Ozarelix
A GnRH-blocking decapeptide tried in prostate cancer and enlarged prostate, whose trials were never published
Ozarelix (D-63153) is a ten-residue synthetic peptide that blocks the GnRH receptor and so lowers testosterone. Spectrum Pharmaceuticals ran phase 2 trials in benign prostatic hyperplasia and prostate cancer between 2007 and 2015, but no results were posted or published. What is in print about the molecule is laboratory work: one study on prostate cancer cells and four on how it aggregates and binds polymers.
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What is it?
Ozarelix is a synthetic decapeptide that blocks the receptor for gonadotropin-releasing hormone (GnRH), the brain signal that tells the pituitary to release the hormones driving testosterone production. Its development code was D-63153. Structurally it belongs to the same family as cetrorelix and degarelix: ten residues, several of them unnatural or mirror-image amino acids, an acetylated start and an amidated end. FDA's substance registry gives its sequence as Ac-D-2Nal-D-4Cpa-D-3Pal-Ser-N-Me-Tyr-D-Hci-Nle-Arg-Pro-D-Ala-NH2 and its formula as C72H96ClN17O14, a molecular weight of about 1,459. ChEMBL files it as a GnRH receptor antagonist.
Blocking the GnRH receptor lowers testosterone within days, without the first surge in hormone levels that GnRH agonists such as goserelin cause, which is why antagonists were studied for hormone-sensitive prostate cancer. A second idea of the period was to use partial, temporary hormone suppression to shrink an enlarged prostate in benign prostatic hyperplasia (BPH) and ease urinary symptoms. Ozarelix was taken into both settings by Spectrum Pharmaceuticals.
No published study reports what ozarelix did in people. ClinicalTrials.gov lists three Spectrum trials, all phase 2: two placebo-controlled trials in men with urinary symptoms from BPH (one completed in 2008 with 74 men, one terminated in 2010 after enrolling 28) and an open-label comparison with goserelin in 203 men with prostate cancer that finished in 2015. None posted results to the registry, and no paper reporting any of them appears in PubMed. ChEMBL records a highest phase of 3, but no phase 3 record appears on ClinicalTrials.gov under the name, and the drug was approved nowhere.
What the published literature does hold is laboratory work. One cell study tested ozarelix on prostate cancer cells that no longer respond to hormones. The other four papers are about its physical behaviour: like other antagonists that carry a naphthylalanine residue, it clumps together in water above a certain concentration, and it binds tightly to negatively charged polysaccharides such as alginate and hyaluronate to form fine nanofibres. Those properties matter to formulation scientists, because aggregation shapes how a peptide depot releases its drug after injection. None of them says whether ozarelix helps patients.
At a Glance
Every value in this table is printed from the Grey Peptides dataset, the same record the compound explorer and comparison tool read, so the three can never disagree.
Mechanism of Action
Ozarelix works the way the other peptide GnRH antagonists do, by occupying the receptor rather than switching it on. The steps below are the class mechanism plus what its own cell study added.
No initial hormone surge
Agonists such as goserelin first stimulate the receptor, so testosterone rises before it falls. An antagonist skips that step; the prostate cancer trial's secondary outcomes included a check for any testosterone surge in the first two weeks (NCT01252693).
Self-association in water
Above a threshold concentration ozarelix molecules clump together, and they bind strongly to negatively charged polymers to form nanofibres (PMIDs 20514551, 21999929). Behaviour like this is what lets some antagonists form a slow-release depot under the skin.
Published Research
All five studies below are laboratory work on ozarelix itself, read from their PubMed abstracts in November 2026: one on prostate cancer cells, four on its physical chemistry. There is no human card because no human study of ozarelix has been published; its three registered trials are described under Legal status. Findings in these papers about other antagonists tested alongside it belong to those drugs.
Measuring when it starts to clump in solution
German biochemists used the fluorescence of the peptides themselves, plus a dye that lights up when it slips into aggregates, to find the concentration at which ozarelix and two related antagonists begin to self-associate. A GnRH analogue known not to aggregate served as the control. The point of the work was a practical method for reading off that critical aggregation concentration, and it treated ozarelix as one of the aggregating, naphthylalanine-bearing decapeptides.
Effects on hormone-independent prostate cancer cells
Italian researchers added ozarelix to cultures of DU145 and PC3, two prostate cancer cell lines that grow without androgens and lack a working androgen receptor. It slowed their growth and held more cells at the G2/M stage of division. The cells died by apoptosis through caspase-8 and then caspase-3, with lower levels of the brake protein c-FLIP(L). Ozarelix also raised the death receptors DR4, DR5 and Fas, which made cells that had resisted the death signal TRAIL sensitive to it. The authors called for more work on how antagonists act in such cells; this was a cell culture result only.
Binding alginate and forming nanofibres
Mixing positively charged ozarelix with alginate, a negatively charged seaweed polysaccharide, produced ordered complexes visible to the eye. Electron microscopy showed a branched network of rod-like fibres 4 to 8 nm across, and calorimetry measured a binding constant of around a million per mole, a strong attraction. Light-absorption and circular dichroism readings showed the peptide changing shape on binding, and modelling pointed to new stacking between the aromatic side chains of neighbouring ozarelix molecules. The authors proposed this charge-plus-stacking binding as a way to stretch out release from a hydrogel.
Fibre shape depends on the partner polymer
The same Regensburg group paired ozarelix with three other negatively charged polysaccharides. With carboxymethylcellulose it formed flexible fibres 10 to 16 nm wide and up to a micrometre long; with the large hyaluronate molecule, aggregates longer than 5 micrometres built from 8 nm strands; with branched xanthan gum, tightly coiled fibres 15 to 25 nm wide. Binding was strong with every partner, on the order of a million per mole, though it differed between them, which the authors saw as a possible handle for designing delivery systems.
Aggregation tracked by NMR and computer simulation
A Swedish academic and industry team compared four GnRH-related peptides known to differ in how readily they aggregate, ozarelix among them. Proton NMR in water followed the aggregates as concentration and time changed and showed that acetate counter-ions from the salt were taken into ozarelix's aggregates. Atom-by-atom computer simulations placed the peptides in the same order of aggregation tendency as the NMR did and showed which parts of the molecule drive the clumping. The authors presented the two methods as early screening tools for peptide formulations.
Side Effects & Contraindications
Reported Side Effects
No safety data for ozarelix in people have been published. Its three trials collected adverse events as an outcome, but none posted results.
Contraindications & Cautions
There is no label, so there are no official contraindications. The trial records show who was kept out.
Legal Status
Ozarelix is an investigational drug that was never approved anywhere.
Frequently Asked Questions
?What is ozarelix?
A synthetic ten-residue peptide, code D-63153, that blocks the receptor for gonadotropin-releasing hormone and so lowers testosterone. It belongs to the same family as cetrorelix and degarelix and was tested in benign prostatic hyperplasia and prostate cancer.
?Did it work in its trials?
Nobody outside the sponsor can say. Spectrum Pharmaceuticals registered three phase 2 trials between 2007 and 2010, one of them stopped early, and none has posted results on ClinicalTrials.gov or been published in a journal indexed by PubMed.
?Why are all the studies on this page laboratory studies?
Because that is all that has been published about the molecule itself. One paper tested it on prostate cancer cells and four studied how it aggregates and binds polymers. Human data on other GnRH antagonists belong to those drugs and are on their own pages.
?How is it different from goserelin?
Goserelin is an agonist: it first stimulates the GnRH receptor, causing a brief rise in testosterone, before suppressing it. Ozarelix blocks the receptor directly. Its prostate cancer trial compared the two head to head (NCT01252693), but the results are not public.
?Can anyone get it now?
No. It is approved in no country and no trial is recruiting. Anything sold under the name outside a trial has no established identity, purity or dose.
References
- [pubmed] Schneider A, et al. "Fluorescence spectroscopic determination of the critical aggregation concentration of the GnRH antagonists Cetrorelix, Teverelix and Ozarelix." J Fluoresc, 2010;20(6):1233-40. PMID: 20514551.
- [pubmed] Festuccia C, et al. "Ozarelix, a fourth generation GnRH antagonist, induces apoptosis in hormone refractory androgen receptor negative prostate cancer cells modulating expression and activity of death receptors." Prostate, 2010;70(12):1340-9. PMID: 20623634.
- [pubmed] Ferstl M, et al. "Nanofibers resulting from cooperative electrostatic and hydrophobic interactions between peptides and polyelectrolytes of opposite charge." Langmuir, 2011;27(23):14450-9. PMID: 21999929.
- [pubmed] Ferstl M, et al. "The impact of polyelectrolyte structure on the shape of nanoassemblies with cationic peptides." J Pharm Sci, 2013;102(8):2599-607. PMID: 23728779.
- [pubmed] Hjalte J, et al. "Aggregation Behavior of Structurally Similar Therapeutic Peptides Investigated by (1)H NMR and All-Atom Molecular Dynamics Simulations." Mol Pharm, 2022;19(3):904-917. PMID: 35104408.
- [pubmed] Bayés M, et al. "Gateways to clinical trials." Methods Find Exp Clin Pharmacol, 2007;29(10):697-735. PMID: 18200333.
- [pubmed] Bayes M, et al. "Gateways to clinical trials." Methods Find Exp Clin Pharmacol, 2007;29(8):547-83. PMID: 18040531.
- [registry] ClinicalTrials.gov NCT00427219, read November 4, 2026: 'A Randomized, Double-Blind, Placebo-Controlled Trial of the Safety and Efficacy of Ozarelix, in Patients With Lower Urinary Tract Symptoms (LUTS) Due to Benign Prostatic Hypertrophy (BPH)'; Spectrum Pharmaceuticals, phase 2, 74 enrolled at 13 US sites, January 2007 to February 2008, completed; two intramuscular injections 14 days apart against placebo, primary outcome the change in IPSS at 12 weeks; no results posted. https://clinicaltrials.gov/study/NCT00427219
- [registry] ClinicalTrials.gov NCT00743184, read November 4, 2026: a multicentre, randomised, double-blind, placebo-controlled phase 2 trial of ozarelix for LUTS due to BPH; Spectrum Pharmaceuticals, 28 enrolled, December 2008 to January 2010, terminated; two dose arms and placebo, each course two injections on days 0 and 14, repeated after six months; primary outcome the change in IPSS at week 14; no results posted. https://clinicaltrials.gov/study/NCT00743184
- [registry] ClinicalTrials.gov NCT01252693, read November 4, 2026: 'Phase 2 Study Assessing the Safety and Efficacy of a Monthly Dosing Regimen of Ozarelix Versus Goserelin Depot (Zoladex) in Men With Prostate Cancer'; Spectrum Pharmaceuticals, open-label, randomised 1:1, 203 enrolled, November 2010 to May 2015, completed; subcutaneous ozarelix on days 1, 8, 28 and 56 against goserelin every 28 days; primary outcome the share of men with testosterone at or below 0.5 ng/mL from day 28 to day 84; no results posted. https://clinicaltrials.gov/study/NCT01252693
- [other] US FDA Global Substance Registration System, substance OZARELIX (UNII Q1IF8M2YL3), read November 4, 2026: Ac-D-2-Nal-D-4-Cpa-D-3-Pal-Ser-N-Me-Tyr-D-Hci-Nle-Arg-Pro-D-Ala-NH2, molecular formula C72H96ClN17O14, molecular weight 1459.09; synonyms D-63153 and ozarelix [INN]; codes INN 8668, CAS 295350-45-7, ChEMBL CHEMBL2108312. The acetate salt is UNII 05D1DCL744. https://gsrs.ncats.nih.gov/ginas/app/beta/substances/Q1IF8M2YL3
- [other] ChEMBL molecule CHEMBL2108312 (OZARELIX), read November 4, 2026: synonyms D-63153 and D63153, INN stem -relix, highest clinical phase recorded 3; mechanism record: gonadotropin-releasing hormone receptor antagonist. https://www.ebi.ac.uk/chembl/compound_report_card/CHEMBL2108312/
- [other] US FDA, Drugs@FDA and the drug label endpoint (openFDA), searched November 4, 2026: no application and no label record for ozarelix.
Sources & Citations
The five studies carded above and the two References-only bibliographies were read from their PubMed records on November 4, 2026; they are every PubMed record on ozarelix or D-63153 apart from two plant papers that match the code only as a gene accession. Sequence and formula come from FDA's substance registry; mechanism and phase from ChEMBL; trial details from ClinicalTrials.gov; United States status from Drugs@FDA and the openFDA label endpoint, all read the same day. Grey Peptides sells nothing and links to no vendor.
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