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  • Title: Investigations of pharmacokinetics of levonorgestrel to specific consideration of a possible first-pass effect in women.
    Author: Hümpel M, Wendt H, Pommerenke G, Weiss C, Speck U.
    Journal: Contraception; 1978 Mar; 17(3):207-20. PubMed ID: 648145.
    Abstract:
    This investigation, using levonorgestrel, estimated the extent of a first-pass effect and established a pharmacokinetic model to describe drug concentrations after intravenous and oral administration. 3 women received 30 mcg of levonorgestrel in succession by intravenous and oral routes; another 6 were given 150 mcg (as Microgynon) in similar fashion. Radiolabeled levonorgestrel was used for intravenous administration of 30 mcg and oral administration for 150 mcg. Radioimmunoassay was used to determine plasma drug level after all treatments. Urine and feces eliminations were also recorded. After intravenous and oral administration, drug concentration and total plasma radioactivity declined in 2 disposition phases, with half-lives in the range of 1.5 hours/day. After intravenous administration, an early phase with a half-life of about 10 minutes was observed. Levonorgestrel was rapidly absorbed, with a half-life of about 20 minutes. Orally administered doses were completely absorbed. Intraindividual comparison showed that intravenous and oral administration of levonorgestrel is not subject to any first-pass effect.
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