Source: Health Products Regulatory Authority (ZA) Revision Year: 2022 Publisher: Litha Pharma (Pty) Ltd, 106 16th Road, Midrand, 1686
Pharmacological classification: A 21.8.2 2 Progesterones with or without oestrogen
The precise mode of action of levonorgestrel is not known. Emergency hormonal contraception is thought to work mainly by preventing fertilisation by altering tubal transport of sperm and/or ova. It may also cause endometrial changes that discourage implantation.
It has been estimated that levonorgestrel prevents 84% of expected pregnancies. At the recommended regimen, levonorgestrel is not expected to induce significant modification of blood clotting factors, and lipid and carbohydrate metabolism.
Orally administered levonorgestrel is rapidly and almost completely absorbed.
The results of a pharmacokinetic study carried out with 16 healthy women showed that following ingestion of one tablet of levonorgestrel, maximum drug serum level of 18,5 ng/ml was found at 2 hours. The mean value of area under the levonorgestrel concentration-time curve (0-infinity) was calculated as 310,2 ng.h/ml. After reaching maximum serum levels, concentration of levonorgestrel decreased with mean elimination half-life of about 26 hours.
Levonorgestrel is not excreted in unchanged form but as metabolites. Levonorgestrel metabolites are excreted in about equal proportions with urine and faeces. The biotransformation follows the known pathways of steroid metabolism; the levonorgestrel is hydroxylated in the liver and the metabolites are excreted as glucuronide conjugates. No pharmacologically active metabolites are known.
Levonorgestrel is bound to serum albumin and sex hormone binding globulin (SHBG). Only about 1,5% of the total serum levels are presented as free steroid, but 65% are specifically bound to SHBG.
Following ingestion of one tablet of levonorgestrel the mean value of SHBG was about 40 nmol/l. Serum levels of SHBG tend to stay at these levels (or slightly increased) up to 24 hours, and then gradually decrease to reach levels of approximately 30 nmol/L at 192 hours.
The absolute bioavailability of levonorgestrel was determined to be almost 100% of the dose administered.
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