Seite - 16 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Condeetal. Biofunctionalizationandsurfacechemistryof inorganicnanoparticles propertiesof thebiological cargowithoutany indicationofnon- specificbindingorparticleaggregation(Wangetal., 2005). The stability conferred by peptide ligands usually depends on their length, hydrophobicity, and charge and in some cases resulted in further improved stability. Actually, Levy et al. designed a pentapeptide ligand, CALNN,which converts citrate stabilizedAuNPsintoextremelystable,water-solubleAuNPswith some chemical properties analogous to those of proteins. These peptide-capped AuNPs can be freeze-dried and stored as pow- ders that canbe subsequently redissolved toyield stable aqueous dispersions(Levyetal., 2004). Biofunctional peptide sequences include “membrane translo- cation signals” like theHIV-TATpeptide sequence (de la Fuente andBerry, 2005; Berry et al., 2007; Conde et al., 2012a), which is capable of transporting nanoscale materials across cellular membranes and “nuclear locating signals” that could be used for further intracellular targeting (Nativo et al., 2008;Chithrani, 2010). In fact, de la Fuente et al. developed AuNPs functional- izedwithHIV-Tatpeptideused toachievecytoplasmandthecell nucleus. One of the major drawbacks of these cell penetrating peptides is that theyarenotcell specificandthat theycanremain entrappedinendosomes(GumpandDowdy,2007).Toovercome these limitations, fusogenicpeptides that are able to escape from endosomes or homing peptides capable of reaching specific tis- suesor cellshavebeendeveloped(Li et al., 2004;Ruoslahti et al., 2010). QDslabeledwiththese typesofpeptideshavebeenextensively prepared using various strategies (Chen and Gerion, 2004; Cai andChen,2008;Curnisetal.,2008).Forexample,Akermanetal. show that ZnS-capped CdSe QDs coated with a lung-targeting peptide accumulate in the lungs ofmice after intravenous injec- tion,whereas twootherpeptides specificallydirectQDs toblood or lymphatic vessels in tumors (Akerman et al., 2002). Curnis etal. also foundthatacyclicCisoDGRCpeptidecoupled toQDs couldbindalphavbeta3integrinandco-localizewithseveralanti- bodies in human renal cell carcinoma tissue sections, indicating that this peptide could efficiently recognize endothelial cells of angiogenic vessels (Curnis et al., 2008).Cai et al. alsouseda thi- olatedarginine-glycine-aspartic acid(RGD)peptide toconjugate to theQDsandappliedtheQDspeptidebioconjugates for tumor vasculature targeted imaging (Cai andChen, 2008).However, if crossing thevascularwall is needed, theseRGDpeptidesneed to beimproved.TheRuoshlatigroupreportedacyclicpeptideiRGD thatcancombine the tumor-homingRGDsequencewitha tissue penetrationmotif (Sugahara et al., 2009).Therefore, thehoming sequencedirects thepeptide to the tumorvascular endothelium, while the tissue penetration motif, once activated by a pro- tease,bindstoadifferentreceptor(neuropilin-1),whichmediates extravasationandtissuepenetration.Asaproofofconcept, iRGD peptide-linked ironoxidenanowormscouldbedetectedbyMRI throughout a tumor once injected in vivo tomice. Recently, the same group combined twodifferent peptideswith themagnetic nanoworms to image and treat mice with glioblastoma, one of the most difficult tumors to treat (Agemy et al., 2011). While theCGKRGpeptide targets theNPs to tumor vascular cells and intotheirmitochondria, theotherpeptideactsasapro-apoptotic drug. By co-injecting theseNPswith iRGD,most of the tumors wereeradicatedortheirdevelopmentdelayedintwoglioblastoma mousemodels. Despite the extraordinary rapiddevelopment in strategies for nanoparticle conjugationwithpeptides, relatively little is known aboutNPbehavior in the immune system,which is responsible for maintaining body integrity and preventing external inva- sion. Bastus et al. described the interaction between murine bonemarrowmacrophages and gold nanoparticle peptide con- jugates. In the presence of conjugates, macrophage prolifera- tionwas stoppedandpro-inflammatorycytokineswere induced. Furthermore,macrophageactivationbyAuNPsconjugatedtodif- ferent peptides appeared to be rather independent of peptide length and polarity, but dependent on peptide pattern at the nanoparticle surface(Bastusetal., 2009). Proteins/antibodies In Bionanotechnology, specific functions of proteins such as antibody–antigen detectionmay also be very useful. Antibodies (Abs)orimmunoglobulinsareagroupofproteinsthathaveavery similar structurewith fourchainsassembled inaYshaped form, containing two identical domains for antigen recognition (Fab fragment), andtwoidenticaldomainswitheffector functions(Fc fragment).ThemainadvantageofAbsor their fragments is that theantigen-bindingregion ishighly specificanddifferentamong Abs (Arrueboet al., 2009).Therefore, different specificity canbe obtained using distinct Abs.However, attachment of Abs to the surface of NPs can impair this function if the antigen binding sites are stericallyblockeduponconjugation.For this reason, the Abs orientation is extremely important to produce effective and bioactiveantibody-nanoparticles. In fact,de laFuenteetal. recentlydemonstratedthat thecom- bination of a goodAb orientation alongwith the property of a goldnanoprism togenerateheatwhen illuminatedwith the cor- rectwavelengthenablevisualdetectionofcarcinoembryonicanti- gen(CEA)byplasmonic-driventhermalsensingwithsensitivities upto theattomolar range inserumsamples (Poloetal., 2013). Immunoassays use the specificity and sensitivity of the antibody-antigen interaction inorder to detect andquantify the amountof a specificanalytepresent ina sample.Effective conju- gatedantibodiescanbeusedtoconstitute thedesired functional- ityof thenanocarrier itself for immunoassays (Hanet al., 2003). Actually, Putman et al. described the use of immunogold labels as cell-surface markers of human lymphocytes in atomic force microscopy.TheAFMimagesrevealthecolloidalgoldparticleson thecell surface,withandwithout silver enhancement. Individual immunogold particles are clearly resolved from the cell surface thusdetermining the locationofantigens(Putmanetal., 1993). Ni et al. described an immunoassay readout method based on surface enhanced Raman scattering (SERS). The method exploits the SERS-derived signal from reporter molecules that are co-immobilized with biospecific species on gold colloids. This concept is demonstrated in a dual analyte sandwich assay, in which two different antibodies covalently bound to a solid substratespecificallycapturetwodifferentantigensfromanaque- ous sample. The captured antigens in turn bind selectively to theircorrespondingdetectionantibodies.Thedetectionantibod- iesareconjugatedwithgoldcolloidsthatarelabeledwithdifferent www.frontiersin.org July2014 |Volume2 |Article48 | 16