Radioimmunoassay of Meterelin and Pharmacokinetics after Single Injection and Implant Administration in Dogs

Abstract

A sensitive and specific radioimmunoassay for a novel luteinizing-hormone-releasing-hormone (LHRH) agonist, [2-Me-D-Trp6, DesGly10]LHRH ethylamide (meterelin), was developed for documenting the pharmacokinetic parameters of this peptide following its intravenous (iv) and subcutaneous (sc) administration in dogs. The assay was also used for monitoring meterelin in plasma following its release from d,l-lactide-glycolide implants in the same species. Rabbit antisera generated against [DespyroGlu1] meterelin and conjugated to bovine serum albumin with glutaraldehyde showed high specificity, whereas crossreactivity to LHRH and its fragments and to analogs with substitutions at residues 6 and 10 was found insignificant. The assay was validated in terms of accuracy (recovery range, 94.0–105.4%), in terms of precision (intra-and interassay variations of 10.0–12.4% and 8.6–11.3%, respectively), and in terms of sensitivity (minimum detectable dose of 2.7pg/assay). Following iv acute administration, a biexponential decline of plasma meterelin levels was observed, with distribution and elimination half-lives of 5.9±2.5 and 106±22 min, respectively. After sc acute administration, the elimination half-life was in the range of 103 to 173min. The systemic clearance (CLT) ranged from 1.6 to 2.6mL/min/kg, and the volume of distribution at steady state (Vdss) varied from 285 to 438mL/kg. The elimination half-life (T1/2 β), Vdss, and ClT were not significantly different after both routes of administration over the 1–100-µg/kg dose range of peptide studied. Castrate levels of testosterone were attained 10days after sc administration of the implant, lasted for up to 247days, and were well correlated with plasma levels of meterelin.