Seite - 125 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Liuet al. AuNS forcancer imagingand therapy et al., 2013). SERS imageswere acquiredat 633nmwitha2-µm step size by using a RenishawRamanmicroscope. The 633nm resonant dye, DTDC was used to label the AuNS for SERS detection. SERS imaging shows that TAT-functionalized AuNS nanoprobeshavemuch stronger signal inside cells. Recently,we have reported the synthesis of silver-coatedAuNS (AuNS@Ag) that provide over an order of magnitude increase in SERS signal when compared to AuNS alone (Fales et al., 2014).We have applied the AuNS@Ag for in vitro homogenous nucleic aciddetection,whichdemonstratedAuNS’ strong capability for molecular sensing with the SERSmethod (Wang et al., 2015). With such flexibility, AuNS thus are a promising nanoplatform for multimodality imaging from whole body scan to high- resolutionoptical imagingaswell asmolecular sensing. BrainTumor Imaging Thepotential ofAuNSas an efficient contrast agent has further been investigated in animal brain tumor models (Yuan et al., 2014).Throughcranialwindowchambers implantedwith tumor cells in live animals,AuNShavebeenemployed for angiography with high spatial resolution; cerebral capillaries were clearly visiblewithminimal tissueautofluorescencebackground.Unlike conventionalcontrastagents(e.g.,FITC-dextran)showingsignal decay within 30min, AuNS can be coated with polyethylene glycol (PEG) to achieve a much longer intravascular signal stability due to its extended serumhalf-life (several hours) and lowerdegreeofextravasation. The main challenge of tumor imaging is delivering the contrast sufficiently to the target. Several physiological barriers exist between the injection site and the target tumor cells. For example, nanoparticles need to survive immunoclearance, extravasate tumor vessels, permeate the blood brain barrier (BBB), diffuse through interstitium, and penetrate the plasma membrane into cells (Chrastina et al., 2011; Goldberg et al., 2013).Forbrain tumor imaging, thegreatest challenge lies in the permeationof theBBB.Tight junctionsbetween the endothelial cells and podocytes from the astrocytes form a highly selective barrier that prevents large molecules from passing through. To date, the typical 24-h post-injection brain accumulation of nanoparticles may still be less than 0.1% of the initial dosage (KhlebtsovandDykman, 2011); this is similar to thoseobtained aftermonoclonal antibody infusion. Several nanoparticle-based platforms have been studied, including transferrin-containing gold nanoparticles (Wiley et al., 2013), polysorbate 80-coated poly(n-butyl cyanoacrylate) dextran polymers (Koffie et al., 2011), and angiopep2 peptide-functionalized dendrimers (Yan etal.,2012).SinceAuNSareastrongcontrastagent, it istherefore prudent to investigate the effect of surface functionalization on brain tumortargeting. Nanoparticlesaccumulate intumors typically throughpassive and activemechanisms.While surface PEGylation achieves the former, thelatterrequiressurfacefunctionalizationofpeptidesor proteins.Uponsystemicexposure,PEGylatedAuNSaccumulate passively near the tumor periphery and around blood vessels. Hyper-neovascularity along the tumor edge and interstitial fluid pressure gradient at the boundary attenuate the penetration of AuNS deep into the tumor. PEG-AuNS also accumulate in vascular endothelial cells butwithminimal true transcytosis(Ye et al., 2013), where paracellular extravasation is most likely attributed to the defective tight junction at the tumor site. With extravasation depending on the nanoparticle size and incubation time (Yuanet al., 1994;Popovic et al., 2010), smaller nanoparticles or longer incubation may further increase the tumoraccumulationorextravasationdepth. Comparing the brain tumorAuNS delivery through passive and active mechanisms, we coated 80-nm AuNS with PEG (PEG-AuNS)andTATpeptides (TAT-AuNS), and50-nmAuNS withangiopep2peptides(angiopep2-AuNS).Angiopep2-peptide has recently been exploited to deliver nano-drugs into the brain via lipoprotein receptor related protein (Gabathuler, 2010). PEG-AuNS accumulated in the tumor periphery and parenchyma but much less in the normal brain. They also accumulated inside endothelial cells and perivascular spaces, particularly in the tumor regions of defective vascular integrity. AsshowninFigure2,TAT-AuNSaccumulated less in tumorbut extensively in liver compared to PEG-AuNS. Positively charged TATpossibly attracted opsonins, causing greater entrapment in the reticuloendothelial system. In contrast, smaller angiopep2- AuNS had a deeper distribution inside tumor parenchyma with relatively less liver accumulation. Calculated from the AuNS intensity ratio from the images collected under the same microscopic settings, the average tumor/liver AuNS density ratios are 0.32, 0.03 and 1.2 for PEG-, TAT- and angiopep2- coatedAuNS.These findings suggest that enhanced tumorBBB permeationwith selected intratumoral delivery can be achieved withpropercontrolofAuNSsizesandselectionofsurface ligand chemistry. AuNSforPhototherapy AuNShavealsobeenappliedforPDTandphotothermal therapy (PTT). PDT uses a photosensitizer that generates ROS upon irradiation to kill cells, while photothermal therapy transduces light to heat for cancer hyperthermia or ablation. The effect of laser type on PTT has been investigated before and the results showed that pulsed lasers could use 10 times less energy to kill cancer cells than that used for continuous-wavelength (CW) lasers (Huang andEl-Sayed, 2011). That is because pulsed-laser with gold nanoparticles can lead to a variety of therapeutic effects including protein denaturation, cell cavitation, bubble formation fromshockwavesandplasmageneration (Pustovalov et al., 2008).With the nanoparticle’s tumor targeting effect and focused irradiation to the tumor site, phototherapy can achieve greater tumor therapeutic specificity with less off-target effects. The therapeutic potentials have been demonstrated on gold nanoshells,nanocages,andnanorods(VonMaltzahnetal., 2009; Xie et al., 2010;Day et al., 2011). Gold nanoshells were used to performPTT(800-nmdiode laser,4W/cm2 intensity for3min.) with murine xenograft models and results showed significant survival rate improvement for the treatment group with 57% of mice remaining tumor free at the end of 90 days, while the mean survival date for the control group was only 13.3 days. Gold nanocages functionalizedwithHER2-antibody have Frontiers inChemistry |www.frontiersin.org August2015 |Volume3 |Article51 125|