«COLLOIDAL GOLD NANOPARTICLES FOR CANCER THERAPY: EFFECTS OF PARTICLE SIZE ON TREATMENT EFFICACY, TOXICOLOGY, AND BIODISTRIBUTION A Dissertation ...»
Spatial Distribution of Au-DOX-PEG within Tumor (a) Collection of extracted tumor from Control, Doxorubicin, Au-DOX-PEG, and Au-PEG groups. Bottom picture indicates the halved Au-DOX-PEG tumor. (b) Brightfield and darkfield microscopy images of Au-DOX-PEG present within tumor (arrows and orange chunks indicates Au-DOX-PEG) (c) TEM images of Au-DOX-PEG inside the tumor cell (d) TEM images of Au-DOX-PEG outside the tumor cell Finally, the successful therapeutic efficacy of our Au-DOX-PEG resulted from the cellular uptake of Au-DOX-PEG by cancer cells (with the assumption that doxorubicin is released inside the cancer cells after cellular uptake of Au-DOX-PEG). It has been reported that particles transport to the tissues by convection in healthy tissues . However, due to lack of lymphatic drainage, the interstitial hydrostatic pressure increases to severely restrict convective transport [181, 182]. As a result, diffusion becomes the dominant means of transport of nanoparticles in tumor tissues. The rate of transport through the extracellular matrix is determined by the effective interstitial diffusion coefficient, which decreases as molecular weight is increased . Thus, as high concentrations of Au-DOX-PEG was injected to the system, concentration gradient acts as a driving force to result in the uptake of gold nanoparticle system by cancer cells.
Spatially, the TEM images in Figure 4.6 confirm that Au-DOX-PEG system is present both inside and outside of tumor cells. The uptake of Au-DOX-PEG by the tumor cells seems to be non-specific endocytosis, as they are usually found inside a vacuole or endosome-looking vesicles inside the cells. Within the cells, Au-DOX-PEG system is mostly present within the outer membrane of the endosome due to diffusion-limited, heavy gold core of Au-DOX-PEG system.
Three mechanisms can be suggested for the successful therapeutic efficacy of our Au-DOX-PEG system: (1) most of the Au-DOX-PEG was accumulated in the tumor stroma and doxorubicin was released from the acidic tumor stroma to be uptaken by tumor cells or extracellular components (it has been reported that drug carriers larger than 10kDa can localize near the vascular surface and release the drug from its carrier when it is near the vascular surface so that drug could penetrate much deeper into the tumor tissue , (2) most of the Au-DOX-PEG was uptaken by the tumor cells by concentration gradient-driven diffusion process, where doxorubicin was released inside the tumor cells to exert its therapeutic efficacy, or (3) Au-DOX-PEG exerted its therapeutic efficacy by releasing doxorubicin both inside and outside the tumor cell (i.e.
combination of both (1) and (2)). It is hard to conclude whether most of Au-DOX-PEG was inside or outside the tumor cell to exert therapeutic efficacy from Figure 4.6 TEM images. Kirpotin et al. reported that non-targeted (passive) colloidal gold encapsulating liposome drug delivery system were predominantly present within the tumor stroma, either in extracellular space or within tumor-resident macrophage but not within the tumor cells themselves, after 7 days of injection . However, Kirpotin et al. used a larger nanoparticle of 86 nm in mean diameter for the experiment, whereas our Au-DOXPEG is 18 nm in mean diameter. Thus, small-size scale of Au-DOX-PEG resulted in uptake of our system by the cancer cells via passive targeting.
ICP-MS Analysis of Gold in Tumor After Single Injection Additionally, PEGylation of our gold nanoparticle drug delivery system has affected the overall uptake of the Au-DOX-PEG. It has been reported that as the nanoparticles are transported within the tumor interstitium, nanoparticles can nonspecifically interact with various proteins and tissue compartments in the tumor interstitium or get metabolized . PEGylation is known to render non-fouling surfaces that minimize nonspecific binding of proteins . Also, PEGylation of our gold nanoparticle, along with attachment of modified doxorubicin, resulted in a nearneutrally charged nanoparticle that decrease the binding of proteins and increased the uptake of Au-DOX-PEG by the tumor cells.
In summary, the therapeutic efficacy of Au-DOX-PEG resulted from its “smallsize” and “near neutral charge” that led to effective transport of the nanoparticle across the tumor microenvironment and reaching the cancer cells at optimal concentrations.
Moreover, it is the combination of the EPR effect (leaky tumor vasculature and decreased clearance rate due to defective lymphatic drainage) along with the concentration-driven, diffusion-limited transport of Au-DOX-PEG system that resulted in successful high accumulation of Au-DOX-PEG at the tumor site. We believe that acidic tumor environment and endosome trigger the release of the bound doxorubicin through cleavage of pH-sensitive hydrazone bond to result in therapeutic efficacy .
Biodistribution of Au-DOX-PEG in Various Organs and Toxicity To further understand the therapeutic efficacy of Au-DOX-PEG, biodistribution of Au-DOX-PEG in tumor and various organs were investigated through InductiveCoupled Plasma Mass Spectroscopy (ICP-MS) analysis of gold. We observed significant accumulation of Au-DOX-PEG in tumor compared to other normal organs (Figure 4.8 (a)). High accumulation of Au-DOX-PEG is due to the size scale and prolonged circulation of Au-DOX-PEG, taking advantage of the EPR effect. Nanoparticle plasma retention time is one of the primary driving forces for nanoparticle tumor accumulation by the EPR effect  that drug concentration in plasma must remain high for more than 6 hours to satisfy the EPR effect in solid tumors [25, 36, 185]. For our Au-DOX-PEG system, PEGylation contributed to the prolonged blood circulation time of ~1.6 days compared to that of pure doxorubucin with few minutes. This prolonged circulation time promotes EPR effect and results in successful accumulation of Au-DOX-PEG at tumor site, where the small-sized Au-DOX-PEG easily extravasate out of the leaky tumor vessel to ultimately result in therapeutic efficacy.
The key to a successful anti-tumor efficacy of a drug system relies on the accumulation and spatial distribution within the tumor. Especially for passive targeting, molecular weight (or size) and charge become the dominant factors that govern the accumulation of drug delivery system at the tumor site . The accumulation of AuDOX-PEG at the tumor site was enhanced by the small molecular weight of the system (but greater than 40kDa to satisfy EPR condition ) that resulted in longer blood circulation half time of ~1.6 days. Moreover, the PEGylation of the small-sized gold nanoparticle core has enhanced the circulation time and minimized any non-specific binding by the serum proteins for RES clearance . PEGylation also resulted in a slightly negative, near-neutrally charged Au-DOX-PEG system. It has been reported that anionic and neutral particles have prolonged blood circulation half-life . In contrast to positively-charged particles, negatively-charged particles results in prolonged blood circulation half-life due to reduced interactions between tissue and cells . Slightly negative charge resulting from PEGylation was effective to give longer plasma half-life for Au-DOX-PEG, which ultimately led to higher accumulation at the tumor site.
Additionally, it is the ultra-structural differences between the normal and tumor vasculature that resulted in a higher concentrations of Au-DOX-PEG system in tumor site compared to other organs. The permeability of normal vasculature decreases with the increasing hydrodynamic diameter of 3.6 nm, which is below the size limit of our system . On the other hand, the permeability/ transport across the tumor vasculature is poorly regulated that tumor vasculature allows molecules up to 2 μm, which allows our Au-DOX-PEG system to easily extravasate and penetrate into the tumor interstitium [25, 27, 189-191]. Thus, the leaky vasculature and the increased permeability of Au-DOXPEG within the tumor interstitium ultimately led to high accumulation of Au-DOX-PEG at the tumor site. Similar to the STEALTH liposomal drugs [192, 193], we believe that our Au-DOX-PEG initially accumulated at the tumor site via EPR effect over the course of few days (as indicated by the blood circulation half-life of ~1.6 days). Then, the drug is slowly released over the few weeks in a controlled manner (as seen in Figure 4.3), where the drug penetrates deeper into the tumor tissue due to creation of diffusion gradient.
Here, we want to emphasize the fact that therapeutic efficacy of Au-DOX-PEG system resulted from “passive” transport of Au-DOX-PEG. Both active targeting and passive targeting require passage through the leaky tumor blood vessel and extravasate into the tumor interstitium or the perivascular region . There has been a controversy where several works have shown that the use of tumor-targeting ligands does not increase the total accumulation of the nanoparticles in solid tumors [17, 184, 194, 195]. The targeting-ligands rather increase the receptor-mediated internalization of the nanoparticles for improved therapeutic efficacy. According to Huang, X. et al, the targeted and non-targeted gold nanorods displayed “marginal” difference in terms of total gold accumulation in xenograft tumor models. However, targeted nanorods altered the intra- and extra-cellular distribution compared to non-targeted nanorods . Moreover, targeting ligands shorten the blood circulation life through opsonization (adsorption of blood proteins) .
It is interesting to see our passive targeting Au-DOX-PEG system resulted in a high accumulation at the tumor site to exert its therapeutic efficacy. As seen in Figure 4.8 (a), the amount of gold accumulated per gram of organ at tumor was similar to that of spleen and greater than liver. In contrast, targeted nanorods (hydrodynamic diameters of 51nm for nanorod and ~80nm for coated nanorod) resulted in fewer amounts of gold accumulated per gram of organ at tumor compared to spleen and liver . Indeed, majority of gold was taken up by the liver and spleen (60-90% of the total injected gold) and less than 2% of the total injected gold was taken up by tumor for targeted nanorods.
Similar trend was seen for TNF-α coated gold nanoparticle (hydrodynamic diameter of 27nm with coating), where amount of gold accumulated per gram of organ at tumor site was significantly less than that of spleen and liver . For our Au-DOX-PEG system, passive targeting resulted in 7.0% of the total single injected gold to be uptaken by the tumor itself. The high accumulation and uptake of Au-DOX-PEG is rendered from the size of our system (5nm core and 18nm for coated). However, we cannot exclude the fact that the higher doses used in this study may have altered the accumulation rate of AuDOX-PEG at tumor site, which could have led to increased nonspecific uptake of our system by the cancer cells.
(a) Amount of Gold per Gram of Organ (ppb) (b) (c) (d) (e) Figure 4.8. Biodistribution of Au-DOX-PEG in Various Organs and Low Toxicity After 2 weeks of Treatment (a) ICP-MS analysis of gold: the size resulted in high accumulation of Au-DOX-PEG at tumor site compared to other non-tumor sites (b) Au-DOX-PEG uptake in spleen and liver (c) Hematoxylin & Eosin staining of extracted organs after
treatment (d) TEM images of various organ after Au-DOX-PEG administration [Spleen:
Red blood cells are seen (polychromatophillic). ; Liver: Dark spots are glycogen granules.
Gold nanoparticles are mostly inside the blood vessel cells (outside the cell) and some are seen inside the liver cell near the nucleus. ; Kidney: Some are inside the blood vessel cell and others are inside some cell. ; Heart: Gold nanoparticles are inside an irregular-shaped cell in between the heart muscle fiber. Some gold nanoparticles are in the cavity/muscle lignin of the heart.; Lung: Gold nanoparticles are overall inside the blood vessel (outside the lung cell).] (e) Blood serum analysis [*=P0.05; LDH=lactate dehydrogenase; ALT= alanin transaminase; AST= aspartate aminotransferase; Alk Phos= alkaline phohphatase] A successful drug delivery system not only exerts therapeutic efficacy but also reduces systemic toxicity when administered. Compared to pure doxorubicin, it is important to note that Au-DOX-PEG system lacked any apparent toxicity in vital organs.
Due to its small size, Au-DOX-PEG accumulated throughout the various organs, including tumor, in the system (Figure 4.8(a), (b)), consistent with the literature results.
The smaller size and longer circulation time results in accumulation of Au-DOX-PEG at non-tumor sitesand concerns for toxicity at normal, healthy cells arises. However, as shown in Figure 4.8 (c), Au-DOX-PEG in spleen, liver, heart, kidney, and lung did not display any apparent toxicity compared to the Control (untreated) mice. Pure doxorubicin also did not display any apparent toxicity in the spleen and liver. However, pure doxorubicin treated mice displayed toxicity particularly in the heart, kidney, and lung.
Edema, swelling of cells, was present in kidney for Doxorubicin group where nephric tubules exhibited swelling, along with congestion of renal glomeruli and narrowed Bowman’s space. For heart, interstitial edema was seen for Doxorubicin group that swelling was present for heart muscle fibers. For lungs, red blood cells were present within the alveoli sac space for Doxorubicin group, in contrast to clear alveoli sac space of the Control and Au-DOX-PEG groups.
The inflammatory and oxidative response can be obtained by analyzing the blood serum. For example, when there is damage to the liver, there is an increased expression of certain immunological proteins in the blood that can be detected by serum analysis.
Serum protein analysis results seen in Figure 4.8 (e) complements our toxicity results found in histological results. Serum proteins such as ALT, AST, and Alkaline phosphatase can be used to measure liver toxicity. Here, there were no significant differences amongst control (untreated), doxorubicin, Au-DOX-PEG, and Au-PEG groups for each ALT, AST, and Alkaline phosphatase level. This indicates minimal toxicity to the liver, which is consistent with the histology results shown in Figure 4.8 (c).