The use of competitive displacement agents to resolve albumin binding problems observed during the development of a radioimmunoassay for ICI 215001.

A radioimmunoassay has been developed for the analysis of ICI 215001, a carboxylic acid metabolite of ICI D7114. The level of binding and sensitivity of the assay were good in the absence of plasma. However, the addition of plasma to the incubation medium reduced the antibody binding of radiolabelled tracer (I) from 41 to 9%. This was attributed to the high (> 99%) plasma albumin binding of ICI 215001. A series of compounds (DL-tryptophan, octanoic acid, bilirubin, ponalrestat, warfarin, phenylbutazone and salicylic acid) which all bind to plasma albumin, were examined for their effect on the tracer-antibody interaction. Warfarin and phenylbutazone were the only compounds to specifically displace the iodinated tracer from albumin; warfarin was the only compound to restore antibody binding to control levels. The warfarin concentration and the pH of the incubation medium also had a substantial effect on the magnitude of the displacement. The optimized method for the analysis of ICI 215001 in human plasma (20 microliters) used phosphate buffer (pH 6.0, 0.1 M) containing warfarin (50 micrograms ml-1), which gave an assay with the desired specificity, precision and sensitivity.