Page - 40 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Dawidczyketal. Nanomedicines for cancer therapy sites, high selectivity, and high binding affinity (Chames et al., 2009).Antibodiesare the largestof the targeting ligands, approx- imately 150kDaor about 15nm long andabout 5nm indiame- ter. The binding (dissociation) constants for antibody—antigen interactions vary over a wide range from 10−6 to 10−9M, but can be as high as 10−12M for high affinity antibodies (Dill et al., 1994).For targetingapplications, theFc regionof theanti- body can be a disadvantage if it is accessible to Fc receptors on macrophages, which can lead to increased accumulation in the liverandspleen(Allen,2002). Antibodyfragments Antigen binding sites represent only a small part of the overall size of antibodies. F(ab′)2 fragments retain both antigen bind- ing sites of the antibody coupled by disulfide linkages. Cleavage of the disulfide bondunder reducing conditions yields twoFab′ fragmentswithsulfhydrylgroupsthatcanbeusedforcouplingto the targetingplatform. Single chain variable fragmentsmaintain only thevariable regions (variable light chainandvariableheavy chain)ofonearmofanantibody. Aptamers Aptamers are folded single strand oligonucleotides, 25–100 nucleotides in length (8–25kDa) that bind tomolecular targets (Tuerk and Gold, 1990; Keefe et al., 2010). High throughput screeningmethodscanbeusedforrapidselectionofaptamersfor specific targets (Bunka and Stockley, 2006).Macugen, approved for use in the treatment of macular degeneration in 2004, is currently theonlyFDAapprovedaptamer(Adamisetal., 2006). Smallmolecules Small molecules for targeting include peptides, growth factors, carbohydrates, ureas, and receptor ligands (Weissleder et al., 2005). Specific examples include folic acid, transferrin, and the RGDpeptide sequence. Folic acid (441Da) is recognized by the folic acid receptor and is expressed in normal epithelial cells but is overexpressed in many cancer types, especially ovarian, brain, and lung cancers (Kamen and Smith, 2004; Hilgenbrink and Low, 2005; Parker et al., 2005; Chames et al., 2009;Muller and Schibli, 2013; Naumann et al., 2013). Folic acid is essen- tial for amino acid synthesis and hence for cell survival and proliferation, andhas ahigh affinity (Kd<10−9M) (Hartmann et al., 2007). Transferrin is a chelating protein that regulates the supply of iron into cells via receptor-mediated endocyto- sis (Kresse et al., 1998). The transferrin receptor is expressed at low levels in most normal tissues but is overexpressed in many tumor types (Daniels et al., 2012). The RGD (Arg-Gly- Asp) peptide is a target for integrins (e.g., αvβ3) on the cell surface (Ruoslahti, 1996;Hynes, 2002). RGD is a component of the extracellular matrix protein fibronectin and promotes cell adhesion and regulates cellmigration, growth, andproliferation (Ruoslahti, 1996; Hynes, 2002). A cyclic peptide containing the RGDsequence iswidelyused for targeting to integrins (Haubner etal., 1996).Theupregulationof integrins ispromotedbyangio- genic factors in several cancer types (Dechantsreiter et al., 1999; Hosotanietal.,2002;Furgeretal.,2003;SheldrakeandPatterson, 2009). TUMORACCUMULATIONANDTARGETINGEFFICIENCY In preclinical studies the efficacy of a drug is often determined from the time dependence of tumor size or from the fraction of animals that survive after a candidate therapy. These param- eters areparticularlyuseful in assessing thepotential therapeutic benefit of a newdelivery systembut integratemany factors. An additional parameter that is important in assessing thepotential efficacy of delivery systems is the tumor accumulation or tar- geting efficiency—the fraction of an intravenously administered dose that accumulates in a tumor (%ID). Despite the impor- tance of this parameter, very fewmeasurements are reported in the literature. We have reviewed 40 pre-clinical studies of delivery systems employing passive targeting (Supplementary Table S1), and 34 pre-clinical studies employing active targeting (Supplementary Table S2). Only studies reporting quantitative results of tumor accumulationwere selected.Analysis of thesepre-clinical studies highlightstheneedforguidelinestoimprovetheoverall impactof research in this field.Despite the importanceof pharmacokinet- icsandtumoraccumulation inassessingtheefficiencyofdelivery systems, very few preclinical studies report quantitative results thatcanbeusedtodevelopdesignrules fornanomedicines. PASSIVETARGETING Delivery systems used in pre-clinical studies exploiting passive targeting include liposomes (Harringtonet al., 2000;Wangetal., 2006;Soundararajanetal., 2009;Zhengetal., 2009;Huangetal., 2011; Chen et al., 2012a; Coimbra et al., 2012;Hsu et al., 2012; Mahakian et al., 2014) (Kheirolomoom et al., 2010), micelles (Yokoyama et al., 1999; Le Garrec et al., 2002; Kawano et al., 2006; Reddy et al., 2006; Rijcken et al., 2007; Kim et al., 2008; Hoang et al., 2009; Shiraishi et al., 2009; Blanco et al., 2010; Sumitani et al., 2011;Wang andGartel, 2011; Zhao et al., 2012; Milleretal., 2013;Zhuetal., 2013),goldnanoparticles (Hainfeld et al., 2006; VonMaltzahn et al., 2009; Puvanakrishnan et al., 2012), ironoxidenanoparticles(Ujiieetal.,2011),silicananopar- ticles (Chen et al., 2012b;Di Pasqua et al., 2012), carbon-based nanostructures (Liu et al., 2011; Robinson et al., 2012; Rong et al., 2014), quantumdots (Sun et al., 2014), andhybridnano- materials (Balogh et al., 2007; Tinkov et al., 2010; Yang et al., 2012) (Paraskar et al., 2012) (Ohnoet al., 2013) (Supplementary TableS1). Ofthe40pre-clinicalstudies,onlyafew(4/40)reportedtumor accumulation as%ID,while the remainder reportednormalized accumulation as %ID/g or %ID/cc. The tumor accumulation varies over a wide range from 0.1 to 35%ID/g at 24h post- injection. Passive delivery systems are generally pegylated and have sizes in the range from 2 to 200nm. However, there are noclear trends in termsof identifyingphysico-chemicalparame- ters that influence thepharmacokineticsor tumoraccumulation. Althoughpegylation is generally assumed to increase circulation time and hence increase tumor accumulation, there is no con- sistent difference in tumor accumulationbetweenpegylated and non-pegylateddelivery systems. Similarly, there is no obvious dependence on the size or shape of the delivery system. For example, the tumor accumu- lationofpegylatedliposomesaround100nmindiameter inthree Frontiers inChemistry | ChemicalEngineering August2014 |Volume2 |Article69 | 40