Intramuscularly injected long-acting testosterone-cholesterol prodrug suspension with three different particle sizes: extended in vitro release and enhanced in vivo safety

The testosterone undecanoate oil solution is the most widely used injection of testosterone for long-acting effects on the market, whereas the formulation carries the potential risk of causing pulmonary vascular embolism, inflammation, and pain at the injection site. Therefore, a sustained-released long-acting injection of testosterone with strong security is urgently exploited. Herein, a poorly water-soluble testosterone-cholesterol prodrug (TST-Chol) was synthesized by esterification. The water solubility of TST-Chol was decreased by 644 folds in comparison to that of testosterone (TST). Moreover, suspensions of TST and TST-Chol were prepared and analyzed in vitro, utilizing three distinct particle sizes: small-sized nanocrystals (SNCs) measuring 300 nm, medium-sized microcrystals (MMCs) measuring 12 μm, and large-sized microcrystals (LMCs) measuring 20 μm. The findings from the in vitro release study indicated that the sustained release of the drug was significantly influenced by the solubility and particle sizes of the suspension. Notably, the suspensions with low water solubility and larger particle sizes exhibited a more desirable sustained-release effect in vitro. Furthermore, the study on pharmacokinetics exhibited that TST-Chol SNCs produced a sustained TST plasma concentration in vivo for up to 40 days and no obvious pathological changes in lung tissue were found. Our study indicated that solubility and particle sizes of suspensions had made a difference in pharmacokinetics and provided a valuable reference for the advancement of long-acting injections. Graphical abstract