Radiolabeling Strategies of Micron- and Submicron-Sized Core–Shell Carriers for In Vivo Studies

Core-shell particles made of calcium carbonate and coated with biocompatible polymers using Layer-by-Layer technique can be considered as a unique drug delivery platform that enables to load different therapeutic compounds, exhibits a high biocompatibility and can integrate several stimuli-responsive mechanisms for drug release. However, before implementation for diagnostic or therapeutic purposes, such core-shell particles require a comprehensive in vivo evaluation in terms of physicochemical and pharmacokinetic properties. Positron Emission Tomography (PET) is an advanced imaging technique for the evaluation of in vivo biodistribution of drug carriers; nevertheless, an incorporation of positron emitters in these carriers is needed. Here, for the first time we demonstrate the radiolabeling approaches of calcium carbonate core-shell particles with different sizes (micrometric: MicCSPs and submicrometric: SubCSPs) in order to precisely determine their in vivo biodistribution after intravenous administration in rats. For this, several methods of radiolabeling have been developed, where the positron emitter (68Ga) was incorporated inside the particle's core (co-precipitation approach) or onto the surface of the shell (either with layer coating or adsorption approaches). According to the obtained data, the radiochemical bounding and stability of 68Ga strongly depend on the used radiolabeling approach, and the co-precipitation method has shown the best radiochemical stability in human serum (96-98.5 % for both types of core-shell particles). Finally, we demonstrate the size-dependent effect of core-shell particles distribution on the specific-organ uptake, using a combination of imaging techniques, PET and Computerized Tomography (CT), as well as radiometry of separate organs. Thus, our findings open up new perspectives of CaCO3 radiolabeled core-shell particles for their further implementation into clinical practice.