Jinxia Xu Lifang Ling Yanhua Lan Ye Yuan Long Ran Jiaxin Jiang Xianhui Huang ^*

• College of Veterinary Medicine, South China Agricultural University, Guangzhou, China

Altrenogest (ALT) is widely used to regulate the estrous cycle of sows and mares; however, currently used oral solutions of ALT are deficient in terms of dose accuracy and stability during use. To resolve these problems, we aimed to prepare softgel capsules of ALT with a unit dose equal to the clinically administered dose. The shell of the softgel capsule was mainly composed of gelatin and glycerol, with titanium dioxide and red iron oxide as masking agents. The concentration of ALT was 2% in oil solution, and the solvent was based on soybean oil, and ethyl acetate was selected as the lipophilic solubilizing carrier by the shake flask method. The contents were automatically filled and pressed into pills by a softgel capsule machine to obtain ALT softgel capsules with a content volume of 1 mL, and its specification of 20 mg/capsule was indicated by content testing. In in vitro dissolution experiments, the softgel capsules were rapidly disintegrated and released in three different pH buffers, with a cumulative release rate of nearly 100% at 1 h. The softgel capsules were stable at high temperature and under strong light for 10 days, and the concentration of ALT was >99% in the 6-month accelerated and long-term tests. In the bioequivalence study, Tmax of the softgel capsules was 2.20 ± 0.77 h, t1/2 was 6.36 ± 1.74 h, and Cmax was 64.65 ± 20.69 ng/mL. The main pharmacokinetic parameters Tmax, Cmax, AUC0-t, and AUC0-∞, did not differ significantly between the softgel capsules and the commercially available ALT oral solution (P > 0.05), and bioequivalence was demonstrated within the 90% confidence interval. In conclusion, our softgels have the advantages of higher content, ease of use with accurate dosing, good stability, and equivalence to ALT oral solution.