FRET as a tool for the investigation of the fate of Lipidots contrast agents in vivo

Different approaches have been reported in the recent years for the in vivo delivery and targeting of poorly soluble contrast agents and active ingredients in diseased tissues. In this context, we developed new lipid nanoparticles (Lipidots®) with size being easily varied from 25 to 120 nm. Lipidots® display numerous advantages: they are composed of low-cost and biocompatible lipids; they can be stored in injection-ready formulations for long duration; their manufacturing process is versatile and up-scalable. Several indocyanines have been efficiently encapsulated in the particles while retaining their spectroscopic properties, with emission wavelengths ranging from 500 to 820 nm. Thus, dye loaded-Lipidots® have been proved suitable for both in vitro and in vivo applications. To better understand Lipidots®' behavior in biological systems, formulations based on Forster Resonance Energy Transfer (FRET) have been studied. Different pairs of the selected indocyanines were co-encapsulated and calculations proved that transfer efficiency within nanoparticles (i) behaves as in a continuous medium; (ii) depends on local acceptor concentration. Thanks to the local dye concentration dependence of FRET, these formulations are used to understand where and when the particles are assimilated in biological systems. FRET-based Lipidots® contrast agents are also promising tools to enhance imaging contrast in vivo by making a clear difference between circulating and uptaken particles.