Cetrorelix
Cetrorelix acetate, SB-75
Cetrorelix is a synthetic decapeptide GnRH receptor antagonist used in assisted reproduction to prevent a premature luteinising hormone surge during ovarian stimulation. By blocking the receptor rather than desensitising it, it works within hours and can be started partway through stimulation, shortening treatment compared with the older agonist long protocols.
Mechanism
Cetrorelix competitively blocks the pituitary GnRH receptor. Endogenous GnRH is displaced, gonadotroph signalling ceases, and LH and FSH secretion fall within hours, a dose-dependent, immediately reversible effect with no stimulatory phase. This is the key difference from GnRH agonists, which must first overstimulate the pituitary for one to three weeks before suppression is achieved. Structurally, cetrorelix carries five substitutions relative to native GnRH, including three D-amino acids at the N-terminus (3-(2-naphthyl)-D-alanine, 4-chloro-D-phenylalanine and 3-(3-pyridyl)-D-alanine), D-citrulline at position 6 and D-alanine at position 10. The N-terminal cluster abolishes agonist activity while preserving high receptor affinity, and the neutral D-citrulline substitution was chosen to reduce the histamine-releasing potential associated with the strongly basic residues of earlier antagonist generations.
In assisted reproduction the clinical logic follows directly from the speed of onset. Rather than downregulating the pituitary before stimulation begins, cetrorelix can be introduced once follicles reach a size at which a spontaneous LH surge becomes a risk, typically around day 5 or 6 of stimulation, or when the lead follicle reaches a threshold diameter. It holds the surge off until oocytes are mature, at which point ovulation is triggered deliberately. Because the pituitary is never desensitised, it remains responsive to a GnRH agonist trigger, which is the basis of the agonist trigger strategy used to reduce ovarian hyperstimulation risk in high responders, an option structurally unavailable in agonist long protocols. Cetrorelix has also been studied in hormone-dependent conditions including benign prostatic hyperplasia and uterine fibroids, though these applications did not reach registration.
What the research shows
The pivotal European phase 3 trial randomised women undergoing ovarian stimulation with human menopausal gonadotrophin to cetrorelix or a buserelin long protocol. Cetrorelix prevented premature LH surges effectively and produced comparable clinical pregnancy rates while requiring a shorter overall treatment course and less gonadotrophin, the practical advantages that drove its adoption. A companion analysis from the same programme reported a reduction in the incidence of ovarian hyperstimulation syndrome with cetrorelix compared with agonist protocols, which has proved the more clinically important finding.
The definitive synthesis is the Cochrane review of GnRH antagonists for assisted reproductive technology, which pooled a large body of randomised trials comparing antagonist protocols against the agonist long protocol. Its conclusions were nuanced and worth stating precisely: live birth rates were comparable between the two approaches, while ovarian hyperstimulation syndrome, including severe cases, was substantially and consistently reduced with antagonists. Earlier versions of the same review had suggested a small reduction in ongoing pregnancy rate with antagonists, a signal that attenuated as more trials accumulated and as clinicians became more practised with the protocols. The honest reading is that antagonists are not more effective than agonist long protocols at producing babies; their advantage is that they achieve the same result with fewer injections, less gonadotrophin, a shorter cycle and markedly less risk of a potentially dangerous complication. That is a real and meaningful benefit, but it is a safety and convenience benefit rather than an efficacy one.
Evidence assessment
High-quality evidence
Cetrorelix holds EU approval (1999) and FDA approval (2000) with regulator-reviewed labelling, granted on the basis of prospective randomised phase 3 trials against the then-standard agonist long protocol. The class has since been evaluated in an updated Cochrane systematic review pooling many randomised trials, which found comparable live birth rates with substantially reduced ovarian hyperstimulation syndrome. Replicated randomised evidence plus approved labelling meets the strong criterion. AUDIT NOTE: all three PMIDs verified against PubMed. The stated half-life was corrected, and the alias "NS-75A" was removed because it could not be confirmed.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Ovarian stimulation with HMG: results of a prospective randomized phase III European study comparing the luteinizing hormone-releasing hormone (LHRH)-antagonist cetrorelix and the LHRH-agonist buserelin Preclinical only
Cetrorelix effectively prevented premature LH surges with comparable clinical pregnancy rates, while requiring a shorter treatment course and less gonadotrophin than the agonist long protocol.
Significant reduction of the incidence of ovarian hyperstimulation syndrome (OHSS) by using the LHRH antagonist Cetrorelix (Cetrotide) in controlled ovarian stimulation for assisted reproduction Preclinical only
Cetrorelix protocols were associated with a lower reported incidence of ovarian hyperstimulation syndrome than agonist long protocols, the finding that has proved most clinically consequential for antagonist adoption.
Gonadotrophin-releasing hormone antagonists for assisted reproductive technology Preclinical only
Live birth rates were comparable between antagonist and agonist long protocols, while ovarian hyperstimulation syndrome including severe cases was substantially reduced with antagonists. The advantage is one of safety and convenience rather than efficacy.
Safety
Cetrorelix is generally well tolerated. The commonest adverse effects are local injection site reactions (redness, itching and swelling), which are usually mild and transient. Nausea and headache occur. Hypersensitivity reactions including rare anaphylactoid events have been reported; the D-citrulline substitution was introduced to reduce the histamine release that limited earlier antagonists, but the risk is diminished rather than eliminated. The most important safety consideration is contextual rather than intrinsic: cetrorelix is used within ovarian stimulation, and ovarian hyperstimulation syndrome remains the principal hazard of that process. Antagonist protocols reduce this risk substantially relative to agonist long protocols but do not abolish it, and severe hyperstimulation can cause ascites, haemoconcentration, thromboembolism and rarely death. Cetrorelix is contraindicated in pregnancy and lactation, in known hypersensitivity to GnRH or its analogues or to any excipient, and in moderate to severe renal or hepatic impairment. It should be used only by clinicians experienced in assisted reproduction, where cycle monitoring allows the timing of administration and of the ovulation trigger to be individualised.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | Licensed prescription-only medicine (POM), originally authorised through the European Medicines Agency in 1999 and now regulated in the United Kingdom by the MHRA, for prevention of premature ovulation in controlled ovarian stimulation. The UK medicines compendium currently lists only the 0.25 mg powder-and-solvent presentation. |
| United States | Approved by the FDA in 2000. Prescription-only, licensed for the inhibition of premature luteinising hormone surges in women undergoing controlled ovarian stimulation. |
| WADA (sport) | Not listed on the WADA Prohibited List. As a GnRH antagonist it suppresses rather than stimulates gonadotrophin secretion and so falls outside the section S2 prohibition covering GnRH agonist analogues. |
Questions
Cetrorelix blocks the GnRH receptor directly, so LH suppression begins within hours and can be started partway through stimulation. Agonists must first overstimulate the pituitary for one to three weeks before suppression takes hold. The practical result is a shorter cycle with fewer injections and less gonadotrophin.
No, and this is worth being clear about. The Cochrane review found live birth rates comparable between antagonist and agonist long protocols. The genuine advantage of cetrorelix is a substantial reduction in ovarian hyperstimulation syndrome, plus a shorter and simpler cycle. That is a safety and convenience benefit, not an efficacy one.
Because the pituitary is blocked rather than desensitised, it remains capable of responding to stimulation. A GnRH agonist can therefore be used to trigger the final LH surge, which produces a shorter-lived surge than the conventional trigger and markedly reduces hyperstimulation risk in high responders. This option does not exist in agonist long protocols, where the pituitary is already downregulated.
Published half-life figures for cetrorelix are inconsistent between sources, so this should be treated with caution. The European product information gives a mean terminal half-life of about 30 hours after subcutaneous injection of the 0.25 mg presentation and about 12 hours intravenously, while some other sources quote considerably shorter values. In practice the 0.25 mg presentation is given daily during the relevant window of stimulation.