A non-chromatographic radioimmunoassay for 3-oxo desogestrel

A non-chromatographic radioimmunoassay for 3-oxo desogestrel (13 beta-ethyl-17-hydroxy-11-methylene-18,19-dinor-pregn-4-en-20-yn-3- one), the biologically-active metabolite of desogestrel (13 beta-ethyl-11-methylene-18,19-dinor-pregn-4-en-20-yn-17-ol), has been developed to facilitate studies of the pharmacokinetics of this steroid. The method uses an antiserum raised against levonorgestrel (13 beta-ethyl-17-hydroxy-18,19-dinor-pregn-4-en-20-yn-3-one). None of the steroids tested which showed significant cross-reactions are believed to be present in plasma after ingestion of desogestrel; furthermore, dilutions of standards and unknowns gave parallel responses in the assay. Intra- and inter-assay coefficients of variation were 12.9 and 11.8% respectively. The sensitivity of the assay was approx 0.02 ng/ml. The peak concentrations of 3-oxo desogestrel after a 150 micrograms dose of desogestrel in three subjects were between 0.48-0.71 ng/ml, and in two subjects 3-oxo desogestrel was still detectable 24 h after dosing.