Source: Health Products Regulatory Authority (ZA) Revision Year: 2022 Publisher: Pfizer Laboratories (Pty) Ltd, 85 Bute Lane, Sandton 2196, South Africa Tel: +27(0)11 320 6000 / 0860 734 937 (Toll-free South Africa)
Category and class: A 18.8 Ovulation controlling agents
The hormonal components of ethinylestradiol and gestodene inhibit ovulation by suppressing gonadotropin release. Secondary mechanisms include changes in the cervical mucous (which increases the difficulty of sperm penetration) and changes in the endometrium (which reduce the likelihood of implantation).
The pharmacological and biochemical profile of gestodene is very similar to that of progesterone. Due to the high binding affinity and biological activity of gestodene, there is an effective inhibition of ovulation at an exceptionally low dose.
Ethinylestradiol and gestodene are rapidly and almost completely absorbed from the gastrointestinal tract.
Peak plasma levels of each medicine are reached within 1-2 hours. Post maximum concentration curves show two phases with half-lives of 1 and 15 hours in the case of gestodene, and 1-3 and approximately 24 hours in the case of ethinylestradiol.
Gestodene is extensively plasma protein bound to sex hormone binding globulin (SHBG). Ethinylestradiol is bound in plasma to albumin and enhances the binding capacity of SHBG.
After oral administration, gestodene, unlike ethinylestradiol is not subject to first-pass metabolism. Following oral administration, gestodene is completely bioavailable, ethinylestradiol about 40%.
The elimination half-life for ethinylestradiol is approximately 25 hours. It is primarily metabolised by aromatic hydroxylation but a wide variety of hydroxylated and methylated metabolites are formed, and these are present both free and as conjugates with glucuronide and sulfate.
Conjugated ethinylestradiol is excreted in bile and is subject to enterohepatic recirculation. About 40% of the medicine is excreted in the urine and 60% is eliminated in the faeces.
The elimination half-life for gestodene is approximately 16-18 hours after multiple oral doses. The medicine is primarily metabolised by reduction of the A ring, followed by glucuronidation. About 50% of gestodene is excreted in the urine and 33% is eliminated in the faeces.
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