Stapleton-2017-Radiation effects on the tumor.pdf

Radiation effects on the tumor microenvironment:Implications fornanomedicine deliveryShawn Stapletona,David Jaffraya,b,Michael Milosevica,b,aRadiation Medicine Program,Princess Margaret Cancer Centre and University Health Network,Toronto,ON,CanadabDepartment of Radiation Oncology,University of Toronto,Toronto,ON,Canadaa b s t r a c ta r t i c l ei n f oArticle history:Received 2 December 2015Received in revised form 22 April 2016Accepted 24 May 2016Available online 1 June 2016Thetumormicroenvironmenthasanimportantinfluenceoncancerbiologicalandclinicalbehaviorandradiationtreatment(RT)response.However,RT also influences the tumor microenvironment in a complex and dynamicmanner that can either reinforce or inhibit this response and the likelihood of long-term disease control in pa-tients.Itisincreasinglyevident thattheinterplay betweenRTandthe tumor microenvironment canbeexploitedto enhance the accumulation and intra-tumoral distribution of nanoparticles,mediated by changes to the vascu-lature and stroma with secondary effects on hypoxia,interstitial fluid pressure(IFP),solid tissue pressure(STP),and the recruitment and activation of bone marrow-derived myeloid cells(BMDCs).The use of RT to modulatenanoparticle drug delivery offers an exciting opportunity to improve antitumor efficacy.This review exploresthe interplay between RT and the tumor microenvironment,and the integrated effects on nanoparticle drugdelivery and efficacy.2016 Elsevier B.V.All rights reserved.Keywords:RadiotherapyNanomedicineNanoparticlesTumor microenvironmentInterstitial fluid pressure(IFP)Tumor-associated macrophages(TAMs)Drug transportEnhanced permeability and retention(EPR)effectContents1.Introduction.1202.Pathophysiology of the tumor microenvironment.1202.1.Tumor hypoxia.1212.2.Interstitial fluid pressure(IFP)and solid tissue pressure(STP).1212.3.Bone marrow-derived myeloid cells(BMDCs).1223.Radiation effects on the tumor microenvironment.1223.1.Radiation effects on the tumor vasculature,hypoxia,and IFP.1223.2.Radiation-induced recruitment of BMDCs.1234.Nanomedicine-based radio-chemotherapy.1244.1.Nanoparticle transport and the EPR effect.1244.2.Radio-modulation of nanoparticle drug delivery.1265.Conclusions and future direction.127Acknowledgments.127References.127Advanced Drug Delivery Reviews 109(2017)119130Abbreviations:Ang2,Angiopoietin 2;BMDCs,Bone marrow-derived myeloid cells;CSF1,Colony stimulating factor 1;CSF1R,Colony stimulating factor 1 receptor;CT,Computedtomography;CXCR4,C-X-C motif receptor 4;CXCR7,C-X-C motif receptor 7;CXCL12,C-X-C motif ligand 12;ECM,Extracellular matrix;EPR,Enhanced permeability and retention;Gy,Gray;HIF1,Hypoxia inducible factor 1;IFP,Interstitial fluid pressure;MVP,Microvascular pressure;RT,Radiotherapy;SDF1,Stromal cell-derived factor 1;STP,Solid tissue pressure;TAMs,Tumor-associated macrophages;VEGF,Vascular endothelial growth factor.This review is part of the Advanced Drug Delivery Reviews theme issue on“Radiotherapy for Cancer:Present and Future”.Corresponding author at:Princess Margaret Cancer Centre and University Health Network,610,University Ave,Toronto,Ontario M5G 2M9,Canada.Tel.:+1 416 946 2932;fax:+1416 946 2227.E-mail address:mike.milosevicrmp.uhn.ca(M.Milosevic).http:/dx.doi.org/10.1016/j.addr.2016.05.0210169-409X/2016 Elsevier B.V.All rights reserved.Contents lists available at ScienceDirectAdvanced Drug Delivery Reviewsjournal homepage: used to treat approximately 50%of all cancerpatients and contributes to long-term disease control and cure in asubstantial proportion 1.The therapeutic benefit of RT is optimizedbased on the balance between tumor control and toxicity.Advances intechnology,including image-guided and intensity-modulated RT,havesubstantially improved the ability to precisely deliver high doses of RTto tumors while minimizing dose to neighboring normal tissues andmaintaining treatment side effects at acceptable levels.Nevertheless,tumor recurrence after RT remains a significant problem.There has been extensive interest in combining RT with systemictreatment,either cytotoxic chemotherapy or biologically targetedagents as a means of further enhancing treatment efficacy.Much ofthis effort has focused on theuse of chemotherapy to improve the cura-tive potential of RT by offsetting accelerated tumor cell repopulationduring a prolonged treatment course,sensitizing or directly killingradioresistantcells,targetingoccult metastases outside of theirradiatedvolume,or protecting normal tissues from injury 2,3.Combined treat-ment with RT and concurrent weekly cisplatin is now the standard ofcare for head and neck,lung,esophageal,cervical,and bladder cancersamong others,based on evidence from phase III trials demonstratingimproved primary tumor control and/or patient survival compared toRT alone.However,the potential for further,significant improvementsin clinical outcome using currently available cytotoxic chemotherapeu-tics in combination with RT is limited because of additive toxicity.Instead,the focus of investigation has shifted to better understandingthe biological mechanisms that drive tumor recurrence after RT,includ-ingtheinterplayamonggenetic,microenvironmental,andimmunologiceffects,to guide more strategic molecular targeting of radioresistancepathways using drugs with non-overlapping toxicities.Abnormalvascular morphology and physiology,hypoxia,high interstitial fluidpressure(IFP),and tumor-infiltrating bone marrow-derived myeloidcells(BMDCs)have all been implicated as important drivers of tumorrecurrence after RT and are potential therapeutic targets 4.Despite pastand continuingeffortsover manyyears to usecytotoxicor molecular chemotherapeutics to enhance radiation response,there has been relatively little investigation of the role of RT tomodify chemotherapy efficacy.RT is known to have profound,time-dependent effects on tumor,endothelial,and stromal cells that,inturn,would be expected to influence drug delivery to tumors,distribu-tion within tumors,and uptake by cancer cells.This is likely to be evenmore relevant with new,long-circulating nanotherapeutics,includingliposomal drug carriers.The biophysical principles that most stronglyinfluence the transport of these agents are recognized to be differentthan for conventional,low-molecular-weight chemotherapeutics,resulting in a greater accumulation in tumors than in normal tissues.RT has been shown to enhance this accumulation and improves theintra-tumoral distribution of nanoparticles,leading to even greatertherapeutic effect 5,6.This appears to be mediated by RT-inducedchanges to the tumor microenvironment including the vasculatureand stroma,with secondary effects on hypoxia,IFP,and BMDC recruit-ment and activation.It has been proposed that nanomedicine-basedradio-chemotherapy may leverage synergies between these two thera-peutic approaches,with RT improving the tumor accumulation of drugdelivery systems harboring payloads designed,in turn,to enhanceradiation treatment response and further improve drug delivery 6.This review explores the dynamic interplay between RT and thetumor microenvironment with a particular focus on RT to enhancenanoparticle transport,as summarized in Fig.1.The effects of RT onthe tumor vasculature and stroma,and the resultant change in hypoxia,IFP,and BMDCrecruitment,are discussed in thecontext of nanoparticledelivery,uptake,and distribution.Perspectives on the current state ofthe art,potential clinical applicability,and limitations of using RT incombination with nanoparticle-based therapies are highlighted.2.Pathophysiology of the tumor microenvironmentSolid tumors are composed of cancer cells surrounded by an extra-cellular matrix(ECM)of cross-linked collagen,hyaluronic acid,andglycoproteins that supports the tumor vasculature and a wide range ofhost-derived cells,including fibroblasts,lymphocytes,and myeloidcells that coexist in a dynamic and adaptive environment 79.Thevasculature in solid tumors is structurally and functionally abnormalFig.1.Summary of the interplay between RT and the tumor microenvironment,including vascular and stromal effects leading to hypoxia,decreased IFP and decreased STP,and theintegrated impact on tumor cell survival,treatment resistance,and nanomedicine delivery.120S.Stapleton et al./Advanced Drug Delivery Reviews 109(2017)119130because of an imbalance between pro-and antiangiogenic effects andloss of the normal regulatory processes inherent in vessel growth andmaturation.Tumor vessels are often incompletely formed,abnormallypermeable to water and(macro-)molecules and interconnected in achaotic and disorganized manner 10.An important functional conse-quence is inefficient delivery of nutrients and removal of cellularwaste,which results in a starved and toxic metabolic milieu character-ized by hypoxia,acidosis,and high IFP 11.These pathophysiologicfeatures of the tumor microenvironment drive a complex and dynamiccompensatory response to enable continued cell survival 12 andare associated with increased tumor aggressiveness and treatmentresistance 13.2.1.Tumor hypoxiaHypoxia exists in most solid tumors and arises as a consequence ofimpaired oxygen delivery from the abnormal vasculature that cannotkeep pace with increasing oxygen consumption by the expandingmass of tumor cells 11.Hypoxia is highly variable from one region toanother in the same tumor and also over time.Any particular micro-region of a tumor may be characterized by a basal level of chronichypoxia determined by local vascular architecture and cell density,with superimposed acute hypoxia due to fluctuations in blood flowand oxygen delivery 14.Acute hypoxia in particular leads to up-regulation of hypoxia inducible factor 1(HIF1)at tissue oxygen partialpressures levels b10 mmHg,with corresponding increases in the ex-pression of downstream genes that support cellular adaptation in thefaceofhypoxiaandothernutrientdeprivation.Thisisdiscussedindetailelsewhere in this issue(Wouters et al.).Tumor hypoxia is an important determinant of radiation treatmentresponse.At the biochemical level,molecular oxygen is required tomake permanent the DNA damage caused by free radicals that are pro-duced when radiation interacts with water 15.At very low oxygenlevels b5 mmHg,DNA damage is more likely to be rapidly repairedthrough reduction by sulfhydryl-containing compounds leading to lessresidual damage.As a result,cells irradiated under well-oxygenatedcondition are 23 times more sensitive to the effects of radiation thanhypoxic cells.Furthermore,there is increasing evidence to indicatethat chronic hypoxia in particular,coupled to slow tumor proliferationrates and long cell cycle times,is associated with decreased repair ofresidual DNA damage and increased genetic instability 16.In general,genetic instability with hypoxia-dependent selection of resistant clonescanleadtotheemergenceoftumorswithaggressivebiologicalandclin-icalphenotypes17,18.HIF1influencesradiationtreatmentresponseina complex manner that includes the regulation of metabolism,prolifer-ation,angiogenesis,and apoptosis.In addition,cycling hypoxia and re-oxygenation have been shown to have important HIF1-dependenteffects on the tumor vasculature during and after RT,including up-regulation of VEGF and other cytokines that promote endothelial cellsurvival,angiogenesis,and treatment resistance 14.There is indisputable clinical evidence to indicate that hypoxia is animportant driver of biological aggressiveness and RT response in manyhuman cancers,including head and neck cancer,cervical cancer,andprostate cancer,and a key therapeutic target for improving treatmenteffectiveness 13.Several strategies have been proposed to overcomethe adverse clinical consequences of tumor hypoxia,including improv-ing oxygen delivery to tumors(hyperbaric oxygen,carbogen)and theuse of drugs to either sensitize hypoxic cells to the direct effects of radi-ation(nitroimidazole compounds)or kill them directly(tirapazamine,evofosfamide)13.A meta-analysis of almost 100 randomized clinicaltrials of patients receiving potentially curative RT demonstratedimproved local control and survival with the addition of one of thesehypoxia-targeted treatments 19.There has also been considerableinterest in“normalizing”the tumor vasculature using antiangiogenicagents to improve oxygen delivery efficiency and radiation treatmentresponse 20.Early preclinical studies demonstrated increasedtumor blood flow,reduced hypoxia,and greater radio-responsivenessfollowing antiangiogenic treatment 2123.However,in general,these finding have not translated well to the clinic,with some studiesreporting increased(rather than decreased)hypoxia and increasedtoxicity in patients undergoing curative treatment 24.Furthermore,it is increasingly acknowledged that the clinical impact of hypoxia,and therefore also the potential benefit of hypoxia-targeted treatment,is not the same in all patients or in all tumor types,but rather is coupledto the genetic state of the tumor.In one recent report,prostate cancerhypoxia was associated with poor clinical outcomes only in patientswith tumors that also displayed a high level of genetic instability 25.Going forward,there is an important need to better understand thecomplex,dynamic interplay between hypoxia and the tumor genomeas a means of identifying patients most likely to benefit from hypoxia-targeted treatments.2.2.Interstitial fluid pressure(IFP)and solid tissue pressure(STP)IFPiselevatedintherangeof10100mmHginmostsolidmalignanttumor in animal models and patients because of abnormal vascular andstromalstructureandfunction26.Themorphologyandfunctionofthetumor microcirculation is highly chaotic and volatile 10,driven by aviscous cycle of hypoxia-induced chronic up-regulation of growth fac-tors that stimulates angiogenesis and abnormal vascular development27.Abnormal vascular morphology and permeability result in highgeometricandviscousbloodflowresistanceandelevatedmicrovascularpressure(MVP)10.High permeability causes excessive leakage offluidandproteinsintotheinterstitialspace.Theosmoticpressuregradi-entthat existsin mostnormal tissues is abolished in tumors 28,whichcontributes further to fluid extravasation.In general,trans-vascularfluidflowmaybeupto15timeshigherintumorsthaninnormaltissues7.This fluid accumulates in the tumor interstitiumbecause of physicalbarriers to interstitial flow(such as high collagen content)and a lack offunctionallymphatic vessels7,29,distendstheelasticECM,andcausesthe pressure to rise.Biomechanical modeling and measurements in alimited number of animal tumor models have shown that IFP nearlyequals the MVP and is relatively uniform through the central tumorvolume,dropping precipitously at the tumor periphery to near zero30,31.These relationships are depicted in Fig.2 as a function of keybiomechanical tissue properties and have important implicationsfor nanoparticle uptake and distribution in tumors,as discussed inSection 4.In addition to the local mass effect of high intersti