Page - 56 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Cooperet al. Nanoparticles for radiation therapy porphyrin (TPP) grafted to the polysiloxane shells after rather than during their formation (Bulin et al., 2013). The NPs and NP-PS were used in DEG solution. Upon excitation at 300nm (primarily resulting in4f8→4f75d1 transitions inTb3+), acon- currentdecreaseof theTb3+ lifetimes (measuredat545nm)and appearance of long PL lifetimes of the grafted PS (measured at 650nm)were taken to be indicative of excited Tb3+-PS nonra- diative energy transfer. Interestingly, the polysiloxane layer was implicated in the appearance of a broad emission component from the NPs with a peak∼425nm that was also involved in efficient, fast energy transfer to TPP under optical excitation, but didnot appearunderX-ray excitation. Singlet oxygen yields under 44kVX-ray excitation (froma tungsten anode, providing adoserateof5.4mGy/s)wereevaluatedwiththechemicalprobes singletoxygensensorgreen(SOSG)and3′-p-(aminophenyl)flu- orescein (APF). SOSG showed a steady increase in signal with both PS alone and NP-PS, with the NP-PS showing a relative increase for irradiation times>10min. TheAPF probe corrob- oratedthe formationofsingletoxygenbytheNP-PSsystem,sup- ported by competitive quenching of singlet oxygen by addition ofNaN3. A small number of nanoscintillator-PS conjugate systems have demonstrated measurable enhancements of X-ray irradi- ation in cancer cell lines. In one study, commercially available Y2O3 NPs were modified with 2-chloroethylphosphonic acid (2-CEP) ligands, which were used to form thioether linkages to fragments of the HIV-1 TAT cell-penetrating/nuclear target- ing peptide bound to the PS psoralen (Scaffidi et al., 2011). A small but significant downward trend in the growth of PS- 3 prostate cancer cells with 2Gy of 160kVp or 320kVpX-rays was seen as a function of particle dose. Another study reported activity of a terbium-doped gadoliniumoxysulfide-Photofrin II mixture against glioblastoma cells irradiatedwith 120kVpdiag- nostic X-rays. Radiation alone produced 20% cell suppression, andradiationplustheNP-PScombinationover90%suppression. Interestingly, the particles alone (without Photofrin) protected thecells againstX-irradiation. A theoretical paper investigated the conditions required for a nanoscintillator-photosensitizer conjugate system to produce therapeutically-relevantresults,usingphysicalparametersinclud- ingnanoparticleuptakeintocells,enhancementofradiationdose, scintillation lightyields, andenergy transferefficiencies (Morgan et al., 2009). These parameters were used to estimate the over- all singletoxygenyieldofaNP-PSsystemwithX-ray irradiation. As singlet oxygen is considered to be the primary effector of PDT, its production was taken to be indicative of the potential of conjugates to damage malignant tissue through PS activa- tion. Overall singlet oxygen production 1O2 was determined from the product of the scintillation yield ϕs, characteristic of thematerial and given in photons perMeV of absorbed radia- tion, theNP-PSenergy transfer efficiencyϕET, and thePSsinglet oxygen yield ϕp. For an extremely generous value of ϕs> 105 photons/MeV (derived from the energy output of bulk crys- tals ofhygroscopicLuI3:Ce3+) and somewhat generousvaluesof ϕET = 0.75 andϕp = 0.89, andusing the relativeX-ray absorp- tion of the NPs, it was determined that to deliver the “Niedre killing dose” of singlet oxygen (reduction of a cell population to 1/e fraction, based on in vitromeasurements of OCI-AML5 leukemia) (Niedre et al., 2002, 2003), onlyX-ray energies below ∼200keV (with peak efficiency∼50keV)would be effective for reasonable total radiation doses. These results suggest that it would be difficult to produce a dramatic outcome with PDT effectsalone. IthasbeenestablishedthattheefficacyofPDT invivodepends on threeprimarymechanisms: direct tumor-cell killing; damage to tumor vasculature; and provocation of an immune response (in contrast to the immunosuppressive effects of radiotherapy and chemotherapy (Dolmans et al., 2003). If these observations hold true for nanoscintillator-photosensitizer systems, it is con- ceivable that the optimal targeting and cell-level distributions of suchsystemsmaybedifferentfromthosethatrelysolelyonradia- tiondoseenhancementbynanoparticles(whicharemosteffective in close proximity to cell nuclei). It would also be reasonable to expect that preserving the amphiphilicity of bioconjugatedpho- tosensitizersmightbebeneficial,as thetendencytoassociatewith lipidmembranes isknowntobeakeyfactor intheactivityof free PSmolecules (Kessel et al., 1987; Jori andReddi, 1993).Whether active targeting to tumors improves nanoparticle accumulation inhumancancersand/or treatmentoutcomesremainsdebatable. There are certainly circumstances inwhichpassive accumulation is insufficient due to the physical properties of the tumor, but the ideal target for human tumors has not beenwell established (Kobayashi et al., 2013;MoghimiandFarhangrazi, 2014;Nichols andBae,2014). OTHERALTERNATIVES:CHEMOTHERAPY-NANOPARTICLE CONSTRUCTS A largenumber of nanoparticle conjugates to chemotherapeutic agents have been reported, but few of these have been used for radiosensitization.This is somewhat surprising, since traditional chemotherapeuticagentsoftenactasradiosensitizers,andproba- bly just reflects theemergingstateof thefield.A fewreportshave targetedmetal nanoparticles to cells or tumors usingmolecules thatplayanactiverole indestroyingthe targetcells. Inonestudy, radioresistantmelanomacellswereexposedtoAunanorodscon- jugated to the RGDpeptide (Xu et al., 2012). Exposure toMV X-rays decreased integrin expression and rendered the cells sus- ceptible toradiation-inducedapoptosis. Another study showed that nanoparticle preparations of epithelial growth factor receptor (EGFR) antisense oligonu- cleotides radiosensitized SCCVII murine squamous carcinoma cells(Xuetal.,2012).However, thenanoparticlesthemselveswere a delivery vehicle only, so no synergywas being sought between theparticlesandtheircargo. In another approach, doxorubicin conjugated toDNA-coated largeAunanoparticleswas loaded intoMCF-7breast cancercells (Starkewolf et al., 2013). Irradiation with X-rays improved cell inhibitionby33%at10Gyrelative toDoxaloneorAunanopar- ticles alone.The authors attributed this observation to releaseof Doxbytheradiation. SUMMARYANDCONCLUSION Dense inorganic nanoparticles show considerable promise for dose enhancement of radiation therapy and enabling synergistic www.frontiersin.org October2014 |Volume2 |Article86 | 56