Clinical Pharmacokinetics of Depot Leuprorelin

Leuprorelin acetate is a synthetic agonist analogue of gonadotropin-releasing hormone. Continued leuprorelin administration results in suppression of gonadal steroid synthesis, resulting in pharmacological castration. Since leuprorelin is a peptide, it is orally inactive and generally given subcutaneously or intramuscularly. Sustained release parenteral depot formulations, in which the hydrophilic leuprorelin is entrapped in biodegradable highly lipophilic synthetic polymer microspheres, have been developed to avoid daily injections. The peptide drug is released from these depot formulations at a functionally constant daily rate for 1, 3 or 4 months, depending on the polymer type [polylactic/glycolic acid (PLGA) for a 1-month depot and polylactic acid (PLA) for depot of >2 months], with doses ranging between 3.75 and 30mg. Mean peak plasma leuprorelin concentrations (C(max)) of 13.1, 20.8 to 21.8, 47.4, 54.5 and 53 microg/L occur within 1 to 3 hours of depot subcutaneous administration of 3.75, 7.5, 11.25, 15 and 30 mg, respectively, compared with 32 to 35 microg/L at 36 to 60 min after a subcutaneous injection of 1mg of a non-depot formulation. Sustained drug release from the PLGA microspheres maintains plasma concentrations between 0.4 and 1.4 microg/L over 28 days after single 3.75, 7.5 or 15mg depot injections. Mean areas under the concentration-time curve (AUCs) are similar for subcutaneous or intravenous injection of short-acting leuprorelin 1mg; a significant dose-related increase in the AUC from 0 to 35 days is noted after depot injection of leuprorelin 3.75, 7.5 and 15mg. Mean volume of distribution of leuprorelin is 37L after a single subcutaneous injection of 1mg, and 36, 33 and 27L after depot administration of 3.75, 7.5 and 15mg, respectively. Total body clearance is 9.1 L/h and elimination half-life 3.6 hours after a subcutaneous 1mg injection; corresponding values after intravenous injection are 8.3 L/h and 2.9 hours. A 3-month depot PLA formulation of leuprorelin acetate 11.25mg ensures a C(max) of around 20 microg/L at 3 hours after subcutaneous injection, and continuous drug concentrations of 0.43 to 0.19 microg/L from day 7 until before the next injection. Recently, an implant that delivers leuprorelin for 1 year has been evaluated. Serum leuprorelin concentrations remained at a steady mean of 0.93 microg/L until week 52, suggesting zero-order drug release from the implant. In general, regular or depot leuprorelin treatment is well tolerated. Local reactions are more common after application of the 3- or 4-month depot in comparison with the 1-month depot.