Absorption
No specific investigation of the absolute bioavailability of levonorgestrel and ethinyl estradiol tablets, and ethinyl estradiol tablets in humans has been conducted. However, literature indicates that levonorgestrel is rapidly and completely absorbed after oral administration (bioavailability nearly 100%) and is not subject to first-pass metabolism. Ethinyl estradiol is rapidly and almost completely absorbed from the gastrointestinal tract but, due to first-pass metabolism in gut mucosa and liver, the systemic bioavailability of ethinyl estradiol is approximately 43%.
The mean plasma pharmacokinetic parameters of levonorgestrel and ethinyl estradiol tablets, and ethinyl estradiol tablets following a single oral dose of three levonorgestrel and ethinyl estradiol combination tablets in normal healthy women under fasting conditions are reported in Table 3.
Table 3: Mean (SD) Pharmacokinetic Parameters Following a Single Dose Administration of Three Tablets of Levonorgestrel and Ethinyl Estradiol and Ethinyl Estradiol in 30 Healthy Women under Fasting Conditions | AUC0-∞ | Cmax | Tmax | T½ |
Levonorgestrel | 76.5 ± 24.9 ng*hr/mL | 6 ± 1.6 ng/mL | 1.6 ± 0.6 hours | 28.5 ± 8.7 hours |
Ethinyl estradiol | 1335.8 ± 365.3 pg*hr/mL | 122.8 ± 39.5 pg/mL | 1.8 ± 0.7 hours | 17.5 ± 7.4 hours |
AUC0-∞ = area under the drug concentration curve from time 0 to infinity
Cmax = maximum concentration
Tmax = time to maximum concentration
The effect of food on the rate and the extent of levonorgestrel and ethinyl estradiol absorption following oral administration of levonorgestrel and ethinyl estradiol tablets, and ethinyl estradiol tablets has not been evaluated.
Distribution
The apparent volume of distribution of levonorgestrel and ethinyl estradiol is reported to be approximately 1.8 L/kg and 4.3 L/kg, respectively. Levonorgestrel is about 97.5 to 99% protein-bound, principally to sex hormone binding globulin (SHBG) and, to a lesser extent, serum albumin. Ethinyl estradiol is about 95 to 97% bound to serum albumin. Ethinyl estradiol does not bind to SHBG, but induces SHBG synthesis, which leads to decreased levonorgestrel clearance. Following repeated daily dosing of combination levonorgestrel and ethinyl estradiol OCs, levonorgestrel plasma concentrations accumulate more than predicted based on single-dose pharmacokinetics, due in part, to increased SHBG levels that are induced by ethinyl estradiol, and a possible reduction in hepatic metabolic capacity.
Metabolism
Following absorption, levonorgestrel is conjugated at the 17β-OH position to form sulfate conjugates and, to a lesser extent, glucuronide conjugates in plasma. Significant amounts of conjugated and unconjugated 3α, 5β-tetrahydrolevonorgestrel are also present in plasma, along with much smaller amounts of 3α, 5α-tetrahydrolevonorgestrel and 16β-hydroxylevonorgestrel. Levonorgestrel and its phase I metabolites are excreted primarily as glucuronide conjugates. Metabolic clearance rates may differ among individuals by several-fold, and this may account in part for the wide variation observed in levonorgestrel concentrations among users.
First-pass metabolism of ethinyl estradiol involves formation of ethinyl estradiol-3-sulfate in the gut wall, followed by 2-hydroxylation of a portion of the remaining untransformed ethinyl estradiol by hepatic cytochrome P-450 3A4 (CYP3A4). Levels of CYP3A4 vary widely among individuals and can explain the variation in rates of ethinyl estradiol hydroxylation. Hydroxylation at the 4-, 6-, and 16- positions may also occur, although to a much lesser extent than 2-hydroxylation. The various hydroxylated metabolites are subject to further methylation and/or conjugation.
Excretion
About 45% of levonorgestrel and its metabolites are excreted in the urine and about 32% are excreted in feces, mostly as glucuronide conjugates. Ethinyl estradiol is excreted in the urine and feces as glucuronide and sulfate conjugates, and then undergoes enterohepatic recirculation.
Race
The effect of race on the pharmacokinetics of levonorgestrel and ethinyl estradiol tablets, and ethinyl estradiol tablets has not been evaluated.
Renal and Hepatic Impairment
No formal studies were conducted to evaluate the effect of hepatic or renal disease on the disposition of levonorgestrel and ethinyl estradiol tablets, and ethinyl estradiol tablets. However, steroid hormones may be poorly metabolized in patients with impaired liver function.