In vitro activity of pentavalent antimony derivatives on promastigotes and intracellular amastigotes of Leishmania infantum strains from humans and dogs in Spain

Challenges and Tools for In Vitro Leishmania Exploratory Screening in the Drug Development Process: An Updated Review

Leishmaniases are a group of vector-borne diseases caused by infection with the protozoan parasites Leishmania spp. Some of them, such as Mediterranean visceral leishmaniasis, are zoonotic diseases transmitted from vertebrate to vertebrate by a hematophagous insect, the sand fly. As there is an endemic in more than 90 countries worldwide, this complex and major health problem has different clinical forms depending on the parasite species involved, with the visceral form being the most worrying since it is fatal when left untreated. Nevertheless, currently available antileishmanial therapies are significantly limited (low efficacy, toxicity, adverse side effects, drug-resistance, length of treatment, and cost), so there is an urgent need to discover new compounds with antileishmanial activity, which are ideally inexpensive and orally administrable with few side effects and a novel mechanism of action. Therefore, various powerful approaches were recently applied in many interesting antileishmanial drug development programs. The objective of this review is to focus on the very first step in developing a potential drug and to identify the exploratory methods currently used to screen in vitro hit compounds and the challenges involved, particularly in terms of harmonizing the results of work carried out by different research teams. This review also aims to identify innovative screening tools and methods for more extensive use in the drug development process.

Phenotype evaluation of human and canine isolates of Leishmania infantum

Human visceral leishmaniasis (VL) and canine leishmaniasis (CanL) in countries of South and Central America are caused by Leishmania infantum and has been endemic in Brazil for several years. The parasite biodiversity as well as the pharmacologic properties of drugs and the host species, are involved in the efficacy or inefficacy of leishmaniasis treatments. Although there are substantial number of reports describing the genetic characterization of the clinical field isolates of L. infantum,the phenotypic parameters have been less studied. In this study isolates from human and canine leishmaniasis (Hum1 and Can1) obtained in Campinas, São Paulo state, Brazil were identified as L. infantum. The Hum1 and Can1 isolates exhibited typical promastigote growth pattern. Regarding morphological features Can1 isolate differed in cell size. The infectivity in vitro of both isolatesis lower compared to the reference strain of L. infantum. Moreover, the in vivo infectivity of the three parasites is similar in Balb/c mice. The Hum1 isolate is more sensitive to leishmanial drugs (amphotericin B, miltefosine and glucantime) than the Can1 isolate when inside human macrophages, but not when inside canine macrophages. These findings indicated that L. infantum isolates differs in some phenotypic characteristics.

Topical Amphotericin B Semisolid Dosage Form for Cutaneous Leishmaniasis: Physicochemical Characterization, Ex Vivo Skin Permeation and Biological Activity

Amphotericin B (AmB) is a potent antifungal successfully used intravenously to treat visceral leishmaniasis but depending on the Leishmania infecting species, it is not always recommended against cutaneous leishmaniasis (CL). To address the need for alternative topical treatments of CL, the aim of this study was to elaborate and characterize an AmB gel. The physicochemical properties, stability, rheology and in vivo tolerance were assayed. Release and permeation studies were performed on nylon membranes and human skin, respectively. Toxicity was evaluated in macrophage and keratinocyte cell lines, and the activity against promastigotes and intracellular amastigotes of Leishmania infantum was studied. The AmB gel remained stable for a period of two months, with optimal properties for topical use and no apparent toxic effect on the cell lines. High amounts of AmB were found in damaged and non-damaged skin (1230.10 ± 331.52 and 2484.57 ± 439.12 µg/g/cm², respectively) and they were above the IC₅₀ of AmB for amastigotes. Although there were no differences in the in vitro anti-leishmanial activity between the AmB solution and gel, the formulation resulted in a higher amount of AmB being retained in the skin, and is therefore a candidate for further studies of in vivo efficacy.

Development and Characterization of a Semi-Solid Dosage Form of Meglumine Antimoniate for Topical Treatment of Cutaneous Leishmaniasis

Cutaneous leishmaniasis (CL) is treated with painful intralesional injections of meglumine antimoniate (MA). With the aim of developing an alternative topical treatment for CL, a gel-based formulation with 30% MA was prepared and its physicochemical properties, stability and rheological behavior were studied. The following were assessed: drug release on propylene hydrophilic membranes ex vivo human skin permeation, tolerance in healthy volunteers, cytotoxicity in three cell lines and anti-leishmanial activity against Leishmania infantum promastigotes and amastigotes. The MA gel formulation was found to have suitable pH, and good spreadability and stability. Low quantities of pentavalent antimony (Sb^V) were observed in release and permeation tests, whereas retention was high in both non-damaged and damaged skin (71,043.69 ± 10,641.57 and 10,728 ± 2254.61 µg/g/cm² of Sb^V, respectively). The formulation did not have a toxic effect on the cell lines, and presented lower Sb^V IC₅₀ values against amastigotes (15.76 ± 4.81 µg/mL) in comparison with the MA solution. The high amount of drug retained in the skin and the Sb^V IC₅₀ values obtained suggest that this semi-solid dosage form has potential as an alternative treatment of CL.

Cross-resistance of Leishmania infantum isolates to nitric oxide from patients refractory to antimony treatment, and greater tolerance to antileishmanial responses by macrophages

Visceral leishmaniasis is a life-threatening disease characterized by intense parasitism of the spleen, liver, and bone marrow. Antimonials have served as front-line antileishmanial therapeutics for decades, but the increasing failure rates under antimonial treatment have challenged the continued use of these drugs. Pentavalent antimonials are known to reinforce the killing mechanisms of macrophages, although the associated mechanism remains unclear. Here, for the first time, we determined whether Leishmania infantum strains isolated from patients refractory to antimony treatment (relapse cases) were cross-resistant to antimonials, liposomal amphotericin B, and/or nitric oxide, and also whether these strains modulate macrophage infection. We selected four clinical isolates from relapse cases and two clinical isolates from antimony-responsive patients (control group) for the present study. The L. infantum promastigotes from all four relapse cases were resistant to trivalent antimonial treatment and nitric oxide, while only one isolate was resistant to liposomal amphotericin B. We evaluated whether the resistant strains from relapse cases showed enhanced infectivity and amastigote survival in macrophages, or macrophage-killing mechanisms in macrophages activated by lipopolysaccharide plus interferon gamma. Infection indexes calculated using macrophages infected with isolates from relapse were higher than those observed with control strains that were stimulated independently. Macrophage infection was higher with L. infantum isolates from relapse cases and correlated with enhanced interleukin 1-β production but showed similar nitrite production. Our results demonstrate that L. infantum field isolates from relapse cases were resistant to antimonials and nitric oxide and that these parasites stimulated inflammatory cytokines and were resistant to macrophage-killing mechanisms, factors that may contribute to disease severity.

In vitro drug susceptibility of Leishmania infantum isolated from humans and dogs

Visceral leishmaniasis (VL) caused by parasites of Leishmania donovani complex is a severe human disease which often leads to death if left untreated. Domestic dogs are the main reservoir hosts for zoonotic human visceral infection caused by Leishmania infantum. In the absence of effective human and dog vaccines, the only feasible way to treat and control leishmaniasis is through the use of suitable medications. To know the drug susceptibility of human and canine Leishmania strains from Lisbon-Portugal, a study on a panel of strains was conducted by testing the susceptibility of promastigotes and intracellular amastigotes to the common drugs used in canine leishmaniasis (CanL) and human VL (meglumine antimoniate, amphotericin B, miltefosine and allopurinol). Although a high heterogeneity of susceptibilities was obtained to each drug on both axenic promastigote and intracellular amastigote assays, intracellular amastigotes system correlated better with treatment outcome. Parasites isolated from the refractory human case were the least susceptible to the drugs used highlighting that the emergence of cross-resistance to the drugs available for human therapy should not be neglected. Furthermore, parasites isolated from dogs showed low susceptibility to the main drugs used in CanL treatment. Our results focus the importance of reducing/avoiding the emergence and spread of resistant parasites in the canine and human populations, a factor that requires special consideration when dogs are treated using the same available anti-Leishmania drugs for human VL. In addition, efforts should be made in order to standardize the conditions used to test drug susceptibility (methodologies, drug formulations and media) in order to compare results between laboratories.

Antimony resistance in leishmania, focusing on experimental research

Leishmaniases are parasitic diseases that spread in many countries with a prevalence of 12 million cases. There are few available treatments and antimonials are still of major importance in the therapeutic strategies used in most endemic regions. However, resistance toward these compounds has recently emerged in areas where the replacement of these drugs is mainly limited by the cost of alternative molecules. In this paper, we reviewed the studies carried out on antimonial resistance in Leishmania. Several common limitations of these works are presented before prevalent approaches to evidence antimonial resistance are related. Afterwards, phenotypic determination of resistance is described, then confronted to clinical outcome. Finally, we detail molecular mechanisms and targets involved in resistance and already identified in vitro within selected mutant strains or in clinical isolates.

One-year clinical and parasitological follow-up of dogs treated with marbofloxacin for canine leishmaniosis

The purpose of this international, multicentric, and non-comparative field trial was to obtain complementary data on long-term clinical and parasitological follow-up of dogs treated with marbofloxacin for canine leishmaniosis (CanL). Seventy-four dogs with clinical signs of CanL and without severe renal failure were recruited in France, Spain and Italy, and 61 of them were part of the analysis. Each dog was treated with palatable tablets of marbofloxacin at 2 mg/kg once a day for 28 days. A clinical and parasitological follow-up was performed regularly up to 12 months. Efficacy was demonstrated in 42 dogs (68.9%), within 51 days (mean value), 10 of them (23.8%) being clinically cured after 3 months. A decrease of 61% in the sum of clinical scores was observed after 3 months. Haemato-biochemical parameters improved in general, supporting the observed clinical efficacy. Relapse was observed in 20/38 dogs (52.6%) approximately 5.5 months after treatment completion. The blood parasite load generally developed in conformity with the clinical outcome, even if exceptions were not rare. Lymph nodes remained positive by culture or PCR for a long time, even in dogs for which a good clinical response was observed. Despite the incomplete parasite clearance, as is also the case with other anti-leishmanial drugs, these results nevertheless confirm the relevance of marbofloxacin as a CanL treatment.

Use of Leishmania donovani field isolates expressing the luciferase reporter gene in in vitro drug screening

Currently available primary screens for the selection of candidate antileishmanial compounds are not ideal. These techniques are time-consuming, laborious, and difficult to scale and require macrophages, which limit their use for high-throughput screening. We have developed Leishmania donovani field isolates that constitutively express the firefly luciferase reporter gene (luc) as a part of an episomal vector. An excellent correlation between parasite number and luciferase activity was observed. luc expression was stable, even in the absence of drug selection, for 4 weeks. The transfectants were infective to macrophages, and intracellular amastigotes exhibited luciferase activity. The suitability of these recombinant field isolates for in vitro screening of antileishmanial drugs was established. The luciferase-expressing sodium stibogluconate-resistant cell lines offer a model for the screening of compounds for resistance. The system is in routine use at the Central Drug Research Institute, Lucknow, India, for high-throughput screening of newly synthesized compounds.

In vitro assays for evaluation of drug activity against Leishmania spp

Leishmania is a parasitic protozoan which causes severe disease in humans and dogs. Screening of new compounds against leishmaniasis is particularly needed to determine the toxicity of some existing drugs and the appearance of drug-resistant strains. Reliable and simple in vitro models are required for large-scale initial screenings. In this review different methods for in vitro evaluation of drug activity against Leishmania spp. are summarized.

In vitro susceptibility to pentavalent antimony in Leishmania infantum strains is not modified during in vitro or in vivo passages but is modified after host treatment with meglumine antimoniate

BACKGROUND

Leishmaniasis is a common parasitic disease in Southern Europe, caused by Leishmania infantum. The failures of current treatment with pentavalent antimonials are partially attributable to the emergence of antimony-resistant Leishmania strains. This study analyses the in vitro susceptibility to pentavalent antimony of intracellular amastigotes from a range of L. infantum strains, derived from the same infected animal, during in vitro and in vivo passages and after host treatment with meglumine antimoniate.

RESULTS

SbV-IC50 values for strains from two distinct isolates from the same host and one stock after two years of culture in NNN medium and posterior passage to hamster were similar (5.0 +/- 0.2; 4.9 +/- 0.2 and 4.4 +/- 0.1 mgSbV/L, respectively). In contrast, a significant difference (P < 0.01, t test) was observed between the mean SbV-IC50 values in the stocks obtained before and after treatment of hosts with meglumine antimoniate (4.7 +/- 0.4 mgSbV/L vs. 7.7 +/- 1.5 mgSbV/L). Drug-resistance after drug pressure in experimentally infected dogs increased over repeated drug administration (6.4 +/- 0.5 mgSbV/L after first treatment vs. 8.6 +/- 1.4 mgSbV/L after the second) (P < 0.01, t test).

CONCLUSIONS

These results confirm previous observations on strains from Leishmania/HIV co-infected patients and indicate the effect of the increasing use of antimony derivatives for treatment of canine leishmaniasis in endemic areas on the emergence of Leishmania antimony-resistant strains.