Design and characterisation of novel GnRH analogues conjugated to hapten carriers

GnRH analogues are extremely useful pharmacological agents, both in the investigation of the hypothalamic-pituitary axis and in the manipulation of gonadotropins for the treatment of reproductive cancers, polycystic ovarian syndrome, endometriosis and precocious puberty, to name but a few. Most GnRH analogues are based on the substitution of the decapeptide sequence of GnRH with unnatural amino acids to modify receptor binding affinity, receptor activation and to decrease proteolysis. Nevertheless these peptides tend to be rapidly cleared from the circulation and in most cases have to be administered by injection. The aim of this study was to modify GnRH agonists and antagonists to enhance half-life by conjugation to a steroid, thereby conferring plasma protein binding affinity and also to enhance oral absorption by further conjugation a carrier molecule to achieve oral absorption. [DLys6]GnRH analogue-progesterone conjugates were designed and synthesized and the products were analysed by HPLC and mass spectrometry. The most successful method of conjugation was with N,N'- dicyclohexylcarbodiimide (DCC) in the presence of hydroxybenzotriazole (HOBt). The pharmacological properties of five GnRH antagonist-21- ... (continues)