A specific and sensitive assay for gefitinib using the enzyme-linked immunosorbent assay in human serum

The epidermal growth factor tyrosine kinase inhibitor gefitinib is a novel, molecularly targeted agent that has been approved for the treatment of advanced non-small cell lung cancer. This paper reports a specific and sensitive enzyme-linked immunosorbent assay (ELISA) for pharmacokinetic studies of gefitinib. Anti-gefitinib antibody was obtained by immunizing rabbits with an antigen conjugated with bovine serum albumin using diazotized 4-amino-2-methoxy-1-[3-(morpholin-4-yl)propoxy]benzene. Enzyme labeling of gefitinib with beta-D-galactosidase was similarly performed using a diazotized 4-amino-2-methoxy-1-[3-(morpholin-4-yl)propoxy]benzene. A simple ELISA for gefitinib was developed using the principle of direct competition between gefitinib and the enzyme marker for anti-gefitinib antibody which had been adsorbed to the plastic surface of a microtiter plate. Gefitinib concentrations in serum lower than 800 pg/ml were measurable reproducibly using the ELISA. Cross-reactivity data showed that the antibody well recognizes both the 3-morpholinopropoxy and methoxy moieties well, and thus is sufficiently specific for the structure of gefitinib. Using this assay, drug levels were easily measured in the serum of rabbits after oral administration of gefitinib at a single dose of 5 mg/kg. The specificity and sensitivity of the ELISA for gefitinib should provide a valuable new tool for use in pharmacokinetic and toxicity studies of gefitinib.