EQUINE VETERINARY JOURNAL Equine vet. J. (2005) 37 (3) 257-262 257 Summary Reasons for performing the study: The study of novel pharmacological strategies to control parasitism in horses is required since many parasite species have developed resistance to anthelmintic drugs. Objectives: To evaluate the effects of piperonyl butoxide (PB) (a metabolic inhibitor) on the plasma availability and enantiomeric behaviour of oxfendazole (OFZ) given orally to horses, and to compare the clinical efficacy of OFZ given either alone or co-administered with PB in naturally parasitised horses. Methods: Fifteen naturally parasitised crossbred male ponies were allocated into 3 groups (n = 5) and treated orally as follows: Group I (control) received distilled water as placebo; Group II was dosed with OFZ (10 mg/kg bwt); and Group III was treated with OFZ (10 mg/kg bwt) co-administered with PB (63 mg/kg bwt). Jugular blood samples were obtained over 120 h post treatment. Three weeks after treatments, all experimental horses were subjected to euthanasia. Results: The observed maximum plasma concentration (Cmax) and area under the concentration vs. time curve (AUC) values for OFZ increased 3- and 5-fold, respectively, in the presence of PB. The plasma concentration profiles of fenbendazole (FBZ), a metabolite generated from OFZ, were significantly lower after the treatment with OFZ alone (AUC = 0.8 μg.h/ml) compared to those obtained after the OFZ + PB treatment (AUC = 2.7 μg.h/ml). The enhanced pharmacokinetic profiles correlated with increased anthelmintic efficacy. The combination OFZ + PB showed 100% efficacy against mature nematode parasites. The efficacy against cyathostome L 3 larvae increased from 94% (Group II) to 98.7% (Group III). Consistently, the number of L 4 larvae recovered from OFZ + PB treated horses (Group III) (n = 146) was significantly lower (P<0.05) than that recovered from Group II (n = 1397). Conclusions: The use of PB as a metabolic inhibitor may be useful to enhance OFZ activity against mature and migrating larvae of different parasite species in horses. Potential relevance: Metabolic inhibitors may be used to enhance the activity of benzimidazole anthelmintics and extend the effective lifespan of benzimidazole drugs in the face of increasing resistance. Introduction In spite of developments in anthelmintic therapy, parasitic diseases are still a dilemma in equine husbandry because of poor efficacy against specific life cycle stages (i.e. inhibited larval stages of cyathostominae within the large intestinal mucosa) and resistance to drugs. There are many drugs available on the veterinary market to control parasitic diseases in horses, the benzimidazole (BZD) methylcarbamates being one of the most used chemical groups. Over the course of more than 20 years, the cyathostomes (‘small strongyles’), actually considered the principal parasitic pathogens of horses (Love et al. 1999), have developed resistance to BZD anthelmintics worldwide (Herd 1990; Kaplan 2002; Sangster 2003). Understanding the relationship between the pharmacological properties of the antiparasitic drugs, parasite epidemiology and drug efficacy contributes to increased efficacy and helps prevent development of resistance. Benzimidazoles are extensively metabolised in all animal species studied (Lanusse and Prichard 1993). The sequential oxidation of the sulphides into sulphoxide and sulphone metabolites leads to more polar and less active compounds (Prichard et al. 1985). Oxfendazole (OFZ) is metabolised either irreversibly to fenbendazole sulphone (FBZSO 2) by a cytochrome P-450-mediated sulphonation, or reversibly by microbial reduction to fenbendazole (FBZ) in the gastrointestinal (GI) tract (Beretta et al. 1987; Murray et al. 1992; Lanusse and Prichard 1993). Fenbendazole is then oxidised to OFZ in a flavin monooxigenase (FMO) -mediated reaction. Oxfendazole is a chiral molecule and 2 OFZ enantiomers, (+) OFZ and (-) OFZ, have been identified in sheep (Delatour et al. 1990) and horse (McKellar et al. 2002) plasma after FBZ or OFZ administration, respectively. The characterisation of the disposition kinetics of OFZ enantiomers may contribute to understanding the relationship between chiral behaviour and anthelmintic activity of BZD carbamates. Since the length of time exposure of the target parasite to toxic drug concentrations is relevant for the pharmacological effect of Changes to oxfendazole chiral kinetics and anthelmintic efficacy induced by piperonyl butoxide in horses S. F. SÁNCHEZ BRUNI* †‡§ , L. A. FUSÉ † , L. MORENO †‡ , C. A. SAUMELL † , L. I. ÁLVAREZ †‡ , C. FIEL † , Q. A. MCKELLAR § and C. E. LANUSSE †‡ † Facultad de Ciencias Veterinarias, Universidad Nacional del Centro de la Provincia de Buenos Aires, Campus Universitario, 7000, Tandil; ‡ Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina; and § Moredun Research Institute, Pentland Science Park, Bush Loan, Penicuik EH26 0PZ, UK. Keywords: horse; pharmacokinetics; efficacy; chirality; benzimidazoles; piperonyl butoxide *Author to whom correspondence should be addressed. [Paper received for publication 28.04.04; Accepted 13.09.04]