Deep eutectic solvent combined with permeation enhancer strategy to convert tandospirone from oral to transdermal formulations improving drug bioavailability

Tandospirone(Tan) is a commonly used drug for anxiety treatment. However, it has a significant first-pass effect and needs to be taken three times a day. To increase the bioavailability of the drug and reduce the number of administrations, this work amid to prepare a Tan patch that can be administered once a day by using the strategy of therapeutic deep eutectic solvent(THEDES) in cooperation with chemical permeation enhancer(CPE). In this study, four organic acids and five permeation enhancers were selected, and the optimized formulation was obtained by single-factor investigation and Box-Behnken design. The optimized formulation could significantly enhance drug loading by 2.5-fold and skin permeation up to 586.6 ± 17 μg/cm² in rats. Based on pharmacokinetic results, compared to oral administration, the drug exhibited a substantially elevated bioavailability, registering a 17-fold increase(from 3.01 % to 52.17 %), alongside a 10-fold rise in the mean residence time(MRT). Meanwhile, the patch was not irritating. The results of the mechanistic study showed that levulinic acid(LeA) acted as a bridge to increase the interaction between the Tan and the matrix and inhibited the crystallization of the drug in the patch, and THEDES together with CPE improved the matrix fluidity and skin permeability. This study provides a reference for the joint application of THEDES and CPEs in patch development.