To COIN a Term: Functional Composite Organic-Inorganic Nanoparticles (COINs) for Biomedical Applications

To COIN a Term: Functional Composite Organic-Inorganic Nanoparticles (COINs) for Biodistribution Analysis

Brian K. Wilson¹ and Robert K. Prud’homme¹

¹Princeton University, Princeton NJ, USA

Abstract

Nanoparticles have received considerable attention as delivery vehicles for therapeutic, diagnostic or mixed theranostic agents targeted against a wide variety of disease conditions. Understanding the systemic biodistribution and the local distribution (within tissues or sub-cellular compartments) are of interest for understanding nanoparticle trafficking, uptake and targeting efficacy in vivo. Incorporating a tracer for the nanoparticle vehicle itself allows quantification of both the nanoparticle distribution and the releasable payload, for example a chemotherapeutic drug. Flash NanoPrecipitation (FNP) provides an easy, scalable platform for producing polymeric nanoparticles that encapsulate hydrophobic species in a core stabilized by an amphiphilic block copolymer corona. Small, primary colloids of rare earth metal oxides can be readily prepared with a hydrophobic surface coating, enabling efficient encapsulation via the FNP process. Producing rare earth metal oxide tagged nanoparticles is of interest because of the inert nature of these oxides and the possible multimodal quantification of their biodistribution by: mass spectrometry, fluorescence microscopy and electron microscopy.

Rare earth metals and their oxides (for example: europium, terbium and gadolinium) have negligible endogenous background signal in vivo, enabling quantification of bulk concentrations in digested tissue by plasma mass spectrometry (ICP-MS). Furthermore, these rare earth metal oxide colloids exhibit strong, element-characteristic fluorescence under electron beam or ultraviolet irradiation, enabling them to be used as fluorescent probes that display excellent stability and a lack of concentration-dependent quenching behavior.  Lastly, the metal oxide colloids provide a substantial degree of electron contrast, allowing for their use as markers in TEM sections containing nanoparticles; the specific identity of the metal can be determined by conventional elemental analysis methods such as S/TEM EDX in situ.

Here we demonstrate the synthesis of rare earth metal oxide colloid loaded polymeric nanoparticles and their use in tracking biodistribution by ICP-MS and fluorescence microscopy. Preparing the primary colloids with a small diameter (< 5 nm) and a hydrophobic surface coating is critical for efficient encapsulation and composite organic-inorganic nanoparticle (COIN) production. Thermal decomposition synthesis from metal acetylacetonates and metal tris-oleates produces more uniform colloids in the desired size range when compared to solution colloidal precipitation methods. These primary metal oxide colloids can then be encapsulated in poly(styrene) or poly(lactic acid) based nanoparticles stabilized with a surface layer of poly(ethylene glycol) (PEG) to loadings of at least ~50% weight and ~30% volume of the primary inorganic colloid. The free ends of the PEG chains in the stabilizing corona can be modified with targeting ligands to direct specific uptake of nanoparticles. We explore the use of folic acid, cyclic RGDyK and internalizing iRGD ligands to target nanoparticles to KB and A549 tumor lines, where bulk nanoparticle uptake is quantified by plasma mass spectrometry. In vitro analysis of nanoparticle cell binding and cellular distribution is performed using the metal oxides as fluorescent markers in confocal microscopy.