Buserelin

Buserelin ([D-Ser(tBu)6,des-Gly-NH2-10]GnRH ethylamide; CAS 57982-77-1, free base; 68630-75-1, acetate salt) is a synthetic nonapeptide analog of the hypothalamic decapeptide gonadotropin-releasing hormone (GnRH, also designated luteinizing hormone-releasing hormone, LHRH) first described by Sandow and colleagues at Hoechst AG in 1976 and approved for clinical use in 1984 [1, 2]. The compound incorporates two structural modifications to native GnRH that collectively confer superagonist potency and metabolic stability: replacement of glycine at position 6 with D-serine bearing a tert-butyl ether on the side-chain hydroxyl, which eliminates the principal endopeptidase cleavage site and introduces conformational rigidity favorable to receptor binding; and replacement of the C-terminal glycinamide (position 10) with an ethylamide, which further resists carboxypeptidase degradation. The resulting peptide binds the type I GnRH receptor with affinity approximately 20- to 170-fold greater than native GnRH and produces a biphasic pharmacological response that is the mechanistic foundation for all clinical applications [3, 4]. Acute administration stimulates pituitary gonadotroph secretion of luteinizing hormone and follicle-stimulating hormone, producing transient elevations in gonadal steroid output (the "flare" phase, lasting 7 to 14 days). Chronic continuous administration produces homologous desensitization of the GnRH receptor through receptor internalization, uncoupling from Gq/11-phospholipase C signaling, and transcriptional downregulation of GnRH receptor expression, resulting in profound and sustained suppression of gonadotropin secretion and a hypogonadal state equivalent to surgical castration in both sexes [5, 6]. This medical castration is reversible on cessation of treatment. Buserelin was the first GnRH agonist demonstrated to achieve medical castration in humans via intranasal administration, an observation reported by Sandow and colleagues in 1980 that established the clinical viability of non-injectable GnRH agonist therapy [2]. The compound is registered in approximately 40 jurisdictions across Europe, the United Kingdom, Canada, New Zealand, South Africa, Latin America, and Asia, but is not approved in the United States or Australia. Registered indications include hormone-responsive prostate cancer, endometriosis, uterine fibroids, premenopausal breast cancer, and pituitary downregulation as an adjunct to controlled ovarian hyperstimulation in assisted reproduction [7, 8, 9]. The compound is additionally used off-label for central precocious puberty and as a component of gender-affirming hormone therapy. Pharmacokinetics are characterized by negligible oral bioavailability due to gastrointestinal peptidase degradation, approximately 2.5 to 3.3 percent intranasal bioavailability, and approximately 70 percent subcutaneous bioavailability [10]. The plasma elimination half-life is 50 to 80 minutes after intravenous or subcutaneous administration and approximately 1 to 2 hours after intranasal dosing. Protein binding is low (approximately 15 percent). Metabolism occurs principally through pyroglutamyl peptidase and chymotrypsin-like endopeptidase activity in the liver, kidneys, and gastrointestinal tract, with approximately 50 percent of the administered dose recovered unchanged in urine [10, 11]. Formulations include aqueous solution for subcutaneous injection and intranasal spray (requiring multiple daily administrations) and sustained-release subcutaneous implants providing 2- or 3-month depot delivery. The adverse-event profile is dominated by the pharmacological consequences of gonadal steroid suppression: hot flashes, reduced libido, erectile dysfunction or vaginal dryness, and long-term bone mineral density reduction. The initial flare phase carries specific risk in metastatic prostate cancer (bone pain exacerbation, spinal cord compression, ureteral obstruction), which is mitigated clinically by co-administration of an antiandrogen during the first 2 to 4 weeks of therapy. This monograph reviews the chemistry and synthesis, the biphasic GnRH receptor pharmacology, the comprehensive human pharmacokinetic record, the clinical evidence base across prostate cancer, endometriosis, uterine fibroids, breast cancer, precocious puberty, and assisted reproduction indications, sourcing and quality verification, reconstitution and handling, stack interactions, adverse-event signal, and a comparative assessment of five GnRH agonist candidates against buserelin on five competency standards.