Validating the Use of Boron Nanoparticles to Quantify Tumor Oxygen Tension in Irradiated Breast Tumors

Abstract

Although hypoxia in tumors has long been identified as a key contributor to poor patient outcome, clinical solutions to lessen its deleterious effects have not been forthcoming. One reason is that there have been few means to directly quantify hypoxia kinetics with adequate spatial and temporal resolution. Dual-emissive boron nanoparticles show promise in this regard. Fluorescence imaging of these dual-emissive boron nanoparticles allows quantification of oxygen tension in a tumor microenvironment; this is accomplished by calculating the ratio of oxygen-independent fluorescence signal to oxygen-dependent phosphorescence signal. In this work, we demonstrate the ability of these nanoparticles to, in conjunction with hyperspectral imaging of hemoglobin saturation, quantitatively characterize the oxygenation state of irradiated murine tumors in vivo. Mice were implanted with E0771 tumors in dorsal window chambers, and tumors were irradiated with 12Gy. Fluorescence images of dual-emissive nanoparticles injected into the tumors, as well as hyperspectral images of hemoglobin saturation, were obtained 1 day before irradiation and 2 days after irradiation. 2-way ANOVA statistical analysis revealed that while oxygen supply to the tumors – indicated by hemoglobin saturation – did not change significantly after irradiation, fluorescence-to-phosphorescence ratios – which indicate oxygen tension in the tumor – increased significantly post-irradiation. Through its success in demonstrating radiation-induced reoxygenation in E0771 tumors, the nanoparticles show promise for further applications in characterizing oxygenation states of various tumors post-treatment. This finding also further underscores the importance of using direct techniques in quantitative studies of tumor oxygen tension. This work presents a second technological objective: to determine if folic acid-conjugated nanoparticles can successfully target folate receptor-expressing cell lines. This is a key step towards more practical delivery of the nanoparticles to deep tumor tissue via intravenous administration, as well as to enable intracellular quantification of oxygen tension. MDA-MB-231 cells were incubated overnight with varying concentrations of folic acid-conjugated nanoparticles. Fluorescence images were obtained to determine uptake of nanoparticles. The results have been encouraging; nanoparticle internalization was observed in almost all cells at all tested concentrations, and higher concentrations of nanoparticles appeared to increase the amounts of nanoparticles internalized per cell. This work has validated that dual-emissive boron nanoparticles can directly quantify tumor oxygen tension post-irradiation, and that folic acid-conjugated boron nanoparticles can successfully target folate receptor-overexpressing cells. Hence, the immediate next step is to introduce folate receptor-targeting abilities to dual-emissive nanoparticles for improved performance in tumor oxygen-sensing applications.