A Semi-Mechanistic Integrated Pharmacokinetic/Pharmacodynamic Model of the Testosterone Effects of the Gonadotropin-Releasing Hormone Agonist Leuprolide in Prostate Cancer Patients

Background and Objective: Leuprolide is a gonadotropin-releasing hormone (GnRH) agonist, which inhibits gonadotropin secretion by down-regulating pituitary GnRH receptor when administered continuously at therapeutic doses. The objectives of this analysis were to develop a population model that can describe the pharmacokinetics of the 6-month depot formulation of leuprolide acetate in patients with prostate cancer and to characterize the relationship of leuprolide plasma concentrations and serum testosterone concentrations. Methods: The pharmacokinetic and pharmacodynamic analyses were performed using a non-linear mixed-effect modeling approach. Observations were pooled from studies on healthy male volunteers and prostate cancer patients, who were administered a single 1 mg intravenous dose of immediate-release leuprolide acetate and two intramuscular doses of 45 mg of the depot formulation, respectively. The covariates that were screened for the pharmacokinetic model included body weight, creatinine clearance, liver function markers (total bilirubin, blood urea nitrogen, AST, alanine aminotransferase), age, and body mass index. Results: A two-compartment model with parallel first- and zero-order absorption processes and a delayed first-order process well-characterized the multi-phasic absorption profile of leuprolide acetate depot formulation. Typical population values of the absorption rate constant of the immediate and delayed processes were estimated to be 0.357 and 0.017 day, respectively, with a mean transit time of 9.5 days. No covariates were significant in this analysis. A semi-mechanistic model, which accounts for down-regulation of the GnRH receptor via an inhibitory maximum effect (E) model and the stimulatory effect of activated receptors on testosterone levels, adequately described serum testosterone profiles following dosing. The equilibrium dissociation constant of leuprolide and the typical leuprolide plasma concentration required to achieve a castration testosterone level of ≤0.5 ng/mL were 0.3 and 0.03 ng/mL, respectively. Conclusion: Population pharmacokinetics and pharmacodynamics of the leuprolide depot formulation were characterized using an integrated semi-mechanistic model. The developed model adequately describes the leuprolide–testosterone relationship and can potentially be used to facilitate design of clinical studies for new formulations, to aid in the selection of candidate formulations, and for the optimization of doses and dosing schemes.