Nanostructured lipid carriers with exceptional gastrointestinal stability and inhibition of P-gp efflux for improved oral delivery of tilmicosin.

Tilmicosin (TMS) is widely applied to treat porcine bacterial respiratory diseases in veterinary medicine. However, oral administration of TMS is greatly limited due to its physicochemical properties, such as poor water solubility, gastric acid sensitivity and bitterness. Therefore, nanostructured lipid carriers (NLCs) were developed as an oral delivery system for TMS by the high shear method combined with ultrasonic techniques in this study. The results showed that TMS-NLCs were approximately spherical with a hydrodynamic diameter of 283.03 nm and a zeta potential of -30.04 mV. TMS was almost entirely encapsulated in the NLCs by interacting with the lipid matrix, as characterized by differential scanning calorimetry and fourier transform infrared spectroscopy. Thus, TMS-NLCs had an excellent encapsulation efficiency and loading capacity with values of 93.46% and 9.23%, respectively. TMS-NLCs maintained good stability not only during storage at 4 ℃, 25 ℃ and 40 ℃ for 90 days but also in stimulated gastrointestinal (GI) fluids at 37 ℃ for 7 days. Therefore, TMS-NLCs displayed low and sustained release in vitro without an initial burst release in stimulated GI fluids. Furthermore, TMS-NLCs showed higher oral bioavailability in piglets compared to the API suspension. Subsequently, Caco-2 cell monolayers were utilized to analyze the mechanism of NLC-enhanced oral adsorption of TMS. The data revealed that NLCs not only increased cellular uptake of TMS but also inhibited the efflux of P-gp in Caco-2 cells. Additionally, TMS-NLCs mainly entered Caco-2 cells via the caveolae/lipid raft-mediated endocytosis pathway. Moreover, nanoparticles were transported across Caco-2 cell monolayers in the intact form to the basolateral side, as identified by transmission electron microscopy, indicating that TMS-NLCs escape lysosome degradation. Taken together, these results indicate that NLCs are a potential delivery carrier for improving the solubility, permeability and oral bioavailability of TMS.