When administered to patients with prostatic cancer, GnRH antagonists act by direct inhibition of GnRH receptors and therefore LH secretion, leading to a faster onset of the action (hours instead of days) and avoiding the initial rise of testosterone levels induced by GnRH receptor agonists.
GnRH antagonists currently in clinical use are peptidomimetics obtained by extensive modification of the natural GnRH hormone. The main problems to be overcome were the tendency of the first compounds to induce the release of histamine as well as their low solubility and propension to form gels, which severely limits their formulation.44 The main modifications that have been explored are the following:
a. Replacement of the first three amino acids by d-amino acids with unnatural side chains. The most widely employed replacements are N-acetyl-d-(p-naphthyl)alanine for the first residue, d-(4-chloro)phenylalanine for the second, and d-(2-pyridyl)alanine or d-Trp for the third one.
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