Cetrorelix

Cetrorelix is a synthetic decapeptide GnRH receptor antagonist, substantially more heavily modified from native GnRH than the agonist analogs profiled elsewhere in this catalog. While Triptorelin, Alarelin, and Leuprolide Acetate each carry a single modified position (position 6, plus a C-terminal ethylamide modification), Cetrorelix carries substitutions at five positions: 1, 2, 3, 6, and 10, using several non-standard amino acids including D-2-naphthylalanine, D-4-chlorophenylalanine, D-3-pyridylalanine, and D-citrulline.

At Verified Peptides, we want to directly flag the mechanistic category this compound belongs to, since it is easy to lose track of within a broader 'GnRH analog' framing: Cetrorelix is an ANTAGONIST, not an agonist. This is a fundamentally different pharmacological mechanism from Gonadorelin, Triptorelin, Alarelin, and Leuprolide Acetate (all agonists, also built on this site), despite all of these compounds targeting the same GnRH receptor and all being loosely grouped under 'GnRH analog' terminology in casual usage.

Chemically, Cetrorelix (free base) has molecular formula C70H92ClN17O14, molecular weight approximately 1431.06 g/mol, CAS number 120287-85-6 (free base) or 145672-81-7 (acetate salt, the commercially standard form), PubChem CID 16130924. It was developed originally by ASTA Medica and later Serono/EMD Serono, and is marketed under the brand name Cetrotide, with generic versions FDA-approved starting in 2022.

The non-standard amino acids used in Cetrorelix's sequence each serve a specific structural role: D-2-naphthylalanine and D-4-chlorophenylalanine at positions 1 and 2 are bulky, hydrophobic aromatic residues that occupy the receptor-binding pocket without triggering the conformational shift needed for activation; D-3-pyridylalanine at position 3 was specifically developed to replace earlier antagonist candidates' D-histidine or other residues that had been associated with histamine-release side effects; and D-citrulline at position 6 (replacing the native glycine or the D-amino acids used in agonist analogs) further stabilizes the antagonist conformation. All positions use D-amino acids or D-amino-acid-like structures at the modified sites, a pattern shared with the agonist analogs' D-amino-acid position-6 substitutions, though serving an antagonistic rather than agonistic structural purpose in Cetrorelix's case.

At Verified Peptides, we think Cetrorelix has a substantial, genuine clinical evidence base supporting its FDA-approved indication. The key pivotal evidence comes from Olivennes, Diedrich, and colleagues (published in Human Reproduction, PMID 12831588), reporting two large multicentre, multinational Phase IIIb clinical experiences comparing a multiple-dose protocol (0.25 mg/day, n=1,066) against a single-dose protocol (3 mg, n=541) — a combined sample of over 1,600 patients. Over 90% of patients in both groups reached criteria for hCG administration and underwent oocyte retrieval, with embryo transfer performed in 83-84% of cycles and pregnancy rates per embryo transfer of 27% (multiple-dose) and 28% (single-dose). Severe ovarian hyperstimulation syndrome occurred in fewer than 1% of cycles in both groups, and injection-site reactions (12% multiple-dose, 8% single-dose) were common but never serious or treatment-discontinuing.

We also want to note real comparative research directly relevant to understanding Cetrorelix relative to other GnRH antagonists: a randomized clinical trial found no significant difference between cetrorelix and ganirelix (also built elsewhere in this phase) in preventing LH surge or in IVF outcomes and pregnancy rates, though cetrorelix required significantly fewer injections per patient (median of 1) compared to ganirelix (median of 4) in that specific comparison, reflecting differences in typical dosing protocol between the two antagonists rather than necessarily a difference in fundamental receptor-blocking potency. We also identified a separate comparative study examining cetrorelix combined with mid-cycle recombinant LH against a leuprolide-based comparison protocol for inhibiting premature LH surges in assisted reproduction, illustrating that Cetrorelix has also been studied in combination with other reproductive hormone therapies beyond its standalone antagonist use, reflecting ongoing research interest in optimizing controlled ovarian stimulation protocols using antagonist-based approaches alongside other supportive hormonal interventions.

At Verified Peptides, we sell research-grade Cetrorelix exclusively for laboratory research, and we think this compound is a useful anchor point for understanding the GnRH antagonist mechanistic class as mechanistically distinct from, though clinically complementary to, the GnRH agonist class represented by several other compounds in this catalog.

Cetrorelix was among the first GnRH antagonists to reach clinical use, historically representing a significant pharmaceutical chemistry achievement: early GnRH antagonist research in the 1970s-1980s was hampered by compounds that, despite blocking the receptor, also triggered histamine release and allergic-type reactions due to their chemical structure — a problem that took years of medicinal chemistry refinement (including the specific non-standard amino acid substitutions used in Cetrorelix) to substantially resolve before a clinically usable GnRH antagonist could be developed.

The five-position substitution pattern in Cetrorelix (versus the single-position substitution used in agonist analogs like Triptorelin, Alarelin, and Leuprolide) reflects a broader structure-activity principle in GnRH receptor pharmacology: converting an agonist into an antagonist at this receptor required substantially more extensive modification of the N-terminal region (positions 1-3) specifically, since this region is critical for the conformational change that triggers receptor activation, not merely for receptor binding itself.

Because GnRH antagonists like Cetrorelix act immediately without requiring the 1-3 week downregulation period needed by agonists, they are sometimes described in reproductive endocrinology as enabling 'antagonist protocols' that are shorter in overall treatment duration than traditional 'long agonist protocols' for IVF ovarian stimulation, a treatment-burden consideration independent of the two approaches' comparable efficacy and safety profiles.

Cetrorelix's development history also intersects with a broader research area examining GnRH receptor expression outside the classic hypothalamic-pituitary axis: GnRH receptors have been identified on some non-pituitary tissues, including certain reproductive tract and, in some research, tumor tissues, motivating separate investigational research into GnRH antagonists (including Cetrorelix specifically) for potential direct anti-proliferative effects independent of pituitary-mediated hormone suppression, an active but distinct research area from its established ovarian-stimulation indication.

At Verified Peptides, we require independent third-party HPLC purity testing and mass spectrometry identity confirmation for every batch of research-grade Cetrorelix we offer, with a Certificate of Analysis (COA) available for each lot. Given Cetrorelix's five distinct modified positions and several non-standard amino acids, identity confirmation is particularly important, since incomplete or incorrect synthesis at any one of these positions could yield a related but functionally different molecule.

We source exclusively from manufacturers operating under Good Manufacturing Practice (GMP)-aligned quality systems, and every lot is independently verified rather than accepted solely on a supplier's internal documentation. Researchers should always request and review the specific COA for the lot they receive, confirming purity percentage and molecular identity before use in any experimental protocol. Researchers should also confirm whether a given lot is the free base (CAS 120287-85-6) or the acetate salt (CAS 145672-81-7), since both forms circulate under the general 'cetrorelix acetate' commercial name despite being distinct chemical registrations.

Lyophilized Cetrorelix is stable when stored at -20°C, protected from light and moisture. At Verified Peptides, we ship lyophilized peptide with appropriate cold-chain packaging to preserve stability in transit.

Once reconstituted, solution should be stored refrigerated at 2–8°C and used within the timeframe indicated on the product's documentation, avoiding repeated freeze-thaw cycles, consistent with standard handling practices for the other peptides in this catalog. Given Cetrorelix's more complex, multiply-modified structure, researchers should also be attentive to solubility characteristics during reconstitution, since the bulky aromatic and non-standard side chains in this compound's modified positions can affect aqueous solubility differently than the simpler single-substitution GnRH agonist analogs elsewhere in this catalog.