Resistance formation to nitro drugs in Giardia lamblia: No common markers identified by comparative proteomics

Investigating Antiprotozoal Chemotherapies with Novel Proteomic Tools-Chances and Limitations: A Critical Review

Identification of drug targets and biochemical investigations on mechanisms of action are major issues in modern drug development. The present article is a critical review of the classical "one drug"-"one target" paradigm. In fact, novel methods for target deconvolution and for investigation of resistant strains based on protein mass spectrometry have shown that multiple gene products and adaptation mechanisms are involved in the responses of pathogens to xenobiotics rather than one single gene or gene product. Resistance to drugs may be linked to differential expression of other proteins than those interacting with the drug in protein binding studies and result in complex cell physiological adaptation. Consequently, the unraveling of mechanisms of action needs approaches beyond proteomics. This review is focused on protozoan pathogens. The conclusions can, however, be extended to chemotherapies against other pathogens or cancer.

Assessment of Drug Activities against Giardia Using Hyperspectral Raman Microscopy

This study demonstrates the capability of Raman microscopy for detecting structural differences in Giardia cells exposed to different drugs and incubation times. While metronidazole (MTZ) visibly affects the cells by inducing extracellular vesicle releases of toxic iron intermediates and modified triple-bond moieties, oseltamivir (OSM) alters the phenylalanine and lipid structures. Modifications in the heme protein environment and the transformation of iron from ferric to ferrous observed for both drug treatments are more notable for MTZ. Different contents and amounts of vesicle excretion are detected for 24 h or 48 h with MTZ incubation. At a shorter drug exposure, releases of altered proteins, glycogen, and phospholipids dominate. Agglomerates of transformed iron complexes from heme proteins and multiple-bond moieties prevail at 48 h of treatment. No such vesicle releases are present in the case of OSM usage. Drug incorporations into the cells and their impact on the plasma membrane and the dynamics of lipid raft confirmed by confocal fluorescence microscopy reveal a more destructive extent by OSM, corroborating the Raman results. Raman microscopy provides a broader understanding of the multifaceted factors and mechanisms responsible for giardiasis treatment or drug resistance by enabling a label-free, simultaneous monitoring of structural changes at the cellular and molecular levels.

Phylogenetic Analysis of Pyruvate-Ferredoxin Oxidoreductase, a Redox Enzyme Involved in the Pharmacological Activation of Nitro-Based Prodrugs in Bacteria and Protozoa

The present frontrunners in the chemotherapy of infections caused by protozoa are nitro-based prodrugs that are selectively activated by PFOR-mediated redox reactions. This study seeks to analyze the distribution of PFOR in selected protozoa and bacteria by applying comparative genomics to test the hypothesis that PFOR in eukaryotes was acquired through horizontal gene transfer (HGT) from bacteria. Furthermore, to identify other putatively acquired genes, proteome-wide and gene enrichment analyses were used. A plausible explanation for the patchy occurrence of PFOR in protozoa is based on the hypothesis that bacteria are potential sources of genes that enhance the adaptation of protozoa in hostile environments. Comparative genomics of Entamoeba histolytica and the putative gene donor, Desulfovibrio vulgaris, identified eleven candidate genes for HGT involved in intermediary metabolism. If these results can be reproduced in other PFOR-possessing protozoa, it would provide more validated evidence to support the horizontal transfer of pfor from bacteria.

Development of New PCR Protocols to Detect Genetic Diversity in the Metronidazole Metabolism Genes in Susceptible and Refractory Clinical Samples of Giardia duodenalis

PURPOSE

Investigating the genetic variation in thioredoxin reductase (TrxR) and nitroreductase (NR) genes in both treatment-resistant and -sensitive Giardia duodenalis isolates can provide valuable information in identifying potential markers of resistance to metronidazole. The rapid increase in metronidazole treatment failures suggests the presence of genetic resistance mechanisms. By analyzing these genes, researchers can gain insights into the efficacy of metronidazole against G. duodenalis and potentially develop alternative treatment strategies. In this regard, four G. duodenalis isolates (two clinically sensitive and two clinically resistant to metronidazole) were collected from various hospitals of Shiraz, southwestern Iran.

METHODS

Parasitological methods including sucrose flotation and microscopy were employed for the primary confirmation of G. duodenalis cysts in stool samples. Microscopy-positive samples were approved by SSU-PCR amplification of the parasite DNA. All four positive G. duodenalis specimens at SSU-PCR were afterward analyzed utilizing designed primers based on important metronidazole metabolism genes including TrxR, NR1, and NR2.

RESULTS

Unlike TrxR gene, the results of NR1 and NR2 genes showed that there are non-synonymous variations between sequences of treatment-sensitive and -resistant samples compared to reference sequences. Furthermore, the outcomes of molecular docking revealed that there is an interaction between the protein sequence and spatial shape of treatment-resistant samples and metronidazole in the position of serine amino acid based on the NR1 gene.

CONCLUSION

This issue can be one of the possible factors involved in the resistance of Giardia parasites to metronidazole. To reach more accurate results, a large sample size along with simulation and advanced molecular dynamics investigations are needed.

Comparative Proteomic Analysis of Toxoplasma gondii RH Wild-Type and Four SRS29B (SAG1) Knock-Out Clones Reveals Significant Differences between Individual Strains

In T. gondii, as well as in other model organisms, gene knock-out using CRISPR-Cas9 is a suitable tool to identify the role of specific genes. The general consensus implies that only the gene of interest is affected by the knock-out. Is this really the case? In a previous study, we generated knock-out (KO) clones of TgRH88_077450 (SRS29B; SAG1) which differed in the numbers of the integrated dihydrofolate-reductase-thymidylate-synthase (MDHFR-TS) drug-selectable marker. Clones 18 and 33 had a single insertion of MDHFR-TS within SRS29B. Clone 6 was disrupted by the insertion of a short unrelated DNA-sequence, but the marker was integrated elsewhere. In clone 30, the marker was inserted into SRS29B, and several other MDHFR-TS copies were found in the genome. KO and wild-type (WT) tachyzoites had similar shapes, dimensions, and vitality. This prompted us to investigate the impact of genetic engineering on the overall proteome patterns of the four clones as compared to the respective WT. Comparative shotgun proteomics of the five strains was performed. Overall, 3236 proteins were identified. Principal component analysis of the proteomes revealed five distinct clusters corresponding to the five strains by both iTop3 and iLFQ algorithms. Detailed analysis of the differentially expressed proteins revealed that the target of the KO, srs29B, was lacking in all KO clones. In addition to this protein, 20 other proteins were differentially expressed between KO clones and WT or between different KO clones. The protein exhibiting the highest variation between the five strains was srs36D encoded by TgRH_016110. The deregulated expression of SRS36D was further validated by quantitative PCR. Moreover, the transcript levels of three other selected SRS genes, namely SRS36B, SRS46, and SRS57, exhibited significant differences between individual strains. These results indicate that knocking out a given gene may affect the expression of other genes. Therefore, care must be taken when specific phenotypes are regarded as a direct consequence of the KO of a given gene.

Multilocus genotyping of Giardia duodenalis in pre-weaned calves with diarrhea in the Republic of Korea

Giardia duodenalis is a protozoan parasite that infects humans, companion animals, livestock, and wildlife. Infections in cattle caused by this parasite are often asymptomatic, but such infections can cause diarrhea, reduced weight gain, and ill-thrift in young calves. Although G. duodenalis causes diarrhea in calves, only a few studies have been conducted on calves in the Republic of Korea (ROK). Here, we aimed to determine the prevalence and distribution of G. duodenalis assemblages in pre-weaned calves with diarrhea in the ROK, identify the association between the occurrence of G. duodenalis and the age of calf, and perform molecular characterization of G. duodenalis. We collected 455 fecal samples from pre-weaned native Korean calves (≤60 days old) with diarrhea in four different regions. G. duodenalis was detected using nested PCR targeting the beta-giardin (bg) gene, and positive samples were further genotyped for the glutamate dehydrogenase (gdh) and triosephosphate isomerase (tpi) genes. The overall prevalence of G. duodenalis in calves with diarrhea was 4.4% (20/455) based on the analysis of bg. The highest prevalence was observed in calves aged 11-30 days (7.5%; 95% confidence interval: 3.7%-11.3%), whereas the lowest prevalence was observed in neonatal calves. From the 20 samples that were positive for bg, 16, 5, and 6 sequences were obtained following genotyping of bg, gdh, and tpi, respectively. Sequencing analysis of the bg gene revealed the presence of assemblage E (n = 15) and sub-assemblage AⅠ (n = 1) in the samples. Moreover, we detected mixed infections with assemblages E and A in two calves for the first time. Among the sequences obtained herein, two new subtypes of assemblage E were detected in gdh and tpi sequences each. The results suggest that G. duodenalis is an infectious agent causing diarrhea in calves, and pre-weaned calves are at a higher risk of infection than neonatal calves. Multilocus genotyping should be performed to confirm the presence of potentially zoonotic genotypes. These results highlight the importance of cattle as a source of zoonotic transmission of G. duodenalis to humans.

Application of Proteomics to the Study of the Therapeutics and Pathogenicity of Giardia duodenalis

Giardia duodenalis remains a neglected tropical disease. A key feature of the sustained transmission of Giardia is the ability to form environmentally resistant cysts. For the last 38 years, proteomics has been utilised to study various aspects of the parasite across different life cycle stages. Thirty-one articles have been published in PubMed from 2012 to 2022 related to the proteomics of G. duodenalis. Currently, mass spectrometry with LC-MS/MS and MALDI-TOF/TOF has been commonly utilised in proteomic analyses of Giardia, which enables researchers to determine potential candidates for diagnostic biomarkers as well as vaccine and drug targets, in addition to allowing them to investigate the virulence of giardiasis, the pathogenicity mechanisms of G. duodenalis, and the post-translational modifications of Giardia proteins throughout encystation. Over the last decade, valuable information from proteomics analyses of G. duodenalis has been discovered in terms of the pathogenesis and virulence of Giardia, which may provide guidance for the development of better means with which to prevent and reduce the impacts of giardiasis. Nonetheless, there is room for improving proteomics analyses of G. duodenalis, since genomic sequences for additional assemblages of Giardia have uncovered previously unknown proteins associated with the Giardia proteome. Therefore, this paper aims to review the applications of proteomics for the characterisation of G. duodenalis pathogenicity and the discovery of novel vaccine as well as drug targets, in addition to proposing some general directions for future Giardia proteomic research.

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.

MDR1 protein (ABC-C1) Over Expression in Giardia Intestinalis Incubated with Albendazole and Nitazoxanide

BACKGROUND

Giardia intestinalis is a worldwide parasite. Drugs used for the treatment of giardiasis are metronidazole, albendazole and nitazoxanide. The development of drug resistance is an obstacle to the effective treatment. Resistance mechanisms in some parasites involve the participation of ATP-binding cassette (ABC) transporter superfamily.

PURPOSE

To find if the ATP-binding cassette genes are overexpressed in trophozoites treated with albendazole or nitazoxanide.

METHODS

A search for ATP-binding cassette genes in Giardia sequence database (GiardiaDB) was done and six genes were selected. Trophozoites treated with albendazole or nitazoxanide and the expression of these six ABC genes was quantitated by real-time RT-PCR. The ABC-C1 gene was selected, and a fragment cloned. The ABC-C1 protein was expressed, and polyclonal antibodies were elicited in mice to detect the protein in treated trophozoites, finally a docking analysis was performed for ABC-C1 and tizoxanide interaction.

RESULTS

Bioinformatics analysis showed that the ATP-binding cassette (ABC) topology is present in the six proteins. The qRT-PCR revealed that the ABC-C1 gene was overexpressed in cells incubated with nitazoxanide or albendazole. Confocal analysis showed that ABC-C1 protein levels increased in trophozoites with both treatments but was higher with nitazoxanide. The mark was detected heavily in the periphery of the cells. Using a docking analysis, it was found that the nitazoxanide metabolite, tizoxanide was docked close to the ATP-binding region as well as in the exit tunnel, located in the transmembrane region.

CONCLUSION

These findings in Giardia intestinalis, support the possible role of ABC-C1 in drug efflux.

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.

Giardiasis treatment: an update with a focus on refractory disease

PURPOSE OF REVIEW

Giardiasis remains a common cause of diarrhea and intestinal enteropathy globally. Here we give an overview of clinical treatment studies and discuss potential mechanisms and molecular targets for in-vitro testing of drug resistance.

RECENT FINDINGS

Giardia is a cause of disease both in diarrheal and nondiarrheal cases. The prevalence of treatment refractory giardiasis is increasing. Recent studies reveal 5-nitroimidazole refractory infection occurs in up to 50% of cases. Mechanisms of drug resistance are not known. Placebo controlled studies of drug efficacy, taking the self-limiting course of giardiasis into account, has not been reported. No randomized controlled trials of treatment of refractory infection have been performed the last 25 years. Based on the clinical studies reported, combination treatment with a 5-nitroimidazole and a benzimidazole is more effective than repeated courses of 5-nitroimidazole or monotherapies in refractory cases. Quinacrine is effective in refractory cases, but potentially severe side effects limit its use.

SUMMARY

A combination of a 5-nitroimidazole and albendazole or mebendazole, and quinacrine monotherapy, are rational choices in nitroimidazole refractory infections, but randomized controlled studies are needed. Further research into more recent clinical isolates is necessary to uncover mechanisms for the increase in metronidazole refractory giardiasis observed during the last decade.

Nitroreductase Activites in Giardia lamblia: ORF 17150 Encodes a Quinone Reductase with Nitroreductase Activity

The intestinal diplomonadid Giardia lamblia is a causative agent of persistent diarrhea. Current treatments are based on nitro drugs, especially metronidazole. Nitro compounds are activated by reduction, yielding toxic intermediates. The enzymatic systems responsible for this activation are not completely understood. By fractionating cell free crude extracts by size exclusion chromatography followed by mass spectrometry, enzymes with nitroreductase (NR) activities are identified. The protein encoded by ORF 17150 found in two pools with NR activities is overexpressed and characterized. In pools of fractions with main NR activities, previously-known NRs are identified, as well as a previously uncharacterized protein encoded by ORF 17150. Recombinant protein 17150 is a flavoprotein with NADPH-dependent quinone reductase and NR activities. Besides a set of previously identified NRs, we have identified a novel enzyme with NR activity.

A proteomic glimpse into the effect of antimalarial drugs on Plasmodium falciparum proteome towards highlighting possible therapeutic targets

There is no effective vaccine against malaria; therefore, chemotherapy is to date the only choice to fight against this infectious disease. However, there is growing evidences of drug-resistance mechanisms in malaria treatments. Therefore, the identification of new drug targets is an urgent need for the clinical management of the disease. Proteomic approaches offer the chance of determining the effects of antimalarial drugs on the proteome of Plasmodium parasites. Accordingly, we reviewed the effects of antimalarial drugs on the Plasmodium falciparum proteome pointing out the relevance of several proteins as possible drug targets in malaria treatment. In addition, some of the P. falciparum stage-specific altered proteins and parasite-host interactions might play important roles in pathogenicity, survival, invasion and metabolic pathways and thus serve as potential sources of drug targets. In this review, we have identified several proteins, including thioredoxin reductase, helicases, peptidyl-prolyl cis-trans isomerase, endoplasmic reticulum-resident calcium-binding protein, choline/ethanolamine phosphotransferase, purine nucleoside phosphorylase, apical membrane antigen 1, glutamate dehydrogenase, hypoxanthine guanine phosphoribosyl transferase, heat shock protein 70x, knob-associated histidine-rich protein and erythrocyte membrane protein 1, as promising antimalarial drugs targets. Overall, proteomic approaches are able to partially facilitate finding possible drug targets. However, the integration of other 'omics' and specific pharmaceutical techniques with proteomics may increase the therapeutic properties of the critical proteins identified in the P. falciparum proteome.

Transfection With Plasmid Causing Stable Expression of a Foreign Gene Affects General Proteome Pattern in Giardia lamblia Trophozoites

Giardia lamblia is an important causative agent of persistent diarrhea in humans, domestic animals, and cattle. Basic research is usually performed with the strain WBC6 and includes genetic manipulations such as transfections. Here, we investigate how transfection with a plasmid causing stable expression of a foreign gene affects the whole proteome pattern. Using shotgun mass spectrometry, we compare the proteomes of untransfected trophozoites to trophozoites transfected with Escherichia coli glucuronidase A (GusA). Besides GusA, which is detected in the transfected trophozoites only, the proteomes of untransfected and transfected trophozoites differ by 132 differentially expressed proteins. In particular, transfection induces antigenic variation. Since transfection causing stable expression affects the proteome pattern, transfection experiments should take into account this effect. Due to a unique peptide panel, GusA is an example for a suitable internal standard for experiments involving transfected cells. Data are available via ProteomeXchange with identifier PXD022565.

Genetic Diversity of the Flavohemoprotein Gene of Giardia lamblia: Evidence for High Allelic Heterozygosity and Copy Number Variation

Purpose

The flavohemoprotein (gFlHb) in Giardia plays an important role in managing nitrosative and oxidative stress, and potentially also in virulence and nitroimidazole drug tolerance. The aim of this study was to analyze the genetic diversity of gFlHb in Giardia assemblages A and B clinical isolates.

Methods

gFlHb genes from 20 cultured clinical Giardia isolates were subjected to PCR amplification and cloning, followed by Sanger sequencing. Sequences of all cloned PCR fragments from each isolate were analyzed for single nucleotide variants (SNVs) and compared to genomic Illumina sequence data. Identical clone sequences were sorted into alleles, and diversity was further analyzed. The number of gFlHb gene copies was assessed by mining PacBio de novo assembled genomes in eight isolates. Homology models for assessment of SNV's potential impact on protein function were created using Phyre2.

Results

A variable copy number of the gFlHb gene, between two and six copies, depending on isolate, was found. A total of 37 distinct sequences, representing different alleles of the gFlHb gene, were identified in AII isolates, and 41 were identified in B isolates. In some isolates, up to 12 different alleles were found. The total allelic diversity was high for both assemblages (>0.9) and was coupled with a nucleotide diversity of <0.01. The genetic variation (SNVs per CDS length) was 4.8% in sub-assemblage AII and 5.4% in assemblage B. The number of non-synonymous (ns) SNVs was high in gFIHb of both assemblages, 1.6% in A and 3.0% in B, respectively. Some of the identified nsSNV are predicted to alter protein structure and possibly function.

Conclusion

In this study, we present evidence that gFlHb, a putative protective enzyme against oxidative and nitrosative stress in Giardia, is a variable copy number gene with high allelic diversity. The genetic variability of gFlHb may contribute metabolic adaptability against metronidazole toxicity.

In vitro-induction of metronidazole-resistant Giardia duodenalis is not associated with nucleotide alterations in the genes involved in pro-drug activation

Giardiasis is an infectious disease caused by Giardia duodenalis. The pro-drug metronidazole (MTZ) is the first-line treatment for giardiasis. Parasite’s proteins as pyruvate:ferredoxin oxidoreductase (PFOR), ferredoxin (Fd), nitroreductase-1 (NR-1) and thioredoxin reductase (TrxR) participate in MTZ activation. Here, we showed Giardia trophozoites long-term exposed to MTZ presented higher IC₅₀ than controls, showing the drug influenced the parasite survival. That reduction in MTZ’s susceptibility does not seem to be related to mutations in the genes pfor, fd, nr-1 or trxr. It points that different mechanism as alterations in other metabolic pathways can account for Giardia resistance to MTZ therapy.

Molecular Epidemiology of Giardia Infections in the Genomic Era

Giardia duodenalis is a major gastrointestinal parasite of humans and animals across the globe. It is also of interest from an evolutionary perspective as it possesses many features that are unique among the eukaryotes, including its distinctive binucleate cell structure. While genomic analysis of a small number of isolates has provided valuable insights, efforts to understand the epidemiology of the disease and the population biology of the parasite have been limited by the molecular tools currently available. We review these tools and assess the impact of affordable and rapid genome sequencing systems increasingly being deployed in diagnostic settings. While these technologies have direct implications for public and veterinary health, they will also improve our understanding of the unique biology of this fascinating parasite.

Antibacterial Discovery: 21st Century Challenges

It has been nearly 50 years since the golden age of antibiotic discovery (1945–1975) ended; yet, we still struggle to identify novel drug targets and to deliver new chemical classes of antibiotics to replace those rendered obsolete by drug resistance. Despite herculean efforts utilizing a wide range of antibiotic discovery platform strategies, including genomics, bioinformatics, systems biology and postgenomic approaches, success has been at best incremental. Obviously, finding new classes of antibiotics is really hard, so repeating the old strategies, while expecting different outcomes, seems to boarder on insanity. The key questions dealt with in this review include: (1) If mutation based drug resistance is the major challenge to any new antibiotic, is it possible to find drug targets and new chemical entities that can escape this outcome; (2) Is the number of novel chemical classes of antibacterials limited by the number of broad spectrum drug targets; and (3) If true, then should we focus efforts on subgroups of pathogens like Gram negative or positive bacteria only, anaerobic bacteria or other group where the range of common essential genes is likely greater?. This review also provides some examples of existing drug targets that appear to escape the specter of mutation based drug resistance, and provides examples of some intermediate spectrum strategies as well as modern molecular and genomic approaches likely to improve the odds of delivering 21st century medicines to combat multidrug resistant pathogens.

Comparative proteomics of three Giardia lamblia strains: investigation of antigenic variation in the post-genomic era

Giardia lamblia is a causative agent of persistent diarrhoea widespread in regions with low hygienic standards. Laboratory research is based on cloned lines issuing from various patient isolates typed in the late 1980s and 90s using restriction analysis and serology. In the present study, we compared the well-characterized strain WBC6 with another clone of the parent WB isolate termed WBA1 and with a clone from another isolate, GS/M-83-H7, using shotgun mass spectrometry proteomics. We identified 398 proteins differentially expressed between the GS and both WB isolates and 97 proteins differentially expressed between the two WB isolates. We investigated the expression levels of the predominant variant-specific surface proteins (VSPs) in each clone and matched the previously described major VSPs of each strain to the corresponding open reading frame sequences identified by whole-genome sequencing efforts. Furthermore, since the original WB isolate comes from a patient treated with metronidazole, we compared the susceptibilities of the strains to nitro compounds, as well the expression levels of enzymes involved in nitro reduction and on the corresponding enzyme activities and found distinct differences between the three strains.

Metronidazole and Secnidazole Carbamates: Synthesis, Antiprotozoal Activity, and Molecular Dynamics Studies

We prepared a series of 10 carbamates derivatives based on two common antiprotozoal drugs: metronidazole (1–5) and secnidazole (6–10). The compounds were tested in vitro against a set of two amitochondriate protozoa: Giardia duodenalis and Trichomonas vaginalis. Compounds 1–10 showed strong antiprotozoal activities, with potency values in the low micromolar-to-nanomolar range, being more active than their parent drugs. Metronidazole carbamate (1) was the most active of the series, with nanomolar activities against G. duodenalis (IC₅₀ = 460 nM) and T. vaginalis (IC₅₀ = 60 nM). The potency of compound 1 was 10 times greater than that of metronidazole against both parasites. None of compounds showed in vitro cytotoxicity against VERO cells tested at 100 µM. Molecular dynamics of compounds 1–10, secnidazole, and metronidazole onto the ligand binding site of pyruvate–ferredoxin oxidoreductase of T. vaginalis and the modeled β-tubulin of G. duodenalis revealed putative molecular interactions with key residues in the binding site of both proteins implicated in the mode of action of the parent drugs.

Metabolomic Profiling of Wildtype and Transgenic Giardia lamblia Strains by 1H HR-MAS NMR Spectroscopy

Giardia lamblia, a causative agent of persistent diarrhea in humans, domestic animals, and cattle, is usually treated with nitro compounds. Consequently, enzymes involved in anaerobic nitro reduction have been investigated in detail as potential targets. Their role within the normal metabolic context is, however, not understood. Using ¹H high-resolution magic angle spinning (HR-MAS) NMR spectroscopy, we analyzed the metabolomes of G. lamblia trophozoites overexpressing three nitroreductases (NR1–NR3) and thioredoxin reductase (TrxR), most likely a scavenger of reactive oxygen species, as suggested by the results published in this study. We compared the patterns to convenient controls and to the situation in the nitro drug resistant strain C4 where NR1 is downregulated. We identified 27 metabolites in G. lamblia trophozoites. Excluding metabolites of high variability among different wildtype populations, only trophozoites overexpressing NR1 presented a distinct pattern of nine metabolites, in particular arginine catabolites, differing from the respective controls. This pattern matched a differential pattern between wildtype and strain C4. This suggests that NR1 interferes with arginine and thus energy metabolism. The exact metabolic function of NR1 (and the other nitroreductases) remains to be elucidated.

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.

Comparative Pathobiology of the Intestinal Protozoan Parasites Giardia lamblia, Entamoeba histolytica, and Cryptosporidium parvum

Protozoan parasites can infect the human intestinal tract causing serious diseases. In the following article, we focused on the three most prominent intestinal protozoan pathogens, namely, Giardia lamblia, Entamoeba histolytica, and Cryptosporidium parvum. Both C. parvum and G. lamblia colonize the duodenum, jejunum, and ileum and are the most common causative agents of persistent diarrhea (i.e., cryptosporidiosis and giardiasis). Entamoeba histolytica colonizes the colon and, unlike the two former pathogens, may invade the colon wall and disseminate to other organs, mainly the liver, thereby causing life-threatening amebiasis. Here, we present condensed information concerning the pathobiology of these three diseases.

Nitroreductases of bacterial origin in Giardia lamblia: Potential role in detoxification of xenobiotics

The anaerobic parasite Giardia lamblia, causative agent of persistent diarrhea, contains a family of nitroreductase genes most likely acquired by lateral transfer from anaerobic bacteria or archaebacteria. Two of these nitroreductases, containing a ferredoxin domain at their N-terminus, NR1, and NR2, have been characterized previously. Here, we present the characterization of a third member of this family, NR3. In functional assays, recombinant NR1 and NR3 reduced quinones like menadione and the antibiotic tetracycline, and-to much lesser extents-the nitro compound dinitrotoluene. Conversely, recombinant NR2 had no activity on tetracycline. Escherichia coli expressing NR3 were less susceptible to tetracycline, but more susceptible to the nitro compound metronidazole under semi-aerobic growth conditions. G. lamblia overexpressing NR1 and NR3, but not lines overexpressing NR2, are more susceptible to the nitro drug nitazoxanide. These findings suggest that NR3 is an active quinone reductase with a mode of action similar to NR1, but different from NR2. The biological function of this family of enzymes may reside in the use of xenobiotics as final electron acceptors. Thereby, these enzymes may provide at least two evolutionary advantages namely a higher potential to recycle NAD(P) as electron acceptors for the (fermentative) energy and intermediary metabolism, and the possibility to inactivate toxic xenobiotics produced by microorganisms living in concurrence inside the intestinal habitat.