Abarelix
The first GnRH antagonist approved for prostate cancer, withdrawn in the US amid allergic reactions
Abarelix is a synthetic peptide that blocks the GnRH receptor, so testosterone falls without the surge agonists cause. Approved in the US as Plenaxis on November 25, 2003, it left the US market in 2005 amid concerns including hypersensitivity reactions; degarelix and relugolix now fill its place.
What is it?
Abarelix proved the antagonist idea in prostate cancer. In phase 3 it reached castrate testosterone faster than leuprolide, given alone or with an antiandrogen, and avoided the testosterone surge, with similar suppression afterwards: no man on abarelix had a surge against 82% on leuprolide, and 78% were castrate within a week. It was a 100 mg depot injection on days 1, 15 and 29, then every four weeks.
It also lowered FSH at once, where agonists first raise it, and that led to trials in cancer that had escaped hormone therapy, on the idea that FSH might drive it. FSH fell by half or more, but PSA did not respond, whether after agonist failure or after orchiectomy.
Its undoing was allergy. Trials recorded more immediate-onset systemic allergic reactions than with agonist therapy, FDA rejected the first application in 2001 partly over them, and the drug left the US market in 2005. Laboratory work later compared the antagonists: in human skin, abarelix released much more histamine than degarelix, the antagonist that followed it.
Mechanism of Action
Abarelix is a GnRH antagonist: it blocks pituitary GnRH receptors directly, so testosterone falls without the initial surge that agonists cause.
Receptor blockade
Competitively blocks GnRH receptors on pituitary gonadotropes without releasing gonadotropins first; the effect is reversible.
No surge
LH, FSH, testosterone and DHT fall from the first days, with no flare.
Histamine release
Releases histamine in human skin more than degarelix, consistent with its immediate allergic reactions.
Published Research
Trials and reviews read from their PubMed abstracts; no Plenaxis label could be read.
Against LH-RH agonists
In 242 men starting hormonal therapy, 75% on abarelix depot were castrate within the first week against none on agonists, with a faster PSA fall and no flare.
Against leuprolide and an antiandrogen
In 255 men, abarelix alone reached castration faster than leuprolide plus a daily antiandrogen, without a surge, and both kept testosterone castrate equally well.
Men who could not risk a flare
In symptomatic advanced prostate cancer, PSA fell a median 75% by day 15 and over 95% from day 57, and no patient needed orchiectomy.
Pharmacokinetics
After continuous subcutaneous infusion in prostate cancer, abarelix reached steady state at about 28 days and its mean observed half-life was 10.0 days, with testosterone suppressed by 94.2%.
Every two weeks in resistant cancer
In 21 men whose prostate cancer progressed on GnRH agonists, abarelix every 2 weeks for 12 weeks lowered FSH from 3.5 to 2.0 mIU/mL; one patient had an immediate allergic reaction.
Phase 3 against leuprolide
In 269 men, no one on abarelix had a testosterone surge against 82% on leuprolide; 78% were castrate by day 7 against none on leuprolide, and castration was maintained equally from day 29 to 85.
FSH falls at once
Men on LHRH agonists had an FSH surge on day 2 before it declined, while men on abarelix depot had an immediate and sustained fall in FSH.
A longer-acting depot
In healthy older men, a new depot formulation had a terminal half-life of 13.2 days against 5.3 hours for the solution and reversibly suppressed testosterone, DHT, LH and FSH.
After agonist failure: no responses
In 20 men whose cancer progressed on GnRH agonists, abarelix depot lowered FSH by more than half but produced no PSA responses; median time to progression was 8 weeks.
After orchiectomy: FSH targeted
In men with androgen-independent cancer after orchiectomy, abarelix cut FSH by nearly 90%, but no PSA response met criteria and median time to progression was 12 weeks; one had a grade 3 allergic reaction.
Antagonists against agonists
A review explains that agonists cause an LH and testosterone surge needing antiandrogen cover, while the pure antagonists abarelix and degarelix bring testosterone to castrate levels quickly without it.
The development programme
A 2000 technology evaluation records phase 3 prostate cancer trials, a sustained-release formulation in early trials for endometriosis, and successive licensing deals in Europe and the US.
Allergic reactions
A review of the approval data found castration without a surge and similar efficacy to agonist combinations, but more immediate-onset systemic allergic reactions with abarelix than in the control arms.
The regulatory path
A drug profile recording that FDA rejected the application in June 2001, asking for analysis of the allergic reactions seen in some patients and raising concern about maintaining suppression.
Histamine release in human skin
In an ex vivo human skin model, abarelix and cetrorelix released significantly more histamine than degarelix, which released the least, offering an indirect clue to the allergy risk.
Dosing Data
Abarelix is withdrawn in the US. It was given as a 100 mg intramuscular depot on days 1, 15 and 29, then every four weeks.
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Side Effects & Contraindications
Reported Side Effects
From the studies.
Contraindications & Cautions
No current label could be read.
Legal Status
Approved in 2003, now withdrawn in the US.
Frequently Asked Questions
?Why was abarelix withdrawn?
Concerns included hypersensitivity: trials recorded more immediate-onset systemic allergic reactions than with agonists, and a 2014 review dates its US withdrawal to 2005.
?Which antagonist came after it?
Degarelix, another pure GnRH antagonist. In a human skin model it released the least histamine of the antagonists tested, while abarelix released significantly more.
?Did it work?
Yes: it lowered testosterone faster than leuprolide, without the surge, and kept it suppressed as well.
?Why was a testosterone surge a problem?
GnRH agonists such as leuprolide first stimulate the pituitary, so testosterone briefly rises before it falls; that flare can accelerate prostate cancer for a time, which is why agonists were started with an antiandrogen. Abarelix and later degarelix avoid the surge.
?Did lowering FSH help hormone-resistant cancer?
No. In men whose cancer progressed on agonists or after orchiectomy, abarelix cut FSH by half or more, but no PSA response met the trial criteria and disease progressed within about 8 to 12 weeks.
?Which antagonist came after it?
Degarelix, another pure GnRH antagonist. In a human skin model it released the least histamine of the antagonists tested, while abarelix released significantly more.
?How long does one depot injection last?
In healthy older men, a newer depot formulation had a terminal half-life of 13.2 days, against 5.3 hours for the injectable solution, and its hormone suppression reversed once the drug cleared. The marketed schedule was an injection on days 1, 15 and 29, then every four weeks.
References
- [clinical-trial] Tomera K, et al. "The gonadotropin-releasing hormone antagonist abarelix depot versus luteinizing hormone releasing hormone agonists leuprolide or goserelin: initial results of endocrinological and biochemical efficacies in patients with prostate cancer." J Urol, 2001;165(5):1585-9. PMID: 11342922.
- [clinical-trial] Trachtenberg J, et al. "A phase 3, multicenter, open label, randomized study of abarelix versus leuprolide plus daily antiandrogen in men with prostate cancer." J Urol, 2002;167(4):1670-4. PMID: 11912385.
- [clinical-trial] Koch M, et al. "An open-label study of abarelix in men with symptomatic prostate cancer at risk of treatment with LHRH agonists." Urology, 2003;62(5):877-82. PMID: 14624912.
- [clinical-trial] Wong SL, et al. "Pharmacokinetics and pharmacodynamics of abarelix, a gonadotropin-releasing hormone antagonist, after subcutaneous continuous infusion in patients with prostate cancer." Clin Pharmacol Ther, 2003;73(4):304-11. PMID: 12709720.
- [review] Debruyne F, et al. "Abarelix for injectable suspension: first-in-class gonadotropin-releasing hormone antagonist for prostate cancer." Future Oncol, 2006;2(6):677-96. PMID: 17155895.
- [review] "Abarelix: abarelix-depot-F, abarelix-depot-M, abarelix-L, PPI 149, R 3827." Drugs R D, 2003;4(3):161-6. PMID: 12757402.
- [review] Moul JW. "Utility of LHRH antagonists for advanced prostate cancer." Can J Urol, 2014;21(2 Supp 1):22-7. PMID: 24775720.
- [pubmed] Beer TM, et al. "Dose-escalated abarelix in androgen-independent prostate cancer: a phase I study." Anticancer Drugs, 2006;17(9):1075-9. PMID: 17001181.
- [pubmed] McLeod D, et al. "A phase 3, multicenter, open-label, randomized study of abarelix versus leuprolide acetate in men with prostate cancer." Urology, 2001;58(5):756-61. PMID: 11711355.
- [pubmed] Garnick MB, et al. "Abarelix Depot, a GnRH antagonist, v LHRH superagonists in prostate cancer: differential effects on follicle-stimulating hormone. Abarelix Depot study group." Mol Urol, 2000;4(3):275-7. PMID: 11062384.
- [pubmed] Wong SL, et al. "Pharmacokinetics and pharmacodynamics of a novel depot formulation of abarelix, a gonadotropin-releasing hormone (GnRH) antagonist, in healthy men ages 50 to 75." J Clin Pharmacol, 2004;44(5):495-502. PMID: 15102870.
- [pubmed] Beer TM, et al. "Phase II study of abarelix depot for androgen independent prostate cancer progression during gonadotropin-releasing hormone agonist therapy." J Urol, 2003;169(5):1738-41. PMID: 12686821.
- [pubmed] Beer TM, et al. "Targeting FSH in androgen-independent prostate cancer: abarelix for prostate cancer progressing after orchiectomy." Urology, 2004;63(2):342-7. PMID: 14972486.
- [pubmed] Koechling W, et al. "Degarelix, a novel GnRH antagonist, causes minimal histamine release compared with cetrorelix, abarelix and ganirelix in an ex vivo model of human skin samples." Br J Clin Pharmacol, 2010;70(4):580-7. PMID: 20840449.
- [pubmed] Kirby RS, et al. "Abarelix and other gonadotrophin-releasing hormone antagonists in prostate cancer." BJU Int, 2009;104(11):1580-4. PMID: 20053189.
- [pubmed] Doehn C, et al. "Technology evaluation: Abarelix, Praecis pharmaceuticals." Curr Opin Mol Ther, 2000;2(5):579-85. PMID: 11249760.
Sources & Citations
Studies were read from their PubMed abstracts; the US product is withdrawn. Read October 6, 2026.
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