CLINICAL PHARMACOLOGY
Emergency contraceptives are not effective if the woman is already pregnant. Plan B® is believed to act as an emergency contraceptive principally by preventing ovulation or fertilization (by altering tubal transport of sperm and/or ova). In addition, it may inhibit implantation (by altering the endometrium). It is not effective once the process of implantation has begun.
PHARMACOKINETICS
ABSORPTION
No specific investigation of the absolute bioavailability of Plan B® in humans has been conducted. However, literature indicates that levonorgestrel is rapidly and completely absorbed after oral administration (bioavailability about 100%) and is not subject to first pass metabolism.
After a single dose of Plan B® (0.75 mg) administered to 16 women under fasting conditions, maximum serum concentrations of levonorgestrel are 14.1 ± 7.7 ng/mL (mean ± SD) at an average of 1.6 ± 0.7 hours. No formal study of the effect of food on the absorption of levonorgestrel has been undertaken.
Table 1 Pharmacokinetic Parameter Values Following
Single Dose Administration of Plan B® (Levonorgestrel)
Tablets, 0.75 mg to Healthy Female Volunteers
| N |
Mean (± S.D.) |
| Cmax (ng/mL) |
Tmax (h) |
CL (L/h) |
Vd (L) |
T1/2 (h) |
AUC0-(infinity) (ng/mL/h) |
| 16 |
14.1± 7.7 |
1.6 ± 0.7 |
7.7 ± 2.7 |
260.0 |
24.4 ± 5.3 |
123.1 ± 50.1 |
|
DISTRIBUTION
Levonorgestrel in serum is primarily protein bound. Approximately 50% is bound to albumin and 47.5% is bound to sex hormone binding globulin (SHBG).
METABOLISM
Following a single oral dosage, levonorgestrel does not appear to be extensively metabolized by the liver. The primary metabolites are 3(alpha),5(beta)- and 3(alpha),5(infinity)-tetrahydrolevonorgestrel with 16(beta)-hydroxynorgestrel also identified. Together, these account for less than 10% of parent plasma levels. Urinary metabolites hydroxylated at the 2(alpha) and 16(beta) positions have also been identified. Small amounts of the metabolites are present in plasma as sulfate and glucuronide conjugates.
EXCRETION
The elimination half-life of levonorgestrel following single dose administration as Plan B® (0.75 mg) is 24.4 ± 5.3 hours. Excretion following single dose administration as emergency contraception is unknown, but based on chronic, low-dose contraceptive use, levonorgestrel and its metabolites are primarily excreted in the urine, with smaller amounts recovered in the feces.
SPECIAL POPULATIONS
GERIATRIC
This product is not intended for use in geriatric (age 65 years or older) populations and pharmacokinetic data are not available for this population.
PEDIATRIC
This product is not intended for use in pediatric (premenarcheal) populations, and pharmacokinetic data are not available for this population.
RACE
No formal studies have evaluated the effect of race. However, clinical trials demonstrated a higher pregnancy rate in the Chinese population with both Plan B® and the Yuzpe regimen (another form of emergency contraception consisting of two doses of ethinyl estradiol 0.1 mg + levonorgestrel 0.5 mg). The reason for this apparent increase in the pregnancy rate of emergency contraceptives in Chinese women is unknown.
HEPATIC INSUFFICIENCY AND RENAL INSUFFICIENCY
No formal studies have evaluated the effect of hepatic insufficiency or renal insufficiency on the disposition of emergency contraceptive tablets.
DRUG-DRUG INTERACTIONS
No formal studies of drug-drug interactions were conducted.
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