Gonadotropin-releasing hormone antagonist

A gonadotropin-releasing hormone antagonist (GnRH antagonist) is a synthetic peptide that competes with the neurohormone GnRH for its receptor, thus decreasing or blocking GnRH action. As a result endogenous pituitary output of FSH and LH is shut down. GnRH antagonists are also derivatives of the natural GnRH decapeptide with multiple amino acid substitutions. These substitutions modify the agent so that it blocks the receptor and decreases FSH and LH secretions within hours. In contrast to GnRH agonists, antagonists have no flare effect, thus their therapeutic effect is immediately apparent. However, there action is short-lived and daily injections are necessary to maintain their effect. Typically endogenous FSH and LH activity returns about 40 hours after cessation of GnRH antagonist administration, although with a higher dose the return to normal pituitary function will be postponed for longer. Unlike the GnRH agonists, long acting or depot forms of the agent are not currently available, thus GnRH antagonists are not used in the long term therapy of patients with cancer where hormone levels need to be kept low for a long time. As of 2006, Histrelin (as Supprelin-LA) is awaiting approval for use as a 12-month depot injection.
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