Pharmacokinetic Evaluation of Buserelin Acetate (Receptal) in Gilts

Buserelin acetate, a synthetic analog of gonadotropin-releasing hormone (GnRH), is used to induce ovulation and enable fixed-time artificial insemination (FTAI) in swine. Evaluating the pharmacokinetics of buserelin acetate is crucial for optimizing its application in precisely controlling ovulation timing and enhancing the effectiveness of FTAI in pigs. This study investigates the pharmacokinetics of buserelin acetate following intramuscular administration in gilts. Ten healthy prepuberty gilts (Landrace × Yorkshire × Duroc) with an average body weight of 72.0 ± 3.4 kg were included in the study. Before treatment, all ten gilts underwent surgical implantation of an indwelling venous catheter to allow repeated blood samplings for pharmacokinetic assessment. On Day 3 after surgery, each gilt received an intramuscular injection of 100 μg buserelin acetate (25 mL of 4 μg/mL Receptal, MSD Animal Health, USA). Blood samples (5 mL each) were collected 14 times from each gilt via the jugular catheter at the following time points: 0, 2, 5, 10, 15, 20, 30, and 45 min, as well as 1, 1.5, 2, 3, 4, and 6 h post-injection. Blood samples were collected into heparinized tubes, centrifuged to separate the plasma, and stored at -20°C until analysis. Plasma buserelin concentrations were then determined using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS). The LC-MS/MS system operated in positive ion mode using electrospray ionization with multiple reaction monitoring. Buserelin was quantified based on precursor/product ion pairs identified in the chromatogram. The calibration curve for buserelin acetate in pig plasma showed high linearity (R² ≥ 0.99), with a lower limit of quantification of 0.30 μg/L. The findings showed that the average maximum plasma concentration (C_max) of buserelin acetate was 2.21 ± 0.72 μg/L, occurring at 0.30 ± 0.10 h (T_max) after administration. The elimination half-life (T_1/2) was 0.51 ± 0.03 h. The area under the concentration-time curve from 0 to the last measurement point (AUC_0-t) was 7.30 ± 0.87 μg·h/L. The clearance rate (CL) was 0.20 ± 0.03 L/h/kg, and the apparent volume of distribution (Vd) was 0.13 ± 0.05 L/kg. These results suggest that buserelin acetate from Receptal is rapidly metabolized and eliminated in gilts, emphasizing the need for precise timing of administration to maintain effective drug concentrations.