Development of A Gadolinium-Based Nanotheranostics Platform for Irradiation Activatable MRI-Radiosensitization and Doxorubicin Release in Cancer Therapy

Abstract: The ultimate goal of this thesis research was to design, fabricate, and characterize an integrated nanotheranostics platform for activatable doxorubicin delivery and simultaneous MRI- radiosensitization for synergistic cancer therapy, targeting breast cancer treatment. As shown in the diagram, once these nanocomplexes enter into tumour sites through the EPR (enhanced permeability and retention) effect, an external radiation source can be applied with extreme precision to activate the cleavage of the mPGA shells (the outer fluorescent blue layer in the diagram). This exposes the positively charged Gd:Mn-Dox cores which are readily adsorbed by the negatively charged cancer cell membrane and internalized through endocytosis. The acidic environment within endosomes then triggers the dissociation of the Gd:Mn-Dox nanocomplexes contained within them, and resulting release of doxorubicin and gadolinium for simultaneous real time MRI-radiosensitization and synergistic therapy. Gadolinium, as one of lanthanide metal elements, has been well investigated and its chelates are commonly used as T1 contrast agents for clinical magnetic resonance imaging (MRI). Anthracycline antibiotics, such as the cancer chemotherapeutic drug doxorubicin, can form complexes with transition metals. Gadolinium and doxorubicin drug-metal complexes were selected for study, and were fabricated as nanostructures through a simple one-step homogeneous precipitation method. They were designed for acidic environment-triggered doxorubicin and gadolinium release. A partially modified poly-glutamic acid polymer (which was esterified to prevent the polymer material from undergoing auto-hydrolysis) was used to coat the surface of the harvested spherical Gd:Mn-Dox nanocomplexes. This surface coating was designed to be radiation activatable and biocompatible. The first nanostructures synthesized in this thesis were Gd(OH)3 and Gd(OH)3:Mn crystalline nanorods, and rod-shaped crystalline Gd(OH)3:Mn-Dox nanocomplexes with a 7.85wt% drug loading capacity of doxorubicin. The longitudinal dimension of these nanorods and nanocomplexes ranged from 200 nm to 400 nm. Next, the reaction process was modified to successfully produce spherical amorphous Gd:Mn and Gd:Mn-Dox nanocomplexes with a higher doxorubicin loading capacity (10.1wt%). The diameter of these nanospheres could be tuned within the range of 100 nm to 500 nm by using different quantities of glycerol in the reaction process. Both the rod-shaped and spherical nanocomplexes were demonstrated to undergo dissociation and release of doxorubicin and gadolinium in acidic environments. The amorphous spherical nanocomplexes had a shorter release time relative to the crystalline rod-shaped nanocomplexes. Confocal and TEM micrographs demonstrated that the synthesized nanocomplexes were actively taken up via endocytosis by human breast cancer cells. A radiation-activated radiosensitization effect, as well as dose-dependent trends in the percentage of apoptotic cells, were measured in the nanocomplex-treated cancer cells in vitro. MRI traceability of this nanotheranostics platform was demonstrated both in vitro and in vivo, using clinical MRI and PET-MRI. These nanocomplexes were well tolerated in rats at the highest tested dose of 240 mg/kg administered intravenously, with negligible histological changes observed. The in vivo biodistribution of these intravenously injected nanocomplexes was observed to be mainly in the liver, lungs and spleen. Overall, a novel and smart doxorubicin-loaded gadolinium-based amorphous cancer nanotheranostic system was developed with a very simple and environmentally-friendly fabrication process. This system demonstrated the capability to deliver gadolinium and doxorubicin for theranostic MRI-radiosensitization and doxorubicin chemotherapy. This proposed nanotheranostics platform represents an increasing trend in cancer nanotheranostics towards the research and development of novel and much more effective drug delivery platforms which pave the way for individualized cancer medicine. This thesis work has explored the feasibility of fabricating a new amorphous-phase gadolinium-based drug/metal theranostic nanocomplex doped with manganese through an environmentally friendly process. The research performed in this thesis takes advantage of the fact that doxorubicin drug molecules can form drug-metal complexes with gadolinium and manganese ions, allowing one to avoid incorporating other complex chelating materials. This work also uses a “smart” radiation-activable strategy for theranostic cargo delivery. This new design of a nanotheranostic solution paves the way for new clinical workflows where real-time MRI-guided synergistic therapy can be carried out during treatment. This work also inspired the consideration of other suitable drugs that may be studied using a similar work flow to extend their potential applications into theranostics and brought about new insights regarding the fabrication of theranostic nanoplatforms.