GnRH analogues Flashcards
Continuous mode of GnRH action
Continuous low-dose/OR single high-dose = Shutting down the axis:
Downregulation of gonadotrophin secretion
Used when gonadal inhibition required i.e. ‘selective medical hypophysectomy’
Equivalent of removing the hypothalamus
NOTE: single high dose will shutdown the axis as there is so much GnRH it remains in the circulation so remains bound to the receptor, so same effect as a continuous low dose.
Pulsatile mode of GnRH action
Switching on:
Upregulation of gonadotrophin secretion
Used when stimulation of gonads is required
Rationale for native GnRH
Native GnRH
Binds to GnRHR
This stimulates a cellular response -> cellular pathways that results in secrfetion of LH and FSH.
Rationale for GnRH analogues: agonists
Bind to GnRHR
Initially triggers same response as native GnRH
HOWEVER this response is only short lived as after a while the response is then terminated and there will be no downstream signalling producing FSH and LH.
SO… the Agonist binds to the GnRH receptor, has same effects as GnRH initially, GnRH has a short half life, GnRH agonist has a longer half life, so the agonist stays there causing a desensitisation causing shutdown of the axis. Agonist does not dissociate from the receptor.
The agonist is initially an activator.
Rationale for GnRH analogues: antagonists
Binds to and blocks the GnRHR
There are no downstream events
Why do we need GnRH analogues if we are able to synthesise GnRH?
As GnRH has a very short half life (t1/2), so it is degraded shortly after its release.
If administering exogenously we need something with a longer half life
So we need analogues to increase potency & duration of GnRH → analogues created ⇒ agonists or antagonists
They are used to manipulate the HPG axis in clinical practice Eg: IVF, Hormone responsive cancers, endometriosis
Native GnRH
Native GnRH is a decapeptide with an amide group attached to a glycine at position 10.
This is highly conserved.
Synthetic GnRH- same primary sequence as endogenous GnRH
Pulsatile mode of delivery = Switching on
GnRH structure manipulation
GnRH is a decapeptide with an amide group attached to a glycine at position 10.
This is highly conserved.
Positions 1-4 and 9+10 are highly conserved, these are important residues for GnRHR binding and activation. Amino acid substitutions rarely occur in these areas.
When GnRH has been synthesised and protein folding has occurred, it usually has a horse shoe formation:
Positions 1-3 are crucial for…
receptor binding and activation
Positions 8-10 are crucial for
receptor binding
Position 6, where you have glycine is crucial for
the stability and activation of GnRH
what is a d amino acid?
A d amino acid is a stereoisomer of the L amino acid
Position 8, where you have arginine, is where it is most variable across species, where amino acid substitutions occur.
The manipulations and changes being made to create analogues:
D amino-acid substitutions are being made in the regions crucial for receptor binding and activation. They also replace the glycine in position 6 to enhance the stability.
GnRH agonists
Making agonists is straightforward
Substitution of Gly (pos 6) with D-amino acids
All agonists & antagonists have substitution of Gly with D-aa at position 6 ⇒ stabilises conformation & enhances activity
In some cases Replacement of Gly-NH2 by NH2-ethylamide binding to Pro (pos 9/10
Replacement of glycine amide with ethylamide at pos 10 to enhance affinity for receptor
Making GnRH Agonist brands
Lupron – At position 6 replaced glycine with a d-leucin and replaced the glycine amide with ethylamide. These two changes result in a 10 fold increase in GnRH activity
Buserelin (most popular agonist in IVF) – replaced glycine woth D serine and ethylamide at pos 10. Results in 100 fold increase
Aim is to shutdown he HPG axis so they can take control and start to administer FSH to stimulate follicle growth.
An added benefit in the manipulation of the GnRH structure is that it helps to avoid proteolytic cleavage
Making GnRH antagonists
Are a lot more difficult to make. Took 30 years:
1st generation replaced His & Trp at pos 2 & 3 woth d amino acid substitutions, but there was low suppressive activity of the HPG axis.
2nd generation potency increased by D-aa substitution in pos 6 but was withdrawn due to anaphylaxis by histamine release
3rd generation replaced D-Arg by D-ureidoalkayl aa
Usually in position 1-4 and 5-10. So there is a lot more manipulation.
In all the brands, positions 1-3 have been swapped. There is a change in postion 6. Some have a change in position 8. ALL have changes in position 10. There is more manipulation that occurs.
This maintains high binding affinity, blocking GnRHR activation
MoA of native GnRH
Binds to the GnRHR
There is an activation of downstream signalling
There is a stimulation of FSH and LH synthesis and secretion
There is dissociation of GnRH from the GnRHR
GnRHR is now responsive to the next GnRH pulse
This is due to the short half life of native GnRH