Comparison of monoclonal and polyclonal antibodies to cotinine in nonisotopic and isotopic immunoassays

Abstract

Monoclonal antibodies (McAb) were used to develop nonisotopic and radioimmunoassays (RIA) for quantitative determination of the major nicotine metabolite, cotinine, in physiological fluids. ELISAs and fluorescence immunoassays were carried out in microtiter plate wells coated with a conjugate of cotinine 4′-carboxylic acid bound covalently to poly- L-lysine. The detection systems were horseradish peroxidase (HRP)-labeled staphylococcal protein A, HRP-streptavidin-biotin, and biotinylated alkaline phosphatase-4-methylumbelliferyl phosphate. With the three McAb tested, I 50 values ranged between 0.024–0.063 ng cotinine and as little as 0.005–0.015 ng gave 15% inhibition. These assays were 5–20 times more sensitive than similar assays using six rabbit antisera. With McAb the standard inhibition curves were steeper and complete inhibition of immune binding was achieved with approximately 1 ng cotinine. In contrast, 100–500 ng cotinine failed to give greater than 80–90% inhibition with rabbit antibodies eiter in the plate assays or in RIA using a 125I-labeled tyramine derivative of cotinine as the tracer. In this RIA, the sensitivity with McAb (mean I 50 of 0.55 ng cotinine) was over three-fold greater than with rabbit antisera (mean I 50 of 1.84 ng). The presence of antibodies directed to the amide linkage group common to the polylysine conjugate, 125I-tyramine derivative and the immunogen likely accounts for the inferior quality of assays using rabbit antisera. Consistent with this conclusion, superimposable inhibition curves were obtained in the RIA when monoclonal or rabbit antibodies were used with [ 3H]cotinine. Cotinine levels in saliva, serum and plasma from smokers and non-smokers determined with McAb-based assays showed a strong correlation with values obtained by RIA using rabbit antisera or by gas chromatography. Properly selected McAb offer distinct advantages over conventional antisera in nonisotopic immunoassays and RIAs for cotinine as a biochemical marker of active or passive smoking.