Effect of the ionophore antibiotic monensin on the ruminal biotransformation of benzimidazole anthelmintics

The benzimidazole (BZD) anthelmintics, netobimin (NTB) pro-drug and albendazole sulphoxide (ABZSO) are reduced to albendazole (ABZ) by ruminal microflora. The aim of the current work was to evaluate the influence of the ionophore monensin (MON) on the in vitro biotransformation of NTB and ABZSO by sheep ruminal fluid. Ruminal fluid, collected from Corriedale sheep, was preincubated (24 h) either without (control) or with known MON concentrations (0.5, 1.5 and 3.0 mug/mL) at 38 degreesC under a CO2 atmosphere. Afterwards, aliquots from both MON-pretreated and control ruminal fluid samples were incubated (30 and 60 min) with 2 mug/mL of either NTB or ABZSO. Incubated samples were chemically extracted and analysed by High Performance Liquid Chromatography to quantify the metabolites formed. The rate of ABZ production after 30 min of NTB incubation with control ruminal fluid was 0.023 mug/min. Conversely, the rates of ABZ formation were significantly (P < 0.05) lower (0.009, 0.011 and 0.013 mug/min) when NTB was incubated with ruminal fluid pretreated with MON (at 0.5, 1.5 and 3.0 mug/mL, respectively). After both incubation periods, the reduction of ABZSO to ABZ was 22 to 70% lower when the ruminal fluid was preincubated with the different MON concentrations. The lower ABZ production observed in the presence of MON may result in a modified availability of this molecule in the gastrointestinal (GI) tract and hence, on its anthelmintic efficacy against GI nematodes. (C) 2003 Elsevier Ltd. All rights reserved.