Avorelin
A potent, long-acting LHRH agonist built into a depot that held testosterone at castrate levels for six months from one injection — but was not taken to approval
Avorelin, also called meterelin, is a nonapeptide copy of the brain hormone LHRH (also called GnRH), changed to bind its receptor far more strongly and resist breakdown. Like the other LHRH agonists used in prostate cancer, it first stimulates and then exhausts the signal that drives testosterone, so after an early flare the body's testosterone falls to the low levels used to slow hormone-driven prostate cancer. Its distinctive feature was a slow-release depot: in a phase 2 trial a single subcutaneous injection kept testosterone suppressed for about forty weeks, raising the prospect of dosing just twice a year. It reached phase 2 and was not developed to approval; it is not a marketed medicine.
Used in human trials: 10–15 mg, single long-acting depot, subcutaneous (Studied dose) Details ↓
Recommended Dosing
Used in human trials
Typical range10–15 mg · single long-acting depot Human studyPMID 10569560compare across peptides →
No manufacturer dose exists for this use: these are amounts given in published trials, not recommendations.
What is it?
Prostate cancer is usually driven by testosterone, so a mainstay of treating advanced disease is to shut testosterone down. The body makes it on a chain of command: the brain releases LHRH in pulses, the pituitary answers with luteinizing hormone, and the testes answer with testosterone. Drugs that copy LHRH but bind harder, the LHRH agonists, jam that chain. Given steadily rather than in pulses they first over-stimulate the pituitary (a testosterone 'flare' in the first week) and then switch it off, so testosterone drops to the low, 'castrate' range within a few weeks. Leuprolide and goserelin are the familiar examples; avorelin is another.
What set avorelin apart was not the molecule alone but its formulation. Its developers built it into a subcutaneous depot designed to release the peptide slowly over many months. In the phase 2 trial, a single 10 or 15 milligram depot drove testosterone into the castrate range within four weeks and held it there for about forty weeks, with blood levels of the drug proportional to the dose. That pointed to a six-month injection, longer than the one- and three-month depots common at the time.
The molecule itself is a nonapeptide, the LHRH sequence with two changes that are standard for this class: a modified, unnatural tryptophan at position six that makes it bind harder and resist enzymes, and the removal of the last residue with an ethylamide cap in its place. Those changes are why it acts as a superagonist and why it lasts. A radioimmunoassay developed in dogs documented its pharmacokinetics after injection and from slow-release implants.
Avorelin also appears in a quite different literature. Male primates, like male humans, have a surge of testosterone in the first months of life, and researchers used avorelin (under its older name meterelin) to block that surge in infant rhesus monkeys and ask what the surge does. Suppressing it altered later social behaviour and the development of the external genitalia, work that used the drug as a tool to study development rather than to treat anything.
Avorelin was never approved and its clinical development did not continue past these phase 2 studies; the published record ends around 2000. The approved long-acting LHRH agonists that followed, and six-month depots of leuprolide, filled the role avorelin's depot was aimed at. It is of historical and research interest, is not a medicine, and nothing here is a dose to use.
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.
Educational information, not medical advice. The doses on this page come from approved labels, published studies and, where no study exists, amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
Mechanism of Action
Avorelin is a stronger, longer-lasting copy of LHRH. Given steadily it drives the testosterone-making chain into shutdown, the same paradoxical route the other LHRH agonists take.
A depot that lasts months
Formulated as a slow-release subcutaneous depot, one injection held testosterone suppressed for about forty weeks in the phase 2 trial (PMID 10569560), the feature that distinguished it from shorter LHRH-agonist depots.
Published Research
These are the studies of avorelin read from their PubMed abstracts on November 5, 2026. The clinical record is a single phase 2 depot study in prostate cancer; the rest is a dog pharmacokinetic study and a set of primate-development experiments that used the drug (under its older name meterelin) to block the infant testosterone surge. Three of those titles name no drug and are carded because their abstracts name it.
Phase 2 depot in prostate cancer
Sixty men with prostate cancer were randomised to a single subcutaneous depot of avorelin at 10 milligrams (31 men) or 15 milligrams (29 men). After the expected first-week flare, testosterone fell to castrate levels (below 1.735 nmol/L) within four weeks and stayed suppressed for a median of about 40 weeks in the 10 mg group and 39 weeks in the 15 mg group; LH fell in parallel and FSH less so. Prostate-specific antigen normalised by six months in 80 and 88 percent of the two groups. Side effects were those of low testosterone (hot flushes, reduced libido, impotence) or of the disease; the depot was well tolerated locally. The authors judged it could be the first LHRH-agonist depot given at six-month intervals.
Pharmacokinetics in dogs
A radioimmunoassay specific for meterelin was developed and used to follow the drug in dogs after injection into the vein and under the skin, and after its slow release from lactide-glycolide implants. The study documented the long-acting pharmacokinetics that the depot formulation was built to exploit, and confirmed the molecule's identity as the LHRH agonist [2-Me-D-Trp6, DesGly10]LHRH ethylamide.
Blocking the infant testosterone surge in monkeys
Male rhesus monkeys were given the LHRH agonist (then called meterelin) during their first six months to block the normal early-life surge of testosterone, and their social behaviour was compared as yearlings with untreated males and females. The study used the drug as a tool to ask whether the early testosterone surge shapes later sex differences in behaviour.
Postnatal testosterone and infant behaviour
A further rhesus-monkey study in which male infants were treated with avorelin from the first week of life to suppress the postnatal testosterone surge, testing its role in the development of sex differences in behaviour. An investigative use of the drug's testosterone-blocking action, not a treatment.
Postnatal testosterone and genital development
Infant rhesus monkeys were treated with avorelin to manipulate circulating testosterone in the first six months of life, and the development of the external genitalia was measured. Blocking the testosterone surge affected phallic and clitoral development, showing the surge's role in shaping the genitalia — again a tool use of the drug in a developmental experiment.
Side Effects & Contraindications
Reported Side Effects
From the single phase 2 trial in prostate cancer; there is no approved-label safety profile.
Contraindications & Cautions
Avorelin has no label and no approved use. These points come from the trial and from the drug class.
Legal Status
Avorelin is an investigational compound that was never approved in any country and is not marketed.
Frequently Asked Questions
?What is avorelin?
An experimental drug, also called meterelin, that copies the brain hormone LHRH (GnRH) but binds its receptor much more strongly. Given steadily it shuts down testosterone, which is why it was tested as a long-acting depot for prostate cancer.
?Was it ever approved?
No. Avorelin reached phase 2 testing (ChEMBL records phase 2) but was not developed further and was never approved or marketed anywhere.
?What was special about it?
Its depot. In the phase 2 trial a single injection held testosterone at castrate levels for about forty weeks, pointing to a twice-a-year dose at a time when most LHRH-agonist depots lasted one or three months (PMID 10569560).
?How is it different from leuprolide or goserelin?
All three are LHRH agonists that lower testosterone by the same route. Leuprolide and goserelin were approved and are widely used; avorelin was not taken to approval. The approved agonists now include their own six-month depots.
?Is it available to buy?
No. Avorelin is not a medicine and is not sold as one. Grey Peptides lists no sellers and recommends no source; this page is a record of the published science.
References
- [clinical-trial] Kaisary AV, et al. "Pharmacodynamics of a long acting depot preparation of avorelin in patients with prostate cancer. Avorelin Study Group." J Urol, 1999;162(6):2019-23. PMID: 10569560.
- [animal] Hudon E, et al. "Radioimmunoassay of meterelin and pharmacokinetics after single injection and implant administration in dogs." J Pharm Sci, 1997;86(2):172-8. PMID: 9040091.
- [animal] Nevison CM, et al. "Effects of altering testosterone in early infancy on social behaviour in captive yearling rhesus monkeys." Physiol Behav, 1997;62(6):1397-403. PMID: 9383131.
- [animal] Brown GR, et al. "Investigation of the role of postnatal testosterone in the expression of sex differences in behavior in infant rhesus macaques (Macaca mulatta)." Horm Behav, 1999;35(2):186-94. PMID: 10202126.
- [animal] Brown GR, et al. "Manipulation of postnatal testosterone levels affects phallic and clitoral development in infant rhesus monkeys." Int J Androl, 1999;22(2):119-28. PMID: 10194644.
- [other] FDA Global Substance Registration System, Avorelin, UNII FSB5837STQ (acetate 3575L4873R; INN 7080, CAS 140703-49-7, PubChem 25074428; codes meterelin, EP-23904, MF-6001), read November 5, 2026: a nonapeptide analogue of gonadotropin-releasing hormone, [2-Me-D-Trp6, DesGly10]LHRH ethylamide (pGlu-His-Trp-Ser-Tyr-(2-methyl)-D-tryptophyl-Leu-Arg-Pro-ethylamide). https://gsrs.ncats.nih.gov/ginas/app/beta/substances/FSB5837STQ
- [other] ChEMBL molecule CHEMBL2103984 (AVORELIN), read November 5, 2026: synonyms Avorelin, Avorelina, EP-23904, Meterelin, MF-6001; highest clinical phase recorded 2, no first approval. https://www.ebi.ac.uk/chembl/compound_report_card/CHEMBL2103984/
- [other] US FDA, Drugs@FDA and the drug label endpoint (openFDA), searched November 5, 2026: no application and no label record for avorelin. It has an FDA substance record (UNII FSB5837STQ) and an INN, which name a molecule and do not approve it.
- [registry] ClinicalTrials.gov API v2, searched November 5, 2026: 0 records for avorelin, meterelin or EP-23904. The registry began in 2000, around when the published trials ended, so the absence is expected. https://clinicaltrials.gov/search?term=avorelin
Sources & Citations
The five studies carded above were read from their PubMed records on November 5, 2026. Structure, codes and identifiers come from the FDA Global Substance Registration System (UNII FSB5837STQ) and ChEMBL (CHEMBL2103984); the absence of approval from Drugs@FDA and openFDA, and of registered trials from ClinicalTrials.gov, all checked the same day. Grey Peptides sells nothing and links to no vendor.
Medical Disclaimer: This content is for educational and informational purposes only. It is not medical advice, and nothing here is an instruction to obtain or use any compound. Consult a licensed healthcare provider before making health decisions.