GnRH Analogues Flashcards
What does a pulsatile GnRH release cause?
LH/FSH upregulation
What is the effect of continuous GnRH release on gonadotrophin release?
Continuous GnRH = LH/FSH cessation
What can be administered to upregulate LH/FSH?
GnRH
What is administered to downregulate gonadotropin release?
GnRH analogues
How can we shut down HPG axis?
Continuous low-dose / single high-dose of GnRH
When may gonadal inhibition be required?
Selective medical hypophysectomy
Describe the effects of GnRH agonists on HPG axis
GnRH agonists initially produce same cell response but GnRHR gets desensitised = no effect
How does GnRH antagonist work?
GnRH antagonist blocks GnRHR
Descrieb the structure of native GnRH
Synthetic GnRH- same primary sequence as endogenous GnRH
How is native GnRH administered?
Pulsatile mode of delivery🡪 Switching on
What is GnRH’s half life normally in circulation?
GnRH t1/2 in circulation is 2-4 mins
What is the purpose of GnRH analogues?
To increase potency & duration of GnRH → analogues created ⇒ agonists or antagonists
What is the role of GnRH analogues?
Manipulate the HPG axis in clinical practice- IVF, Hormone responsive cancers, endometriosis
How consistent is GnRH structure across species?
Highly conserved in all mammals - important residues for GnRHR binding and activation
1 a.a differentiates between them
substitution usually occurs at (Arg) pos.8
Describe the structure of GnRH post-translationally
Once post-translational modifications have occurred, GnRH takes a horseshoe configuration
Which regions of GnRH are most manipulated for administration?
N terminus and C terminus regions are most manipulated of peptide sequence to form (ant)agonists
How is GnRH manipulated to form GnRH agonists?
Straightforward to make agonist
- Substitution of Gly by D-amino acids
- Replacement of Gly-NH2 by NH2-ethylamide binding to Pro (pos 9/10)
How is GnRH agonist affinity to its receptor increased?
Replacement of glycine amide with ethylamide (at pos 10) enhances affinity for receptor
Where are the common substitutions of GnRH to form GnRH agonists?
Most substitutions among GnRH agonists proprietary brands is at position 6 and C terminus
What is the advantage of GnRH agonist substitutions?
Substitutions avoid proteolytic cleavage and enhance stability
How long did it take to form a GnRH antagonist?
30 years to make antagonist due to anaphylaxis that was occurring
What was the problem with the first generation GnRH antagonist created?
1st generation replaced His & Trp at pos 2 & 3, but low suppressive activity
Why was 2nd generation GnRH antagonist not the final product?
2nd generation potency increased by D-aa substitution in pos 6 but anaphylaxis by histamine release
Describe the manipulation to produce the 3rd generation GnRH antagonist
3rd generation replaced D-Arg by D-ureidoalkayl aa
What is the benefit of 3rd generation GnRH antagonist structure?
Maintains high binding affinity, blocks GnRHR activation
Describe the mechanism of action of GnRH
- Binds to receptor
- Activation of signalling
- Stimulation of gonadotropin synthesis and secretion
- Dissociation from GnRHR
- GnRHR responsive to next GnRH pulse
Describe the mechanism of action of GnRH agonists
- Binds to receptor
- Activation of signalling
- Stimulation of gonadotropin synthesis and secretion
- Desensitisation of GnRHR
- GnRHR non-responsive to GnRH
What is the mechanism of action of GnRH antagonists?
- Binds to receptor
- Blockage of receptor
- No downstream effects
What are the diagnostic uses of Native GnRH?
Diagnostic tests:
- Distinguish between 1° & 2° hypogonadism
- Diagnose and treat hypogonadotrpic hypogonadism
What is hypogonadism?
Hypogonadism defined as impaired gonadal function with resultant decreased sex steroids
Where does primary hypogonadism start?
Primary hypogonadism starts in ovary/testes
What are the clinical consequences of primary hypogonadism?
low gonadal steroids
high LH & FSH
Where does secondary hypogonadism originate?
Secondary hypogonadism indicates problem in hyp/pituitary axis.
Why may LH/FSH levels be normal in secondary hypogonadism?
Normalish FHS/LH = gonadal failure due to ovaries or testes (lack of gonadal steroids)
What does high gonadotropin levels in secondary hypogonadism indicate?
High LH & FSH = hypothalamic dysfunction
What does low LH/FSH levels indicate in secondary hypogonadism?
low FSH/LH response = hypothalamus / pituitary gland.- levels vary during puberty
How is hypogonadism tested for?
IV GnRH administered or subcutaneously Plasma LH and FSH are measured at 0, 15, 30, 45 and 60 minutes
What disorders may cause gonadotropin deficiency?
- Large pituitary tumors
- Endocrine deficiency
- Hemochromatosis
- Kallmann syndrome
- Hyperprolactinemia
- Amenorrhea
- Anorexia nervosa
- Starvation
How is delayed puberty classified in boys?
Boys, when testicular growth (volume >4 ml) has not started at 14yrs
Describe delayed puberty in girls
Girls, when breast development is not present at 13yrs or menarche did not occur 15-18 years of age
Why is it difficult to distinguish between HH and delayed puberty?
Difficult to distinguish between delayed puberty & HH ⇒ pre-pubertal pituitary is unresponsive
What are the clinical uses of GnRH analogues?
- IVF
- Dysfunctional uterine bleeding
- Precocious puberty
- Hormone-dependent cancers
Breast cancer
Prostate cancer - Hirsutism and virilisation
Endometriosis
Outline the normal mechanism of the HPG axis
Normal: pulsatile GnRH from hyp→ pit = LH/FSH release
How is HPG axis manipulated in IVF?
GnRH agonist administered to uncouple HPG axis - abolished endogenous gonadotrophin production
⇒ exogenous LH/FSH production to stimulate follicular growth
What are the effects of HPG manipulation in IVF?
Multiple follicles reach maturity
Follicles monitored via US (>3 ~18mm) give hCG to trigger maturation and ovulation
Why is hCG administered in IVF instead of LH?
hCG used as has LH-like properties and has a longer half life (can collect oocytes 36hrs later)
Why do follicles have to be aspirated after 36 hrs in IVF?
If follicles are not aspirated after 36hrs they will all ovulate and will be lost
How are follicles aspirated in IVF?
Transvaginal egg collection to puncture follicle and aspirate oocyte into tubes passed onto embryologists
How are the eggs collected in IVF fertilised?
Eggs placed in culture and inseminated - fertilisation assessed next day
How are GnRH agonists used in IVF?
GnRH agonist + gonadotrophins used extensively for follicle growth stimulation in IVF
What are the benefits of using GnRH agonists in IVF?
Improved follicular recruitment
⇒ larger no. oocytes recovered (not in all patients)
Prevent premature LH surge
⇒ lower cancellation rate
Improvement in routine organisation
How are GnRH agonists used in pre-menopausal women to reduce breast cancer?
Premenopausal women → chemical castration (reduce oestrogen output)
Why are GnRH agonists used for breast cancer?
GnRHR present in breast cancer tissue (50-60%)
Direct anti-proliferative effect of GnRHa in BCa cell lines
How common is Prostate cancer in men?
Prostate Cancer (PCa) is 2nd most frequent tumour in men
How androgen dependent is prostate cancer?
80% of PCa are androgen dependent
How can GnRH agonists aid in prostate cancer
GnRH agonist → desensitisation →↓↓ T (chemical castration)
downside: “Flare-effect” results ↑T
What is a major consequence of cancer treatments in female fertility?
Large percentage develop POF due to follicular damage
How do cancer treatments affect female fertility?
Chemotherapeutic agents directly attack DNA in dividing and dormant germ cells
How can fertility be preserved during cancer treatment?
- Cryopreserve embryos or MII oocytes after IVF and before chemotherapy
- Cryopreserve ovarian tissue for transplantation later
What is a major limitation of GnRH agonist use?
Temporary solution - symptoms can return
What side effects may present while using GnRH agonists?
- Reduced libido
- Erectile dysfunction
- Increased LDL / decreased HDL cholesterol
- Insomnia
- Headaches
Why is GnRH not tissue specific?
Extra pituitary sites of action? (e.g. oocyte, embryo, uterus) in animals - humans??
GnRHR present on these sites – role in implantation?
Inadvertently administered during pregnancy
What is the consequence of chronic GnRH agonist treatment?
Chronic treatment (>6 months) Osteoporosis, Heart disease
What is the benefit of using GnRH antagonists for prostate cancer?
- No “flare” / microsurges
- Reduces testosterone to castrate levels by day 3
Name an example of GnRH antagonist used in prostate cancer
Degarelix ⇒ rapid & sustained reduction in Testo & PSA (prostate specific antigen) routinely used now in advanced prostate cancer
What are the advantages of GnRH antagonist use
- Rapid action (rapid pain relief) 4-6hrs post administered.
- Rapid reversal
- Shorter treatment regime
- No “flare effect”
- Dose-dependent
- Partial pituitary-gonadal inhibition
- Can adjust level of hypogonadism as desired
What are the disadvantgaes of GnRH antagonist use?
- Limited licenses available for wider use
- More expensive than agonists
- Need higher dose than agonist 100mg/month versus 3-5mg
- Competitive inhibitor, therefore less effective over time
