Abarelix (Plenaxis)

Prescription

Abarelix (brand name Plenaxis) was the first gonadotropin-releasing hormone (GnRH) antagonist approved for prostate cancer, and it marks an important chapter in the history of hormone therapy. Unlike GnRH agonists such as leuprolide and goserelin — which first stimulate the pituitary (causing a testosterone 'flare') before suppressing it — abarelix blocks the GnRH receptor directly and competitively, producing an immediate fall in LH, FSH, and testosterone with no flare. This made it attractive for men in whom a testosterone surge would be dangerous, for example those with impending spinal cord compression or severe urinary obstruction. Structurally, abarelix is a synthetic decapeptide with several non-natural amino acid substitutions that convert the GnRH scaffold from an agonist into a pure antagonist. Because it occupies the receptor rather than triggering and then exhausting it, testosterone can reach castrate levels within days rather than the two-to-four weeks agonists require, and without the need for a protective anti-androgen at the start. Abarelix received FDA approval in 2003, but its clinical use was limited by a specific safety signal: immediate-onset systemic allergic reactions (including rare anaphylaxis), which appeared to increase with cumulative dosing. Because of this, and commercial factors, the manufacturer voluntarily withdrew Plenaxis from the US market in 2005; it remained available in a few countries. Its regulatory story is instructive rather than current — the antagonist concept it pioneered was carried forward more successfully by degarelix, which has a more favorable tolerability profile and remains in use. This page is provided as an educational reference on a historically important peptide. Abarelix is not a current first-line therapy in most countries, and any decisions about androgen-deprivation therapy for prostate cancer are made with an oncologist or urologist using presently available, approved options.