Pharmacokinetics and pharmacodynamics studies of Relugolix long-acting microcrystalline formulation in male Beagle dogs

Relugolix is the first oral GnRH receptor antagonist. However, its oral formulation requires daily administration, leading to poor patient adherence and suboptimal treatment outcomes for long-term users. To address these issues, we developed a long-acting microcrystalline formulation of relugolix administered once every 28 days.This study systematically evaluated the pharmacokinetics (PK), pharmacodynamics (PD), and safety profile of the drug following intramuscular administration in male Beagle dogs. In this study, we established a HPLC-MS/MS method for blood concentration detection, used a highly specific and stable ELISA method to measure serum testosterone levels, and assessed the safety of the drug in male Beagle dogs. The HPLC-MS/MS method showed good linearity (R²=0.9991). The intra-day precision varied between 2.8 % and 10.9 %, while the inter-day precision ranged from 1.6 % to 5.4 %. In terms of accuracy, the intra-day values ranged from -7.3 % to -3.2 %, and the inter-day accuracy ranged from -10.2 % to 0.8 %. Furthermore, key parameters such as stability, extraction recovery, and matrix effects met the FDA bioanalytical method validation guidelines. PK studies showed that the AUC and Tmax exhibited significant sustained-release characteristics, and the half-life was notably extended compared to Orgovyx. PD analysis showed a significant negative correlation between blood drug concentration and testosterone suppression (testosterone levels began to decline after administration, and castration levels were maintained between 504–1008 h). Safety assessments revealed that although no treatment-related deaths or significant weight changes were observed, some hematological parameters (RBC, HGB increases), liver function indicators (ALT, AST increases), and triglyceride (TG) levels showed statistically significant changes (p < 0.05). This suggests potential concerns regarding hemodynamic effects, hepatotoxicity, and lipid metabolism abnormalities. This study is the first to systematically evaluate the PK/PD characteristics of the long-acting micronized formulation of relugolix, providing key data for its clinical translation, but further validation through large sample and cross-species studies is needed.