Levonorgestrel concentrations during use of levonorgestrel rod (LNG ROD) implants

Area of Science:

• Contraception
• Pharmacokinetics
• Reproductive Health

Background:

• Levonorgestrel (LNG) implants are a common form of long-acting reversible contraception.
• Understanding drug release profiles and factors influencing them is crucial for contraceptive efficacy.
• Norplant-2 implants represent an earlier formulation compared to newer two-rod systems.

Purpose of the Study:

• To compare levonorgestrel concentrations between a new two-rod implant system and Norplant-2 implants over three years.
• To investigate the relationship between levonorgestrel concentrations, time, body weight, and ponderal index.
• To establish the minimum levonorgestrel concentration required for effective contraception.

Main Methods:

• A three-year randomized trial involving 398 women.
• Blood samples collected at multiple time points (1, 3, 6, 9, 12 months, then semiannually).
• Assay of levonorgestrel concentrations and statistical analysis correlating with time, body weight, and implant type.

Main Results:

• Levonorgestrel concentrations decreased over time and with increasing body weight and ponderal index for both implant types.
• Higher levonorgestrel concentrations were observed in the third year for the new LNG ROD implants compared to Norplant-2.
• No pregnancies were reported in either group throughout the three-year study period.

Conclusions:

• The new LNG ROD implant system demonstrates sustained contraceptive efficacy over three years.
• The minimum effective levonorgestrel concentration for pregnancy prevention is likely below 200 pg/ml, possibly below 175 pg/ml.
• Body weight and ponderal index significantly influence levonorgestrel pharmacokinetics, but efficacy remains high.