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Mechanism of Action

The contraceptive effect of IMPLANON is achieved by suppression of ovulation, increased viscosity of the cervical mucus, and alterations in the endometrium.

Pharmacodynamics

Exposure-response relationships of IMPLANON are unknown.

Pharmacokinetics

Absorption

After subdermal insertion of the etonogestrel implant, etonogestrel is released into the circulation and is approximately 100% bioavailable.

The mean peak serum concentrations in 3 pharmacokinetic studies ranged between 781 and 894 pg/mL and were reached within the first few weeks after insertion. The mean serum etonogestrel concentration decreases gradually over time declining to 192 to 261 pg/mL at 12 months (n=41), 154 to 194 pg/mL at 24 months (n=35), and 156 to 177 pg/mL at 36 months (n=17).

The pharmacokinetic profile of IMPLANON from 1 of 3 pharmacokinetic studies is shown in Figure 24.

Figure 24 : Mean Serum Concentration-time Profile of Etonogestrel During 2 Years of IMPLANON Use and After Removal in 20 Healthy Women

Distribution

The apparent volume of distribution averages about 201 L. Etonogestrel is approximately 32% bound to sex hormone binding globulin (SHBG) and 66% bound to albumin in blood.

Metabolism

In vitro data shows that etonogestrel is metabolized in liver microsomes by the cytochrome P450 3A4 isoenzyme. The biological activity of etonogestrel metabolites is unknown.

Excretion

The elimination half-life of etonogestrel is approximately 25 hours. Excretion of etonogestrel and its metabolites, either as free steroid or as conjugates, is mainly in urine and to a lesser extent in feces. After removal of the implant, etonogestrel concentrations decreased below sensitivity of the assay by 1 week.

Clinical Studies

Pregnancy

In clinical trials of up to 3 years duration that involved 923 subjects, 18 - 40 years of age at entry, and 1,756 women-years of IMPLANON use, the total exposures expressed as 28-day cycle equivalents by study year were:

Year 1: 10,866 cycles
Year 2: 8,581 cycles
Year 3: 3,442 cycles

The clinical trials excluded women who:

• Weighed more than 130% of their ideal body weight
• Were chronically taking medications that induce liver enzymes

In the subgroup of women 18 to 35 years of age at entry, 6 pregnancies during 20,648 cycles of use were reported. Two pregnancies occurred in each of Years 1, 2 and 3. Each conception was likely to have occurred shortly before or within 2 weeks after IMPLANON removal. With these 6 pregnancies, the cumulative Pearl Index was 0.38 pregnancies per 100 women-years of use.

Return to Ovulation

In clinical trials with IMPLANON, the etonogestrel levels in blood decreased below sensitivity of the assay by one week after removal of the implant. In addition, pregnancies were observed to occur as early as 7 to 14 days after removal. Therefore, a woman should re-start contraception immediately after removal of the implant if continued contraceptive protection is desired.

Last reviewed on RxList: 2/28/2012
This monograph has been modified to include the generic and brand name in many instances.