Achiral and chiral high-performance liquid chromatographic determination of flubendazole and its metabolites in biomatrices using UV photodiode-array and mass spectrometric detection

Flubendazole, methyl ester of [5-(4-fluorobenzoyl)-1H-benzimidazol-2-yl]carbamic acid, belongs to the group of benzimidazole anthelmintics, which are widely used in veterinary and human medicine. The phase I flubendazole biotransformation includes a hydrolysis of the carbamoyl methyl moiety accompanied by a decarboxylation (hydrolysed flubendazole) and a carbonyl reduction of flubendazole (reduced flubendazole). Flubendazole is a prochiral drug, hence a racemic mixture is formed during non-stereoselective reductions at the carbonyl group. Two bioanalytical HPLC methods were developed and validated for the determination of flubendazole and its metabolites in pig and pheasant hepatic microsomal and cytosolic fractions. Analytes were extracted from biomatrices into tert-butylmethyl ether. The first, achiral method employed a 250 mm × 4 mm column with octylsilyl silica gel (5 μm) and an isocratic mobile phase acetonitrile–0.025 M KH2PO4 buffer pH 3 (28:72, v/v). Albendazole was used as an internal standard. The whole analysis lasted 27 min at a flow rate of 1 ml/min. The second, chiral HPLC method, was performed on a Chiralcel OD-R 250 mm × 4.6 mm column with a mobile phase acetonitrile–1 M NaClO4 (4:6, v/v). This method enabled the separation of both reduced flubendazole enantiomers. The enantiomer excess was evaluated. The column effluent was monitored using a photodiode-array detector (scan or single wavelength at λ = 246 nm). Each of the analytes under study had characteristic UV spectrum, in addition, their chemical structures were confirmed by high-performance liquid chromatography–mass spectrometry (HPLC–MS) experiments. Stereospecificity in the enzymatic carbonyl reduction of flubendazole was observed. While synthetic racemic mixture of reduced flubendazole was separated to equimolar amounts of both enantiomers, practically only one enantiomer was detected in the extracts from all incubates.