Energy-dependent efflux from Leishmania promastigotes of substrates of the mammalian multidrug resistance pumps

ABC transporter inhibition by beauvericin partially overcomes drug resistance in Leishmania tropica

Leishmaniasis is a neglected tropical disease infecting the world's poorest populations. Miltefosine (ML) remains the primary oral drug against the cutaneous form of leishmaniasis. The ATP-binding cassette (ABC) transporters are key players in the xenobiotic efflux, and their inhibition could enhance the therapeutic index. In this study, the ability of beauvericin (BEA) to overcome ABC transporter-mediated resistance of Leishmania tropica to ML was assessed. In addition, the transcription profile of genes involved in resistance acquisition to ML was inspected. Finally, we explored the efflux mechanism of the drug and inhibitor. The efficacy of ML against all developmental stages of L. tropica in the presence or absence of BEA was evaluated using an absolute quantification assay. The expression of resistance genes was evaluated, comparing susceptible and resistant strains. Finally, the mechanisms governing the interaction between the ABC transporter and its ligands were elucidated using molecular docking and dynamic simulation. Relative quantification showed that the expression of the ABCG sub-family is mostly modulated by ML. In this study, we used BEA to impede resistance of Leishmania tropica. The IC50 values, following BEA treatment, were significantly reduced from 30.83, 48.17, and 16.83 µM using ML to 8.14, 11.1, and 7.18 µM when using a combinatorial treatment (ML + BEA) against promastigotes, axenic amastigotes, and intracellular amastigotes, respectively. We also demonstrated a favorable BEA-binding enthalpy to L. tropica ABC transporter compared to ML. Our study revealed that BEA partially reverses the resistance development of L. tropica to ML by blocking the alternate ATP hydrolysis cycle.

In Vitro and In Vivo Antileishmanial Activity of Thioridazine

INTRODUCTION

Leishmaniasis is a neglected disease with high prevalence and incidence in tropical and subtropical areas. Existing drugs are limited due to cost, toxicity, declining efficacy and unavailability in endemic places. Drug repurposing has established as an efficient way for the discovery of drugs for a variety of diseases.

PURPOSE

The objective of the present work was testing the antileishmanial activity of thioridazine, an antipsychotic agent with demonstrated effect against other intracellular pathogens.

METHODS

The cytotoxicity for mouse peritoneal macrophages as well as the activity against Leishmania amazonensis, Leishmania mexicana and Leishmania major promastigotes and intracellular amastigotes, as well as in a mouse model of cutaneous leishmaniasis, were assessed.

RESULTS

Thioridazine inhibited the in vitro proliferation of promastigotes (50% inhibitory concentration-IC50-values in the range of 0.73 µM to 3.8 µM against L. amazonensis, L. mexicana and L. major) and intracellular amastigotes (IC50 values of 1.27 µM to 4.4 µM for the same species). In contrast, in mouse peritoneal macrophages, the 50% cytotoxic concentration was 24.0 ± 1.89 µM. Thioridazine inhibited the growth of cutaneous lesions and reduced the number of parasites in the infected tissue of mice. The dose of thioridazine that inhibited lesion development by 50% compared to controls was 23.3 ± 3.1 mg/kg and in terms of parasite load, it was 11.1 ± 0.97 mg/kg.

CONCLUSIONS

Thioridazine was effective against the promastigote and intracellular amastigote stages of three Leishmania species and in a mouse model of cutaneous leishmaniasis, supporting the potential repurposing of this drug as an antileishmanial agent.

Do phenothiazines possess antimicrobial and efflux inhibitory properties?

Antibiotic resistance is a global health concern; the rise of drug-resistant bacterial infections is compromising the medical advances that resulted from the introduction of antibiotics at the beginning of the 20th century. Considering that the presence of mutations within individuals in a bacterial population may allow a subsection to survive and propagate in response to selective pressure, as long as antibiotics are used in the treatment of bacterial infections, development of resistance is an inevitable evolutionary outcome. This, combined with the lack of novel antibiotics being released to the clinical market, means the need to develop alternative strategies to treat these resistant infections is critical. We discuss how the use of antibiotic adjuvants can minimise the appearance and impact of resistance. To this effect, several phenothiazine-derived drugs have been shown to potentiate the activities of antibiotics used to treat infections caused by Gram-positive and Gram-negative bacteria. Outside of their role as antipsychotic medications, we review the evidence to suggest that phenothiazines possess inherent antibacterial and efflux inhibitory properties enabling them to potentially combat drug resistance. We also discuss that understanding their mode of action is essential to facilitate the design of new phenothiazine derivatives or novel agents for use as antibiotic adjuvants.

Enhancing the copy number of Ldrab6 gene in Leishmania donovani parasites mediates drug resistance through drug-thiol conjugate dependent multidrug resistance protein A (MRPA)

Visceral leishmaniasis (VL) is a neglected tropical disease caused by protozoan Leishmania donovani parasite which may be fatal if left untreated. While drug-sensitive parasites are able to live and multiply within the host macrophages, they develop resistance to drugs used against them for survival and multiplication in the infected patients undergoing routine treatment. Development of new agents devoid of such drug resistance potential is achievable by identifying new drug targets in the parasite. One such target is the key regulator of intracellular vesicular trafficking protein, RabGTPase which belongs to the Ras GTPase superfamily. We recently elucidated whole genome sequence (WGS) of L. donovani (clinical Indian isolate; BHU 1220, GenBank: AVPQ00000000.1) and identified Ldrab6 gene. We now provide experimental evidence for this gene's ability to impart drug-resistant phenotype to wild-type (sensitive) Leishmania upon transfection. trans-Dibenzalacetone (DBA), a synthetic analog of curcumin, was used to determine its antileishmanial activity in wild-type parasites and parasites transfected with Ldrab6 gene. Dose-response study showed that DBA had no effect on transfected parasites at 20 µg/mL dose, whereas wild-type promastigotes showed 50% inhibition (IC₅₀) at the same dose. This indicates the development of resistant mechanism in the transfected parasites due to enhancement of the copy number of Ldrab6 gene in L. donovani parasites. Flow cytometric analysis revealed elevated level of thiols in transfectants when compared to wild-type parasites treated with DBA. To assess the functional activity of multidrug resistance-associated protein (MRP) pump in transfectants, the accumulation of calcein, a known MRP pump substrate and probenecid, a known MRP pump regulator, were analyzed. The results indicate that Ldrab6 gene in Leishmania conferred resistance by the well-established mechanism of drug-thiol conjugation and sequestration by ABC transporter multidrug resistance-protein A (MRPA). Accordingly, Leishmania parasites transfected with Ldrab6 gene can be used as an experimental cell line for the screening of new lead molecules for their propensity to develop drug resistance.

Increased thiol levels in antimony-resistant Leishmania infantum isolated from treatment-refractory visceral leishmaniasis in Brazil

BACKGROUND

Treatment-refractory visceral leishmaniasis (VL) has become an important problem in many countries.

OBJECTIVES

We evaluated the antimony-resistance mechanisms of Leishmania infantum isolated from VL patients refractory or responsive to treatment with pentavalent antimony.

METHODS

Strains isolated from antimony-refractory patients (in vitro antimony-resistant isolates) and antimony-responsive patients (in vitro antimony-sensitive isolates) were examined. Morphological changes were evaluated by transmission electron microscopy after trivalent antimony exposure. P-glycoprotein (P-gp) efflux pump activity was evaluated using the pump-specific inhibitor verapamil hydrochloride, and the role of thiol in trivalent antimony resistance was investigated using the enzymatic inhibitor L-buthionine sulfoximine.

FINDINGS

Antimony treatment induced fewer alterations in the cellular structure of L. infantum resistant isolates than in that of sensitive isolates. P-gp efflux activity was not involved in antimony resistance in these isolates. Importantly, the resistant isolates contained higher levels of thiol compared to the sensitive isolates, and inhibition of thiol synthesis in the resistant isolates recovered their sensitivity to trivalent antimony treatment, and enhanced the production of reactive oxygen species in promastigotes exposed to the drug.

MAIN CONCLUSIONS

Our results demonstrate that isolates from patients with antimony-refractory VL exhibited higher thiol levels than antimony-sensitive isolates. This indicates that redox metabolism plays an important role in the antimony-resistance of New World VL isolates.

Calcein+/PI- as an early apoptotic feature in Leishmania

Although leishmaniases are responsible for high morbidity and mortality all over the world, no really satisfying treatment exists. Furthermore, the corresponding parasite Leishmania undergoes a very characteristic form of programmed cell death. Indeed, different stimuli can induce morphological and biochemical apoptotic-like features. However, the key proteins involved in mammal apoptosis, such as caspases and death receptors, are not encoded in the genome of this parasite. Currently, little is known about Leishmania apoptosis, notably owing to the lack of specific tools for programmed cell death analysis in these parasites. Furthermore, there is a need for a better understanding of Leishmania programmed cell death in order (i) to better understand the role of apoptosis in unicellular organisms, (ii) to better understand apoptosis in general through the study of an ancestral eukaryote, and (iii) to identify new therapeutic targets against leishmaniases. To advance understanding of apoptosis in Leishmania, in this study we developed a new tool based on the quantification of calcein and propidium iodide by flow cytometry. This double labeling can be employed to distinguish early apoptosis, late apoptosis and necrosis in Leishmania live cells with a very simple and rapid assay. This paper should, therefore, be of interest for people working on Leishmania and related parasites.

Glucose-6-phosphate dehydrogenase and Trypanothione reductase interaction protects Leishmania donovani from metalloid mediated oxidative stress

Exploration of metabolons as viable drug target is rare in kinetoplastid biology. Here we present a novel protein-protein interaction among Glucose-6-phosphate dehydrogenase (LdG6PDH) and Trypanothione reductase (LdTryR) of Leishmania donovani displaying interconnection between central glucose metabolism and thiol metabolism of this parasite. Digitonin fractionation patterns observed through immunoblotting indicated localisation of both LdG6PDH and LdTryR in cytosol. In-silico and in-vitro interaction observed by size exclusion chromatography, co-purification, pull-down assay and spectrofluorimetric analysis revealed LdG6PDH and LdTryR physically interact with each other in a NADPH dependent manner. Coupled enzymatic assay displayed that NADPH generation was severely impaired by addition of Sb^III, As^III and Te^IV extraneously, which hint towards metalloid driven structural changes of the interacting proteins. Co-purification patterns and pull-down assays also depicted that metalloids (Sb^III, As^III and Te^IV) hinder the in-vitro interaction of these two enzymes. Surprisingly, metalloids at sub-lethal concentrations induced the in-vivo interaction of LdG6PDH and LdTryR, as analyzed by pull-down assays and fluorescence microscopy signifying protection against metalloid mediated ROS. Inhibition of LdTryR by thioridazine in LdG6PDH^-/- parasites resulted in metalloid induced apoptotic death of the parasites due to abrupt fall in reduced thiol content, disrupted NADPH/NADP⁺ homeostasis and lethal oxidative stress. Interestingly, clinical isolates of L.donovani resistant to SAG exhibited enhanced interaction between LdG6PDH and LdTryR and showed cross resistivity towards As^III and Te^IV. Thus, our findings propose the metabolon of LdG6PDH and LdTryR as an alternate therapeutic target and provide mechanistic insight about metalloid resistance in Visceral Leishmaniasis.

Biophysical and Pharmacological Characterization of Energy-Dependent Efflux of Sb in Laboratory-Selected Resistant Strains of Leishmania (Viannia) Subgenus

The growing resistance of leishmaniasis to first-line drugs like antimonials in some regions limits the control of this parasitic disease. The precise mechanisms involved in Leishmania antimony resistance are still subject to debate. The reduction of intracellular Sb^III accumulation is a common change observed in both laboratory-selected and field isolated resistant Leishmania strains, but the exact transport pathways involved in antimony resistance have not yet been elucidated. In order to functionally characterize the antimony transport routes responsible for resistance, we performed systematic transport studies of Sb^III in wild-type and resistant strains of L. (Viannia) guyanensis and L. (V.) braziliensis. Those include influx and efflux assays and the influence of ABC transporters and metabolism inhibitors: prochlorperazine, probenecid, verapamil, BSO, and sodium azide. The mRNA levels of genes associated with antimony resistance (MRPA, GSH1, ODC, AQP1, ABCI4, and ARM58) were also investigated in addition to intracellular thiol levels. A strong reduction of Sb influx was observed in L. guyanensis resistant mutant (LgSbR), but not in L. braziliensis (LbSbR). Both mutants showed increased energy-dependent efflux of Sb^III, when compared to their respective parental strains. In LgSbR, BSO and prochlorperazine inhibited antimony efflux and resistance was associated with increased MRPA and GSH1 mRNA levels, while in LbSbR antimony efflux was inhibited by probenicid and prochlorperazine in absence of resistance-associated gene modulation. Intracellular thiol levels were increased in both Sb-resistant mutants. An energy-dependent Sb^III efflux pathway sensitive to prochlorperazine was clearly evidenced in both Sb-resistant mutants. In conclusion, the present study allowed the biophysical and pharmacological characterization of energy-dependent Sb efflux pathway apparently independent of MRPA, ABCI4, and ARM58 upregulation, in Leishmania (Vianna) mutant selected in vitro for resistance to Sb^III. Prochlorperazine has also been identified as an effective chemosensitizer in both Sb resistant mutants, which acts through inhibition of the active efflux of Sb.

The overexpression of genes of thiol metabolism contribute to drug resistance in clinical isolates of visceral leishmaniasis (kala azar) in India

Background

Visceral leishmaniasis (VL), also called Kala Azar (KA) or black fever in India, claims around 20,000 lives every year. Chemotherapy remains one of the most important tools in the control of VL. Current chemotherapy for Kala Azar in India relies on a rather limited arsenal of drugs including sodium antimony gluconate and amphotericin B in addition to the very expensive drug miltefosine. Pentavalent antimonials have been used for more than half a century in the therapy of leishmaniasis as it is relatively safe and inexpensive, however, the spread of resistance to this drug is forcing clinicians in India to abandon this treatment. Consequently, improvement of antimonial chemotherapy has become a major challenging area of study by leishmaniacs worldwide. The alarming emergence of resistance to the commonly used antleishmanial drug, sodium antimony gluconate, in India, has led us to elucidate the resistance mechanism(s) in clinical isolates. Studies on laboratory mutants have shown that resistance to antimonials is highly dependent on thiol levels. The parasite evades cytotoxic effects of antimonial therapy by enhanced efflux of drug upon conjugation with thiols, through overexpressed membrane proteins belonging to the superfamily of ABC transporters.

Methods

We have carried out functional studies to determine the activity of the efflux pumps in antimonial resistant clinical isolates collected from disease endemic areas in India and also carried out molecular characterization of thiol levels in these parasites.

Results

Overexpression of the gene coding for γ glutamylcysteine synthetase was observed in these resistant clinical isolates thereby establishing that thiols represent the key determinants of antimonial resistance. The SbIII/thiol conjugates can be sequestered by ABC transporter multidrug resistance protein A (MRPA) into intracellular organelles or can be directly pumped out by an uncharacterized transporter.

Conclusions

Our studies investigating antimonial resistance in different L. donovani clinical isolates suggest that over functioning of MRP plays a role in generation of antimony resistance phenotype in some L. donovani clinical isolates.

Antimony transport mechanisms in resistant leishmania parasites

Antimonial compounds have been used for more than a century in the treatment of the parasitic disease leishmaniasis. Although pentavalent antimonials are still first-line drugs in several developing countries, this class of drugs is no longer recommended in the Indian sub-continent because of the emergence of drug resistance. The precise mechanisms involved in the resistance of leishmania parasites to antimony are still subject to debate. It is now well documented that drug resistance in leishmania parasites is a multifactorial phenomenon involving multiple genes whose expression pattern synergistically leads to the resistance phenotype. The reduction of intracellular antimony accumulation is a frequent change observed in resistant leishmania cells; however, no comprehensive transport model has been presented so far to explain this change and its contribution to Leishmania resistance. The present review firstly covers the actual knowledge on the metabolism of antimonial drugs, the mechanisms of their transmembrane transport and intracellular processing in Leishmania. It further describes both the functional and molecular changes associated with Sb resistance in this organism. Possible transport models based on the actual knowledge are then presented, as well as their functional implications. Biophysical and pharmacological strategies are finally proposed for the precise identification of the transport pathways.

Identification of specific reversal agents for Leishmania ABCI4-mediated antimony resistance by flavonoid and trolox derivative screening

OBJECTIVES

To identify reversal agents for the Leishmania ABCI4 transporter that confers resistance to antimony.

METHODS

Selective ABCI4 inhibitors among a series of 15 flavonoid and trolox derivatives or analogues were investigated by evaluating their ability to reverse antimony resistance in Leishmania parasites overexpressing ABCI4. Among the compounds screened, N-ethyltrolox carboxamide (compound D2) produced the highest reversal activity. In order to optimize the activity of D2, we synthesized a series of 10 derivatives by condensation of various amines with trolox.

RESULTS

Analysis of antimony resistance reversal activity showed that N-propyltrolox carboxamide (compound D4) was the most potent ABCI4 inhibitor, with reversal activity being maintained in the intracellular amastigote stage. In addition, trolox derivatives significantly reverted the resistance to zinc protoporphyrin. The mechanism of action of these active derivatives was found to be related to significant reversion of Sb(III) and zinc protoporphyrin accumulation and to a decrease in drug efflux.

CONCLUSIONS

Our findings suggest that trolox derivatives D2 and D4 could be considered to be specific reversal agents targeting the Leishmania ABCI4 transporter. The structure-activity relationship obtained in the present study highlights the importance of the size and length of the alkyl substituent linked to trolox. Furthermore, the structural data obtained provide valuable information for the further development of new, even more specific and potent Leishmania ABCI4 reversal agents.

Drug resistance in leishmaniasis: current drug-delivery systems and future perspectives

Leishmaniasis is a complex of diseases with numerous clinical manifestations for instance harshness from skin lesions to severe disfigurement and chronic systemic infection in the liver and spleen. So far, the most classical leishmaniasis therapy, despite its documented toxicities, remains pentavalent antimonial compounds. The arvailable therapeutic modalities for leishmaniasis are overwhelmed with resistance to leishmaniasis therapy. Mechanisms of classical drug resistance are often related with the lower drug uptake, increased efflux, the faster drug metabolism, drug target modifications and over-expression of drug transporters. The high prevalence of leishmaniasis and the appearance of resistance to classical drugs reveal the demand to develop and explore novel, less toxic, low cost and more promising therapeutic modalities. The review describes the mechanisms of classical drug resistance and potential drug targets in Leishmania infection. Moreover, current drug-delivery systems and future perspectives towards Leishmaniasis treatment are also covered.

Role of efflux pumps and intracellular thiols in natural antimony resistant isolates of Leishmania donovani

BACKGROUND

In view of the recent upsurge in the phenomenon of therapeutic failure, drug resistance in Leishmania, developed under natural field conditions, has become a great concern yet little understood. Accordingly, the study of determinants of antimony resistance is urgently warranted. Efflux transporters have been reported in Leishmania but their role in clinical resistance is still unknown. The present study was designed to elucidate the mechanism of natural antimony resistance in L. donovani field isolates by analyzing the functionality of efflux pump(s) and expression profiles of known genes involved in transport and thiol based redox metabolism.

METHODOLOGY/PRINCIPAL FINDINGS

We selected 7 clinical isolates (2 sensitive and 5 resistant) in addition to laboratory sensitive reference and SbIII resistant mutant strains for the present study. Functional characterization using flow cytometry identified efflux pumps that transported substrates of both P-gp and MRPA and were inhibited by the calmodulin antagonist trifluoperazine. For the first time, verapamil sensitive efflux pumps for rhodamine 123 were observed in L. donovani that were differentially active in resistant isolates. RT-PCR confirmed the over-expression of MRPA in isolates with high resistance index only. Resistant isolates also exhibited consistent down regulation of AQP1 and elevated intracellular thiol levels which were accompanied with increased expression of ODC and TR genes. Interestingly, γ-GCS is not implicated in clinical resistance in L. donovani isolates.

CONCLUSIONS/SIGNIFICANCE

Here we demonstrate for the first time, the role of P-gp type plasma membrane efflux transporter(s) in antimony resistance in L. donovani field isolates. Further, decreased levels of AQP1 and elevated thiols levels have emerged as biomarkers for clinical resistance.

Development of an in vitro drug sensitivity assay for Trichuris muris first-stage larvae

BACKGROUND

Trichuriasis represents a major public health problem in the developing world and is regarded as a neglected disease. Albendazole and mebendazole, the two drugs of choice against trichuriasis display only moderate cure rates, hence alternative drugs are needed. To identify candidate compounds, in vitro drug sensitivity testing currently relies on the adult Trichuris muris motility assay. The objective of the present study was to develop a simple and cost-effective drug sensitivity assay using Trichuris muris first-stage larvae (L1).

METHODS

Several potential triggers that induce hatching of T. muris were studied, including gastrointestinal enzymes, acidic environment and intestinal microflora. Next, optimal culture conditions for T. muris L1 were determined assessing a wide range of culture media. T. muris L1 were incubated in the presence of mebendazole, ivermectin, nitazoxanide, levamisole or oxantel pamoate at 37°C. The viability of the parasites was evaluated microscopically after 24 hours. The usefulness of fluorescent markers (resazurin, calcein AM, ethidium homodimer-1 or fluorescein-conjugated albumin) in drug sensitivity testing was also assessed.

RESULTS

The established L1 motility assay provided accurate and reproducible drug effect data in vitro. IC50 values for oxantel pamoate, levamisole and nitazoxanide were 0.05, 1.75 and 4.43 μg/mL, respectively. Mebendazole and ivermectin failed to show any trichuricidal effect on L1. No correlation was found between data from the four fluorescent markers and the comparative motility assay.

CONCLUSIONS

The motility assay based on L1 was found suitable for drug sensitivity screening. It is rather simple, cost-effective, time-saving and sustains medium-throughput testing. Furthermore, it greatly reduces the need for the animal host and is therefore more ethical. None of the viability markers assessed in this study were found to be satisfactory.

Characterization of Leishmania (Leishmania) amazonensis promastigotes resistant to pentamidine

Pentamidine is a second-line agent used in the treatment of leishmaniasis and its mode of action and mechanism of resistance is not well understood. It was previously demonstrated that transfection of promastigotes and amastigotes with the ABC transporter PRP1 gene confers resistance to pentamidine. To further clarify this point, we generated Leishmania amazonensis mutants resistant to pentamidine. Our results indicated that this ABC transporter is not associated with pentamidine resistance in lines generated by drug pressure through amplification or overexpression mechanisms of PRP1 gene.

Role of the ABC transporter PRP1 (ABCC7) in pentamidine resistance in Leishmania amastigotes

Pentamidine is a second-line agent in the treatment of leishmaniasis whose mode of action and resistance mechanism are not well understood. In this work, we show that the intracellular ABC protein PRP1 (pentamidine resistance protein 1) (ABCC7) can confer resistance to pentamidine in Leishmania sp. parasites in the intracellular stage.

Leishmania spp.: proficiency of drug-resistant parasites

Leishmaniasis is a disease caused by at least 17 different species of protozoan Leishmania parasites and currently affects around 12 million people living mostly in tropical and subtropical areas. Failure to treat leishmaniasis successfully is often due to drug resistance. However, there are no cellular and molecular markers of chemoresistance against leishmanicidal drugs and the only reliable method for monitoring resistance of individual isolates is the in vitro amastigote/macrophage model. It is thus necessary to find cellular and molecular markers that can be used systematically to identify the drug-resistant phenotype of the infecting parasites. Until now, whether drug resistance in Leishmania compromises parasite proficiency, e.g. in terms of infectivity or metabolism, has not been systematically evaluated. Therefore, here we examine whether the physiological changes expressed by drug-resistant Leishmania reflect a modification of parasite vitality in drug-resistant compared with drug-sensitive parasites. Finally, the clinical implications of drug resistance in Leishmania are also discussed.

Leishmania amazonensis: Metabolic adaptations induced by resistance to an ABC transporter blocker

We compared growth rate, cell glucose turnover and expression of ATP-binding-cassette (ABC) transporters in Leishmania amazonensis (LTB0016; LTB) versus LTB(160) selected for resistance against the ABC transporter blocker glibenclamide. Additionally, we evaluated the influence of drug-resistance on Leishmania sensitivity against 2-mercaptoacetate and 2-deoxyglucose. Our data demonstrate that (1) LTB(160) and LTB constitutively express ABC transporters for neutral substrates, (2) glibenclamide resistance induces the expression of organic anion ABC transporters, members of the drug resistance associated transporters subfamily, (3) LTB(160) parasites use less glucose as energy substrate and exhibit a slower glucose uptake than LTB cells, and (4) LTB(160) parasites are less sensitive to 2-mercaptoacetate and 2-deoxyglucose than the glibenclamide-sensitive Leishmania LTB. Together these and previous results indicate that the metabolic adaptations expressed in drug-resistant LTB(160) differ from those described for mammalian drug resistant cells and constitute general mechanisms that underlie drug resistance in Leishmania and may be helpful for identifying alternative strategies to circumvent drug resistance in leishmaniasis.

Roles for mitochondria in pentamidine susceptibility and resistance in Leishmania donovani

Pentamidine resistant Leishmania donovani was raised in the laboratory by stepwise exposure to increasing drug pressure until a line capable of growth in 8 microM pentamidine (R8) had been selected. An IC(50) value of 40 microM was determined for this line, some 50-fold higher than that recorded for the parental wild-type line. The pentamidine resistant promastigotes were cross-resistant to other toxic diamidine derivatives but not to antimonials or substrates of multidrug resistance pumps. Decreased mitochondrial transmembrane potential was observed in pentamidine resistant promastigotes. A substantial net decrease in accumulation of [(3)H]-pentamidine accompanied the resistance phenotype. Inhibitors of P-glycoprotein pumps, including prochlorperazine and trifluoperazine, did not reverse this decreased drug uptake, which distinguishes the L. donovani resistant line studied here from L. mexicana promastigotes previously studied for pentamidine resistance. Kinetic analysis identified a carrier with an apparent K(m) value of 6 microM for pentamidine. No significant difference between wild-type and resistant parasites could be detected with respect to this transporter in rapid uptake experiments. However, in longer-term uptake experiments and also using concentrations of pentamidine up to 1mM, it was demonstrated that wild-type cells, but not resistant cells, could continue to accumulate pentamidine after apparent saturation via the measured transporter had been reached. Agents that diminish the mitochondrial membrane potential inhibited this secondary route. A fluorescent analogue of pentamidine, 2,5-bis-(4-amidophenyl)-3,4-dimethylfuran (DB99), accumulated in the kinetoplast of wild-type but not resistant parasites indicating that uptake of this cationic compound into mitochondria of wild-type cells was more pronounced than in the resistant line. These data together indicate that resistance to pentamidine in L. donovani is associated with alterations to the mitochondria of the parasites, which lead to reduced accumulation of drug.

The phenothiazinium chromophore and the evolution of antimalarial drugs

The phenothiazinium salt methylene blue [3,7-bis(dimethylamino)phenothiazinium chloride] is the oldest known synthetic antimalarial drug, its clinical efficacy having been reported in 1891. The role of methylene blue in the evolution of the modern antimalarial armoury is often unappreciated, yet it can be linked directly to standard drugs such as chloroquine and its congeners. Also, in the face of increasing plasmodial resistance to modern antimalarials, phenothiazinium derivatives have again featured as lead compounds in drug research. The precise mode of action of methylene blue and its commercial analogues against Plasmodium spp. remains a cause for conjecture, having been variously described as nucleic acid intercalation, food vacuole basification, parasite redox cycle interference and haem polymerization inhibition. That the activity of the series may be due to more than one route - i.e. a multifactorial activity - underlines the utility of these compounds in antimalarial research either as single drugs or as adjuvants (partners in a drug combination), particularly in the face of resistant parasitic strains.

Activity of megazol, a trypanocidal nitroimidazole, is associated with DNA damage

DNA damage associated with the trypanocidal activity of megazol [2-amino-5-(1-methyl-5-nitro-2-imidazolyl)-1,3,4-thiadiazole] was shown in experiments in which DNA repair-deficient RAD51(-/-) Trypanosoma brucei mutants were found to be hypersensitive to the drug. Parasites resistant to megazol were selected and showed modest cross-resistance to other trypanocides, although neither drug efflux nor changes to intracellular thiols correlated with resistance.

The in vivo susceptibility of Leishmania donovani to sodium stibogluconate is drug specific and can be reversed by inhibiting glutathione biosynthesis

Resistance to pentavalent antimonial (Sb(v)) agents such as sodium stibogluconate (SSG) is creating a major problem in the treatment of visceral leishmaniasis. In the present study the in vivo susceptibilities of Leishmania donovani strains, typed as SSG resistant (strain 200011) or SSG sensitive (strain 200016) on the basis of their responses to a single SSG dose of 300 mg of Sb(v)/kg of body weight, to other antileishmanial drugs were determined. In addition, the role of glutathione in SSG resistance was investigated by determining the influence on SSG treatment of concomitant treatment with a nonionic surfactant vesicle formulation of buthionine sulfoximine (BSO), a specific inhibitor of the enzyme gamma-glutamylcysteine synthetase which is involved in glutathione biosynthesis, and SSG, on the efficacy of SSG treatment. L. donovani strains that were SSG resistant (strain 200011) and SSG sensitive (strain 200016) were equally susceptible to in vivo treatment with miltefosine, paromomycin and amphotericin B (Fungizone and AmBisome) formulations. Combined treatment with SSG and vesicular BSO significantly increased the in vivo efficacy of SSG against both the 200011 and the 200016 L. donovani strains. However, joint treatment that included high SSG doses was unexpectedly associated with toxicity. Measurement of glutathione levels in the spleens and livers of treated mice showed that the ability of the combined therapy to inhibit glutathione levels was also dependent on the SSG dose used and that the combined treatment exhibited organ-dependent effects. The SSG resistance exhibited by the L. donovani strains was not associated with cross-resistance to other classes of compounds and could be reversed by treatment with an inhibitor of glutathione biosynthesis, indicating that clinical resistance to antimonial drugs should not affect the antileishmanial efficacies of alternative drugs. In addition, it should be possible to identify a treatment regimen that could reverse antimony resistance.

The Drosophila melanogaster multidrug-resistance protein 1 (MRP1) homolog has a novel gene structure containing two variable internal exons

Drosophila melanogaster has a gene very similar to human MRP1 that encodes a full ABC-transporter containing three membrane-spanning domains and two nucleotide-binding domains. This 19 exon insect gene, dMRP (FBgn0032456), spans slightly more than 22 kb. The cDNA SD07655 representing this gene was sequenced and found to contain sequences from 12 exons including single copies of two exons having multiple genomic copies. The gene contains two variant copies of exon 4 and seven of exon 8. While a cDNA contains only one version of each variable exon, all forms of these variable exons were detected in adult fly mRNA. These results predict that Drosophila could make 14 different MRP isoforms from a single gene by substituting different variable exons. This is the first report of any organism using differential splicing of alternative, internal exons, to produce such a large array of MRP isoforms having the same size, but with limited and defined internal variations. Defining the functional differences in the dMRP isoforms should provide clues to the structure/function relationships of the amino acids in these MRP domains, both for the insect enzyme and for those of other species.

Optimizing chemotherapy by measuring reversal of P-glycoprotein activity in plasma membrane vesicles

The appearance of multidrug resistance (MDR) of cancer cells is a major obstacle to successful chemotherapy. Several proteins have been identified that pump chemotherapeutic drugs out of cells, thus bringing about MDR. One representative pump is the P-glycoprotein, whose function can be inhibited by blockers (also known as reversers, modulators or chemosensitizers). In clinical application, many of these blockers are often not effective because they become bound to the plasma of the patients. The extent of plasma binding of the blocker varies in different persons and we have developed a 96-well kit to assay such inter-person differences. The assay uses membrane vesicles isolated from a human lymphoblastoid cell line (CEM Col1000). Uptake of rhodamine into the vesicles was measured with different concentrations of the blockers verapamil and XR9576 in presence of human plasma. The reverser XR9576 is nearly 30 times more effective than the classical blocker verapamil, the relevant K(m) values ranging from 2.66 to 45 nM for XR 9576 and 0.7 to 5.5 microM for verapamil. An even greater difference between these two drugs, nearly 1,000-fold, could be shown also in intact cells by calcein AM uptake experiments.

Resistance to pentamidine in Leishmania mexicana involves exclusion of the drug from the mitochondrion

The uptake of [(3)H]pentamidine into wild-type and drug-resistant strains of Leishmania mexicana was compared. Uptake was carrier mediated. Pentamidine-resistant parasites showed cross-resistance to other toxic diamidine derivatives. A substantial decrease in accumulation of the drug accompanied the resistance phenotype, although the apparent affinity for pentamidine by its carrier was not altered when initial uptake velocity was measured. The apparent V(max), however, was reduced. An efflux of pentamidine could be measured in both wild-type and resistant cells. Only a relatively small proportion of the total accumulated pentamidine was available for efflux in wild-type cells, while in resistant cells the majority of loaded pentamidine was available for release. Pharmacological reagents which diminish the mitochondrial membrane potential reduced pentamidine uptake in wild-type parasites, and the mitochondrial membrane potential was shown to be reduced in resistant cells. A fluorescent analogue of pentamidine, 4',6'-diamidino-2-phenylindole, accumulated in the kinetoplast of wild-type but not resistant parasites. These data together indicate that diamidine drugs accumulate in the Leishmania mitochondrion and that the development of the resistance phenotype is accompanied by lack of mitochondrial accumulation of the drug and its exclusion from the parasites.

Genetic rescue of Leishmania deficiency in porphyrin biosynthesis creates mutants suitable for analysis of cellular events in uroporphyria and for photodynamic therapy

Leishmania was found deficient in at least five and most likely seven of the eight enzymes in the heme biosynthesis pathway, accounting for their growth requirement for heme compounds. The xenotransfection of this trypanosomatid protozoan led to their expression of the mammalian genes encoding delta-aminolevulinate (ALA) dehydratase and porphobilinogen deaminase, the second and the third enzymes of the pathway, respectively. These transfectants still require hemin or protoporphyrin IX for growth but produce porphyrin when ALA was supplied exogenously. Leishmania is thus deficient in all first three enzymes of the pathway. Uroporphyrin I was produced as the sole intermediate by these transfectants, further indicating that they are also deficient in at least two porphyrinogen-metabolizing enzymes downstream of porphobilinogen deaminase, i.e. uroporphyrinogen III co-synthase and uroporphyrinogen decarboxylase. Pulsing the transfectants with ALA induced their transition from aporphyria to uroporphyria. Uroporphyrin I emerged in these cells initially as diffused throughout the cytosol, rendering them sensitive to UV irradiation. The porphyrin was subsequently sequestered in cytoplasmic vacuoles followed by its release and accumulation in the extracellular milieu, concomitant with a reduced photosensitivity of the cells. These events may represent cellular mechanisms for disposing soluble toxic waste from the cytosol. Monocytic tumor cells were rendered photosensitive by infection with uroporphyric Leishmania, suggestive of their potential application for photodynamic therapy.

Chemosensitization of a multidrug-resistant Leishmania tropica line by new sesquiterpenes from Maytenus magellanica and Maytenus chubutensis

Parasite resistance to drugs has emerged as a major problem in current medicine, and therefore, there is great clinical interest in developing compounds that overcome these resistances. In an intensive study of South American medicinal plants, herein we report the isolation, structure elucidation, and biological activity of dihydro-beta-agarofuran sesquiterpenes from the roots of Maytenus magellanica (1-14) and M. chubutensis (14-17). This type of natural products may be considered as privileged structures. The structures of 10 new compounds, 1, 3, 6-9, and12-15, were determined by means of (1)H and (13)C NMR spectroscopic studies, including homonuclear (COSY and ROESY) and heteronuclear correlation experiments (HMQC and HMBC). The absolute configurations of eight hetero- and homochromophoric compounds, 1, 3,6-9, 12, and 13, were determined by means of CD studies. Fourteen compounds, 1-3 and 6-16, have been tested on a multidrug-resistant Leishmania tropica line overexpressing a P-glycoprotein-like transporter to determine their ability to revert the resistance phenotype and to modulate intracellular drug accumulation. From this series, 1, 2, 3, 14, and 15 showed potent activity, 1 being the most active compound. The structure-activity relationships of the different compounds are discussed.

Inhibition of the P-glycoprotein- and multidrug resistance protein-mediated efflux of anthracyclines and calceinacetoxymethyl ester by PAK-104P

Multidrug resistance phenotype in mammalian cells is often correlated with overexpression of P-glycoprotein or Multidrug Resistance-Associated protein (MRP(1)). Both proteins are energy-dependent drug efflux pumps that efficiently reduce the intracellular accumulation and hence the cytotoxicity of many natural cytotoxins. Overexpression of these transporters by tumor cells is thought to be a significant factor in both intrinsic and acquired resistance to anticancer drugs. Consequently a great deal of interest is focused on identifying chemical agents that can either antagonise drug transport by these proteins or that can inhibit the proliferation of tumors cells despite the expression of these transporters. P-glycoprotein-mediated multidrug resistance is reversed by a variety of compounds, but surprisingly, few agents reverse the MRP(1)-mediated multidrug resistance. However, it has recently been shown that 2-[4-(diphenylmethyl)-1-piperazinyl]ethyl-5-(trans-4,6-dimethyl-1, 3, 2-dioxaphosphorinan-2-yl)-2, 6-dimethyl-4-(3-nitrophenyl)-3-pyridinecarboxylate P oxide (PAK-104P) was able to inhibit the P-glycoprotein and MRP(1)-mediated efflux of several compounds. Understanding of the interactions between transporters and multidrug resistance reversing agents is important in the design of more effective multidrug resistance modulators. We now examined the effect of PAK-104P on Pgp-and MRP1-mediated efflux of three anthracyclines, daunorubicin, pirarubicin, hydroxydoxorubicin and of calcein acetoxymethyl ester and calcein. Our data show that PAK-104P non-competitively inhibits the P-glycoprotein-mediated efflux of anthracycline derivatives and calcein acetoxymethyl ester with an inhibitory constant K(I)=0. 25+/-0.05 microM. PAK-104P also non-competitively inhibits the MRP(1)-mediated efflux of daunorubicin, pirarubicin, hydroxyrubicin, calcein acetoxymethyl ester and calcein. However, surprisingly, in this case the K(I) values obtained were very different ranging from 0.06 for hydroxyrubicin to 10 microM for calcein. These data strongly suggested the existence of two different mechanisms for the inhibition by PAK-104P of the MRP(1)-mediated efflux of molecules: a first mechanism, involving a low-affinity site for PAK-104P, and which would concern molecules such as calcein, cysteinyl leukotriene LCT(4) etc. whose efflux do not depend on glutathione. A second mechanism involving a high-affinity site for PAK-104P and which would concern molecules such as anthracyclines, calcein acetoxymethyl ester whose efflux depends on the presence of glutathione.