1、Peptide ligand-modified nanomedicines for targeting cells at thetumor microenvironmentAyelet DavidDepartmentofClinicalBiochemistryandPharmacology,FacultyofHealthSciences,andtheIlseKatzInstituteforNanoscaleScienceandTechnology,Ben-GurionUniversityoftheNegev,Beer-Sheva8410501,Israela b s t r a c ta r
2、t i c l ei n f oArticle history:Received 22 December 2016Received in revised form 17 March 2017Accepted 9 May 2017Available online 12 May 2017Since their initial discovery more than 30 years ago,tumor-homing peptides have become an increasinglyuseful tool for targeted delivery of therapeutic and dia
3、gnostic agents into tumors.Today,it is well acceptedthat cells at the tumor microenvironment(TME)contribute in many ways to cancer development and pro-gression.Tumor-homing peptide-decorated nanomedicines can interact specifically with surface receptorsexpressed on cells in the TME,improve cellular
4、uptake of nanomedicines by target cells,and impair tumorgrowth and progression.Moreover,peptide ligand-modified nanomedicines can potentially accumulate inthe target tissue at higher concentrations than would small conjugates,thus increasing overall targettissue exposure to the therapeutic agent,enh
5、ance therapeutic efficacy and reduce side effects.Thisreview describes the most studied peptide ligands aimed at targeting cells in the TME,discusses majorobstacles and principles in the design of ligands for drug targeting and provides an overview of homingpeptides in ligand-targeted nanomedicines
6、that are currently in development for cancer therapy anddiagnosis.2017 Published by Elsevier B.V.Keywords:Cancer therapyDrug delivery systemsDrug targetingNanomedicinesPeptide ligandsTumor-homing peptidesContents1.Introduction.1212.Targeting the tumor microenvironment.1213.Peptide ligand discovery.1
7、223.1.Focused antibody libraries.1233.2.One-beadone-compound peptide(OBOC)libraries.1233.3.Phage display libraries.1234.Tumor targeting peptides.1234.1.Peptides homing to the tumor vasculature.1234.2.Peptides homing to tumor lymphatics.1264.3.Peptides homing to tumor cells.126Advanced Drug Delivery
8、Reviews 119(2017)120142Abbreviations:Abs,Antibodies;Ags,Antigens;ADCs,Antibodydrugconjugates;ASGPR,Asialoglycoproteinreceptor;BM,Basementmembrane;BMDCCs,Bonemarrow-deriveddendriticcells;BMDCs,Bone marrow-derived cells;CDRs,Complementary-determining regions;CRC,Colorectal cancer;CLIO,Cross-linked dex
9、tran-coated iron oxide;CRD,Carbohydrate recognitiondomain;CS,Chitosan;DCs,Dendriticcells;DOX,Doxorubicin;ECs,Endothelialcells;ECM,Extracellularmatrix;EGFR,Epidermal growthfactorreceptor;Esbp,E-selectin-bindingpeptide;FR,Folate receptors;GAG,glycosaminoglycan;hTf,Human transferrin;HPMA,N-(2-hydroxypr
10、opyl)methacrylamide;ICAM1,Intercellularadhesionmolecule 1;IONPs,Ironoxide nanoparticles;LCL,Long-circulatingliposomes;3LL,Lewislungcarcinoma;MRI,Magneticresonanceimaging;Mw,Molecularweight;mAbs,Monoclonalantibodies;NPs,Nanoparticles;NIRF,Nearinfraredfluorescence;NRP-1,Neuropilin-1;NSCLC,Non-small-ce
11、ll lung carcinoma;OBOC,One-bead one-compound;PET,Positron emission tomography;PDGFR,Platelet-derived growth factorreceptor;P(Asp),Poly(aspartic acid);PBCA,Poly n-butylcyanoacrylate;PCL,Poly(-caprolactone);PEG,Polyethylene glycol;pHPMA,Poly(N-(2-hydroxypropyl)methacrylamide);PGA,Poly(glutamic acid);P
12、LA,Poly(D,L-lactide);PLGA,Poly(D,L-lactide-co-glycolide);POE,Poly(orthoester);PTMC,Poly(trimethylene carbonate);PTX,Paclitaxel;PVP,Poly(N-vinylpyrrolidone);QDs,Quantum dots;SR-A,Scavenger receptor-A;SPECT,single photon emission computed tomography;SSTR2,somatostatin receptor 2;SPIONs,Superparamagnet
13、iciron oxide nanoparticles;Tf,Transferrin;Tf-R,Transferrin receptor;TAMs,Tumor-associated macrophages;TME,Tumor microenvironment;uMUC-1,Underglycosylated mucin-1;VEGFR,Vascular endothelial growth factor receptor.This review is part of the Advanced Drug Delivery Reviews theme issue on“Emerging nanome
14、dical solutions for angiogenesis regulation”.Corresponding author at:Department of Clinical Biochemistry and Pharmacology,Faculty of Health Sciences,Ben-Gurion University of the Negev,P.O.Box 653,Beer-Sheva 8410501,Israel.E-mail address:ayeletdabgu.ac.il.http:/dx.doi.org/10.1016/j.addr.2017.05.00601
15、69-409X/2017 Published by Elsevier B.V.Contents lists available at ScienceDirectAdvanced Drug Delivery Reviewsjournal homepage: homing to macrophages and dendritic cells.1275.Peptide ligand-conjugated nanomedicines for targeting the TME.1285.1.Nanomedicines:main principles and obstacles.1285.2.Pepti
16、de ligand-conjugated nanomedicines for cancer therapy.1295.3.Peptide ligand-conjugated nanomedicines for cancer imaging.135Conclusions and future prospects.137Acknowledgments.137References.1371.IntroductionCancer cells in primary tumors are surrounded by a complex micro-environment comprising numero
17、us cells,including endothelial cells ofthe blood and lymphatic circulation,stromal fibroblasts and a varietyof bone marrow-derived cells.Nowadays we know that the cancercells and the cancer-associated tissues such as tumor vasculature ofthe blood and lymphatic circulation express different molecular
18、 targets(antigens and/or receptors)than do normal cells and tissues.Whilesome of these molecules are down-regulated,others are either newlyexpressed or significantly up-regulated on the surface of target cells,thus offering possible targets for tumor-homing molecules 1.The nat-uralhomingproteinsof t
19、heorganismare antibodies,providingantigen-specificbindingaffinity,andindeed,manyantibodieshaveenteredclin-ical trials or are in use as proven therapeutic agents 2.However,anti-bodies are large,multi-chain proteins that are difficult and expensive toproduce.Moreover,thelowtumortissuepenetratingabilit
20、yduetotheirlarge size and the non-specific uptake by the mononuclear phagocytesystem further limittheapplicationofantibodiesin the clinical settings.As such,the notion that short peptides can mimic the homing propertyassociated with antibodies is promising.Indeed,the first homing pep-tidestobeidenti
21、fiedandalargeproportionofhomingpeptidescurrent-ly known are targeted to various vasculature motifs 3.These motifscan be over-expressed on angiogenic,tumor-associated blood vesselendothelialcells(ECs)orlymphatic vesselECs.Furthermore,itwaspro-posed that vasculature-related cells,such as pericytes,cou
22、ld also repre-sentpossible targets 4.Moreover,tumor cell-associated antigens havealso been used as targets for homing peptides 5,although such pep-tides could encounter a more difficult path to reach a target cell thanwould be the case were the target is expressed in the vasculature.This review is a
23、imed to describe known examples of peptide ligandsthat react actively with specific cell surface receptors expressed on themembrane of cells in the tumor microenvironment(TME),primarilycancercellsandECsofthebloodandlymphaticcirculation,forincreasedcellular uptake of nanomedicines.The main barriers f
24、or activelytargeting drugs to solid tumors and the reasons for the limited clinicalsuccessofthisstrategy,ascomparedtopassively-targetednanomedicines,are discussed.In the second half of this review,recentstudies on peptide ligand-directed nanomedicines for targeted therapyand molecular imaging in can
25、cer are considered.2.Targeting the tumor microenvironmentThe developmentofcancer,from the initial mutational aberration tofull-blown malignancy,comprises a complex series of events,many ofwhich are not performed by the tumor cells themselves,but ratherwhichare highly dependent on thetumor stroma or
26、microenvironment6.Althoughtheconceptof“seedandsoil”,initiallyproposedbytheEn-glish surgeon Stephen Paget,dates back to 1889,clear understanding ofwhat drives tumor cells(seeds)to thrive and populate a distant organ(soil)via the process of malignancy has only been extensivelyresearched over the last
27、forty years or so 7.Today,it is well acceptedthat tumors,similar to normal tissues,interact with their surroundingcells and extracellular matrix(ECM),both at the initial site of tumori-genesis and at the site of tumor implantation.The TME includes ECsand their precursors,pericytes,smooth muscle cell
28、s,fibroblasts of vari-ous phenotypes,myofibroblasts,neutrophils and other granulocytes,mast cells,T,B and natural killer cells,as well as antigen-presentingcells,such as macrophages and dendritic cells(Fig.1).These cells con-tribute to tumor development in many ways,such as the release of en-zymes,c
29、ytokines,chemokines,and growth factors,which,in turn,enhance angiogenesis,immune escape,ECM breakdown and more.Therefore,in addition to the ongoingsearch for new cancer cell-specificdrugs,manynewdrugsintheclinicandindevelopmentaredesignedtotarget different TME cell types and motifs.Cancer cells expr
30、ess a large number of receptors on their surface.Over-expression of some receptors is associated with tumor growth,migration,invasion and metastasis.The over-expression of surface re-ceptors on cancer cells may allow specific delivery of ligand-targetedanti-cancer therapies in vivo.Receptors on canc
31、er cells that have beenexplored clinically for drug and/or radiotherapy targeting include theepidermal growth factor receptor(EGFR)8,9,the human epidermalgrowth factor receptor 2(HER2)10,11,the folate receptor(FR)12,13,the transferrin receptor(Tf-R)14,15 and the asialoglycoproteinreceptor(ASGPR)16.M
32、oreover,several integrin receptor sub-types17,intercellular adhesion molecule 1(ICAM1)18,under-glycosylat-ed mucin-1 antigen(uMUC-1)19,somatostatin receptor 2(SSTR2)20,luteinizing hormone-releasing hormone receptor(LHRH-R)21,22,and galectins 23 are over-expressed on some tumors,and thusmay have a po
33、tential use in ligand-targeted therapies.Antibodies orfragments thereof are the most common targeting agents currentlyused for the specific delivery of anti-cancer drugs and imaging agentsto tumor sites.Antibodydrug conjugates(ADCs)targeted to,for in-stance,CD20,CD25andCD33,proteinmarkersthatare(ove
34、r)expressedin non-Hodgkins lymphoma,T-cell lymphoma and acute myeloid leu-kaemia,respectively,have been approved for clinical use for deliveringradionuclides(Zevalin)24,25,immunotoxins(Ontak)26,27 andanti-tumor antibiotics(Mylotarg)2830 more selectively to tumorcells.Some ADCs e.g.trastuzumab emtans
35、ine 31 and brentuximabvedotin 32,are already on the market and many more similar agentsare in clinical trials.Therealization that tumor cells are notsole players but rather majorfactors in a tightly orchestrated series of events led investigators tolookfor drugs that target other cell populations an
36、d different molecules in-volved in carcinogenesis and metastasis.Today,it is widely acceptedthat the main limitation facing a cancerous growth is the abilityto induce the formation of new capillaries from pre-existing vessels(a physiological process termed angiogenesis).Our ever-increasing un-dersta
37、nding of angiogenesis as a crucial process in tumorigenesis hasencouraged researchers to invest a considerable time in developingdrugs that target newly established blood vessels,effectively“suffocat-ing”the tumor and hence leading to tumor shrinkage.Among theseare several antibodies or antibody-fus
38、ion proteins,like Avastin(bevacizumab)33,34 and VEGF-Trap 35,respectively,for bindingvascular endothelial growth factor(VEGF)-A,that have alreadyreceived FDA approval.The list also includes other low Mw drugs,liketyrosine kinase inhibitors,that interfere with signal transduction inECs 3639.Anotherim
39、portantcomponentoftheTMEistheECMthatsurroundscells and which is composed of numerous macromolecules,including121A.David/Advanced Drug Delivery Reviews 119(2017)120142collagen,laminin,fibronectin and heparin sulfate proteoglycans 40.These molecules are linked in an intricate,three-dimensional matrix.
40、One specialized ECM,the basement membrane(BM),separates the ep-ithelium from the stroma,and underlines ECs,pericytes,and other celltypes.Maintaining organ homeostasis can prevent neoplastic transfor-mationinnormaltissuesbyensuringfirmcell-cellcontacts,asmediatedby the proteins of tight junctions and
41、 cell adhesion molecules,such as1 integrins and E-cadherin.In tumors,these protective constraintscan be over-ridden by remodeling the BM/ECM.Thus,a class of drugsin development targets these BM/ECM components.For examplevitaxin 41 and volociximab 42 are two humanized monoclonal anti-bodies targeting
42、 v3 and 51 integrins,respectively.Still,despite many decades of effort,receptor-mediated targeting ofcancer has met little clinical success.Even antibody-targeted radiother-apy,with very high binding affinity to its receptor,localizes well below0.01%of the dose administered to a tumor 43.Monoclonal
43、antibodies(mAbs)and large protein ligands have two major limitations,as com-pared to peptides,namely low tumor tissue penetration ability,due totheir large size,and dose-limiting toxicity to the liver,spleen and bonemarrow,duetonon-specificuptakeintothereticulo-endothelialsystem(RES)44,45.Assuch,the
44、useofmAbshasbeenrestrictedtoeithervas-culartargetspresentontheluminalsideoftumorvesselendotheliumorhematological malignancies 1,4649.Other problems associated withtheclinicaluseofmAbsarethedifficulties andexpenses ofcommercial-scale production,and the possibilities of anti-idiotypic antibody induc-t
45、ion and immune complex formation(even after humanization).Small,high affinity peptide ligands that react specifically withtumor membrane receptors are being pursued as potential tumor-targeting agents.Peptide ligands offer many advantages over mAbs,oligosaccharides and protein scaffolds in terms of
46、cellular targeting,owing to their small size(b30 amino acids on average)and precisechemical structure.Peptide manufacturing is scalable and the prod-ucts are highly pure and relatively inexpensive to produce.Further-more,peptide ligands exhibit good biocompatibility 5052 andtheir degradation by prot
47、eolysis can be prevented by chemical mod-ifications,such as the incorporation of D-amino acids or cyclization51.Moreover,peptide ligands can be easily manipulated for conju-gation with other agents including chemical drugs,cytokines,thera-peutic genes and radionuclides 45,5355.In addition,somerecent
48、ly developed peptide ligands(i.e.,iRGD,Lyp-1 and F3,discussed below)can not only target the cognate receptor but alsofacilitate deep tumor penetration 56.The use of tumor-homingpeptides for targeting therapeutics and nanomedicines in cancertherapy is receiving increasing attention and is the subject
49、 of thisreview.3.Peptide ligand discoveryCell surface-targeting peptides can be developed through molecularmodeling(if an X-ray structure of the receptor or a related receptor isavailable)or by screening combinatorial peptide libraries againstknown molecular targets.Combinatorial libraries can be di
50、vided intothree general classifications,focused libraries of distinct compounds(natural peptides or antibodies),one-bead one-compound(OBOC)li-braries as a chemical approach 5759,and phage-display peptide li-braries as a biological approach 60.The latter two methods are thetwo most popular and have b