Radioimmunological determination of levonorgestrel and ethinylestradiol

A sensitive and specific radioimmunoassay for ethinylestradiol (EE2) and levonorgestrel (LN) has been developed. Antisera were raised in rabbits after injection of EE2-6-carboxymethyloxime-aminocaproic acid and LN-3-carboxy-methyloxime linked to bovine serum albumin. The sensitivity of immunoassay was 15 pg/ml for EE2 and 10 pg/ml for LN. The cross-reactions showed a high specificity of the assay, and the accuracy and precision of this method satisfied the requirements for the investigation of pharmacokinetics. With this assay, although the recovery of the assay was not very high (the coefficient of variation 14.02% for EE2 and 10.27% for LN) when used to analyse diethylether extracts of biological fluids, a microquantitative metabolite of norethisterone (NET) and norethisterone-3-oxime (NETO), EE2 analogue in incubation solution with rat hepatocytes, was satisfactorily detected. The result showed that in vitro the amount of NETO transferred to EE2 analogue was three times as much as that of NET. It is necessary to further establish the identity and activity of the analogue, but these data will be found advantageous to the research of steroid-oxime.