Seite - 97 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Pearsonetal. Nanoparticlebiomoleculecorona Recentreportshavesupportedthecorrelationbetweensurface charge ofNPs andbiomolecule association. Poly(vinyl alcohol)- coated superparamagnetic iron oxideNPs (SPIONs)with nega- tive and neutral surface charges adsorbedmore serumproteins than dextran-coated SPIONs, leading to increased circulation times(Sakulkhuetal.,2014a).BiomoleculeassociationtoPStNPs with different sizes (50 and 100nm) and three different surface charges [charge neutral (plain), negatively charged (carboxyl- modified), andpositively charged (amine-modified)]were stud- ied to elucidate the effect of size and surface charge of NPs on protein adsorption (Lundqvist et al., 2008). A size dependency inbiomolecularcoronacompositionwasobservedforbothtypes of chargedPStNPs. For example, 100nmnegatively chargedPSt NPs displayed a higher fractionof unique proteins, including Ig mu chain C region, apolipoprotein L1, and complement C1q, present in their coronas, as demonstrated by low homology in biomoleculecompositioncomparedtosimilar50nmNPs. TheconnectionbetweenNPhydrophobicityandproteinasso- ciation has been also demonstrated to be of great importance. Isothermal titration calorimetry was used to assess the sto- ichiometry, affinity, and enthalpy of NP-protein interactions (Cedervall et al., 2007; Lindman et al., 2007). Titration of human serum albumin into solutions ofNPs comprised of dif- ferentcompositionsofN-isopropylacrylamine(NIPAM):N-tert- butylacrylamide(BAM),itwasfoundthatmorehydrophobicNPs (50:50)boundhighernumbersofalbuminthanmorehydrophilic NPs (85:15). LargerNPsboundmore albumins than smallerNP counterparts. Importantly, it was also shown that apolipopro- teinA-I associationwas50-foldgreater for50:50NPs than65:35 NPs, demonstrating favorable interactions of the proteins with thehydrophobicNPs. Althoughcorrelationshavebeen foundwith thoseproperties, it should be noted that they could only act as predictive indica- tors of biomolecule association toNPs. This is important since thecompositionofbiomoleculesassociatedwithNPs invitrohas been shown tobedifferent than in vivo (Sakulkhuet al., 2014b). Nonetheless, thefindings suggested that the surfacepropertiesof NPareresponsible fordrivingbiomoleculeadsorptiontotheNP. Therefore, to further realize thepotentialofNPsasdrugdelivery vehicles, it iscritical tocoat theirsurfacewithanon-fouling layer, e.g., poly(ethyleneglycol) (PEG),polyoxazoline, poly(vinyl alco- hol), or polyglycerol, tominimize biomolecule association and therefore achieve more controllable cellular responses (Owens and Peppas, 2006; Romberg et al., 2008; Amoozgar and Yeo, 2012). IMPACTOFPEGLAYERSONBIOMOLECULARCORONA FORMATION Modification of the surface of NPs with a layer of PEG, or PEGylation, isknowntoreduceopsonizationandenhanceblood circulation timeofNPsbyprovidinga“stealth” effect, i.e., invis- ible to immune cell recognition (Owens and Peppas, 2006). Recently, a number of studies have been reported to character- ize the role of thePEGconformation (i.e., brushormushroom) andits impactonbiomolecularcoronaformation. TheeffectofPEGdensityoncoronaformationhasbeenevalu- atedonnumerousoccasions.Forexample,NPspreparedfromthe particle replication in non-wetting templates (PRINT)method were prepared with two different PEG densities corresponding to thebrush(0.083PEG/nm2)andmushroom(0.028PEG/nm2) regimes (Perry et al., 2012). Brush NPs displayed lower bind- ing of bovine serum albumin (BSA) by nearly three-fold and four-fold less than non-PEGylated NPs. Significant differences betweenNPswiththetwoPEGconformations intermsofdimin- ishedmacrophageuptakeor increasedcirculationhalf-liveswere not directly measured, but brush NPs performed better than mushroomNPsonaverage.At constant size, a similar resultwas obtained using AuNPs, where an increase in PEG grafting den- sities resulted in decreased serum protein adsorption (Walkey et al., 2011). In contrast, distinct differences were observed in termsofproteinadsorptionwhensizewas considered.The same study foundan inversecorrelationbetweenparticle sizeandpro- tein adsorption. The increased protein binding onto the smaller NPswas attributed to higher surface curvature and lower PEG- PEG steric interactions, which allowed a greater amount of the bare surface of the AuNP exposed (Figure1B) (Walkey et al., 2011).Whenmacrophageuptakewasconsidered,twotrendswere observed. First, increased PEG density on similarly sized NPs resulted in decreased uptake. Second, at similar PEG densities, smaller NPs were taken up to a lesser extent than larger ones. Contrary to those results, in a study using PEGylated single- walledcarbonnanotubes (SWCNT),brushSWCNTswere found to display shortened blood circulation times, faster renal clear- ance, and increased spleen vs. liver uptake, compared tomush- room SWCNTs (Sacchetti et al., 2013). Although these studies presentedcontrastingresultswithregardtoPEGconformation, it is clear that thepresenceofPEGminimizedbiomolecularcorona formation that was translated to enhanced pharmacokinetics of variousNPs.However,todistinctlydeterminetheroleofPEGand PEGdensity inNPformulations, it isnecessary toverify thebio- logical properties ofNPs in a case-by-casemanner toobtain the desiredresponse. CONFORMATIONALCHANGESOFADSORBEDPROTEINS CAUSEDBYNPs Achieving control over the toxicity of NPs is critical to ensure their optimal therapeutic effects.When aNP enters the body, it can alter the proteins that form its protein corona, and there- foreinducetoxicityduringtherapy.Someofthesechangesinclude alterations inproteinconformation,protein function, anddefec- tive transport leading to the overexpression of inflammatory factors (BaughandDonnelly,2003;Wolframetal., 2014a). Many physicochemical properties of NPs affect protein adsorption, which influences how NPs interact with cells and tissues. The proteins adsorbed on the surface of the NPs can still be recognized as the native proteins by an interacting cell, and as a result, these denatured ormisfolded proteins can trig- ger inappropriate cellular processes (Lynch et al., 2006). In a study investigating protein stability using silicaNPs, conforma- tional changes in protein variants of carbonic anhydrase II on NPsurfaces occurred in a step-wisemanner,where the least sta- ble variants exhibited the quickestmisfolding kinetics (Karlsson etal., 2000).Whenexposed toNPs for longerperiodsof time, all variantseventually folded into thesameunstable state. www.frontiersin.org November2014 |Volume2 |Article108 | 97