Intracellular amastigote replication may not be required for successful in vitro selection of miltefosine resistance in Leishmania infantum

DNDI-6174 is a preclinical candidate for visceral leishmaniasis that targets the cytochrome bc1

New drugs for visceral leishmaniasis that are safe, low cost, and adapted to the field are urgently required. Despite concerted efforts over the last several years, the number of new chemical entities that are suitable for clinical development for the treatment of Leishmania remains low. Here, we describe the discovery and preclinical development of DNDI-6174, an inhibitor of Leishmania cytochrome bc1 complex activity that originated from a phenotypically identified pyrrolopyrimidine series. This compound fulfills all target candidate profile criteria required for progression into preclinical development. In addition to good metabolic stability and pharmacokinetic properties, DNDI-6174 demonstrates potent in vitro activity against a variety of Leishmania species and can reduce parasite burden in animal models of infection, with the potential to approach sterile cure. No major flags were identified in preliminary safety studies, including an exploratory 14-day toxicology study in the rat. DNDI-6174 is a cytochrome bc1 complex inhibitor with acceptable development properties to enter preclinical development for visceral leishmaniasis.

Identification of Resistance Determinants for a Promising Antileishmanial Oxaborole Series

Current treatment options for visceral leishmaniasis have several drawbacks, and clinicians are confronted with an increasing number of treatment failures. To overcome this, the Drugs for Neglected Diseases initiative (DNDi) has invested in the development of novel antileishmanial leads, including a very promising class of oxaboroles. The mode of action/resistance of this series to Leishmania is still unknown and may be important for its further development and implementation. Repeated in vivo drug exposure and an in vitro selection procedure on both extracellular promastigote and intracellular amastigote stages were both unable to select for resistance. The use of specific inhibitors for ABC-transporters could not demonstrate the putative involvement of efflux pumps. Selection experiments and inhibitor studies, therefore, suggest that resistance to oxaboroles may not emerge readily in the field. The selection of a genome-wide cosmid library coupled to next-generation sequencing (Cos-seq) was used to identify resistance determinants and putative targets. This resulted in the identification of a highly enriched cosmid, harboring genes of chromosome 2 that confer a subtly increased resistance to the oxaboroles tested. Moderately enriched cosmids encompassing a region of chromosome 34 contained the cleavage and polyadenylation specificity factor (cpsf) gene, encoding the molecular target of several related benzoxaboroles in other organisms.

Transmission potential of paromomycin-resistant Leishmania infantum and Leishmania donovani

OBJECTIVES

Former studies demonstrated quick selection of paromomycin resistance for Leishmania infantum and Leishmania donovani accompanied by increased fitness. The present study aimed to interpret these findings in an epidemiological context by comparing infection of WT and experimentally derived paromomycin-resistant strains in the sand fly vector.

METHODS

Depending on the Leishmania species, Lutzomyia longipalpis and Phlebotomus perniciosus or Phlebotomus argentipes sand flies were artificially infected with procyclic promastigotes of WT and paromomycin-resistant L. infantum (MHOM/FR/96/LEM3323-cl4) or L. donovani (MHOM/NP/03/BPK275/0-cl18). The infection rate and gut/stomodeal valve colonization were determined to monitor parasite phenotypic behaviour within the vector. The impact of the previously described gain of fitness in the vertebrate host on infectivity for the vector was assessed by feeding L. longipalpis on Syrian golden hamsters heavily infected with either WT or paromomycin-resistant parasites.

RESULTS

WT and paromomycin-resistant Leishmania of both species behaved similarly in terms of infection and parasite location within the studied sand fly species. Blood feeding on infected hamsters did not reveal differences in acquisition of WT and paromomycin-resistant parasites, despite the higher organ burdens observed for the paromomycin-resistant strain. Strains remained resistant after passage in the vector.

CONCLUSIONS

Although paromomycin-resistant parasites show an increased parasite fitness in vitro and in laboratory rodents, the intrinsic infection potential of paromomycin-resistant parasites remains unaltered in the sand fly. Of importance is the fact that paromomycin-resistant Leishmania are able to complete development in the natural vectors and produce stomodeal infection with metacyclic forms, which clearly suggests their potential to spread and circulate in nature.

Antileishmanial Aminopyrazoles: Studies into Mechanisms and Stability of Experimental Drug Resistance

Current antileishmanial treatment is hampered by limitations, such as drug toxicity and the risk of treatment failure, which may be related to parasitic drug resistance. Given the urgent need for novel drugs, the Drugs for Neglected Diseases initiative (DNDi) has undertaken a drug discovery program, which has resulted in the identification of aminopyrazoles, a highly promising antileishmanial chemical series. Multiple experiments have been performed to anticipate the propensity for resistance development.

Current antileishmanial treatment is hampered by limitations, such as drug toxicity and the risk of treatment failure, which may be related to parasitic drug resistance. Given the urgent need for novel drugs, the Drugs for Neglected Diseases initiative (DNDi) has undertaken a drug discovery program, which has resulted in the identification of aminopyrazoles, a highly promising antileishmanial chemical series. Multiple experiments have been performed to anticipate the propensity for resistance development. Resistance selection was performed by successive exposure of Leishmania infantum promastigotes (in vitro) and intracellular amastigotes (both in vitro and in golden Syrian hamsters). The stability of the resistant phenotypes was assessed after passage in mice and Lutzomyia longipalpis sandflies. Whole-genome sequencing (WGS) was performed to identify mutated genes, copy number variations (CNVs), and somy changes. The potential role of efflux pumps (the MDR and MRP efflux pumps) in the development of resistance was assessed by coincubation of aminopyrazoles with specific efflux pump inhibitors (verapamil, cyclosporine, and probenecid). Repeated drug exposure of amastigotes did not result in the emergence of drug resistance either in vitro or in vivo. Selection at the promastigote stage, however, was able to select for parasites with reduced susceptibility (resistance index, 5.8 to 24.5). This phenotype proved to be unstable after in vivo passage in mice and sandflies, suggesting that nonfixed alterations are responsible for the elevated resistance. In line with this, single nucleotide polymorphisms and indels identified by whole-genome sequencing could not be directly linked to the decreased drug susceptibility. Copy number variations were absent, whereas somy changes were detected, which may have accounted for the transient acquisition of resistance. Finally, aminopyrazole activity was not influenced by the MDR and MRP efflux pump inhibitors tested. The selection performed does not suggest the rapid development of resistance against aminopyrazoles in the field. Karyotype changes may confer elevated levels of resistance, but these do not seem to be stable in the vertebrate and invertebrate hosts. MDR/MRP efflux pumps are not likely to significantly impact the activity of the aminopyrazole leads.

Interferon Alpha Favors Macrophage Infection by Visceral Leishmania Species Through Upregulation of Sialoadhesin Expression

Type I interferons (IFNs) induced by an endogenous Leishmania RNA virus or exogenous viral infections have been shown to exacerbate infections with New World Cutaneous Leishmania parasites, however, the impact of type I IFNs in visceral Leishmania infections and implicated mechanisms remain to be unraveled. This study assessed the impact of type I IFN on macrophage infection with L. infantum and L. donovani and the implication of sialoadhesin (Siglec-1/CD169, Sn) as an IFN-inducible surface receptor. Stimulation of bone marrow-derived macrophages with type I IFN (IFN-α) significantly enhanced susceptibility to infection of reference laboratory strains and a set of recent clinical isolates. IFN-α particularly enhanced promastigote uptake. Enhanced macrophage susceptibility was linked to upregulated Sn surface expression as a major contributing factor to the infection exacerbating effect of IFN-α. Stimulation experiments in Sn-deficient macrophages, macrophage pretreatment with a monoclonal anti-Sn antibody or a novel bivalent anti-Sn nanobody and blocking of parasites with soluble Sn restored normal susceptibility levels. Infection of Sn-deficient mice with bioluminescent L. infantum promastigotes revealed a moderate, strain-dependent role for Sn during visceral infection under the used experimental conditions. These data indicate that IFN-responsive Sn expression can enhance the susceptibility of macrophages to infection with visceral Leishmania promastigotes and that targeting of Sn may have some protective effects in early infection.

Miltefosine enhances the fitness of a non-virulent drug-resistant Leishmania infantum strain

Objectives

Miltefosine is currently the only oral drug for visceral leishmaniasis, and although deficiency in an aminophospholipid/miltefosine transporter (MT) is sufficient to elicit drug resistance, very few naturally miltefosine-resistant (MIL-R) strains have yet been isolated. This study aimed to make a detailed analysis of the impact of acquired miltefosine resistance and miltefosine treatment on in vivo infection.

Methods

Bioluminescent versions of a MIL-R strain and its syngeneic parental line were generated by integration of the red-shifted firefly luciferase PpyRE9. The fitness of both lines was compared in vitro (growth rate, metacyclogenesis and macrophage infectivity) and in BALB/c mice through non-invasive bioluminescence imaging under conditions with and without drug pressure.

Results

This study demonstrated a severe fitness loss of MT-deficient parasites, resulting in a complete inability to multiply and cause a typical visceral leishmaniasis infection pattern in BALB/c mice. The observed fitness loss could not be rescued by host immune suppression with cyclophosphamide, whereas episomal reconstitution with a wild-type MT restored parasite virulence, hence linking parasite fitness to MT mutation. Remarkably, in vivo miltefosine treatment or in vitro miltefosine pre-exposure significantly rescued MIL-R parasite virulence. The in vitro pre-exposed MIL-R promastigotes showed a longer and more slender morphology, suggesting an altered membrane composition.

Conclusions

The profound fitness loss of MT-deficient parasites most likely explains the low frequency of MIL-R clinical isolates. The observation that miltefosine can reverse this phenotype indicates a drug dependency of the MT-deficient parasites and emphasizes the importance of resistance profiling prior to miltefosine administration.

Phenotypic adaptations of Leishmania donovani to recurrent miltefosine exposure and impact on sand fly infection

Background

Since the introduction of miltefosine (MIL) as first-line therapy in the kala-azar elimination programme in the Indian subcontinent, treatment failure rates have been increasing. Since parasite infectivity and virulence may become altered upon treatment relapse, this laboratory study assessed the phenotypic effects of repeated in vitro and in vivo MIL exposure.

Methods

Syngeneic Leishmania donovani lines either or not exposed to MIL were compared for drug susceptibility, rate of promastigote multiplication and metacyclogenesis, macrophage infectivity and behaviour in the sand fly vector, Lutzomyia longipalpis.

Results

Promastigotes of both in vitro and in vivo MIL-selected strains displayed a slightly reduced drug susceptibility that was associated with a reduced MIL-accumulation linked to a lower copy number (disomic state) of chromosome 13 harboring the miltefosine transporter (LdMT) gene. In vitro selected promastigotes showed a lower rate of metacyclogenesis whereas the in vivo derived promastigotes displayed a moderately increased growth rate. Repeated MIL exposure did neither influence the parasite load nor metacyclogenesis in the sand fly vector.

Conclusions

Recurrent in vitro and in vivo MIL exposure evokes a number of very subtle phenotypic and genotypic changes which could make promastigotes less susceptible to MIL without attaining full resistance. These changes did not significantly impact on infection in the sand fly vector.

In-depth comparison of cell-based methodological approaches to determine drug susceptibility of visceral Leishmania isolates

Monitoring the drug susceptibility of Leishmania isolates still largely relies on standard in vitro cell-based susceptibility assays using (patient-isolated) promastigotes for infection. Although this assay is widely used, no fully standardized/harmonized protocol is yet available hence resulting in the application of a wide variety of host cells (primary cells and cell lines), different drug exposure times, detection methods and endpoint criteria. Advocacy for standardization to decrease inter-laboratory variation and improve interpretation of results has already repeatedly been made, unfortunately still with unsatisfactory progress. As a logical next step, it would be useful to reach at least some agreement on the type of host cell and basic experimental design for routine amastigote susceptibility determination. The present laboratory study using different L. infantum strains as a model for visceral leishmaniasis species compared primary cells (mouse peritoneal exudate (PEC), mouse bone marrow derived macrophages and human peripheral blood monocyte derived macrophages) and commercially available cell lines (THP-1, J774, RAW) for either their susceptibility to infection, their role in supporting intracellular amastigote multiplication and overall feasibility/accessibility of experimental assay protocol. The major findings were that primary cells are better than cell lines in supporting infection and intracellular parasite multiplication, with PECs to be preferred for technical reasons. Cell lines require drug exposure of >96h with THP-1 to be preferred but subject to a variable response to PMA stimulation. The fast dividing J774 and RAW cells out-compete parasite-infected cells precluding proper assay read-out. Some findings could possibly also be applicable to cutaneous Leishmania strains, but this still needs cross-checking. Besides inherent limitations in a clinical setting, susceptibility testing of clinical isolates may remain problematic because of the reliance on patient-derived promastigotes which may exhibit variable degrees of metacyclogenesis and infectivity.

Leishmaniasis is a neglected tropical disease caused by parasites belonging to the genus of Leishmania and transmitted by the bite of infected female sand flies. Concerns about the effective control of the disease are rising in view of the increasing number of treatment failures that may be related to drug resistance. Monitoring of drug susceptibility in the field should become an essential asset, however, there is still insufficient harmonization in the laboratory assays. This study focused on the standard intracellular amastigote susceptibility assay and compared protocol variables, such as type of macrophage host cell (primary versus cell lines), multiplicity of infection and duration of drug exposure. Primary cells perform best with little difference between cells derived from Swiss mice or BALB/c mice. From a practical point of view, mouse peritoneal exudate cells can be recommended. If mice would not be available, THP-1 cells are the best alternative. For field strains, metacyclic promastigotes should be used at a multiplicity of infection of 10–15 parasites per cell with drug exposure starting at 24h post-infection and continued for 120h. Unfortunately, susceptibility testing of clinical isolates will remain problematic because of the reliance on promastigotes which may exhibit variable degrees of metacyclogenesis and infectivity. Opting for cell-based assays may be complicated by the fact that dedicated laboratory infrastructure may sometimes be lacking in disease-endemic countries.

Impaired development of a miltefosine-resistant Leishmania infantum strain in the sand fly vectors Phlebotomus perniciosus and Lutzomyia longipalpis

OBJECTIVES

To gain insight into the propagation of miltefosine (MIL) resistance in visceral leishmaniasis, this laboratory study explored development of resistant parasites with a defective miltefosine transporter (MT) in sand flies.

METHODS

Infectivity, colonization of stomodeal valve and metacyclogenesis of a MIL-resistant (MIL-R) Leishmania infantum LEM3323 line with a defective MT were assessed in the natural sand fly vectors Phlebotomus perniciosus and Lutzomyia longipalpis. Given our recent description of partial drug dependency of the MT-deficient line, the impact of MIL pre-exposure on sand fly infectivity was explored as well.

RESULTS

A significant reduction in sand fly infection, stomodeal valve colonization and differentiation into metacyclics (determined by a lower flagellum/cell body length ratio) was observed in both vectors for MIL-R as compared to the isogenic parent MIL-susceptible line. Re-introduction of the wildtype MT gene into MIL-R tended to partially rescue the capacity to infect sand flies. Pre-exposure to MIL did not alter infectivity of the MIL-R line.

CONCLUSION

The MIL resistant L. infantum LEM3323 line is significantly hampered in its development and transmissibility potential in two sand fly vector species. Additional studies are warranted to evaluate whether this applies to other visceral Leishmania parasites with acquired MIL-resistance.

Immunosuppression of Syrian golden hamsters accelerates relapse but not the emergence of resistance in Leishmania infantum following recurrent miltefosine pressure

Although miltefosine (MIL) has only been approved for the treatment of visceral leishmaniasis (VL) in 2002, its application in monotherapy already led to the development of two confirmed MIL-resistant isolates by 2009. Although liposomal amphotericin B is recommended as first-line treatment in Europe, MIL is still occasionally used in HIV co-infected patients. Since their immune system is incapable of controlling the infection, high parasite burdens and post-treatment relapses are common. Linked to the particular pharmacokinetic profile of MIL, successive treatment of recurrent relapses could in principle facilitate the emergence of drug resistance. This study evaluated the effect of immunosuppression (cyclophosphamide 150 mg/kg once weekly) on the development of MIL-resistance in Syrian golden hamsters infected with Leishmania infantum. The hamsters were treated with MIL (20 mg/kg orally for 5 days) whenever clinical signs of infection or relapse were observed. The immunosuppression resulted in a significant depletion of CD4⁺ lymphocytes and MHCII-expressing cells in peripheral blood, and a concomitant increase in tissue parasite burdens and shorter time to relapse, but the strain's susceptibility upon repeated MIL exposure remained unaltered. This study demonstrates that immunosuppression accelerates the occurrence of relapse without expediting MIL resistance development.

Impact of primary mouse macrophage cell types on Leishmania infection and in vitro drug susceptibility

Primary mouse macrophages are frequently used to provide an in vitro intracellular model to evaluate antileishmanial drug efficacy. The present study compared the phenotypic characteristics of Swiss, BALB/c, and C57BL/6 mouse bone marrow-derived macrophages and peritoneal exudate cells using different stimulation and adherence protocols upon infection with a Leishmania infantum laboratory strain and two clinical isolates. Evaluation parameters were susceptibility to infection, permissiveness to amastigote multiplication, and impact on drug efficacy. Observed variations in infection of peritoneal exudate cells can mostly be linked to changes in the inflammatory cytokine profiles (IL-6, TNF-α, KC/GRO) rather than to differences in initial production of nitric oxide and reactive oxygen species. Optimization of the cell stimulation and adherence conditions resulted in comparable infection indices among peritoneal exudate cells and the various types of bone marrow-derived macrophages. BALB/c-derived bone marrow-derived macrophages were slightly more permissive to intracellular amastigote replication. Evaluation of antileishmanial drug potency in the various cell systems revealed minimal variation for antimonials and paromomycin, and no differences for miltefosine and amphotericin B. The study results allow to conclude that drug evaluation can be performed in all tested primary macrophages as only marginal differences are observed in terms of susceptibility to infection and impact of drug exposure. Combined with some practical considerations, the use of 24-h starch-stimulated, 48-h adhered, Swiss-derived peritoneal exudate cells can be advocated as an efficient, reliable, relatively quick, and cost-effective tool for routine drug susceptibility testing in vitro whenever the use of primary cells is feasible.

Genomic and transcriptomic alterations in Leishmania donovani lines experimentally resistant to antileishmanial drugs

Leishmaniasis is a serious medical issue in many countries around the World, but it remains largely neglected in terms of research investment for developing new control and treatment measures. No vaccines exist for human use, and the chemotherapeutic agents currently used are scanty. Furthermore, for some drugs, resistance and treatment failure are increasing to alarming levels. The aim of this work was to identify genomic and trancriptomic alterations associated with experimental resistance against the common drugs used against VL: trivalent antimony (Sb^III, S line), amphotericin B (AmB, A line), miltefosine (MIL, M line) and paromomycin (PMM, P line). A total of 1006 differentially expressed transcripts were identified in the S line, 379 in the A line, 146 in the M line, and 129 in the P line. Also, changes in ploidy of chromosomes and amplification/deletion of particular regions were observed in the resistant lines regarding the parental one. A series of genes were identified as possible drivers of the resistance phenotype and were validated in both promastigotes and amastigotes from Leishmania donovani, Leishmania infantum and Leishmania major species. Remarkably, a deletion of the gene LinJ.36.2510 (coding for 24-sterol methyltransferase, SMT) was found to be associated with AmB-resistance in the A line. In the P line, a dramatic overexpression of the transcripts LinJ.27.T1940 and LinJ.27.T1950 that results from a massive amplification of the collinear genes was suggested as one of the mechanisms of PMM resistance. This conclusion was reinforced after transfection experiments in which significant PMM-resistance was generated in WT parasites over-expressing either gene LinJ.27.1940 (coding for a D-lactate dehydrogenase-like protein, D-LDH) or gene LinJ.27.1950 (coding for an aminotransferase of branched-chain amino acids, BCAT). This work allowed to identify new drivers, like SMT, the deletion of which being associated with resistance to AmB, and the tandem D-LDH-BCAT, the amplification of which being related to PMM resistance.

Combined treatment of miltefosine and paromomycin delays the onset of experimental drug resistance in Leishmania infantum

Background

Since miltefosine monotherapy against visceral leishmaniasis (VL) caused by Leishmania donovani has been discontinued in the Indian subcontinent due to an increase in the number of treatment failures, single dose liposomal amphotericin B is now advocated as a treatment option of choice. Paromomycin-miltefosine combination therapy can be used as substitute first-line treatment in regions without cold-chain potential. Previous laboratory studies in the closely related species Leishmania infantum have demonstrated that paromomycin monotherapy fairly rapidly selects for resistance producing a phenotype with increased fitness. Given the possible clinical implications of these findings for the current field situation, the present study aimed to identify the potential hazards of paromomycin-miltefosine combination therapy.

Principal findings

Drug interaction studies using the fixed-ratio isobologram method revealed an indifferent interaction between paromomycin and miltefosine. In hamsters infected with L. infantum, the combination resulted in cumulative efficacy in reducing parasite burdens in the liver, spleen and bone marrow. Selected resistant lines against the single drugs did not display cross-resistance. When the intracellular amastigote stage was repeatedly exposed to the paromomycin-miltefosine combination, either in vitro or in vivo, no significant susceptibility decrease towards either drug was noted.

Conclusions

These results suggest that implementation of paromomycin-miltefosine combination therapy indeed could represent a safe and affordable treatment option for L. donovani VL as miltefosine appears to overrule the anticipated rapid development of PMM resistance.

Liposomal amphotericin B is presently being used as first-line treatment option against visceral leishmaniasis in the Indian subcontinent. However, the need for temperature-controlled transport and storage limits its widespread use in rural areas. Previous studies already suggested that paromomycin-miltefosine combination therapy could be a valuable alternative, side passing some of the disadvantages associated with monotherapy, such as development of drug resistance. As the first reports of miltefosine resistant clinical isolates have already surfaced and paromomycin resistance could be easily induced under laboratory conditions, it remains essential to assess the risk of developing resistance against both drugs upon combination therapy. This study evaluated the efficacy of combined therapy against a Leishmania species closely related to the agent found in the Indian subcontinent, using both in vitro and in vivo models with the aim to select multidrug-resistant species by simultaneous exposure to paromomycin and miltefosine. The combination of both drugs in the hamster model resulted in a cumulative efficacy but did not lead to a significant susceptibility decrease, indicating that paromomycin-miltefosine combination therapy may represent a safe and affordable treatment option for visceral leishmaniasis.

Evaluating drug resistance in visceral leishmaniasis: the challenges

For decades antimonials were the drugs of choice for the treatment of visceral leishmaniasis (VL), but the recent emergence of resistance has made them redundant as first-line therapy in the endemic VL region in the Indian subcontinent. The application of other drugs has been limited due to adverse effects, perceived high cost, need for parenteral administration and increasing rate of treatment failures. Liposomal amphotericin B (AmB) and miltefosine (MIL) have been positioned as the effective first-line treatments; however, the number of monotherapy MIL-failures has increased after a decade of use. Since no validated molecular resistance markers are yet available, monitoring and surveillance of changes in drug sensitivity and resistance still depends on standard phenotypic in vitro promastigote or amastigote susceptibility assays. Clinical isolates displaying defined MIL- or AmB-resistance are still fairly scarce and fundamental and applied research on resistance mechanisms and dynamics remains largely dependent on laboratory-generated drug resistant strains. This review addresses the various challenges associated with drug susceptibility and -resistance monitoring in VL, with particular emphasis on the choice of strains, susceptibility model selection and standardization of procedures with specific read-out parameters and well-defined threshold criteria. The latter are essential to support surveillance systems and safeguard the limited number of currently available antileishmanial drugs.

Genomic and Molecular Characterization of Miltefosine Resistance in Leishmania infantum Strains with Either Natural or Acquired Resistance through Experimental Selection of Intracellular Amastigotes

During the last decade miltefosine (MIL) has been used as first-line treatment for visceral leishmaniasis in endemic areas with antimonial resistance, but a decline in clinical effectiveness is now being reported. While only two MIL-resistant Leishmania infantum strains from HIV co-infected patients have been documented, phenotypic MIL-resistance for L. donovani has not yet been identified in the laboratory. Hence, a better understanding of the factors contributing to increased MIL-treatment failure is necessary. Given the paucity of defined MIL-resistant L. donovani clinical isolates, this study used an experimental amastigote-selected MIL-resistant L. infantum isolate (LEM3323). In-depth exploration of the MIL-resistant phenotype was performed by coupling genomic with phenotypic data to gain insight into gene function and the mutant phenotype. A naturally MIL-resistant L. infantum clinical isolate (LEM5159) was included to compare both datasets. Phenotypically, resistance was evaluated by determining intracellular amastigote susceptibility in vitro and actual MIL-uptake. Genomic analysis provided supportive evidence that the resistance selection model on intracellular amastigotes can be a good proxy for the in vivo field situation since both resistant strains showed mutations in the same inward transporter system responsible for the acquired MIL-resistant phenotype. In line with previous literature findings in promastigotes, our data confirm a defective import machinery through inactivation of the LiMT/LiRos3 protein complex as the main mechanism for MIL-resistance also in intracellular amastigotes. Whole genome sequencing analysis of LEM3323 revealed a 2 base pair deletion in the LiMT gene that led to the formation an early stop codon and a truncation of the LiMT protein. Interestingly, LEM5159 revealed mutations in both the LiMT and LiRos3 genes, resulting in an aberrant expression of the LiMT protein. To verify that these mutations were indeed accountable for the acquired resistance, transfection experiments were performed to re-establish MIL-susceptibility. In LEM3323, susceptibility was restored upon expression of a LiMT wild-type gene, whereas the MIL-susceptibility of LEM5159 could be reversed after expression of the LiRos3 wild-type gene. The aberrant expression profile of the LiMT protein could be restored upon rescue of the LiRos3 gene both in the LEM5159 clinical isolate and a ΔLiRos3 strain, showing that expression of LdMT is dependent on LdRos3 expression. The present findings clearly corroborate the pivotal role of the LiMT/LiRos3 complex in resistance towards MIL.

Evidence of a drug-specific impact of experimentally selected paromomycin and miltefosine resistance on parasite fitness in Leishmania infantum

OBJECTIVES

Although miltefosine and paromomycin were only recently introduced to treat visceral leishmaniasis, increasing numbers of miltefosine treatment failures and occasional primary resistance to both drugs have been reported. Understanding alterations in parasite behaviour linked to drug resistance is essential to assess the propensity for emergence and spread of resistant strains, particularly since a positive effect on fitness has been reported for antimony-resistant parasites. This laboratory study compared the fitness of a drug-susceptible parent WT clinical Leishmania infantum isolate (MHOM/FR/96/LEM3323) and derived miltefosine and paromomycin drug-resistant lines that were experimentally selected at the intracellular amastigote level.

METHODS

Parasite fitness of WT, paromomycin-resistant and miltefosine-resistant strains, in vitro and in vivo parasite growth, metacyclogenesis, infectivity and macrophage stress responses were comparatively evaluated.

RESULTS

No significant differences in promastigote fitness were noted between the WT and paromomycin-resistant strain, while clear benefits could be demonstrated for paromomycin-resistant amastigotes in terms of enhanced in vitro and in vivo growth potential and intracellular stress response. The miltefosine-resistant phenotype showed incomplete promastigote metacyclogenesis, decreased intracellular growth and weakened stress response, revealing a reduced fitness compared with WT parent parasites.

CONCLUSIONS

The rapid selection and fitness advantages of paromomycin-resistant amastigotes endorse the current use of paromomycin in combination therapy. Although a reduced fitness of miltefosine-resistant strains may explain the difficulty of miltefosine resistance selection in vitro, the growing number of miltefosine treatment failures in the field still requires further exploratory research.