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 were 14.1 + 7.7 ng/mL (mean + SD) at an average of 1.6 + 0.7 hours.
Table 2: Pharmacokinetic Parameter Values Following Single Dose Administration of Plan B (Levonorgestrel) Tablets 0.75 mg to Healthy Female Volunteers under Fasting Conditions | Mean (± SD) |
| Cmax (ng/mL) | Tmax (h) | CL (L/h) | Vd (L) | t½ (h) | AUCinf (ng/mL.h) |
| Levonorgestrel | 14.1 (7.7) | 1.6 ( 0.7) | 7.7 (2.7) | 260.0 | 24.4 (5.3) | 123.1 (50.1) |
Cmax = maximum concentration Tmax = time to maximum concentration CL = clearance Vd = volume of distribution t1/2 = elimination half life AUCinf = area under the drug concentration curve from time 0 to infinity |
Effect of Food: The effect of food on the rate and the extent of levonorgestrel absorption following single oral administration of Plan B has not been evaluated.
Distribution
The apparent volume of distribution of levonorgestrel is reported to be approximately 1.8 L/kg. It is about 97.5 to 99% protein-bound, principally to sex hormone binding globulin (SHBG) and, to a lesser extent, serum albumin.
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 16βhydroxylevonorgestrel. 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.
Excretion
About 45% of levonorgestrel and its metabolites are excreted in the urine and about 32% are excreted in feces, mostly as glucuronide conjugates.
Specific Populations
Pediatric: This product is not intended for use in the pediatric (pre-menarcheal) population, and pharmacokinetic data are not available for this population.
Geriatric: This product is not intended for use in postmenopausal women and pharmacokinetic data are not available for this population.
Race: No formal studies have evaluated the effect of race on pharmacokinetics of Plan B. However, clinical trials demonstrated a higher pregnancy rate in Chinese women with both Plan B and the Yuzpe regimen (another form of emergency contraception). The reason for this apparent increase in the pregnancy rate with emergency contraceptives in Chinese women is unknown [see USE IN SPECIFIC POPULATIONS (8.6)].
Hepatic Impairment: No formal studies were conducted to evaluate the effect of hepatic disease on the disposition of Plan B.
Renal Impairment: No formal studies were conducted to evaluate the effect of renal disease on the disposition of Plan B.
Drug-Drug Interactions
No formal drug-drug interaction studies were conducted with Plan B [see DRUG INTERACTIONS (7)].