Title: Gonadotropin-releasing hormone antagonist: new tools vs. old habits
Abstract: Until recently, the only GnRH analogues available were GnRH agonists. Their range of indications is now well established and includes the suppression of gonadotropin-dependent disorders. The first antagonist analogue is now available in Europe (cetrorelix), and its use is thus far limited to the suppression of the LH surge in ovarian stimulation protocols for assisted reproductive techniques (IVF). Gonadotropin-releasing hormone antagonists (GnRH-a) are competitive inhibitors of the binding of GnRH to its receptors. Subcutaneous administration results in the rapid decrease in bioactive as well as immunoreactive LH levels, followed by a decrease in FSH levels. The latter effect is significant after several injections of antagonists, probably because of the long half-life of FSH. This confirms that there is no differential control for LH and FSH and justifies the use of the term “GnRH” rather than “LH-releasing hormone.” Gonadotropin-releasing hormone antagonists are produced by the significant substitution of four to six amino acids for unnatural D amino acids. Since 1972, when the first antagonist was synthesized by removal of the histidine in position 2 (an amino acid essential for biologic action), many antagonists have been produced. Their development was limited, however, because of side effects related to their ability to release histamine from the skin at the injection site or eventually to provoke systemic reactions. New generations of GnRH-a devoid of significant histamine-releasing properties are now available for clinical use. The more advanced compounds include cetrorelix (Cetrotide; Asta Medica AG, Frankfurt, Germany) and ganirelix (Orgalutran; NV Organon, Oss, the Netherlands). Finally, antagonists, unlike agonists, suppress sex steroids to castration levels at milligram doses. This difference is probably due to the fact that competitive inhibitors need to occupy all GnRH receptors continuously. In clinical practice, GnRH-a does not produce desensitization of GnRH receptors. As discussed previously, these antagonists are able to suppress gonadotropin secretion in a rapid and dramatic manner. It is also clear that GnRH-a is more potent than agonists in terms of suppressing LH and FSH. The suppression of gonadotropin production is observed within hours and lasts 10–100 hours depending on the dose administered, because the half-lives of antagonists increase in relation to doses administered. This phenomenon is generally considered to be caused by precipitation of the product and a subsequent depot effect. Obviously there is no initial flare after antagonist administration, and the resulting gonadotropin suppression can be overridden by exogenous administration of native GnRH or GnRH agonist. In women with normal cycles, GnRH-a administered at the end of the follicular phase before the LH surge (1Dubourdieu S. Charbonnel B. D’Acremont M.F. Carreau S. Spitz I.M. Bouchard P. Effect of administration of a gonadotropin-releasing hormone (GnRH) antagonist (Nal-Glu) during the preovulatory period the luteinizing hormone surge requires the secretion of GnRH.J Clin Endocrinol Metab. 1994; 78: 343-347Crossref PubMed Scopus (62) Google Scholar) or even at the time of the surge can postpone or interrupt the LH surge. Although it is impossible to directly measure GnRH levels in the portal circulation in humans, this phenomenon is clearly the consequence of the key role of GnRH during the surge, as has been demonstrated in rats, ewes, and, more recently, nonhuman primates (all species in which secretion of GnRH can be directly determined). The only outstanding controversy concerns the amount of GnRH that is secreted at the time of the surge. There is recent evidence that GnRH has a permissive action and that high levels are not required to trigger the gonadotropin surge. On the basis of this information, it seems likely that GnRH-a will be used essentially in circumstances in which the need for gonadotropin suppression is profound and immediate. Long-term applications must wait until depot preparations are available, provided that the final cost of antagonists remains acceptable. Clinical applications therefore include controlled ovarian stimulation for IVF, in which one or two injections of GnRH-a (2Frydman R. Cornel C. de Ziegler D. Taieb J. Spitz I.M. Bouchard P. Prevention of premature luteinizing hormone and progesterone rise with a gonadotropin-releasing hormone antagonist, Nal-Glu, in controlled ovarian hyperstimulation.Fertil Steril. 1991; 56: 923-927Abstract Full Text PDF PubMed Google Scholar, 3Olivennes F. Alvarez S. Bouchard P. Franchin R. Salat-Baroux J. Frydman R. The use of a GnRH antagonist (Cetrorelix) in a single dose protocol in IVF-embryo transfer a dose finding study of 3 versus 2 mg.Hum Reprod. 1998; 13: 2411-2414Crossref PubMed Scopus (144) Google Scholar) or repeated daily administration (4Diedrich K. Diedrich C. Santos E. Zoll C. al-Hasani S. Reissmann T. et al.Suppression of the endogenous luteinizing hormone surge by the gonadotropin-releasing hormone antagonist Cetrorelix during ovarian stimulation.Hum Reprod. 1994; 9: 788-791PubMed Google Scholar) prevents LH surges. The advantage of the first approach is that the LH surge is suppressed without a long-lasting depletion of endogenous gonadotropins. The multiple-dose regimen (using low doses) permits a deeper suppression of LH, that can eventually produce lower plasma E2 levels and, as a consequence, reduced endometrial quality when gonadotropin preparations devoid of LH are used. The single-dose regimen requires more attention in that follicular development or E2 production must be monitored, but it allows completion of multifollicular growth without the use of high doses of hMG or FSH. In all studies, the number of ampules of hMG or FSH was reduced by about 40%. In most cases, one injection of antagonist on day 8 of the stimulation cycle is enough. In slow responders, however, a repeat injection may be needed. With the use of cetrorelix, each injection might allow 3–4 days of treatment with gonadotropins without an LH surge. The dose that seems the least effective in this regimen is 3 mg of cetrorelix SC. Some investigators, in particular Diedrich et al. (4Diedrich K. Diedrich C. Santos E. Zoll C. al-Hasani S. Reissmann T. et al.Suppression of the endogenous luteinizing hormone surge by the gonadotropin-releasing hormone antagonist Cetrorelix during ovarian stimulation.Hum Reprod. 1994; 9: 788-791PubMed Google Scholar), have proposed prolonged treatment with the antagonist, starting on day 7 of stimulation and continuing until the triggering of oocyte maturation with hCG. The results obtained by Diedrich and colleagues (4Diedrich K. Diedrich C. Santos E. Zoll C. al-Hasani S. Reissmann T. et al.Suppression of the endogenous luteinizing hormone surge by the gonadotropin-releasing hormone antagonist Cetrorelix during ovarian stimulation.Hum Reprod. 1994; 9: 788-791PubMed Google Scholar) are also excellent in terms of pregnancy rate. With this regimen, the dose of hMG or FSH required is more than in the single- or dual-administration protocol. More studies are needed to compare these two methods and the use of agonists. Recent data suggest that in this “long” antagonist protocol, the dose of antagonist required may be reduced dramatically to daily doses far less than 1 mg (e.g., 0.25 mg) (5Albano C. Smitz J. Camus M. Riethmuller-Winzen H. Van Steirteghem A. Devroey P. Comparison of different doses of gonadotropin-releasing hormone antagonist Cetrorelix during controlled ovarian hyperstimulation.Fertil Steril. 1997; 67: 917-922Abstract Full Text PDF PubMed Scopus (228) Google Scholar, 6A double-blind, randomized, dose-finding study to assess the efficacy of the gonadotropin-releasing hormone antagonist ganirelix (Org 37462) to prevent premature luteinizing hormone surges in women undergoing ovarian stimulation with recombinant follicle-stimulating hormone (Puregon). The ganirelix dose-finding study group.Hum Reprod. 1998; 13: 3023-3031Crossref PubMed Scopus (335) Google Scholar). Other advantages of using antagonists in IVF include the possibility of triggering ovulation with native GnRH or GnRH agonists, the possible avoidance of luteal supplementation resulting from the short half-life of the compound and the rapid recovery after cessation of treatment with the antagonist, and the possibility of using the antagonist in minimal-stimulation or natural (unstimulated) cycles. Such procedures are promising, because women do not need to receive high doses of gonadotropins, and intracytoplasmic sperm injection can probably improve efficacy. In a natural cycle, monofollicular development is associated with a high risk of cancellation because of premature LH surge. As proposed by Rongières-Bertrand et al. (7Rongières-Bertrand C. Olivennes F. Righini C. Fanchin R. Taı̈eb J. Hamamah S. et al.Revival of the natural cycles in in-vitro fertilization with the use of a new gonadotropin-releasing hormone antagonist (Cetrorelix) a pilot study with minimal stimulation.Hum Reprod. 1999; 14: 683-688Crossref PubMed Scopus (150) Google Scholar), this can be prevented by administration of an antagonist and a low dose of hMG or FSH at the same time on day 8 or 9 of the follicular phase. Gonadotropin administration would compensate for the decrease in endogenous gonadotropin levels after antagonist administration. The minimal-stimulation approach involves administration of low-dose hMG or FSH in the midfollicular to late follicular phase (8De Jong D. Macklon N.S. Fauser B.C.J.M. Minimal ovarian stimulation for IVF extending the FSH window.in: Jansen R. Mortimer D. Towards reproductive certainty? Fertility and genetics beyond 1999. Parthenon, New York1999: 195-199Google Scholar, 9Fauser B.C.J.M. Devroey P. Yen S.S.C. Gosden R. Crowley Jr, W.F. Baird D.T. et al.Minimal ovarian stimulation for IVF appraisal of potential benefits and drawbacks.Hum Reprod. 1999; 14 ([editorial]): 2681-2686Crossref PubMed Scopus (194) Google Scholar). The use of antagonist analogues in IVF needs to be studied further. Although they allow flexible stimulation procedures, until now results of their use may have appeared, in some centers at least, slightly less positive than those observed with agonists. This seems due mainly to the necessity for the physician to learn a new procedure and to adapt gonadotropin doses. In relation to the potent suppression of LH and FSH, the potentially poorer results may be related to the use of recombinant FSH devoid of any LH activity, which might impair follicular growth or the increase of E2 levels as well as its subsequent impact on the endometrium. This may lead to the necessity of careful titration of the antagonist. Finally, comparison with agonist analogues remains difficult because the reasons behind the efficacy of these drugs (a 70% increase in pregnancy rate compared with protocols without agonists) (10Barbieri R.L. Hornstein M.D. Assisted reproduction-in vitro fertilization success is improved by ovarian stimulation with exogenous gonadotropins and pituitary suppression with gonadotropin-releasing hormone analogues.Endocr Rev. 1999; 20: 249-252Crossref PubMed Scopus (41) Google Scholar) remain incompletely elucidated. For instance, it is still unknown whether agonists modify the kinetics of follicular growth in a significant manner. In other words, the concept of synchronization of the follicles before stimulation, possibly related to the initial flare effect, is still speculative, and such a phenomenon does not fit with the more recent data on the physiology of follicular growth (11Fauser B.C.J.M. Van Heusden A.M. Manipulation of human ovarian function physiological concepts and clinical consequences.Endocr Rev. 1997; 18: 71-106Crossref PubMed Scopus (432) Google Scholar). It is well accepted that agonists—especially in the so-called long protocols, the present gold standard—allow an easy organization of the procedure. However, when short, ultrashort, and long protocols were compared, the only significant difference convincingly shown was that the long protocol is preferred for practical reasons. Also, the duration of the period of gonadotropin suppression has not been proved to affect follicular recruitment and subsequent growth. A careful titration of GnRH agonist doses is still lacking. Gonadotropin-releasing hormone antagonists appear to be promising drugs because of their potency and their flexibility, which allows an infinite number of protocols. These drugs should elicit a significant change in our habits, and every clinical IVF team must adjust to new parameters of follicular growth. The magnitude of adaptation is probably similar to that of the adjustments required when hMG was replaced by recombinant FSH. We must also compare the results of single- and multiple-dose regimens of antagonists. Concerning the latter, it will also be interesting to try regimens that start at different times in the stimulation process; it may not always be appropriate to start treatment on day 7. In addition, investigators should also focus on different starting days for treatment with exogenous gonadotropins. The follicular dynamics must be studied in all of these protocols. In the present era of evidence-based medicine, more mechanistic studies of both agonists and antagonists must be conducted before a true comparison can be done. The potential direct effects of GnRH agonists and antagonists on ovarian function and endometrium are unknown. No data supporting a significant deleterious effect of these drugs have thus far been produced. If the absence of proof is not the proof of absence of effect, there is no reason to believe that antagonists would behave differently from agonists. Recent data suggest that embryos obtained during antagonist-treated cycles and frozen are normal and lead to ongoing pregnancies whose number is not different from that associated with embryos obtained during agonist-treated cycles (12Kol S. Lightman A. Hillensjo T. Devroey P. Fauser B. Tarlatzis B. et al.High doses of gonadotropin-releasing hormone antagonist in in-vitro fertilization cycles do not adversely affect the outcome of subsequent freeze-thaw cycles.Hum Reprod. 1999; 14: 2242-2244Crossref PubMed Scopus (79) Google Scholar). In addition to the successful use of antagonists in IVF, gonadotropin ovarian-stimulation protocols for IUI may also have favorable outcomes. This has not yet been studied. It is also unknown whether chronic administration of antagonists produces better results than administration of agonists in hormone-dependent diseases such as polycystic ovary syndrome, breast cancer, prostate cancer, endometriosis, and uterine myomatosis. However, preliminary observations suggest that antagonists are more potent suppressors of bioactive LH and FSH. These results need to be confirmed, and a depot preparation of antagonist is required. Alternatively, new therapeutic regimens such as repeated treatments with an antagonist, separated by treatment-free intervals, must be assessed. One of the keys for the future of development of antagonist therapy in these areas is the development of a depot preparation. Such a long-acting form is still under investigation and requires further development. Other indications might include preparations for hysteroscopy or for mammograms to eventually reduce breast density. In conclusion, GnRH-a is a powerful suppressor of LH and FSH. New generations of antagonists, including cetrorelix and ganirelix, appear to be useful in ovarian stimulation protocols by suppressing LH surges. Although their use is already standardized and is associated with good pregnancy rates, more research is needed to achieve the consistently good results they are likely to produce. Their other qualities also need further study: absence of flare, triggering of the surge by GnRH, absence of luteal support, and, above all, flexibility of use, allowing protocols with reduced doses of gonadotropins and at lower cost (9Fauser B.C.J.M. Devroey P. Yen S.S.C. Gosden R. Crowley Jr, W.F. Baird D.T. et al.Minimal ovarian stimulation for IVF appraisal of potential benefits and drawbacks.Hum Reprod. 1999; 14 ([editorial]): 2681-2686Crossref PubMed Scopus (194) Google Scholar). Orally active antagonists remain to be synthesized, but the search for a nonpeptidic ligand of the GnRH receptor has already begun. No product has yet been introduced into clinical practice. Let us prepare for these new chapters by trying to understand better the mechanisms behind the efficiency, rather than by blindly applying existing regimens. This approach may take longer, but reproducible success is at the end of the road.