Determination of ivermectin in bovine liver by optical immunobiosensor Jeanne V. Samsonova a, *, G. Andrew Baxter b , Steven R.H. Crooks b , Amanda E. Small b , Christopher T. Elliott a,b a Department of Veterinary Science, Queen’s University of Belfast, Stoney Road, Stormont, BT4 3SD Belfast, UK b Veterinary Sciences Division, Department of Agriculture and Rural Development for Northern Ireland, Stoney Road, Stormont, BT4 3SD Belfast, UK Received 31 January 2001; received in revised form 14 August 2001; accepted 20 December 2001 Abstract A rapid and sensitive biosensor immunoassay was developed for residues of the antiparasitic agent ivermectin in bovine liver. A detection limit of 19.1 ng g 1 was achieved. The sensor employed was a Biacore optical instrument based on surface plasmon resonance. 5-O -succinoylivermectin-apo-transferrin conjugate was used to produce monoclonal antibody while a second derivative, ivermectin-oxime, was immobilised onto the surface of a sensor chip. A range of assay parameters (flow rate, injection time, temperature) and extraction techniques were investigated. In the final assay procedure, ivermectin was extracted with acetonitrile followed by C 8 SPE clean-up. Matrix effect was minimised by increasing the flow rate to 25 ml min 1 and reducing the sample injection time to 2 min. The average value for liver samples spiked at 100 ng g 1 (the MRL for the drug) and 50 ng g 1 concentrations were 93.7 and 43.2 ng g 1 , respectively. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ivermectin; Avermectins; Biosensor; Bovine liver 1. Introduction Ivermectin is a semi-synthetic 22,23-dihydro deriva- tive of avermectin B1 (abamectin) which is the major and most important product of the fermentation of Streptomyces avermitilis (Campbell, 1989). Ivermectin consists of a mixture of two homologous compounds (‘H 2 B 1a ’ and ‘H 2 B 1b ’) in an average ratio of 80:20 (Table 1). These compounds differ only by one methylene group at the C-25 position. Since the drug’s introduction to the market in 1981 it has found wide usage as an antiparasitic agent against endo- and ectoparasites of animals (cattle, sheep, swine, horses, dogs). It is also used as a very effective treatment for human filarial worm infections (Onchocerca volvulus , River Blindness). Ivermectin is used at substantially lower doses than other veterinary medicines (up to 300 mg kg 1 ; Camp- bell, 1989). This makes the detection of residues in animal tissue, which have resulted from treatment, very challenging. To enable residue detection very sensitive analytical assays are required. During the last 15 years a number of chromatographic methods (LC, HPLC) were developed for determination of ivermectin in different matrices such as liver, muscle (Abjean and Gaugain, 1995; Nordlander and Johnsson, 1990; Reuvers et al., 1993; Scott and McKellar, 1992), milk, serum, plasma (Chiou et al., 1987; De Montigny et al., 1990; Dick- inson, 1990), salmon (Kennedy et al., 1993) and others. While sensitive these methods can be described as lengthy and tedious to perform. As routine screening methods they accomplish the analytical need but take much longer to perform than is desired. Immunoassay, as a screening technique for residue analysis, has proven to be a promising alternative to conventional physicochemical methods. Both monoclo- nal (Schmitt et al., 1990) and polyclonal (Mitsui et al., 1996; Crooks et al., 1998) anti-ivermectin antibodies have been produced and successfully applied to ELISA based techniques. It is a logical progression to try to further develop ivermectin residue tests using immunosensor- * Corresponding author. Present address: Department of Chemical Enzymology, Chemistry Faculty, M.V. Lomonosov Moscow State University, 119899 Moscow, Russia. Tel.: 7-095-939-3407; fax: 7- 095-939-2742. E-mail address: jvs@enzyme.chem.msu.ru (J.V. Samsonova). Biosensors and Bioelectronics 17 (2002) 523 /529 www.elsevier.com/locate/bios 0956-5663/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0956-5663(02)00016-7