The kinetic disposition of pyrantel citrate and pamoate and their efficacy against pyrantel-resistant Oesophagostomum dentatum in pigs

Horse and donkey parasitology: differences and analogies for a correct diagnostic and management of major helminth infections

In June 2022, at the XXXII Conference of the Italian Society of Parasitology, the parallels of the main endoparasitic infections of horses and donkeys were discussed. Although these 2 species are genetically different, they can be challenged by a similar range of parasites (i.e. small and large strongyles, and Parascaris spp.). Although equids can demonstrate some level of resilience to parasites, they have quite distinct helminth biodiversity, distribution and intensity among different geographical locations and breeds. Heavily infected donkeys may show fewer clinical signs than horses. Although parasite control is primarily provided to horses, we consider that there may be a risk of drug-resistance parasitic infection through passive infection in donkeys when sharing the same pasture areas. Knowing the possible lack of drug efficacy (<90 or 80%), it is advocated the use of selective treatment for both species based on fecal egg counts. Adult horses should receive treatment when the threshold exceeds 200–500 eggs per gram (EPG) of small strongyles. Moreover, considering that there are no precise indications in donkeys, a value >300 EPG may be a safe recommendation. We have highlighted the main points of the discussion including the dynamics of helminth infections between the 2 species.

The narrow-spectrum anthelmintic oxantel is a potent agonist of a novel acetylcholine receptor subtype in whipworms

In the absence of efficient alternative strategies, the control of parasitic nematodes, impacting human and animal health, mainly relies on the use of broad-spectrum anthelmintic compounds. Unfortunately, most of these drugs have a limited single-dose efficacy against infections caused by the whipworm, Trichuris. These infections are of both human and veterinary importance. However, in contrast to a wide range of parasitic nematode species, the narrow-spectrum anthelmintic oxantel has a high efficacy on Trichuris spp. Despite this knowledge, the molecular target(s) of oxantel within Trichuris is still unknown. In the distantly related pig roundworm, Ascaris suum, oxantel has a small, but significant effect on the recombinant homomeric Nicotine-sensitive ionotropic acetylcholine receptor (N-AChR) made up of five ACR-16 subunits. Therefore, we hypothesized that in whipworms, a putative homolog of an ACR-16 subunit, can form a functional oxantel-sensitive receptor. Using the pig whipworm T. suis as a model, we identified and cloned a novel ACR-16-like subunit and successfully expressed the corresponding homomeric channel in Xenopus laevis oocytes. Electrophysiological experiments revealed this receptor to have distinctive pharmacological properties with oxantel acting as a full agonist, hence we refer to the receptor as an O-AChR subtype. Pyrantel activated this novel O-AChR subtype moderately, whereas classic nicotinic agonists surprisingly resulted in only minor responses. We observed that the expression of the ACR-16-like subunit in the free-living nematode Caenorhabditis elegans conferred an increased sensitivity to oxantel of recombinant worms. We demonstrated that the novel Tsu-ACR-16-like receptor is indeed a target for oxantel, although other receptors may be involved. These finding brings new insight into the understanding of the high sensitivity of whipworms to oxantel, and highlights the importance of the discovery of additional distinct receptor subunit types within Trichuris that can be used as screening tools to evaluate the effect of new synthetic or natural anthelmintic compounds.

Prevalence of gastrointestinal helminths and parasites in smallholder pigs reared in the central Free State Province

Pigs are kept by farmers as a source of livelihood and food. Unfortunately, helminthiasis and other internal parasites are major setbacks to profitable pig production in Africa. There is a lack of information on the prevalence and intensity of gastrointestinal helminths and parasites plaguing resource-poor pig farmers in the Free State. Knowledge of these endemic parasites can be used as baseline data to help design future intervention plans. The aim of this study was to identify and quantify the types of gastrointestinal helminths and parasites prevalent in smallholder pigs reared in the central Free State Province. Faecal samples were randomly collected from 77 pigs and parasitologically analysed. Quantification was done using the McMaster counting technique. Farming system, age, gender and health status were the risk factors considered. The study was conducted between January and March 2016. Overall, results showed that 61 samples (79.2%) tested positive for one or more gastrointestinal parasites, which were observed as single or mixed infections. Amongst the positive samples, 44.5% were infected with Ascaris suum, 50.6% with Trichuris suis, 26.0% and 72.7% were infected with Oesophagostomum dentatum and coccidia, respectively. There were significant differences (p < 0.05) between the rate of infection in the intensive and semi-intensive systems and between the dewormed and non-dewormed pigs. Piglets and female pigs recorded a higher prevalence in their categories. Pigs excreted mostly low (eggs per gram [EPG] ≤ 100) to moderate (EPG > 100 < 500) levels of helminth eggs. It is concluded that different species of gastrointestinal parasites are present in most pigs reared by smallholder farmers in this study area.

Keywords: gastrointestinal helminths and parasites; smallholder pig farmers; pigs; prevalence; Central Free State Province.

Anthelmintic drugs used in equine species

Internal parasites of horses comprise an intractable problem conferring disease, production and performance losses. Parasitism can rarely be controlled in grazing horses by management alone and anthelmintic drugs have formed the basis of therapy and prophylaxis for the last sixty years. The pharmacology of the anthelmintic drugs available dictate their spectrum of activity and degree of efficacy, their optimal routes of administration and characteristics which prevent some routes of administration, their safety tolerance and potential toxicities and as a consequence of their persistence in the body at effective concentrations their use in epidemiological control programmes. Their use has also resulted in the selection of parasites with genetically controlled characteristics which reduce their susceptibility to treatment, characteristics which are often common to whole chemical classes of anthelmintics. Pharmacological properties also confer compatibility in terms of safety and persistence with other anthelmintic drugs and thus the potential of combinations to treat parasites from different phylogenetic groups such as nematodes, cestodes and trematodes and also the potential by agency of their different molecular mechanisms of action to delay the selection of resistant genes. The major groups of anthelmintics now available, the benzimidazoles (BZD), macrocyclic lactones (MLs) and tetrahydropyrimidines are all highly effective against their targeted parasites (primarily nematodes for BZD's and ML's and cestodes for tetrahydropyrimidines) easily administered orally to horses and are well tolerated with wide margins of safety. Nevertheless, some parasitic stages are inherently less susceptible such as hypobiotic stages of the small strongyles (cyathostomins) and for some such as the adult stages of cyathostomins resistance has developed. Furthermore, for some less common parasites such as the liver fluke unlicensed drugs such as the salicylanilide, closantel have been used. A deep understanding of the pharmacology of anthelmintic drugs is essential to their optimal use in equine species.

Pharmacokinetics of praziquantel and pyrantel pamoate combination following oral administration in cats

Objectives

The pharmacokinetics of praziquantel and pyrantel pamoate has never been reported in cats. The present study was designed to establish the plasma concentration–time profile and to derive pharmacokinetic data for a combined formulation of praziquantel and pyrantel in cats, after a single, oral administration.

Methods

Twenty-two clinically healthy adult cats were used, each receiving a single oral dose of praziquantel (8.5 mg/kg) and pyrantel (100 mg/kg). Blood samples were collected at regular time points up to 48 h post-dosing. Plasma concentrations of praziquantel and pyrantel were measured using a liquid chromatography–mass spectrometry–high-throughput screening method.

Results

Clinical examination of all cats did not reveal any side effects after oral administration of these medications. The terminal half-life for praziquantel and pyrantel was 1.07 and 1.36 h, respectively. Praziquantel peak concentration (Cmax) was 1140 μg/ml, reached at 1.22 h. The plasma concentrations of pyrantel after oral administration were low with a mean Cmax of 0.11 μg/ml, reached at a Tmax of 1.91 h. Pyrantel showed a very limited absorption as pamoate salt, suggesting permanence and efficacy inside the gastrointestinal tract, where the adult stages of most parasitic nematodes reside.

Conclusions and relevance

Pyrantel showed a very limited absorption as pamoate salt. Praziquantel was rapidly absorbed following oral administration and the concentrations achieved suggest that praziquantel could be an effective and safe medication in cats. Although some resistance problems are arising as a result of their long use, these anthelminthic products can still play a major role in parasitic control, especially in geographical areas where the high cost of newer treatments or necessity of parenteral administration could decrease the number of treated animals.

Assessment of inhibition of porcine hepatic cytochrome P450 enzymes by 48 commercial drugs

Drug interactions due to inhibition of hepatic cytochrome P450 (CYP450) enzymes are not well understood in veterinary medicine. Forty-eight commercial porcine medicines were selected to evaluate their potential inhibition on porcine hepatic CYP450 enzymes at their commercial doses and administration routes. Those drugs were first assessed through a single point inhibitory assay at 3 µM in porcine liver microsomes for six specific CYP450 metabolisms (phenacetin o-deethylation, coumarin 7-hydroxylation, tolbutamide 4-hydroxylation, bufuralol 1-hydroxylation, chlorozoxazone 6-hydroxylation and midazolam 1'-hydroxylation). When the inhibition was > 10% in the single point inhibitory assay, IC50 values (inhibitory concentrations that decrease biotransformation of selected substrate by 50%) were determined. Overall, 17 drugs showed in vitro inhibition on one or more porcine hepatic CYP450 metabolisms with different IC50 values. The potential in vivo porcine hepatic CYP450 inhibition by those drugs was assessed by combining the in vitro data and in vivo Cmax (maximum plasma concentrations from pharmacokinetic studies of the porcine medicines at their commercial doses and administration routes). Three drugs showed high potential inhibition to one or two porcine hepatic CYP450 isoforms at their commercial doses and administration routes, while seven drugs had medium risk and seven had low risk of such in vivo inhibition. These data are useful to prevent potential drug interactions in veterinary medical practice.

Pyrantel in small animal medicine: 30 years on

Pyrantel, a tetrahydropyrimidine nicotinic agonist anthelmintic, has been used in companion animal medicine since the 1970s to control two important nematode groups, the hookworms and the roundworms. Given the zoonotic potential of these parasites, pyrantel has served a dual role in helping to protect the health of both companion animals and the public for more than 30 years. This review describes the history and mechanism of action of this drug, and collates evidence that resistance to pyrantel has developed in at least one canine nematode, the hookworm Ancylostoma caninum. The role of in vitro diagnosis tests in managing anthelmintic resistance in companion animal parasites is discussed, as are management practices that may reduce the rate at which resistance develops.

Computational models to assign biopharmaceutics drug disposition classification from molecular structure

PURPOSE

We applied in silico methods to automatically classify drugs according to the Biopharmaceutics Drug Disposition Classification System (BDDCS).

MATERIALS AND METHODS

Models were developed using machine learning methods including recursive partitioning (RP), random forest (RF) and support vector machine (SVM) algorithms with ChemDraw, clogP, polar surface area, VolSurf and MolConnZ descriptors. The dataset consisted of 165 training and 56 test set molecules.

RESULTS

RF model 3, RP model 1, and SVM model 1 can correctly predict 73.1, 63.6 and 78.6% test compounds in classes 1, 2 and 3, respectively. Both RP and SVM models can be used for class 4 prediction. The inclusion of consensus analysis resulted in improved test set predictions for class 2 and 4 drugs.

CONCLUSIONS

The models can be used to predict BDDCS class for new compounds from molecular structure using readily available molecular descriptors and software, representing an area where in silico approaches could aid the pharmaceutical industry in speeding drugs to the patient and reducing costs. This could have significant applications in drug discovery to identify molecules that may have future developability issues.

The disposition of pyrantel in the gastrointestinal tract and effect of digesta flow rate on the kinetic behaviour of pyrantel in the pig

Pigs consuming a diet with slow digesta transit time were orally administered with molar equivalent doses of pyrantel as the citrate and pamoate salt. At appropriate intervals pigs were killed and the quantitative-time distribution of pyrantel throughout the gut contents was determined. Compared with the citrate, there appeared to be greater quantities of the less soluble pamoate salt in the small and large intestine. An additional group of pigs fed on diets with "slow" or "fast" digesta transit time were orally treated with molar equivalent amounts of pyrantel as the citrate and pamoate salt and the respective kinetic disposition of pyrantel in peripheral plasma and quantitative excretion in faeces was determined. The diet type had little effect on pyrantel availability after administration of the less soluble pamoate salt. However, the maximum concentration of pyrantel in plasma was lower and there appeared to be greater quantities of pyrantel retained in the gut and excreted in faeces when the citrate salt was orally administered to pigs fed the "fast" compared to the "slow" diet. Since it is the quantity of pyrantel contained in the gut lumen which is believed to affect efficacy against gastrointestinal parasites, greater efficacy with this anthelmintic should be obtained when pigs are consuming a high fibre diet.

Frontiers in anthelmintic pharmacology

Research in anthelmintic pharmacology faces a grim future. The parent field of veterinary parasitology has seemingly been devalued by governments, universities and the animal industry in general. Primarily due to the success of the macrocyclic lactone anthelmintics in cattle, problems caused by helminth infections are widely perceived to be unimportant. The market for anthelmintics in other host species that are plagued by resistance, such as sheep and horses, is thought to be too small to sustain a discovery program in the animal health pharmaceutical industry. These attitudes are both alarming and foolish. The recent history of resistance to antibiotics provides more than adequate warning that complacency about the continued efficacy of any class of drugs for the chemotherapy of an infectious disease is folly. Parasitology remains a dominant feature of veterinary medicine and of the animal health industry. Investment into research on the basic and clinical pharmacology of anthelmintics is essential to ensure chemotherapeutic control of these organisms into the 21st century. In this article, we propose a set of questions that should receive priority for research funding in order to bring this field into the modern era. While the specific questions are open for revision, we believe that organized support of a prioritized list of research objectives could stimulate a renaissance in research in veterinary helminthology. To accept the status quo is to surrender.

The influence of high- and low-fibre diets on the activity of piperazine against Oesophagostomum spp. in pigs

An in vivo study evaluated the effect of diet on the efficacy of piperazine against nodular worms of pigs. Twenty pigs, later allocated into five groups, were each infected (and 37 days later re-infected) with 3000 infective larvae of a mixed isolate of Oesophagostomum dentatum and Oesophagostomum quadrispinulatum. Beginning on day 23 post infection (p.i.), pigs in groups 1 and 2 were fed a low-fibre diet consisting of 70% barley flour and 30% protein concentrate, while pigs in groups 3, 4 and 5 were fed a high-fibre diet consisting of 55% barley flour, 21% oat-husk meal and 24% protein concentrate. On day 42 p.i., pigs in groups 1 and 3 were orally dosed with 200 mg piperazine dihydrochloride (Ascarex D, 53%) per kg bodyweight, the recommended dose, while pigs in group 4 were given 100 mg kg-1. Groups 2 and 5 served as non-treated controls for the respective dietary regimens. Eight days after treatment, the pigs were slaughtered and worms recovered from the caecum and large intestine (divided into five sections) and counted. The mean worm count reduction (WCR) in group 1 (full-dose piperazine with low-fibre diet) was 89.8%, while the high-fibre diet in group 3 increased the WCR to 99.4%. In group 4, where the pigs were fed the high-fibre diet and treated with only 100 mg piperazine kg-1, the WCR was 90.9%, identical to the "low fibre" group 1 treated with twice this piperazine dose. There was a zero efficacy recorded against immature worms in all three treated groups. The high-fibre diet improved the efficacy of piperazine against more pathogenic and generally more tolerant O. quadrispinulatum to 99.2% compared with 84.3% at the low-fibre diet.

Modifying the formulation or delivery mechanism to increase the activity of anthelmintic compounds

The development of resistance to current chemical classes of broad-spectrum anthelmintics poses an undeniable threat to the long-term viability of the animal health industry. Alternative treatment strategies including vaccines, biological control and breeding of parasite-resistant animals are unlikely to be widely available in the near future and even then, they will be integrated with chemotherapy. To compound the severity of the situation there appears to be no new chemical class of anthelmintics, with unique mode of action, on the horizon. The significant cost of drug research and the development costs of a drug that is to be used in food-producing animals, together with the small market share of animal health products compared to human pharmaceutical/medical and cosmetic products, provide little incentive for anthelmintic development. The chemical actives that are currently available, are all that we are likely to have for the foreseeable future. If effective parasite treatment is to continue, existing actives must be used more efficiently. Recognising the potential for the animal's physiological behaviour to assist drug action is of significant value. Reduction of feed intake before oral anthelmintic treatment slows ruminant digesta flow, prolongs and extends the availability and therefore increases efficacy, of the benzimidazole and ivermectin compounds. This is a cost effective option that can be employed which not only increases efficacy of 'older' compounds, but will be instrumental in prolonging the useful life of the 'newer' drugs. In a related approach the co-administration of metabolic inhibitors can prolong drug clearance and extend availability and increase the action of existing anthelmintics. However, given the large costs which would be associated with this development (host toxicity, residue safety) it is probable that the value of such combinations would be more appropriate for use in the treatment of non-food producing animals. The most promising approach for improved formulation lies in innovative delivery systems using chemical or physical carriers. Solubility-defining salts, oils, solid/drug matrices, liposomes and related microparticles that reduce drug absorption/metabolism and can specifically direct large quantities of active, over an extended or pre-determined period, to the site(s) of parasitic infection. The use of lipophilic actives/vehicles which deposit in and are released from body fat is of particular value in extending drug availability. The prophylactic action of extended drug residence time, when used with effective grazing/treatment management programmes, provides opportunities for sustainable antiparasitic action. Clearly, with the paucity of new chemical classes of anthelmintics, the use of 'intelligent' but still relatively inexpensive carriers/delivery systems for existing actives will form the basis of future parasite control.

Physiology, pharmacology and parasitology

The developing resistance to current chemical classes of broad-spectrum anthelmintics and insecticides presents an undeniable threat to the long-term viability of the animal health industry. Alternative treatment strategies including vaccines, biological control and breeding of parasite-resistant animals are unlikely to be widely available in the near future and even then they will be integrated with chemotherapy. The significant cost of research and development of new therapeutics for food-producing animals, together with the small market share of animal health products, particularly in Australia and New Zealand, is a positive disincentive for drug development. The chemical actives that are currently available are all that we are likely to have for the foreseeable future and they must be used more efficiently. Understanding the pharmacokinetic behaviour of antiparasitics and recognising the potential for the animal's physiological characteristics to assist drug action is crucial. Careful administration, coupled with a reduction of feed intake before oral anthelmintic treatment, maximises drug availability and therefore increases efficacy of the benzimidazole and ivermectin compounds. This is a cost-effective option that can be employed immediately, which not only increases efficacy of "older" compounds but will be instrumental in prolonging the useful life of the newer drugs. Taking care to apply topical insecticide formulations directly along the backline immediately after shearing will maximise even diffusion of active around the sheep flanks to contact lice inhabiting sites remote from the point of drug application. The use of "intelligent" formulation and delivery of existing compounds, based on knowledge of host physiological and pharmacological responses, holds the key to effective antiparasitic treatment.