Evaluation of luteinizing hormone-releasing hormone antagonistic activity in vitro.

Abstract

Antagonistic analogues of luteinizing hormone-releasing hormone (LHRH) belong to a class of compounds that can be utilized for treatment of some hormonedependent cancers and gynecologic disorders. Recently, we synthesized and tested a large number of LHRH analogues for LHRH antagonistic activity in the dispersed pituitary cell superfusion system. This fast, reliable, and dynamic system made it possible for us not only to evaluate the relative amounts of an analogue required for suppression of the LH-releasing activity of exogenous LEIRH but also provided quantitative data on dynamic interactions between the LHRH analogue, LHRH receptors, and LH secretion. Three experimental paradigms were used: (i) LHRH responses after preincubation with the antagonist, (ii) pulsatile, simultaneous infusion of LHRH and the antagonistic analogue, and (iW) effects of the analogues on ongoing, continuous LH secretion induced by prolonged stimulation with LHRH. From the data obtained, we conclude that (') the suppression of the LHRH-induced LH release was more effective and longer lasting when the cells were preincubated with the antagonistic analogues before the LBRH stimulation than in the case of simultaneous exposure; (ui) not only the potency but also the time of onset and the duration of the LH release-suppressing activity varied according to the different peptides used, resulting in different shapes of response curves; and (ii) from the accurate data obtained in this dynamic system, quantitative parameters of the in vivo interactions between the antagonists and LBRH on the LHRH receptor can be calculated.In the past few years, a large number of antagonists of luteinizing hormone-releasing hormone (LHRH) has been synthesized and evaluated for therapeutic use (1-6). These analogues may have various practical applications, including treatment of gynecologic disorders and hormone-sensitive tumors such as prostate and breast cancers (1, 2, 7-9). Although chronic administration of LHRH agonists is necessary for inhibition of LH, a single dose of a potent LHRH antagonist is sufficient to evoke the same effect (1, 2, 5-13).

Antiovulatory Doses of Antagonists of LH‐RH Inhibit LH and Progesterone but not FSH and Estradiol Release

“…Since only the LH-RH and the LH-RH antagonist were present in the in vitro system without any other ovarian or hypothalamic hormones stimulating the FSH release in vivo, the much weaker FSH-releasing potency of the LH-RH could be antagonized more easily than the LH-releasing effect of it. Our observation, that the in vitro potency of the LH-RH antagonists is dependent on the mode of administration (after or without preincubation with the antagonist before LH-RH exposure), confirms previous data obtained with other potent LH-RH antagonists (21,34,35) and can be explained by the much higher association-than dissociation rate of the analogues to the binding sites of the receptors (36).…”

Comparative response of rams and bulls to long-term treatment with gonadotropin-releasing hormone analogs

“…Hence, under the influence of agonist, even large doses of GnRH cannot induce gonadotropin secretion. In contrast, it appears that smaller doses of GnRH antagonist do not affect GnRH receptor population or signal transduction pathways in gonadotrope cells to the extent of the larger doses (Conn and Crowley, 1991;Csernus and Schally, 1992); therefore, amounts of GnRH sufficient to displace antagonist from GnRH receptors can override the antagonistic effect and induce the pituitary to respond (Caraty et al, 1990;Weinbauer et al, 1992;Li et al, 1994). Relatively large doses of cetrorelix in rats, however, can induce downregulation of GnRH receptors in a similar way as the treatment with GnRH agonists (Srkalovic et al, 1990;Pinski et al, 1996).…”