Ganirelix (Antagon®)

Ganirelix (Antagon) is a GnRH antagonist given as a 0.25 mg daily subcutaneous injection during ovarian stimulation in IVF. It provides immediate, reversible suppression of LH to prevent premature ovulation, is easy to use, reduces OHSS risk, and is equally effective as Cetrorelix.

What is Ganirelix?

• Ganirelix acetate is a synthetic GnRH antagonist.

• Brand name: Antagon (though generic versions are also available).

• Used in controlled ovarian stimulation (COS) for IVF/ICSI.

• Its main role: prevent premature LH surge and thus prevent premature ovulation during stimulation.

Mechanism of Action

• Ganirelix competitively binds to GnRH receptors in the pituitary gland.

• This blocks GnRH stimulation → immediate suppression of LH and FSH secretion.

• No initial flare effect (unlike GnRH agonists).

• Effect is rapid, reversible, and dose-dependent.

Outcome: The ovaries continue to respond to exogenous gonadotropins (FSH ± LH), but ovulation does not occur prematurely.

Formulation & Dosage

• Pre-filled syringes with 0.25 mg solution.

• Administered as a subcutaneous injection (usually in the lower abdomen).

IVF protocol use:

• 0.25 mg daily starting around day 5–6 of stimulation OR when the lead follicle reaches ~12–14 mm.

• Continued once daily until the day of trigger injection (hCG or GnRH agonist).

Timing in IVF Cycle

1. Controlled ovarian stimulation begins with FSH injections on cycle day 2–3.

2. Ganirelix is started mid-cycle (day 5–6 or follicle size criteria).

3. It’s continued daily to block LH surge until final oocyte maturation is triggered.

4. Trigger is then given → egg retrieval follows ~36 hours later.

Clinical Benefits

• Immediate suppression of LH (no delay).

• Shorter treatment compared to GnRH agonist protocols (no downregulation phase).

• Reduces risk of OHSS when combined with a GnRH agonist trigger instead of hCG.

• Flexible start (fixed or flexible antagonist protocol possible).

• Well-studied, widely used, and effective.

Side Effects

Generally mild and transient:

• Injection site reactions (redness, swelling, itching).

• Headache.

• Abdominal discomfort/bloating (mostly due to stimulation itself, not Ganirelix).

• Nausea.

• Rare: hypersensitivity or allergic reaction.

Contraindications

• Known hypersensitivity to Ganirelix, GnRH, or similar peptides.

• Pregnancy or breastfeeding.

• Moderate to severe liver or kidney disease (caution advised).

Ganirelix vs. Cetrorelix

Both are GnRH antagonists and very similar in effect.

• Onset of action: Immediate for both.

• Dosing: Both typically 0.25 mg daily.

• Formulation: Ganirelix is usually supplied in ready-to-use prefilled syringes, whereas Cetrorelix may come as powder (reconstituted) or prefilled syringes depending on the product.

• Protocols: Essentially interchangeable, clinic choice often depends on availability and physician preference.

Clinical Notes

• Ganirelix is considered safe, effective, and patient-friendly.

• Especially preferred for patients at high risk of OHSS (e.g., PCOS, high AMH, young women).

• When a GnRH agonist trigger is used in combination with Ganirelix, the risk of severe OHSS is extremely low.