Hrishikesh D. Pai1 , Pritimala Bhalchandra Gangurde1, Nandita P. Palshetkar1 and Rishma Dhillon Pai2
(1)
In Vitro Fertilization Unit, Department of Obstetrics and gynecology, Bloom IVF Centre, Lilavati Hospital and Research Centre, Mumbai, Maharashtra, India
(2)
Department of Obstetrics and gynecology, Bloom IVF Centre, Lilavati Hospital and Research Centre, Mumbai, Maharashtra, India
Hrishikesh D. Pai
Email: hdpai@hotmail.com
Abstract
In a rapidly advancing era of science, we are witnessing inventions of new drugs with discovery of newer applications. GnRH agonists are one of the good examples of the same. The short half-life of native GnRH triggered the need for more stable and long-acting molecules with similar actions and effects. That was subsequently achieved by the modification of the original structure. The use of GnRH agonist started in ovarian hyperstimulation for suppression of HPO axis and prevention of premature LH surge. Subsequently with wide use of antagonist protocol, it is found to be applicable as ovulation trigger, preventing OHSS. With this wonderful discovery, the concept of OHSS free clinic appears to be possible. Apart from this, it also appears to be useful for fertility preservation in patients undergoing treatment for cancer. The role in luteal phase support is still controversial and needs further studies.
Keywords
AgonistHalf lifeSurgeTriggerLuteolysisDepotPOF
9.1 Introduction
Gonadotropin-releasing hormone agonists (GnRHas) are widely used in controlled ovarian hyperstimulation (COH). For over two decades gonadotropin preparations are extensively applied for ovarian stimulation in ovulatory women for empirical treatment of unexplained subfertility.
The GnRH agonists were developed with the goal of use for the treatment of anovulation. But soon their paradoxical ability of inhibition of reproductive function was demonstrated in animals. They cause rapid desensitization of pituitary gland as a result of prolonged and non-pulsatile administration [1].
In IVF cycles multi-follicular development is targeted with use of gonadotropins. But incidence of premature LH surge was found to be ranging between 20 and 50 % leading to increased cycle cancellation rates and compromised IVF outcome [1].
The major advantage offered by agonist is efficient prevention of premature LH surge and reduced cycle cancellation rates. It has also shown increase in oocyte yield and number of embryos. So eventually, pregnancy rates are better [1].
9.2 What Is GnRH?
GnRH is one of the four hypothalamic hormones, regulating the function of anterior pituitary. It is a decapeptide. It regulates the production and release of LH and FSH from anterior pituitary.
It was first isolated, characterized and synthesized independently in 1971 by Andrew Shally and Roger Guillman. They were awarded the Nobel Prize for their achievement [1].
GnRH is produced and released from groups of loosely connected neurons in arcuate nucleus of medial basal hypothalamus and preoptic area of ventral hypothalamus [1].
9.2.1 Structure of GnRH
Structure of GnRH is common to all mammals including humans. Action is similar in both males and females [1].
GnRH is a single-chain peptide comprising of ten amino acids with important functions at positions 1, 2, 3, 6 and 10 (Fig. 9.1).
Fig. 9.1
Amino acid sequence of native GnRH
Position 6 is involved in enzymatic cleavage. Positions 2 and 3 are involved in gonadotropin release, and positions 1, 6 and 10 are important for three-dimensional structure [1].
9.2.2 Structure of GnRH Agonist
Native GnRH has a short half-life due to rapid cleavage of bonds between amino acids 5–6, 6–7, 9–10. By altering amino acids at this position, analogues of GnRH can be synthesized with different properties (Fig. 9.2) [2].
Fig. 9.2
Structural modification of GnRH, leading to formation of GnRH agonist
Substitution of amino acid glycine at position 10 at ‘c’ terminus was a first major modification. This was for increasing the potency. But 90 % of its biological activity was lost with splitting of glycine at 10. It was restored by attachment of NH2-ethylamide to proline at position 9 [1].
Replacement of glycine at postion 6 by D amino acids decreases enzymatic degradation. Hence it renders more stability. These modifications also have higher receptor binding affinity.
The introduction of larger, hydrophobic and more lipophilic D amino acids at position 6 can further increase the affinity. Increased lipophilicity is associated with prolonged half-life [1].
9.2.3 Structure of Antagonist
Substitution of amino acids at positions 1, 2, 3, 6, 8 and 10 produces antagonist (Figs. 9.3 and 9.4) [3].
Fig. 9.3
Amino acid sequence of Cetrorelix
Fig. 9.4
Amino acid sequence of Ganirelix
9.2.4 Mode of Action of GnRH
Native GnRH has got a half-life of 2–4 min. GnRH neuronal system releases it in pulsatile fashion. It is necessary for rhythmic secretion of FSH and LH. The pulse frequency is approximately 1 per hour during follicular phase and 1 per 3 h in luteal phase.
It results in gonadal stimulation without down-regulation of anterior pituitary.
GnRH can produce its biological effect if it covers receptors episodically. Hence it gives time for replenishment of receptors. Receptors have three important segments which include hormone-specific external binding, transmembranous region and internal site controlling the process of internalization.
Gonadotrophin will be secreted only in response to pulsatile release of GnRH. A change in frequency or amplitude or both is associated with irregular gonadotropin release. Continuous delivery is ineffective and can lead to suppression of gonadotropic pituitary function. Similarly, prolonged stimulation of receptor by GnRH molecule results in down-regulation. (loss of ability of receptor to respond with original sensitivity). The receptor after being internalized does not return to cell surface for further action. So GnRH limits its own activity by down-regulation.
9.3 GnRH Analogues
They are structural modifications of natural GnRH. There are two types of GnRH analogues that are used: GnRH agonist and GnRH antagonist.
9.4 GnRH Agonists
A gonadotropin-releasing hormone agonist is a synthetic peptide modelled after the hypothalamic neurohormone GnRH. It interacts with the gonadotropin-releasing hormone receptors resulting in released gonadotropins (FSH and LH) from pitutary.
The agonist was developed with the idea of increasing the stability, potency and receptor affinity.
An increased potency could be achieved by replacing glycine for D amino acids at position 6 and by replacing gly-NH2 at position 10 by ethyl amide. It has 100–200 times more affinity to the receptors than native GnRH. Such structural modifications render these compounds more hydrophobic and more resistant to enzymatic degradation.
In 1978, it was discovered that repeated administration of GnRH agonist produced a transient increase in gonadal function. The mechanism of action is ‘flare effect’ followed by down-regulation. Within 12 h of administration it induces liberation of high amounts of LH and FSH. It also increases the number of receptors (fivefold increase in FSH, tenfold increase in LH and fourfold increase in E2 receptors). This is known as up-regulation. This is rationale for using GnRH agonist as trigger in antagonist cycles.
The continuous occupation of the receptors leads to desensitization due to clustering and internalization of receptors resulting in fall of FSH and LH levels. This is known as down-regulation which results in arrest of follicles and fall in sex steroids. This effect is completely reversible as soon as therapy is stopped. This is a basis for clinical use of agonist in ovulation induction and controlled ovarian hyperstimulation. GnRH agonists, when chronically administered, result in marked reductions in blood levels of testosterone and oestrogen.
9.4.1 Available Preparations
The preparations available include leuprolide acetate, the first GnRH agonist to be approved in the United States, nafarelin acetate, histerelin, triptorelin, buserelin and goserelin acetate.
9.4.2 Routes of Administration
GnRH agonists need to be administered parenterally, as they would be susceptible to gastrointestinal proteolysis. Preparations are available for intramuscular, nasal and subcutaneous administration. The preferred route of administration is the subcutaneous route. As the absorption is rapid, blood concentration remains elevated for many hours without long-term pituitary desensitization.
9.4.2.1 Nasal Spray
Buserelin and naferelin are available as nasal preparations.
By nasal route of administration, the absorption is unpredictable. Considerable losses occur by proteolysis and swallowing, giving a fluctuating desensitization levels. The systemic absorption of nasal buserelin is estimated to be 5 % only. So it needs to be administered two to four times a day to maintain an effective drug concentration. The only advantage is that it is a convenient alternative to parenteral route of administration [4].
In most of the cases it is sufficient to prevent premature LH surge.
9.4.2.2 Subcutaneous Injections (Daily Doses)
This can be given once a day. They are given preference because of more stable effect. After subcutaneous administration agonist is rapidly absorbed and blood concentrations remain elevated for several hours.
Buserelin, histerelin, leuprolide and triptorelin can be effectively used as subcutaneous daily administrations. Histerelin is used in treatment of central precocious puberty.
In controlled ovarian hyperstimalation subcutaneous daily preparations are started in luteal phase of previous cycle or follicular phase of stimulation cycle according to the protocol used (long, short or ultrashort protocol). Commonly used preparation for this is leuprolide.
9.4.2.3 Intramuscular Depot Preparations
Depot preparations are useful where long-term pituitary desensitization is needed. So they are given preference for treatment of endometriosis, adenomyosis or fibroids.
Depot preparations are not first choice of treatment in ART because of long duration of action. Hypogonadotropic hypogonadal state may be sustained for 8 weeks after single depot in regularly cycling women.
It is used in ART practice in cases of frozen embryo transfer cycles, egg donation or embryo donation cycles for suppression of endogenous hormones.
Leuprolide and triptorelin are available as intramuscular depot preparations.
Goserelin acetate is available as 3.6 mg depot preparation for subcutaneous use.
Albuquerque LE found no evidence of a significant difference between depot and daily GnRHa use for pituitary down-regulation in IVF cycles using the long protocol, but substantial differences could not be ruled out [5]. Since depot GnRHa requires more gonadotropins and a longer duration of use, it may increase the overall costs of IVF treatment [5]. Hence, daily administration of GnRH agonist seems to be a more cost-effective option as compared to depot preparation.
9.4.3 Applications of GnRH Agonists in ART
1.
2.
3.
4.
9.4.3.1 In Controlled Ovarian Hyperstimulation
Gonadotropin-releasing hormone agonists are used in assisted reproduction technology (ART) cycles to prevent a premature luteinizing hormone surge. After use of GnRH agonist IVF, cycle cancellation rates dropped from 20 to 2 % and the fertilization and implantation rates significantly improved.
The protocols are
1.
2.
3.
4.
A Cochrane database review in 2011 published analysis of gonadotrophin-releasing hormone agonist protocols for pituitary suppression in assisted reproduction [6]. There was no evidence of a difference in the live birth rate, but this outcome was only reported by three studies. There was evidence of a significant increase of 50 % in clinical pregnancy rate in a long protocol when compared to a short protocol. This difference could range from 16 to 93 % increased chance of pregnancy. There was evidence of about 60 % increased number of oocytes obtained when a long protocol was used as compared to a short protocol. However, gonadotropin requirement was also increased in a long protocol. There was no difference in any of the outcome measures for luteal versus follicular start of GnRHa and stopping versus continuation of GnRHa at the start of stimulation.
Long protocol is an original protocol described and is still considered as the gold standard.
9.4.3.2 An Ovulation Trigger to Prevent OHSS
GnRH agonist trigger instead of human chorionic gonadotropin was introduced in the early 1990s as a means to prevent OHSS.
The GnRH agonist preparations used in practice for triggering ovulation are Triptorelin 0.2 mg SC or Leuprolide 1 mg SC. Buserelin use as trigger is also mentioned in few studies.
GnRH antagonist protocols for pituitary down-regulation in IVF and ICSI allow the use of GnRH agonists for triggering final oocyte maturation. Currently, human chorionic gonadotropin (HCG) is still the standard medication for this purpose. The effectiveness of triggering with a GnRH agonist compared to HCG measured as pregnancy and ovarian hyperstimulation (OHSS) rates are unknown.
Cochrane database review in 2011 showed that, in fresh autologous cycles, GnRH agonist was less effective than HCG in terms of the live birth rate and ongoing pregnancy rate per randomized woman [7]. Incidence of ovarian hyperstimulation syndrome (OHSS) was significantly lower in the GnRH agonist group compared to the HCG group. In donor recipient cycles, there was no evidence of a statistical difference in the live birth rate per randomized woman. In conclusion they did not recommend GnRH agonists to be routinely used as a final oocyte maturation trigger in fresh autologous cycles because of lowered live birth rates and ongoing pregnancy rates. An exception could be made for women with high risk of OHSS, after appropriate counselling [8] stated that this recommendation was too premature and more studies are required before coming to a conclusion on GnRH agonist trigger in ovulation induction [8].
Haas et al. studied GnRH agonist vs. hCG for triggering of ovulation – differential effects on gene expression in human granulosa cells [9]. The fertilization rate was similar in the two groups. The mRNA expression of CYP19A1, CYP11A1 and 3 beta hydroxysteroid-dehydrogenase was significantly lower in the GnRH group. The expression of VEGF and inhibin β B was lower in the GnRH analogue triggered group. Expression of genes related to steroidogenesis is lower at the time of oocyte retrieval in patients triggered with GnRH agonist. The decreased expression of VEGF and inhibin β B in the GnRH agonist group can explain the mechanism of early OHSS prevention.
GnRHa trigger offers important advantages, including virtually complete prevention of ovarian hyperstimulation syndrome (OHSS), the introduction of a surge of FSH in addition to the LH surge and finally the possibility to individualize luteal-phase supplementation based on ovarian response to stimulation. Virtually complete elimination of OHSS is one of the major benefits of GnRHa trigger. The mechanism behind this is the luteolysis. As endogenous LH released after bolus of GnRHa has got short half-life as compared with HCG (Table 9.1).
Table 9.1
Comparison of hCG and GnRH agonist as trigger for ovulation
|
HCG |
GnRH agonist |
|
|
Mechanism of action |
Surrogate of LH; acts on LH receptors and causes resumption of meiosis in oocytes |
Acts on GnRH receptor, and within 12 h induces liberation of high amounts of LH and FSH, which is known as ‘flare effect’. |
|
Half life |
8–10 days |
24–48 h |
|
Effect of luteal phase |
No luteal phase insufficiency |
Due to shorter duration of action there is early luteolysis. So deficient luteal phase |
|
OHSS |
Increased incidence |
Decreased incidence |
Once the endogenous HCG production from the trophoblast reaches measurable serum concentrations around day 8 after ovulation, it is too late to rescue the corpora luteae, which results in virtual elimination of the late-onset pregnancy-associated OHSS [10, 11]. Taken together, the combination of GnRH antagonist co-treatment and GnRHa trigger is the tool by which the concept of a future OHSS-free clinic could become a reality [11].
9.4.3.3 GnRH Agonist as Luteal Phase Support
Use of GnRH agonist is suggested on day 5/6 after the ICSI procedure for luteal phase support. The effect remains controversial.
A prospective randomized control study in 2009 of ‘single-dose GnRH agonist administration in the luteal phase of GnRH antagonist cycles’ was designed to evaluate the effect of luteal-phase administration of single-dose GnRH agonist on pregnancy, implantation and live birth rates [12].
The patients in the luteal-phase agonist group had significantly higher rates of implantation and clinical pregnancy rates. There were also statistically significant differences in multiple pregnancy and live birth rates. Administration of single-dose GnRH agonist as a luteal-phase support in ovarian stimulation-GnRH antagonist cycles in addition to standard luteal support seems to be effective in all cycle outcome parameters.
Olieveira et al. in 2010 published the meta-analysis on administration of ‘single-dose GnRH agonist in the luteal phase in ICSI cycles’ [13]. The outcomes analyzed were implantation rate, clinical pregnancy rate (CPR) per transfer and ongoing pregnancy rate. In all trials, a single dose of GnRHa was administered at day 5/6 after ICSI procedures. All cycles presented statistically significant higher rates of implantation, CPR per transfer and ongoing pregnancy in the group that received luteal-phase GnRHa administration than in the control group. In trials with long GnRHa protocol, CPR per transfer and ongoing pregnancy rates were not significantly different between the groups, but implantation rate was significantly higher in the group that received luteal-phase-GnRHa administration. On the other hand, the results from trials with GnRH antagonist multi-dose ovarian stimulation protocol showed statistically significant higher implantation, CPR per transfer and ongoing pregnancy rate in the luteal-phase GnRHa administration group.
These findings demonstrate that the luteal-phase single-dose GnRHa administration can increase implantation rate in all cycles. It increases CPR per transfer and ongoing pregnancy rate in cycles with GnRH antagonist ovarian stimulation protocol.
Nevertheless, by considering the heterogeneity between the trials, it seems premature to recommend the use of GnRHa in the luteal phase. Additional randomized controlled trials are necessary before evidence-based recommendations can be provided.
9.4.3.4 For Fertility Preservation in Patients Undergoing Cancer Treatments
In day-to-day practice, we come across multiple patients diagnosed with cancers and hence undergoing radiotherapy and chemotherapy for treatment. In these patients fertility is affected by these therapies to variable extent. But prevention is possible to some extent. GnRH agonists are used to suppress gonadotropins. Rendering the follicular development quiescent does reduce the ovarian damage [14].
A meta-analysis of studies of ovarian preservation by GnRH agonists during chemotherapy was published in 2009. It showed that 93 % women treated with GnRHa during chemotherapy maintained ovarian function as compared to 48 % of women not treated with GnRHa. The use of a GnRHa during chemotherapy was associated with a 68 % increase in the rate of preserved ovarian function compared with women not receiving a GnRHa. Among the GnRHa-treated women, 22 % achieved pregnancy following treatment compared with 14 % of women without GnRHa therapy [15].
The analysis of randomized studies, published in 2014, also shows that the temporary ovarian suppression induced by GnRHa significantly reduces the risk of chemotherapy-induced POF in young cancer patients [16].
9.5 Summary
GnRH agonist is a useful tool to prevent premature LH surge in ovulation induction. Hence, it is increasing the oocyte yield and clinical pregnancy rate. It can also be used along with antagonist protocols as preventive measure for OHSS. It is also coming up as a new hope for fertility preservation in cancer patients undergoing treatment. Its role in luteal phase as support is still controversial and requires further studies.
References
1.
Huirne JAF, Schats R. The use of GnRh agonists. In: Gardner DK, Weissman A, Howles CM, Shoham Z, editors. Textbook of assisted reproductive technologies: laboratory and clinical perspectives. 3rd ed. London: Informa Healthcare; 2009.
2.
Speroff L. Neuroendocrinology. In: Speroff L, Fritz MA, editors. Clinical gynecologic endocrinology and infertility. 8th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2011. p. 157–98.
3.
Ludwig M. GnRh antagonists. In: Gardner DK, Weissman A, Howles CM, Shoham Z, editors. Textbook of assisted reproductive technologies: laboratory and clinical perspectives. 3rd ed. London: Informa Healthcare; 2009. p. 539.
4.
Weissman A, Shoham Z. GnRH its agonistic analogues. Basic knowledge. In: Shoham Z, Howels CM, Jacobs JS, editors. Female infertility therapy–current practice. London: Martin Dunitz; 1999. p. 157–66.
5.
Albuquerque LE, Tso LO, Saconato H, Albuquerque MC, Macedo CR. Depot versus daily administration of gonadotrophin-releasing hormone agonist protocols for pituitary down regulation in assisted reproduction cycles. Cochrane Database Syst Rev. 2013;CD002808.
6.
Maheshwari A, Gibreel A, Siristatidis CS, Bhattacharya S. Gonadotrophin-releasing hormone agonist protocols for pituitary suppression in assisted reproduction. Cochrane Database Syst Rev. 2011;(8):CD006919.
7.
Youssef MAFM, Van der Veen F, Al-Inany HG, Griesinger G, Mochtar MH, Aboulfoutouh I, Khattab SM, van Wely M. Gonadotropin-releasing hormone agonist versus HCG for oocyte triggering in antagonist assisted reproductive technology cycles. Cochrane Database Syst Rev;(1):CD008046.
8.
Humaidan P, Papanikolaou EG, Tarlatzis BC. GnRHa to trigger final oocyte maturation: a time to reconsider. Hum Reprod. 2009;24(10):2389–94.CrossRefPubMed
9.
Haas J, Ophir L, Barzilay E, Yerushalmi GM, Yung Y, Kedem A, et al. GnRH Agonist vs. hCG for triggering of ovulation – differential effects on gene expression in human granulosa cells. PLoS One. 2014;9(3), e90359.PubMedCentralCrossRefPubMed
10.
Speroff L. Sperm and egg transport, fertilization, and implanatation. In: Speroff L, Fritz MA, editors. Clinical gynecologic endocrinology and infertility. 8th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2011. p. 243–68.
11.
Speroff L, Devroey P, Nikolaos P. Polyzos, Blockeel C. An OHSS –free clinic by segmentation of IVF treatment. Hum Reprod. 2011;26(10):2593–7.CrossRef
12.
Isik AZ, Caglar GS, Sozen E, Akarsu C, Tuncay G, Ozbicer T, Vicdan K. Single-dose GnRH agonist administration in the luteal phase of GnRH antagonist cycles: a prospective randomized study. Reprod Biomed Online. 2009;19(4):472–7.CrossRefPubMed
13.
Oliveira JB, Baruffi R, Petersen CG, Mauri AL, Cavagna M, Franco Jr JG. Administration of single-dose GnRH agonist in the luteal phase in ICSI cycles: a meta-analysis. Reprod Biol Endocrinol. 2010;8:107.PubMedCentralCrossRefPubMed
14.
Chhabra S, Kutchi I. Fertility preservation in gynecological cancers. Clin Med Insights Reprod Health. 2013;7:49–59.PubMedCentralCrossRefPubMed
15.
Clowse ME, Behera MA, Anders CK, Copland S, Coffman CJ. Ovarian preservation by GnRH agonists during chemotherapy: a meta-analysis. J Womens Health (Larchmt). 2009;18(3):311–9.CrossRef
16.
Del Mastro L, Ceppi M, Poggio F, Bighin C, Peccatori F. Gonadotropin-releasing hormone analogues for the prevention of chemotherapy-induced premature ovarian failure in cancer women: systematic review and meta-analysis of randomized trials. Cancer Treat Rev. 2014;40(5):675–83.CrossRefPubMed