Antitrypanosomal activity of fluoroquinolones

In vitro trypanocidal activities and structure-activity relationships of ciprofloxacin analogs

Tropical diseases, such as African trypanosomiasis, by their nature and prevalence lack the necessary urgency regarding drug development, despite the increasing need for novel, structurally diverse antitrypanosomal drugs, using different mechanisms of action that would improve drug efficacy and safety. Traditionally antibacterial agents, the fluoroquinolones, reportedly possess in vitro trypanocidal activities against Trypanosoma brucei organisms. During our research, the fluroquinolone, ciprofloxacin (1), and its analogs (2-24) were tested against bloodstream forms of T. brucei brucei, T. b. gambiense, T. b. rhodesiense, T. evansi, T. equiperdum, and T. congolense and Madin-Darby bovine kidney cells (cytotoxicity). Ciprofloxacin [CPX (1)] demonstrated selective trypanocidal activity against T. congolense (IC₅₀ 7.79 µM; SI 39.6), whereas the CPX derivatives (2-10) showed weak selective activity (25 < IC₅₀ < 65 µM; 2 < SI < 4). Selectivity and activity of the CPX and 1,2,3-triazole (TZ) hybrids (11-24) were governed by their chemical functionality at C-3 (carboxylic acid, or 4-methylpiperazinyl amide) and their electronic effect (electron-donating or electron-withdrawing para-benzyl substituent), respectively. Trypanocidal hits in the micromolar range were identified against bloodstream forms of T. congolense [CPX (1); CPX amide derivatives 18: IC₅₀ 8.95 µM; SI 16.84; 22: IC₅₀ 5.42 µM; SI 25.2] and against T. brucei rhodesiense (CPX acid derivative 13: IC₅₀ 4.51 µM; SI 10.2), demonstrating more selectivity toward trypanosomes than mammalian cells. Hence, the trypanocidal hit compound 22 may be optimized by retaining the 4-methylpiperazine amide functional group (C-3) and the TZ moiety at position N-15 and introducing other electron-withdrawing ortho-, meta-, and/or para-substituents on the aryl ring in an effort to improve the pharmacokinetic properties and increase the trypanocidal activity. Structure-activity relationships of ciprofloxacin-1,2,3-triazole hybrids were governed by the chemical functionality at C-3 and electronic effect.

Synthesis and in vitro antitrypanosomal evaluation of novel 6-heteroarylidene-substituted quinolone derivatives

Human African trypanosomiasis is a vector-borne tropical disease of African origin. Presently, due to human migration and climate change, the disease might present global health and economic burdens as current chemotherapy of trypanosomiasis remains a challenge due to limited existing drugs, which are of poor efficacy, cause severe adverse events and are very costly. Recently, Beteck and co-workers identified a small library of 1,3,6-substituted non-fluoroquinolones that showed moderate to weak trypanocidal activity without cytotoxic effects. The current study further explored SARs of the quinolone scaffold in search for more potent trypanocidal agents. Fifteen novel quinolone derivatives bearing a heteroarylidene moiety at positon-6 and varied chemical entities at positions -1 and -3 of the quinolone scaffold were synthesized and evaluated in vitro for antitrypanosomal activity. The compounds exhibit exceptionally good antitrypanosomal activity with IC₅₀ values in the low-micromolar to sub-micromolar range (0.08-15.26 μM), with compound 6d being the most active having an IC₅₀ value of 80 nM against T.b. brucei. Compounds in this study generally have molecular weight less than 600Da, ClogP value of 2-4 and a BBB score of 1-5, hence they could be potentially effective against both stages of trypanosomiasis.

Repositioned Drugs for Chagas Disease Unveiled via Structure-Based Drug Repositioning

Chagas disease, caused by the parasite Trypanosoma cruzi, affects millions of people in South America. The current treatments are limited, have severe side effects, and are only partially effective. Drug repositioning, defined as finding new indications for already approved drugs, has the potential to provide new therapeutic options for Chagas. In this work, we conducted a structure-based drug repositioning approach with over 130,000 3D protein structures to identify drugs that bind therapeutic Chagas targets and thus represent potential new Chagas treatments. The screening yielded over 500 molecules as hits, out of which 38 drugs were prioritized following a rigorous filtering process. About half of the latter were already known to have trypanocidal activity, while the others are novel to Chagas disease. Three of the new drug candidates—ciprofloxacin, naproxen, and folic acid—showed a growth inhibitory activity in the micromolar range when tested ex vivo on T. cruzi trypomastigotes, validating the prediction. We show that our drug repositioning approach is able to pinpoint relevant drug candidates at a fraction of the time and cost of a conventional screening. Furthermore, our results demonstrate the power and potential of structure-based drug repositioning in the context of neglected tropical diseases where the pharmaceutical industry has little financial interest in the development of new drugs.

Novel lead compounds in pre-clinical development against African sleeping sickness

Human African trypanosomiasis (HAT), also known as African sleeping sickness, is caused by parasitic protozoa of the genus Trypanosoma. As the disease progresses, the parasites cross the blood brain barrier and are lethal for the patients if the disease is left untreated. Current therapies suffer from several drawbacks due to e.g. toxicity of the respective compounds or resistance to approved antitrypanosomal drugs. In this review, the different strategies of drug development against HAT are considered, namely the target-based approach, the phenotypic high throughput screening and the drug repurposing strategy. The most promising compounds emerging from these approaches entering an in vivo evaluation are mentioned herein. Of note, it may turn out to be difficult to confirm in vitro activity in an animal model of infection; however, possible reasons for the missing efficacy in unsuccessful in vivo studies are discussed.

Nuclear DNA replication and repair in parasites of the genus Leishmania: Exploiting differences to develop innovative therapeutic approaches

Leishmaniasis is a common tropical disease that affects mainly poor people in underdeveloped and developing countries. This largely neglected infection is caused by Leishmania spp, a parasite from the Trypanosomatidae family. This parasitic disease has different clinical manifestations, ranging from localized cutaneous to more harmful visceral forms. The main limitations of the current treatments are their high cost, toxicity, lack of specificity, and long duration. Efforts to improve treatments are necessary to deal with this infectious disease. Many approved drugs to combat diseases as diverse as cancer, bacterial, or viral infections take advantage of specific features of the causing agent or of the disease. Recent evidence indicates that the specific characteristics of the Trypanosomatidae replication and repair machineries could be used as possible targets for the development of new treatments. Here, we review in detail the molecular mechanisms of DNA replication and repair regulation in trypanosomatids of the genus Leishmania and the drugs that could be useful against this disease.

Synthesis and biological evaluation of quinoxaline di-N-oxide derivatives with in vitro trypanocidal activity

We report the synthesis and in vitro activity against Trypanosoma cruzi epimastigotes of 15 novel quinoxaline derivatives. Ten of the derivatives presented IC50 values lower than the reference drugs Nfx and Bzn; four of them standed out with IC50 values lower than 1.5 μM. Moreover, unspecific cytotoxicity and genotoxicity studies are also reported. Compound 14 showed a SI higher than 24, whereas compound 10 was the only one that was negative in the genotoxicity screening.

Antiviral, antifungal, and antiparasitic activities of fluoroquinolones optimized for treatment of bacterial infections: a puzzling paradox or a logical consequence of their mode of action?

This review summarizes evidence that commercially available fluoroquinolones used for the treatment of bacterial infections are active against other non-bacterial infectious agents as well. Any of these fluoroquinolones exerts, in parallel to its antibacterial action, antiviral, antifungal, and antiparasitic actions at clinically achievable concentrations. This broad range of anti-infective activities is due to one common mode of action, i.e., the inhibition of type II topoisomerases or inhibition of viral helicases, thus maintaining the selective toxicity of fluoroquinolones inhibiting microbial topoisomerases at low concentrations but mammalian topoisomerases at much higher concentrations. Evidence suggests that standard doses of the fluoroquinolones studied are clinically effective against viral and parasitic infections, whereas higher doses administered topically were active against Candida spp. causing ophthalmological infections. Well-designed clinical studies should be performed to substantiate these findings.

1,2-substituted 4-(1H)-quinolones: synthesis, antimalarial and antitrypanosomal activities in vitro

A diverse array of 4-(1H)-quinolone derivatives bearing substituents at positions 1 and 2 were synthesized and evaluated for antiprotozoal activities against Plasmodium falciparum and Trypanosoma brucei rhodesiense, and cytotoxicity against L-6 cells in vitro. Furthermore, selectivity indices were also determined for both parasites. All compounds tested showed antimalarial activity at low micromolar concentrations, with varied degrees of selectivity against L-6 cells. Compound 5a was found to be the most active against P. falciparum, with an IC50 value of 90 nM and good selectivity for the malarial parasite compared to the L-6 cells. Compound 10a, on the other hand, showed a strong antitrypanosomal effect with an IC50 value of 1.25 µM. In this study side chain diversity was explored by varying the side chain length and substitution pattern on the aliphatic group at position-2 and a structure-antiprotozoal activity study revealed that the aromatic ring introduced at C-2 contributed significantly to the antiprotozoal activities.

Trypanosomatids topoisomerase re-visited. New structural findings and role in drug discovery

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There is an urgent need of new treatments against trypanosomatids-borne diseases.

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DNA topoisomerases are pointed as potential drug targets against unicellular parasites.

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Trypanosomatids have a full set of DNA topoisomerases in both nucleus and kinetoplast.

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TopII and TopIII are located in the kinetoplast and fully involved in kDNA replication.

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Tritryps TopIB differ in structure from mammalian’s pointing to an attractive target.

The Trypanosomatidae family, composed of unicellular parasites, causes severe vector-borne diseases that afflict human populations worldwide. Chagas disease, sleeping sickness, as well as different sorts of leishmaniases are amongst the most important infectious diseases produced by Trypanosoma cruzi, Trypanosoma brucei and Leishmania spp., respectively. All these infections are closely related to weak health care services in low-income populations of less developed and least economically developed countries. Search for new therapeutic targets in order to hit these pathogens is of paramount priority, as no effective vaccine is currently in use against any of these parasites. Furthermore, present-day chemotherapy comprises old-fashioned drugs full of important side effects. Besides, they are prone to produce tolerance and resistance as a consequence of their continuous use for decades. DNA topoisomerases (Top) are ubiquitous enzymes responsible for solving the torsional tensions caused during replication and transcription processes, as well as in maintaining genomic stability during DNA recombination. As the inhibition of these enzymes produces cell arrest and triggers cell death, Top inhibitors are among the most effective and most widely used drugs in both cancer and antibacterial therapies. Top relaxation and decatenation activities, which are based on a common nicking–closing cycle involving one or both DNA strands, have been pointed as a promising drug target. Specific inhibitors that bind to the interface of DNA-Top complexes can stabilize Top-mediated transient DNA breaks. In addition, important structural differences have been found between Tops from the Trypanosomatidae family members and Tops from the host. Such dissimilarities make these proteins very interesting for drug design and molecular intervention. The present review is a critical update of the last findings regarding trypanosomatid’s Tops, their new structural features, their involvement both in the physiology and virulence of these parasites, as well as their use as promising targets for drug discovery.

Selection of molecular targets for drug development against trypanosomatids

Trypanosomatid parasites are a group of flagellated protozoa that includes the genera Leishmania and Trypanosoma, which are the causative agents of diseases (leishmaniases, sleeping sickness and Chagas disease) that cause considerable morbidity and mortality, affecting more than 27 million people worldwide. Today no effective vaccines for the prevention of these diseases exist, whereas current chemotherapy is ineffective, mainly due to toxic side effects of current drugs and to the emergence of drug resistance and lack of cost effectiveness. For these reasons, rational drug design and the search of good candidate drug targets is of prime importance. The search for drug targets requires a multidisciplinary approach. To this end, the completion of the genome project of many trypanosomatid species gives a vast amount of new information that can be exploited for the identification of good drug candidates with a prediction of "druggability" and divergence from mammalian host proteins. In addition, an important aspect in the search for good drug targets is the "target identification" and evaluation in a biological pathway, as well as the essentiality of the gene in the mammalian stage of the parasite, which is provided by basic research and genetic and proteomic approaches. In this chapter we will discuss how these bioinformatic tools and experimental evaluations can be integrated for the selection of candidate drug targets, and give examples of metabolic and signaling pathways in the parasitic protozoa that can be exploited for rational drug design.

Synthesis and structure-activity relationships of new quinolone-type molecules against Trypanosoma brucei

Human African trypanosomiasis (HAT) or sleeping sickness is caused by two subspecies of Trypanosoma brucei , Trypanosoma brucei gambiense , and Trypanosoma brucei rhodesiense and is one of Africa's old plagues. It causes a huge number of infections and cases of death per year because, apart from limited access to health services, only inefficient chemotherapy is available. Since it was reported that quinolones such as ciprofloxacin show antitrypanosomal activity, a novel quinolone-type library was synthesized and tested. The biological evaluation illustrated that 4-quinolones with a benzylamide function in position 3 and cyclic or acyclic amines in position 7 exhibit high antitrypanosomal activity. Structure-activity relationships (SAR) are established to identify essential structural elements. This analysis led to lead structure 29, which exhibits promising in vitro activity against T. b. brucei (IC(50) = 47 nM) and T. b. rhodesiense (IC(50) = 9 nM) combined with low cytotoxicity against macrophages J774.1. Screening for morphological changes of trypanosomes treated with compounds 19 and 29 suggested differences in the morphology of mitochondria of treated cells compared to those of untreated cells. Segregation of the kinetoplast is hampered in trypanosomes treated with these compounds; however, topoisomerase II is probably not the main drug target.

State of the art in African trypanosome drug discovery

African sleeping sickness is endemic in sub-Saharan Africa where the WHO estimates that 60 million people are at risk for the disease. Human African trypanosomiasis (HAT) is 100% fatal if untreated and the current drug therapies have significant limitations due to toxicity and difficult treatment regimes. No new chemical agents have been approved since eflornithine in 1990. The pentamidine analog DB289, which was in late stage clinical trials for the treatment of early stage HAT recently failed due to toxicity issues. A new protocol for the treatment of late-stage T. brucei gambiense that uses combination nifurtomox/eflornithine (NECT) was recently shown to have better safety and efficacy than eflornithine alone, while being easier to administer. This breakthrough represents the only new therapy for HAT since the approval of eflornithine. A number of research programs are on going to exploit the unusual biochemical pathways in the parasite to identify new targets for target based drug discovery programs. HTS efforts are also underway to discover new chemical entities through whole organism screening approaches. A number of inhibitors with anti-trypanosomal activity have been identified by both approaches, but none of the programs are yet at the stage of identifying a preclinical candidate. This dire situation underscores the need for continued effort to identify new chemical agents for the treatment of HAT.

Newly identified antibacterial compounds are topoisomerase poisons in African trypanosomes

Human African trypanosomiasis, caused by the Trypanosoma brucei protozoan parasite, is fatal when left untreated. Current therapies are antiquated, and there is a need for new pharmacologic agents against T. brucei targets that have no human ortholog. Trypanosomes have a single mitochondrion with a unique mitochondrial DNA, known as kinetoplast DNA (kDNA), a topologically complex network that contains thousands of interlocking circular DNAs, termed minicircles (approximately 1 kb) and maxicircles (approximately 23 kb). Replication of kDNA depends on topoisomerases, enzymes that catalyze reactions that change DNA topology. T. brucei has an unusual type IA topoisomerase that is dedicated to kDNA metabolism. This enzyme has no ortholog in humans, and RNA interference (RNAi) studies have shown that it is essential for parasite survival, making it an ideal drug target. In a large chemical library screen, two compounds were recently identified as poisons of bacterial topoisomerase IA. We found that these compounds are trypanocidal in the low micromolar range and that they promote the formation of linearized minicircles covalently bound to protein on the 5' end, consistent with the poisoning of mitochondrial topoisomerase IA. Surprisingly, however, band depletion studies showed that it is topoisomerase IImt, and not topoisomerase IAmt, that is trapped. Both compounds are planar aromatic polycyclic structures that intercalate into and unwind DNA. These findings reinforce the utility of topoisomerase IImt as a target for development of new drugs for African sleeping sickness.

Leishmania panamensis: Comparative inhibition of nuclear DNA topoisomerase II enzymes from promastigotes and human macrophages reveals anti-parasite selectivity of fluoroquinolones, flavonoids and pentamidine

Certain model inhibitors exerted selective action against the catalytic activity of nuclear DNA topoisomerase II (TOPII) of Leishmania panamensis promastigotes. The second-generation fluoroquinolones enoxacin and ciprofloxacin exhibited extraordinarily high anti-parasite selectivity displaying 582- and 40-fold greater potencies against L. panamensis TOPII as compared with the human macrophage enzyme. The flavonoids quercetin and ellagic acid showed inverse specificities, the former being 161-fold more potent against L. panamensis TOPII, and the latter 15.7-fold more active against macrophage TOPII. The protoberberine coralyne was a potent inhibitor of both Leishmania and macrophage TOPII. Bis-benzimidazoles and the diamidine diminazene aceturate exhibited uniformly high potencies against parasite and host TOPII, but a second diamidine pentamidine showed 17.6-fold greater specificity for Leishmania TOPII. The antimonial sodium stibogluconate was an ineffective inhibitor of parasite TOPII showing 4.3-fold greater potency against the macrophage enzyme. These findings suggest that the leishmanicidal activities of certain fluoroquinolones and pentamidine may be mediated partly through TOPII inhibition.

Novel antitrypanosomal agents

Trypanosomes are the causative agents of Chagas' disease in Central and South America and sleeping sickness in sub-Saharan Africa. The current chemotherapy of the human trypanosomiases relies on only six drugs, five of which were developed > 30 years ago. In addition, these drugs display undesirable toxic side effects and the emergence of drug-resistant trypanosomes has been reported. Therefore, the development of new drugs in the treatment of Chagas' disease and sleeping sickness is urgently required. This article summarises the recent progress in identifying novel lead compounds for antitrypanosomal chemotherapy. Particular emphasis is placed on those agents showing promising, selective antitrypanosomal activity.

SELECTIVE ACTION OF FLUOROQUINOLONES AGAINST INTRACELLULAR AMASTIGOTES OF LEISHMANIA (VIANNIA) PANAMENSIS IN VITRO

We have demonstrated that fluoroquinolones, a class of antibacterial agents that act through inhibition of type II DNA topoisomerases, exert selective action against intracellular amastigotes of Leishmania (Viannia) panamensis at concentrations that are achievable in vivo. Drug cytotoxicity assays employing the luciferase reporter gene revealed that intracellular amastigotes were 6.6- to 25.9-fold more sensitive than human macrophages (P < 0.05) to second-generation fluoroquinolones in vitro. The most selective agents (enoxacin and ciprofloxacin) exhibited 2 orders of magnitude greater potency against parasites (50% effective dose [ED50] = 54.9-83.4 microM) than host cells (ED50 = 1,425-1,740 microM). Linear regression analysis of ED50 data confirmed a complete lack of correlation (r = 0.001) between the relative drug sensitivities of parasites and host cells. A potential relationship between the structures of fluoroquinolones and their relative leishmanicidal activities was observed. The key substituents of the basic pyridone beta-carboxylic acid nucleus accounting for enhanced antiparasite potency and selectivity appear to be a nitrogen at position 8 of the bicyclic nucleus (enoxacin), a cyclopropyl substituent at the R1 site (ciprofloxacin), and linkage of the R1 and X8 groups by a CH3CHO bridge to form a tricyclic compound (ofloxacin). These findings support the potential of fluoroquinolones and derivatives as novel antileishmanials and encourage their clinical evaluation.

Antimicrosporidial activity of (fluoro)quinolones in vitro and in vivo

Microsporidia are a cause of emerging and opportunistic infections in humans and animals. Although two drugs are currently being used to treat microsporidiosis, concerns exist that albendazole is only selective for inhibiting some species of microsporidia that infect mammals, and fumagillin appears to have been found to be toxic. During a limited sequence survey of the Vittaforma corneae genome, a partial gene encoding for the ParC topoisomerase IV subunit was identified. The purpose of this set of studies was to determine if fluoroquinolones, which target topoisomerase IV, exert activity against Encephalitozoon intestinalis and V. corneae in vitro, and whether these compounds could prolong survival of V. corneae-infected athymic mice. Fifteen fluoroquinolones were tested. Of these, norfloxacin and ofloxacin inhibited E. intestinalis replication by more than 70% compared with non-treated control cultures, while gatifloxacin, lomefloxacin, moxifloxacin, and nalidixic acid (sodium salt) inhibited both E. intestinalis and V. corneae by at least 60% at concentrations not toxic to the host cells. These drugs were tested in vivo also, where gatifloxacin, lomefloxacin, norfloxacin, and ofloxacin prolonged survival of V. corneae-infected athymic mice (P < 0.05), whereas moxifloxacin and nalidixic acid failed to prolong survival. Therefore, these results support continued studies for evaluating the efficacy of the fluoroquinolones for treating microsporidiosis and for characterizing the target(s) of these fluoroquinolones in the microsporidia.

Topoisomerases of kinetoplastid parasites as potential chemotherapeutic targets

The protozoan parasites Trypanosoma, Leishmania and Crithidia, which belong to the order kinetoplastidae, emerge from the most ancient eukaryotic lineages. The diversity found in the life cycle of these organisms must be directed by genetic events, wherein topoisomerases play an important role in cellular processes affecting the topology and organization of intracellular DNA. Topoisomerases are valuable as potential drug targets because they have indispensable function in cell biology. This review summarizes what is known about topoisomerase genes and proteins of kinetoplastid parasites and the roles of these enzymes as targets for therapeutic agents.

Antitrypanosomal activities of fluoroquinolones with pyrrolidinyl substitutions

Fluoroquinolones with pyrrolidinyl substitutions were tested against Trypanosoma brucei and mammalian cells. Bulky substituents at C-7 or a 1-2-bridging thiazolidine ring increased antitrypanosomal activity and selective toxicity. These compounds trap protein-DNA complexes and inhibit nucleic acid biosynthesis in trypanosomes, characteristics of topoisomerase II inhibition.

Evaluation of quinolone derivatives for antitrypanosomal activity

About 160 fluoroquinolones and derivatives were tested for antitrypanosomal activity in a drug sensitivity assay followed by fluorometric evaluation. The most active quinolone compounds had IC50 values in the range from 100 to 900 ng/ml, while several derivatives were not active at a concentration of 100 microg/ml. In a structure activity relationship study, modification of the quinolones at position R1, R2, R3 and R8 did not influence trypanocidal activity. An exchange of the fluor at position 6 may contribute to an increase in activity but does not entirely control it. Pyrrolidine substituents at position R7 generally were more active than other substituents at this position. Tetracyclic quinolone derivatives were amongst the most active compounds with IC50 values in the range of 0.3-8.8 microg/ml. The in vitro cytotoxicity on HT-29 cells was determined for active compounds with IC50 values below 1 microg/ml. In addition, six drugs with an IC50 below 1 microg/ml and a selectivity index of more than 10 were chosen for in vivo experiments. Dose escalation experiments with a maximum dose of 100 mg/kg/bid were performed in a mouse model without central nervous system involvement. For unknown reasons the in vitro effect of the drugs could not be confirmed in vivo, but the class of compound remains of interest for their mode of action, the low toxicity, pharmacological properties and the availability of a large number of synthesized compounds.

New drugs for the treatment of human African trypanosomiasis: research and development

Chemotherapy of human African trypanosomiasis is problematic because of the high frequency of severe adverse events, the long duration and high cost of treatment, and an increasing number of treatment-refractory cases. New cost-efficient, easy-to-use drugs are urgently needed. Whereas basic research on potential drug targets is anchored in academia, the complex, highly regulated and very expensive process of preclinical and clinical drug development is almost exclusively in the hands of pharmaceutical companies. Jennifer Keiser, August Stich and Christian Burri here review, from the angle of industrial drug research and development, the past ten years of research activities at different stages of the development of trypanocidal drugs, and assess future prospects. The absence of compounds in clinical development Phases I-III indicates no new drugs will become available in the next few years.

Novelties in the field of anti-infective compounds in 1999

In 1999 the number of new compounds reported in the anti-infective field decreased significantly in comparison with previous years, especially for antifungals. The reported new compounds are mainly directed against Staphylococcus aureus isolates resistant to methicillin. Few derivatives were reported in the field of anti-infectives for gram-negative bacteria. At the moment, we are in a period of discovery as we await novel compounds that could issue from new engineering.