Accumulation of monepantel and its sulphone derivative in tissues of nematode location in sheep: Pharmacokinetic support to its excellent nematodicidal activity
Introduction
Gastrointestinal nematodes control programs are mainly based on a combination of animal management practices and the use of antiparasitic drugs. During the last 30 years, antiparasitic treatment has been mainly restricted to three anthelmintic groups: the imidazothiazoles, the benzimidazoles and the macrocyclic lactones. After years of intensive use to optimize animal productivity, the widespread appearance of resistant parasites in different areas of the world was inevitable. In this context, the need of novel drugs acting at novel target sites has been highlighted on many occasions.
The amino-acetonitrile derivatives (AADs) represent one of the newest anthelmintic classes (Kaminsky et al., 2008) introduced in veterinary medicine. From many compounds evaluated, the racemic molecule AAD 96, was selected and the active s-enantiomer of this molecule, named monepantel (MNP), was launched into the veterinary pharmaceutical market for oral administration to sheep in 2009 (Hosking et al., 2010). MNP acts at a new target as a positive allosteric modulator of the nematode specific receptor MPTL-1, which belongs to the DEG-3 subfamily of acetylcholine receptors (Rufener et al., 2010). MNP binding to receptor accounts for an alteration in ion flux and leads to the paralysis of nematodes (Epe and Kaminsky, 2013). This new mechanism of action explains the high efficacy of MNP against nematodes resistant to other anthelmintic classes (Baker et al., 2012).
The plasma disposition kinetics of MNP has been assessed in sheep after its intravenous and oral administration (Karadzovska et al., 2009). Monepantel sulphone (MNPSO2) was the main metabolite detected in the bloodstream after MNP administration. As this metabolite is also active against nematodes, the pharmacokinetic behavior of MNPSO2 is relevant for the interpretation of residue and efficacy studies (Karadzovska et al., 2009). Although the evaluation of drug concentration profiles in the bloodstream contributed with useful information (Karadzovska et al., 2009), MNP and MNPSO2 exert their anthelmintic effects in some non-vascular target tissues such as the gastrointestinal (GI) tract (Kaminsky et al., 2009), where nematode parasites are located. The characterization of MNP and MNPSO2 concentration profiles attained at specific GI sites of parasite location and the establishment of the relationship between their plasma and gastrointestinal content/tissues availabilities were the main goals of the experiment described here.
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Animals
The study was conducted in clinically healthy and parasite-free sheep. Twenty two (22) Romney Marsh (15–20 kg) lambs were used. The animals were kept under field conditions during the experimental period. Their health was monitored prior to and throughout the experiment. Animals were in optimal body condition, grazed on a lucerne/red clover pasture with free access to water during the study. Animal procedures and management protocols were approved by an Ethics Committee according to the Animal
Monepantel and monepantel sulphone quantitation
The extraction of MNP and MNPSO2 from spiked and experimental plasma and tissue samples was carried out following the technique described by Karadzovska et al. (2009). Briefly, 0.5 ml of plasma and 0.3–0.5 g of GI content/mucosa mixed with 0.5 ml of water and 1.3 ml of acetonitrile. After mixing for 2 min, the solvent-sample mixture was centrifuged at 2000g for 15 min. The supernatant was manually transferred into a tube, mixed with 5.0 ml of water and injected onto a polymeric sorbent solid phase
Pharmacokinetic analysis of the data
The plasma concentration versus time curves obtained after treatment of each individual animal were fitted with the PK Solutions 2.0 (Ashland, OH, USA) computer software. Pharmacokinetic parameters were determined using a non-compartmental model method. The peak concentration (Cmax) was read from the plotted concentration-time curve of each individual animal. The area under the concentration vs. time curves (AUC) were calculated by the trapezoidal rule (Gibaldi and Perrier, 1982) and further
Statistical analysis
Mean pharmacokinetic parameters for MNP and MNPSO2 were statistically compared using Student's t-test. The assumption that the data obtained after treatments have the same variance was assessed. A non-parametric Mann–Whitney test was used where significant differences among standard deviations were observed. A similar procedure was used to compare MNP and MNPSO2 concentrations measured in plasma and in different GI contents and tissues. The statistical analysis was performed using the Instat
Results
Low concentrations of MNP were measured in plasma up to 48 h post-administration. A significantly higher concentration profile was observed for the MNPSO2 derivative in the bloodstream compared to that of the parent compound. The persistence of the sulphone metabolite was present longer in the bloodstream; up to 216 h (9 days) after the oral administration of MNP to sheep. The comparative MNP and MNPSO2 plasma concentration profiles are shown in Fig. 1. The observed differences were reflected in
Discussion
Data on plasma kinetic profiles can help to explain the comparative efficacy and persistence of different anthelmintic drugs. However, the characterization of drug concentration profiles at the tissue sites of parasite habitation permits a more direct interpretation and provides a basis for understanding the therapeutic action of nematodicidal compounds. The work described here assessed the accumulation and disposition kinetics of the novel anthelmintic MNP and its active sulphone metabolite in
Acknowledgments
This work was supported by CONICET and Agencia Nacional de Promoción Científica y Técnica (PICT 1432), all from Argentina. We thank Novartis Animal Health for providing MNP and MNPSO2 pure analytical standards.
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