Antiovulatory Activity Research Articles

Antagonists of luteinizing hormone releasing hormone with novel unnatural amino acids at position six

Five new antagonists of luteinizing hormone releasing hormone (LHRH) containing novel unnatural amino acids at position six are reported. They are very effective in the rat antiovulatory assay. Using saline as vehicle, antagonist-[N-Ac-D-2-Nal1, D-4-Cl-Phe2, D-3-Pal3, Arg5, D-A26, D-Ala10]-LHRH inhibited ovulation completely at 1 micrograms/rat and three of the other antagonists showed some antiovulatory activity at 0.5 micrograms/rat.

The Next Generation of Oral Contraceptives in Clinical Practice: Chairman's Summary

Oral contraceptives with low-dose formulations have mitigated cardiovascular (CV) disease risk. High-density lipoprotein (HDL) had cardioprotective properties whereas low-density lipoprotein (LDL) is positively correlated with high risk. Half of the women have a lipid problem by the age of 30. The benefits of early lipid modification will accrue later in life because of reduced CV morbidity and mortality. The evolution of OCs improved the benefit-to-risk ratio. The risk of venous thromboembolic events has dropped by lowering the dose of estrogen to 30 or 35 mcg. Beta 1 and 2 agonists to H2 receptor blockers are a promising solution to the problem of OC-related CV risk. Norgestimate a selective new progestin has strong progestational potency and antiovulatory activity. 250 mcg norgestimate (NGM) was combined with 35 mcg ethinyl estradiol (EE) in a new formulation NGM/EE and compared to a norgestrel (NG) containing compound Lo/Ovral (NG/EE). The effect of NGM/EE on ovulation suppression was equivalent to NG/EE. The LDL/HDL ratio changed as a result of the increase of HDL in NGM/EE patients and the rise of LDL in NG/EE patients. NGM/EE positively affected lipid parameters increasing HDL levels with a concomitant improvement of the LDL/HDL ratio. Obstetricians and gynecologists have the duty of assessing cardiovascular risk for the sake of the long-term health of women. Effects of lipid-lowering OCs on cardiovascular morbidity and morality are still a long way off from exact quantification. Thus the progestational selectivity of norgestimate will be a valuable alternative by obviating problems associated with oral contraception.

Comparative studies on the hypotensive effect of LHRH antagonists in anesthetized rats

Certain neuropeptides are known to cause a hypotensive response, thought to be due to mast cell degranulation. The effects of five antagonists of luteinizing hormone-releasing hormone on blood pressure and heart rate were compared in the anesthetized rat. When given intravenously, all five compounds induced hypotensive and bradycardiac effects. The order of potency for these effects was Nal-Arg Antagonist ≈ detirelix ([N-Ac-D-Nal (2) 1, D-pCl-Phe 2, D-Trp 3, D-hArg(Et 2) 6, D-Ala 10]LHRH) > [N-Ac-D-Nal(2) 1, D-pCl-Phe 2, D-Pal(3) 3, D-hArg(Et 2) 6, L-hArg (Et 2) 8, D-Ala 10]LHRH (RS-26306) ≈ antide > [N-Ac-D-Nal(2) 1, D-pCl-Phe 2, D-Pal(3) 3,6, L-hArg(Et 2) 8, D-Ala 10]LHRH (RS-15378) and did not parallel the order of antiovulatory potencies of these compounds. The hypotensive activity of LHRH antagonists, therefore, appeared dissociable from their antiovulatory activity. RS-26306 and RS-15378 appeared to have the greatest therapeutic ratios.

Antide and related antagonists of luteinizing hormone release with long action and oral activity.

Antide is the decapeptide N-Ac-D-Nal(2),D-Phe(pCl),D-Pal(3),Ser,Lys(Nic),D-Lys(Nic),Leu,Lys(iPr),P ro,D- Ala-NH2 [Nal(2) represents 3-(2-naphthyl)alanine; Phe(p-Cl) represents 3-(4-chlorophenyl)alanine; Pal(3) represents 3-(3-pyridyl)alanine; Lys(Nic) represents N epsilon-nicotinoyllysine; Lys-(iPr) represents N epsilon-isopropyllysine], which is an antagonist of luteinizing hormone-releasing hormone (LHRH), which has high antiovulatory activity, releases negligible histamine, and is scheduled for scale-up, safety testing, and evaluation in the experimental primate and in clinical medicine. Thirty-five more peptides were synthesized, designed on antide with variations in positions 5-8. Of these, N-Ac-D-Nal(2),D-Phe(pCl),D-Pal(3),Ser,Lys(Pic),cis-D- Ala(PzAC),Leu,Lys(iPr),Pro,D-Ala-NH2 [Lys(Pic) represents N epsilon-picoloyllysine; Ala(PzAC) represents 3-(4-pyrazinylcarbonylaminocyclohexyl)alanine] was not only the most potent but also had higher antiovulatory activity than antide--i.e., 73% per 0.25 microgram and 100% per 0.5 microgram vs. 36% per 0.5 microgram and 100% per 1.0 microgram. Antide showed significant (P less than 0.001) duration of action when injected at a dose of 10 micrograms 44 hr before injection of 50 ng of the agonist [D-Qal(3)6]LHRH [Qal(3) represents 3-(3-quinolyl)alanine]. Antide showed oral antiovulatory activity at 600 micrograms (73%) and at 1200 micrograms (100%) with negligible difference between water and corn oil oral formulations.

Effect of the CH2NH and CH2NAc peptide bond isosteres on the antagonistic and histamine releasing activities of a luteinizing hormone-releasing hormone analog

The effect of the CH2NH and CH2NAc peptide bond isosteres on the antagonistic and histamine releasing activities of the LH-RH antagonist [N-Ac-D-Nal1,D-Phe2,3,D-Arg6,Phe7,D-Ala10] LH-RH was investigated. The moieties were introduced by facile, racemization-free solid-phase synthesis in an attempt to reduce the histamine releasing activity inherent to the most potent analogues while retaining high antiovulatory activity. The psi [CH2NH] isostere was incorporated at each CONH site with the exception of 8-9, which involves Pro, by reductive alkylation with a protected amino acid aldehyde in the presence of NaBH3CN during conventional solid-phase peptide synthesis. The psi [CH2NH] group was extremely resistant to derivatization and could only be partially acetylated to give psi [CH2NAc]. The analogues were cleaved from the resin with simultaneous deprotection by anhydrous hydrogen fluoride and purified to homogeneity in two stages: gel permeation followed by preparative reversed-phase liquid chromatography. The analogues were assayed in standard rat antiovulatory and in vitro histamine release assays. The isosteres caused a loss of the antiovulatory activity of the antagonist at the 50-microgram dose when incorporated at the positions 1-2, 2-3, 3-4, and 7-8. Incorporation at the other positions resulted in a less marked reduction in activity relative to the unmodified parent analogue. No significant effect was noted on the potent histamine releasing activity of the analogues.

Acitretin (RO10-1670) and oral contraceptives: interaction study.

Acitretin (RO 10-1670) the main metabolite of ethylester etretinate is used in the treatment of hyperkeratoses. Since it is teratogenic it is always used in association with oral contraceptives in women of childbearing age. This study was done in order to determine whether or not acitretin interacts with and reduces the efficacy of oral contraceptives. 10 patients were given acitretin (25-40 mg/day) for at least 2 cycles in combination with 1 of 3 contraceptive regimens. 3 patients were given a normal dose combined oral contraceptive (levonorgestrel .50 mg-ethinyl estradiol .05 mg); 6 patients were given low-dose combined oral contraceptives (either levonorgestrel .15/.20 mg-ethinyl estradiol .03/.04 mg or levonorgestrel .15 mg-ethinyl estradiol .03 mg); and 1 patient was given a very low dose progestin (levonorgestrel .03 mg). By the end of the study a total of 52 cycles had been controlled for plasma progesterone levels. Only in the patient taking the very low-dose progestin (levonorgestrel .03 mg) was there a significant increase in progesterone level. It is concluded that acitretin does not interfere with antiovulatory activity of combined oral contraceptives.

Anaphylactoid and anti-ovulatory activities of LHRH antagonists in rats

Studies were conducted with LHRH antagonists examining the relationship of structure to anaphylactoid-like activity and the relationship of anaphylactoid-like activity to anti-ovulatory activity in rats. Substitution of basic amino acids appeared to enhance the anaphylactoid-like activity of these peptides but other complex structural characteristics may also be involved. Anaphylactoid and anti-ovulatory activities were clearly independent and potent LHRH antagonists with minimal anaphylactoid-like activity were identified.

Effect of reductive alkylation of lysine in positions 6 and/or 8 on the histamine-releasing activity of luteinizing hormone-releasing hormone antagonists

The solid-phase reductive alkylation of the Lys side chain in positions 6 (D) and 8 (L) and position 8 alone of the LH-RH antagonist [N-Ac-D-Nal,D-Ph2,3,D-Arg6,Phe7,D-Ala10]LH-RH was investigated in an attempt to reduce the histamine-releasing activity inherent to most potent antagonists while retaining high antiovulatory activity. The protected parent analogues were prepared by conventional solid-phase peptide synthesis. After selective removal of the Lys Fmoc side chain protection, the resin-bound peptides were readily and conveniently alkylated at the epsilon amino groups with various aldehydes and ketones in the presence of NaBH3CN. The analogues were then cleaved from the resin with simultaneous deprotection by anhydrous hydrogen fluoride and purified to homogeneity in two stages: gel permeation followed by preparative reversed-phase liquid chromatography. The analogues were assayed in standard rat antiovulatory and in vitro histamine-release assays.

Design, synthesis and bioassays of antagonists of LHRH which have high antiovulatory activity and release negligible histamine

Potent antagonists of the luteinizing hormone releasing hormone have been achieved which release negligible histamine. [N-Ac-D-2-Nal 1, D-pClPhe 2, D-3-Pal 3, NicLys 5, D-NicLys 6, ILys 8, D-Ala 10]-LHRH showed 100%AOA/1μg and 36%/0.5μg; ED 50 >300. [N-Ac-D-2-Nal 1, D-pClPhe 2, D-3-Pal 3, PicLys 5, D-PicLys 6, ILys 8, D-Ala 10]-LHRH showed 100% AOA/0.5 μg and 40%/0.25 μg; ED 50, 93±11, and is the most potent of 52 new peptides. These antagonists feature designs with weakly basic acylated D-Lys 6, notably D-NicLys 6 and D-PicLys 6 and alkylated Lys 8 or Orn 8, e.g., ILys 8 and IOrn 8, and NicLys 5 and PicLys 5. Concepts included balanced overall basicity, superiority of ILys 8 and IOrn 8 which are sequence dependent and sensitivity of positions 5 and 6 for potency.

Effect of reductive alkylation of D-lysine in position 6 on the histamine-releasing activity of luteinizing hormone-releasing hormone antagonists

The reductive alkylation of the D-Lys side chain in position 6 of the LH-RH antagonist [N-Ac-D-Nal1,D-Phe2,3,D-Arg6,Phe7,D-Ala10]LH-RH was investigated in an attempt to reduce the histamine-releasing activity inherent to most potent antagonists while retaining high antiovulatory activity. The protected parent analogue was prepared by conventional solid-phase peptide synthesis. After selective removal of the Lys Fmoc side-chain protection, the resin-bound peptide was readily and conveniently alkylated at the epsilon amino group with various aldehydes and ketones in the presence of NaCNBH3. The analogues were then cleaved from the resin with simultaneous deprotection by anbydrous hydrogen fluoride and purified to homogeneity in two stages: gel permeation followed by preparative reversed-phase liquid chromatography. The analogues were assayed in standard rat antiovulatory and in vitro histamine-release assays. Simple alkyl groups such as ethyl, isopropyl, neopentyl, and cyclohexyl caused little reduction in histamine-releasing activity while exhibiting antiovulatory activity similar to that of the parent peptide. The presence of benzyl and substituted benzyl groups resulted in substantial losses of both histamine-releasing and antiovulatory activities. Thus, results showed that alterations in the hydrophobicity and size of the position-6 side chain have little effect on histamine-releasing activity or antiovulatory activity as long as a high degree of basicity is retained.

Relative Potencies of Antagonists of the Luteinizing Hormone Releasing Hormone with Lys8 and Arg8 and Substitutions in Positions 3 ,5 ,6 ,7 and 8

Antagonists of the luteinizing hormone releasing hormone (LHRH) of increased potency is a goal for control of ovulation. In the design and synthesis of 26 decapeptides, emphasis was given to analogs with Lys8 and Arg8 and with various substitutions in positions 3, 5, 6, 7 and 8. Two antagonists, [N-Ac-D-2-Nal1,D-pClPhe2,D-3-Pal3,Ser4,Tyr5,D-Ar g6,Leu7,Lys8, Pro9,D-Ala10]-NH2 and [N-Ac-D-2-Nal1,D-pClPhe2,D-3-Pal3,Ser4,Arg5++ +,D-3-Pal6,Leu7,Arg8,Pro9, D-Ala10]-NH2 showed 80-85% antiovulatory activity (AOA) at 0.25 micrograms in the rat. The latter antagonist showed 60% AOA at 0.125 micrograms. Of four pairs of analogs with Arg8 and Lys8, respectively, two pairs favored Lys8 over Arg8 for potency. One pair showed negligible difference and another pair favored Arg8 over Lys8. There is specificity of substitution for potency. In other antagonists, D-3-Pal3, Tyr5 or Phe5, D-Arg6 and Leu7 or Nle7 or Val7 and Arg8 were variously effective substitutions for increase of potency and reduction of histamine release.

ChemInform Abstract: IMPROVED ANTAGONISTS OF LUTEINIZING HORMONE‐RELEASING HORMONE MODIFIED IN POSITION 7

The structure-activity relationship of position 7 in the antagonist [N-Ac-D-Nal1,D-pClPhe2,D-Trp3,D-Arg6,D-Ala10]-LH-RH has been investigated by the incorporation of a series of amino acids at this position. The analogues were prepared by solid-phase peptide synthesis. All purifications were performed in two stages: gel permeation followed by preparative reversed-phase high-performance liquid chromatography. The analogues were assayed in the standard rat antiovulatory assay using a 40% propylene glycol-saline vehicle. The results demonstrated that position 7 requires a hydrophobic aromatic amino acid for greatest antiovulatory activity. The compound [N-Ac-D-Nal1,D-pClPhe2,D-Trp3,D-Arg6,Phe7,D-Ala10]-LH- RH caused 65% blockade of ovulation at the 500-ng dose and is approximately twice as active as the parent analogue in this assay system. The enhanced activity may indicate the stabilization of the active conformation via intramolecular hydrophobic or tau-tau interactions.

Antagonists of the Luteinizing Hormone Releasing Hormone with Emphasis on Amino Acids in Position Five

Abstract Seventeen analogs of the luteinizing hormone releasing hormone (LHRH) have been synthesized, bioassayed, and compared for antiovulatory activity (AOA) in rats. The emphasis of design was replacement of Tyr5 of LHRH. Position 5 has not been extensively studied. [N-Ac-D-2-Nal1 , D-pClPhe2 , D-3-Pal3 , D-Arg6 , D-Ala10 ]-LHRH was the baseline for new designs. Comparison of the AOA's of the 17 analogs with the baseline revealed the two peptides with Phe5 and 3-Pal5 had equivalent AOA's, and were the best of the 17, and about 45% more potent than the baseline. Analogs with pClPhe5 , oClPhe5 , α-MepClPhe5 , 2-Nal5 , Trp5 , and His5 were less potent than the Phe5 -and 3-Pal5 -analogs. Based on the Phe5 -analog, eight other analogs were synthesized with changes in positions 1, 2, 3 and 7 and although none were better than the baseline, 5/8 showed 20-60% AOA's at 250 ng and revealed optimum positions for new designs.

Analogs of the Luteinizing Hormone Releasing Hormone having the Azagly10 Moiety with Antiovulatory Activity

Twenty-four new analogs of the luteinizing hormone releasing hormone (LHRH) were synthesized and bioassayed for antiovulatory activity in rats. [N-Ac-3⊿Pro1, pF-D-Phe2, D-Trp3,6, Azagly10]-LHRH com pletely inhibited ovulation at 6 μg and was the most potent of the 24, and is a relatively potent antagonist in the field. The Azagly- and Ac-NHNH- and D-Alamoieties in position 10, and D-Arg in position 6, and diverse substitutions in position 1 were emphasized. D-Arg6 was inferior to D-Trp6, and pCl-D-Phe6 appeared superior to D-Trp6. D-Trp3,6 was superior to D-2-Nal3,6 and D-His3,6.

Synthesis and Antiovulatory Activities in Rats of Analogs of the Luteinizing Hormone Releasing Hormone Having a Moiety of /?-(3-Quinolyl)-D-a-alanine in Positions 3 and 6

Abstract Analogs of the luteinizing hormone releasing hormone (LHRH) having a moiety (D-Qal-) of β-(3-quinolyl)-D-α-alanine were synthesized toward more effective inhibitors of ovulation. [N-Ac-pCl-D-Phe1,2, D-3-Pal3, D-3-Qal6, D-Ala10]-LHRH was the most effective, and had an antiovulatory activity of 40% at 1 μg/rat. D-3-Qal6 was as effective as D-Trp6 in combination with N-Ac-3⊿Pro1, pF-D-Phe2, D-Trp3. D-Arg6 was superior to D-3-Qal6) in combination with N-Ac-pCl-D-Phe1,2, D-Trp3 (or D-3-Pal3), D-Ala10. The introduction of D-3-Qal3, D-His3 and D-Arg3 in comparable analogs caused substantial loss of activities.

Synthesis and biological activity of LH-RH antagonists modified in position 1

As part of our studies on the design of more potent antagonists of the LH-RH (luteinizing hormone-releasing hormone) decapeptide, twelve new highly soluble D-Arg 6-analogs have been synthesized. These peptides contain modifications in position 1 and are typified by the general formula (N-acetyl-X 1, D-p-Cl-Phe 2, D-Trp 3, D-Arg 6, D-Ala 10) LH-RH. We have found that a lypophilic, aromatic substituent is required in position 1 in order to elicit antiovulatory activity at a dose as low as 3 μg. The larger the hydrophobic amino acid (X: p-Br-Phe, β-Nal-2) in position 1, the higher is the antiovulatory activity that can be attained. Analogs with non-aromatic or hydrophilic amino acids (X: Gly, Leu, Arg, His, Glu) in position 1 generally have much lower activities in this series of LH-RH antagonists.

Suppression of ovulation in the rat by an orally active antagonist of luteinizing hormone-releasing hormone.

A synthetic antagonist of luteinizing hormone-releasing hormone blocked ovulation in rats in a dose-dependent manner when given by gavage on the afternoon of proestrus. Ovulation was delayed for at least 1 day in all animals given 2 milligrams of antogonist and in some of the animals treated with 1 or 0.5 milligram. Oral administration of 2 milligrams also blocked the preovulatory surge of luteinizing hormone. This demonstration that antagonists of luteinizing hormone-releasing hormone can have oral antiovulatory activity clearly enhances their therapeutic potential.

Antiovulatory Potency and Conformation of an Antagonist of the Luteinizing Hormone-Releasing Hormone Having Six D-Amino Acids

Abstract [N-Ac-Thr 1 ,D-Phe 2,D-Trp 3,6]-LHRH was the model antagonist of LHRH, which was the basis for tho design, synthesis and bioassay of seven peptides having four, five and six D-amino acids, which resulted from three single, three double, and one triple introductions of D-amino acids in positions 4, 5 and 8 of the model. Only the analog with six D-amino acids, [N-Ac-Thr 1,D-Phe 2 ,D-Trp 3,D-Ser 4, D-Tyr 5 ,D-Trp 6 ,D-Arg 8]-LHRH, had antiovulatory activity which was higher than that of the model antagonist, i.e., 70% antiovulatory activity at 25 μg/rat compared with 50% activity at 50 μg/rat, respectively. Empirical energy calculations gave a conformational structure for [N-Ac-Thr 1,D-Phe 2,D-Trp 3, D-Ser 4,D-Tyr 5,D-Arg 6,D-Arg 8]-LHRH which is similar to that calculated for previous potent antagonists. These results are a basis of new designs of antagonists having D-sub-stituents in up to ten positions toward effective inhibitors of ovulation by the parenteral and oral routes of administration.

Peptide antagonists of LH-RH: large increases in antiovulatory activities produced by basic D-amino acids in the six position.

It has been assumed, usually with good reason, that D-amino acids with large aromatic side-chains must be present in position 6 of both LH-RH superagonists and antagonists for the highest levels of biological activity to be reached. However, using one of a recent generation of potent lH-RH inhibitory analogs as a model, we have found that the insertion of D-lysine or, better still, D-arginine in this position results in greater antiovulatory activity in the rat over corresponding D-phenylalanine6- and D-tryptophan6-analogs. For instance, [Ac-D-p-Cl-Phe1,2,D-Trp3,D-Arg6,D-Ala10]-LH-RH exhibits antiovulatory activity at a dose of 750 ng per animal and appears to be the first competitive antagonist with activity in the nanogram region in vivo. This effect seems to be highly dependent on the degree of basicity of the amino acid side-chain since D-amino acids with neutral or acidic groups produced far less active compounds.

Evolution of Design and Achievement of Inhibitors of the Luteinizing Hormone-Releasing Hormone as Inhibitors of Ovulation

Abstract Detailed structure-activity studies on inhibitors of the luteinizing hormone releasing hormone (LH-RH) have been described. The most potent ovulation inhibitors have substitutions in positions 1, 2, 3, and 6. Currently four basic structural requirements for potent antiovulatory activity are: a D-aromatic amino acid, such as D-Trp or D-Phe, in position 6; a D-Phe residue in position 2; substitutions in positions 1 and 3. For inhibitors based on substitutions in positions 2, 3, and 6, the substitution of a Pro, N-Me-Leu or D-Trp residue in position 3 is equally acceptable, and gives analogues which inhibit ovulation at 750 ^g/rat. For inhibitors based on substitutions in positions 1, 2, 3, and 6, D-Trp appears necessary in position 3 in order for ovulation to be inhibited at 200 μ/rat. Many analogues based on the [residue1, D-Phe2, D-Trp3, D-Trp6]-LH-RH sequence are known which inhibit ovulation at 200 μ/rat. These include those analogues having D- <Glu, Ac-Pro, N-Ac-Hyp and N-Ac-Thr in position 1. The choice between L- or Dresidues in this position is structure dependent (Ac-L-Pro > Ac-D-Pro, D- <Glu >L- <Glu, etc.). In addition, a "protected" N-terminal residue having some polar character appears to be important. Substitution of the dipeptide residue, <Glu-Pro-, into position 1 has produced a new category of potent ovulation inhibitors based on linear peptides longer than decapeptides. Continued studies on other analogues in this later class could provide more potent inhibitors by (1) utilizing new binding sites on or in the vicinity of the LH-RH receptor(s); (2) altering transportation properties; (3) producing "pro-drugs". The substitution of N-Me-Leu into position 7 was not advantageous, presumably because of the presence of bulky D-aromatic amino acids in position 6. Nonapeptide ethylamide analogues also had very low antiovulatory potencies. The analogue [chlorambucil1, Leu2, Leu3, D-Ala6]-LH-RH acted as an agonist, but also inhibited in a modified assay in vitro. Comparative assays measuring the inhibition of LH-RH, and inhibition of ovulation have emphasized other factors of importance to inhibitor design. Although all ovulation inhibitors active at 750 or 200 /μg/rat strongly inhibited in vivo, at a ratio of analogue to LH-RH of 166:1, other analogues of comparable in vitro potency have displayed a range of antiovulatory activities. Similar discrepancies have been observed in the results of in vivo LH-RH inhibition assays. The most potent ovulation inhibitors always inhibited LH-RH at 333:1 in adult male chimpanzees, and at 100:1 in adult male rats. The dissociation of the results of the LH-RH and antiovulatory assays have been rationalized in two cases. The Cpc-analogues were active in inhibiting LH-RH in rats and in chimpanzees when given i.V., but were inactive in rats when given s.c. which is the mode of administration in the antiovulatory assay. The results for inhibition of LH-RH in vivo paralleled the results for inhibition of ovulation, and raised a question as to differences in absorption of peptides though the lipid layers of subcutaneous tissue. The reduced in vivo activities of the L-Trp3 analogues in both the LH-RH and antiovulatory assays suggest an increase in enzymatic inactivation for these compounds. [D-Phe2, Pro3, D-Phe6]-LH-RH can inhibit endogenous LH-RH in the Rhesus monkey and inhibit ovulation. Infusion of [D-Phe2, Pro3, D-Trp6]-LH-RH at 375 ^ug/day for 4 days from a s.c. implanted minipump completely inhibited ovulation in cycling female rats and decreased serum LH levels in castrated rats. In contrast with LH-RH or des-Gly10- [D-Ala6]-LH-RH ethylamide the Pro3 analogue did not block uterine implantation sites of mated rats, indicating a difference in the mechanism of contraception for LH-RH agonists and inhibitors