Characterization of a Giardia lamblia WB C6 clone resistant to the isoflavone formononetin

Characterization of Metronidazole-Resistant Giardia intestinalis Lines by Comparative Transcriptomics and Proteomics

Metronidazole (MTZ) is a clinically important antimicrobial agent that is active against both bacterial and protozoan organisms. MTZ has been used extensively for more than 60 years and until now resistance has been rare. However, a recent and dramatic increase in the number of MTZ resistant bacteria and protozoa is of great concern since there are few alternative drugs with a similarly broad activity spectrum. To identify key factors and mechanisms underlying MTZ resistance, we utilized the protozoan parasite Giardia intestinalis, which is commonly treated with MTZ. We characterized two in vitro selected, metronidazole resistant parasite lines, as well as one revertant, by analyzing fitness aspects associated with increased drug resistance and transcriptomes and proteomes. We also conducted a meta-analysis using already existing data from additional resistant G. intestinalis isolates. The combined data suggest that in vitro generated MTZ resistance has a substantial fitness cost to the parasite, which may partly explain why resistance is not widespread despite decades of heavy use. Mechanistically, MTZ resistance in Giardia is multifactorial and associated with complex changes, yet a core set of pathways involving oxidoreductases, oxidative stress responses and DNA repair proteins, is central to MTZ resistance in both bacteria and protozoa.

Antimicrobial resistance of the enteric protozoon Giardia duodenalis - A narrative review

Introduction

As therapy-refractory giardiasis is an emerging health issue, this review aimed at summarizing mechanisms of reduced antimicrobial susceptibility in Giardia duodenalis and strategies to overcome this problem.

Methods

A narrative review on antimicrobial resistance in G. duodenalis was based upon a selective literature research.

Results

Failed therapeutic success has been observed for all standard therapies of giardiasis comprising nitroimidazoles like metronidazole or tinidazole as first line substances but also benznidazoles like albendazole and mebendazole, the nitrofuran furazolidone, the thiazolide nitazoxanide, and the aminoglycoside paromomycin. Multicausality of the resistance phenotypes has been described, with differentiated gene expression due to epigenetic and post-translational modifications playing a considerable bigger role than mutational base exchanges in the parasite DNA. Standardized resistance testing algorithms are not available and clinical evidence for salvage therapies is scarce in spite of research efforts targeting new giardicidal drugs.

Conclusion

In case of therapeutic failure of first line nitroimidazoles, salvage strategies including various options for combination therapy exist in spite of limited evidence and lacking routine diagnostic-compatible assays for antimicrobial susceptibility testing in G. duodenalis. Sufficiently powered clinical and diagnostic studies are needed to overcome both the lacking evidence regarding salvage therapy and the diagnostic neglect of antimicrobial resistance.

Drug resistance in Giardia: Mechanisms and alternative treatments for Giardiasis

The use of chemotherapeutic drugs is the main resource against clinical giardiasis due to the lack of approved vaccines. Resistance of G. duodenalis to the most used drugs to treat giardiasis, metronidazole and albendazole, is a clinical issue of growing concern and yet unknown impact, respectively. In the search of new drugs, the completion of the Giardia genome project and the use of biochemical, molecular and bioinformatics tools allowed the identification of ligands/inhibitors for about one tenth of ≈150 potential drug targets in this parasite. Further, the synthesis of second generation nitroimidazoles and benzimidazoles along with high-throughput technologies have allowed not only to define overall mechanisms of resistance to metronidazole but to screen libraries of repurposed drugs and new pharmacophores, thereby increasing the known arsenal of anti-giardial compounds to some hundreds, with most demonstrating activity against metronidazole or albendazole-resistant Giardia. In particular, cysteine-modifying agents which include omeprazole, disulfiram, allicin and auranofin outstand due to their pleiotropic activity based on the extensive repertoire of thiol-containing proteins and the microaerophilic metabolism of this parasite. Other promising agents derived from higher organisms including phytochemicals, lactoferrin and propolis as well as probiotic bacteria/fungi have also demonstrated significant potential for therapeutic and prophylactic purposes in giardiasis. In this context the present chapter offers a comprehensive review of the current knowledge, including commonly prescribed drugs, causes of therapeutic failures, drug resistance mechanisms, strategies for the discovery of new agents and alternative drug therapies.

Novel giardicidal compounds bearing proton pump inhibitor scaffold proceeding through triosephosphate isomerase inactivation

Giardiasis is a worldwide parasitic disease that affects mainly children and immunosuppressed people. Side effects and the emergence of resistance over current used drugs make imperative looking for new antiparasitics through discovering of new biological targets and designing of novel drugs. Recently, it has determined that gastric proton-pump inhibitors (PPI) have anti-giardiasic activity. The glycolytic enzyme, triosephosphate isomerase (GlTIM), is one of its potential targets. Therefore, we employed the scaffold of PPI to design new compounds aimed to increase their antigiardial capacity by inactivating GlTIM. Here we demonstrated that two novel PPI-derivatives (BHO2 and BHO3), have better anti-giardiasic activity than omeprazole in concentrations around 120–130 µM, without cytotoxic effect on mammal cell cultures. The derivatives inactivated GlTIM through the chemical modification of Cys222 promoting local structural changes in the enzyme. Furthermore, derivatives forms adducts linked to Cys residues through a C-S bond. We demonstrated that PPI can be used as scaffolds to design better antiparasitic molecules; we also are proposing a molecular mechanism of reaction for these novel derivatives.

Drug Resistance in the Microaerophilic Parasite Giardia lamblia

The microaerophilic parasite Giardia lamblia is a causative agent of dysentery affecting hundreds of millions of people around the globe every year. The symptoms of the disease, commonly referred to as giardiasis, are diarrhea, nausea, and malabsorption. Treatment of giardiasis is exclusively based on chemotherapy with antigiardial drugs, including metronidazole, albendazole, and nitazoxanide. In this review, all drugs currently used in the treatment of Giardia infections are discussed with a special emphasis on treatment failure and drug resistance.

High sensitivity of Giardia duodenalis to tetrahydrolipstatin (orlistat) in vitro

Giardiasis, a gastrointestinal disease caused by Giardia duodenalis, is currently treated mainly with nitroimidazoles, primarily metronidazole (MTZ). Treatment failure rates of up to 20 percent reflect the compelling need for alternative treatment options. Here, we investigated whether orlistat, a drug approved to treat obesity, represents a potential therapeutic agent against giardiasis. We compared the growth inhibitory effects of orlistat and MTZ on a long-term in vitro culture adapted G. duodenalis strain, WB-C6, and on a new isolate, 14-03/F7, from a patient refractory to MTZ treatment using a resazurin assay. The giardiacidal concentration of the drugs and their combined in vitro efficacy was determined by median-effect analysis. Morphological changes after treatment were analysed by light and electron microscopy. Orlistat inhibited the in vitro growth of G. duodenalis at low micromolar concentrations, with isolate 14-03/F7 (IC50_24h = 2.8 µM) being more sensitive than WB-C6 (IC50_24h = 6.2 µM). The effect was significantly more potent compared to MTZ (IC50_24h = 4.3 µM and 11.0 µM, respectively) and led to specific undulated morphological alterations on the parasite surface. The giardiacidal concentration of orlistat was >14 µM for 14-03/F7 and >43 µM for WB-C6, respectively. Importantly, the combination of both drugs revealed no interaction on their inhibitory effects. We demonstrate that orlistat is a potent inhibitor of G. duodenalis growth in vitro and kills parasites at concentrations achievable in the gut by approved treatment regimens for obesity. We therefore propose to investigate orlistat in controlled clinical studies as a new drug in giardiasis.

New approaches for the identification of drug targets in protozoan parasites

Antiparasitic chemotherapy is an important issue for drug development. Traditionally, novel compounds with antiprotozoan activities have been identified by screening of compound libraries in high-throughput systems. More recently developed approaches employ target-based drug design supported by genomics and proteomics of protozoan parasites. In this chapter, the drug targets in protozoan parasites are reviewed. The gene-expression machinery has been among the first targets for antiparasitic drugs and is still under investigation as a target for novel compounds. Other targets include cytoskeletal proteins, proteins involved in intracellular signaling, membranes, and enzymes participating in intermediary metabolism. In apicomplexan parasites, the apicoplast is a suitable target for established and novel drugs. Some drugs act on multiple subcellular targets. Drugs with nitro groups generate free radicals under anaerobic growth conditions, and drugs with peroxide groups generate radicals under aerobic growth conditions, both affecting multiple cellular pathways. Mefloquine and thiazolides are presented as examples for antiprotozoan compounds with multiple (side) effects. The classic approach of drug discovery employing high-throughput physiological screenings followed by identification of drug targets has yielded the mainstream of current antiprotozoal drugs. Target-based drug design supported by genomics and proteomics of protozoan parasites has not produced any antiparasitic drug so far. The reason for this is discussed and a synthesis of both methods is proposed.

New approaches to the treatment of giardiasis

PURPOSE OF REVIEW

Giardiasis is one of the most common causes of diarrheal disease worldwide, yet existing antimicrobial therapies are not always effective and drug resistance occurs in vivo and in vitro. The review focuses on recent advances in the development of new antigiardial drug candidates.

RECENT FINDINGS

Modification of existing drug leads is a major strategy to develop new high-potency drugs. Complex derivatives of 5-nitroimidazole, the core structure of the most commonly used antigiardial drug, metronidazole, have shown significantly improved activities against Giardia and the ability to overcome metronidazole resistance. Derivatives of benzimidazole, the structural core of the effective antigiardial albendazole, are also exhibiting promising new activities. Beyond lead modifications, several new classes of antigiardial drug candidates have recently been identified by high-throughput screening of large compound libraries, and first efforts have been reported on the development of drugs tailored to known molecular targets in Giardia.

SUMMARY

The pipeline of new antigiardial drug candidates has significantly expanded over the last few years, but this expansion has so far not been accompanied by demonstration of efficacy in animal models or by a clear understanding of the action mechanisms, particularly in regard to new nitro antimicrobials. Many challenges are still to be expected before clinical utility of new antigiardial drugs can be established.

Rapid detachment of Giardia lamblia trophozoites as a mechanism of antimicrobial action of the isoflavone formononetin

OBJECTIVES

Attachment to the small intestinal mucosa is crucial for initiating and maintaining Giardia infection. We tested the effect of isoflavones on Giardia attachment.

METHODS

We evaluated the effect of formononetin on trophozoite attachment to glass, to intestinal epithelial cell layers in vitro and to murine small intestinal explants, and on the intestinal load in mice.

RESULTS

We found that the isoflavone formononetin inhibits both attachment and flagellar motility within minutes and reduces the trophozoite load of Giardia in mice within 1.5 h after treatment.

CONCLUSIONS

The antigiardial activity of formononetin is at least partially due to its capacity to rapidly detach trophozoites.

Stable expression of Escherichia coli beta-glucuronidase A (GusA) in Giardia lamblia: application to high-throughput drug susceptibility testing

OBJECTIVES

In order to create a suitable model for high-throughput drug screening, a Giardia lamblia WB C6 strain expressing Escherichia coli glucuronidase A (GusA) was created and tested with respect to susceptibility to the anti-giardial drugs nitazoxanide and metronidazole.

METHODS

GusA, a well-established reporter gene in other systems, was cloned into the vector pPacVInteg allowing stable expression in G. lamblia under control of the promoter from the glutamate dehydrogenase (gdh) gene. The resulting transgenic strain was compared with the wild-type strain in a vitality assay, characterized with respect to susceptibility to nitazoxanide, metronidazole and -- as assessed in a 96-well plate format -- to a panel of 15 other compounds to be tested for anti-giardial activity.

RESULTS

GusA was stably expressed in G. lamblia. Using a simple glucuronidase assay protocol, drug efficacy tests yielded results similar to those from cell counting.

CONCLUSIONS

G. lamblia WB C6 GusA is a suitable tool for high-throughput anti-giardial drug screening.

Vaccination of mice with recombinant NcROP2 antigen reduces mortality and cerebral infection in mice infected with Neospora caninum tachyzoites

Rhoptry antigens are involved in a variety of cellular functions related to host cell invasion, formation of the parasitophorous vacuole and parasite-host cell interplay. The cDNA sequence of one of these antigens, NcROP2 was identified from Neospora caninum expressed sequence tags (ESTs), amplified by reverse transcription-PCR, expressed in Escherichia coli as a (His)(6)-tagged recombinant protein (recNcROP2) and purified over Ni(2+)-affinity chromatography. Both recNcROP2 and antibodies directed against recNcROP2 had a negative impact on N. caninum tachyzoite host cell invasion in vitro, indicating that this protein participates in the host cell entry process. Subsequently, the protective efficacy of NcROP2 as a potential vaccine candidate was evaluated in a C57BL/6 mouse cerebral disease model. Mice were vaccinated three times at 2-week intervals with recNcROP2 emulsified either in Freund's incomplete adjuvants (FIA) or saponin, and control groups were treated with adjuvants alone (adjuvants control) or PBS (infection control). Subsequently, mice were challenged with 2x10(6)N. caninum tachyzoites. Nine mice, all belonging to the infection control or adjuvants control groups, exhibited clinical signs of cerebral neosporosis and succumbed to infection, whilst no clinical signs were noted for recNcROP2-vaccinated mice. For all other animals, the experiment was terminated 35 days p.i. Cerebral parasite burdens were assessed by quantitative PCR in all mice, and were revealed to be significantly reduced in the recNcROP2-vaccinated mice. ELISA of sera revealed IgG1 to be elevated in recNcROP2-saponin vaccinated mice, whilst IgG2a was higher in recNcROP2-FIA vaccinated animals. This shows that, depending on the adjuvants used, vaccination with NcROP2 induces a protective Th-1- or Th-2-biased immune response against experimental N. caninum infection.