GnRH Antagnonists (GnRHa)
The primary mechanism of action of GnRH antagonist analogues of is to compete with the natural GnRH hormone. GnRH antagonists bind to the GnRH receptor which causes a suppression from the very beginning of their administration, due to the occupation (and not a reduction in the numbers) of GnRH receptors. They thus result in the reduction of FSH and LH gonadotropin production. The suppression of the pituitary gland function is achieved in minimum time (almost straight after) following their administration.
The pharmaceutical production of GnRH antagonist analogues occurred after that of GnRH agonist analogues. After setting up the daily GnRH antagonist dosage, five multicenter phase III studies followed (2000-2001) in Europe, Middle East and USA, in order to extract conclusions for their effectiveness in clinical practice, the comparison of their results with GnRH agonists, etc. Since then, numerous original studies and meta-analyses have been published with regard to the experience of administering GnRH antagonists on a preselected day (typically, the 6th day of stimulation) and the so called flexible administration protocol.
When are they used
Antagonists are administered during the treatment with gonadotropins, in the current menstrual cycle. The main innovation of antagonists is that their administration, even for a few days prior to follicular rupture, causes an immediate suppression of pituitary function that results in the prevention of the LH surge.
GnRH antagonists are available with the following commercial names: Orgalutran (Ganirelix) and Cetrotide (Cetrorelix), in a pre-mixed dilution ready to be used by subcutaneous injection.