GnRH analgoues

Continuous low-dose/single high-dose- Shutting down: Downregulation of gonadotrophin secretion When gonadal inhibition required i.e. ‘selective medical hypophysectomy’ Pulsatile mode of delivery- Switching on: Upregulation of gonadotrophin secretion When stimulation of gonads required

Native GnRH binds to GnRH receptor and produces downstream effects resulting in secretion of gonadotrophins such as FSH and LH GnRH agonists bind to the GnRH receptor and initially produces downstream effects but then the receptor becomes desensitised with no downstream effects The GnRH antagonist blocks the receptor from the beginning so there is no LH/FSH production at all Mimicking pulsatile GnRH functions- switching on HPG axis Inhibition of GnRH functions and shutting down of HPG axis

Switches on HPG axis, decapeptide with amide attached at C terminus synthetic/native GnRH has same primary sequence as endogenous GnRH chain on 10 amino acids starting with glutamine ending with glycine ending with nh2

GnRH t1/2 in circulation is 2-4 mins To increase potency & duration of GnRH → analogues created ⇒ agonists or antagonists Manipulate the HPG axis in clinical practice- IVF, Hormone responsive cancers, endometriosis

The structure of GnRH is highly conserved in all mammals and most species- important residues for GnRH binding and activation 4,4,2 N shaped structure of 10 amino acids left side: pglu, his, trp—- D amino acid substitutions here are present in antagonists middle section: Ser, tyr, gly, leu—- in this area d amino substitutions enhance activity and stability right side: Arg,Pro, Gly, NH2. The Arg part is most variable across species. The right side is for receptor binding only

to make agonist Substitution of Gly by D-amino acids Replacement of Gly-NH2 by NH2-ethylamide binding to Pro (pos 9/10) ll agonists & antagonists have substitution of Gly with D-aa at position 6  stabilises conformation & enhances activity

30 years to make antagonist! 1st generation replaced His & Trp at pos 2 & 3, but low suppressive activity 2nd generation potency increased by D-aa substitution in pos 6 but anaphylaxis by histamine release 3rd generation replaced D-Arg by D-ureidoalkayl aa This maintains high binding activity and blocks GnRHr activation

Test: GnRH is administered intravenously or subcutaneously and plasma LH and FSH are measured at 0, 15, 30, 45 and 60 minutes. Hypogonadism defined as impaired gonadal function with resultant decreased sex steroids To distinguish between 1° & 2° hypogonadism Diagnostic tests Hypogonadism defined as impaired gonadal function with resultant decreased sex steroids To distinguish between 1° & 2° hypogonadism Hypogonadotrophic hypogonadism (HH) to diagnose and treat 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

IVF (hCG also required for final maturation- ovulatory process mimic Oocytes retrieval) Dysfunctional uterine bleeding Precocious puberty Hormone-dependent cancers Breast cancer Prostate cancer Hirsutism and virilisation Endometriosis

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

Premenopausal women → chemical castration (reduce oestrogen output) GnRHR present in breast cancer tissue (50-60%) Direct anti-proliferative effect of GnRHa in BCa cell lines

Prostate Cancer (PCa) is 2nd most frequent tumour in men in West 80% of PCa are androgen dependent GnRH agonist → desensitisation →↓↓ T (chemical castration) “Flare-effect” results ↑T Micro-surges of T, LH & FSH with continued use Co-administer with anti-androgens

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? Before chemotherapy, GnRH agonists on their own result in pre-menopause and fertility If GnRH agonist and FSH are administered together, follicles are protected with normal oestrogen production- retained ovarian function

Temporary solution - symptoms can return Side-effects -pseudo-menopause in women (with associated symptoms): reduced libido, erectile dysfunction, increased LDL / decreased HDL cholesterol, insomnia, headaches 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 (>6 months) Osteoporosis, Heart disease

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 used now in advanced prostate cancer.

Advantages: 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. Disadvantages: 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.