Development and use of a radioimmunoassay for D-(-)-norgestrel 17β-cyclopentane-carboxylate

Abstract The synthesis of the 6α-carboxymethylmercapto BSA and homologous histamine conjugate of D-(-)-norgestrel 17β-cyclopentanecarboxylate is reported. Using the BSA conjugate as an immunogen for the development of antibody in the rabbit and the 125 l-histamine conjugate as the radioligand, a radioimmunoassay (RIA) for the ester was developed. Serum profiles of the free alcohol and ester were determined following IV or IM injection in macaques. Peak values for the ester (about 12 ng/mL) were observed 2 min following an IV bolus of 0.5 mg in one rhesus monkey. Blood levels dropped rapidly within the first 30 min and were barely detectable at 24 h. Serum levels of the free alcohol rose to a peak at 30 min and then declined slowly to very low values by 24 h. Following IM injection of 20 mg in cynomolgus monkeys, peak levels of the ester were observed within a few days while the free alcohol reached a maximum about day 30.Serum concentrations of D-(-)-norgestrel had fallen to about 0.4 ng/mL 160 days post-injection when levels of the ester fell below 0.2 ng/mL.