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Biomaterials 27 (2006) 2171–2177

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Leading Opinion

Calcium phosphate cements: Competitive drug carriers for themusculoskeletal system?$

Maria-Pau Ginebra�, Tania Traykova, Josep A. Planell

Center of Reference for Bioengineering of Catalonia (CREBEC), Division of Biomaterials, Biomechanics and Tissue Engineering, Department of Materials

Science and Metallurgical Engineering, Technical University of Catalonia (UPC), Av. Diagonal 647, 08028 Barcelona, Spain

Received 23 June 2005; accepted 14 November 2005

Available online 2 December 2005

Abstract

This paper attempts to provide an insight in the application of calcium phosphate cements (CPC) in the field of drug delivery devices

for the musculoskeletal system. Their ability to set once implanted within the body, giving a highly microporous material, allows

incorporation of many types of drugs and biologically active molecules, without losing activity and denaturalization. Additionally, by

being injectable these materials can be used in the growing market for new technologies of minimally invasive surgery, and in the

treatment of difficult accessible sites. All these characteristics, together with the excellent biological behaviour of CPC, make them good

candidates for drug delivery devices to be used in the pharmacological treatment of a great number of diseases of the bone tissue.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Drug delivery; Calcium phosphate cements; Bone regeneration; Bone healing; Bone cements

1. Introduction

The incidence of musculoskeletal disorders, such asosteoporosis and osteoarthritis has increased strongly inthe last decades, due to the increasing life expectancy. Inparallel, also the number of medications to treat and evenprevent these diseases has expanded dramatically in recentyears [1]. When once only surgical options were availableto treat some diseases at their end-stage, there are nowtreatments based on new drugs and active substances,targeted at the early steps of these musculoskeletal diseases.A key issue in these treatments is to maximize the drugaccess to specific bone sites, and to be able to control therelease of drugs, in order to maintain a desired drug

e front matter r 2005 Elsevier Ltd. All rights reserved.

omaterials.2005.11.023

ote: Leading Opinions: This paper is one of a newly

of scientific articles that provide evidence-based scientific

pical and important issues in biomaterials science. They

res of an invited editorial but are based on scientific facts,

tures of a review paper, without attempting to be

These papers have been commissioned by the Editor-in-

wed for factual, scientific content by referees.

ing author.

ess: maria.pau.ginebra@upc.edu (M.-P. Ginebra).

concentration level for long periods of time withoutreaching a toxic level or dropping below the minimumeffective level [2]. For this reason, a major effort has beendone focused on the development of materials that arecapable of releasing drugs by a reproducible and pre-dictable kinetics. A potential substrate to be used as drugcarrier must have the ability to incorporate a drug, toretain it in a specific target site, and to deliver itprogressively with time in the surrounding tissues.A range of materials has been employed as drug carriers,

most of them being stable or biodegradable polymers.However, in the field of the pharmacological treatment ofskeletal disorders, the specific characteristics of bone tissueshould be considered. Thus, an ideal drug carrier for boneshould be bioactive, which would ensure the ability of thematerials to bond to bone tissue, and resorbable to allowits progressive substitution by newly formed bone. Addi-tional advantages are provided if the material is injectable,since it would improve ease of administration, by allowingminimally invasive surgical techniques. All these propertiesare very well fitted by calcium phosphate cements (CPC),and therefore they should be competitive candidates to beused for this application.

ARTICLE IN PRESSM.-P. Ginebra et al. / Biomaterials 27 (2006) 2171–21772172

2. CPC as drug-carrier materials

The development of CPC was an important break-through in the field of bioceramics for bone regeneration,since it supplied a material which was mouldable andwhich had the capacity of self-setting in vivo, within thebone cavity [3,4]. In addition, the development of injectablecalcium phosphate cement formulations established goodprospects for minimally invasive surgical techniques devel-oped in recent years, less aggressive than the classicalsurgical methods. Their ability to set once implanted withinthe body, giving a highly microporous material, allowsincorporation of many types of drugs and biologicallyactive molecules, without losing activity and denaturaliza-tion. Therefore, the possibility to use CPCs not only asbone substitutes, but as carriers for local and controlled

Table 1

Some representative studies on CPC as drug carriers, for different drugs and

Cement formulation Drug

b-TCP, MCPM (brushite cement) Gentamicin

TTCP, DCP Flomoxef sodium

TTCP, DCP Gentamicin, Amikacin an

Ceftiofur

TTCP, DCP Gentamicin

TTCP, DCP, chitosan Flomoxef sodium

TTCP, a-TCP Tetraciclin

ACP, DCPD Gentamicin

TTCP, DCPD Vancomicin

Non specified Tobramicin

a-TCP, TTCP, DCPD Albecacin sulphate

TTCP, DCPD Mercatopurine

TTCP, DCPD Estradiol

TTCP, DCPD Estradiol

TTCP, DCPD, HA Salicylic acid

TTCP, DCPD, HA Indomethacine

a-TCP Metacrilamide derived fro

salicylic acid

Non specified Nifedipine

a-TCP, TTCP, DCPD rhTGF-b1a-TCP, TTCP, DCPD rhTGF-b1a-TCP, DCP, CaCO3, HA rhTGF-b1a-TCP, TTCP, DCPD rhTGF-b1b-TCP, MCPM, calcium sulphate (brushite

cement)

rhBMP-2

a-TCP, DCP, CaCO3, HA, PLGA

microparticles

rhBMP-2

ACP, DCPD rhBMP-2

a-TCP, DCP, CaCO3, HA Osteocalcin and collagen

TTCP, DCP Insulin and albumin

CPCs are apatitic cements, unless otherwise stated.

Abbreviations: DCP, dicalcium phosphate, CaHPO4; DCPD, brushite,

Ca10(PO4)6(OH)2; MCPM, monocalcium phosphate monohydrate Ca(H2PO

tricaclium phosphate, b-Ca3(PO4)2;TTCP, tetracalcium phosphate, Ca4(PO4)2acid); rhTGF-b1, human recombinant-transforming growth factor-b1; rhBMP

supply of drugs is very attractive and can be useful intreatments of different skeletal diseases, which normallyrequire long and painful therapies, as well as for accelerat-ing the rate of bone fracture healing.Unlike calcium phosphate ceramics employed as drug-

delivery systems, where the drugs are usually absorbed onthe surface, in CPCs the drugs can be incorporatedthroughout the whole material volume, by addingthem into one of the two cement phases. This fact canfacilitate the release of drugs for more prolongedtimes. Several studies related to the application of bothcommercial and experimental CPCs as drug carriers havebeen published. Some of the most relevant are summarizedin Table 1, which cover different carrier/drug combina-tions, in terms of both CPC formulations, and drugsystems.

CPC formulations

Group Type of study References

Antibiotic In vitro [5,6]

Antibiotic In vitro [7]

d Antibiotic In vitro [8]

Antibiotic In vivo [9]

Antibiotic In vitro [10]

Antibiotic In vitro [11]

Antibiotic In vitro [12]

Antibiotic In vitro

In vivo [13]

Antibiotic In vitro [14]

Antibiotic In vivo [15]

Anticancer drug In vitro [16,17]

Hormone In vitro

In vivo [18]

Hormone In vitro

In vivo [19]

Analgesic,

antiinflammatory

In vitro [20]

Antiinflammatory,

non- steroid

In vitro

In vivo [21–24]

m Analgesic,

antiinflammatory

In vitro [25]

Calcium antagonist In vitro [26]

Growth factor In vitro [27]

Growth factor In vitro [28]

Growth factor In vitro [29]

Growth factor In vivo [30]

Growth factor In vivo [31]

Growth factor In vivo [32]

Growth factor In vivo [33–35]

Proteins In vitro [36]

Proteins In vitro [37]

dicalcium phosphate dihydrate CaHPO4 � 2H2O; HA, hydroxyapatite,

4)2 �H2O; a-TCP, alpha-tricalcium phosphate, a-Ca3(PO4)2; b-TCP beta-

O; ACP, amorphous calcium phosphate; PLGA, poly (lactic-co-glycolic

-2, human recombinant-bone morphogenetic protein-2.

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2.1. Antibiotics

Major attention has been paid to antibiotics, due to theirwide areas of application: either as prophylactics to preventinfections produced during surgical interventions, or ingeneral in the treatment of bone infections. In fact, one ofthe key factors for the success of surgical interventionsaimed at the implantation of a prosthesis or of anosteoconductive material is the prevention from bacterialinfections. Wound contamination, or postoperative infec-tions following fracture repair, implantation of jointprosthesis or spine surgery, can cause serious problems.For this reason antibiotics are often provided as prophy-lactics, either orally or intravenously. However, the littleaccessibility of the site of infection to antibiotics deliveredsystemically lengthens often the treatment of bone infec-tions over 1 year.

Traditionally, a method applied to control locally thebone tissue infections has been the implantation ofpolymethylmethacrylate spheres (PMMA) loaded withgentamicin sulphate in the infection site. However, PMMAspheres are nonresorbable and must be removed after somemonths and replaced with other materials able to facilitatebone regeneration. Antibiotics can be likewise incorpo-rated in hydroxyapatite or b-tricalcium phosphate ceramicblocks, although their resorption rates are slow, and it isdifficult to shape ceramics with complex form in order tobe fitted into any type and size of bone defect. Analternative material proposed some decades ago wascalcium sulphate hemihydrate, which can be used in theform of cement [38]. The main disadvantage of thismaterial comes from its low mechanical strength and veryhigh resorption rate. In this context, the use of CPCs,combined with different antibiotics can be very helpful inorder to overcome the different drawbacks mentionedabove.

2.2. Antiinflammatory, anticancer and other drugs

CPC have been considered as appropriate matrixes forthe incorporation of other drugs with potential applicationin the musculoskeletal system, such indomethacine, ananti-inflammatory, nonsteroidal drug with wide applica-tion in different pathologies, such as chronical jointrheumatism [21–24], or some anticancer drugs such asmercatopurine [16,17]. Other studies have investigated theincorporation of some hormones, such as estradiol, afeminine sexual hormone with estrogenic activity, whichcan be used in the treatment of symptoms caused by thedeficit of estrogens during menopause, such as mineralresorption and bone loss [19].

2.3. Growth factors

A large number of osteogenic factors, peptides and smallmolecules have been associated with accelerated bonehealing in animal models and human clinical trials. Growth

factors are a large group of polypeptides, able to transmitsignals which affect cellular activity [39]. Among them, thesuperfamily of b-trasforming growth factors (TGFb-SF) isespecially relevant for bone regeneration. It includes thetransforming growth factors b1, b2, b3, and the bonemorphogenetic proteins (BMP). In the last years therecombination techniques have made possible the indus-trial production of human growth factors in big quantitiesand high purity. However, injection of this type ofsubstances alone cannot induce tissue formation andregeneration, since protein diffuses very fast from theimplantation site. Therefore, it is necessary to haveavailable carriers which allow for the controlled adminis-tration of these factors at adequate therapeutic levels, andtheir vectoring towards local tissue targets and cells. This isnecessary to allow large quantities of bone-formingprecursor cells to migrate proliferate and differentiate intoosteogenic cells that induce healing at the fracture site.Some studies in vitro demonstrated that combinatory useof these factors with calcium phosphate ceramics resultedin improved bone growth due to adsorption of big doses ofthe osteogenic factors on the ceramic surface, which weresubsequently released [40,41]. The self-setting ability andosteoconductivity of CPCs are very relevant for thisapplication. Additionally, if an appropriate resorption rateis assured, the incorporation of growth factors into CPCscan supply a material with a great regenerative potential,applicable in several bone diseases such as osteoporosis, orto enhance bone healing in fractures.In general, the studies on CPC as drug delivery systems

tackle very different aspects, all of them relevant for thisissue. In the first place, it is necessary to verify to whichextent the addition of the drug (either to the liquid or thesolid phases of the cement), interferes with the settingreaction, modifying either the rheological behaviour or thephysico-chemical properties of the cement. Secondly, it isnecessary to characterize the kinetics of drug release invitro. Subsequently, the effectiveness of the cement to actas carrier for drug delivery in vivo must be assessed andcarefully characterized. And as the final step, still to arrive,the clinical performance of the drug delivery system mustbe evaluated.

3. Effect of drug incorporation on the physico-chemical

properties of CPCs

The setting reaction of CPCs can be affected or modifiedby introducing a drug either to the powder phase or to theliquid phase, and as a consequence the physico-chemicaland mechanical properties can change [5–7,11,14]. Ingeneral, in apatitic cements antibiotics tend to increasetheir setting times and reduce the mechanical strength[7,11,14]. This decrease of mechanical strength can beattributed to different factors, such as increased porosity orto some inhibition of the setting reaction, as suggested bythe presence of certain amount of reactants in the setcements when the antibiotic quantity increases.

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In other cases the change in the setting properties arecaused by some chemical interaction with the dug, which canmodify the kinetics of the dissolution-precipitation reactionand the morphology of the precipitated crystals. Thus, Ratieret al. [11] attributed the reduction of mechanical properties inan apatitic cement caused by the addition of tetracycline, tothe ability this antibiotic to chelate Ca-atoms. The hypothesiswas supported by the fact that when the antibiotic wasintroduced already complexed with calcium, the interactionsantibiotic-cement could be limited, and bigger amounts couldbe added without influencing the setting and mechanicalproperties of the CPC. In the same direction, a strong effectwas observed in the rheological and mechanical properties ofan a-TCP-based cement when an amino salicylic acid derivedmethacrylamide with calcium complexation ability was added[25]. In another CPC, in this case a brushite one, the increaseof setting time and mechanical strength was attributed to thepresence of sulphate ions in the antibiotic, which modified themorphology of the precipitated crystals [5].

4. Drug release kinetics from CPCs

It is well known that the release of drugs from any drugdelivery device depends on different factors such as themicrostructure, the type of bond between the drug and thematrix which holds it, and the mechanism of degradation(if any) of the matrix. The drug delivery devices are usuallyclassified in three types depending on the mechanism whichcontrols drug liberation [42,43]: (a) Devices controlled by

diffusion. The drug is incorporated into a nonbiodegrad-able matrix, or is surrounded by a stable membranethrough which it should diffuse. In this case, the releasekinetics is dependent only on the physical diffusion processthrough the matrix or through the membrane outwards.(b) Devices controlled by chemical processes. The drug isintroduced in a biodegradable matrix. The liberationkinetics is linked to the kinetics of matrix degradation.(c) Devices controlled externally and/or electronically.

Generally speaking, CPC could be ascribed to the firsttype of devices, where drug release is diffusion-controlled.Although some CPC are resorbable, in most of the CPCstudied as drug-carriers the rate of matrix degradation (e.g.the cement itself) is much lower than the rate of drugliberation. For that reason it is possible to assume that thedrug release is mainly controlled by the process of diffusionthough the cement matrix and not by the degradation ofthe same.

During cement setting, dissolution of calcium phos-phates from the powder phase takes place, which isfollowed by precipitation of the new phase, which is inmost cases precipitated hydroxyapatite. Despite the factthat the drug can be partly dissolved in this precipitatedphase, as a rule it can be considered that the concentrationof the drug is higher than its solubility in the matrix, andtherefore the most part of the drug will be dispersed in thematrix. In this particular case, the drug-release kinetics

follows Higuchi’s law [44], at least at the initial stages (untilaround 60% drug is released).In addition, we have to consider that the matrix

containing the dispersed drug, i.e. the CPC, is very porous.Indeed, it is formed by a mesh of interlocking of crystalswhich creates an open microporosity. This porous structurecan vary depending on the starting materials and condi-tions, but normally it is higher than 30%. Pore sizes candiffer as well, but generally they are in the nano-microrange, usually lower than 10 mms. In this case, it can beassumed that the effective diffusion coefficient of thematrix will be proportional to CPC porosity and inverselyproportional to its tortuosity. Therefore, the drug libera-tion kinetics will be controlled by the following expression,as it has been shown by a series of works from Otsuka et al.[16,17,20,21,23] with different cements and drugs, andconfirmed by other authors [5,6]:

Mt ¼ AM0D0et

CSð2C0 � eCSÞt

� �1=2, (1)

where Mt is the amount of drug released for time t, A is thesurface area of the device, M0 is the total amount of drug,D0 is the diffusion coefficient of the cement structure, e isthe cement porosity and t is the tortuosity of the cement,Cs is the solubility of the drug in the matrix, and C0 is theinitial concentration of the drug in the matrix.

4.1. Effect of porosity

According to Eq. (1), the rate of drug liberation iscontrolled by drug diffusion through pores, and willdepend on the microstructural features of CPC. Indeed, ithas been shown for different drugs that it increases withhigher porosity of the CPC [16,17,20], which can be easilycontrolled by liquid-to-powder ratio.In this sense it must be pointed out that any modification

of the geometric pore structure of the cement matrix duringthe drug release period will affect the delivery pattern. Thischange in pore structure can be produced either by aresorption of the CPC, or by the formation of an apatiticlayer on the cement surface or even within the pores, due toits bioactive character, as reported in several studies byOtsuka et al. [16,21,24].Any resorption of the CPC will modify the pore

structure and affect the drug release pattern. Indeed, upto now it has been considered that CPC do not degradewhile drug is released, or, in other words, that the porosityof the matrix is maintained constant during drug delivery.However, some authors have shown certain degree ofdegradation of CPC during drug liberation. For example,Otsuka [22] showed an increase of porosity in a carbonatedhydroxyapatite cement during the release of indometha-cine, which was ascribed to the cement degradation. In thiscase, the higher resorption rate of the cement was explainedby the fact that the addition of different amounts ofNaHCO3 to a TTCP/DCPD cement resulted in a

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carbonated apatite more soluble than the stoichiometricHA used in previous studies. Resorption of carbonated HAwith time produced an increase of porosity, whichincreased the drug release rate. If porosity does not remainconstant, drug release kinetics does not follow Higuchi’slaw, and the drug diffusion through the CPC matrix is notany longer the only mechanism which controls drugliberation. In the case of brushite cements, the degradationrate is much higher than that of apatite cements, andtherefore is more important to consider these phenomena.

4.2. Effect of the drug nature

Another factor that affects the release kinetics is the drugthat is incorporated in the CPC. It is difficult to comparethe drug release kinetics reported by different studies, dueto the great differences in the experimental conditions. Inmost cases not only the drugs and the concentrationsstudied are different, but also the CPCs used as carriers, thegeometry, the conditions of measurement of drug release,etc. However, it seems clear that the release kineticsdepends strongly on the type of drug incorporated in thecement, as it conditions the interaction drug/matrix. Ingeneral, the release of antibiotics from the cements appearsto be fast. Thus, Takechi reported a total amount ofliberated antibiotic (flomoxef sodium) in a TTCP/DCPcement between 55% and 60% after 72 h [7]. Bohner et al.reported a total amount of antibiotic released within 7days, in the case of gentamicin incorporated in a brushitecement [5].

Moreover, this can vary also depending on the type ofantibiotic. In a comparative study the release kinetics ofthree antibiotics, i.e. gentamicin, amikacin and ceftiofurincorporated either in a commercial CPC or in PMMAspheres was analysed [8]. The results after 30 days showedfaster antibiotic liberation from CPC than from thepolymer. In both carriers the concentrations of liberatedgentamicin and amikacin were higher than the criticaldoses for preventing bacterial effects. However, theliberation of ceftiofur from both materials remained withinthe right levels only during 7 days, being thereforeinadequate when the bactericide effect has to be prolongedin time.

In the cases where antibiotic release is considered to beexcessively fast, a possible strategy is to incorporate somepolymers in the CPC, in order to retard drug liberation. Inthis sense some studies have been carried out aiming atthe formation of a gel into the cement pores which servedas matrix for the antibiotic, by the addition of some gel-forming substance such as sodium alginate or chitosan[7,10]. Another strategy consists in the formation of acomplex with the antibiotic, for instance by the addition ofpolyacrylic acid (PAA) to the CPC [6].

The situation is very different in the case of growthfactors, where the release kinetics seems to be very slow.This can be related to the high binding affinity of theprotein for calcium phosphate ceramics. Recently, Blom et

al. analysed the release kinetics of human recombinantTGF-b1 (rhTGF-b1) in two CPCs [28,29]. Initially therewas a fast elution of rh TGF-b1 during the first day,followed by a very slow release during the following weeks.Interestingly, the results suggested that the growth factorwas released only from the superficial layer in contact withthe surrounding medium, and not from the whole volumeof the CPC. Similar trends are observed in the drug releasebehaviour of rh-BMP-2 loaded in a poly(DL-lactic-co-glycolic acid, PLGA)/CPC composite [32], where the nano-porosity of the CPC not only did not facilitate the releaseof the protein, but further limited it because of the highbinding affinity of the protein for CPC.

4.3. Autoregulation

One of the most exciting challenges in controlled drugdelivery lies in the field of responsive delivery systems, withwhich it will be possible for instance to deliver drugsthrough implantable devices in response to a measuredblood level. This is the approach adopted by Otsuka et al.who studied the rate of release of estradiol incorporated ina CPC, both in vitro and in vivo, by subcutaneousimplantation in rats [18,19]. The rate of estradiol liberationin vitro was inversely proportional to calcium concentra-tion in solution. Consistently, in vivo release of estradiolwas faster in rats which had lower concentrations ofvitamin D and Ca, compared to healthy rats, suggesting theautoregulatory mechanism of estradiol liberation. Thebone mass of the recovery model rats was greater afterthe experiment than before, suggesting that the severity ofosteoporosis in these animals could be reduced by theimplantation of this estradiol-loaded apatite cement.

5. In vivo assessment

The number of in vivo studies is still low, and most ofthem have been performed in small animals, with theintrinsic limitations for results extrapolation to humans.The efficacy of several CPCs as carriers for antibiotics hasbeen assessed in different animal models. Himanashi et al.[13] showed that the therapeutic level of vancomicin loadedin a CPC in different concentrations, and implanted intibial condyles of rabbits, was maintained after severalweeks. Stallmann et al. [9] verified the efficacy of agentamicin-loaded commercial CPC to reduce the devel-opment of osteomyelitis in Stafilococus aureus-vaccinatedrabbits, when it was implanted in the femoral channel.A relevant question is whether it is reliable to extrapolate

the in vitro drug delivery kinetics results to the in vivobehaviour of the drug carrier, even more in a material, suchas CPC, which is far from inert when implanted. Indeed,surface changes in the cement such as the formation of anapatitic layer on the surface of the material caused by itsbioactive character, or either by protein adsorption mustbe expected, that can modify the drug release kinetics. Inthis sense, different liberation rates were observed in an

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indomethacine-loaded CPC when the concentrations ofreleased drug in vitro and in vivo, in a rat model, were

compared. A linear relation at the initial stage wasobserved in both cases, but liberation in vivo was muchslower than liberation in vitro during the last stage of thestudy [23]. Moreover, the half-life of indomethacine inplasma was much higher when drug was introduced viacement implantation, rather than when it was injectedsubcutaneously.

The in vivo results obtained by osteogenic factor-loadedCPC are very encouraging. A complete series of in vivostudies has been published by Seeherman et al., wheredifferent injectable osteogenic factor/carrier combinationshave been reviewed in different large-animal models[33–35]. Among all the carriers studied, a commercialresorbable CPC is considered to be the best one as carrierof recombinant human BMP-2 in terms of bone healingafter 10 weeks, in a fibular osteotomy, in a nonhumanprimate model. One of the main advantages of this carrieris that it can be implanted by a single percutaneousinjection. Bone healing was accelerated by approximately40%, as compared to untreated osteotomy sites. Similarresults were found by Ohura et al. who incorporateddifferent amounts of rhBMP-2 to a resorbable cement andimplanted them in critical fractures created in the femurs ofrats, observing an accelerated fracture consolidation [31].

6. Conclusions

CPC have specific properties which make them compe-titive drug carriers for the musculoskeletal system. To-gether with their injectability and low-temperature self-setting ability, their bioactivity ensures an optimuminteraction with the bone tissue. The drug release kineticsis affected by the variable resorption rate of CPCs, and bythe morphological changes caused by its bioactive char-acter. There is still a lot to be done in terms of adjusting itto different therapeutical needs and obtaining predictabledrug delivery systems. The incorporation of osteogenicfactors increases their potential as bone regenerationmaterials, provided an appropriate resorption rate of theCPCs is ensured.

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