Multiple drug resistance in the pathogenic protozoa

Apicoplast-targeting antibacterials inhibit the growth of Babesia parasites

The apicoplast housekeeping machinery, specifically apicoplast DNA replication, transcription, and translation, was targeted by ciprofloxacin, thiostrepton, and rifampin, respectively, in the in vitro cultures of four Babesia species. Furthermore, the in vivo effect of thiostrepton on the growth cycle of Babesia microti in BALB/c mice was evaluated. The drugs caused significant inhibition of growth from an initial parasitemia of 1% for Babesia bovis, with 50% inhibitory concentrations (IC(50)s) of 8.3, 11.5, 12, and 126.6 μM for ciprofloxacin, thiostrepton, rifampin, and clindamycin, respectively. The IC(50)s for the inhibition of Babesia bigemina growth were 15.8 μM for ciprofloxacin, 8.2 μM for thiostrepton, 8.3 μM for rifampin, and 206 μM for clindamycin. The IC(50)s for Babesia caballi were 2.7 μM for ciprofloxacin, 2.7 μM for thiostrepton, 4.7 μM for rifampin, and 4.7 μM for clindamycin. The IC(50)s for the inhibition of Babesia equi growth were 2.5 μM for ciprofloxacin, 6.4 μM for thiostrepton, 4.1 μM for rifampin, and 27.2 μM for clindamycin. Furthermore, an inhibitory effect was revealed for cultures with an initial parasitemia of either 10 or 7% for Babesia bovis or Babesia bigemina, respectively. The three inhibitors caused immediate death of Babesia bovis and Babesia equi. The inhibitory effects of ciprofloxacin, thiostrepton, and rifampin were confirmed by reverse transcription-PCR. Thiostrepton at a dose of 500 mg/kg of body weight resulted in 77.5% inhibition of Babesia microti growth in BALB/c mice. These results implicate the apicoplast as a potential chemotherapeutic target for babesiosis.

In vitro activity of the F-6 fraction of oregano against Giardia intestinalis

Giardiosis is a neglected parasitic disease that produces diarrhoea and different degrees of malabsorption in humans and animals. Its treatment is based on derivatives of 5-nitroimidazoles, benzimidazoles, nitrofuranes, acridine and nitrotiazoles. These drugs produce undesirable secondary effects, ranging from a metallic taste in the mouth to genetic damage and the selection of resistant strains; therefore, it is necessary to develop new therapeutic alternatives. We demonstrated that a 2-h treatment with 2·87 μg ml(-1) of fraction 6 of Lippia graveolens (F-6) was sufficient to kill half of an experimental Giardia intestinalis (Syn. G. duodenalis, G. lamblia) population, based on the reduction of MTT-tetrazolium salt levels. F-6 breaks the nuclear envelope and injures the ventral suckling disc. The major compounds of F-6 were characterized as naringenin, thymol, pinocembrin and traces of compounds not yet identified. The results suggest that Lippia is a potential source to obtain compounds with anti-Giardia activity. This knowledge is an important starting point to develop new anti-giardial drugs. Future studies will be required to establish the efficacy of F-6 in vivo using an animal model.

Regulation of ABC transporter function via phosphorylation by protein kinases

ATP-binding cassette (ABC) transporters are multispanning membrane proteins that utilize ATP to move a broad range of substrates across cellular membranes. ABC transporters are involved in a number of human disorders and diseases. Overexpression of a subset of the transporters has been closely linked to multidrug resistance in both bacteria and viruses and in cancer. A poorly understood and important aspect of ABC transporter biology is the role of phosphorylation as a mechanism to regulate transporter function. In this review, we summarize the current literature addressing the role of phosphorylation in regulating ABC transporter function. A comprehensive list of all the phosphorylation sites that have been identified for the human ABC transporters is presented, and we discuss the role of individual kinases in regulating transporter function. We address the potential pitfalls and difficulties associated with identifying phosphorylation sites and the corresponding kinase(s), and we discuss novel techniques that may circumvent these problems. We conclude by providing a brief perspective on studying ABC transporter phosphorylation.

Cloning and heterologous expression of the ovine (Ovis aries) P-glycoprotein (Mdr1) in Madin-Darby canine kidney (MDCK) cells

P-glycoprotein (P-gp) plays a crucial role in the multidrug resistance of pathogenic helminths in sheep (Ovis aries) as well as in antiparasitic drug pharmacokinetics in the host. We cloned sheep P-gp cDNA and expressed it stably in Madin-Darby canine kidney (MDCK) cells. The open reading frame consists of 3858 nucleotides coding for a 1285 amino acids containing protein. The sequence shows high homology to the orthologs of other mammalian species, especially cattle. Both ruminant DNA sequences show a 9 bp insertion that is lacking in all other investigated sequences. Expressed in MDCK cells, the protein displays a size of 170 kDa on Western analysis. Transfection of MDCK cells with sheep P-gp resulted in 10- to 50-fold resistance to the cytotoxic P-gp substrates colchicin and daunorubicin, and in reduced digoxin accumulation.

Drug resistance in Giardia: clinical versus laboratory isolates

The advantages and limitations of determining mechanisms of drug resistance in Giardia duodenalis laboratory isolates, which have been generated in a number of ways, is weighed against the difficulty of analysing mechanisms in clinical isolates with a large diversity of genetic and expression capabilities. Using isogenic strains to follow changes in enzyme regulation involved in drug resistance, we have been able to assess the full capability of the parasite in developing drug resistance mechanisms. The complementarity of the two approaches, clinical versus laboratory induced drug resistance, and continuing comparison with other organisms, particularly the anaerobic bacteria with which Giardia has strong affiliations, is emphasized. These considerations lead to the study of the population genetics of drug resistance, and strategies critical for rational drug usage, design and therapy.

Oregano (Lippia spp.) kills Giardia intestinalis trophozoites in vitro: antigiardiasic activity and ultrastructural damage

In the world, giardiosis is still a very important parasitic disease; only in Asia, Africa and America, there are more than 200 million of infected people in a year. The usual treatments are drugs that produce undesirable secondary effects, perhaps favouring the resistant strain selection. One alternative is to research compounds from plants used as antidiarrhoeic or antiparasitic in the traditional medicine. In a previous work, we found that Lippia beriandieri (Oregano) revealed to be more potent than tinidazole, a common antigiardiasic drug. In this current work, we tested the cell viability by re-culture and reduction of MTT-tetrazolium salts to MTT-formazan, and we showed the effect of oregano ethanolic extracts against Giardia intestinalis (synonyms: Giardia duodenalis, Giardia lamblia) trophozoites at concentrations ranging form 58 to 588 microg. We demonstrated the ultrastructural injury produced by oregano extracts in this parasite. Trophozoites lost their size and shape and showed damage in nucleus structure, perhaps by breaking the pattern of nucleoskeleton proteins.

Cryptosporidium parvum: effect of multi-drug reversing agents on the expression and function of ATP-binding cassette transporters

In the present study, the gene expression of three multidrug resistance (MDR) and resistance-associated protein (MRP) transport proteins or efflux pumps was characterized and the phenotypic evidence for such pumps was demonstrated in cultured Madin-Darby canine kidney (MDCK) cells. A gradient for the fluorescent probe calcein was established between parasite and host cell suggestive of a parasite extrusion pump at the parasite-host interface. This gradient was decreased in a glucose-free medium containing 2-deoxyglucose or 3-O-methylglucose, by probenecid, and by the isoflavonoid, narigenin, suggesting that the calcein extrusion was energy-dependent and involved an MRP-like pump. While neither MDR or MRP inhibiters significantly affected transcript levels of any of the ABC transporters, transcript levels of the Cryptosporidium parvum ABC protein (CpABC1), an MRP transporter, were consistently expressed 4 logs higher than either CpABC3 or CpABC2, suggesting a prominent role in the intracellular stages of the parasite.

Excretion of fluorescent substrates of mammalian multidrug resistance-associated protein (MRP) in the Schistosoma mansoni excretory system

The protonephridium of platyhelminths including Schistosoma mansoni plays a pivotal role in their survival by excretion of metabolic wastes as well as xenobiotics, and can be revealed in the living adult parasite by certain fluorescent compounds which are concentrated in excretory tubules and collecting ducts. To determine the presence of the multidrug resistance-associated protein (MRP) as a possible transporter in protonephridial epithelium, adult schistosomes were exposed to a fluorescent Ca2+ indicator, fluo-3 acetyloxymethyl ester, which is a potential substrate of mammalian MRP. Specific fluorescence related to fluo-3/Ca2+ chelate delineated the whole length of the protonephridial system. Simultaneously, a fluorescent substance was accumulated in the posterior part of collecting ducts and the excretory bladder. Similarly, when other fluorogenic substrates for mammalian MRP such as monoclorobimane, fluorescein diacetate, and 5(6)-carboxyfluorescein diacetate were applied to adult schistosomes, these fluorescent markers were observed in the excretory tubules through to the excretory bladder. The excretory system of mechanically-transformed schistosomula was not labelled with any of these 4 fluorescent markers. These findings suggest that the protonephridial epithelium of adult schistosomes, but not schistosomula, might express the homologue of the mammalian MRP transporting organic anionic conjugates with glutathione, glucuronate or sulphate as well as unconjugated amphiphilic organic anions.

Efficacy of antigiardial drugs

The flagellated protozoa Giardia duodenalis is the most commonly detected parasite in the intestinal tract of humans. Infections with the parasite result in diarrhoeal disease in humans and animals, with infants at risk from failure-to-thrive syndrome. The incidence of giardiasis worldwide may be as high as 1000 million cases. Current recommended treatments include the nitroheterocyclic drugs tinidazole, metronidazole and furazolidone, the substituted acridine, quinacrine, and the benzimidazole, albendazole. Paromomycin is also used in some situations, and nitazoxanide is proving to be useful. However, treatment failures have been reported with all of the common antigiardial agents, and drug resistance to all available drugs has been demonstrated in the laboratory. In addition, clinical resistance has been reported, including cases where patients failed both metronidazole and albendazole treatments. The identification of new antigiardial drugs is an important consideration for the future, but maintaining the usefulness of the existing drugs is the most cost-effective measure to ensure the continued availability of antigiardial drugs.

P-glycoprotein-like protein contributes to cadmium resistance in Euglena gracilis

Selective pressures from polluted environments have led to the development of resistance systems in aquatic organisms. Using different techniques, this study examined a cadmium defense mechanism of the freshwater unicellular protozoa Euglena gracilis, and found it to be an efflux pump similar to the multidrug resistance P-glycoprotein. Cd(2+)-treated E. gracilis were able to extrude Rhodamine 123 at 21 degrees C, but not at 4 degrees C. Furthermore, verapamil, a P-glycoprotein modulator, partially blocked the efflux process (at 21 degrees C), and enhanced the Cd(2+) toxic effects on these cells. Western immunoblots of cell lysates, using the anti-P-glycoprotein antibody JSB-1, revealed a 120-KDa protein, which was expressed, in high amounts on Cd(2+)-exposed cells (74% above the control values). Moreover, cells treated with JSB-1 became more sensitive to the harmful effects of cadmium, showing a decreased survival rate. Taken together, these results suggest that a MDR phenotype has evolved in Euglena as one of the mechanisms for cadmium detoxification.

Drug resistance in the sexually transmitted protozoan Trichomonas vaginalis

Trichomoniasis is the most common, sexually transmitted infection. It is caused by the flagellated protozoan parasite Trichomonas vaginalis. Symptoms include vaginitis and infections have been associated with preterm delivery, low birth weight and increased infant mortality, as well as predisposing to HIV/AIDS and cervical cancer. Trichomoniasis has the highest prevalence and incidence of any sexually transmitted infection. The 5-nitroimidazole drugs, of which metronidazole is the most prescribed, are the only approved, effective drugs to treat trichomoniasis. Resistance against metronidazole is frequently reported and cross-resistance among the family of 5-nitroimidazole drugs is common, leaving no alternative for treatment, with some cases remaining unresolved. The mechanism of metronidazole resistance in T. vaginalis from treatment failures is not well understood, unlike resistance which is developed in the laboratory under increasing metronidazole pressure. In the latter situation, hydrogenosomal function which is involved in activation of the prodrug, metronidazole, is down-regulated. Reversion to sensitivity is incomplete after removal of drug pressure in the highly resistant parasites while clinically resistant strains, so far analysed, maintain their resistance levels in the absence of drug pressure. Although anaerobic resistance has been regarded as a laboratory induced phenomenon, it clearly has been demonstrated in clinical isolates. Pursuit of both approaches will allow dissection of the underlying mechanisms. Many alternative drugs and treatments have been tested in vivo in cases of refractory trichomoniasis, as well as in vitro with some successes including the broad spectrum anti-parasitic drug nitazoxanide. Drug resistance incidence in T. vaginalis appears to be on the increase and improved surveillance of treatment failures is urged.

Drug resistance in Trypanosoma brucei spp., the causative agents of sleeping sickness in man and nagana in cattle

Drug resistance in pathogenic trypanosomes threatens successful control of fatal sleeping sickness in man and hinders economic livestock production in sub-Saharan Africa. We report on the occurrence and development of drug resistance, and discuss the genetic basis of such resistance in Trypanosoma brucei. Understanding these mechanisms at the molecular level will enable improved management of existing drugs and provide valuable clues to the development of new trypanocides.

Drug targets and mechanisms of resistance in the anaerobic protozoa

The anaerobic protozoa Giardia duodenalis, Trichomonas vaginalis, and Entamoeba histolytica infect up to a billion people each year. G. duodenalis and E. histolytica are primarily pathogens of the intestinal tract, although E. histolytica can form abscesses and invade other organs, where it can be fatal if left untreated. T. vaginalis infection is a sexually transmitted infection causing vaginitis and acute inflammatory disease of the genital mucosa. T. vaginalis has also been reported in the urinary tract, fallopian tubes, and pelvis and can cause pneumonia, bronchitis, and oral lesions. Respiratory infections can be acquired perinatally. T. vaginalis infections have been associated with preterm delivery, low birth weight, and increased mortality as well as predisposing to human immunodeficiency virus infection, AIDS, and cervical cancer. All three organisms lack mitochondria and are susceptible to the nitroimidazole metronidazole because of similar low-redox-potential anaerobic metabolic pathways. Resistance to metronidazole and other drugs has been observed clinically and in the laboratory. Laboratory studies have identified the enzyme that activates metronidazole, pyruvate:ferredoxin oxidoreductase, to its nitroso form and distinct mechanisms of decreasing drug susceptibility that are induced in each organism. Although the nitroimidazoles have been the drug family of choice for treating the anaerobic protozoa, G. duodenalis is less susceptible to other antiparasitic drugs, such as furazolidone, albendazole, and quinacrine. Resistance has been demonstrated for each agent, and the mechanism of resistance has been investigated. Metronidazole resistance in T. vaginalis is well documented, and the principal mechanisms have been defined. Bypass metabolism, such as alternative oxidoreductases, have been discovered in both organisms. Aerobic versus anaerobic resistance in T. vaginalis is discussed. Mechanisms of metronidazole resistance in E. histolytica have recently been investigated using laboratory-induced resistant isolates. Instead of downregulation of the pyruvate:ferredoxin oxidoreductase and ferredoxin pathway as seen in G. duodenalis and T. vaginalis, E. histolytica induces oxidative stress mechanisms, including superoxide dismutase and peroxiredoxin. The review examines the value of investigating both clinical and laboratory-induced syngeneic drug-resistant isolates and dissection of the complementary data obtained. Comparison of resistance mechanisms in anaerobic bacteria and the parasitic protozoa is discussed as well as the value of studies of the epidemiology of resistance.

Role of P glycoprotein in the course and treatment of Encephalitozoon microsporidiosis

Encephalitozoon microsporidia are obligate intracellular protozoan parasites that proliferate and differentiate within a parasitophorous vacuole inside host cells that are usually epithelial in nature. Isolates of the three species of the Encephalitozoon microsporidia, E. cuniculi, E. hellem, and E. intestinalis, were obtained from AIDS patients and cultured in green monkey (E6) kidney cells. Anti-P-glycoprotein (anti-Pgp) and anti-multidrug resistance-associated protein (anti-MRP) monoclonal antibodies were used to probe for multidrug resistance (MDR) pump epitopes and verapamil- or cyclosporin A- and probenecid-modulated intracellular calcein fluorescence were used to assess the expression of Pgp and MRP respectively in uninfected and infected cells. Pgp, but not MRP, was detected immunocytochemically and by verapamil- and cyclosporin A-potentiated intracellular fluorescence in both host cells and parasite developing stages. When an in vitro infection assay was employed, verapamil and cyclosporin A acted as chemosensitizing agents for the antiparasitic drug albendazole. These observations suggest that inhibiting host cell and perhaps parasite MDR pumps may increase the efficacy of antiparasitic agents in these and other microsporidia species.

My favorite cell: Giardia

The gut protozoan parasite, Giardia duodenalis, is the best characterized example of the most ancient eukaryotes, which are anaerobic and appear to be primitively amitochondrial. Apart from its obvious medical importance, Giardia is fascinating in its own right. Its prokaryotic-like anaerobic metabolism renders it selectively sensitive to some bacterial drugs, especially the nitroimidazoles, which are activated to form toxic radicals. Other features, including an enzyme that reduces oxygen directly to water, cysteine as the keeper of redox balance, a plasmid, and toxin-like genes are also distinctly prokaryotic-like. But, unlike prokaryotes, Giardia has a sophisticated, highly developed cytoskeleton, bounded nuclei, linear chromosomes capped with telomeric repeats, and telomere positional regulation of gene expression.

Characterisation and purification of pyruvate:ferredoxin oxidoreductase from Giardia duodenalis

The major 2-oxoacid oxidoreductase (2-OR), pyruvate:ferredoxin oxidoreductase (PFOR) from Giardia duodenalis has been purified to apparent homogeneity. A second 2-OR with a preference for alpha-ketobutyrate as substrate was identified and was removed from PFOR containing fractions during purification. Only PFOR and the second 2-OR were identified in gels of crude Giardia extracts assayed for 2-OR activity. The native form of PFOR which is membrane associated, is a homodimer of 138 kDa subunits. Pyruvate is the preferred substrate: alpha-ketobutyrate and oxaloacetate, but not phenyl-pyruvate or alpha-ketoglutarate, are decarboxylated. PFOR from Giardia is more stable than PFOR from most other organisms and purified PFOR can be stored without deterioration at -70 degrees C. Purified PFOR donates electrons to Giardia ferredoxin (Fd I) with concomitant reduction of metronidazole. However, two other Giardia ferredoxins did not accept electrons from PFOR. Consistent with the involvement of PFOR in metronidazole activation, the activity of pyruvate dependent 2-OR activity was decreased in all metronidazole-resistant lines tested but not in furazolidone-resistant lines. The presence of three different ferredoxins and two 2-ORs in Giardia suggests that a number of different electron transport pathways operate in this organism providing unusual metabolic flexibility for a eukaryote.

Quinacrine-resistant Giardia duodenalis

Quinacrine resistance has been induced in 3 Giardia duodenalis laboratory stocks and 4 lines resistant to other drugs. The quinacrine-resistant lines were maintained in normally lethal levels of 5 microM quinacrine and some lines are viable in 20 microM levels of the drug. Fluorescence studies indicated that quinacrine was taken up by sensitive cells but was actively excluded from resistant trophozoites. The nuclei were not a site of drug accumulation and no specific fluorescence in the trophozoite could be attributed to any structure. Blebs of concentrated drug appeared prior to disintegration of the membrane of drug-sensitive trophozoites exposed to drug overnight. Parasite lines already resistant to furazolidone adapted more readily to quinacrine exposure than drug-sensitive stocks. This multiple drug resistant phenotype was not as marked with metronidazole-resistant lines.

Dreamtime, devastation and deviation: Australia's contribution to the chemotherapy of human parasitic infections

There are three areas in which Australian scientists have made outstanding contributions to the study of the chemotherapy of human parasitic infections. Naturally occurring products of plants have great potential as antiparasitic agents and although several native species have been shown to have antimalarial and anthelmintic activity, their potential as chemotherapeutic agents has not been fully realised; secondly, the demands of war ensured that the Army Malaria Unit at Cairns carried out meticulous and exceptional studies to evaluate new antimalarial compounds. Not only were they able to prove the effectiveness of atebrin, Proguanil and chloroquine as prophylactics, they also obtained much new information on the pharmacokinetics of antimalarials and about the infection itself. Full recognition of these pioneering studies involving over 1000 volunteers infected with malaria, which can never be repeated, has not been appreciated. The third significant contribution is the molecular studies on the mechanisms of drug resistance Plasmodium falciparum of both the antifolate- and quinoline-containing drugs and the identification and subsequent biochemical and molecular analysis of drug resistance in Giardia intestinalis infections.