In-Cell Activation of Organo-Osmium(II) Anticancer Complexes.

Angewandte Communications Chemie InternationalEdition:DOI:10.1002/anie.201610290 Anticancer Agents GermanEdition: DOI:10.1002/ange.201610290 In-Cell Activation of Organo-Osmium(II) Anticancer Complexes RussellJ. Needham+, Carlos Sanchez-Cano+, Xin Zhang, Isolda Romero-Canel(cid:2)n, Abraha Habtemariam, Margaret S. Cooper, Levente Meszaros, GuyJ. Clarkson, PhilipJ. Blower, and PeterJ. Sadler* Abstract:ThefamilyofiodidoOsIIarene phenylazopyridine and 2-I·PF (R1=H, R2=NMe) show long Os(cid:2)I bonds 6 2 complexes [Os(h6-p-cym)(5-R1-pyridylazo-4-R2-phenyl))I]+ (2.6974(2) and 2.7083(2)(cid:2), respectively), and relatively flat (where p-cym=para-cymene) exhibit potent sub-micromolar N,N-chelated phenylazopyridine ligands (Figure1). Unlike antiproliferative activity towards human cancer cells and are activeinvivo.Theirchemicalbehaviorisdistinctfromthatof cisplatin: they do not readily hydrolyze, nor bind to DNA bases.Wereporthereamechanismbywhichtheyareactivated incancercells,involvingreleaseoftheI(cid:2)ligandinthepresence of glutathione (GSH). The X-ray crystal structures of two activecomplexesarereported,1-I(R1=OEt,R2=H)and2-I (R1=H,R2=NMe).Theywerelabelledwiththeradionuclide 2 131I (b(cid:2)/g emitter, t 8.02d), and their activity in MCF-7 1/2 human breast cancer cells was studied. 1-[131I] and 2-[131I] exhibit good stability in both phosphate-buffered saline and blood serum. In contrast, once taken up by MCF-7 cells, the iodide ligand is rapidly pumped out. Intriguingly, GSH catalyzestheirhydrolysis.Theresultinghydroxidocomplexes canformthiolatoandsulfenatoadductswithGSH,andreact with HO generating hydroxyl radicals. These findings shed 2 2 newlightonthemechanismofactionoftheseorgano-osmium Figure1. Complexesstudiedhere,HPLCretentiontimes,andORTEP complexes. diagramsforcomplexes1-Iand2-I.Ellipsoidsareshownatthe50% O probabilitylevel.Hatoms,counterionsandsolventmoleculesare rganometallic complexes show promise as a new gener- omittedforclarity. ation of anticancer drugs. These include cyclopentadienyl complexesofFeII,RhIIIandIrIII,andarenecomplexesofRuII, OsII,RhIIIandIrIII.[1]Theyoffertheprospectofmechanisms cisplatin,1-Iand2-Iarerelativelyinert.Theydonotreadily ofactionthatdifferfromPtIIcomplexes,whicharecurrently hydrolyze nor bind to DNA in vitro.[2] However, 1-I and 2-I the most widely-used drugs in the clinic. Organometallic exhibit activity that is equal to or greater than that of the drugs have the potential to expand the range of treatable clinical drug cisplatin in a wide range of cancer cell lines, cancers,causefewerside-effects,andprovideactivityagainst despite having a contrasting profile of chemical reactivity Pt-resistance, a current clinical problem. OsII arene com- (Table1),andarecapableofovercomingresistancetoclinical plexes containing a chelated phenylazopyridine ligand, [Os- platinum drugs.[2] Complex 2-I is 49(cid:3) more potent than (h6-p-cym)(5-R1-pyridylazo-4-R2-phenyl))I]+, possess typical cisplatin in a panel of 809 cancer cell lines,[3] and active in half-sandwich “piano-stool” structures. The X-ray crystal vivo.[4]Wehaveshownthatthisdrug-candidateiscapableof structuresofcomplexes1-I·PF·0.5EtOH(R1=OEt,R2=H) modulating the cellular redox balance, increasing dramati- 6 cally the production of reactive oxygen species (ROS) in [*] R.J.Needham,[+]Dr.C.Sanchez-Cano,[+]X.Zhang, Dr.I.Romero-Canel(cid:2)n,Dr.A.Habtemariam,Dr.G.J.Clarkson, Prof.Dr.P.J.Sadler Table1: Antiproliferativeactivityofcomplexes1and2towardsA2780 DepartmentofChemistry,UniversityofWarwick humanovarianandMCF-7breastcancercells. Coventry,CV47AL(UK) Complex IC [mm] E-mail:p.j.sadler@warwick.ac.uk 50 A2780 MCF-7 Dr.M.S.Cooper,Dr.L.Meszaros,Prof.Dr.P.J.Blower DivisionofImagingSciencesandBiomedicalEngineering 1-I 0.92(cid:3)0.02 1.2(cid:3)0.2 King’sCollegeLondon 1-Cl 15.1(cid:3)0.5 n.d. St.ThomasHospital,London,SE17EH(UK) 1-OH 0.27(cid:3)0.02 14.3(cid:3)0.3 [+] Theseauthorscontributedequallytothiswork. 2-I 0.15(cid:3)0.02[a] 0.20(cid:3)0.01[a] 2-Cl 1.8(cid:3)0.3[a] 1.1(cid:3)0.8[a] SupportinginformationandtheORCIDidentificationnumber(s)for cisplatin 1.2(cid:3)0.2 7.4(cid:3)0.2 theauthor(s)ofthisarticlecanbefoundunderhttp://dx.doi.org/10. 1002/anie.201610290. [a]Ref.[2]. Angew.Chem.Int.Ed.2016,55,1–5 (cid:2)2016Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim 1 (cid:2)(cid:2) These are not the final page numbers! Angewandte Communications Chemie cancer cells, and has enhanced potency when used in combination with low doses of l-buthionine sulfoximine (l- BSO),whichdepletescellularlevelsofglutathione(GSH,g- l-Glu-l-Cys-Gly).[5]GSHisanimportantcellularantioxidant that plays a key role in the detoxification of ROS[6] and platinum drugs.[7] The organo-Ru analogue of 2-I binds to GSHandcatalyzesitsoxidationtoGSSG,probablythrough redoxmediationbytheazogroup.[8]Furthermore,theRu–SG thiolateadductformedfromchloridoRuIIareneethylenedi- amine complexes can be oxidized to Ru-SOG sulfenate speciesinthepresenceofO ;thesereactiveadductsfacilitate 2 the interaction between the Ru complexes and guanine/ DNA.[9]OtherRucomplexescanalsobeactivatedbyGSH.[10] Additionally, the coupling of anticancer activity to redox reactionsofmetalsandligandsinorganometalliccomplexes Figure2. Releaseoffree131Iintothesupernatantofcellculture provides intriguing possibilities for novel mechanisms of mediumatvarioustimesafterincubationwith1-[131I](^)or2-[131I] action,asisillustratedespeciallybytheferrociphenseriesof (&),intheabsence(dashedlines)orpresence(solidlines)ofMCF-7 complexes.[11]Weshowherethat,surprisingly,theantioxidant breastcancercells.PercentagesweredeterminedbyHPLCpeak GSH not only promotes activation of iodido OsII arene integrals. azopyridine complexes through aquation, but also forms sulfenateadducts. Initially, we used the b(cid:2)/g emitter 131I (t 8.02d) as 1/2 aradiotracertolabel1-I and2-Iandstudytheircelluptake andefflux.Complexes1-[131I]and2-[131I]weresynthesizedby exchange of Cl(cid:2) in 1-Cl and 2-Cl (Figure1), by reacting a large excess of complex with Na131I. The exchange was completein2hfor1-Cl,butrequired18hfor2-Cl.Reactions werefollowedbyradio-TLCchromatograms(FigureS1inthe Supporting Information), and reverse-phase HPLC with simultaneousdetectionat254nmandg-emission(FigureS2). Os-OH,Os-ClandOs-IspecieswereidentifiedusingLC-MS, andtheformationofOs-131Iwasconfirmedbyperformingthe radio-labelling in presence of 0.5mol equiv of cold NaI (Figures1 and S2). Complexes 1-[131I] and 2-[131I] were purified via preparative HPLC to remove residual chlorido Figure3. Cellularaccumulationof131IinMCF-7breastcancercellsat complex,dilutedinPBStopH6–7,thenfrozenimmediately varioustimesafterincubationwith2-[131I]. and stored at 193K to minimize decomposition until required. Both1-[131I]and2-[131I]exhibitedgoodstabilityafter24h readily under either intracellular (23mm) or intranuclear at 310K in human blood serum (ca. 25%, and 11% iodide (4mm) levels of chloride (FigureS5), and therefore the released, respectively), and cell culture medium (ca. 27%, observedlossoftheiodidoligandmightarisefrominteraction and 14%, FigureS3). However, when MCF-7 breast cancer withintracellularbiomolecules. cells were treated with the tracer-complexes, we observed These surprising results led us to study the reactions a rapid release of free 131I into the supernatant. After 24h, between 1-I or 2-I (75mm) and GSH (1mol. equiv) using 97% and 99% of 1-[131I] and 2-[131I], respectively, lost the HPLC. Surprisingly, these experiments showed that the radioactiveiodideligand,therateoflossbeinggreaterfor2- hydrolysisoftheOs(cid:2)IbondispromotedbyGSH:anewset [131I](Figure2).Interestingly,themaximumamountofintra- of peaks identified as 1-OH and 2-OH by LC-MS began to cellular131Iwasobservedafteronly5minforboth1-[131I]and appear after 3h of incubation and no GSH adducts were 2-[131I] (0.8% and 1.8% of total 131I/106 cells, respectively). detected.Theextentofhydrolysiswasgreaterfor1-I(71%) Afterthistime,thelevelofaccumulated131Ideclinedsteadily than for 2-I (33%) after 24h (Figure4 and S7). In contrast, (Figure3andS4). lessthan1%of1-OHand2-OHwasobservedwhen1-Iand We have shown previously that significant amounts of 2-Iwereincubatedunderthesameconditionsintheabsence complex 2-I are taken up by cancer cells within the first of GSH (FigureS8), indicating that hydrolysis is induced by 30min and the amount of Os accumulated increases with thepresenceofthethiol-containingtripeptide.ThepK of1- a longerincubationtimes.[12]Thusthefreeiodidedetectedhere OH was determined by NMR titration to be 4.55(cid:3)0.01, 2 might arise from the efflux of 131I, perhaps via chloride indicating that the more stable Os-OH species will predom- transport channels.[13] In general, iodide transport mecha- inateatphysiologicalpHvalues(ca.7.4)overthemorelabile nisms appear to be little studied apart from cells in the Os-OH species (FigureS9). Furthermore, incubation of 1-I 2 thyroid. However, complex 1-I was found not to hydrolyze or2-IwithalargeexcessofGSH(100mol.equiv)accelerated 2 www.angewandte.org (cid:2)2016Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim Angew.Chem.Int.Ed.2016,55,1–5 (cid:2)(cid:2) These are not the final page numbers! Angewandte Communications Chemie Figure4. HPLCseparationofproductsfromreactionsofcomplexeswithGSHin7.5mmphosphatebuffer(pH7.4)at310Kforvarioustimes. A)1mm1-OHwith1mol.equivGSHin0.1mphosphatebuffer.B)75mm1-Iwith1molequivGSH,andC)75mm1-Iwith100molequivGSH. theirhydrolysisratesdramatically(completein3hfor1-Iand 6h for 2-I; Figures4 and S7). Moreover, the formation of thiolato (GS(cid:2)) and sulfenato (GSO(cid:2)) adducts of 1-I and 2-I wereobserved(Figures4andS7),andconfirmedbyLC-MS (FigureS10andTableS1).Thisappearstobethefirstreport of OsII–sulfenato adducts. Similar adducts were observed when 1-OH was incubated with just 1mol. equiv of GSH, suggesting that hydrolysis of the Os(cid:2)I bond is essential for GSHbindingto1-Iand2-I.Complexes1-Cland2-Clbehaved in a similar way, but showed faster rates of hydrolysis and GSH binding, indicating greater reactivity than their Os-I analogues (FigureS11). Similar results were observed when 1mol.equivN-acetyl-l-cysteine(NAC)orascorbicacid(also reducingagents)wereusedinsteadofGSH(FiguresS12and S13). Interestingly, the presence of 30%v/v acetone almost completely hindered the reaction of 2-I with NAC and hydrolysis of 2-I (FigureS14), accounting for our previous observationofalackofreactionbetween2-IandNAC.[2] Figure5. X-bandEPRspectrashowingA)trappingofOHradicalsby GSH-mediated hydrolysis of 1-I and 2-I can therefore thespin-trapDEPMPO(6mm)fromreactionofHO (10mm)with1- 2 2 generate Os species that are more reactive than the parent OH(1mm)in75mmphosphatebuffer,pH7.4,andB)quenchingin iodidocomplexes.Whenthereactionof2-I(75mm)andGSH presenceofethanol(10mm).TheEPRparametersofthetrapped radicalaretypicaloftrappedHOC(g:2.01,aN :14.02G,aP:47.01G, (0.08–7.5mm)wasrepeatedinthepresenceofanintracellular NO aH :13.22G).[14] concentrationofNaCl(25mm),anewpeakcorrespondingto b 2-Cl was observed after 24h incubation with GSH concen- trations up to ca. 2mm. Incubations at higher GSH concen- trationsledtotheformationof2-OH,thiolatoandsulfenato 1-IinA2780butnotMCF-7cells.Thismightbeduetohigher adductsonly(FigureS15). levels of GSH in MCF-7 compared to A2780 cells (ca. 40 Complexes 1-I and 2-I and their Cl and hydroxido versus30nmolGSH/mg(cid:2)1protein,respectively)[15]butother analogues react with HO, a ROS overproduced in cancer factorsmayalsoplayarole. 2 2 cells, to generate hydroxyl radicals detected by EPR spec- In conclusion, we show that, in contrast to platinum troscopyusingthespin-trap5-diethoxyphosphoryl-5-methyl- anticancer drugs, GSH can provide a route for intracellular 1-pyrroline N-oxide (DEPMPO, Figures5 and S16). The activationofosmiumprodrugs1-Iand2-I(FigureS17).This efficiency of OHC radical generation followed the trend Os- involveshydrolysisoftheOs(cid:2)Ibond,mostprobablyviaredox OH > Os-Cl > Os-I. This is the same reactivity trend mediation by the azo group and transformation into more observed for reactions with GSH, and suggests that intra- reactive hydroxido forms 1-OH and 2-OH, releasing the I(cid:2) cellularhydrolysisof1-Iand2-Imightbeakeyactivationstep ionsfromthecellintheprocess.SuchOs-OHcomplexescan fortheirbiologicalactivity.Additionally,thelowreactivityof bindtoCl(cid:2)ionsorGSH,formingchlorido,thiolatoandalso iodidocomplexes1-Iand2-Icomparedto1-Cland2-Clmight Os-sulfenatoadducts,andcanalsocatabolizeHO generat- 2 2 explain why chlorido derivatives are 10(cid:3) less active in ingOHCradicals.ThefacileabilityofGSHtoformsulfenato inhibiting the proliferation of cancer cells than their iodido adducts(comparedtoNAC)wasespeciallynotable.Organo- analogues (Table1). The chlorido complexes might undergo osmiumcomplexescancausedramaticchangesintheredox side-reactions more readily, leading to detoxification before state of cancer cells[5] and these new insights into their reachingtargetsites.Interestingly,1-OHismoreactivethan activationmechanismpavethewayforfurtheringourunder- Angew.Chem.Int.Ed.2016,55,1–5 (cid:2)2016Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim www.angewandte.org 3 (cid:2)(cid:2) These are not the final page numbers! Angewandte Communications Chemie standing of their target sites, which might involve attack by osmiumonproteinsinthecytoplasmandmitochondria. [1] a)A.M.Pizarro,A.Habtemariam,P.J.Sadler,Top.Organomet. Chem.2010,32,21–56;b)C.G.Hartinger,P.J.Dyson,Chem. Soc. Rev. 2009, 38, 391–401; c)G. Gasser, I. Ott, N. Metzler- Nolte, J. Med. Chem. 2011, 54, 3–25; d)S.S. Braga, A.M.S. Experimental Section Silva, Organometallics 2013, 32, 5626–5639; e)Z. Liu, P.J. Synthesis of 1-I·PF 6 : [Os(h6-p-cym)I 2 ] 2 (100mg, 86.5mmol) was Sadler, Acc. Chem. Res. 2014, 47, 1174–1185; f)S.J. Lucas, dissolved in ethanol (10mL), and a solution of 2-(phenylazo)-5- R.M. Lord, A.M. Basri, S.J. Allison, R.M. Phillips, A.J. ethoxypyridine (41.3mg, 181.6mmol) in ethanol (5mL) was added Blacker, P.C. McGowan, Dalton Trans. 2016, 45, 6812–6815; drop-wise.Themixturewasstirredfor18hatambienttemperature, g)W. Kandioller, E. Balsano, S.M. Meier, U. Jungwirth, S. and ammonium hexafluorophosphate (140.9mg, 0.86mmol) was Gçschl, A. Roller, M.A. Jakupec, W. Berger, B.K. Keppler, added.Themixturewasconcentratedunderreducedpressuretoca. C.G. Hartinger, Chem. Commun. 2013, 49, 3348–3350; h)M. 3mL and placed in a freezer overnight. The dark crystalline Gras,B.Therrien,G.S(cid:5)ss-Fink,A.Casini,F.Edafe,P.J.Dyson, precipitatewascollectedviavacuumfiltration,washedwithice-cold J. Organomet. Chem. 2010, 695, 1119–1125; i)M. Streib, K. ethanol(2(cid:3)1mL)anddiethylether(2(cid:3)5mL),anddriedovernight Kr(cid:6)ling, K. Richter, X. Xie, H. Steuber, E. Meggers, Angew. inavacuumdesiccator.Yield:119.6mg(84%).1HNMR(CD 3 OD): Chem.Int.Ed.2014,53,305–309;Angew.Chem.2014,126,311– d=9.07 (d, 1H, J=2.6Hz), 8.84 (d, 1H, J=9.1Hz), 8.04–8.01(m, 315; j)S. Chatterjee, S. Kundu, A. Bhattacharyya, C.G. Har- 2H),7.94(dd,1H,J=9.1,2.6Hz),7.73–7.63(m,3H),6.47–6.46(m, tinger, P.J. Dyson, J. Biol. Inorg. Chem. 2008, 13, 1149–1155. 1H),6.16–6.15(m,1H),6.03–6.02(m,1H),5.96–5.95(m,1H),4.50– [2] Y. Fu, A. Habtemariam, A.M. Pizarro, S.H. vanRijt, D.J. 4.38(m,2H),2.70(s,3H),2.45(sept.,1H,J=6.9Hz),1.55(t,3H,J= Healey,P.A.Cooper,S.D.Shnyder,G.J.Clarkson,P.J.Sadler, 7.0Hz),0.94–0.92ppm(2xd,6H,J=6.9Hz).ESI-MScalculatedfor J.Med.Chem.2010,53,8192–8196. C 23 H 27 IN 3 OOs+:m/z680.1.Found:679.9.CHNanalysis:Found:C, [3] J.M. Hearn, I. Romero-Canel(cid:7)n, A.F. Munro, Y. Fu, A.M. 33.30%;H,3.26%;N,4.98%.CalculatedforC 23 H 27 F 6 IN 3 OOsP:C, Pizarro, M.J. Garnett, U. McDermott, N.O. Carragher, P.J. 33.54%;H,3.30%;N,5.10%. Sadler,Proc.Natl.Acad.Sci.USA2015,112,E3800–E3805. Radiolabelling: 50mL of 1-Cl·PF 6 or 2-Cl·PF 6 in methanol [4] S.D. Shnyder, Y. Fu, A. Habtemariam, S.H. VanRijt, P.A. (5mgmL(cid:2)1) was transferred into a 2mL plastic sealable tube and Cooper,P.M.Loadman,P.J.Sadler,MedChemComm2011,2, combined with Na131I (30–70 MBq) in 0.1mm NaOH solution. 666–668. Furtherwaterwasaddedtogiveawater:methanol(1:1,v/v)solvent [5] I.Romero-Canel(cid:7)n,M.Mos,P.J.Sadler,J.Med.Chem.2015,58, matrix,thenthemixturewasheatedfor18hat333Kwith300rpm 7874–7880. stirring.Theradio-labelledcomplexeswere purified bypreparative [6] G.K. Balendiran, R. Dabur, D. Fraser, Cell Biochem. Funct. radio-HPLCanddilutedwiththreepartsphosphatebufferedsaline 2004,22,343–352. (PBS)andstoredat193K. [7] L. Galluzzi, L. Senovilla, I. Vitale, J. Michels, I. Martins, O. Kepp,M.Castedo,G.Kroemer,Oncogene2012,31,1869–1883. [8] a)S.J.Dougan,A.Habtemariam,S.E.McHale,S.Parsons,P.J. Acknowledgements Sadler, Proc. Natl. Acad. Sci. USA 2008, 105, 11628–11633; b)H. Petzold, P.J. Sadler, Chem. Commun. 2008, 4413–4415; c)H.Petzold,J.Xu,P.J.Sadler,Angew.Chem.Int.Ed.2008,47, WethanktheERC(GrantNo.247450),ScienceCity(AWM/ 3008–3011;Angew.Chem.2008,120,3050–3053. ERDF), EPSRC (Grant No. EP/F034210/1), the Wellcome [9] F. Wang, J. Xu, A. Habtemariam, J. Bella, P.J. Sadler, J. Am. Trust (Grant No. 107691/Z/15/Z) for support, Dr. Florian Chem.Soc.2005,127,17734–17743. Kampmeier and Putthiporn Charoenphun (KCL) for assis- [10] G.Jaouen,A.Vessieres,S.Top,Chem.Soc.Rev.2015,44,8802– tance with cell culture, and members of EC COST Action 8817. CM1105 for stimulating discussions. Research at King(cid:4)s [11] a)M.Matczuk,M.Przadka,S.S.Aleksenko,Z.Czarnocki,K. Pawlak,A.R.Timerbaev,M.Jarosz,Metallomics2014,6,147– College London was supported by Bloodwise (L.M.), the 153; b)A.M. Palmer, B. PeÇa, R.B. Sears, O. Chen, M. Centre of Excellence in Medical Engineering funded by the ElOjaimi, R.P. Thummel, K.R. Dunbar, C. Turro, Philos. WellcomeTrust(GrantNo.WT088641/Z/09/Z),EPSRC,the Trans.R.Soc.LondonSer.A2013,371,20120135. King(cid:4)s College London and University College London [12] S.H. vanRijt, I. Romero-Canel(cid:7)n, Y. Fu, S.D. Shnyder, P.J. Comprehensive Cancer Imaging Centre funded by CRUK Sadler,Metallomics2014,6,1014–1022. andEPSRCinassociationwiththeMRCandDoH(England) [13] K.J. Long, K.B. Walsh, Methods Cell Sci. 1997, 19, 207–212. (M.S.C.), and the National Institutes for Health Research [14] a)J. V(cid:8)squez-Vivar, B. Kalyanaraman, P. Mart(cid:8)sek, N. Hogg, B.S. SilerMasters, H. Karoui, P. Tordo, K.A. Pritchard,Jr., (NIHR)BiomedicalResearchCentrebasedatGuy(cid:4)sandSt. Proc.Natl.Acad.Sci.USA1998,95,9220–9225;b)C.Frejaville, Thomas(cid:4)NHSFoundationTrustandKing(cid:4)sCollegeLondon. H. Karoui, B. Tuccio, F. leMoigne, M. Culcasi, S. Pietri, R. Lauricella,P.Tordo,J.Chem.Soc.Chem.Commun.1994,1793– 1794. Conflict of interest [15] a)S.Okuno,H.Sato,K.Kuriyama-Matsumura,M.Tamba,H. Wang,S.Sohda,H.Hamada,H.Yoshikawa,T.Kondo,S.Bannai, Br.J.Cancer2003,88,951–956;b)A. Ozkan,K.Fiskin,Exp. Theauthorsdeclarenoconflictofinterest. Oncol.2006,28,86–88. Keywords: anticanceragents·bioinorganicchemistry· glutathione·metallodrugs·organo-osmiumcomplexes Manuscriptreceived:October20,2016 FinalArticlepublished:&&&&,&&&& 4 www.angewandte.org (cid:2)2016Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim Angew.Chem.Int.Ed.2016,55,1–5 (cid:2)(cid:2) These are not the final page numbers! Angewandte Communications Chemie Communications Anticancer Agents Os(cid:2)Ibondactivation:IodidoOsIIarene phenylazopyridinecomplexesshow R.J.Needham,C.Sanchez-Cano, promisinganticancerpropertiesbothin X.Zhang,I.Romero-Canel(cid:3)n, vitroandinvivo.Surprisinglytheycanbe A.Habtemariam,M.S.Cooper, activatedincellsbyhydrolysisoftheir L.Meszaros,G.J.Clarkson,P.J.Blower, Os(cid:2)Ibondinthepresenceofglutathione P.J.Sadler* &&&&—&&&& (GSH).Thenewlyformedhydroxido complexesaremorereactiveandcan In-CellActivationofOrgano-Osmium(II) formchlorido,thiolatoandsulfenato AnticancerComplexes adductswithGSH,andreactwithHO, 2 2 generatinghydroxylradicals. Angew.Chem.Int.Ed.2016,55,1–5 (cid:2)2016Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim www.angewandte.org 5 (cid:2)(cid:2) These are not the final page numbers!