Plasma levels of ethinyl estradiol were measured by radioimmunoassay in five women after oral administration of 30 microgram and of 50 microgram of ethinyl estradiol. Peak levels of ethinyl estradiol were mostly observed within two hours after the tablet intake. The peak concentrations found were 50--90 pg and 95--135 pg for the two doses administered. Twenty-four hours after the administration of the tablets, the plasma concentration of ethinyl estradiol did not exceed the plasma blank values (25 pg/ml) in most women.
PIP: To determine levels of ethinyl estradiol (EE) in peripheral plasma following oral administration of the drug, 5 healthy women were given a 50 mcg tablet after breakfast, and blood samples were taken at intervals from 1 to 24 hours postadministration for radioimmunoassay; the same procedure was followed 14 days later with a tablet containing 30 mcg EE. Under the 50 mcg dosage, plasma EE in 4 of the 5 women within 1-2 hours at values of 95-135 pg/ml. A 5th subject showed peak value at 8 hours. At 30 mcg dosage, peak levels of 50-70 pg/ml were achieved within 1-3 hours in 4 of the women, and at 4 hours in the same subject who had previously shown a delay in peak level. As the lower detection limit of the assay was 25 pg/ml, it was impossible to determine plasma half-life. Since in 2 women EE could not be detected after 12 hours, it is concluded that levels of EE above 25 pg/ml are not necessary over a full 24 hours for their biological effect.