GnRH Analogues Flashcards
Mode of GnRH action - Continuous (3)
low-dose/single high-dose- Shutting down of HPG:
» Downreg. of gonadotrophin secretion
» useful when gonadal inhibition required i.e. ‘selective medical hypophysectomy’- IVF shut down ovaries
Mode of GnRH action - Pulsatile (3)
mode of delivery- Switching on:
» Upreg. of gonadotrophin secretion
» When stimulation of gonads required - need feedback
What is the rationale for native GnRH versus GnRH analogues? (2)
-Mimicking pulsatile GnRH functions- switching on HPG axis (agonist)
-Inhibition of GnRH functions and shutting down of HPG axis (antagonist)
image
Native GnRH (3)
-Synthetic GnRH- same primary sequence as endogenous GnRH
-NH2 group at end
-Pulsatile mode of delivery -> Switching on
Why do we need GnRH analogues (think clinically)? (3)
- GnRH t1/2 in circulation is 2-4 mins (short half life = inc. potency)
-To increase potency & duration of GnRH → analogues created ⇒ agonists or antagonists
-Manipulate the HPG axis in clinical practice- IVF, Hormone responsive cancers, endometriosis
GnRH structure manipulation (5)
Images
- 1st 4 A.A’s + 9-10( Glu, His, Trp, Ser) (Pro, Gly) - highly conserved in all mammals and most species
-changes/ substitutions occur in position 8 - Arg (most variable across species)
1) post translation + protein folding = horse shoe shape
2) A.A 1-3: receptor binding + activation , 8-10 receptor binding only
3) (substititions made by agonists + antagonists) - Ago: gly (6) replaced w/ D-AA (stereoisomer)
How do you create GnRH agonists? (3)
Straightforward to make agonist
1)Substitution of Gly by D-amino acids
2)Replacement of Gly-NH2 by NH2-ethylamide binding to Pro (pos 9/10) = increased stability + resistance to proteolytic cleavage
= 10/100x potency in GnRH activity
image and info
How do you create GnRH antagonists? (5)
30yrs to make:
1) tried replacing His + Try (2,3) = low suppressive activity
2) inc. potnecy by D-A.A. sub. in (6) but anaphylaxis by histamine release
3)replaced D-Arg by D-ureidoalkayl AA
=maintains high binding affinity, blocks GnRHr activation
Mechanisms of action of GnRH and GnRH analogues (4)
diagram
1) all bind to receptor
2) G + Ag activate signalling, Antag blocks receptor
3) G + Ag stim. gonad synth + sec., Antag no downstream effects (uncoupled GS + Gq signalling = shutdown)
4) G disassociation of GnRh from receptor (rec- responsive to next pulse), Ag desensitised GnRHr = non-responsive to GnRH
Clinical uses of native GnRH -Hypogonadism (6)
- Diagnostic tests
- Hypogonadism: defined as impaired gonadal function with resultant decreased sex steroids
- To distinguish between 1° & 2° hypogonadism:
-1∘ hypogonadism arises from gonadal failure (hyper gonadotrophins - no feedback form gonads)
-2∘ hypogonadism arises from abnormalities of hypo-pituitary axis (downreg. of FSH + LH) = affects gonadal function
Test: GnRH is administered intravenously or subcutaneously and plasma LH and FSH are measured at 15 min intervals (0, 15, 30, 45 and 60 minutes)
- high: primary
-low/no = secondary
Clinical uses of Native GnRH- Hypogonadotrophic hypogonadism vs puberty (HH) (4)
HH: to diagnose and treat (pump w/ generator of intermittent pulses of GnRH)
Delayed puberty:
- Boys, when testicular growth (volume >4 ml) has not started at 14yrs,
-Girls, when breast development is not present at 13yrs or menarche did not occur 15-18 years of age
Difficult to distinguish between delayed puberty & HH ⇒ pre-pubertal pituitary is unresponsive
no response of GnRH pump = delayed puberty
Clinical uses of GnRH analogues -illnesses e.g (6)
*IVF
* Dysfunctional uterine bleeding
* Precocious puberty
*Hormone-dependent cancers : Breast + Prostate cancer
* Hirsutism and virilisation
* Endometriosis
Manipulation of HPG in IVF (7)
images
normal HPG:
1) GnRH sec.
2) FSH/LH
3) ovaries- follicular growth + selection
IVF: trying to take control of the ovaries to help recruit multiple oocytes ( = Shutdown HPG)
1) analogue applied = HPG shutdown (7-14days)
2) Dose ovaries w/ FSH (+ LH) - to recruit multiple antral follicles (increased chances of success)
3) growth of follicles tracked via Ultrasounds + blood oestrogen levels
4) hCG administered = final maturation of oocytes in those follicles triggering ovul.
5) need 3-8 follicles, diameter of 16-18mm = oocyte retrieval - has to take place 36-48hrs after hCG to not lose them
6) Sperm and embryo transfer
GnRH agonists IVF benefits (3)
-GnRH agonist + gonadotrophins used extensively for follicle growth
stimulation in IVF
Major benefit:
-improved follicular recruitment = larger no. oocytes recovered (not
in all patients)
-prevent premature LH surge = lower cancellation rate
-Improvement in routine organisation
GnRH agonists & Breast Cancer + premeno (3)
-Premenopausal women → chemical castration (reduce oestrogen output)
-GnRHR present in breast cancer tissue (50-60%) - E2 dependent = shutdown HPG axis = shutdown E (needed for prolif.)
-Direct anti-proliferative effect of GnRHa in BCa cell lines
GnRH agonists & Prostate Cancer (6)
-Prostate Cancer (PCa) is 2nd most frequent tumour in men in West
-80% of PCa are androgen dependent
-GnRH agonist → desensitisation →↓↓ T (chemical castration - shutdown HPG)
-“Flare-effect”/ initial result: ↑T (upreg. of LH +FSH) - aggravates symp.’s
- Micro-surges of T, LH & FSH w/ continued use
-Co-administer with anti-androgens (to combat flare effect)
images
GnRH agonists & fertility preservation – female cancers (5)
Last 20 years survival rates for young women > 80-90%
- Large percentage develop POF due to follicular damage:
(Chemotherapeutic agents directly attack DNA in dividing and dormant germ cells)
To preserve fertility either:
-Cryopreserve embryos or MII oocytes after IVF and before chemotherapy
-Cryopreserve ovarian tissue for transplantation later
Administer adjuvant therapy to minimise gonadal damage?
GnRH agonists & chemotherapy (4)
image - study done suggesting possible results - popular mice study
1) control vs GnRH agonist before chemo
2)control = - A lot of follicular damage -POF, no E2 no viable oocytes = premature menopause + infertility
GnRH agonists = good protection of oocytes + normal E2 + retained ovarian function
but mice X Humans = refuted for uses and viability
Limitations of GnRH agonists (6)
-Temporary solution - symptoms can return
-Side-effects -pseudo-menopause in WOMEN =
reduced libido, erectile dysfunction ( + in MEN)
-increased LDL / decreased HDL cholesterol, insomnia, headaches (BOTH)
-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
-“Flare effect”
-Chronic treatment(>6months) = Osteoporosis + Heart disease
GnRH antagonists & Prostate Cancer (4)
Prostate Cancer
-No “flare” or micro-surges.
-Reduces testosterone to castrate levels by day 3.
-1st antagonist Abarelix withdrawn due to systemic allergic reaction.
-Degarelix => rapid & sustained reduction in Testo & PSA (prostate specific antigen) routinely (special) used now in advanced prostate cancer.
GnRH antagonists advantages (5)
» Rapid action (= rapid pain relief) – 4-6hrs post administered.
» Rapid reversal
» Shorter treatment regime compared to 7-10 days for pituitary down-regulation with agonists.
» No “flare effect”.
» Dose-dependent.
– Partial pituitary-gonadal inhibition.
– Can adjust level of hypogonadism as desired.
GnRH antagonists disadvantages (4)
» 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.
