Efficacy of cyclooctadepsipeptides and aminophenylamidines against larval, immature and mature adult stages of a parasitologically characterized trichurosis model in mice

Another decade of Trichuris muris research: An update and application of key discoveries

The mouse whipworm, Trichuris muris, has been used for over 60 years as a tractable model for human trichuriasis, caused by the related whipworm species, T. trichiura. The history of T. muris research, from the discovery of the parasite in 1761 to understanding the lifecycle and outcome of infection with different doses (high versus low dose infection), as well as the immune mechanisms associated with parasite expulsion and chronic infection have been detailed in an earlier review published in 2013. Here, we review recent advances in our understanding of whipworm biology, host-parasite interactions and basic immunology brought about using the T. muris mouse model, focussing on developments from the last decade. In addition to the traditional high/low dose infection models that have formed the mainstay of T. muris research to date, novel models involving trickle (repeated low dose) infection in laboratory mice or infection in wild or semi-wild mice have led to important insights into how immunity develops in situ in a multivariate environment, while the use of novel techniques such as the development of caecal organoids (enabling the study of larval development ex vivo) promise to deliver important insights into host-parasite interactions. In addition, the genome and transcriptome analyses of T. muris and T. trichiura have proven to be invaluable tools, particularly in the context of vaccine development and identification of secreted products including proteins, extracellular vesicles and micro-RNAs, shedding further light on how these parasites communicate with their host and modulate the immune response to promote their own survival.

Emodepside for Trichuris trichiura and Hookworm Infection

BACKGROUND

Current treatments for soil-transmitted helminth infections in humans have low efficacy against Trichuris trichiura. Emodepside - a drug in veterinary use and under development for the treatment of onchocerciasis in humans - is a leading therapeutic candidate for soil-transmitted helminth infection.

METHODS

We conducted two phase 2a, dose-ranging, randomized, controlled trials to evaluate the efficacy and safety of emodepside against T. trichiura and hookworm infections. We randomly assigned, in equal numbers, adults 18 to 45 years of age in whom T. trichiura or hookworm eggs had been detected in stool samples to receive emodepside, at a single oral dose of 5, 10, 15, 20, 25, or 30 mg; albendazole, at a single oral dose of 400 mg; or placebo. The primary outcome was the percentage of participants who were cured of T. trichiura or hookworm infection (the cure rate) with emodepside 14 to 21 days after treatment, determined with the use of the Kato-Katz thick-smear technique. Safety was assessed 3, 24, and 48 hours after the receipt of treatment or placebo.

RESULTS

A total of 266 persons were enrolled in the T. trichiura trial and 176 in the hookworm trial. The predicted cure rate against T. trichiura in the 5-mg emodepside group (85% [95% confidence interval {CI}, 69 to 93]; 25 of 30 participants) was higher than the predicted cure rate in the placebo group (10% [95% CI, 3 to 26]; 3 of 31 participants) and the observed cure rate in the albendazole group (17% [95% CI, 6 to 35]; 5 of 30 participants). A dose-dependent relationship was shown in participants with hookworm: the observed cure rate was 32% (95% CI, 13 to 57; 6 of 19 participants) in the 5-mg emodepside group and 95% (95% CI, 74 to 99.9; 18 of 19 participants) in the 30-mg emodepside group; the observed cure rates were 14% (95% CI, 3 to 36; 3 of 21 participants) in the placebo group and 70% (95% CI, 46 to 88; 14 of 20 participants) in the albendazole group. In the emodepside groups, headache, blurred vision, and dizziness were the most commonly reported adverse events 3 and 24 hours after treatment; the incidence of events generally increased in a dose-dependent fashion. Most adverse events were mild in severity and were self-limited; there were few moderate and no serious adverse events.

CONCLUSIONS

Emodepside showed activity against T. trichiura and hookworm infections. (Funded by the European Research Council; ClinicalTrials.gov number, NCT05017194.).

Development of emodepside as a possible adulticidal treatment for human onchocerciasis-The fruit of a successful industrial-academic collaboration

Current mass drug administration (MDA) programs for the treatment of human river blindness (onchocerciasis) caused by the filarial worm Onchocerca volvulus rely on ivermectin, an anthelmintic originally developed for animal health. These treatments are primarily directed against migrating microfilariae and also suppress fecundity for several months, but fail to eliminate adult O. volvulus. Therefore, elimination programs need time frames of decades, well exceeding the life span of adult worms. The situation is worsened by decreased ivermectin efficacy after long-term therapy. To improve treatment options against onchocerciasis, a drug development candidate should ideally kill or irreversibly sterilize adult worms. Emodepside is a broad-spectrum anthelmintic used for the treatment of parasitic nematodes in cats and dogs (Profender and Procox). Our current knowledge of the pharmacology of emodepside is the result of more than 2 decades of intensive collaborative research between academia and the pharmaceutical industry. Emodepside has a novel mode of action with a broad spectrum of activity, including against extraintestinal nematode stages such as migrating larvae or macrofilariae. Therefore, emodepside is considered to be among the most promising candidates for evaluation as an adulticide treatment against onchocerciasis. Consequently, in 2014, Bayer and the Drugs for Neglected Diseases initiative (DNDi) started a collaboration to develop emodepside for the treatment of patients suffering from the disease. Macrofilaricidal activity has been demonstrated in various models, including Onchocerca ochengi in cattle, the parasite most closely related to O. volvulus. Emodepside has now successfully passed Phase I clinical trials, and a Phase II study is planned. This Bayer–DNDi partnership is an outstanding example of “One World Health,” in which experience gained in veterinary science and drug development is translated to human health and leads to improved tools to combat neglected tropical diseases (NTDs) and shorten development pathways and timelines in an otherwise neglected area.

Onchocerca volvulus is the causative agent of human river blindness, and current elimination programs rely on the use of ivermectin to kill microfilariae. Since no adulticidal drug is available and adult worms have a life span of up to 15 years, elimination programs need to be sustained over several decades. Emodepside is an anthelmintic that is licensed as a dewormer for cats and dogs. Due to its ability to eliminate nematodes located in various extraintestinal host tissues, including migrating larvae and adult filarial worms, it is considered to be an excellent candidate for the treatment of onchocerciasis. Intense collaboration between academia and the pharmaceutical industry has led to a deep understanding of the novel mode of action of the drug and of its parasite target spectrum. Phase I clinical trials with emodepside have demonstrated its safety and adulticide activity against the closely related cattle parasite Onchocerca ochengi. Currently, Phase II clinical trials are planned to confirm that emodepside, developed initially to improve animal health, has also the potential to improve human health by tackling a very important neglected tropical disease (NTD).

Evaluation of Commercially Available Anthelminthics in Laboratory Models of Human Intestinal Nematode Infections

Drug repurposing from veterinary to human medicine has been the main strategy to develop the four recommended human anthelminthics, albendazole, mebendazole, levamisole, and pyrantel pamoate, for the treatment of soil-transmitted helminthiasis. A systematic, head-to-head comparison of the anthelminthic activity profile of derivatives of these drugs and other anthelminthics developed in succession has not been conducted to date. We studied eight benzimidazoles, five macrocyclic lactones, tribendimidine, levamisole, and pyrantel pamoate in laboratory models of human intestinal nematode infections. In vitro studies were performed on Trichuris muris L1 larval stage and adults, as well as Ancylostoma ceylanicum, Necator americanus, Heligmosomoides polygyrus, and Strongyloides ratti L3 larvae and adults. The benzimidazoles showed pronounced differences against larval and adult stages, with low activity against larvae and the highest activity observed against adult N. americanus (IC₅₀ of flubendazole 1.1 μM). The macrocyclic lactones, on the other hand, revealed a higher activity on the larval stages, with the lowest IC₅₀ values observed against N. americanus L3 (IC₅₀ values of 0.03-3 μM). In vivo studies were performed in the T. muris and H. polygyrus mice models, with moxidectin and milbemycin oxime showing the highest activity against H. polygyrus (ED₅₀ values of 0.009 and 0.006 mg/kg, respectively) and moxidectin and abamectin being the most effective drugs against T. muris (ED₅₀ values of 0.2 and 0.5 mg/kg, respectively). Laboratory models for soil-transmitted helminthiasis can assist characterizing potential drug candidates. Drugs should be evaluated against different species, and both the adult and larval stages as activities could differ considerably.

Population Pharmacokinetics and Exposure-Response Analysis of Tribendimidine To Improve Treatment for Children with Hookworm Infection

Tribendimidine has been successful in treating hookworm infections and may serve as an alternative to albendazole should resistance arise. Our aims were to (i) characterize the pharmacokinetics (PK) of tribendimidine's primary metabolite, deacetylated amidantel (dADT), and secondary metabolite, acetylated derivative of amidantel (adADT), in school-aged children and adolescents, (ii) link exposure to efficacy against hookworm, and (iii) evaluate whether tribendimidine pharmacotherapy in children could be further improved. First, a population PK model was developed based on dried-blood-spot samples collected from 155 school-aged children and adolescents with hookworm infections, following tribendimidine doses ranging from 100 to 400 mg. Second, an exposure-response analysis was conducted to link the active metabolite dADT to cure rates (CRs) and egg reduction rates (ERRs). Third, simulations were performed to identify a treatment strategy associated with >90% CRs. A two-compartmental model with transit compartments describing observed delay in absorption adequately described PK data of dADT and adADT. Allometric scaling was included to account for growth and development. The absorption rate was 56% lower with 200-mg tablets than with 50-mg tablets, while the extent of absorption remained unaffected. The identified E _max models linking dADT exposure to ERRs and CRs showed shallow curves, as increasing exposure led to marginal efficacy increase. Combination therapy should be considered, as a 12-fold-higher dose would be needed to achieve 95% ERRs and CRs >90% with tribendimidine alone. Further studies are warranted to evaluate safety of higher tribendimidine doses and combination therapies with other anthelmintic agents to improve treatment strategy for children with hookworm infection.

Pharmacometric Analysis of Tribendimidine Monotherapy and Combination Therapies To Achieve High Cure Rates in Patients with Hookworm Infections

In the treatment of hookworm infections, pharmacotherapy has been only moderately successful and drug resistance is a threat. Therefore, novel treatment options including combination therapies should be considered, in which tribendimidine could play a role. Our aims were to (i) characterize the pharmacokinetics of tribendimidine's metabolites in adolescents receiving tribendimidine monotherapy or in combination with ivermectin or oxantel pamoate, (ii) evaluate possible drug-drug interactions (DDI), (iii) link exposure to response, and (iv) identify a treatment strategy associated with high efficacy, i.e., >90% cure rates (CRs), utilizing model-based simulations. A population pharmacokinetic model was developed for tribendimidine's primary and secondary metabolites, dADT and adADT, in 54 hookworm-positive adolescents, with combination therapy evaluated as a possible covariate. Subsequently, an exposure-response analysis was performed utilizing CRs as response markers. Simulations were performed to identify a treatment strategy to achieve >90% CRs. A two-compartmental model best described metabolite disposition. No pharmacokinetic DDI was identified with ivermectin or oxantel pamoate. All participants receiving tribendimidine plus ivermectin were cured. For the monotherapy arm and the arm including the combination with oxantel pamoate, E _max models adequately described the correlation between dADT exposure and probability of being cured, with required exposures to achieve 50% of maximum effect of 39.6 and 15.6 nmol/ml·h, respectively. Based on our simulations, an unrealistically high monotherapy tribendimidine dose would be necessary to achieve CRs of >90%, while combination therapy with ivermectin would meet this desired target product profile. Further clinical studies should be launched to develop this combination for the treatment of hookworm and other helminth infections.

Evaluation of emodepside in laboratory models of human intestinal nematode and schistosome infections

Background

Helminthiases are very prevalent worldwide, yet their treatment and control rely on a handful of drugs. Emodepside, a marketed broad-spectrum veterinary anthelminthic with a unique mechanism of action, undergoing development for onchocerciasis is an interesting anthelmintic drug candidate. We tested the in vitro and in vivo activity of emodepside on nematode species that serve as models for human soil-transmitted helminth infection as well as on schistosomes.

Methods

In vitro viability assays were performed over a time course of 72 hours for Trichuris muris, Necator americanus, Ancylostoma ceylanicum, Heligmosomoides polygyrus, Strongyloides ratti, Schistosoma mansoni and Schistosoma haematobium. The drug effect was determined by the survival rate for the larvae and by phenotypical scores for the adult worms. Additionally, mice infected with T. muris and hamsters harboring hookworm infection (N. americanus or A. ceylanicum) were administered orally with emodepside at doses ranging from 1.25 to 75 mg/kg. Expelled worms in the feces were counted until 3 days post-drug intake and worms residing in the intestines were collected and counted after dissection.

Results

After 24 hours, emodepside was very active in vitro against both larval and adult stages of the nematodes T. muris, A. ceylanicum, N. americanus, H. polygyrus and S. ratti (IC₅₀ < 4 µM). The good in vitro activity was confirmed in vivo. Hamsters infected with the hookworms were cured when administered orally with 2.5 mg/kg of the drug. Emodepside was also highly active in vivo against T. muris (ED₅₀ = 1.2 mg/kg). Emodepside was moderately active on schistosomula in vitro (IC₅₀ < 8 µM) 24 h post-drug incubation and its activity on adult S. mansoni and S. haematobium was low (IC₅₀: 30–50 µM).

Conclusions

Emodepside is highly active against a broad range of nematode species both in vitro and in vivo. The development of emodepside for treating soil-transmitted helminth infections should be pursued.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3476-x) contains supplementary material, which is available to authorized users.

In Vitro and In Vivo Drug-Drug Interaction Study of the Effects of Ivermectin and Oxantel Pamoate on Tribendimidine

Soil-transmitted helminth (STH) infections still remain a major health problem in poor rural settings. The lack of efficacious drugs against all STH species raises interest in drug combinations. Drug-drug interactions (DDIs) are, however, of major concern, so careful in vitro and in vivo characterization is needed. The combination of tribendimidine with either ivermectin or oxantel pamoate targets a broad range of STHs and thus represents a promising treatment alternative. Drug-drug interactions, however, have not yet been investigated. Therefore, the effects of combinations of ivermectin, oxantel pamoate, and tribendimidine's active metabolite deacylated amidantel (dADT) on cytochrome P450 (CYP450) metabolism were evaluated, followed by a pharmacokinetic analysis of tribendimidine and ivermectin alone and in combination in healthy rats. Oxantel pamoate is only poorly absorbed and was therefore excluded from pharmacokinetic analysis. No evident effect was observed for tribendimidine-oxantel pamoate at the CYP450 metabolism level, whereas a combination of tribendimidine and ivermectin led to moderately increased CYP2D6 inhibition compared to ivermectin or tribendimidine alone. Coadministration of tribendimidine with ivermectin altered neither the time to maximum concentration of drug in plasma (T _max) nor the elimination half-lives of dADT, the acetylated derivative of amidantel (adADT), and ivermectin. While the area under the concentration-versus-time curve (AUC) and maximum concentration of drug in plasma (C _max) values of dADT, adADT, and ivermectin are reduced by coadministration, the change is insufficient to declare that a DDI has been detected. Further studies are necessary to understand the observed interaction of tribendimidine and ivermectin, which is not related to P450 metabolism, and its significance for the situation in humans.

A BRIEF REVIEW ON THE MODE OF ACTION OF ANTINEMATODAL DRUGS

Anthelmintics are some of the most widely used drugs in veterinary medicine. Here we review the mechanism of action of these compounds on nematode parasites. Included are the older classes of compounds; the benzimidazoles, cholinergic agonists and macrocyclic lactones. We also consider newer anthelmintics, including emodepside, derquantel and tribendimidine. In the absence of vaccines for most parasite species, control of nematode parasites will continue to rely on anthelmintic drugs. As a consequence, vigilance in detecting drug resistance in parasite populations is required. Since resistance development appears almost inevitable, there is a continued and pressing need to fully understand the mode of action of these compounds. It is also necessary to identify new drug targets and drugs for the continued effective control of nematode parasites.

Identification of novel splice variants of the voltage- and Ca²⁺-dependent K⁺-channel SLO-1 of Trichuris muris

The anthelmintic cyclooctadepsipeptide emodepside is effective against nematodes showing resistance against established drug classes. Emodepside exerts its nematicidal effects mainly through its validated target, the tetrameric voltage- and calcium-activated potassium channel SLO-1. Two slo-1 genes were described in Trichuris muris. Alternative splicing is known to alter SLO-1 properties. Here, 16 T. muris splice variants for slo-1.1 and three variants for slo-1.2 were identified in addition to previously described variants. Splice variants caused by intron retentions and/or exon exclusions encode varyingly truncated subunits. Depending on the subunit composition, channels might have altered physiological and pharmacological properties including different modulation by calcium and/or voltage or reduced emodepside susceptibility which might lead to emodepside resistance as observed in Caenorhabditis elegans expressing only similarly truncated Slo-1. The comprehensive characterisation of splice variants is a prerequisite for functional analysis and confirmed conservation of remarkable differences found between both slo-1 paralogs in Trichuris suis.

Characterization of the Ca2+-gated and voltage-dependent K+-channel Slo-1 of nematodes and its interaction with emodepside

The cyclooctadepsipeptide emodepside and its parent compound PF1022A are broad-spectrum nematicidal drugs which are able to eliminate nematodes resistant to other anthelmintics. The mode of action of cyclooctadepsipeptides is only partially understood, but involves the latrophilin Lat-1 receptor and the voltage- and calcium-activated potassium channel Slo-1. Genetic evidence suggests that emodepside exerts its anthelmintic activity predominantly through Slo-1. Indeed, slo-1 deficient Caenorhabditis elegans strains are completely emodepside resistant. However, direct effects of emodepside on Slo-1 have not been reported and these channels have only been characterized for C. elegans and related Strongylida. Molecular and bioinformatic analyses identified full-length Slo-1 cDNAs of Ascaris suum, Parascaris equorum, Toxocara canis, Dirofilaria immitis, Brugia malayi, Onchocerca gutturosa and Strongyloides ratti. Two paralogs were identified in the trichocephalids Trichuris muris, Trichuris suis and Trichinella spiralis. Several splice variants encoding truncated channels were identified in Trichuris spp. Slo-1 channels of trichocephalids form a monophyletic group, showing that duplication occurred after the divergence of Enoplea and Chromadorea. To explore the function of a representative protein, C. elegans Slo-1a was expressed in Xenopus laevis oocytes and studied in electrophysiological (voltage-clamp) experiments. Incubation of oocytes with 1-10 µM emodepside caused significantly increased currents over a wide range of step potentials in the absence of experimentally increased intracellular Ca2+, suggesting that emodepside directly opens C. elegans Slo-1a. Emodepside wash-out did not reverse the effect and the Slo-1 inhibitor verruculogen was only effective when applied before, but not after, emodepside. The identification of several splice variants and paralogs in some parasitic nematodes suggests that there are substantial differences in channel properties among species. Most importantly, this study showed for the first time that emodepside directly opens a Slo-1 channel, significantly improving the understanding of the mode of action of this drug class.

Recent advances in candidate-gene and whole-genome approaches to the discovery of anthelmintic resistance markers and the description of drug/receptor interactions

Anthelmintic resistance has a great impact on livestock production systems worldwide, is an emerging concern in companion animal medicine, and represents a threat to our ongoing ability to control human soil-transmitted helminths. The Consortium for Anthelmintic Resistance and Susceptibility (CARS) provides a forum for scientists to meet and discuss the latest developments in the search for molecular markers of anthelmintic resistance. Such markers are important for detecting drug resistant worm populations, and indicating the likely impact of the resistance on drug efficacy. The molecular basis of resistance is also important for understanding how anthelmintics work, and how drug resistant populations arise. Changes to target receptors, drug efflux and other biological processes can be involved. This paper reports on the CARS group meeting held in August 2013 in Perth, Australia. The latest knowledge on the development of molecular markers for resistance to each of the principal classes of anthelmintics is reviewed. The molecular basis of resistance is best understood for the benzimidazole group of compounds, and we examine recent work to translate this knowledge into useful diagnostics for field use. We examine recent candidate-gene and whole-genome approaches to understanding anthelmintic resistance and identify markers. We also look at drug transporters in terms of providing both useful markers for resistance, as well as opportunities to overcome resistance through the targeting of the transporters themselves with inhibitors. Finally, we describe the tools available for the application of the newest high-throughput sequencing technologies to the study of anthelmintic resistance.

Repurposing drugs for the treatment and control of helminth infections

•

Drug repurposing continues to be the central drug discovery strategy for helminths.

•

Most repurposed drugs come from veterinary medicine and known drug classes.

•

Only a handful of drugs have advanced clinically.

•

More collaborations and funding are needed to advance discoveries to the market.

Helminth infections are responsible for a considerable public health burden, yet the current drug armamentarium is small. Given the high cost of drug discovery and development, the high failure rates and the long duration to develop novel treatments, drug repurposing circumvents these obstacles by finding new uses for compounds other than those they were initially intended to treat. In the present review, we summarize in vivo and clinical trial findings testing clinical candidates and marketed drugs against schistosomes, food-borne trematodes, soil-transmitted helminths, Strongyloides stercoralis, the major human filariases lymphatic filariasis and onchocerciasis, taeniasis, neurocysticercosis and echinococcosis. While expanding the applications of broad-spectrum or veterinary anthelmintics continues to fuel alternative treatment options, antimalarials, antibiotics, antiprotozoals and anticancer agents appear to be producing fruitful results as well. The trematodes and nematodes continue to be most investigated, while cestodal drug discovery will need to be accelerated. The most clinically advanced drug candidates include the artemisinins and mefloquine against schistosomiasis, tribendimidine against liver flukes, oxantel pamoate against trichuriasis, and doxycycline against filariasis. Preclinical studies indicate a handful of promising future candidates, and are beginning to elucidate the broad-spectrum activity of some currently used anthelmintics. Challenges and opportunities are further discussed.