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
