Nanomedicines inability to penetrate throughout the entire tumor volume due to heterogeneous distribution within the tumor mass remains a crucial limiting factor for a vast range of theranostic applications, including image-guided radiation therapy. Despite innumerable studies conducted on the topic have shown efficacy and biocompatibility of colloidal gold nanoparticles (GNPs), the biological effects of GNPs in the tumor microenvironment, including the particle-protein interaction and the consequent impact on cellular pathways and contrast enhancement remain unclear. In this regard, further investigations on how GNP surface passivation effects X-ray attenuation as well as in vivo biodistribution will clarify several aspects still under discussion from the scientific community, which so far have limited the clinical translation of their applications cancer-related.
We aim to evaluate the influence of protein surface adsorption on the GNP biodistribution in Lewis Lung Carcinoma (LLC) tumor-bearing mice using high resolution Computed Tomography (CT) preclinical imaging. We hypothesize that by controlling the adsorption of proteins on the GNP surface, we can influence the intratumoral distribution and retention of the particles.
GNPs approximately 34 nm in diameter are synthesized with a surface plasmon peak at ~530 nm, surface passivated with Bovine Serum Albumin (BSA) to reduce opsonization and improve colloidal stability, and characterized with standard methods. Modulation of BSA adsorption on the GNPs is observed by tuning the pH of the immobilization medium from acidic to alkaline, which we quantify using Langmuir isotherms. CT phantom imaging is used to determine X-ray attenuation as a function of GNP concentration and surface functionalization. The in vitro study for evaluating the uptake of GNPs by Lewis Lung Carcinoma cells highlighted a difference in the internalization depending on the surface functionalization. In both cases, macropinocytosis is the trafficking mechanism, but while endosomes with citrate-GNPs can be found in different stages of maturation, cells treated with BSA-GNPs presented larger vesicles up to 1μm in diameter. The in vivo study is performed by injecting intratumorally, concentrated GNPs into Lewis Lung Carcinoma (LLC) solid tumors grown on the right flank of 6-week old female C57BL/6 mice. Ten days post-injection, follow-up assessments with CT imaging show the distribution and retention of the particles in the tumor. CT attenuation quantification based on bioimaging analysis for each time point is conducted.
In vivo results show significant heterogeneity in the intratumoral biodistribution of GNPs dependent on surface passivation. BSA-GNPs perfuse predominately along the tumor periphery with few depositions throughout the entire tumor volume. This response can be explained by the abnormal and heterogeneous vascular structure of the LLC tumor, suggesting perfusion rather than permeability as the limiting factor for tumor accumulation of the GNPs. Despite the perivascular cluster accumulation, the BSA-GNP distribution diverges from that obtained after unpassivated, citrate-GNP intratumoral injections.
In conclusion, our investigations have shown that surface passivation of GNPs is able to influence the mechanism of cellular uptake in vitro and their in vivo intratumoral diffusion highlighting the spatial heterogeneity of the solid tumor.