A specific radioimmunoassay for the measurement of ICI 200,880, an elastase inhibitor, in human serum

Abstract

ICI 200,880, 4-(4-chlorophenylsulphonylcarbamoyl)benzoyl- l-valyl- l-proline-1( RS)-(1-trifluoroacetyl-2-methylpropyl)amide ( I), is a human neutrophil elastase inhibitor in development by ICI Pharmaceuticals Group. A specific and sensitive radioimmunoassay has been developed for this compound in human serum. An hydroxysuccinimide ester analogue of ICI 200,880 was coupled to the lysine residues of bovine serum albumin, characterized by gel electrophoresis and injected into rabbits to stimulate antibody formation; a highly specific, high titre antibody was obtained. An 125I-diiodinated 4-hydroxy-phenyl derivative of ICI 200,880 was utilized as the radiolabelled antigen. Satisfactory zero and non-specific binding were achieved with a 2-h pre-incubation of ICI 200,880 and antiserum at 37°C, followed by the addition of radiolabelled antigen and a second 2-h incubation. Separation of bound and free radiolabel was achieved by employing PEG-goat anti-rabbit IgG separant. Sensitivity of 25 pg ml −1 ICI 200,880 in human serum was achieved ( n = 12), with quantitative recovery of 106%, inter-assay precision of 6.3% and an average relative binding statistically different than that of pooled human serum. Serum quality control samples spiked 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-assay precision of 9.2% RSD ( n = 24). Cross-reactivity of the ICI 200,880 antibody to three known metabolites and several analogues was negligible. Over 1200 clinical samples following aerosol administration of ICI 200,880 have been analysed by this procedure. After maximum exposure 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 generally up to 16 h post-dose.