The development of a sensitive and specific enzyme immunoassay for FK480, a novel cholecystokinin type-A receptor antagonist, in human plasma.

A sensitive and specific enzyme immunoassay for FK480, a novel cholecystokinin type-A (CCK-A) receptor antagonist, was developed to study the pharmacokinetics of the drug at low-dose administration using a specific monoclonal antibody. The high performance liquid chromatography (HPLC) method had been used for studying toxicokinetics, but its determination limit (2.5 ng ml-1) was too high for use in clinical studies. Subsequently we developed an enzyme immunoassay (EIA) using rabbit anti-FK480 serum (polyclonal antibody). It had higher sensitivity (0.1 ng ml-1) when 0.5 ml of plasma was used but its specificity was low because of the cross-reactivity of the metabolites of FK480. Therefore we produced several monoclonal antibodies for FK480 by cell fusion, and selected the antibody which was least cross-reactive for the isolated metabolites of FK480. Finally we developed a sensitive and specific EIA using this monoclonal antibody. The lower limit of quantification of this method was 0.2 ng ml-1 when 0.2 ml of human plasma was used. The coefficient of variation over the calibration range (0.2-10 ng ml-1) was less than 15%. We used this method for clinical studies, and it showed a good correlation to the HPLC method when plasma concentration was 2.5 ng ml-1 or more.