essential amino acids for catalytic activity of LdTopIB, interrupted the enzymatic activity. Next, we examined 151 compounds selected from virtual screening in a functional assay and identified three LRL-TP compounds with a significant decrease in LdTopIB activity (IC₅₀ of LRL-TP-85: 1.3 μM; LRL-TP-94: 2.9 μM; and LRL-TP-101: 35.3 μM) and no effects on Homo sapiens (Hs) TopIB activity. Based on molecular docking, the protonated tertiary amine of inhibitors formed key interactions with S415 of the large subunit. The EC₅₀ values of LRL-TP-85, LRL-TP-94, and LRL-TP-101 were respectively 4.9, 1.4, and 27.8 μM in extracellular promastigote assay and 34.0, 53.7, and 11.4 μM in intracellular amastigote assay. Overall, we validated the protein-protein interaction site of LdTopIB as a potential drug target and identified small molecule inhibitors with anti-leishmanial activity.

An adamantamine derivative as a drug candidate for the treatment of visceral leishmaniasis

BACKGROUND

This study aimed to investigate compounds acting on the host cell machinery to impair parasite installation with the possible advantage of limiting drug resistance. The strategy therefore consisted of selecting compounds that are poorly active on the axenic parasite, but very active on the intramacrophage form of Leishmania.

OBJECTIVES

To identify a drug candidate from focused screening of adamantamine derivatives that can inhibit the development of Leishmania infantum in macrophages.

METHODS

In vitro screening was performed on a library of 142 adamantamine derivatives with axenic and intramacrophage forms of L. infantum, as well as cytotoxicity assays, allowing selection of the most promising compound. Absorption, distribution, metabolism and excretion (ADME) experiments, including pharmacokinetics and microsomal stability, were performed and finally the physicochemical stability of the compound was investigated to assess its suitability for further drug development.

RESULTS

VP343 was identified first in vitro, with a CC50 value of 63.7 μM and an IC50 value of 0.32 μM for L. infantum intramacrophage amastigotes and then in vivo, with a 59% reduction of the liver parasite burden after oral administration at 10 mg/kg/day for 5 days. In addition, the ADME data were compatible with moving this compound further through the antileishmanial drug candidate pipeline.

CONCLUSIONS

VP343 has the properties of a good drug candidate and merits further investigations.

Identification of Resistance Determinants for a Promising Antileishmanial Oxaborole Series

Current treatment options for visceral leishmaniasis have several drawbacks, and clinicians are confronted with an increasing number of treatment failures. To overcome this, the Drugs for Neglected Diseases initiative (DNDi) has invested in the development of novel antileishmanial leads, including a very promising class of oxaboroles. The mode of action/resistance of this series to Leishmania is still unknown and may be important for its further development and implementation. Repeated in vivo drug exposure and an in vitro selection procedure on both extracellular promastigote and intracellular amastigote stages were both unable to select for resistance. The use of specific inhibitors for ABC-transporters could not demonstrate the putative involvement of efflux pumps. Selection experiments and inhibitor studies, therefore, suggest that resistance to oxaboroles may not emerge readily in the field. The selection of a genome-wide cosmid library coupled to next-generation sequencing (Cos-seq) was used to identify resistance determinants and putative targets. This resulted in the identification of a highly enriched cosmid, harboring genes of chromosome 2 that confer a subtly increased resistance to the oxaboroles tested. Moderately enriched cosmids encompassing a region of chromosome 34 contained the cleavage and polyadenylation specificity factor (cpsf) gene, encoding the molecular target of several related benzoxaboroles in other organisms.

Revisiting Pyrazolo[3,4-d]pyrimidine Nucleosides as Anti-Trypanosoma cruzi and Antileishmanial Agents

Chagas disease and visceral leishmaniasis are two neglected tropical diseases responsible for numerous deaths around the world. For both, current treatments are largely inadequate, resulting in a continued need for new drug discovery. As both kinetoplastid parasites are incapable of de novo purine synthesis, they depend on purine salvage pathways that allow them to acquire and process purines from the host to meet their demands. Purine nucleoside analogues therefore constitute a logical source of potential antiparasitic agents. Earlier optimization efforts of the natural product tubercidin (7-deazaadenosine) involving modifications to the nucleobase 7-position and the ribofuranose 3'-position led to analogues with potent anti-Trypanosoma brucei and anti-Trypanosoma cruzi activities. In this work, we report the design and synthesis of pyrazolo[3,4-d]pyrimidine nucleosides with 3'- and 7-modifications and assess their potential as anti-Trypanosoma cruzi and antileishmanial agents. One compound was selected for in vivo evaluation in an acute Chagas disease mouse model.

3,5-Dimethyl-4-isoxazoyl selenocyanate as promising agent for the treatment of Leishmania infantum-infected mice

Compounds 1 and 2 (selenocyanate and diselenide derivatives, respectively) were evaluated for their potential use in vivo against visceral leishmaniasis (VL). Both entities showed low cytoxicity in vitro in Vero and Caco-2 cell lines. However, the compounds were not suitable for their oral administration, since they exhibited poor values of intestinal permeability in vitro. Microsomal stability assays did not show any metabolite for compound 1 after 120 min, whereas 2 was highly metabolized by the enzyme CYP450. Thus, the in vivo efficacy of compound 1 was assessed in a murine model of L. infantum VL. The daily i.v. administration of 1 mg/kg of compound 1 during 5 consecutive days reduced parasite load in liver, spleen and bone marrow (99.2%, 91.7% and 61.4%, respectively) compared to non-treated mice. To the best of our knowledge, this is the first time that a selenium compound has been tested in vivo against VL. Thus, this work evidences the possible usefulness of selenocyanate derivatives for the treatment of this disease.

Novel Linker Variants of Antileishmanial/Antitubercular 7-Substituted 2-Nitroimidazooxazines Offer Enhanced Solubility

Antitubercular 7-substituted 2-nitroimidazo[2,1-b][1,3]oxazines were previously shown to exhibit potent antileishmanial and antitrypanosomal activities, culminating in a new clinical investigational drug for visceral leishmaniasis (DNDI-0690). To offset development risks, we continued to seek further leads with divergent candidate profiles, especially analogues possessing greater aqueous solubility. Starting from an efficacious monoaryl derivative, replacement of the side chain ether linkage by novel amine, amide, and urea functionality was first explored; the former substitution was well-tolerated in vitro and in vivo but elicited marginal alterations to solubility (except through a less stable benzylamine), whereas the latter groups resulted in significant solubility improvements (up to 53-fold) but an antileishmanial potency reduction of at least 10-fold. Ultimately, we discovered that O-carbamate 66 offered a more optimal balance of increased solubility, suitable metabolic stability, excellent oral bioavailability (100%), and strong in vivo efficacy in a visceral leishmaniasis mouse model (97% parasite load reduction at 25 mg/kg).

Collaborative virtual screening to elaborate an imidazo[1,2-a]pyridine hit series for visceral leishmaniasis

An innovative pre-competitive virtual screening collaboration was engaged to validate and subsequently explore an imidazo[1,2-a]pyridine screening hit for visceral leishmaniasis. In silico probing of five proprietary pharmaceutical company libraries enabled rapid expansion of the hit chemotype, alleviating initial concerns about the core chemical structure while simultaneously improving antiparasitic activity and selectivity index relative to the background cell line. Subsequent hit optimization informed by the structure–activity relationship enabled by this virtual screening allowed thorough investigation of the pharmacophore, opening avenues for further improvement and optimization of the chemical series.

Ligand-based similarity screening of proprietary pharmaceutical company libraries enables rapid hit to lead investigation of a chemotype with anti-leishmania activity.

Pharmacokinetics and pharmacodynamics in the treatment of cutaneous leishmaniasis - challenges and opportunities

Pharmacological efficacy is obtained when adequate concentrations of a potent drug reach the target site. In cutaneous leishmaniasis, a heterogeneous disease characterised by a variety of skin manifestations from simple nodules, skin discoloration, plaques to extensive disseminated forms, the parasites are found in the dermal layers of the skin. Treatment thus involves the release of the active compound from the formulation (administered either topically or systemically), it's permeation into the skin, accumulation by the local macrophages and further transport into the phagolysosome of the macrophage. The pharmacodynamic activity of a drug against the parasite is relatively straight forward to evaluate both in vivo and in vitro. The pharmacokinetic processes taking place inside the skin are more complex to elucidate due to the multi-lamellar structure of the skin, heterogeneous distribution of drugs within the tissue, the difficulty of accessing the site of infection complicating sampling and the lack of surrogate markers reflecting the activity of a drug in the skin. This review will discuss the difficulties encountered when investigating drug distribution, PK PD relationships and efficacy in the skin with a focus on cutaneous leishmaniasis treatment.

Computationally designed synthetic peptides for transporter proteins imparts allostericity in Miltefosine resistant L. major

The emergence of drug resistance is a major concern for combating against Cutaneous Leishmaniasis, a neglected tropical disease affecting 98 countries including India. Miltefosine is the only oral drug available for the disease and Miltefosine transporter proteins play a pivotal role in the emergence of drug-resistant Leishmania major. The cause of resistance is less accumulation of drug inside the parasite either by less uptake of the drug due to a decrease in the activity of P4ATPase-CDC50 complex or by increased efflux of the drug by P-glycoprotein (P-gp, an ABC transporter). In this paper, we are trying to allosterically modulate the behavior of resistant parasite (L. major) towards its sensitivity for the existing drug (Miltefosine, a phosphatidylcholine analog). We have used computational approaches to deal with the conservedness of the proteins and apparently its three-dimensional structure prediction through ab initio modeling. Long scale membrane-embedded molecular dynamics simulations were carried out to study the structural interaction and stability. Parasite-specific motifs of these proteins were identified based on the machine learning technique, against which a peptide library was designed. The protein-peptide docking shows good binding energy of peptides Pg5F, Pg8F and PC2 with specific binding to the motifs. These peptides were tested both in vitro and in vivo, where Pg5F in combination with PC2 showed 50-60% inhibition in resistant L. major's promastigote and amastigote forms and 80-90% decrease in parasite load in mice. We posit a model system wherein the data provide sufficient impetus for being novel therapeutics in order to counteract the drug resistance phenotype in Leishmania parasites.

Trypanosomatid-Caused Conditions: State of the Art of Therapeutics and Potential Applications of Lipid-Based Nanocarriers

Trypanosomatid-caused conditions (African trypanosomiasis, Chagas disease, and leishmaniasis) are neglected tropical infectious diseases that mainly affect socioeconomically vulnerable populations. The available therapeutics display substantial limitations, among them limited efficacy, safety issues, drug resistance, and, in some cases, inconvenient routes of administration, which made the scenarios with insufficient health infrastructure settings inconvenient. Pharmaceutical nanocarriers may provide solutions to some of these obstacles, improving the efficacy-safety balance and tolerability to therapeutic interventions. Here, we overview the state of the art of therapeutics for trypanosomatid-caused diseases (including approved drugs and drugs undergoing clinical trials) and the literature on nanolipid pharmaceutical carriers encapsulating approved and non-approved drugs for these diseases. Numerous studies have focused on the obtention and preclinical assessment of lipid nanocarriers, particularly those addressing the two currently most challenging trypanosomatid-caused diseases, Chagas disease, and leishmaniasis. In general, in vitro and in vivo studies suggest that delivering the drugs using such type of nanocarriers could improve the efficacy-safety balance, diminishing cytotoxicity and organ toxicity, especially in leishmaniasis. This constitutes a very relevant outcome, as it opens the possibility to extended treatment regimens and improved compliance. Despite these advances, last-generation nanosystems, such as targeted nanocarriers and hybrid systems, have still not been extensively explored in the field of trypanosomatid-caused conditions and represent promising opportunities for future developments. The potential use of nanotechnology in extended, well-tolerated drug regimens is particularly interesting in the light of recent descriptions of quiescent/dormant stages of Leishmania and Trypanosoma cruzi, which have been linked to therapeutic failure.

Quinolizidine-Derived Lucanthone and Amitriptyline Analogues Endowed with Potent Antileishmanial Activity

Leishmaniases are neglected diseases that are endemic in many tropical and sub-tropical Countries. Therapy is based on different classes of drugs which are burdened by severe side effects, occurrence of resistance and high costs, thereby creating the need for more efficacious, safer and inexpensive drugs. Herein, sixteen 9-thioxanthenone derivatives (lucanthone analogues) and four compounds embodying the diarylethene substructure of amitriptyline (amitriptyline analogues) were tested in vitro for activity against Leishmania tropica and L. infantum promastigotes. All compounds were characterized by the presence of a bulky quinolizidinylalkyl moiety. All compounds displayed activity against both species of Leishmania with IC₅₀ values in the low micromolar range, resulting in several fold more potency than miltefosine, comparable to that of lucanthone, and endowed with substantially lower cytotoxicity to Vero-76 cells, for the best of them. Thus, 4-amino-1-(quinolizidinylethyl)aminothioxanthen-9-one (14) and 9-(quinolizidinylmethylidene)fluorene (17), with selectivity index (SI) in the range 16-24, represent promising leads for the development of improved antileishmanial agents. These two compounds also exhibited comparable activity against intramacrophagic amastigotes of L. infantum. Docking studies have suggested that the inhibition of trypanothione reductase (TryR) may be at the basis (eventually besides other mechanisms) of the observed antileishmanial activity. Therefore, these investigated derivatives may deserve further structural improvements and more in-depth biological studies of their mechanisms of action in order to develop more efficient antiparasitic agents.

In vitro identification of imidazo[1,2-a]pyrazine-based antileishmanial agents and evaluation of L. major casein kinase 1 inhibition

Leishmaniasis constitutes a severe public health problem, with an estimated prevalence of 12 million cases. This potentially fatal disease has a worldwide distribution and in 2012, the fatal Visceral Leishmaniasis (VL) was declared as new emerging disease in Europe, mainly due to global warming, with expected important public health impact. The available treatments are toxic, costly or lead to parasite resistance, thus there is an urgent need for new drugs with new mechanism of action. Previously, we reported the discovery of CTN1122, a potent imidazo[1,2-a]pyrazine-based antileishmanial hit compound targeting L-CK1.2 at low micromolar ranges. Here, we described structurally related, safe and selective compounds endowed with antiparasitic properties, better than miltefosine, the reference therapy by oral route. L-CK1.2 homology model gave the first structural explanations of the role of 4-pyridyl (CTN1122) and 2-aminopyrimidin-4-yl (compound 21) moieties, at the position 3 of the central core, in the low micromolar to nanomolar L-CK1.2 inhibition, whereas N-methylpyrazole derivative 11 remained inactive against the parasite kinase.

Antileishmanial Aminopyrazoles: Studies into Mechanisms and Stability of Experimental Drug Resistance

Current antileishmanial treatment is hampered by limitations, such as drug toxicity and the risk of treatment failure, which may be related to parasitic drug resistance. Given the urgent need for novel drugs, the Drugs for Neglected Diseases initiative (DNDi) has undertaken a drug discovery program, which has resulted in the identification of aminopyrazoles, a highly promising antileishmanial chemical series. Multiple experiments have been performed to anticipate the propensity for resistance development.

Current antileishmanial treatment is hampered by limitations, such as drug toxicity and the risk of treatment failure, which may be related to parasitic drug resistance. Given the urgent need for novel drugs, the Drugs for Neglected Diseases initiative (DNDi) has undertaken a drug discovery program, which has resulted in the identification of aminopyrazoles, a highly promising antileishmanial chemical series. Multiple experiments have been performed to anticipate the propensity for resistance development. Resistance selection was performed by successive exposure of Leishmania infantum promastigotes (in vitro) and intracellular amastigotes (both in vitro and in golden Syrian hamsters). The stability of the resistant phenotypes was assessed after passage in mice and Lutzomyia longipalpis sandflies. Whole-genome sequencing (WGS) was performed to identify mutated genes, copy number variations (CNVs), and somy changes. The potential role of efflux pumps (the MDR and MRP efflux pumps) in the development of resistance was assessed by coincubation of aminopyrazoles with specific efflux pump inhibitors (verapamil, cyclosporine, and probenecid). Repeated drug exposure of amastigotes did not result in the emergence of drug resistance either in vitro or in vivo. Selection at the promastigote stage, however, was able to select for parasites with reduced susceptibility (resistance index, 5.8 to 24.5). This phenotype proved to be unstable after in vivo passage in mice and sandflies, suggesting that nonfixed alterations are responsible for the elevated resistance. In line with this, single nucleotide polymorphisms and indels identified by whole-genome sequencing could not be directly linked to the decreased drug susceptibility. Copy number variations were absent, whereas somy changes were detected, which may have accounted for the transient acquisition of resistance. Finally, aminopyrazole activity was not influenced by the MDR and MRP efflux pump inhibitors tested. The selection performed does not suggest the rapid development of resistance against aminopyrazoles in the field. Karyotype changes may confer elevated levels of resistance, but these do not seem to be stable in the vertebrate and invertebrate hosts. MDR/MRP efflux pumps are not likely to significantly impact the activity of the aminopyrazole leads.

Of Drugs and Trypanosomatids: New Tools and Knowledge to Reduce Bottlenecks in Drug Discovery

Leishmaniasis (Leishmania species), sleeping sickness (Trypanosoma brucei), and Chagas disease (Trypanosoma cruzi) are devastating and globally spread diseases caused by trypanosomatid parasites. At present, drugs for treating trypanosomatid diseases are far from ideal due to host toxicity, elevated cost, limited access, and increasing rates of drug resistance. Technological advances in parasitology, chemistry, and genomics have unlocked new possibilities for novel drug concepts and compound screening technologies that were previously inaccessible. In this perspective, we discuss current models used in drug-discovery cascades targeting trypanosomatids (from in vitro to in vivo approaches), their use and limitations in a biological context, as well as different examples of recently discovered lead compounds.

Infectivity and Drug Susceptibility Profiling of Different Leishmania-Host Cell Combinations

Protozoan parasites of the genus Leishmania are the causative agents of leishmaniasis, a spectrum of a disease that threatens public health worldwide. Although next-generation therapeutics are urgently needed, the early stage of the drug discovery process is hampered by very low hit rates from intracellular Leishmania phenotypic high-throughput screenings. Designing and applying a physiologically relevant in vitro assay is therefore in high demand. In this study, we characterized the infectivity, morphology, and drug susceptibility of different Leishmania and host cell infection combinations. Primary bone marrow-derived macrophage (BMDM) and differentiated human acute monocytic leukemia (THP-1) cells were infected with amastigote or promastigote forms of Leishmania amazonensis and Leishmania donovani. Regardless of host cell types, amastigotes were generally well phagocytosed and showed high infectivity, whereas promastigotes, especially those of L. donovani, had predominantly remained in the extracellular space. In the drug susceptibility test, miltefosine and sodium stibogluconate (SSG) showed varying ranges of activity with 14 and >10-fold differences in susceptibility, depending on the host-parasite pairs, indicating the importance of assay conditions for evaluating antileishmanial activity. Overall, our results suggest that combinations of Leishmania species, infection forms, and host cells must be carefully optimized to evaluate the activity of potential therapeutic compounds against Leishmania.

Comparative evaluation of nucleic acid stabilizing reagents for RNA- and DNA-based Leishmania detection in blood as proxy for visceral burdens

BACKGROUND

Molecular detection techniques using peripheral blood are preferred over invasive tissue aspiration for the diagnosis and post-treatment follow-up of visceral leishmaniasis (VL) patients. This study aims to identify suitable stabilizing reagents to prevent DNA and RNA degradation during storage and transport to specialized laboratories where molecular diagnosis is performed.

METHODOLOGY

The stabilizing capacities of different commercially available reagents were compared using promastigote-spiked human blood and peripheral blood of Syrian golden hamsters subjected to experimental infection, treatment (miltefosine or aminopyrazole DNDi-1044) and immunosuppression. The impact of various storage temperature conditions was tested in combination with an established kinetoplast DNA (kDNA) qPCR and a recently developed spliced leader RNA (SL-RNA) assay for Leishmania detection.

PRINCIPAL FINDINGS

Irrespective of the blood type and stabilizer used, threshold (cT) values obtained with the SL-RNA qPCR were systematically lower than those obtained with the kDNA assay, confirming the advantage of the SL-RNA assay over the widely used kDNA assay for low-level Leishmania detection. Peripheral blood parasite levels correlated relatively well with hepatic burdens. RNA protect cell reagent provided the most optimal simultaneous DNA and RNA stabilization in both human and hamster blood. However, this stabilizer requires an erythrocyte lysis step, which can be challenging under field conditions. DNA/RNA shield provides a good alternative for downstream kDNA and SL-RNA assays, especially if sample storage capacity at 4 °C can be guaranteed.

CONCLUSIONS/SIGNIFICANCE

The recommended stabilizing reagents are compatible with RNA- and DNA-based Leishmania detection in peripheral blood in the VL hamster model and spiked human blood. Since molecular detection techniques using peripheral blood are less invasive than microscopic assessment of tissue aspirates, the findings of this study may be applied to human VL clinical studies.

Structure-Activity Relationships of Cinnamate Ester Analogues as Potent Antiprotozoal Agents

Protozoal infections are still a global health problem, threatening the lives of millions of people around the world, mainly in impoverished tropical and sub-tropical regions. Thus, in view of the lack of efficient therapies and increasing resistances against existing drugs, this study describes the antiprotozoal potential of synthetic cinnamate ester analogues and their structure-activity relationships. In general, Leishmania donovani and Trypanosoma brucei were quite susceptible to the compounds in a structure-dependent manner. Detailed analysis revealed a key role of the substitution pattern on the aromatic ring and a marked effect of the side chain on the activity against these two parasites. The high antileishmanial potency and remarkable selectivity of the nitro-aromatic derivatives suggested them as promising candidates for further studies. On the other hand, the high in vitro potency of catechol-type compounds against T. brucei could not be extrapolated to an in vivo mouse model.

Novel benzoxaborole, nitroimidazole and aminopyrazoles with activity against experimental cutaneous leishmaniasis

OBJECTIVES

Drugs for Neglected Diseases initiative (DNDi) has identified three chemical lead series, the nitroimidazoles, benzoxaboroles and aminopyrazoles, as innovative treatments for visceral leishmaniasis. The leads discovered using phenotypic screening, were optimised following disease- and compound-specific criteria. Several leads of each series were progressed and preclinical drug candidates have been nominated. Here we evaluate the efficacy of the lead compounds of each of these three chemical classes in in vitro and in vivo models of cutaneous leishmaniasis.

METHODS

The in vitro activity of fifty-five compounds was evaluated against the intracellular amastigotes of L. major, L. aethiopica, L. amazonensis, L. panamensis, L. mexicana and L. tropica. The drugs demonstrating potent activity (EC50 < 5 μM) against at least 4 of 6 species were subsequently evaluated in vivo in different L. major - BALB/c mouse models using a 5 or 10-day treatment with either the oral or topical formulations. Efficacy was expressed as lesion size (measured daily using callipers), parasite load (by quantitative PCR - DNA) and bioluminescence signal reduction relative to the untreated controls.

RESULTS

The selected drug compounds (3 nitroimidazoles, 1 benzoxaborole and 3 aminopyrazoles) showed consistent and potent activity across a range of Leishmania species that are known to cause CL with EC50 values ranging from 0.29 to 18.3 μM. In all cases, this potent in vitro antileishmanial activity translated into high levels of efficacy with a linear dose-response against murine CL. When administered at 50 mg/kg/day, DNDI-0690 (nitroimidazole), DNDI-1047 (aminopyrazole) and DNDI-6148 (benzoxaborole) all resulted in a significant lesion size reduction (no visible nodule) and an approximate 2-log-fold reduction of the parasite load as measured by qPCR compared to the untreated control.

CONCLUSIONS

The lead compounds DNDI-0690, DNDI-1047 and DNDI-6148 showed excellent activity across a range of Leishmania species in vitro and against L. major in mice. These compounds offer novel potential drugs for the treatment of CL.

Pharmacokinetics and Pharmacodynamics of the Nitroimidazole DNDI-0690 in Mouse Models of Cutaneous Leishmaniasis

The nitroimidazole DNDI-0690 is a clinical drug candidate for visceral leishmaniasis (VL) that also shows potent in vitro and in vivo activity against cutaneous leishmaniasis (CL). To support further development of this compound into a patient-friendly oral or topical formulation for the treatment of CL, we investigated the free drug exposure at the dermal site of infection and subsequent elimination of the causative Leishmania pathogen. This study evaluates the pharmacokinetics (PK) and pharmacodynamics (PD) of DNDI-0690 in mouse models of CL. Skin microdialysis and Franz diffusion cell permeation studies revealed that DNDI-0690 permeated poorly (<1%) into the skin lesion upon topical drug application (0.063% [wt/vol], 30 μl). In contrast, a single oral dose of 50 mg/kg of body weight resulted in the rapid and nearly complete distribution of protein-unbound DNDI-0690 from the plasma into the infected dermis (ratio of the area under the curve [0 to 6 h] of the free DNDI-0690 concentration in skin tissue to blood [fAUC_{0-6 h, skin tissue}/fAUC_{0-6 h, blood}] is greater than 80%). Based on in vivo bioluminescence imaging, two doses of 50 mg/kg DNDI-0690 were sufficient to reduce the Leishmania mexicana parasite load by 100-fold, while 6 such doses were needed to achieve similar killing of L. major; this was confirmed by quantitative PCR. The combination of rapid accumulation and potent activity in the Leishmania-infected dermis indicates the potential of DNDI-0690 as a novel oral treatment for CL.

Route map for the discovery and pre-clinical development of new drugs and treatments for cutaneous leishmaniasis

Although there have been significant advances in the treatment of visceral leishmaniasis (VL) and several novel compounds are currently in pre-clinical and clinical development for this manifestation of leishmaniasis, there have been limited advances in drug research and development (R & D) for cutaneous leishmaniasis (CL). Here we review the need for new treatments for CL, describe in vitro and in vivo assays, models and approaches taken over the past decade to establish a pathway for the discovery, and pre-clinical development of new drugs for CL. These recent advances include novel mouse models of infection using bioluminescent Leishmania, the introduction of PK/PD approaches to skin infection, and defined pre-clinical candidate profiles.

Current and promising novel drug candidates against visceral leishmaniasis

Leishmaniasis is a group of zoonotic diseases caused by a trypanosomatid parasite mostly in impoverished populations of low-income countries. In their different forms, leishmaniasis is prevalent in more than 98 countries all over the world and approximately 360-million people are at risk. Since no vaccine is currently available to prevent any form of the disease, the control strategy of leishmaniasis mainly relies on early case detection followed by adequate pharmacological treatment that may improve the prognosis and can reduce transmission. A handful of compounds and formulations are available for the treatment of leishmaniasis in humans, but only few of them are currently in use since most of these agents are associated with toxicity problems such as nephrotoxicity and cardiotoxicity in addition to resistance problems. In recent decades, very few novel drugs, new formulations of standard drugs or combinations of them have been approved against leishmaniasis. This review highlights the current drugs and combinations that are used medical practice and recent advances in new treatments against leishmaniasis that were pointed out in the recent 2nd Conference, Global Challenges in Neglected Tropical Diseases, held in San Juan, Puerto Rico in June 2018, emphasizing the plethora of new families of molecules that are bridging the gap between preclinical and first-in-man trials in next future.

Walking a tightrope: drug discovery in visceral leishmaniasis

The current commitment of the pharma industry, nongovernmental organizations and academia to find better treatments against neglected tropical diseases should end decades of challenge caused by these global scourges. The initial result of these efforts has been the introduction of enhanced combinations of drugs, currently in clinical use, or formulations thereof. Phenotypic screening based on intracellular parasite infections has been revealed as the first key tool of antileishmanial drug discovery, because most first-in-class drugs entering Phase I trials were discovered this way. The professional commitment among stakeholders has enabled the availability of a plethora of new chemical entities that fit the target product profile for these diseases. However, the rate of hit discovery in leishmaniasis is far behind that for other neglected diseases. This review defends the need to develop new screening methods that consider the part played not only by intracellular parasites but also by the host's immune system to generate disease-relevant assays and improve clinical outcomes.

Oral activity of the antimalarial endoperoxide 6-(1,2,6,7-tetraoxaspiro[7.11]nonadec-4-yl)hexan-1-ol (N-251) against Leishmania donovani complex

Visceral leishmaniasis (VL) is a major problem worldwide and causes significant morbidity and mortality. Existing drugs against VL have limitations, including their invasive means of administration long duration of treatment regimens. There are also concerns regarding increasing treatment relapses as well as the identification of resistant clinical strains with the use of miltefosine, the sole oral drug for VL. There is, therefore, an urgent need for new alternative oral drugs for VL. In the present study, we show the leishmanicidal effect of a novel, oral antimalarial endoperoxide N-251. In our In vitro studies, N-251 selectively and specifically killed Leishmania donovani D10 amastigotes with no accompanying toxicity toward the host cells. In addition, N-251 exhibited comparable activities against promastigotes of L. donovani D10, as well as other L. donovani complex parasites, suggesting a wide spectrum of activity. Furthermore, even after a progressive infection was established in mice, N-251 significantly eliminated amastigotes when administered orally. Finally, N-251 suppressed granuloma formation in mice liver through parasite death. These findings indicate the therapeutic effect of N-251 as an oral drug, hence suggest N-251 to be a promising lead compound for the development of a new oral chemotherapy against VL.

Molecular Basis of the Leishmanicidal Activity of the Antidepressant Sertraline as a Drug Repurposing Candidate

Drug repurposing affords the implementation of new treatments at a moderate cost and under a faster time-scale. Most of the clinical drugs against Leishmania share this origin. The antidepressant sertraline has been successfully assayed in a murine model of visceral leishmaniasis. Nevertheless, sertraline targets in Leishmania were poorly defined. In order to get a detailed insight into the leishmanicidal mechanism of sertraline on Leishmania infantum, unbiased multiplatform metabolomics and transmission electron microscopy were combined with a focused insight into the sertraline effects on the bioenergetics metabolism of the parasite. Sertraline induced respiration uncoupling, a significant decrease of intracellular ATP level, and oxidative stress in L. infantum promastigotes. Metabolomics evidenced an extended metabolic disarray caused by sertraline. This encompasses a remarkable variation of the levels of thiol-redox and polyamine biosynthetic intermediates, as well as a shortage of intracellular amino acids used as metabolic fuel by Leishmania Sertraline killed Leishmania through a multitarget mechanism of action, tackling essential metabolic pathways of the parasite. As such, sertraline is a valuable candidate for visceral leishmaniasis treatment under a drug repurposing strategy.

Development of NanoLuc-PEST expressing Leishmania mexicana as a new drug discovery tool for axenic- and intramacrophage-based assays

The protozoan parasite Leishmania causes leishmaniasis; a spectrum of diseases of which there are an estimated 1 million new cases each year. Current treatments are toxic, expensive, difficult to administer, and resistance to them is emerging. New therapeutics are urgently needed, however, screening the infective amastigote form of the parasite is challenging. Only certain species can be differentiated into axenic amastigotes, and compound activity against these does not always correlate with efficacy against the parasite in its intracellular niche. Methods used to assess compound efficacy on intracellular amastigotes often rely on microscopy-based assays. These are laborious, require specialist equipment and can only determine parasite burden, not parasite viability. We have addressed this clear need in the anti-leishmanial drug discovery process by producing a transgenic L. mexicana cell line that expresses the luciferase NanoLuc-PEST. We tested the sensitivity and versatility of this transgenic strain, in comparison with strains expressing NanoLuc and the red-shifted firefly luciferase. We then compared the NanoLuc-PEST luciferase to the current methods in both axenic and intramacrophage amastigotes following treatment with a supralethal dose of Amphotericin B. NanoLuc-PEST was a more dynamic indicator of cell viability due to its high turnover rate and high signal:background ratio. This, coupled with its sensitivity in the intramacrophage assay, led us to validate the NanoLuc-PEST expressing cell line using the MMV Pathogen Box in a two-step process: i) identify hits against axenic amastigotes, ii) screen these hits using our bioluminescence-based intramacrophage assay. The data obtained from this highlights the potential of compounds active against M. tuberculosis to be re-purposed for use against Leishmania. Our transgenic L. mexicana cell line is therefore a highly sensitive and dynamic system suitable for Leishmania drug discovery in axenic and intramacrophage amastigote models.

The protozoan parasite Leishmania causes a spectrum of diseases collectively known as leishmaniasis. The parasite is transmitted to humans by the bite of its vector, the sand fly, following which the parasite invades host white blood cells, particularly macrophages. Leishmaniasis is classified as a neglected tropical disease, and is endemic in 97 countries. Symptoms of the disease depend on the species of Leishmania. These include skin lesions, destruction of the mucosal membranes, and the visceral form which is usually fatal if untreated. Current therapeutic options for leishmaniasis have a number of associated problems that include toxicity, the development of drug resistance and poor patient compliance due to lengthy and painful treatment regimens. New therapeutics are therefore urgently needed. The ability to screen potential drug candidates requires robust screening assays. Currently, screening the intracellular parasite relies on microscopy-based techniques that require expensive equipment, are time consuming and only detect parasite burden, not viability. By using a transgenic cell line that expresses the NanoLuc-PEST luciferase, we show that we have a parasite-specific viability marker that can be used to measure the efficacy of compounds against the intracellular parasite. We validate the potential of this cell line by screening the MMV Pathogen Box.