The relative bioavailability of levonorgestrel and ethinyl estradiol administered as a low-dose combination oral contraceptive

All progestins are not created equal

A variety of progestins are available for therapeutic use. It is convenient to classify them into those related in chemical structure to progesterone or testosterone. Progestins related to progesterone can be subdivided into pregnanes and 19-norpregnanes, whereas those related to testosterone can be subdivided into those with and without a 17-ethinyl group. 17-Ethinylated progestins consist of the families of norethindrone (estranes) and levonorgestrel (13-ethylgonanes). Progestins administered orally undergo extensive hepatic first pass metabolism primarily by reduction and conjugation, and in most instances, relatively high progestin doses are required for therapeutic use. There are limited reliable data on the pharmacokinetics of most progestins. Some progestins are prodrugs, requiring transformation prior to exhibiting progestational activity. Qualitative and quantitative tests utilizing either human or animal species have been used to establish progestin potency. However, profound differences in progestational activity are often observed between human and animal tissues. Also, there is a misconception about androgenicity of progestins due largely to extrapolation of data from rat studies to the human. Progestins differ widely in their chemical structures, structure-function relationships, metabolism, pharmacokinetics, and potencies; they are not created equal.

Bioavailability of orally administered sex steroids used in oral contraception and hormone replacement therapy

The concept of bioavailability is discussed with particular references to the sex steroids. Problems encountered in the measurement of bioavailability of these steroids and the various factors that may affect their bioavailability are briefly described. Information regarding the bioavailability of the estrogens and gestogens, some of which are prodrugs, used in oral contraception and hormone replacement therapy is summarized and the implications regarding the clinical use of these steroids are discussed.

Pharmacokinetics of gestagens: some problems

Various approaches to studying the pharmacokinetics of gestagens and the factors that influence derivation of the parameters are described with levonorgestrel used as an example. Published studies of the pharmacokinetics of levonorgestrel are reviewed, and new information is presented regarding intra- and intersubject variation. Differences between various formulations of levonorgestrel are apparent when the formulations are compared in the same subjects. There is also a marked difference in the parameters when derived under single-dose or steady-state conditions. The role of sex hormone-binding globulin in the metabolism of levonorgestrel is questioned. Large intra- and inter-subject variations in the parameters exist, and a subject may show a large month-to-month variation when one levonorgestrel formulation is used and smaller variations when another formulation is used. This wide variability in the pharmacokinetic parameters, problems that arise in the derivation and interpretation of the parameters, the biologic significance of most of these parameters, and their lack of correlation with pharmacodynamic responses severely limit the usefulness of pharmacokinetic studies of the gestagens.

Potency and pharmacokinetics of gestagens

Serum concentrations of gestagens were compared after single doses and after multiple doses (steady-state conditions) of four widely used oral contraceptives containing norethisterone (NET), levonorgestrel (LNG), desogestrel (DSG) and gestodene (GSD). There were marked differences among the gestagens with respect to the serum concentrations. Under steady-state conditions 12 h after dosing, the relative concentrations were approximately GSD, 4.5:DSG, 1:LNG, 1:NET, 2 compared to ratios of 1:2:2:13.3, respectively, for dose. Thus, there was no correlation between serum concentration and dose. These differences in serum concentrations are determined by the different pharmacokinetic behaviour of the gestagens, which in turn is largely determined by their binding to serum proteins. Calculations suggest that the concentrations of unbound gestagen in serum, and hence probably also at the target organ, are similar (about 35 pg/ml) for LNG, DSG and GSD but may be higher (up to 60 pg/ml) for NET whose half-life of elimination is about half that of the other three gestagens. Measurement of the serum total concentration is unlikely to correlate with their pharmacological activity. A further complication is the multiplicity of pharmacological effects elicited by the gestagens and each of these effects is likely to have its own dose-response relationship.

Pharmacokinetic comparison of two triphasic oral contraceptive formulations containing levonorgestrel and ethinylestradiol

The pharmacokinetics of levonorgestrel (LNG) and ethinylestradiol (EE2) were determined in 24 women (aged 21 to 35 years), following the administration of a single tablet from the second phase of two different triphasic preparations (Triphasil and Trinordiol. Each tablet contained 0.075 mg of LNG and 0.040 mg of EE2. The data were compared to the pharmacokinetics of LNG and EE2 obtained following the oral administration of a hydroalcoholic solution (standard) containing the same steroids and dose. The study consisted of a randomized design in which the three formulations were administered to each of the 24 subjects in a three-period crossover pattern. Blood samples were taken at frequent intervals after dosing. Serum levels of LNG and EE2 were measured by specific radioimmunoassays. The results show that both LNG and EE2 in the Triphasil and Trinordiol tablets are bioequivalent with respect to rate and extent of absorption. Furthermore, LNG, but not EE2, in both tablet formulations was bioequivalent to the solution dose. The serum concentration-time profiles for the three formulations showed that the range of mean peak levels was 2.3-2.8 ng/ml for LNG and 116-159 pg/ml for EE2. These levels were achieved within 2 hours in the majority of subjects. The ranges of mean values calculated for the areas under the curves were 15-16 ng.hr/ml for LNG and 1053-1390 pg.hr/ml for EE2. The ranges of mean values calculated for other pharmacokinetic parameters were: volume of distribution: LNG--1.6-1.8 L/kg, EE2--7.7-9.1 L/kg; clearance: LNG--84-88 ml/hr/kg, EE2--0.67-0.99 ml/hr/kg; half-life: LNG--13-15 hr, EE2--7-12 hrs.

Pharmacokinetics of three bioequivalent norethindrone/mestranol-50 micrograms and three norethindrone/ethinyl estradiol-35 micrograms OC formulations: are "low-dose" pills really lower?

We have examined the pharmacokinetic parameters derived from the analysis of plasma ethinyl estradiol (EE) and norethindrone levels after administration of a single dose of three bioequivalent norethindrone-1mg/mestranol (ME)-50 micrograms formulations (Ortho-NovumR 1/50, NorinylR 1/50 and Norcept-MR 1/50) and three norethindrone-1mg/ethinyl estradiol-35 micrograms formulations (Ortho-Novum 1/35R, NorinylR 1/35, Norcept-ER 1/35) in a randomized crossover design involving 24 women for the 35 micrograms and 27 women for the 50 micrograms agents. Differences between the AUC-EE of pairs from the same manufacturer (1 + 35 and 1 + 50) were not significantly different, indicating that 50 micrograms of mestranol was equivalent to 35 micrograms ethinyl estradiol with respect to this pharmacokinetic parameter. The Cmax values were also similar. Inter-individual coefficients of variation (C.V.) for the AUC-EE were 47% and 57% for the 1 + 35 and 1 + 50 agents, respectively. Intra-individual C.V.s were 41% and 42%, respectively. For norethindrone, the AUC was larger with the 1 + 50 formulations than with the 1 + 35 group (87.9 vs. 72.8 pg hr/ml). Additionally, the Cmax values were larger for the 1/50 group (17.7 vs. 14.0). Since the amount of norethindrone in the two dosage groups was the same, this difference in the pharmacokinetics between the 35 micrograms EE and the 50 micrograms ME formulations remains unexplained. The inter-individual C.V. averaged 56% for both dosage groups. The intra-individual C.V.s were 17% and 46% for the 1 + 35 and 1 + 50 groups, respectively. The large variation in blood levels of ethinyl estradiol and norethindrone between and within individuals may overshadow clinical differences attributable to differences in dosage.