Buserelin: Mechanism, Dosing & Research Data

Buserelin is a synthetic GnRH agonist approximately 100x more potent than native GnRH, available as subcutaneous injection, intranasal spray, and subcutaneous implant. It is widely used in Europe and globally for prostate cancer, endometriosis, uterine fibroids, and IVF downregulation, though not FDA-approved in the US, where leuprolide and other GnRH agonists fill the same roles.

Mechanism of Action

Initial agonist phase: single dose causes LH/FSH surge and transient sex steroid rise (flare effect); clinically relevant in prostate cancer (tumor flare)
Chronic desensitization: within 2-4 weeks of continuous exposure, GnRHR is downregulated, LH/FSH collapse to castrate levels
SC implant delivers constant release for 2-3 months; intranasal dosing (3x/day) required for GnRHR saturation via mucosa
D-Ser(tBu) at position 6 confers resistance to endopeptidase cleavage, extending half-life vs native GnRH
IVF application: buserelin downregulates the pituitary prior to controlled ovarian hyperstimulation, preventing premature LH surge

Research Findings

Prostate cancer (Suprefact): buserelin equivalent to surgical castration in testosterone suppression and clinical outcomes in multiple European RCTs
Endometriosis: 6 months buserelin 900 mcg/day intranasal produced pain resolution comparable to danazol with less androgenic side effects
IVF downregulation: buserelin long protocol (starting luteal phase day 21) reduced premature LH surges from 25-30% to under 5% in multiple RCTs
Uterine fibroids: buserelin 3-6 months pre-operative reduced fibroid volume by 40-60% and corrected anemia, improving surgical outcomes
GnRH test: single dose buserelin 100 mcg SC used to evaluate GnRH/LH axis function (hypopituitary workup)

Research Protocols

Prostate cancer: 500 mcg SC TID for 7 days (flare covered by anti-androgen), then 200 mcg SC TID maintenance; or implant every 2-3 months
Endometriosis: 600-900 mcg/day intranasal (200-300 mcg 3x daily) for 6 months with add-back if needed
IVF long protocol: 500 mcg/day SC starting day 21 of previous cycle; reduce to 250 mcg/day after confirmation of downregulation by ultrasound and E2
GnRH axis testing: 100 mcg SC single dose; measure LH at 0, 30, 60 minutes (stimulation test)

Interactions

Anti-androgens (bicalutamide, cyproterone): co-start for 2-3 weeks in prostate cancer to prevent testosterone flare and tumor progression
FSH/hMG: given after buserelin-induced pituitary suppression in IVF; timing critical for synchronized follicle development
Add-back therapy (tibolone, low-dose HRT): used in long-term buserelin treatment to preserve bone density and reduce vasomotor symptoms

Safety Profile

Well-established safety profile across decades of use. Hot flashes, decreased libido, osteopenia with prolonged use, potential mood changes. Injection site reactions with SC use. Approved in Europe, Canada, Asia; not FDA-approved in USA (leuprolide preferred there). Avoid in pregnancy.