Liposomes Enhance Dye Localization within the Mammary Ducts of Porcine Nipples

Transductal and transepidermal diffusion are two distinct penetration routes of molecules administered via the nipple. To improve the therapeutic potential of this drug administration technique, drug penetration into the mammary ducts should be maximized, which may be accomplished through design optimization of drug delivery vehicles. In this study, we evaluated liposomes, ranging in size from 100 to 3000 nm, to improve ductal penetration of model fluorescent dyes using fluorescence microscopy and image analysis. Liposomes encapsulating a model fluorescent lipophilic dye, nile red, or hydrophilic dye, sulforhodamine B, were applied topically on porcine nipples for 6 h in vitro. Liposome encapsulation of sulforhodamine B significantly reduced the total amount of dye penetrating the nipple, while penetration of liposome-encapsulated nile red varied depending on vesicle size, as compared to their solution controls. However, the fluorescence intensity localized at the ductal epithelium was higher at extended nipple depths in tissues treated with liposomes versus dye solutions, suggesting a higher concentration of dye penetrating the nipple via the ducts. In contrast, the fluorescence intensity measured at the stratum corneum was reduced (sulforhodamine B) or unchanged (nile red) in nipples treated with liposomes versus dye solutions, suggesting a decrease or no change in dye penetration of the nipple via the stratum corneum. Furthermore, the limited penetration distance into the connective tissue beyond the ductal epithelium for both liposome-encapsulated nile red and sulforhodamine B suggests that liposomes remain intact over the 6 h duration of this study when penetrating through the ducts and enhance retention within the ductal lumen. However, the varied penetration profiles into the connective tissue beyond the stratum corneum between liposome-encapsulated nile red and sulforhodamine B suggests that the liposomes destabilize when penetrating the outer tissues layers of the nipple. Overall, liposomes, regardless of size, improved penetration into and retention within the mammary ducts, while limiting penetration into the stratum corneum, indicating their capacity to target the mammary ductal network.