Seite - 42 - in Cancer Nanotheranostics - What Have We Learnd So Far?

Dawidczyketal. Nanomedicines for cancer therapy tumor accumulation, and biodistribution, there are numerous problems in comparingpre-clinical studies. Inparticular, differ- ences in cell line and tumor size, dose, lack of good pharma- cokinetics data, anddifferences in reportingmakemeta-analysis extremely difficult and are a limitation to progress in the field (Table4). Similarly, physico-chemical properties of the delivery system such as size, surface properties (i.e., pegyla- tion), zeta potential, targeting ligand density, and stability in blood or serum at physiological temperature are not uniformly reported. For example, results are usually reported as percent of initial administered dose per gram of tumor (%ID/g), which is only useful if the tumormass is also reported. For example, a tumor accumulation of 10%ID/g is 10% of the initial dose for a 1g Table4 |Summaryof limitations topre-clinical studiesof nanomedicines thathinderbroadassessmentofdesignrules. Problem Solution Total tumoraccumulation (%ID) is not always reported Report tumoraccumulationas%ID (and%ID/g) Inconsistent reportingof tumor size/weight Report tumorsize/weight Inconsistent reportingofdose Reportdoseas total numberof nanoparticles injected Alongwithotherparameterssuch asdrug loading,drugconcentration (and/ordrugamount), andactivity ofdose (gammacounter) Inconsistent reportingof physico-chemical properties Report standardphysico-chemical properties (e.g., size, zetapotential, surfacecoating, stabilityunder physiological conditions) Tumoraccumulation reportedat different timepoints Report tumoraccumulationat standard timepoints (e.g., 1and 24hpost-injection).Detailed pharmacokinetics (concentration in bloodand tumor) atmultiple time points ispreferred Variation in tumorcharacteristics (type, size, vascularization,etc.) Standardize tumor typeandsize (e.g.,C26or4T1;1cmdiameter) Moredifficult for active targeting dependingon targetmolecule Variation incontrolsused inactive targeting Report control studies fordelivery systemwithno targeting ligandand anydifferences inphysico-chemical properties.Reportother control studiesasnecessary Variation inanimalmodels (mouse, rat, etc.) anddifferences indrug concentrationcompared tohumans Usemousexenograftmodel for initial pre-clinical studies Differentdetectionmethodsused toassess tumoraccumulation Performvalidationusingother method(s) tumor but 1% of the initial dose for a 0.1g tumor. These dif- ferences are significant in terms of the efficiency of delivery and minimizingunwantedsideeffects innormal tissue. Insomecases tumor characteristics such as tumor diameter or approximate tumor volumeare reported, however, these parameters canonly beusedtoestimate theabsolutepercentageof the initialdose. Mousemodels arewidely used for research studies of disease progression and the development of new therapies (Frese and Tuveson, 2007). Rat and rabbitmodels are also commonly used for pre-clinical studies. Standard tumormodels include subcu- taneous xenografts of human cell lines or explants, orthotopic xenografts, andgenetically engineeredmousemodels (Frese and Tuveson,2007;Chenetal.,2012a).Whilethesemodelsareinvalu- able forpre-clinical studies, differences inphysiology can lead to differences in circulation and tumor accumulation compared to humans(Steichenetal., 2012). Xenografts represent a relatively straightforward model to study thepharmacokinetics, tumoraccumulation, andbiodistri- bution of a nanomedicine, however, tumor characteristics vary considerablywith cell line and size (Harrington et al., 2000; Jain and Stylianopoulos, 2010). The density and vascularization of tumors of similar size can also vary significantly. Highly inva- sive cell lines often formmore highly vascularized tumors, for examplexenograftsofcoloncancercell lineshavevasculaturethat ismuchmore leaky thanpancreatic cancer cell lines. Therefore, tumoruptakeby theEPReffect is expected tobestronglydepen- dent on the cell line used. Establishing a standard cell line and tumor size for xenografts would greatly enhance compar- ison of pre-clinical trials of delivery systems (Table4). While this is feasible for passive delivery systems, active targeting often requires theuseof specific cell lines thatoverexpress aparticular biomarker. Tumor accumulation is usually measured using a gamma counter, positron emission tomography (PET), or inductively coupledplasmamassspectroscopy(ICP-MS).Themethodsusing a gamma counter or PET require that a suitable radiolabel is conjugated to the drug delivery platform. With a gamma counter, the radioactivity of the resected tumor ismeasured and compared to the radioactivity of the dose. To determine the tumor accumulation fromPET scans, reconstructed 3D regions of interest are drawn around the tumor. The activity per unit mass can then be determined after correcting for decay and tissue density. An alternative to using a radiolabel to measure the tumor accumulation is to use ICP-MS to determine the amount of one or more elemental components in the deliv- ery system and to compare to the initial dose. However, this method requires a component of the delivery system to be dis- tinguishable frombiologicalmatter. Inmost pre-clinical studies only one of themethods is used to determine pharmacokinet- ics and tumor accumulation andhence there is no independent verification. Tumoraccumulation is expected tobedependenton thedose and time post-injection, and hence time-course studies at dif- ferent doses are important for full characterization. In many cases, tumoraccumulation isdeterminedonlyatoneor twotime points therefore limiting analysis of thepharmacokineticswhich iscrucial fordevelopingdesignrules. Frontiers inChemistry | ChemicalEngineering August2014 |Volume2 |Article69 | 42