Comparison of bioanalytical determinations of Iloprost, a chemically stable PGI2 mimetic, by conventional radioimmunoassay (RIA) and scintillation proximity assay (SPA).

The scintillation proximity assay is a novel variant of classical radioimmunoassay. It can be performed as a single tube measurement because the separation of bound and unbound tracer fraction is avoided. In principle, microbeads are coated with anti-species antibodies that can couple with the respective antiserum used for RIA. By means of special cores, light emission takes place if labelled, antiserum-bound tracer is coupled to the anti-species antibody on the fluomicrosphere surface. In the present report, the novel assay was compared to a validated RIA for the bioanalysis of the PGI2 mimetic, Iloprost. Extraction recovery of Iloprost was approximately 90% at pH less than or equal to 4. The detection limit of the novel assay was 2-4 pg/sample, corresponding to 10-20 pg/ml plasma (if 0.2 ml plasma was used). Coefficients of variations were 9, 7 and 6% (within-day, n = 5) and 30, 11 and 10% (day-to-day, n = 10) at 50, 100 and 200 pg/ml. RIA and SPA levels of Iloprost measured in human plasma samples (n = 428) were similar. The SPA method exhibits both a similar specificity and detection limit to RIA and will be used for further analyses.