& Drug Delivery

Drug Delivery Nanocarriers from Fully Degradable PEG-Conjugated Polyester with Reduction-Responsive Backbone

Basit Yameen,[a] Cristian Vilos,[a, b] Won Il Choi,[a] Andrew Whyte,[a, c] Jining Huang,[a, c]

Lori Pollit,[a, c] and Omid C. Farokhzad*[a, d]

Abstract: The remarkably high intracellular concentrationof reducing agents is an excellent endogenous stimulusfor designing nanocarriers programmed for intracellulardelivery of therapeutic agents. However, despite their ex-cellent biodegradability profiles, aliphatic polyesters thatare fully degradable in response to the intracellular reduc-ing environment are rare. Herein, a reduction-responsivedrug delivery nanocarrier derived from a linear polyesterbearing disulfide bonds is reported. The reduction-respon-sive polyester is synthesized via a convenient polyconden-sation process. After conjugation of terminal carboxylicacid groups of polyester to polyethylene glycol (PEG), theresulting polymer self-assembles into nanoparticles thatare capable of encapsulating dye and anticancer drugmolecules. The reduction-responsive nanoparticles displaya fast payload release rate in response to the intracellularreducing environment, which translates into superior anti-cancer activity towards PC-3 cells.

Applying the tools of nanotechnology to address the biomedi-cal challenges in the treatment of severe disease such ascancer has led to the emergence of nanomedicines.[1] The suc-cess of nanomedicines stems from their ability to overcomethe inherent limitations often associated with conventionaltherapeutic agents, such as non-specific biodistribution andtoxicity, poor solubility, and suboptimal pharmacokinetics andpharmacodynamics.[1b] This is achieved by encapsulating thetherapeutic agents in biocompatible and biodegradable nano-

carriers that are undetectable by the immune system and re-lease the cytotoxic drugs at their intended site of action. Theselective, site-specific release of cytotoxic drugs is of particularinterest for reducing collateral damage and improving overalltreatment safety. The differential physiopathological aspects ofcertain diseases have been exploited to improve treatment se-lectivity. In the case of a tumor or other inflammation-causingdiseases, the enhanced permeability and retention (EPR) effectoriginating from the leaky microvasculature provides a passiveway of concentrating nanocarriers in the interstitial spaces ofdiseased tissues.[1a] However, because of disease heterogeneityand the complex nature of biological processes, the therapeu-tic impact based on the EPR effect can be inconsistent. Nano-medicines can be made more selective by employing ligandsthat exhibit high affinity towards disease markers, an approachreferred to as active targeting.[2] Despite concerns related tothe attenuated selectivity of some ligands in biological mediaand the heterogeneity of biomarker expression,[3, 4] the en-hanced selectivity associated with active disease targeting haselicited positive therapeutic outcomes in human clinical stud-ies.[5]

Besides passive and active targeting, nanocarriers can be de-signed to release therapeutic agents specifically in response toa variety of stimuli, both endogenous (pH, reducing agents, re-active oxygen species, enzymes) and exogenous (magneticfield, radiations, ultrasound, temperature).[6] Endogenous stim-uli are of particular interest, as they are specific to disease-re-lated microenvironmental and pathological changes. The dif-ferential intracellular and extracellular concentration of reduc-ing agents has recently attracted increasing attention as an en-dogenous stimulus.[7] The intracellular concentration of reduc-ing agents is ~2–10 mm, whereas their extracellularconcentration is ~2–20 mm.[8] The concentration of reducingagents in tumor cells has been found to be at least fourfoldhigher than in normal cells.[9–11] The remarkably high intracellu-lar reducing environment constitutes an excellent endogenousstimulus for nanocarriers designed for intracellular release oftherapeutic agents. For example, disulfide bonds have beenrecognized as bioreducible, that is, they are reduced to thiolsin the intracellular environment.[7, 9b, 12] Consequently, a varietyof materials bearing disulfide bonds is under investigation forthe development of reduction-responsive drug delivery nano-carriers.

Among the different materials under investigation, polymershave emerged as the most promising materials for fabricatingnanocarriers for cytotoxic drugs. Aliphatic polyesters offer an

[a] Dr. B. Yameen, Dr. C. Vilos, Dr. Won Il Choi, A. Whyte, J. Huang, L. Pollit,Prof. Dr. O. C. FarokhzadLaboratory of Nanomedicine and Biomaterials, Department of Anaesthesi-ology Brigham and Women’s Hospital, Harvard Medical School75 Francis Street, Boston, MA 02115 (USA)E-mail : ofarokhzad@zeus.bwh.harvard.eduHomepage: http ://farokhzad.bwh.harvard.edu/

[b] Dr. C. VilosUniversidad Andres Bello, Facultad de MedicinaCenter for Integrative Medicine and Innovative Science (CIMIS)Echaurren 183, Santiago 8370071 (Chile)

[c] A. Whyte, J. Huang, L. PollitUniverity of Waterloo200 University Avenue West Waterloo, ON, N2L 3G1 (Canada)

[d] Prof. Dr. O. C. FarokhzadKing Abdulaziz University, Jeddah 21589 (Saudi Arabia)

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201502233.

Chem. Eur. J. 2015, 21, 1 – 6 � 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1 &&

These are not the final page numbers! ��

CommunicationDOI: 10.1002/chem.201502233

excellent biodegradability profile, and several members of thisclass have been approved by regulatory authorities such asthe United States Food and Drug Administration (FDA) and theEuropean Medicine Agency (EMA) for animal and human clini-cal trials.[1b] Surprisingly, despite the practical relevance of dif-ferentially high intracellular reducing agents concentration asendogenous stimulus, fully bioreducible polyester-based re-duction-responsive systems are rare.[9] To realize a bench-to-bedside translation of reduction-responsive drug deliverynanocarriers, it is necessary to develop materials that complywith the requirements of regulatory agencies. Herein wereport such a system derived from polyester-based, polymer-bearing disulfide bonds in the main chain repeat units.

The polyester bearing disulfide bonds (4) in the backbonewas synthesized by a polycondensation polymerization reac-tion (Scheme 1 A) between a diacid chloride (1) and a diolbearing a disulfide bond (2). The gel permeation chromatogra-phy (GPC) profile of polymer 4 reveals a monomodal molecularweight distribution at all polymerization times (Figure 1 a). Aslight excess of diacid chloride 1 (1:1.1) was employed, and thecarboxylic acid end groups in polymer 4 were allowed to reactwith polyethylene glycol 6 to form polymer 7 (Mn : 24100, Mw:34400, �: 1.4). The conjugation of polymer 4 to PEG 6 was es-tablished by PEG protons (-O-CH2-) appearing at d= 3.6 ppm in

the 1H NMR spectrum (see Figure S1 in the Supporting Infor-mation). A comparison of the signal integral of PEG protonswith the integrals of polyester proton signals in the 1H NMRspectra estimated that 77 % of the polyester chains were con-jugated to PEG. A shift in the GPC profile of 7 to a lower reten-tion time compared to the GPC profile of 4 further supportedthe synthesis of 7 (Figure 1 b). For comparison, the polyesterpolymer 5 with a backbone lacking the disulfide bonds wasalso synthesized (Scheme 1). The conjugation of polymer 5 toPEG 6 resulted in a version of polymer 7 that lacked disulfidebonds (8, Mn : 19500, Mw: 26100, �: 1.3). The resonance for PEGprotons at d= 3.6 ppm in the 1H NMR spectrum (Figure S1)and the shift of the GPC profile to a lower retention time cor-roborated the synthesis of polymer 8 (Figure 1 b). An estima-tion based on the PEG and polyester proton signal integrals in1H NMR spectra revealed that 46 % of the polyester chainswere functionalized with PEG.

The reduction responsiveness of polymer 7 and the insensi-tivity of polymer 8 were ascertained by treating the polymerswith 2 mm and 10 mm solutions of reducing agent (RA, dl-di-thiothreitol), simulating extracellular and intracellular reducingagent concentrations, respectively. The treatment of polymer 7with 10 mm reducing agent immediately moved the GPC pro-file to the higher retention time, corresponding to the PEG res-idue, reflecting a fast degradation of its reduction-responsivepolyester constituent (Figure 1 c). The treatment of 7 with2 mm reducing agent did not elicit such a dramatic effect (Fig-ure S2). The GPC profile of the reduction-insensitive polymer 8did not change even after 8 h of incubation with 10 mm reduc-ing agent (Figure 1 d).

Scheme 1. A) Synthesis of polyesters with (4) and without (5) disulfidebonds in the backbone. Their subsequent conjugation to PEG followed byB) nanoprecipitation process to fabricate RBOEP-dye or DOX encapsulatednanoparticles. a) Pyridine, dry CH2Cl2, r.t. , 5 h. b) EDC.HCl, NHS, dry CH2Cl2,r.t. , 2 h. c) 6, DIEA, CH2Cl2, r.t. , 20 h.

Figure 1. a) GPC profiles depicting the molecular weight evolution of poly-mer 4 during the polycondensation of diacid chloride 1 and disulfide bond-containing dialcohol 2, and GPC profile of polymer 7 synthesized by conju-gating PEG 6 to polymer 4. b) GPC profile of the disulfide bond lacking poly-mer 5 synthesized by the polycondensation of diacid chloride 1 and dialco-hol 3, and GPC profile of polymer 8 synthesized from the conjugation ofpolymer 5 and PEG 6. c) GPC profiles of polymer 7 before and immediatelyafter treatment with DTT reducing agent at 10 mm concentration. d) GPCprofiles of polymer 8 treated with DTT reducing agent at 10 mm concentra-tion.

Chem. Eur. J. 2015, 21, 1 – 6 www.chemeurj.org � 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2&&

�� These are not the final page numbers!

Communication

Polymers 7 and 8 were subjected to a nanoprecipitationprocess for fabricating nanoparticles (NPs). Briefly, the solutionsof polymers (7 or 8) in acetonitrile were slowly added to deion-ized water under constant stirring to yield nanoparticles (NP7and NP8). TEM images confirmed the fabrication of NPs under100 nm (Figure 2 a and b), which was also supported by dy-

namic light scattering (DLS) studies (Table S3). NP7 and NP8were nicely dispersed in water without any sign of aggregation(Figure 2 c), even after storage for several days at 37 8C in PBSor PBS with 10 % fetal bovine serum (FBS) (DLS data in Fig-ure S3). NPs from both polymers were fully stable against 2 mm

reducing agent over extended periods of time, while a clearchange in the size of NP7 was observed after 30 min of treat-ment with reducing agent at 10 mm concentration (Figure S4).The change was so drastic that the suspension of NP7 becamevisibly turbid (Figure 2 d and e). The NP8 remained unaffectedat 10 mm reducing agent concentration (Figure S4). In the TEMimages, the NP7 treated with 10 mm reducing agent werecompletely broken, confirming their reduction-responsivenature, while the NP8 remained intact.

The nanoprecipitation process employed to prepare Rhoda-mine B octadecyl ester perchlorate (RBOEP) dye-loaded NPs re-sulted in loading efficiencies of (57.49�13.18) % and (36.93�5.81) % and loading capacities of (1.24�0.28) % and (0.76�0.12) % for NP7/dye and NP8/dye nanoparticles, respectively.NP7/dye and NP8/dye were subjected to a release kineticsevaluation at 2 mm and 10 mm reducing agent concentrations(Figure 3 a). NP8/dye exhibited a slow release of loaded dye atboth 2 mm and 10 mm reducing agent concentrations, witha cumulative dye release of 20 % and 45 % after 48 h, respec-tively. NP7/dye, on the other hand, exhibited a much faster re-lease, reaching a cumulative dye release of 65 % after 24 h and

88 % after 48 h. The release kinetics of the NP7/dye was muchslower at 2 mm reducing agent concentration, with a cumulativedye release of 20 % after 48 h. The contents recovered fromthe NP7 release test at 10 mm showed hardly any dye color,while the dye color was clearly visible in all the other samples,corroborating the reduction-responsive release behavior ofNP7 under simulated intracellular RA concentration (Figure 3 band c).

NP7 and NP8 were tolerated by PC-3 cells to concentrationsas high as 2 mg mL�1 (Figure S5). After establishing the nontox-ic nature of NP7 and NP8, PC-3 cells were incubated withNP7/dye and NP8/dye nanoparticles for 24 h, and confocal mi-croscopic imaging revealed a reasonable cellular uptake ofboth types of nanoparticles (Figure 3 d and e). The dye contentin cells incubated with NP7/dye appeared to be more diffusivethan the more particulate appearance of cells treated withNP8/dye. The diffusive nature of the dye was also observedafter 4 h of incubation (Figure S7). We attribute this observa-tion to the fast payload release kinetics of the reduction-re-

Figure 2. TEM images of NP7 (a) and NP8 (b) nanoparticles prepared frompolymers 7 and 8 by nanoprecipitation. The insets in both the images showthe effect of reducing agent at 10 mm concentration (scale bars 200 nm).Digital photographs of the as-prepared nanoparticles NP7 and NP8 (c),nanoparticles NP7 and NP8 treated with the reducing agent at a concentra-tion of 2 mm (d) and 10 mm (e).

Figure 3. a) Dye-release kinetics of dye-loaded NP7 and NP8 nanoparticles.The release experiments were performed in release buffers containing re-ducing agents at 2 mm or 10 mm concentrations. b) Digital photographs ofdye-loaded nanoparticles recovered from dialysis tubes after release tests at2 mm concentration of reducing agent. c) Digital photographs of dye-loadednanoparticles recovered from the dialysis tubes after release tests with10 mm concentration of reducing agent. Confocal microscope images of PC-3 cells treated with dye-loaded NP7 (d) and NP8 (e) nanoparticles (scale bars10 mm). f) Dosage-dependent cytotoxicity of free DOX against PC-3 cellsafter 24 h of incubation. g) Viability of PC-3 cells against treatment with10 mg mL�1 free DOX solution, DOX-loaded NP7 and NP8 formulations con-taining 10 mg mL�1 DOX.

Chem. Eur. J. 2015, 21, 1 – 6 www.chemeurj.org � 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3 &&

These are not the final page numbers! ��

Communication

sponsive NP7/dye under the intracellular reducing environ-ment. To further establish this, doxorubicin (DOX)-loaded NPs,NP7/DOX and NP8/DOX, were prepared by nanoprecipitation.A dosage-dependent cell viability assay showed thata 10 mg mL�1 concentration of free DOX reduces the viability ofthe PC-3 cell line to ~ (35�0.5) % after 24 h incubation (Fig-ure 3 f). PC-3 cells were treated with NP7/DOX and NP8/DOXformulations containing 10 mg mL�1 DOX. After 24 h incuba-tion, the reduction-responsive NP7/DOX formulation reducedthe viability of PC-3 cells to (44�0.3) %, while (74�0.3) % ofcells treated with the reduction-insensitive NP8/DOX formula-tion were still viable, confirming the faster intracellular releaseand superior efficacy of the reduction-responsive NP7 nanocar-rier system. To the best of our knowledge, there are no reportsavailable in the literature on reduction-responsive PEG conju-gated linear polyester with disulfide bonds in the main chain.There are reports on linear polyamido amines and poly(b-amino esters) with disulfide bonds in the main chain, however,these polymers are mainly used for gene delivery applica-tions.[13] A reduction-responsive poly(ester carbonate)-basedsystem reported by Zu et al. represents the closest examplecomparable to our system, however, their polymer responds tothe reducing environment via pendant group cleavage ratherthan main chain degradation.[14]

In summary, we report a reduction-responsive drug deliverynanocarrier system derived from main chain disulfide bondscontaining linear polyester conjugated to PEG. For comparison,a polyester lacking the disulfide bonds was also prepared asa reduction-insensitive control polymer. The reduction-respon-sive polymer is highly sensitive to a concentration of reducingagent that is comparable to the intracellular reducing environ-ment and degrades completely, whereas the reduction-insensi-tive polymer remains completely stable under such conditions.Both reduction-responsive and reduction-insensitive polymersare capable of self-assembling into sub-100 nm nanoparticlesthat can encapsulate dye and anticancer drug. The dye-encap-sulating nanoparticles derived from the reduction-responsivepolymer exhibit a much faster payload release at intracellularconcentrations of reducing agent. Fluorescence images of cellstreated with the dye-loaded reduction-responsive nanoparti-cles show diffuse dye content compared to the particulate ap-pearance of reduction-insensitive nanoparticles. Drug-loadedreduction-responsive nanoparticles reduced cell viability toa greater extent than the reduction-insensitive nanoparticles atequivalent drug content. The simplicity of the polymerizationprocess allows for easy scale-up to the multigram level. Giventhe biocompatibility of polyester polymers and the high sensi-tivity and selectivity of reduction-responsiveness, the approachpresented above offers an attractive path to producing clinical-ly relevant reduction-responsive drug delivery nanocarriers. Tothe best of our knowledge, the work presented is the first ex-ample of a polyethylene glycol conjugated reduction-respon-sive linear polyester with disulfide bonds in the main chainrepeat units. The reported work is a step forward to addressingthe pressing need of reduction-responsive materials that willpotentially comply with the requirements of the regulatoryagencies for bench-to-bedside transition.

Acknowledgements

We thank the Neurobiology Department and the NeurobiologyImaging Facility at Harvard Medical School for consultationand instrument use that supported this work. This facility issupported in part by the Neural Imaging Center as part of anNINDS P30 Core Center grant no. NS072030. This work wassupported by the National Cancer Institute (NCI) (grant U54-CA151884), the National Heart, Lung, and Blood Institute(NHLBI) Program of Excellence in Nanotechnology (PEN) (con-tract no. HHSN268201000045C), the National Institute of Bio-medical Imaging and Bioengineering (NIBIB) R01 grant(EB015419-01), National Research Foundation of KoreaK1A1A2048701, and the David Koch-Prostate Cancer Founda-tion Award in Nanotherapeutics. C.V. acknowledges the sup-port from the Center for the Development of Nanoscience andNanotechnology (Grant FB0807) and the Postdoctoral Programof Becas-Chile/CONICYT. J.H. acknowledges partial financialsupport from the University of Waterloo David Johnston Inter-national Experience Award. O.C.F. has financial interest in Selec-ta Biosciences, Blend Therapeutics, and BIND Therapeutics ; 3biotechnology companies that are developing therapeuticnanoparticles.

Keywords: bioreducible polyester · cancer · cytotoxicity · drugdelivery · nanomedicine

[1] a) N. Bertrand, J. Wu, X. Xu, N. Kamaly, O. C. Farokhzad, Adv. Drug Deliv-ery Rev. 2014, 66, 2 – 25; b) N. Kamaly, Z. Xiao, P. M. Valencia, A. F. Radov-ic-Moreno, O. C. Farokhzad, Chem. Soc. Rev. 2012, 41, 2971 – 3010; c) B.Godin, J. H. Sakamoto, R. E. Serda, A. Grattoni, A. Bouamrani, M. Ferrari,Trends Pharmacol. Sci. 2010, 31, 199 – 205; d) E. M. Pridgen, F. Alexis, T. T.Kuo, E. Levy-Nissenbaum, R. Karnik, R. S. Blumberg, R. Langer, O. C. Far-okhzad, Sci. Transl. Med. 2013, 5, 213ra167; e) J. Wu, N. Kamaly, J. Shi, L.Zhao, Z. Xiao, G. Hollett, R. John, S. Ray, X. Xu, X. Zhang, P. W. Kantoff,O. C. Farokhzad, Angew. Chem. Int. Ed. 2014, 53, 8975 – 8979; Angew.Chem. 2014, 126, 9121 – 9125.

[2] O. C. Farokhzad, S. Jon, A. Khademhosseini, T.-N. T. Tran, D. A. LaVan, R.Langer, Cancer Res. 2004, 64, 7668 – 7672.

[3] A. Salvati, A. S. Pitek, M. P. Monopoli, K. Prapainop, F. B. Bombelli, D. R.Hristov, P. M. Kelly, C. �berg, E. Mahon, K. A. Dawson, Nat. Nanotechnol.2013, 8, 137 – 143.

[4] Z. Cao, R. Tong, A. Mishra, W. Xu, G. C. L. Wong, J. Cheng, Y. Lu, Angew.Chem. Int. Ed. 2009, 48, 6494 – 6498; Angew. Chem. 2009, 121, 6616 –6620.

[5] J. Hrkach, D. Von Hoff, M. M. Ali, E. Andrianova, J. Auer, T. Campbell, D.DeWitt, M. Figa, M. Figueiredo, A. Horhota, S. Low, K. McDonnell, E.Peeke, B. Retnajaran, A. Sabnis, E. Schnipper, J. J. Song, Y. H. Song, J.Summa, D. Tompsett, G. Troiano, T. Van Geen Hoven, J. Wright, P. LoRus-so, P. W. Kantoff, N. H. Bander, C. Sweeney, O. C. Farokhzad, R. Langer, S.Zale, Sci. Transl. Med. 2012, 4, 128ra39.

[6] S. Mura, J. Nicolas, P. Couvreur, Nat. Mater. 2013, 12, 991 – 1003.[7] R. Cheng, F. Feng, F. Meng, C. Deng, J. Feijen, Z. Zhong, J. Controlled Re-

lease 2011, 152, 2 – 12.[8] F. Q. Schafer, G. R. Buettner, Free Radical Biol. Med. 2001, 30, 1191 – 1212.[9] a) G. Saito, J. A. Swanson, K. D. Lee, Adv. Drug Delivery Rev. 2003, 55,

199 – 215; b) M. Huo, J. Yuan, L. Tao, Y. Wei, Polym. Chem. 2014, 5,1519 – 1528; c) J. Chen, X. Qiu, J. Ouyang, J. Kong, W. Zhong, M. M. Q.Xing, Biomacromolecules 2011, 12, 3601 – 3611.

[10] a) G. Wu, Y.-Z. Fang, S. Yang, J. R. Lupton, N. D. Turner, J. Nutr. 2004,134, 489 – 492; b) Y.-M. Go, D. P. Jones, Biochim. Biophys. Acta Gen. Subj.2008, 1780, 1273 – 1290; c) B. Arunachalam, U. T. Phan, H. J. Geuze, P.Cresswell, Proc. Natl. Acad. Sci. USA 2000, 97, 745 – 750.

Chem. Eur. J. 2015, 21, 1 – 6 www.chemeurj.org � 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim4&&

�� These are not the final page numbers!

Communication

[11] T. Kurz, J. W. Eaton, U. T. Brunk, Antioxid. Redox Signaling 2010, 13, 511 –523.

[12] a) S. Son, R. Namgung, J. Kim, K. Singha, W. J. Kim, Acc. Chem. Res. 2012,45, 1100 – 1112; b) S. Bauhuber, C. Hozsa, M. Breunig, A. Gopferich, Adv.Mater. 2009, 21, 3286 – 3306.

[13] a) C. Lin, Z. Zhong, M. C. Lok, X. Jiang, W. E. Hennink, J. Feijen, J. F. Eng-bersen, J. Controlled Release 2006, 116, 130 – 137; b) F. Martello, M. Piest,J. F. Engbersen, P. Ferruti, J. Controlled Release 2012, 164, 372 – 379;c) L. J. van der Aa, P. Vader, G. Storm, R. M. Schiffelers, J. F. Engbersen, J.

Controlled Release 2014, 195, 11 – 20; d) J. Chen, F. Zehtabi, J. Ouyang, J.Kong, W. Zhong, M. M. Q. Xing, J. Mater. Chem. 2012, 22, 7121 – 7129.

[14] W. Zhu, Y. Wang, X. Cai, G. Zha, Q. Luo, R. Sun, X. Li, Z. Shen, J. Mater.Chem. B 2015, 3, 3024 – 3031.

Received: June 8, 2015

Published online on && &&, 0000

Chem. Eur. J. 2015, 21, 1 – 6 www.chemeurj.org � 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5 &&

These are not the final page numbers! ��

Communication

COMMUNICATION

& Drug Delivery

B. Yameen, C. Vilos, Won Il Choi,A. Whyte, J. Huang, L. Pollit,O. C. Farokhzad*

&& –&&

Drug Delivery Nanocarriers from FullyDegradable PEG-Conjugated Polyesterwith Reduction-Responsive Backbone

A reduction-responsive drug deliverynanocarrier system derived froma linear polyester containing disulfidebonds in the main chain is reported.After conjugation to polyethylene glycol(PEG), the polymer self-assembles intonanoparticles capable of encapsulatingdyes and anticancer drugs. The nano-particles are highly sensitive to the con-centration of intracellular reducingagent and exhibit superior anticanceractivity.

Chem. Eur. J. 2015, 21, 1 – 6 www.chemeurj.org � 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim6&&

�� These are not the final page numbers!

Communication