Abarelix
Abarelix was the first GnRH antagonist approved for advanced prostate cancer, achieving flare-free testosterone suppression. It was largely withdrawn from many markets due to rare but serious systemic allergic reactions, and has been superseded by degarelix and oral relugolix.
01 Overview
Abarelix is a synthetic decapeptide GnRH antagonist. As a direct receptor blocker it suppressed testosterone without the initial flare of agonists, making it useful in men with symptomatic metastatic disease.
Its clinical use was curtailed by a small but real risk of immediate systemic allergic reactions, including hypotension and syncope, which cumulated with repeated dosing and required post-injection observation. It was voluntarily withdrawn from the US market and remains historically important rather than widely used.
02 Mechanism
Competitive antagonism of pituitary GnRH receptors immediately suppresses LH/FSH and testosterone without an agonist flare.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Prostate cancer depot | 100 mg | IM | 100 mg IM on days 1, 15, 29, then every 4 weeks |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Flare-free testosterone suppressionLowers testosterone to castrate range without an initial surge. | Clinical | ||
| No anti-androgen cover neededAbsence of a flare removes the need for concurrent anti-androgen therapy at initiation, useful in symptomatic metastatic disease. | Clinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Immediate systemic allergic reactionHypotension and syncope can occur immediately after injection, with risk rising over repeated doses. | Life-threatening | Rare but cumulative | Clinical | |
| Hypogonadal effectsHot flushes, sleep disturbance, and fatigue from androgen deprivation. | Moderate | Most users | Clinical |