Absorption and metabolism of nafarelin, a potent agonist of gonadotropin-releasing hormone.

Nafarelin, a potent gonadotropin-releasing hormone (GnRH) agonist, was absorbed rapidly into systemic circulation (time to reach peak concentration, 20 to 40 minutes) after intranasal but not after sublingual or vaginal administration. Serum elimination half-life was about 2 hours. Nasal absorption of nafarelin was increased by increasing the concentration of the drug in the dose solution and incorporating sodium glycocholate into the nasal formulation. An optimal formulation providing maximum nasal absorption of nafarelin was one containing 1.75 mg nafarelin per ml and 2% sodium glycocholate. Bioavailability of this nasal formulation relative to a single subcutaneous dose averaged 21%. The metabolism and excretion of nafarelin were determined in three subjects after subcutaneous administration of [14C]-nafarelin. Radioactivity was excreted in approximately equal amounts in urine and stool. Six metabolites accounted for most of the radioactivity in urine. Four metabolites were short peptide fragments of nafarelin and the other metabolites were naphthylalanine and 2-naphthylacetic acid.