Cetrorelix
Hormonal / FertilityAlso known as: Cetrotide
Mechanism
A GnRH antagonist used in IVF (in vitro fertilization) to prevent premature ovulation. Unlike GnRH agonists which take weeks to suppress hormones, cetrorelix blocks the GnRH receptor immediately — no initial hormone surge or "flare." This allows doctors to precisely control the timing of egg retrieval during fertility treatment.
Technical detail
Synthetic decapeptide GnRH antagonist with D-amino acid substitutions at positions 1, 2, 3, 6, and 10 (Ac-D-Nal-D-Cpa-D-Pal-Ser-Tyr-D-Cit-Leu-Arg-Pro-D-Ala-NH2). Competitively blocks GnRHR binding without activating Gq-coupled signaling — no initial flare effect. Rapid, dose-dependent suppression of LH (onset 1-2 hours, 70% suppression by 4 hours) and FSH. In IVF protocols (GnRH antagonist protocol): initiated on stimulation day 5-6 at 0.25 mg/day SC or single 3 mg dose, continued until hCG trigger. Maintains follicular growth while preventing premature LH surge. Half-life ~30 hours (single dose) to ~20 hours (multiple dose).
Effects
**Endocrine — HPG Axis (Tier 1 — Human Clinical):** Cetrorelix is a third-generation GnRH antagonist that competitively blocks pituitary GnRH receptors, producing immediate and dose-dependent suppression of LH and FSH without any initial stimulatory flare. At the standard IVF dose (0.25 mg/day), it suppresses the endogenous LH surge that would trigger premature ovulation during controlled ovarian stimulation. **Reproductive — IVF (Tier 1 — Human Clinical):** In antagonist IVF protocols, cetrorelix is started on cycle day 5–6 (or when lead follicle reaches 14 mm) and continued daily until hCG trigger. This "flexible" antagonist protocol has become the dominant IVF stimulation approach worldwide, replacing the older long GnRH agonist protocols in many centers. Shorter treatment duration, fewer injections, and dramatically lower OHSS (ovarian hyperstimulation syndrome) risk. **Reproductive — OHSS Prevention (Tier 1 — Human Clinical):** GnRH antagonist protocols enable GnRH agonist trigger (instead of hCG trigger) for final oocyte maturation, which virtually eliminates severe OHSS. This is a paradigm-shifting safety advance in IVF medicine. **Endocrine — Male (Tier 2 — Limited Human Data):** In men, cetrorelix suppresses testosterone within hours to days (no flare). Investigated for prostate cancer and benign prostatic hyperplasia, though degarelix has become the primary GnRH antagonist for prostate cancer.
Practitioner Guide
**APPROVED INDICATION:** • Prevention of premature LH surge in women undergoing controlled ovarian stimulation for IVF (Cetrotide, FDA-approved 2000). **DOSING — IVF PROTOCOLS:** • Fixed protocol: Cetrorelix 0.25 mg SC daily starting on stimulation day 5 or 6, continuing until day of hCG (or GnRH agonist) trigger. • Flexible protocol: Cetrorelix 0.25 mg SC daily starting when lead follicle reaches 14 mm, continuing until trigger day. This is the most commonly used protocol. • Single-dose protocol: Cetrorelix 3 mg SC on stimulation day 7 (provides ~4 days of suppression, may need additional 0.25 mg doses if trigger is delayed). Less commonly used. • Injection: SC in the lower abdomen, around the umbilicus. Rotate sites. **CLINICAL PEARLS — IVF CONTEXT:** • Antagonist vs agonist protocols: The cetrorelix-based antagonist protocol has largely replaced the leuprolide-based agonist long protocol in modern IVF due to: (1) Shorter treatment duration (fewer days of injections), (2) Lower gonadotropin requirements, (3) Dramatically reduced OHSS risk (enables GnRH agonist trigger), (4) Comparable pregnancy rates in most populations. • GnRH agonist trigger: In antagonist protocols, a GnRH agonist (leuprolide 1–2 mg or triptorelin 0.2 mg) can trigger the LH surge for final oocyte maturation instead of hCG. This is only possible when GnRH receptors are NOT downregulated (i.e., in antagonist protocols, not agonist protocols). The agonist trigger eliminates OHSS risk but requires aggressive luteal support or a freeze-all strategy. • High responders (PCOS patients, young patients with high antral follicle counts): ALWAYS use antagonist protocol with agonist trigger option available. The ability to switch to agonist trigger if estradiol rises excessively is the primary safety advantage. • Breakthrough LH surge: Rare (<1%) if cetrorelix is administered consistently. If a dose is missed, premature ovulation can occur. Emphasize daily injection compliance. **NO-FLARE ADVANTAGE (Relevant to Off-Label Contexts):** • Like degarelix, cetrorelix causes NO testosterone/estrogen flare — immediate suppression upon administration. • This property has led to investigation for uterine fibroids (rapid symptom relief without initial worsening), endometriosis, and prostate conditions. • Clinical pearl: In the bodybuilding context, cetrorelix (and GnRH antagonists in general) should NOT be confused with the "triptorelin PCT" concept. Antagonists suppress the HPG axis immediately and persistently — they are functionally castrating agents, not HPTA restarters. Using cetrorelix for PCT would be counterproductive and harmful. **STORAGE:** • Store in refrigerator. Reconstitute with provided diluent immediately before injection. • The reconstituted solution should be used immediately — do not store.
Research Summary
**Tier 1 (Human Clinical Evidence):** • Multiple Phase III trials establish cetrorelix efficacy for premature LH surge prevention in IVF. Pregnancy rates comparable to GnRH agonist long protocols in meta-analyses. • OHSS reduction: Systematic reviews and meta-analyses consistently show lower OHSS rates with antagonist protocols vs agonist protocols (OR 0.43–0.61). • The flexible dosing protocol (starting based on follicle size rather than fixed day) is supported by RCTs showing equivalent outcomes with fewer injection days. **Tier 2 (Strong Preclinical + Mechanistic):** • GnRH receptor competitive antagonism is well-characterized pharmacologically. No receptor downregulation — suppression is maintained only while drug is present, providing a controllable "on/off switch" for HPG axis suppression. • The pharmacological basis for agonist trigger in antagonist protocols (functioning pituitary GnRH receptors that can respond to exogenous GnRH agonist) is robust and clinically validated. **Tier 3 (Emerging / Theoretical):** • Investigation for uterine fibroids and endometriosis as alternatives to GnRH agonists (avoiding flare). • Oral GnRH antagonists (elagolix, relugolix, linzagolix) are increasingly available and may reduce the need for injectable cetrorelix outside of IVF.