Page - 19 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Condeetal. Biofunctionalizationandsurfacechemistryof inorganicnanoparticles ImagingQDs invivo is arduousdue to theneedofanexternal source of light, which produces strong background autofluo- rescence from ubiquitous endogenous chromophores. So et al. proposed the ideal QD, where it would emit light with no requirement for external excitation (So et al., 2006b). Bymodi- fyingQDswithRenilla reniformis luciferase the authors discard the need of external excitation due to the phenomenon of bio- luminescence resonance energy transfer (BRET). BRET occurs naturallyandit isanalogoustoFRET,butthedonorenergycomes from a chemical reaction catalyzed by the donor enzyme. The polymer coatedCdSe/ZnS core-shell QDs dottedwith carboxy- late groups were incubated with R. reniformis luciferase, where through carbodiimide reaction the aminogroups of the enzyme were coupled to the carboxylates. Thus,with a simplemodifica- tiontheauthorswereabletomimicthenaturalBRETsystemwith self-illuminatingQDs(Soetal., 2006b). Carbohydrates Carbohydrates are, together with nucleic acids and proteins, importantmolecules for life.Much is already known about the structure, interactionsandfunctionofnucleicacidsandproteins, however, the role of carbohydrates in the cell is less clear (de la FuenteandPenades,2006).Acharacteristic featureofthebiologi- cal interactionswherecarbohydratesare involvedis theirextreme low affinity that has to be compensated bymultivalent presen- tation of the ligands. Although individual carbohydrate interac- tions are relativelyweak, natureutilizedmultivalent interactions between the cell surface ligands and their biological receptors to modulate biological events such as the ones related to cell adhe- sion, normal tissue growth and repair, viral/bacterial infection, signaling transduction, trappingof leucocytes, andcancer trans- fer. So the decoding of carbohydrate interactions opens up the possibility to employ nanoparticles in diagnostics and/or ther- apy (Dong, 2011). In fact, the unique physical, chemical and optical properties of thenanocarrierswith carbohydrate coating comprise a series of advantages that range from ensuringwater solubility, biocompatibility and stability to targeting properties (Garciaetal., 2010). Amongthem,goldglyconanoparticles (glycoNPs)havedrawn attention owing to their well-defined features, such as water- soluble carbohydrate-functionalizednanoclusterswithapromis- ingpotential for chemical glycobiology,biomedicine,diagnostics and clinical applications. In the last 10 years, Penades and co- workers have extensively reported a pioneer integration of a glyconanotechnology strategy based on the use of nanoparticles tostudyandevaluatecarbohydrate–carbohydrate, carbohydrate– protein interactions (Figure7) (de la Fuente et al., 2001, 2006; Barrientos et al., 2003; de la Fuente and Penades, 2004, 2006), which could be used as potential tools in anti-adhesive therapy (Rojo et al., 2004), for cell–cell adhesion studies (de la Fuente et al., 2005), prevention of pathogen invasion (Reynolds et al., 2012) and for exploring blood–brain barrier permeability via neuropeptideconjugation(Frigell et al., 2014). Smaller carbohydrates, such as lactose, glucose andmannose (Otsuka et al., 2001;Reynolds et al., 2006; Schofield et al., 2007; Martinez-Avila et al., 2009) can be thiolated for attachment to AuNPsvia ligandexchange.Thesenanoparticlesmaybeusefulas sensitivecolorimetricprobes foravarietyofmetal ions.Mannose and lactose have also been used for the reduction of gold salts andstabilizationof thenanoparticles.Schofieldetal.haveshown that thiolated carbohydrate derivatives can be readily assembled on silver and gold NPs. These metal glycoNPs can be used to developaggregationbasedcolorimetricbioassays(Schofieldetal., 2006). Magnetic glycoNPs with unique properties have also been reported, although in amore limited number (El-Boubbou and Huang, 2011; Marradi et al., 2013). Once carbohydrates are attachedontheMNPs, it iscrucial thattheyretaintheirbiological activity.Toexplorethis,plant lectinscanbeused,as their interac- tionwith carbohydrates is highly selective. The clustering of the MNPsduetotheselectiverecognitionofthelectincanbedetected usingMRSassays (Morosetal., 2010). Carbohydrates canalsobeused to targetdifferent cells and/or enhance the cellular uptake ofNPs in a highly specificway. For instance, Moros et al. functionalized MNPs with glucose and galactoseusingEDCandstudied their interactionwithVerocells in vitro (Moros et al., 2012). Although these monosaccharides sharethesamechemical formula,exceptforthespatialconforma- tion of the hydroxyl group inC-4, the cell entrance patternwas completely different.WhileMNPs-glucose entered all through- out the cell,MNPs-galactose remainedpredominantly in the cell periphery. By preparing a library ofMNPs functionalized with different monosaccharides, El-Boubbou et al. were also able to detect, differentiate cancer cells and quantitatively profile their carbohydratebindingabilitiesbyMRI(El-Boubbouetal., 2010). Carbohydrateshavebeenalsoconjugated toQDs(Chenetal., 2003;Osaki et al., 2004;Kikkeri et al., 2009;Cai et al., 2012;Yang et al., 2012a). For example, Kikkeri et al. synthesized PEGylated QDscappedwithD-mannose,D-galactose,andD-galactosamine to study specific carbohydrate-protein interactions in vitro and invivo.TheseQD-carbohydrateswereproducedthroughcovalent couplingby4-maleimidopropanoic acidNHSester andused for in vitro imaging and in vivo liver targeting (Kikkeri et al., 2009). Shinchi et al. also developed glycol-QDs by preparing stable sugar-chain-immobilized fluorescent nanoparticles (CdTe/CdS core/shell QDs functionalized with sugar-chain-ligand conju- gates, β-galactose- and α-glucose) and their application to the analysis of sugar-chain-protein interactions andcellular imaging (Shinchietal., 2012). BIOMOLECULECOUPLINGSTRATEGIES Functionalization of NPs with biomolecules has to face several hurdles and surface modifications can have significant impacts on their physical-chemical properties and therapeutic efficacy, once theymight alter surface charge, size, hydrophobicity, and targeting skills. One of the biggest challenges is that NPs need to remain stable in solution while the conjugation takes place. However, many NPs may precipitate while being activated, as their stability depends on a delicate balance between attractive and repulsive forces, which can bemodifiedwhen using differ- ent chemicals for their biofunctionalization. Moreover, due to the huge amount of different NPs and biomolecules reported so far, there are no standardizedprotocols forNP functionaliza- tion.Therefore, the choiceof a coupling strategydependson the Frontiers inChemistry | ChemicalEngineering July2014 |Volume2 |Article48 | 19