Selected radiotracers as imaging tools for the investigation of nano-sized delivery systems

Abstract

Developing nanoparticulate delivery systems that will allow easy movement and localisation of a drug to the target tissue and provide more controlled release of the drug in vivo is a challenge for researchers in nanomedicine. The aim of this study was to evaluate the biodistribution of two nano-delivery systems namely, poly(D,L-lactide-co-glycolide) (PLGA) nanoparticles containing samarium-153 oxide ([153Sm]Sm2O3) as radiotracer and solid lipid nanoparticles (SLNs) containing technetium-99m-methylene diphosphonate (99mTc-MDP), after oral and intravenous administration to rats to prove that orally administered nanoparticles indeed alter the biodistribution of a drug as compared to the drug on its own. Stable samarium-152 oxide ([152Sm]Sm2O3) was encapsulated in polymeric PLGA nanoparticles. These were then activated in a nuclear reactor to produce radioactive [153Sm]Sm2O3 loaded-PLGA nanoparticles. Both the stable nanoparticles as well as the fully decayed activated nanoparticles, were characterized for size, Zeta potential and morphology using dynamic light scattering and scanning electron microscopy (SEM) or transmission electron microscopy (TEM), respectively. SLNs were a form of delivery system which was used to encapsulate the radiotracer, 99mTc-MDP. 99mTc-MDP SLNs were characterized before and after encapsulation for size and Zeta potential. Both nanoparticle compounds were orally and intravenously (IV) administered to rats in order to trace their uptake and biodistribution through imaging and ex vivo biodistribution studies. The PLGA nanoparticles containing [153Sm]Sm2O3 were spherical in morphology and smaller than 500 nm, therefore meeting the objective of producing radiolabelled nanoparticles smaller than 500 nm. Various parameters were optimized to obtain an average particle size ranging between 250 and 300 nm, with an average polydispersity index (PDI) ≤ 0.3 after spray drying. The particles had a Zeta potential ranging between 5 and 20 mV. The Sm2O3-PLGA nanoparticles had an average size of 281 ± 6.3 nm and a PDI average of 0.22. The orally administered [153Sm]Sm2O3-PLGA nanoparticles were deposited in various organs which includes bone with a total of 0.3% of the Injected Dose (ID) per gram vs the control of [153Sm]Sm2O3which showed no uptake in any organs except the GI-tract. The IV injected [153Sm]Sm2O3-PLGA nanoparticles exhibit the highest localisation of nanoparticles in the spleen (8.63%ID/g) and liver (3.07%ID/g). The 99mTc-MDP-labelled SLN were spherical and smaller than 500 nm. Optimization of the MDP-loaded SLN emulsions yielded a slightly higher PDI of ≥0.5 and a size range between 150 and 450 nm. The Zeta potential was between -30 and -2 mV. The MDP-loaded SLN had an average size of 256 ± 5.27 and an average PDI of 0.245.The orally administered 99mTc-MDP SLN had the highest localisation of nanoparticles in the kidneys (8.50%ID/g) and stomach (8.04%ID/g) while the control, 99mTc-MDP had no uptake in any organs except the GI-tract. The IV injected 99mTc-MDP SLN also exhibited a high localisation of particles in the kidneys (3.87%ID/g) followed by bone (2.66%ID/g). Both the IV and oral 99mTc-MDP SLN reported significantly low deposition values in the heart, liver and spleen. Based on the imaging and the biodistribution studies, it can be concluded that there was a significant transfer of the orally administrated radiolabelled nanoparticles from the stomach to other organs vs the controls. Furthermore, this biodistribution of the nano carriers warrants surface modification and optimisation of the nanoparticles to avoid higher particle localisation in the stomach.