Degarelix

Degarelix (brand name: Firmagon) is a synthetic 10-amino acid GnRH receptor antagonist developed by Ferring Pharmaceuticals. It received FDA approval in 2008 for the treatment of advanced prostate cancer, offering an alternative to GnRH agonist therapy (leuprolide, goserelin) without the testosterone flare.

Degarelix was designed as a third-generation GnRH antagonist, optimizing upon earlier antagonists (cetrorelix, ganirelix) specifically for the subcutaneous depot formulation required for long-term prostate cancer therapy. Key structural modifications include D-amino acid substitutions at multiple positions and a unique C-terminal modification that reduces histamine release (a problem with first-generation GnRH antagonists).

**Mechanism — competitive antagonist without flare:** Degarelix competitively blocks GnRH receptors in the anterior pituitary within hours of injection: 1. Hour 1-24: Rapid LH and FSH suppression as GnRH receptor blockade takes effect 2. Day 1-3: Testosterone falls to castrate levels (<50 ng/dL in >95% of patients by day 3) 3. No initial testosterone surge — in contrast, GnRH agonists initially stimulate testosterone for 1-2 weeks before suppression

The SC depot forms a gel-like depot at the injection site from which degarelix is slowly released, providing approximately 28 days of sustained GnRH receptor blockade per 80 mg dose.

**Cardiovascular controversy:** Multiple pooled analyses and registry studies (including a meta-analysis by Albertsen et al.) have suggested that degarelix is associated with fewer major adverse cardiovascular events (MACE) vs GnRH agonists in men with pre-existing cardiovascular disease. The proposed mechanism relates to FSH's direct cardiovascular effects (GnRH agonists transiently elevate FSH via the flare before suppression), but the evidence is not from a randomized trial. The PRONOUNCE trial (NCT02663908) was designed to prospectively compare MACE rates but was terminated early due to low event rates.

**CS21 Phase III trial (Klotz et al.):** In 610 treatment-naive prostate cancer patients, degarelix 240/80 mg achieved testosterone <50 ng/dL in 95% of patients by day 3, vs 0% of leuprolide patients (who showed testosterone flare). By day 28, both groups achieved similar castrate rates.