Impoant difference between leuprolide and ganirelix is that ganirelix

Superactive / long-acting GnRH agonists Many analogues of GnRH, e.g. Goserelin, Leuprolide,Nafarelin, Triptorelin, have longer action (t 1/2 2-6 hours) because of high affinity for GnRH receptor and resistance to enzymatic hydrolysis. Because physiological release of GnRH is in pulses, whereas these agonists act continuously; they only initially increase Gn secretion. After 1-2 weeks they cause desensitization and down regulation of GnRH receptors ->inhibition of FSH and LH secretion -> suppression of gonadal function. GnRH antagonists:- Some more extensively substituted GnRH analogues act as GnRH receptor antagonists. They inhibit Gn secretion without causing initial stimulation. The early GnRH antagonists(Leuprolide, gosereline etc.)had the limitation of producing reactions due to histamine release. Later agents like ganirelix and cetrorelix have low histamine releasing potential and are being clinically used. Ref :- kd tripathi; pg num:-242,243