Seite - 65 - in Cancer Nanotheranostics - What Have We Learnd So Far?

MorenoandPêgo AONcancer therapeutics for somesmall drugs.Thiswas attributed toa strongassociation of theAONwith regionsofnecrosis/hypoxiaordue to the effect of thedrugpromotingneovascularizationandthe lesspermeable statusof thenewlyformedvessels.Also,onecouldreasonthatthe tumormatrixandthespecificcollagencontentalongwiththesta- tusofotherfibrilliaryproteins couldaffectdistributionofAONs (Netti et al., 2000;Mocanu et al., 2007), especiallywhendealing withPS-AONsduetotheirunspecificbindingproperties. Incon- trast to tumor normalization, the EPR effect can be transiently augmentedbymodulationofbloodpressureandlocal increaseof bloodflow through theuse of angiotensin-II-inducedhyperten- sion andnitric oxide releasing agents (Fang et al., 2011). In this wayuptakeofnanoparticle systemscouldbe favored. In termsof available systems for vectorizationofAONs these can be divided in nanoparticle systems formed by interactions of different carrier formulations with the AONs or nanoconju- gates where AONs are covalently linked to different functional molecules (e.g., peptides, sugars) (Juliano et al., 2012; Yin et al., 2014). Carrier formulations that have been frequently used for delivery of different nucleic acids comprise cationic lipids and polymers. The basic driving force of complex formation is the electrostatic interaction. In brief, the carrier system needs to (i) protect the nucleic-acid from extracellular and intracellular degradation, until it reaches its target, (ii) achieve a prolonged circulation time in order to be accumulated in the location of interest, (iii) efficiently interact with the cellular membrane to promote uptake (generally through endocytosis processes), (iv) promote escape fromendocytic vesicles andfinally (v)dissociate fromtheactivenucleic-acid inorder for it to function(Yinet al., 2014). Cationic lipids generally used with nucleic acids (forming lipoplexes) comprise DOTMA, DOSPE, DOTAP, but also neu- tral lipids such as the fusogenic DOPE have been incorpo- rated to improve transfection efficiency (Simoes et al., 2005). Some of these lipids have been studied specifically with AONs (Jaaskelainen et al., 2000; Meidan et al., 2001; Gokhale et al., 2002) but fewhave beenutilized inpre-clinical or clinicalwork. A liposome formulation of c-raf antisense oligonucleotide con- stitutes the first example of anAON-lipoplex taken into clinical developmentstages (Zhangetal., 2009). Polymershavebeenalsoused.Thesehavean immensechemi- caldiversity andare easy tochemicallymanipulate thus enabling tuningofpropertiesbyfunctionalization.Someexamplesofpoly- meric systemsthathavebeenutilizedarepoly(L-Lysine) (Stewart et al., 1996) and poly(ethylene imine) (Seong et al., 2006). However, some issues regarding efficiencyand toxicityhavewar- ranted the development of other systems based on natural and biodegradablepolymers suchaschitosan(Gomesetal., 2014). Also worth mentioning are delivery systems based on inor- ganic nanoparticles, an emerging field, of which, gold nanopar- ticlesareperhapsthemostrepresentativeones(Dingetal.,2014). Adetailedviewontheintracellulartransport(e.g.,understanding its endocytic route) (Wu et al., 2014) and careful evaluation of toxicity profile (e.g., genotoxicity,membrane damage) (Alkilany andMurphy, 2010) can provide important information to the advancementof the technology intoclinicaldevelopment. Taking into consideration the previously mentioned tumor features, some design specificities should be taken into account when implementing an AON-nanocomplex strategy as anti- cancertherapeuticplatform.Regardingsize, thesmallertheparti- cle thebetter the intra-tumoral transport (<10nm),however the EPR effect will be less significant than for bigger particles (10– 200nm) (Netti et al., 2000).On theother hand, particles on the higher size range will present a limited capacity to extravasate from vessel pores, but for the same reason will be more spe- cific.Also, bigger sizes determine ahigher clearance by theRES, although this canbe counteractedby steric stabilization through poly(ethylene glycol) surface modification (Van Vlerken et al., 2007). Surface charge also plays a crucial role. While cationic par- ticles tend to target tumor endothelium and exhibit a higher vascular permeability than neutral or anionic ones, the fastest and more homogenously distribution in tumor interstitium is seen for theneutralparticles.Presenceofcharge inparticles con- tributes to aggregationwith different components of the tumor matrix thus hindering transport. Accordingly, neutral or zwit- terionic particles, or even particles with the property to change chargeaccordingtothemicroenvironmentshouldperhapsbethe best options (Chauhan et al., 2011). Shape, anoften-overlooked property, likewise affects transport. Here factors such as rigid- ity and form (spherical vs. rod) come into play with flexible nanometer-sizedparticles showing, inprinciple, better transport characteristics (Chauhanetal., 2011). Inconclusion, thefieldofanti-cancerAONsisrapidlyadvanc- ing, supported inpartby thegrowingnumberof chemicalmod- ifications that conferred superior properties toAONs.However, specific and efficient delivery to tumors is still of uttermost importance. Uniform distribution throughout the tumor is an important challenge particularly due to intra-tumoral regional specificities andaprogressivemicroenvironment.A furtherchal- lengelies inthedynamicnatureoftumorsthatmaycorrelatewith temporal and spatial changes in expression of the AON target genes. Multi-gene targetingAONsandefficient tumor targetingvec- torization systemswill, thus, be of uttermost importance in the developmentofa successfulanti-cancerAONstrategy. ACKNOWLEDGMENTS TheauthorswouldliketoacknowledgetheFEDERfundsthrough the Programa Operacional Factores de Competitividade— COMPETE and the Portuguese funds through FCT—Fundação para a Ciência e a Tecnologia (PTDC/CTM-NAN/115124/2009, HMSP-ICT/0020/2010andPEst-C/SAU/LA0002/2013) that sup- ported this work. PedroM.D.Moreno is supported by aMarie CurieActionof theEuropeanCommunity’s SeventhFramework Program(PIEF-GA-2011-300485). REFERENCES Adiseshaiah, P. P., Hall, J. B., and McNeil, S. E. (2010). Nanomaterial stan- dards for efficacy and toxicity assessment.Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol.2,99–112.doi:10.1002/wnan.66 Agrawal, S., Temsamani, J., Galbraith,W., andTang, J. (1995). Pharmacokinetics of antisense oligonucleotides. Clin. Pharmacokinet. 28, 7–16. doi: 10.2165/00003088-199528010-00002 www.frontiersin.org October2014 |Volume2 |Article87 | 65