Radioimmunoassay of serum d-Norgestrel in women following oral and intravaginal administration

A radioimmunoassay (RIA) method for measuring d-Norgestrel (d-Ng) in serum has been developed utilizing petroleum ether extraction, an antiserum raised against d-Norgestrel-3-(0-carboxymethyl) oxime-epsilon-aminocaproic acid--boxine serum albumin, d-Norgestrel-3-(0-carboxymethyl) imino-[125I]-iodohistamine, and dextran-coated charcoal separation. Control serum blanks were undetectable, 6 pg of d-Ng was measurable in 0.1 ml of serum with a high reliability, and intra- and interassay coefficients of variation were 4.4 and 4.9 percent, respectively. d-Ng added to control serum was quantitatively recovered. This RIA proved to be highly specific for d-Ng, cross-reacting less than 0.01 percent with 1-Ng and some 4 to 8 percent with 5α- and 5β-tetrahydro-d-Ng. Serum d-Ng levels measured in 3 women after ingestion of 75 μg of dl-Ng rose to 1.5 to 2 ng/ml within 1/2 to 2-3 hours after oral intake and fell rapidly thereafter to 0.2 to 0.4 ng/ml within 24 hours. Insertion of Silastic intravaginal rings (IVRs), containing 50 or 100 mg of dl-Ng, into 3 women for periods of 3 weeks resulted in a rapid rise of serum d-Ng after insertion. Serum d-Ng levels were highest 24 to 48 hours after the first insertion of each IVR, reaching peak levels of about 5 and 7 to 11 ng/ml, respectively, and declined gradually during the ensuing 15 to 20 days to some 30 to 40 percent of the peak serum d-Ng concentration. Serum d-Ng levels attained after IVR reinsertion remained relatively constant at 1 to 3 ng/ml during each subsequent cycle in all subjects and fell rapidly after each IVR removal. Serum estradiol and progesterone concentrations, measured about 3 times a week in these patients, indicated that ovulation was consistently inhibited