GnRH Agonist and Antagonist

The two principal gonadotropins are luteinizing hormone (LH) and follicle stimulating hormone (FSH). Both hormones consist of two peptide chains, an alpha chain and a chain, linked by Hydrogen bonds and van der Waals forces. LH and FSH share nearly identical alpha chains, while the chain provides specificity for receptor interactions.

A third human gonadotropin is human chorionic gonadotropin (hCG), produced by the placenta during pregnancy.

Gonadotropin Structure

Gonadotropin receptors are embedded in the surface of the target cell membranes and coupled to the G-protein system. Signals triggered by binding to the receptor are relayed within the cells by the cyclic AMP second messenger system.

Gonadotropins are released under the control of gonadotropin-releasing hormone (GnRH) from the arcuate nucleus and preoptic area of the hypothalamus. The gonads - testes and ovaries - are the primary target organs for LH and FSH. The gonadotropins affect multiple cell types and elicit multiple responses from the target organs. As a simplified generalization, LH stimulates the Leydig cells of the testes and the theca cells of the ovaries to produce testosterone (and indirectly estradiol), while FSH stimulates the spermatogenic tissue of the testes and the granulosa cells of ovarian follicles.

GnRH Analogs / Agonists

GnRH - structure and function

• Is a decapeptide produced by neurons of arcuate nucleus of medio-basal hypothalamus
• Schally et al first isolated and characterized its structure in 1971
• Amino acid sequence in position 2 and 3 facilitates release of gonadotrophins
• Amino acid sequence in position 6 is involved in enzymatic cleavage
• Residues 1, 6 and 10 are important for three- dimensional structure and thus for receptor binding
• GnRH binds to specific trans-membrane receptors in the gonadotrophic cells in the anterior pituitary. The receptors then undergo a process of dimerization and initiate a cascade of intracellular events that culminate in the synthesis and secretion of FSH and LH

What is GnRH?

• Decapeptide neuroendocrine hormone
• Induces both FSH and LH secretion by pituitary gonadotrope cells in orderly way
• Crucial for control of gonadal function and normal ovarian cyclicity
• Half life: 4 min

Physiology

• Once the dimer has completed its signal transduction, it is internalized into the cell
• The dimer then breaks and the receptors recycle to the surface and allow a subsequent pulse of GnRH to repeat the cycle
• The physiological pattern of pulsatile release also maintains GnRH numbers as well as the functional intracellular transduction pathwayson

GnRH pulse generator

• GnRH molecules are transported from the hypothalamus to the anterior pituitary by the hypothalamic -hypophyseal portal system
• It is released rythymically in short bursts, known as the GnRH pulse generator.
• Pulsatile frequency varies from 71 minutes in late follicular phase to 216 minutes in late luteal phase

GnRH metabolism

• Degraded rapidly by endopeptidases and cleared by glomerular filtration
• ½ life in peripheral circulation is only 2-8 minutes

Structure

• Position 6 of amino acid sequence is important for the inactivation of the decapeptide
• Position 2 and 3 of amino acid sequence are needed for gonadotrophin release
• Position 1, 6 and 10 are needed for binding

Routes of administration

• After IV administration, plasma GnRH shows 2 phases of ½ lives, 2-8 minutes for fast and 15-16 minutes for slow phase
• Has to be administered parenterally as they are susceptible to gastrointestinal proteolysis
• Subcutaneous- most common, high bio-availability (75-90%) and depot like effects result in prolonged absorption , blood concentrations remain high for many hours, hence effective as single daily subcutaneous injection
• intranasal- nasal spray of buserelin and nafarelin are widely available
• Disadvantage of marked individual variation in absorption and loss by proteolysis and swallowing
• Nasal dose needs to be administered 2-3 times daily to maintain effective drug serum concentration
• Depot formulations- continuous release, improved efficiency, more convenience to the patient,
• Specially for long term pituitary desensitization as in endometriosis, fibroids, and infertility
• 3.75 mg triptorelin or leuprolide I/M once a m

GnRH Agonist protocols

• Long protocols
• Short protocols
• Ultra-short protocols
• Ultra-long protocols

Long protocol

• Superior to short and ultra-short
• Can be used to plan timing of OPU, and administration of Hcg can be delayed without any detrimental effects on IVF outcome
• Higher no. of oocytes retrieved and higher pregnancy rates as compared to short and ultra-short protocols, although more ampoules of gonadotrophins were needed
• An unwanted side-effect is the induction of formation of functional cysts. Ovarian cyst formation was reduced when pre-treatment with OCP's is done
• Higher pregnancy rates when compared to other protocols
• Mean desensitization phase with an agonist in the long protocol is about 3 weeks
• Not very good for poor responders and PCO's

Overview of IVF cycles

• Pre-treatment with GnRH agonist started from luteal phase of previous cycle
• Controlled ovarian hyperstimulation started from 2nd day of treatment cycle with gonodotropins
• GnRH agonist continued at half-dose in treatment cyle
• Serial USG scans to monitor follicle recruitment and growth
• Ovum pickup and fertilization
• ET after 48 h
• Follow-up with progesterone suppplentation

Advantages of GnRH Agonist

• Long-term treatment possible
• Depot formulations available, dose<10mg/mo
• Depot formulations with long duration of action available (e.g. leuprolide SC implant)
• Relatively low cost (D:200 -250 Euro/mo)

Disadvantages of GnRH Agonist

• Flare-up risk of disease exacerbation in malignant tumors
• + antihormones
• + add-back therapy
• Longer treatment duration compared to antagonists (e.g. uterine fibroid

Flare-up risk of disease exacerbation in malignant tumors

• antihormones
• add-back therapy

Longer treatment duration compared to antagonists (e.g. uterine fibroids, COS)

GnRH Antagonists

• It took nearly 10 years more (after analogues) to develop GnRH antagonists, with their ability to block pituitary function without any intrinsic effect, (the flare-up effect), and which could be used for clinical studies in the field of ovarian stimulation.
• Histamine release and allergic reactions were the initial problems in clinical use.
• Finally, four drugs were developed-
  • nal-glu
  • cetrorelix
  • ganirelix
  • antide
• Only two, cetrorelix and ganirelix have been commercially available
• Cetrorelix has 2 dosages (0.25mg, 3mg) hence 2 protocols - single dose and multiple dose.
• Ganirelix has only 1 dosage (0.25 mg) hence ganirelix can be used only in multiple dose protocols.
• The idea with the development of GnRH antagonists was to have an ovarian stimulation protocol available which is as close to the normal cycle as possible.
• This could not be achieved with the GnRH agonist long protocol since the normal cycle was switched off by this procedure.

Other ways to use GnRH agonists, such as the short protocol, used the normal menstrual cycle; however these protocols have been less effective compared to the long protocol.

• The most convenient way for the patient is to start with spontaneous menstrual blleeding using the endogenous gonadotrophins as efficiently as possible and suppressing pituitary function as late as necessary.
• Hence, two dosage protocols- single-dose and multiple-dose were developed.

GnRH antagonist protocols

• The single-dose protocol
• The multiple-dose protocol

The multi-dose GnRH antagonist protocol

The multiple-dose GnRH antagonist protocol. Ovarian stimulation using gonadotropins starts on day 2 or 3 of the spontaneous menstrual cycle. In a fixed protocol the GnRH antagonist is administered for the first time on day 6 of gonadotropins. This is continued up to and including the day of hCG.

The single- dose antagonist Protocol

The single-dose GnRH antagonist protocol. Ovarian stimulation using gonadotropins starts on day 2 or 3 of the spontaneous menstrual cycle. In a fixed protocol the GnRH antagonist is administered in a single dose on day 7 of gonadotropins. A single injection of Cetrotide 3 mg lasts for 96 hours to prevent a premature LH surge. In case that hCG cannot be administered within this time frame additional dosage of Cetrotide 0.25 mg should be given daily.

GnRH antagonist protocol vs. GnRH agonist long protocol

The main results were-
1. a significantly lower risk of OHSS, (which can be confirmed for cetrorelix alone, for ganirelix the risk seems not to be reduced compared to the GnRH long protocol)
2. a significantly lower pregnancy rate for all studies
How can the lower risk of OHSS be explained?
Why is there a difference overall regarding pregnancy rates?

How can the lower risk of OHSS be explained?

• The number of retrieved oocytes was significantly lower by about 2( Al-inany and Aboulgar)
• The estradiol levels on the day of hCG were lower (with a difference of 570pg/ml to 650 pg/ml)
• The development of less small follicles as compared to GnRH agonist long protocol (Ludwig et al)

Why is there a difference regarding pregnancy rates?

• Early suppression of endogenous LH and thereby detrimental effects on follicular growth and the endometrium

GnRH antagonist effects

Down regulation - Why the need for DR

• To bring down the basal hormone levels

USES OF ANTAGONISTS

• Down regulation
• Endometriosis
• Premature LH surge in IUI and IVF
• Precocious puberty
• Menorrhagia
• Fibroids
• Coasting( in cases of hyperstimulation in egg donors

Antagonist regimes in IVF

• Fixed day regimen- day 6 of cycle- irrespective of size of follicle
• When follicle is 14 mm in size till follicle is mature(18 or 19 mm)
• Give in morning on day of hcg
• Time interval between hcg and antagonist should not be more than 12 hours
• There is severe suppression of LH when we give antagonist, but threshold level of LH is required , hence if we can add recombinant LH (inj leuverin) for better results
• Another regime (rarely used) is - start antagonist along with stimulation (but requires very high doses of gonadotrophins)

Advantages of GnRH Antagonist

• Immediate suppression
• Rapid recovery from suppression
• Shorter treatment periods (e.g single-shot for certain IVF patients)

Disadvantages of GnRH Antagonist

• Markedly higher doses required for sustained complete suppression
• Probably markedly higher Cost for long-term treatment (e.g 3 mg CET ~ 390 Euro ~ 29,000 INR)

Disadvantages of GnRH Antagonist

• Markedly higher doses required for sustained complete suppression
• Probably markedly higher Cost for long-term treatment (e.g 3 mg CET ~ 390 Euro ~ 29,000 INR)