Distribution and percentages of non-protein bound contraceptive steroids in human serum

Abstract

The percentages of non-protein bound ethinyl-estradiol (EE 2), norethisterone (NET), d-norgestrel and cyproterone-acetate (CA) have been measured by centrifugal ultrafiltration-dialysis in undiluted serum at 37°C. The respective percentages (mean ± SD) of these steroids which are non-protein bound in serum from normal men ( n = 5) and women ( n = 10) are as follows: EE 2, 1.44 ± 0.15, and 1.54 ± 0.16; NET, 4.71 ± 0.55 and 3.69 ± 0.90; d-norgestrel, 3.05 ± 0.34 and 2.50 ± 0.68; CA, 6.62 ± 0.43 and 7.22 ± 0.96. Similar values were found for samples taken during follicular and luteal phases of the menstrual cycle. Percentages of non-protein bound serum EE 2 and CA were similar in both sexes, while those for NET ( P < 0.05) and d-norgestrel ( P < 0.2) appear to be higher in serum from men than women. Furthermore, the percentages of non-protein bound EE 2 (1.58 ± 0.14, n = 4) and CA (6.33 ± 1.01, n = 5) in third trimester pregnancy sera were similar to those in serum samples from men and non-pregnant women, while those of NET (1.06 ± 0.21, n = 5) and d-norgestrel (1.19 ± 0.29, n = 4) were much lower ( P < 0.001) in pregnancy serum than in any of the other samples. These differences are probably a consequence of NET and d-norgestrel binding to sex-hormone binding globulin (SHBG), since correlations were observed between serum SHBG binding capacities and percentages of non-protein bound NET and d-norgestrel, and because heat denaturation of SHBG binding capacity by >90% almost completely abolished these differences. By comparing the non-protein bound percentages of steroids in native and heat treated (60° for 1 h) serum, it was calculated that in native serum both EE 2 and CA are almost exclusively bound by the heat stable component (predominantly albumin), which binds >98.5% of EE 2 and >93.0% of CA in both men and women. In contrast, the respective percentages of NET and d-norgestrel bound to SHBG are 20.57 ± 11.38 and 37.73 ± 7.66 in men, and 35.49 ± 13.64 and 47.54 ± 11.69 in women, while the percentages of these steroids bound to albumin are 74.72 ± 10.94 and 59.21 ± 7.33 in men, and 60.82 ± 12.94 and 49.96 ± 11.05 in women, respectively. These data indicate that the distributions of NET and d-norgestrel between various serum binding proteins are determined largely by SHBG binding capacity, and that knowledge of the latter may be useful in assessments of the lowest effective dose of these two progestins in different individuals.