Biodistribution of Near-Infrared Fluorescent Nanoparticles:an in vivo study

Introduction: Research in new fluorescent nanoparticles is today an important field for preclinical studies in the optical imaging technique area. Great expectation concerns new fluorescent markers engeneered for particular applications (conjugated with antibody or pharmaceutical compounds) or alone to study nano-compound intrinsic behaviour in living organisms. In particular, nanosized fluorescent particles (silica nanopaticles1 and quantum dots2-3) are promising tools for the development of luminescent probes and markers provided by great brightness and high photostability respect to traditional organic fluorophores. Here we present an in vivo study of biodistribution in a small laboratory animal model of silica-core / PEG-shell fluorescent nanoparticles (20-30nm) doped with a CY7 NIR emitting dye ((2-((E)-2-((E)-2-chloro-3-((Z)-2-(3-ethyl-1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene) ethylidene)cyclohex-1-enyl)vinyl)-3-ethyl-1,1-dimethyl-1H-benzo[e]indolium iodide). Silica particles, due to the recognized low toxicity of their chemical composition, could be interesting for future clinical applications. Methods: Silica fluorescent nanoparticles biodistribution was studied. We have observed with Optical Imager the biodistribution kinetics and tissue accumulation during three hours immediately after fluorescent tracer administration, in gas anaesthetized mice. Optical images were acquired with IVIS® 200 (Xenogen Corporation, Alameda USA). Data were extracted using Living Image 2.6 software. Silica nanoparticles, with an emission peak around 810 nm, were excited with ICG exc. filter (710-760 nm) and the fluorescent emission acquired with ICG ems. filter (810-875 nm). Results: Biodistribution kinetics and accumulation of the silica nanoparticles was studied in all anatomical districts4 for three hours after injection using the fluorescent signal escaping from the animal surface and acquired in the optical images. The fluorescent emission was measured on anatomical Region of Interest (ROIs) traced on the optical images corresponding to the plane projection of the organs. and directly on the surgically extracted organs. Actually we are analysing section from explanted organs with the aim of histologically localizing the exact accumulation sites and to detect the (nanoparticles) fluorescent signal. Conclusions: Fluorescent silica-core / PEG-shell nanoparticles showed a very good fluorescent efficiency comparable with commercial semiconductors nanocrystals (quantum dots, QDs) actually used in preclinical research. They can be successfully used for in vivo applications allowing to follow the biodistribution for hours in a small animal model. The very low intrinsic toxicity of the chemical composition encourages the employ of such fluorescent markers for many in vivo applications in preclinical research and to investigate the possibility to engineering them with biomelcules for targeting bio-analytical applications.