Lipidots: competitive organic alternative to quantum dots for in vivo fluorescence imaging

Abstract

The use of fluorescent nanostructures can bring several benefits on the signal to background ratio for in vitro microscopy, in vivo small animal imaging, and image-guided surgery. Fluorescent quantum dots (QDs) display outstanding optical properties, with high brightness and low photobleaching rate. However, because of their toxic element core composition and their potential long term retention in reticulo-endothelial organs such as liver, their in vivo human applications seem compromised. The development of new dye-loaded (DiO, DiI, DiD, DiR, and Indocyanine Green (ICG)) lipid nanoparticles for fluorescence imaging (lipidots) is described here. Lipidot optical properties quantitatively compete with those of commercial QDs (QTracker(®)705). Multichannel in vivo imaging of lymph nodes in mice is demonstrated for doses as low as 2 pmols of particles. Along with their optical properties, fluorescent lipidots display very low cytotoxicity (IC(50) > 75 nM), which make them suitable tools for in vitro, and especially in vivo, fluorescence imaging applications.

Synthetic Lipid Nanoparticles Targeting Steroid Organs

“…The lipidots used in the present study were 55 nm in diameter, large enough to offer a sufficient volume for drug loading but small enough to limit uptake by macrophages of the reticuloendothelial system and favor extravasation and cell internalization (21). Lipidots have a high loading ratio for lipophilic molecules (6) and are coated with PEG to prevent surface protein adsorption and macrophage uptake (1,2, 21,22) and to favor long-term colloidal stability, neutrality of charge (4), and long plasma circulation times. Lipidots were loaded with 3 tracers presenting similar physicochemical properties, that is, a slightly polar cyanine or cholesterol head and long C 16 or C 18 hydrophobic chains.…”

“…The 2 radiotracers, 1,2-3 H(N)-cholesteryl-hexadecyl ether ( 3 H-CHE) and cholesteryl-1-14 C-oleate ( 14 C-CHO), both previously used for liposome development (14,15,19), have different metabolic profiles: the ether link between the tritiated cholesteryl moiety and the lipid chain prevents metabolization of 3 H-CHE, whereas 14 C-CHO is hydrolyzed in cells but not significantly in plasma (13). The nearinfrared fluorescent dye 1,19-dioctadecyl-3,3,39,39-tetramethylindotricarbocyanine perchlorate (DiD), a cyanine bound by an amine link to 2 lipophilic C18 chains, is efficiently incorporated in lipidots with long-term stability (6).…”

“…Lipidots are newly described lipid nanoparticles based on oil-in-water nanoemulsion templates made of components generally regarded as safe (4,5), forming colloidal dispersions of lipid droplets with finely tuned core/shell properties and stable for several years (4,5). These particles display no cellular toxicity and can incorporate high amounts of organic fluorescent dyes (6). Systemic administration of fluorescent dye-loaded lipidots carrying the cyclic arginine-glycine-aspartate peptide, a ligand of a y b 3 integrins highly expressed in the tumor vasculature, produced in vivo images of subcutaneously grafted cancer cells in mice (7) originally interpreted as resulting from ligand-specific uptake.…”

Bone morphogenetic protein 9 (BMP9) controls lymphatic vessel maturation and valve formation

“…These neutral dye-loaded oily nanodroplets (with narrow size distribution) dispersed in aqueous buffer have been shown to display high brightness suitable for in vivo fluorescence imaging. 19,20 Moreover, because of their very small size (,100 nm) and their lipid nature, these particles are relevant not only for efficient lymphatic draining through lymph vessels, but also for their partial retention into lymph nodes. [20][21][22] DiD-lipidots' excitation and emission maxima were 646 and 668 nm, respectively.…”

“…The DiD-lipidots' dispersion was prepared as previously described. 19 After injection of 5 mL of 10-mM dye, the mouse was placed inside a whole body in vivo imaging system (IVIS; Caliper Life Sciences). The uptake of DiD-lipidots by dermal lymphatic capillaries that feed lymphatic precollector and collector vessels was followed over a 15-minute period.…”