Coumarin 6 as a fluorescent model drug: How to identify properties of lipid colloidal drug delivery systems via fluorescence spectroscopy?

In the development of colloidal lipid drug delivery systems (DDS), the localization of the drug within the DDS, the protective potential of the DDS as well as the compatibility of the drug with the excipients are of special interest. In this study, Coumarin 6 (C6), a fluorescent dye frequently used to facilitate the traceability of DDS in vitro, is demonstrated as a tool to gain insight into the aforementioned properties by fluorescence spectroscopy. Both ways of loading C6 to solid colloidal DDS (pre-loading to the lipid matrix or post-loading to the ready DDS after its processing) led to a localization of C6 in the interfacial layer between solid lipid nanoparticles (SLN) and aqueous medium. In the case of pre-loading, C6 is homogenously distributed in emulsion droplets and expelled from the lipid matrix due to crystallization. Nanoemulsions distributed C6 in the liquid lipid matrix itself and cause a hypsochromic shift of fluorescence spectra. Larger surface areas of emulsion droplets due to smaller droplet sizes displayed more interaction of C6 with the aqueous phase as a bathochromic shoulder in fluorescence spectra. Measured fluorescence intensity was linearly dependent on light scattering of colloidal DDS of the same excipients. A simple method to evaluate the protective properties of DDS was presented and displayed that (i) nanoemulsions provide more protection compared with SLN and (ii) higher phospholipid concentration improve the protection within SLN.