Pregnancy-terminating action of a luteinizing hormone-releasing hormone agonist d-Ser(But)6desGly10ProEA in baboons**This work was conducted cooperatively with the International Committee for Contraception Research of the Population Council, New York, and was supported by research grants from the Rockefeller Foundation, the International Development Research Centre of Canada, the Family Planning Foundation of India, and the Indian Council of Medical Research

Dose-related action of gonadotropin-releasing hormone on basal prostanoid production from the human term placenta

The dose-related effect of gonadotropin-releasing hormone on placental prostanoids was studied with a perifusion system. Villous tissues were perifused with medium 199 (1 ml/hr) and at the beginning of the fifth hour, either 0, 10(-10), 10(-9), 10(-8), 10(-7), or 10(-6) mol/L gonadotropin-releasing hormone was added to the medium of triplicate chambers. The concentration of prostaglandin E, prostaglandin F, 13,14-dihydro-15-keto-prostaglandin F2 alpha, 6-keto-prostaglandin F1 alpha, and thromboxane B2 in the effluent medium, collected every hour, was determined by specific radioimmunoassay. The cumulative release after gonadotropin-releasing hormone treatment for each chamber was calculated, and replicate chambers were averaged. Linear regression analysis of the average for each dose from three different placentas was used to determine the dose-response relationship. Gonadotropin-releasing hormone significantly inhibited the release of placental prostaglandin E, prostaglandin F, and thromboxane B2 in a dose-dependent fashion. Gonadotropin-releasing hormone had no significant effect on 13,14-dihydro-15-keto-prostaglandin F2 alpha and 6-keto-prostaglandin F1 alpha, although there was an apparent increase in 13,14-dihydro-15-keto-prostaglandin F2 alpha. These data support the hypothesis that chorionic gonadotropin-releasing hormone inhibits prostanoid production from the placenta, which in turn may regulate various functions of prostanoids during pregnancy.

The effect of LHRH analogs on pregnancy in the baboon

Thirty-three baboons were maintained throughout their peri-ovulatory period in group enclosures with fertile males. They received treatment on days 14-19 of presumed pregnancy with either twice daily injection of 50 micrograms of an LHRH agonist or of 2 mg of an LHRH antagonist. Three pregnancies from 14 cycles were maintained in each of the treatment groups as compared to 5 of 14 in controls. Four abortions were noted but as 1 occurred in a control group the response could not be attributed to treatment. The agonist had no apparent effect on urinary progestins in nonpregnant animals but both analogs did cause a transient decline in pregnant animals. Results indicated that LHRH analogs are not suitable agents for early pregnancy termination under the present treatment regimen.

Pregnancy interfering action of LHRH and anti-LHRH

Until recently LHRH was believed to be a product of the hypothalamic origin whose primary function was to regulate the secretion of gonadotropin from the pituitary. In the last few years, a large body of experimental evidence has emerged for the existence of the releasing hormone at extrahypothalamic sites. The placenta is one such organ in which the hormone is made and probably has a role in stimulation of the secretion of chorionic gonadotropin as suggested by both in vivo and in vitro experiments. Superagonists of LHRH as well as monoclonal antibodies raised against the decapeptide administered during early pregnancy bring about a sharp decrease in blood chorionic gonadotropin and progesterone levels followed by termination of pregnancy in baboons. The mechanism of the abortifacient action appears to be curtailment of chorionic gonadotropin secretion by the placenta. Whether the immunointerception through LHRH antibodies will hold true for human beings also has to be investigated.

Comparisons of the potential utility of LHRH agonists and antagonists for fertility control

Prospects for the use of LHRH analogs for human fertility control have been reviewed with particular reference to two highly potent representatives. Nafarelin acetate, the LHRH agonist, has a potency about 200 X that of LHRH and is consistently effective in suppressing gonadal function in females through a desensitization of LHRH receptors in the pituitary. Such agents show promise as ovulation inhibitors for women although concern has been expressed over the dangers of unopposed estrogen or alternatively hypoestrogenemia. Although early studies indicated luteolysis in women and interceptive action in baboons it is now clear that the LHRH agonists will not be useful clinically to terminate pregnancy. Wide species differences in the male response to LHRH agonists exist. Unfortunately azoospermia has not been achieved in men. The LHRH antagonists, typified by [N-Ac-D-Nal(2)1, D-pCl-Phe2, D-Trp3, D-hArg(Et2)6, D-Ala10]LHRH, require high doses to competitively inhibit responses to endogenous LHRH. Their advantages include a rapid induction of the hypogonadal state with apparently little species or sexual variation in response. Based on animal studies, preferable utility of the antagonists would lie in male contraception and pregnancy interception.

Effects of a gonadotropin-releasing hormone antagonist on hormonal levels in the pregnant baboon and on fetal outcome

In order to study the function of a gonadotropin-releasing hormone (GnRH) in pregnancy, a GnRH antagonist was administered to pregnant baboons (35 to 45 days' gestation) over 7 days. A marked suppression of progesterone, estrone, and estradiol was observed during treatment. In two animals, estrogens were still suppressed later in pregnancy (days 95 to 116), and delivery resulted in stillbirths. In one of these animals the placenta was recovered, and it was half normal size. In the third animal, the circulating steroids, including the estrogens, returned to normal levels following antagonist administration, and a small live-born fetus was delivered. These data indicate that interference with GnRH activity during early pregnancy can result in stillbirths, possibly due to induction of placental insufficiency. Thus, these studies support the hypothesis that chorionic GnRH has a physiologic function during pregnancy in the control of placental hormonogenesis and normal fetal outcome.

Luteinizing hormone-releasing hormone (LHRH) and its analogs for contraception in women: a review

In animals, LHRH agonists have multiple sites of action including the pituitary, the gonads, and the reproductive tract. In humans, the major antifertility action of this class of peptides is believed to be mediated via the pituitary. Studies in women have indicated that potent LHRH agonists can block ovulation when administered once daily. In the volunteers who have used these agents no serious side effects were observed, although some women experienced irregular bleeding or amenorrhea. It is anticipated that formal clinical trials could be conducted in the near future to determine the efficacy of continuous LHRH agonist administration. Early attempts to use an LHRH agonist to produce luteal insufficiency, luteolysis, or interruption of pregnancy have either been unsuccessful or the results are still too preliminary to ascertain whether these approaches warrant further trials. LHRH antagonists are believed to act by inhibiting the action of LHRH on the pituitary. Although some of these peptides are known to be active in women, very large doses have been required. Recently several investigators have produced LHRH antagonists with increased potency. In the near future, it should be possible to determine whether these peptides should be considered as potential contraceptives in men or in women.