A reappraisal of the relative sensitivity of nematode pharyngeal and somatic musculature to macrocyclic lactone drugs

Ivermectin Protects Against Experimental Autoimmune Encephalomyelitis in Mice by Modulating the Th17/Treg Balance Involved in the IL-2/STAT5 Pathway

Multiple sclerosis (MS), a T-cell-mediated autoimmune disease that affects the central nervous system (CNS), is characterized by white matter demyelination, axon destruction, and oligodendrocyte degeneration. Ivermectin, an anti-parasitic drug, has anti-inflammatory, anti-tumor, and antiviral properties. However, to date, there are no in-depth studies on the effect of ivermectin on the function effector of T cells in murine experimental autoimmune encephalomyelitis (EAE), an animal model of MS. Here, we conducted in vitro experiments and found that ivermectin inhibited the proliferation of total T cells (CD3+) and their subsets (CD4+ and CD8+ T cells) as well as T cells secreting the pro-inflammatory cytokines IFN-γ and IL-17A; ivermectin also increased IL-2 production and IL-2Rα (CD25) expression, which was accompanied by an increase in the frequency of CD4+CD25+Foxp3+ regulatory T cells (Treg). Importantly, ivermectin administration reduced the clinical symptoms of EAE mice by preventing the infiltration of inflammatory cells into the CNS. Additional mechanisms showed that ivermectin promoted Treg cells while inhibiting pro-inflammatory Th1 and Th17 cells and their IFN-γ and IL-17 secretion; ivermectin also upregulated IL-2 production from MOG35-55-stimulated peripheral lymphocytes. Finally, ivermectin decreased IFN-γ and IL-17A production and increased IL-2 level, CD25 expression, and STAT5 phosphorylation in the CNS. These results reveal a previously unknown etiopathophysiological mechanism by which ivermectin attenuates the pathogenesis of EAE, indicating that it may be a promising option for T-cell-mediated autoimmune diseases such as MS.

Ivermectin: From theory to clinical application

Approximately 250 million people have been using ivermectin (IVM) annually to combat many parasitic diseases including filariasis, onchocerciasis, strongyloidiasis, scabies and pediculosis. Many clinical studies have proven its efficacy against these diseases and have reported the optimum dose and duration of treatment. Moreover, its antiparasitic range has increased to cover more parasitic infections, but it still requires further exploration, e.g. for trichinosis and myiasis. Furthermore, IVM showed high efficacy in killing vectors of disease-causing parasites such as mosquitoes, sandflies and tsetse flies. The World Health Organization (WHO) has managed many control programmes involving the use of IVM to achieve elimination of onchocerciasis and lymphatic filariasis and to reduce malaria transmission. However, IVM is not exempt from the possibility of resistance and, certainly, its intensive use has led to the emergence of resistance in some parasites. Recent research is investigating the possibility of novel drug delivery systems for IVM that increase its potential to treat a new range of diseases and to overcome the possibility of drug resistance. This review highlights the most common human uses of IVM, with special reference to the new and promising properties of IVM.

Motility in the L3 stage is a poor phenotype for detecting and measuring resistance to avermectin/milbemycin drugs in gastrointestinal nematodes of livestock

Motility is a commonly used in vitro phenotype for assessing anthelmintic activity of candidate compounds, and for detecting anthelmintic resistance in nematodes. Third-stage larvae (L3) of parasitic nematodes are commonly used in motility-based assays because L3 are simple to obtain and can remain viable in storage for extended periods. To improve the measurement of motility of microscopic stages of nematodes, our laboratory developed the Worminator, which quantitatively measures motility of parasites. Using the Worminator, we compared the dose-response characteristics of several avermectin/milbemycin (AM) compounds using L3 from both AM-susceptible and AM-resistant Cooperia spp. (abamectin, doramectin, eprinomectin, ivermectin, moxidectin) and Haemonchus contortus (eprinomectin, ivermectin, moxidectin). Concentrations tested with the Worminator ranged from 0.156 to 40 μM. Differences in EC₅₀ between AM-susceptible and AM-resistant isolates of Cooperia spp. and Haemonchus contortus were small, with resistance ratios ranging from 1.00 to 1.34 for Cooperia spp., 0.99 to 1.65 for Haemonchus contortus. Larval migration inhibition assays were conducted using the same isolates and were equally ineffective for detection of resistance with resistance ratios less than 2.0. These results contrast with those of the Larval Development Assay where we obtained a resistance ratio of 16.48 using the same isolates of Haemonchus contortus. Moreover, even at the highest concentration tested (40 μM), 100% inhibition of motility was never achieved and EC₅₀ for Worminator assays were more than 100× higher than peak plasma levels achieved in vivo following treatment. These data demonstrate that dose-response characteristics for inhibition of motility in L3 of gastrointestinal nematodes of livestock do not significantly differ for AM-susceptible and AM-resistant isolates. These data challenge the suitability of motility as a phenotype for detecting and measuring resistance to AM drugs in gastrointestinal nematodes of livestock.

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Motility of L3 is a poor phenotype for detection of avermectin resistance.

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Resistance ratios were less than 2.0 between susceptible and resistant isolates.

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Confidence intervals overlapped between susceptible and resistant isolates.

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Concentration to inhibit L3 motility is 100× peak plasma concentration in vivo.

How do the macrocyclic lactones kill filarial nematode larvae?

The macrocyclic lactones (MLs) are one of the few classes of drug used in the control of the human filarial infections, onchocerciasis and lymphatic filariasis, and the only one used to prevent heartworm disease in dogs and cats. Despite their importance in preventing filarial diseases, the way in which the MLs work against these parasites is unclear. In vitro measurements of nematode motility have revealed a large discrepancy between the maximum plasma concentrations achieved after drug administration and the amounts required to paralyze worms. Recent evidence has shed new light on the likely functions of the ML target, glutamate-gated chloride channels, in filarial nematodes and supports the hypothesis that the rapid clearance of microfilariae that follows treatment involves the host immune system.

Gaining Insights Into the Pharmacology of Anthelmintics Using Haemonchus contortus as a Model Nematode

Progress made in understanding pharmacokinetic behaviour and pharmacodynamic mechanisms of drug action/resistance has allowed deep insights into the pharmacology of the main chemical classes, including some of the few recently discovered anthelmintics. The integration of pharmaco-parasitological research approaches has contributed considerably to the optimization of drug activity, which is relevant to preserve existing and novel active compounds for parasite control in livestock. A remarkable amount of pharmacology-based knowledge has been generated using the sheep abomasal nematode Haemonchus contortus as a model. Relevant fundamental information on the relationship among drug influx/efflux balance (accumulation), biotransformation/detoxification and pharmacological effects in parasitic nematodes for the most traditional anthelmintic chemical families has been obtained by exploiting the advantages of working with H. contortus under in vitro, ex vivo and in vivo experimental conditions. The scientific contributions to the pharmacology of anthelmintic drugs based on the use of H. contortus as a model nematode are summarized in the present chapter.

Haemonchus contortus: Applications in Drug Discovery

Haemonchus contortus is an important pathogen of small ruminants and is therefore a crucially important target for anthelmintic chemotherapy. Its large size and fecundity have been exploited for the development of in vitro screens for anthelmintic discovery that employ larval and adult stages in several formats. The ability of the parasite to develop to the young adult stage in Mongolian jirds (Meriones unguiculatus) provides a useful small animal model that can be used to screen compounds prior to their evaluation in infected sheep. This chapter summarizes the use of H. contortus for anthelmintic discovery, offers a perspective on current strategies in this area and suggests research challenges that could lead to improvements in the anthelmintic discovery process.

Comparative pharmacokinetic and pharmacodynamic response of single and double intraruminal doses of ivermectin and moxidectin in nematode-infected lambs

AIMS

To compare the pharmacokinetics, distribution and efficacy (pharmacodynamic response) of intraruminal ivermectin (IVM) and moxidectin (MXD) administered at 0.2 and 0.4 mg/kg to naturally nematode-infected lambs, and to determine the ex vivo accumulation of these anthelmintics by Haemonchus contortus.

METHODS

Romney Marsh lambs, naturally infected with IVM-resistant H. contortus, were allocated to treatment groups based on faecal nematode egg counts. They received 0.2 or 0.4 mg/kg IVM or MXD (n=10 per group), or no treatment (Control; n=6), on Day 0. Samples from four animals from each treatment group, including abomasal parasites, were obtained on Day 1. Plasma samples were also collected from Day 0 to 14, and a faecal egg count reduction test (FECRT) and a controlled efficacy trial were carried out on Day 14. Concentrations of IVM and MXD in plasma, in abomasal and intestinal tissues and in H. contortus were evaluated by high-performance liquid chromatography. Additionally, the ex vivo drug accumulation of IVM and MXD by H. contortus was determined.

RESULTS

Peak plasma concentrations and the area under the concentration vs. time curve for both IVM and MXD were higher for 0.4 than 0.2 mg/kg treatments (p<0.05), but there were no differences for other parameters. Concentrations of IVM and MXD in the gastrointestinal target tissues and in H. contortus were higher compared to those measured in plasma. Concentrations of both drugs in H. contortus were correlated with those observed in the abomasal content (r=0.86; p<0.0001). The exposure of H. contortus to IVM and MXD was related to the administered dose. Mean FECRT and efficacy for removal of adult H. contortus was 0% for IVM at 0.2 and 0.4 mg/kg. For MXD, FECRT were >95% for both treatments, and efficacy against H. contortus was 85.1% and 98.1% for 0.2 and 0.4 mg/kg, respectively. The ex vivo accumulation of IVM and MXD in H. contortus was directly related to the drug concentration present in the environment and was influenced by the duration of exposure.

CONCLUSION

Administration of IVM and MXD at 0.4 compared with 0.2 mg/kg accounted for enhanced drug exposure in the target tissues, as well as higher drug concentrations within resistant nematodes. The current work is a further contribution to the evaluation of the relationship between drug efficacy and basic pharmacological issues in the presence of resistant parasite populations.

Effect of Moxidectin Treatment at Peripartum on Gastrointestinal Parasite Infections in Ewes Raised under Tropical Andes High Altitude Conditions

This study tested the impact of moxidectin at peripartum on nematode fecal egg count (FEC) and clinical parameters on ewes in the high altitude tropical Andes of Colombia. FEC and clinical evaluations were performed on 9 occasions in 43 naturally infected ewes before and during gestation and after lambing. Moxidectin (Mox, 200 µg kg(-1)) was applied at late pregnancy (T 1, n = 15) or 48 hours after parturition (T 2, n = 14). 14 untreated ewes served as controls (C). Suckling lambs (n = 58) remained untreated and underwent four clinical and parasitological evaluations until 8 weeks after birth. Mox efficacy equaled 99.3% (T 1) and 96.9% (T 2). Highest mean FEC value reflecting periparturient nematode egg rise (PPER) was recorded in C ewes at 4-6 weeks after lambing. Significant FEC reductions were found in T 1 (94.8%) and T 2 (96.7%) ewes (p < 0.05). All lambs showed a significant and ewes-group independent increase in FEC before weaning (p < 0.05). Clinical parameters (anemia and diarrhea) showed time- and treatment-related differences (p < 0.05). Monitoring of FEC and clinical parameters linked to gastrointestinal parasite infections allowed demonstrating that postpartum or preweaning are two critical periods to nematode infection for sheep raised under tropical Andes high altitude conditions. Use of Mox as anthelmintic treatment prevented PPER.

Utilization of computer processed high definition video imaging for measuring motility of microscopic nematode stages on a quantitative scale: "The Worminator"

A major hindrance to evaluating nematode populations for anthelmintic resistance, as well as for screening existing drugs, new compounds, or bioactive plant extracts for anthelmintic properties, is the lack of an efficient, objective, and reproducible in vitro assay that is adaptable to multiple life stages and parasite genera. To address this need we have developed the "Worminator" system, which objectively and quantitatively measures the motility of microscopic stages of parasitic nematodes. The system is built around the computer application "WormAssay", developed at the Center for Discovery and Innovation in Parasitic Diseases at the University of California, San Francisco. WormAssay was designed to assess motility of macroscopic parasites for the purpose of high throughput screening of potential anthelmintic compounds, utilizing high definition video as an input to assess motion of adult stage (macroscopic) parasites (e.g. Brugia malayi). We adapted this assay for use with microscopic parasites by modifying the software to support a full frame analysis mode that applies the motion algorithm to the entire video frame. Thus, the motility of all parasites in a given well are recorded and measured simultaneously. Assays performed on third-stage larvae (L3) of the bovine intestinal nematode Cooperia spp., as well as microfilariae (mf) of the filarioid nematodes B. malayi and Dirofilaria immitis, yielded reproducible dose responses using the macrocyclic lactones ivermectin, doramectin, and moxidectin, as well as the nicotinic agonists, pyrantel, oxantel, morantel, and tribendimidine. This new computer based-assay is simple to use, requires minimal new investment in equipment, is robust across nematode genera and developmental stage, and does not require subjective scoring of motility by an observer. Thus, the "Worminator" provides a relatively low-cost platform for developing genera- and stage-specific assays with high efficiency and reproducibility, low labor input, and yields objective motility data that is not subject to scorer bias.

The in vitro assay profile of macrocyclic lactone resistance in three species of sheep trichostrongyloids

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Ivermectin and its two components contribute to action and resistance.

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Moxidectin tended to have lower resistance ratios than ivermectin in the LDA.

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Moxidectin was the most potent inhibitor of migration in susceptible H. contortus.

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LMIA performs better in detecting resistance to MOX than LDA.

Anthelmintic resistance has emerged as an important problem in animal industries. Understanding resistance mechanisms, especially against macrocyclic lactones (MLs), is the first step in developing better diagnostic tools. Effects of several MLs including ivermectins and milbemycins were tested using two well established in vitro assays: the larval development assay (LDA) and the larval migration inhibition assay (LMIA). These were performed on free-living stages of susceptible and ML-resistant isolates of three trichostrongyloid nematode species of sheep. In general, dose response curves shifted to the right in the resistant isolates. Data showed that resistance was present to ivermectin and its two components suggesting that both components contribute to action and resistance. There were no consistent patterns of potency and resistance of the tested substances for the different isolates in the LDA except that moxidectin (MOX) tended to have lower resistance ratios than ivermectin (IVM). MOX was the most potent inhibitor in the LMIA in susceptible Haemonchus contortus while being less potent in Trichostrongylus colubriformis and particularly in Ostertagia circumcincta. MOX showed high resistance ratios in the LMIA in all three species. Based on these results, resistance to MOX has unique characteristics and the LMIA may perform better in detecting resistance to MOX in these parasite species.

Glutamate-gated chloride channels

Glutamate-gated chloride channels (GluCls) are found only in protostome invertebrate phyla but are closely related to mammalian glycine receptors. They have a number of roles in these animals, controlling locomotion and feeding and mediating sensory inputs into behavior. In nematodes and arthropods, they are targeted by the macrocyclic lactone family of anthelmintics and pesticides, making the GluCls of considerable medical and economic importance. Recently, the three-dimensional structure of a GluCl was solved, the first for any eukaryotic ligand-gated anion channel, revealing a macrocyclic lactone-binding site between the channel domains of adjacent subunits. This minireview will highlight some unique features of the GluCls and illustrate their contribution to our knowledge of the entire Cys loop ligand-gated ion channel superfamily.