Ganirelix

A synthetic decapeptide GnRH receptor antagonist, FDA-approved in 1999 as Antagon and now sold as ganirelix acetate injection and Fyremadel, used in prefilled syringes to prevent premature LH surges during controlled ovarian stimulation.

Also referenced as: Ganirelix acetate, Ganirelix diacetate, Antagon, Orgalutran, Fyremadel, Org 37462, RS-26306

Also appears in: Other

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What is Ganirelix?

Ganirelix is a synthetic decapeptide antagonist of the gonadotropin-releasing hormone (GnRH) receptor, supplied as the acetate salt. It is native GnRH with six of ten positions replaced — N-Ac-D-Nal(2) at 1, D-Phe(4Cl) at 2, D-Pal(3) at 3, D-hArg(Et2) at 6, L-hArg(Et2) at 8, and D-Ala at 10 — with a molecular weight of 1570.4 as the anhydrous free base. The two N,N’-diethyl-homoarginine residues at positions 6 and 8 are the molecule’s signature: they were engineered at Syntex specifically to keep the potency that basic residues confer on GnRH antagonists while blocking the mast-cell degranulation that had sidelined earlier compounds in the class (Nestor et al., Journal of Medicinal Chemistry, 1992;35(21):3942–3948).

It was the first of the “third-generation” GnRH antagonists to reach the U.S. market, approved on July 29, 1999 as Antagon, and it has one indication: inhibition of premature luteinizing hormone surges in women undergoing controlled ovarian hyperstimulation. In the clinic it is a short-course, daily subcutaneous injection from a prefilled syringe, used for a short daily course during stimulation. It appears in research-compound catalogs for the same reason cetrorelix does — it is the receptor-blocking counterpart to the GnRH agonists already circulating there — and because the licensed product is a prescription fertility drug.

How it works

• Competitive blockade of pituitary GnRH receptors, with no flare. Ganirelix occupies the GnRH receptor on gonadotrope cells without activating it, so LH and FSH secretion falls within hours rather than after the days of receptor downregulation a GnRH agonist requires. In the first human study, single subcutaneous doses of 1, 3, and 6 mg of the compound then coded RS-26306 lowered serum LH in five postmenopausal women by up to 70.1 ± 3.6% and FSH by 42.3 ± 2.5% at the top dose, also lowered testosterone, and produced no systemic side effects, with a terminal half-life of 22.8 to 26.9 hours at those multi-milligram doses (Rabinovici et al., Journal of Clinical Endocrinology and Metabolism, 1992;75(5):1220–1225).
• Suppression is occupancy-driven and reverses within days. At the approved 250 mcg daily dose, serum LH is maximally reduced by 74% about four hours after an injection and FSH by 32%, estradiol falls by about 25%, and all three return to pretreatment values within two days of the last dose. Peak concentrations arrive about one hour after injection, steady state within two to three days, and exposure is dose-proportional between 125 and 500 mcg (Oberyé et al., Fertility and Sterility, 1999;72(6):1006–1012). The approved labeling gives an absolute bioavailability of 91.1%, an elimination half-life of 12.8 hours after a single dose and 16.2 hours on repeated dosing, 81.9% plasma protein binding, and excretion mainly in feces (75.1%) as the 1–4 and 1–6 peptide fragments, with 22.1% in urine.
• The LH effect is dose-separable from the ovarian effect, and more is not better. In the six-arm dose-finding trial, a serum LH of 10 IU/L or more during treatment was seen in 16% of women at 62.5 mcg, 9% at 125 mcg, and 1.4% at 250 mcg, and estradiol on the day of hCG fell steadily with dose, from 1,475 pg/mL in the lowest arm to 430 pg/mL at 2 mg — but implantation and pregnancy collapsed at the higher doses. Implantation was 21.9% at 250 mcg and 1.5% at 2 mg, early miscarriage ran 11.9% and 13% in the 1 mg and 2 mg arms against 0 to 3.7% in the lower arms, and vital pregnancy per attempt peaked at 36.8% in the 250 mcg arm (33.8% ongoing at 12 to 16 weeks); the lowest and highest arms were stopped early on the advice of an external independent advisory committee (Ganirelix Dose-Finding Study Group, Human Reproduction, 1998;13(11):3023–3031). That is why the approved strength is a single 250 mcg dose rather than a range.
• Leaving the pituitary responsive is what makes an agonist trigger possible. Because ganirelix blocks the receptor without desensitizing it, a GnRH agonist bolus can still elicit an endogenous LH surge for final oocyte maturation. The first report of this approach was eight ganirelix-treated women at high OHSS risk given 0.2 mg triptorelin, who produced a median LH peak of 219 IU/L four hours later, a mean of 23.4 oocytes, and no OHSS (Itskovitz-Eldor et al., Human Reproduction, 2000;15(9):1965–1968). The strategy, its OHSS benefit, and its fresh-cycle live-birth cost are covered on the cetrorelix profile; the physiology is the same for both drugs.

Research status

Ganirelix has a deep randomized record in exactly one setting and little else.

Approval history. The FDA approved NDA 021057 on July 29, 1999 as a Type 1 new molecular entity under priority review, under the brand name Antagon; the EU authorized the same product as Orgalutran on May 17, 2000. The Antagon brand name was later dropped in the United States, and the originator product has since been sold under its generic name, Ganirelix Acetate Injection, with the current Organon label revised in September 2026. Generic competition arrived through ANDA 204246 (Sun Pharmaceutical, approved November 30, 2018), distributed by Ferring under the brand name Fyremadel — a generic carrying a trade name, which is a frequent source of confusion. Five further abbreviated applications followed: Amphastar (April 7, 2022), Meitheal (June 6, 2022), Gland (February 28, 2023), Lupin (November 16, 2023), and Qilu (April 23, 2025). The indication has never widened.

Pivotal trials. Two open-label phase III trials randomized ganirelix 2:1 against a long agonist protocol. The European and Middle East Orgalutran Study Group assigned 236 women to ganirelix and 119 to triptorelin: the ganirelix regimen was 17 days shorter (9 versus 26 days), used 450 IU less recombinant FSH (median 1,350 versus 1,800 IU), retrieved fewer oocytes (7.9 versus 9.6), produced an identical implantation rate (22.9%), and an ongoing pregnancy rate per attempt of 31.0% versus 33.9%; local skin reactions were half as common (11.9% versus 24.1%) (Human Reproduction, 2001;16(4):644–651). The North American Ganirelix Study Group randomized 313 women against leuprolide: 11.6 versus 14.1 oocytes, implantation 21.1% versus 26.1%, and ongoing pregnancy per attempt 30.8% versus 36.4% (Fluker et al., Fertility and Sterility, 2001;75(1):38–45). Neither trial was powered to show equivalence in pregnancy, and the numerical gap in the North American study is part of why the antagonist protocol was received cautiously in its first decade. In the 463-patient open-label ganirelix study described in the U.S. label, a premature LH surge with progesterone rise occurred in fewer than 1% of subjects.

The class-level evidence. The 2016 Cochrane review pooled 73 randomized trials and 12,212 participants comparing antagonist protocols with the long agonist protocol and found no conclusive live birth difference (OR 1.02, 95% CI 0.85 to 1.23; 12 RCTs, n = 2,303) alongside less OHSS of any grade (OR 0.61, 95% CI 0.51 to 0.72; 36 RCTs, n = 7,944) (Al-Inany et al., Cochrane Database of Systematic Reviews, 2016;4(4):CD001750). The review pools ganirelix and cetrorelix trials together. Head-to-head data between the two are thin: a 185-patient U.S. randomized trial of single-dose 3 mg cetrorelix versus daily 0.25 mg ganirelix in a flexible, pill-programmed protocol found no premature LH surge in either arm and no difference in any IVF or pregnancy outcome, only fewer injections with cetrorelix (Wilcox et al., Fertility and Sterility, 2005;84(1):108–117), and a 2023 systematic review and network meta-analysis covering 75 unique antagonist-protocol RCTs concluded there were insufficient data to compare the two molecules for ongoing pregnancy at all (Venetis et al., Human Reproduction Update, 2023;29(3):307–326).

Long-term safety follow-up. Pooled phase II and III follow-up covered 340 ongoing pregnancies after ganirelix and 134 after an agonist, and 432 versus 184 children. Congenital malformations were recorded in 32 of 424 fetuses (7.5%) in the ganirelix group and 10 of 181 (5.5%) in the agonist group; under a broad major-malformation definition the rates were 4.5% versus 3.3% (OR 1.37, 95% CI 0.54 to 3.48), a difference the authors judged consistent with chance in a small denominator (Boerrigter et al., Human Reproduction, 2002;17(8):2027–2034). A later single-center follow-up of 1,000 fetuses conceived after ganirelix, against a historical buserelin cohort, found major malformations in 5.0% versus 5.4% (OR 0.94, 95% CI 0.62 to 1.42) (Bonduelle et al., Human Reproduction, 2010;25(6):1433–1440); current labeling cites that observational comparison in more than 1,000 newborns as showing anomaly rates comparable to agonist cycles.

Repurposing within reproduction. A Greek group has used 0.25 mg daily antagonist in the luteal phase to manage established severe early OHSS as outpatients: 40 of 353 high-risk patients developed severe early OHSS, and none required hospitalization after luteal antagonist plus freeze-all (Lainas et al., Reproductive Biology and Endocrinology, 2012;10:69); a later cohort that proceeded to fresh transfer reported live birth of 40.9% in 22 ganirelix-treated women versus 43.6% in 172 controls (Lainas et al., Human Reproduction, 2013;28(7):1929–1942). These are single-center observational series, not randomized trials, and the use is off-label.

Exploratory work outside IVF that never advanced. An open-label study gave 2 mg daily to 20 premenopausal women with symptomatic fibroids awaiting surgery (19 evaluable); median time to maximal shrinkage was 19 days, with leiomyoma volume down 42.7% by ultrasound (29.2% by MRI), and 8 of 19 reported hypoestrogenic symptoms (Flierman et al., BJOG, 2005;112(5):638–642). No controlled trial followed. In polycystic ovary syndrome, seven days of 0.125 to 0.5 mg normalized elevated LH in 24 women but did not lower androgens or restore follicle development (Hohmann et al., Journal of Endocrinological Investigation, 2005;28(11):990–997). An intranasal formulation suppressed LH by 62 to 74% in a 14-woman dose-escalation study and was never developed further (Fujimoto et al., Fertility and Sterility, 1997;67(3):469–473). No long-acting depot of ganirelix was ever brought forward — the fibroid authors themselves wrote that one would be needed — which is part of why it never followed cetrorelix into BPH or oncology programs.

Where the field is now. ClinicalTrials.gov lists 105 studies with ganirelix as an intervention (October 2026): 56 return under infertility, 6 under PCOS, 2 each under OHSS and endometriosis, and none in prostate disease. A meaningful fraction are not drug trials at all but physiology studies that use ganirelix as a reversible “pituitary clamp” to study LH–testosterone dynamics in men (for example Takahashi et al., Journal of Clinical Endocrinology and Metabolism, 2007;92(9):3626–3632). The one genuinely new direction is sub-therapeutic dosing in PCOS: a Lille phase I study (NCT05751252, 20 women enrolled, completed February 2025) gave single 0.025 or 0.0625 mg doses and, in the 18 women analyzed (8 and 10 per dose), reported reduced LH pulse frequency, a lower LH/FSH ratio, and a 25 to 30% fall in androstenedione with unchanged estradiol, alongside restored cyclicity and ovulation in a PCOS-like mouse model (Cotellessa et al., EBioMedicine, 2026;130:106398). The authors propose phase II trials; none has reported, and the senior author is an inventor on a patent covering the approach.

Common dosage forms

• Prescription prefilled syringe, 250 mcg/0.5 mL — a disposable, single-dose 1 mL glass syringe of colorless aqueous solution (glacial acetic acid, 23.5 mg mannitol, water for injection, pH 5.0), with a staked 27-gauge × ½-inch needle under a rigid needle shield, for subcutaneous use only. This is the only strength and presentation approved anywhere; there is no 3 mg single-dose form analogous to cetrorelix’s.
• Originator product — Ganirelix Acetate Injection (Organon), the product first approved as Antagon; its labeling states the needle shield is not made with natural rubber latex.
• Fyremadel — the Sun Pharmaceutical generic distributed by Ferring, bioequivalent and at the same strength, but with labeling stating that its rigid needle shield may contain dry natural rubber latex that comes into contact with the product; latex hypersensitivity is folded into its hypersensitivity contraindication.
• Generic ganirelix acetate injection — five further ANDA products at 250 mcg/0.5 mL, all prefilled syringes.
• EU presentation — Orgalutran, 0.25 mg in a prefilled syringe, the same strength and route.
• Research-market vials — lyophilized powder for reconstitution, with the labeled quantity set by the supplier rather than by the 250 mcg unit of the licensed product, and without the ready-to-use solution, matched needle, or release testing of the pharmaceutical presentation.

This section describes formats only and is not dosing guidance.

Key considerations

• This is a licensed prescription drug with a single, narrow indication. Ganirelix has been FDA-approved since 1999 and generic since 2018; its FDA Approved status reflects that an approved drug product containing the molecule exists. Labeling directs that it be prescribed by physicians experienced in infertility treatment, requires that pregnancy be excluded before starting, contraindicates known or suspected pregnancy and hypersensitivity to ganirelix, to GnRH, or to any GnRH analog, and states it should not be used by lactating women; pharmacokinetics have not been characterized in pediatric, geriatric, renally impaired, or hepatically impaired patients. Research-market supply sits entirely outside that framework.
• The mechanism runs opposite to most hormone peptides sold alongside it. Gonadorelin is native GnRH and triptorelin and leuprolide are agonists; all activate the receptor ganirelix blocks. Ganirelix lowers LH by about 74% within hours of a single 250 mcg dose, and multi-milligram doses suppressed testosterone in the earliest human studies. Combining it with an agonist, or treating it as a “milder” one, inverts the intended endocrine effect.
• The dose-response is non-monotonic in the only outcome that matters. Higher doses suppressed LH and estradiol more completely and cut implantation to near zero in the dose-finding trial. The 250 mcg strength is a trade-off chosen from randomized data, not a floor, and this is the clearest documented case in the class of over-suppression harming the result it was meant to protect.
• Naming is crowded, and one of the “generics” has a brand name. Antagon (U.S., discontinued), Orgalutran (EU), Ganirelix Acetate Injection (current U.S. originator), and Fyremadel (a Sun/Ferring generic) are all the same molecule at the same strength; Org 37462 and RS-26306 are its development codes. Cetrorelix (Cetrotide) is the other injectable peptide GnRH antagonist approved for the same IVF indication; it differs at position 6 (D-citrulline instead of D-diethyl-homoarginine) and position 8 (native arginine instead of L-diethyl-homoarginine), has a 3 mg single-dose option that ganirelix lacks, and is supplied as a powder for reconstitution rather than a solution. Degarelix (Firmagon, approved December 24, 2008) is a different decapeptide antagonist approved for advanced prostate cancer as a monthly depot and has no fertility indication. Relugolix and elagolix are oral small-molecule antagonists, not peptides. “GnRH analog” covers agonists and antagonists together and hides exactly the distinction that matters.
• Labeled safety signals, with an attribution caveat. In the 794-patient controlled-trial safety population, gynecological abdominal pain was reported in 4.8%, fetal death in 3.7%, headache in 3.0%, OHSS in 2.4%, vaginal bleeding in 1.8%, and injection-site reactions and nausea in 1.1% each; a neutrophil count at or above 8.3 × 10⁹/L was seen in 11.9%, of undetermined significance. Post-marketing reports include anaphylaxis, anaphylactic shock, angioedema, and urticaria, some with the first dose, and labeling advises against use in women with severe allergic conditions. “Fetal death” and OHSS in that table are outcomes of the IVF cycle ganirelix accompanies rather than effects attributed to the drug; the class-level data show antagonist protocols lowering OHSS risk, not raising it.