Cetrorelix

Cetrorelix substitutes amino acids at positions 1, 2, 3, 6 and 10 of native GnRH, converting it from an agonist into a pure competitive antagonist. Because it simply occupies the receptor rather than driving and then downregulating it, suppression is immediate and dose-dependent, and it reverses within a day or two of stopping. In IVF this allows stimulation to begin on the woman's own cycle and the antagonist to be introduced only when follicles reach a threshold size, shortening the protocol from weeks to days compared with long agonist downregulation. It also preserves the option of triggering with a GnRH agonist instead of hCG, which is the single most effective way to prevent ovarian hyperstimulation syndrome.

Targets: GnRH receptor (GnRHR), Pituitary gonadotrophs, LH surge suppression

• · 0.25 mg per day into the lower abdomen at roughly the same time each day. This is the dominant IVF protocol worldwide.
• · 3 mg once. Less used than the daily protocol because it is less flexible if the cycle runs long.

Work out your exact syringe units →

• commonInjection-site redness or itching— Usually resolves within an hour.
• commonNausea
• commonHeadache
• uncommonOvarian hyperstimulation syndrome— A risk of the stimulation cycle itself rather than of cetrorelix, and antagonist protocols carry lower OHSS risk than agonist ones.
• rareHypersensitivity reaction— The earlier antagonists in this class had histamine-release problems; cetrorelix much less so, but it is not zero.

• Pregnancy and breastfeeding.
• Known hypersensitivity to cetrorelix, extrinsic peptide hormones or mannitol.
• Severe renal impairment.
• Moderate or severe hepatic impairment.

• synergyfollitropin — The core IVF antagonist protocol - FSH stimulates, cetrorelix prevents a premature LH surge.
• conflictgonadorelin — Direct competitive block of the same receptor.
• synergykisspeptin-54 — Antagonist cycles are exactly where kisspeptin triggering has been trialled.
• redundantleuprolide — Both prevent a premature LH surge in IVF, by opposite mechanisms and on different timelines.

• · Serial follicular ultrasound and serum oestradiol through stimulation.
• · LH to confirm no premature surge.
• · OHSS symptoms after the trigger - abdominal distension, rapid weight gain, breathlessness.

Prescription drug in the US, EU and most jurisdictions for controlled ovarian stimulation.

• Cetrotide (cetrorelix acetate) prescribing information (label)
• Al-Inany et al. 2016, GnRH antagonists for assisted reproductive technology, Cochrane Database of Systematic Reviews (review)

Everything interesting about cetrorelix follows from one fact: it occupies the GnRH receptor without activating it. There is no phospholipase C activation, no calcium mobilisation, no gonadotropin release - and therefore no flare, no two-week testosterone or oestradiol surge, and no three-week wait for suppression. LH and FSH fall within hours because the pituitary is simply no longer receiving its signal, and stored gonadotropin is left where it is rather than being dumped into the circulation. That produces a completely different clinical rhythm from the agonists. Suppression is immediate, dose-dependent and, crucially, promptly reversible: stop the drug and the receptor clears, the endogenous pulse generator is still intact underneath, and gonadotropin secretion resumes within a couple of days. In IVF that reversibility is the entire product. An antagonist protocol starts stimulation with the woman's own cycle, adds cetrorelix only when follicles reach the size at which a premature LH surge becomes a risk, and blocks that surge for the handful of days it matters. Compare that to the leuprolide long protocol, which downregulates the axis for weeks beforehand to achieve the same control, and you can see why antagonist cycles took over: fewer injections, shorter cycles, less gonadotropin, no flare, and lower OHSS rates. The 3 mg single dose and the 0.25 mg daily dose are two different strategies for the same job - one shot covering roughly four days, or daily dosing until the trigger.

The failure that matters is a cycle-level one: start the antagonist too late and a premature LH surge luteinises the follicles before retrieval. That is why monitoring is intensive rather than optional and why this is not a drug anyone should be self-directing. The dose-dependent half-life catches people out too - 5 hours at 0.25 mg but 62.8 hours at 3 mg, which is not a linear scaling and means the two presentations behave like different drugs rather than different amounts of one. Injection-site reactions are common and mild. Hypersensitivity is rare but real and is part of the historical baggage of this class, whose first-generation compounds released histamine. Reconstitution matters: the lyophilised powder should be dissolved gently with the supplied diluent and injected promptly, not stored reconstituted.

• GnRH receptor (GnRHR) on pituitary gonadotrophs — High-affinity competitive antagonism; a specific Kd was not resolved here.

  Competitive blockade with no intrinsic activity. LH and FSH secretion falls within hours, with no flare and prompt reversibility on discontinuation.

• Premature LH surge (the clinical target)

  Prevention of premature luteinisation during controlled ovarian stimulation, which is the single thing the drug is there to do in an IVF cycle.

• Histamine release (formulation-related)

  Early-generation GnRH antagonists were limited by histamine-releasing properties causing systemic reactions. Cetrorelix was specifically engineered to minimise this, and it is largely a historical problem rather than a current one - but it is why the class took decades longer to arrive than the agonists.

• Cochrane review: GnRH antagonists for assisted reproductive technology Systematic review · 2016

  Live birth rate and OHSS incidence with antagonist protocols compared against long GnRH agonist protocols across randomised trials. Antagonist protocols showed a substantially lower incidence of ovarian hyperstimulation syndrome with comparable live birth rates, which is the evidence that shifted routine practice.

LH and FSH begin to fall within a few hours of a dose and suppression is established the same day - there is no flare and no waiting period, which is the whole clinical point. The 0.25 mg dose covers roughly 24 hours and is given daily until the trigger. The 3 mg dose covers about four days. On stopping, pituitary responsiveness returns within a couple of days, which is what allows a GnRH agonist trigger to work at the end of an antagonist cycle. Nothing about this drug operates on a scale longer than days.

Cetrorelix belongs inside a stimulated IVF cycle and nowhere else in practice. The standard combination is recombinant FSH or human menopausal gonadotropin for stimulation, cetrorelix added from around day 5 or 6 of stimulation or when the lead follicle reaches about 14 mm, then a trigger - hCG, a GnRH agonist, or in trial settings kisspeptin-54. The agonist trigger option only works because the antagonist has left the pituitary responsive rather than downregulated, which is a neat piece of protocol design and one of the real advantages of antagonist cycles. Running cetrorelix with a GnRH agonist depot is contradictory. Combining it with gonadorelin or kisspeptin makes no sense, since both are trying to stimulate the receptor cetrorelix is blocking. Outside fertility work, cetrorelix has been investigated in benign prostatic hyperplasia and endometriosis but is not established there.

Against ganirelix, cetrorelix is functionally interchangeable in an IVF cycle, and clinics choose between them on price, supply and presentation rather than on outcomes. The pharmacokinetic differences are real but small: ganirelix has 91 percent bioavailability against cetrorelix's 85 percent and is eliminated predominantly in faeces, while cetrorelix uses the biliary route and offers a 3 mg single-dose presentation that ganirelix does not. Against leuprolide long-protocol downregulation, the antagonist wins on almost every axis that matters to a patient - days instead of weeks, no flare, fewer injections, less gonadotropin, lower OHSS - which is why the Cochrane evidence moved practice. Against degarelix, cetrorelix is the short-acting fertility antagonist and degarelix is the depot oncology antagonist; same receptor logic, entirely different jobs.

$200–$800/month. Cetrotide has typically run roughly 100 to 200 dollars per 0.25 mg vial in the US, and an antagonist cycle uses perhaps five to eight of them, so a few hundred to around eight hundred dollars per stimulated cycle. This is a per-cycle cost rather than a monthly one - there is no chronic use. Figures are indicative and were not verified in this session.

• ?The full amino acid sequence with all unnatural residues was not verified from a primary source in this session, so the sequence fields are left descriptive rather than spelled out.
• ?The volume of distribution is recorded as 70 L, which is the label's stated 1 litre per kilogram applied to a 70 kg reference woman rather than a directly reported absolute value.
• ?Clearance is similarly converted from the label's 1.28 mL/min/kg to a 70 kg basis.
• ?The dose-dependent half-life is unusually wide - 5 hours at 0.25 mg, 62.8 hours at 3 mg, with a reported range up to 179 hours on multiple 0.25 mg dosing - and no single half-life figure describes this drug well.
• ?Live birth and OHSS effect sizes from the Cochrane review were not extracted numerically in this session.
• ?Cost figures are indicative and were not verified in this session.

• Gonadotrophin-releasing hormone antagonists for assisted reproductive technology Al-Inany HG, Youssef MA, Ayeleke RO, Brown J, Lam WS, Broekmans FJ, Cochrane Database of Systematic Reviews, 2016 · PMID 27126581

  The systematic review behind the shift from agonist long protocols to antagonist cycles. If you want to know why nobody downregulates with leuprolide for weeks any more, this is the document.

• CETROTIDE (cetrorelix acetate) for injection - FDA prescribing information DailyMed / FDA

  Source of the 85 percent bioavailability, 1 L/kg volume of distribution, 86 percent protein binding, the dose-dependent half-life from 5 hours at 0.25 mg to 62.8 hours at 3 mg, and the biliary elimination route.