A SPECIFIC RADIOIMMUNOASSAY FOR THE MEASUREMENT OF ICI-200,880, AN ELASTASE INHIBITOR, IN HUMAN SERUM

ICI 200,880, line-1(RS)-(1-trifluoroacetyl-2-methylpropyl)amide (I), i s a human neutrophil elastase inhibitor in development by ICI Pharmace uticals Group. A specific and sensitive radioimmunoassay has been deve loped for this compound in human serum. An hydroxy-succinimide ester a nalogue of ICI 200,880 was coupled to the lysine residues of bovine se rum albumin, characterized by gel electrophoresis and injected into ra bbits to stimulate antibody formation; a highly specific, high titre a ntibody was obtained. An I-125-diiodinated 4-hydroxy-phenyl derivative of ICI 200,880 was utilized as the radiolabelled antigen. Satisfactor y zero and non-specific binding were achieved with a 2-h pre-incubatio n of ICI 200,880 and antiserum at 37-degrees-C, followed by the additi on of radiolabelled antigen and a second 2-h incubation. Separation of bound and free radiolabel was achieved by employing PEG-goat anti-rab bit IgG separant. Sensitivity of 25 pg ml-1 ICI 200,880 in human serum was achieved (n = 12), with quantitative recovery of 106%, inter-assa y precision of 6.3% and an average relative binding statistically diff erent than that of pooled human serum. Serum quality control samples s piked at 600 and 100 pg ml-1 ICI 200,880 averaged 104% recovery over 6 validation days, with intra-assay precision of 12.8% RSD and inter-as say precision of 9.2% RSD (n = 24). Cross-reactivity of the ICI 200,88 0 antibody to three known metabolites and several analogues was neglig ible. Over 1200 clinical samples following aerosol administration of I CI 200,880 have been analysed by this procedure. After maximum exposur e ranging from 0.3 to 32 mg ICI 200,880, mean peak levels from 0.08 to 20.5 ng ml-1 were measured, respectively, with compound detected gene rally up to 16 h post-dose.