RTC

(gd

h

68Ga-labeled DOTA-Peptides and 68Ga-labeledadiopharmaceuticals for Positron Emissionomography: Current Status of Research,linical Applications, and Future Perspectives

Wouter A. P. Breeman, PhD, Erik de Blois, MSc, Ho Sze Chan, MSc, Mark Konijnenberg, PhD,Dik J. Kwekkeboom, MD, PhD, and Eric P. Krenning, MD, PhD

In this review we give an overview of current knowledge of 68Ga-labeled pharmaceuticals,with focus on imaging receptor-mediated processes. A major advantage of a 68Ge/68Gagenerator is its continuous source of 68Ga, independently from an on-site cyclotron. Theincrease in knowledge of purification and concentration of the eluate and the complexligand chemistry has led to 68Ga-labeled pharmaceuticals with major clinical impact.68Ga-labeled pharmaceuticals have the potential to cover all today’s clinical options with99mTc, with the concordant higher resolution of positron emission tomography (PET) incomparison with single photon emission computed tomography. 68Ga-labeled analogs ofoctreotide, such as DOTATOC, DOTANOC, and DOTA-TATE, are in clinical application innuclear medicine, and these analogs are now the most frequently applied of all 68Ga-labeled pharmaceuticals. All the above-mentioned items in favor of successful applicationof 68Ga-labeled radiopharmaceuticals for imaging in patients are strong arguments for thedevelopment of a 68Ge/68Ga generator with Marketing Authorization and thus to providepharmaceutical grade eluate. Moreover, now not one United States Food and Drug Admin-istration–approved or European Medicines Agency–approved 68Ga-radiopharmaceutical isavailable. As soon as these are achieved, a whole new radiopharmacy providing PETradiopharmaceuticals might develop.

Semin Nucl Med 41:314-321 © 2011 Elsevier Inc. All rights reserved.

a

edi

al

ab

Interest and research in 68Ge and 68Ga (half-life of 9 monthsand 68 minutes, respectively) started in the early 1950s,

initially as a spinoff from the discovery of 68Ge in fissionproducts.1,2 Most of 68Ge for 68Ge/68Ga generators is now acyclotron-produced radio-metal, formed from 69Ga by ap,2n) reaction (Table 1).3-5 A major advantage of a 68Ge/68Gaenerator is its continuous source of 68Ga (Fig. 1), indepen-ently from an on-site cyclotron.The 68Ge/68Ga generator was first described in 19606; for a

istorical overview see these sources.3,7-9

A revival started in the 1970s, when several other 68Ge/68Ga generator systems were developed that resulted in reli-

Department of Nuclear Medicine, Erasmus MC Rotterdam, Rotterdam, TheNetherlands.

Address reprint requests to Wouter A. P. Breeman, PhD, Department ofNuclear Medicine, Erasmus MC Rotterdam, Department of NuclearMedicine, ‘s Gravendijkwal 230, 3015 Rotterdam, CE, Netherlands.

E-mail: w.a.p.breeman@erasmusmc.nl

314 0001-2998/11/$-see front matter © 2011 Elsevier Inc. All rights reserved.doi:10.1053/j.semnuclmed.2011.02.001

ble sources of the positron emitter 68Ga. Renewed interest in68Ga has recently arisen for several reasons. First, positronmission tomography (PET) has developed during the lastecade from a research tool into a powerful diagnostic and

maging technique for routine clinical application. Second,68Ge/68Ga generators have been developed that produce suit-ble eluates for labeling that can be converted into a 68Ga-abeled pharmaceutical for PET studies.8,10-17 Third, there aremany DOTA-peptides that can be labeled with 68Ga. Fourth,

variety of monofunctional and bifunctional chelators haveeen developed that allow the formation of stable 68Ga3�

complexes and convenient coupling to biomolecules. Fifth,the availability of PET radiolabeled pharmaceuticals by theintroduction of 68Ga in radiopharmacy, independent of anon-site cyclotron, opened new applications and possibilities.Coupling of 68Ga to small peptides and biomolecules wasrecently reviewed,3,7,9,18,19 and 68Ga is potentially an alterna-tive to 18F- and 11C-based radiopharmacy.19

Last but not least, equipment, including generators, puri-

pt

rrish

c

lp

At4

fZwac

a

sp

babFs

licp

ettmaa

E(

f

68Ga-labeled pharmaceuticals 315

fication and concentration of eluate, techniques of radiola-beling, robotics, and PET cameras, has improved during thelast decade. Preclinical application of many 68Ga-labeled

harmaceuticals has been developed, and all of today’s op-ions with 99mTc are potentially covered, for example, myo-

cardial perfusion and function, blood flow, renal function,liver function, and clinical application (Mäcke and Andre,19

Fani, et al,7 Rösch and Riss,3 Wadas, et al9).The term radiopharmaceutical has several meanings: any

adiolabeled molecule intended for human use, and from aegulatory point of view, a radiopharmaceutical must be ster-le, pyrogen free, safe for human use, and efficacious for apecific indication.20 Therefore, we will not use that termere. None of the developed 68Ga-labeled pharmaceuticals

has Food and Drug Administration (FDA) or European Med-icines Agency (EMA) approval. Also, no FDA- or EMA-ap-proved 68Ge/68Ga generator is available. Therefore, all currentlinically applied 68Ga-labeled pharmaceuticals are prepared

on site through a so-called magisterial preparation.21-23

Here we give an overview of the state of current knowledgeand research of many 68Ga-labeled pharmaceuticals of theast decade, with focus on the application of 68Ga-labeledharmaceuticals for imaging receptor-mediated processes.

Physical Characteristics of 68Ge,68Ga, and the 68Ge/68Ga GeneratorThe theoretical ingrowth of 68Ga from the parent nuclide68Ge in a 68Ge/68Ga generator is shown in Fig. 1 and Table 1.

lready 68 minutes after the previous elution of the genera-or, 50% of the maximal activity of 68Ga is reached, and afterhours this is �90% (Fig. 1). 68Ga decays to 89% by positron

emission and 11% via electron capture. The average positronenergy per disintegration is 740 keV (Emax � 1.9 MeV), whichis higher, for example, than that of 18F and potentially leads tolower resolution during scanning.7

68Ge/68Ga generators have been developed with many dif-erent inorganic carriers such as Al2O3, CeO2, SnO2, TiO2,rO2

7,24-31 and organic carriers like pyrogallol32 and elutedith different eluents, including ethylenediaminetetraacetic

cid, 8-hydroxyquinoline, and HCl. All have their pros andons, and most have recently been reviewed.3,7,9,18

Purification andEluate ConcentrationThe major limitations for direct use of 68Ga for radiolabeling

Table 1 68Ge is a cyclotron-produced radio-metal, formedrom 69Ga by a (p,2n) reaction

p, 2n EC ��

69Ga ¡ 68Ge ¡ 68Ga ¡ 68Zn68Ga half-life � 68 minutes; specific activity ' 3.63 � 10�13 gram-

atoms per 37 MBq 68Ga. In theory, 1 nmol DOTA-peptide cancomplex 10�9 gram-atoms Ga, corresponding to 102 GBq 68Ga.After �2.5 hours, equal amounts of 68Zn and 68Ga are present inthe generator.33

of DOTA-peptides for clinical PET applications are the large

volume of generator eluate, high [H�], 68Ge breakthrough,nd potential metal ion impurities.7,33-35 However, although

the concentration of these metals (eg, Zn, Sn, Ti, and Fe) arelow (ie, at sub parts per million level), their concentration canstill be much higher than the concentration of 68Ga. If oneupposes a concentration of Zn2�, Sn4�, Ti4�, or Fe2�/3� of 1pm versus 370 MBq 68Ga in 6 mL, then the ratios, expressed

in gram-atoms, are approximately 1000, 1250, 2500, and2500, respectively.

The presence of 4� metals such as Ti4� is irrelevant,34-38

because Ti4� does not compete with Ga3� in incorporation inDOTA. By contrast, the presence of Fe3� is very relevant,

ecause Fe3� competes with Ga3� for incorporation in DOTAs a result of a very limited difference in physical chemistryetween Ga3� and Fe3�. However, as Pourbaix reported,e3� is hardly present, because Fe2� is the stable oxidationtate of iron instead of Fe3� in acid systems, and Fe2� is much

less a competitor for Ga3� for incorporation in DOTA39 atabeling pH 3-5.34,35 However, the situation is quite differentn neutral or basic solutions.39 More detailed data regardingompetitions, impurities, and kinetics of labeling DOTA-eptides with 68Ga were described previously.18,35 Chemistry

of eluate concentration and purification of TiO2-based gen-rators was recently summarized by Ocak et al.13 Briefly,here are 3 different methods of purification and/or concen-ration of 68Ga: (1) by fractionated elution, (2) by anion chro-atography, and (3) by cation chromatography. Fraction-

ted elution results in a ready-to-use eluate containingpproximately 80% of the elutable 68Ga activity.33,35,37,40 An-

ion and cation exchange resins are used to reduce ionic im-purities, reduce acidity, and to concentrate the generator el-uate.12,33,35,37,40,41 These techniques for purification andconcentration of the eluate can also be applied for the GoodManufacturing Practice-produced SnO2-based 68Ge/68Gagenerator (iThemba Labs, Cap Town, South Africa) andeventually labeling DOTA-peptides successfully.33

68Ge BreakthroughSince the discovery of 68Ge in fission products1,2 and thepossibility of medical application of 68Ga, there has been

Figure 1 Theoretical ingrowth of 68Ga from a 68Ge/68Ga generator.ight hours after the previous elution there is an equilibrium�99%) of formation of 68Ga and decay to 68Zn (Table 1). Already

68 minutes after the previous elution, 50% of the equilibrium value

is reached, whereas after 4 hours, this is �90%.

Ridhitctnt(

aa

p

sdo

m

Ttim

s

cep

o

P

w

mb

mia

mt

iab

316 W.A.P. Breeman et al

renewed interest in its biochemical and pharmacologic prop-erties. The presence of 68Ge activity in the eluate of 68Ge/68Gagenerator, although low, has frequently been an item of con-cern for clinical application of 68Ga-labeled pharmaceuticals.

osenfeld2 reported studies on the metabolism of germaniumn the 1950s. Metabolic studies on the absorption, transport,istribution, storage, and excretion of inorganic Ge in ratsave been conducted. The data demonstrate that Ge is rap-

dly absorbed after oral, subcutaneous, intramuscular, or in-raperitoneal administration. When injected directly into theirculation or absorbed after oral or parenteral administra-ion, Ge is transported unbound by plasma proteins. Germa-ium is rapidly eliminated from the blood and excreted viahe urine and feces; the kidney is the main excretory organratio kidney vs liver in rats is 6).

Rosenfeld2 concluded that Ge is not deposited selectivelybut is widely distributed among all the organs. No accumu-lation was observed by any tissue even after many weeklydoses. The biochemical data support the view that Ge isrelatively inert metabolically and are consonant with its re-markably low (chemical) toxicity. With an excretion of 50%during the first hour after administration and a half-life of 30minutes in the kidneys, the current conclusion is that Gedoes not accumulate in bone.

With the techniques described above, 68Ge activity in elu-te can be reduced by anion or cation exchange. Anotherdditional “safety net” (to reduce 68Ge activity administra-

tion) is frequently incorporated in the preparation of the finalsolution before administration by reversed phase C18 or solid

hase extraction columns.21,33,42 These columns retain 68Ga-DOTA-complexes, whereas 68Ge and uncomplexed 68Ga passthrough the column.21

A typical example of a final 68Ga- and 68Ge-containingolution that is ready for clinical administration has a ra-ionuclide purity (RNP), expressed as the ratio in activitiesf 68Ge over 68Ga, in the range of 10-6 and 10-8.12,33,41,42. If

one supposes a clinical administration of 100 MBq 68Ga-DOTATOC, this will result in an effective dose of 2.3 mSv(0.023 mSv/MBq43), whereas the effective dose of 100 Bq68Ge (supposing RNP of 10-6) will be 0.6*10-5 mSv (0.06

Sv/MBq 68Ge44).

DOTA-Peptides Labeled With 68GaG protein–coupled receptors like somatostatin receptors are fre-quently overexpressed on human tumor cells.22,45,46

Somatostatin receptor–targeted imaging, initially with [123I-yr3]octreotide and later with [111In-DTPA0]octreotide (Oc-

reoScan; Covedien, Hazelwood, MO), was important for imag-ng and diagnostics of neuroendocrine tumors in nuclear

edicine.3,7,19,47

Radiolabeled peptides targeting G protein–coupled recep-tors with DOTA as bifunctional chelator were developed andhave shown in vivo stability, favorable pharmacokinetics(PK), and high and specific receptor-mediated tumor up-take.11 Another boost for nuclear medicine came with theomatostatin receptor–targeting radiopeptide [68Ga-DOTA0,

Tyr3]octreotide (68Ga-DOTATOC; Fig. 2) in the diagnostic

imaging of somatostatin receptor–positive tumors. In pre-clinical studies it has shown superiority over its 111In-labeledongener; however, this is not surprising because among oth-rs, the resolution of PET is higher in comparison with singlehoton emission computed tomography (SPECT).10,11,14-16

Because DOTATOC and other DOTA analogs have provenhigh in vitro and in vivo stability, most of the investigatedradiolabeled peptides have the bifunctional DOTA as chela-tor. However, alternative chelators are still being developed,such as NOTAGA and acyclic chelates, all with their specificadvantages and disadvantages.3,7,18,19,48-50

An additional beneficial effect of the presence of Ga3� inthe DOTA cavity in DOTA-pharmaceuticals is the increasedreceptor affinity, as compared with other trivalent metals inthe DOTA cavity such as In3� or Y3�,8,35,45 probably becausef differences in coordination chemistry.3,9,51,52

PK of 68Ga-labeledharmaceuticals

The uptake kinetics of 68Ga-DOTA-peptides such asDOTATOC, DOTANOC, and DOTA-TATE (for structuralformula see Fig. 2) are rapid11,18 and in good agreement

ith the half-life of 68Ga. The small size of these moleculesdisplays desirable PK properties, because the process ofbinding with their G protein– coupled receptor is rapid, aswell as their clearance, which are both required for suc-cessful imaging.53

Other ligands that bind to G protein–coupled receptors,such as analogs of octreotide, cholecystokinin, and gastrin-releasing peptide, also display rapid PK and clearance andfavorable for the half-life of 68Ga. In addition, successful

odification of PK with spacers between the chelator and theiomolecule is reported.52

Because of the mismatch in half-life of 68Ga and the PK ofost intact monoclonal antibodies (MoAb), imaging (radio-

mmunoimaging) with 68Ga-labeled MoAb is not considereds a potential application.54 The promise of radioimmunoim-

aging by �-scintigraphy has not fully lived up to expectations,ostly because of differences in biodistribution and PK be-

ween animals and humans.55 PK of 68Ga-labeled F(ab=)2

fragments of a mouse MoAb are more rapid and favorable for68Ga,56 showed good target-to-background ratios at approx-mately 2 hours after administration, and were successfullypplied in animal models.57 HER2-expressing tumors inreast cancer patients were successfully imaged with 68Ga-

labeled affibodies ABY-002.10

Specific RadioactivityThere are many factors that influence the interaction of aradioligand with its receptor. In saturable regulatory peptidebinding processes (ie, in vitro radioimmunoassay and recep-tor binding), the signal-to-background ratio is often im-proved by increasing the specific radioactivity (expressed asactivity units per mass units of ligand, eg, MBq per nmol) of

the ligand. In in vivo experiments it was shown that contrary

sbmffiiu

68Ga-labeled pharmaceuticals 317

to what was expected, the percentage uptake of radiolabeledoctreotide analogs in octreotide receptor–positive tissues isnot optimal at the lowest dose of maximum specific radioac-tivity; rather, the uptake is a bell-shaped function of the in-jected mass, initially increasing followed by a decreased up-take. These findings might be the result of 2 opposing effects,ie, a positive effect of increasing ligand concentrations on therate of internalization by ligand-induced receptor clustering

Figure 2 Structural formula of DOTATOC ([DOTA0, TDOTA-TATE ([DOTA0, Tyr3, Thr8]octreotide). Labeled68Ga-labeled pharmaceuticals in nuclear medicine.

and a negative effect because of saturation of the receptor at

increasing ligand concentrations.58 This implies that the sen-itivity of detection of somatostatin receptor–positive tumorsy peptide receptor scintigraphy might be improved by ad-inistration at optimized dose of radioligand, as was found

or other radioligands.59-61 These findings have been con-rmed in patients for [111In-DTPA0]octreotide58,62 and led to

ncreased quality of imaging with a significantly increasedptake in tumors.

reotide), DOTANOC [DOTA0, 1-Nal3]octreotide, and8Ga, these analogs are the most clinically applied of all

yr3]octwith 6

From the above-mentioned data the amount of adminis-

D

ot

itatam

cd

etitifl

mtav

wpe

f

famaap

i

saDa

D

olt

318 W.A.P. Breeman et al

tered radioactivity and the amount of ligand can be con-cluded; thus, specific radioactivity is a potent tool and can beapplied for optimizing personal patient peptide dose in pep-tide receptor radionuclide therapy (PPRT).16,18,63-65

In a PRRT study with [90Y-DOTA0, Tyr3]octreotide (90Y-OTATOC), DOTATOC was radiolabeled with 86Y (86Y is a

positron emitter, with a half-life of 15 hours) for dosimetricmeasurements to replace 90Y.66 Jonard et al67 presented dataon tissue distribution after the administration of 86Y-DOTATOC, labeled with various amounts of DOTATOC(range of 50-500 �g); the kidney dose was not affected; how-ever, tumor dose decreased (in absolute uptake) with higherpeptide amounts. Velikyan et al17 also investigated the impactf peptide mass on binding to neuroendocrine tumor soma-ostatin receptors in vivo by using 68Ga-DOTATOC as tracer

at a constant high specific activity, preceded by injection of 0,50, 250, or 500 �g of octreotide (Sandostatin; Novartis,Stein, Switzerland), administered 10 minutes before thetracer. Nine patients with gastroenteropancreatic neuroen-docrine tumors were included. Six of them underwent 3sequential PET/computed tomography (CT) examinationswith intravenous injections of 68Ga-DOTATOC (after admin-stration of octreotide) and carried out in 1 day. Three pa-ients were examined by dynamic and static PET/CT for PKnd dosimetric calculations. The tracer accumulation in theumors varied and depended on the total amount of the pre-dministered octreotide. In 5 of 6 patients, the highest tu-or-to-normal tissue ratio was found when 50 �g of oc-

treotide was preadministered. Thus again, optimizing massimproved image contrast. However, 1 patient showed a con-tinuously increasing tumor uptake even with higher oc-treotide preadministered. The application of 68Ga-labeled li-gand for optimizing therapeutic applications of concordantradiotherapeutic labeled ligand needs further dosimetricstudies. A relation (such as in PK and clearance) between theligands labeled with 68Ga versus the therapeutic radionuclide(eg, 90Y or 177Lu) at early time points also needs to be estab-lished.

Robotics for Automated LabelingInitially all labeling procedures were done manually. Fingerdosimetry measurements during elution of the generator andradiolabeling revealed doses of 60 �Sv per 555 MBq and 1.5mSv per administration of 10 MBq in rats.35 Introduction ofrobotics increased reproducibility, and it also opened theroutine application of 68Ga-labeled analogs for radiopharma-ies. The development of automated modules included re-uction of 68Ge activity, impurities, and volume in the 68Ga

activity containing eluate of the 68Ge/68Ga generator andventually minimizing the reaction volume for DOTA-pep-ides.12,13,33 Different strategies for radiolabeling with robot-cs were applied, including ion exchange chromatographyechniques, variation in labeling time,35-38 methods of heat-ng,36,37,68 labeling temperature,35-37,68 and application of dif-erent materials to reduce “sticking” or other uncontrolled

oss of ligand.35,41,69 Eventually all reported robotics were N

successful, with labeling yields �95% and specific radioac-tivity �50 MBq/nmol.13,37,38,68,70

To our knowledge, no finger dosimetry data while apply-ing robotics have been reported.

Regulatory AffairsEfforts to obtain approval for Marketing Authorization forradionuclide generators (such as the 68Ge/68Ga generator),labeling kits, etc have recently been reported.22,23,71,72 Be-cause the eluate of a 68Ge/68Ga generator is considered a

edicinal product and all ingredients for clinical administra-ion should be approved for pharmaceutical use, aspects suchs the choice of a buffer for pharmaceutical use are also in-estigated and reported.21 Moreover, not only should the

ingredients be of pharmaceutical grade, the ingredientsthemselves should also be of high chemical purity, becausethey should not interfere with the incorporation of Ga3�, as

as investigated for the choice of the buffer for clinical ap-lication, with 68Ga DOTATOC as model.21 Sterility of theluates from TiO2- and SnO2-based 68Ge/68Ga generator has

been reported.33,35 For overviews to the regulations for radio-pharmaceuticals in early-phase clinical trials in the EuropeanUnion, see the “Guideline to Regulations for Radiopharma-ceuticals in Early Phase Clinical Trials in the EU” by Verbrug-gen et al23 and for the U.S. in “Operation of a Radiopharmacyor a Clinical Trial” by Norenberg et al.71

Considering the increase in knowledge on the chemistry ofGa3�, the developments in robotics, and eventually success-ul application of 68Ga-labeled radiopharmaceuticals for im-ging in patients, all are strong arguments for the develop-ent of a 68Ge/68Ga generator with Marketing Authorization

nd to provide pharmaceutical grade eluate.22 As soon this ischieved, a whole new radiopharmacy providing PET radio-harmaceuticals might develop.

ApplicationsMost neuroendocrine tumors express somatostatin receptorsubtype 2.45,46,73 Somatostatin receptor–targeted imaging ismportant in nuclear medicine.

In general, analogs of octreotide have high affinities foromatostatin receptor subtypes 2, 3, and 5; however, therere differences in affinity per receptor subtype per analog.OTANOC has very high affinity for receptor subtypes 2,3,nd 5.10,14,19,74-76 In an intraindividual clinical study compar-

ing the diagnostic efficacy of 68Ga-DOTA-NOC and 68Ga-OTA–TATE, it was demonstrated that 68Ga-DOTA-NOC is

superior to 68Ga-DOTA–TATE52; thus, Prasad and Baum15

use 68Ga-DOTANOC for routine receptor PET/CT of neu-roendocrine tumors. Prasad et al14 also state that for properstaging of neuroendocrine tumors and also for follow-up, it ismore appropriate to use 68Ga-DOTA-NOC receptor PET/CTrather than CT or 68Ga-DOTA-NOC PET alone. The additionf morphologic information from CT was found to be abso-utely essential in pinpointing the exact site of the primaryumor, particularly in the abdominal region. 68Ga-DOTA-

OC PET/CT caused a therapy modification in more than

s

Df

Dlaa

T

bf

mc

pd

68Ga-labeled pharmaceuticals 319

half of the patients.77 Another possible clinical application of68Ga-labeled pharmaceuticals includes the techniques andet-up as described by Velikyan et al17 with 68Ga-DOTATOC

to optimize the interval in time between withdrawal of oc-treotide and PRRT. In short, the use of variable amounts ofunlabeled octreotide in combination with, for example, 68Ga-

OTATOC at high specific radioactivities might also be a toolor optimizing personal patient peptide dose in PRRT.

This might also be true for other radiodiagnostics such asOTABOC (another octreotide analog, or the -ATE versions

ike DOTANOC-ATE16,18,19,78) and can be investigated, alsoccording to the procedure guidelines for PET/CT tumor im-ging with 68Ga-DOTA–conjugated peptides,79 preferably in

an intraindividual clinical study.Development of other DOTA-peptides with high affinity to

other G protein–coupled receptors with overexpression onhuman tumors (eg, receptors of gastrin-releasing peptide,glucagon-like peptide) and integrins like arginine-glycine-aspartic acid are still ongoing and successfully applied inpatients74 (Dr R. Baum, personal communication, 2010).

here are many other 68Ga-labeled pharmaceuticals devel-oped, such as melatonin-stimulating hormone,80 amino ac-ids81 and siderophores for aspergillosis,82 and other analogswith potential to replace current options in nuclear medicineand labeled with 99mTc. In addition, there are more applica-tions such as the coupling of 68Ga to small peptides and

iomolecules, as was recently reviewed.3,7,9,18,19 68Ga is there-ore potentially an alternative to 18F- and 11C-based radiop-

harmacy.19 However, up to now, not one 68Ga-labeled phar-aceutical has FDA or EMA approval. For a recent overview,

onsult these sources.3,7,19

Future PerspectivesIn a review on the continuing role of radionuclide generatorsystems for nuclear medicine, Knapp and Mirzadeh83 statethat “despite the availability of the 68Ge/68Ga generator appli-cation of 68Ga radiopharmaceuticals may suffer from thecomplex ligand chemistry required for Ga3� complexation touseful tissue-specific radiopharmaceuticals.” Indeed, at thattime (1994) no 68Ga-labeled pharmaceuticals were in clinicalstudies, and currently still not one EMA- or FDA-approved68Ga-labeled pharmaceutical is available.20,72

However, as Nunn84 states, “Molecular imaging must beaccepted as not just good science but also as central to routinepatient management in the personalized medicine of the fu-ture. There is an urgent need to reduce the cost (i.e., time andmoney) of developing imaging agents for routine clinical use.The mismatch between the current regulations and person-alized medicine includes molecular imaging and requires theengagement of the regulatory authorities to correct. This is anew venture in both molecular imaging and targeted drugs.However, there are various regulatory, financial, and practi-cal barriers that must be overcome to achieve this aim.”

We would like to express our vision that with currentknowledge and techniques, applications of 68Ga-labeled

harmaceuticals have a new future; however, there are in-

eed various barriers that must be overcome.

AcknowledgmentsWe would like to thank the COST working groups B12, D18,D38, BM0607, the Training School on Generators in Mainz,and many colleagues involved in these networks for stimu-lating discussions. Thanks to the cooperation and network-ing within COST, more than 40 centers in Europe use 68Ge/68Ga generators, also in clinical studies.

References1. Mirzadeh S, Lambrecht RM: Radiochemistry of germanium. Journal of

Radioanalytical and Nuclear Chemistry 202:7-102, 19962. Rosenfeld G: Studies of the metabolism of germanium. Arch Biochem

Biophys 48:84-94, 19543. Roesch F, Riss PJ: The renaissance of the Ge/Ga radionuclide generator

initiates new developments in Ga radiopharmaceutical chemistry. CurrTop Med Chem 10:1633-1668, 2010

4. Naidoo C, van der Walt TN: Cyclotron production of 67Ga(III) with atandem natGe-natZn target. Appl Radiat Isot 54:915-919, 2001

5. Aardaneh K, van der Walt TN: Ga2O for target, solvent extraction forradiochemical separation and SnO(2) for the preparation of a Ge-68/Ga-68 generator. Journal of Radioanalytical and Nuclear Chemistry268:25-32, 2006

6. Gleason GI: A positron cow. Int J Appl Radiat Isot 8:90-94, 19607. Fani M, Andre JP, Maecke HR: 68Ga-PET: a powerful generator-based

alternative to cyclotron-based PET radiopharmaceuticals. Contrast Me-dia Mol Imaging 3:67-77, 2008

8. Antunes P, Ginj M, Zhang H, et al: Are radiogallium-labelled DOTA-conjugated somatostatin analogues superior to those labelled withother radiometals? Eur J Nucl Med Mol Imaging 34:982-993, 2007

9. Wadas TJ, Wong EH, Weisman GR, et al: Coordinating radiometals ofcopper, gallium, indium, yttrium, and zirconium for PET and SPECTimaging of disease. Chem Rev 110:2858-2902, 2010

10. Baum RP, Prasad V, Müller D, et al: Molecular imaging of HER2-ex-pressing malignant tumors in breast cancer patients using synthetic111In- or 68Ga-labeled affibody molecules. J Nucl Med 51:892-897,2010

11. Hofmann M, Maecke H, Börner R, et al: Biokinetics and imaging withthe somatostatin receptor PET radioligand (68)Ga-DOTATOC: prelim-inary data. Eur J Nucl Med 28:1751-1757, 2001

12. Meyer GJ, Mäcke H, Schuhmacher J, et al: 68Ga-labelled DOTA-deri-vatised peptide ligands. Eur J Nucl Med Mol Imaging 31:1097-1104,2004

13. Ocak M, Antretter M, Knopp R, et al: Full automation of (68)Ga label-ling of DOTA-peptides including cation exchange prepurification.Appl Radiat Isot 68:297-302, 2010

14. Prasad V, Ambrosini V, Hommann M, et al: Detection of unknownprimary neuroendocrine tumours (CUP-NET) using (68)Ga-DOTA-NOC receptor PET/CT. Eur J Nucl Med Mol Imaging 37:67-77, 2010

15. Prasad V, Baum RP: Biodistribution of the Ga-68 labeled somatostatinanalogue DOTA-NOC in patients with neuroendocrine tumors: char-acterization of uptake in normal organs and tumor lesions. Q J NuclMed Mol Imaging 54:61-67, 2007

16. Prasad V, Fetscher S, Baum RP: Changing role of somatostatin receptortargeted drugs in NET: nuclear medicine’s view. J Pharm Pharm Sci10:321s-337s, 2007

17. Velikyan I, Sundin A, Eriksson B, et al: In vivo binding of [68Ga]-DOTATOC to somatostatin receptors in neuroendocrine tumours: im-pact of peptide mass. Nucl Med Biol 37:265-275, 2010

18. Maecke HR, Hofmann M, Haberkorn U: (68)Ga-labeled peptides intumor imaging. J Nucl Med 46(Suppl 1):172S-178S, 2005

19. Maecke HR, Andre JP: 68Ga-PET radiopharmacy: a generator-basedalternative to 18F-radiopharmacy. Ernst Schering Res Found Work-shop 215-242, 2007

20. Vallabhajosula S, Killeen RP, Osborne JR: Altered biodistribution ofradiopharmaceuticals: role of radiochemical/pharmaceutical purity,physiological, and pharmacologic factors. Semin Nucl Med 40:220-

241, 2010

2

3

3

3

3

3

3

3

3

3

3

4

4

4

4

4

4

4

4

4

4

5

5

5

5

5

5

5

5

5

5

6

6

6

6

6

6

320 W.A.P. Breeman et al

21. Bauwens M, Chekol R, Vanbilloen H, et al: Optimal buffer choice of theradiosynthesis of (68)Ga-Dotatoc for clinical application. Nucl MedCommun 31:753-758, 2010

22. Breeman WA, Verbruggen AM: The 68Ge/68Ga generator has highpotential, but when can we use 68Ga-labelled tracers in clinical rou-tine? Eur J Nucl Med Mol Imaging 34:978-981, 2007

23. Verbruggen A, Coenen HH, Deverre JR, et al: Guideline to regulationsfor radiopharmaceuticals in early phase clinical trials in the EU. EurJ Nucl Med Mol Imaging 35:2144-2151, 2008

24. Ehrhardt GJ, Welch MJ: A new germanium-68/gallium-68 generator.J Nucl Med 19:925-929, 1978

25. McElvany KD, Hopkins KT, Welch MJ: Comparison of 68Ge/68Gagenerator systems for radiopharmaceutical production. Int J Appl Ra-diat Isot 35:521-524, 1984

26. Neirinckx RD, Davis MA: Potential column chromatography generatorsfor ionic Ga-68: I–inorganic substrates. J Nucl Med 20:1075-1079,1979

27. Egamediev SK, Khujaev S, Mamatkazina AK: Influence of preliminarytreatment of aluminum oxide on the separation of Ge-68-Ga-68 radio-nuclide chain. Journal of Radioanalytical and Nuclear Chemistry 246:593-596, 2000

28. Bao B, Song M: A new Ge-68/Ga-68 generator based on CeO2. Journalof Radioanalytical and Nuclear Chemistry 213:233-238, 1996

9. Loc’h, C.; Maziere B, Comar D: A new generator for ionic gallium-68.J Nucl Med 21:171-173, 1980

0. Schuhmacher J, Maier-Borst W: A new 68Ge/68Ga radioisotope gen-erator system for production of 68Ga in dilute HCl. Appl Radiat Isot31-36, 1981

1. Chakravarty R, Shukla R, Ram S, et al: Nanoceria-PAN composite-based advanced sorbent material: a major step forward in the field ofclinical-grade 68Ge/68Ga generator. ACS Appl Mater Interfaces2:2069-2075, 2010

2. Nakayama M, Haratake M, Ono M, et al: A new Ge-68/Ga-68 generatorsystem using an organic polymer containing N-methylglucaminegroups as adsorbent for Ge-68. Appl Radiat Isot 58:9-14, 2003

3. De Blois E, Sze Chan H, Naidoo C, et al: Characteristics of SnO2-based68Ge/68Ga generator and aspects of radiolabelling DOTA-peptides.Appl Radiat Isot 69:308-315, 2011

4. Breeman WA, De Jong M, Visser TJ, et al: Optimising conditions forradiolabelling of DOTA-peptides with 90Y, 111In and 177Lu at highspecific activities. Eur J Nucl Med Mol Imaging 30:917-920, 2003

5. Breeman WA, de Jong M, de Blois E, et al: Radiolabelling DOTA-peptides with 68Ga. Eur J Nucl Med Mol Imaging 32:478-485, 2005

6. Velikyan I, Beyer GJ, Bergström-Pettermann E, et al: The importance ofhigh specific radioactivity in the performance of 68Ga-labeled peptide.Nucl Med Biol 35:529-536, 2008

7. Velikyan I, Beyer GJ, Langstrom B: Microwave-supported preparationof (68)Ga bioconjugates with high specific radioactivity. BioconjugChem 15:554-560, 2004

8. Velikyan I, Maecke H, Langstrom B: Convenient preparation of 68Ga-based PET-radiopharmaceuticals at room temperature. BioconjugChem 19:569-573, 2008

9. Pourbaix M, Zhang H, Pourbaix A: Presentation of an atlas of chemicaland electrochemical equilibria in the presence of a gaseous phase. HighTemp Corros Protect Mater 4(parts 1 and 2, 251-2):143-148, 1997

0. Decristoforo C, Knopp R, von Guggenberg E, et al: A fully automatedsynthesis for the preparation of 68Ga-labelled peptides. Nucl MedCommun 28:870-875, 2007

1. Zhernosekov KP, Filosofov DV, Baum RP, et al: Processing of generator-produced 68Ga for medical application. J Nucl Med 48:1741-1748,2007

2. Gabriel M, Decristoforo C, Kendler D, et al: 68Ga-DOTA-Tyr3-oc-treotide PET in neuroendocrine tumors: comparison with somatostatinreceptor scintigraphy and CT. J Nucl Med 48:508-518, 2007

3. Hartmann H, Zöphel K, Freudenberg R, et al: [Radiation exposure ofpatients during 68Ga-DOTATOC PET/CT examinations]. Nuklearme-dizin 48:201-207, 2009

4. ICRP. Limits for intakes of radionuclides by workers, vol 30. ICRP

Publication Ann. ICRP 8, 1978

5. Reubi JC, Maecke HR: Peptide-based probes for cancer imaging. J NuclMed 49:1735-1738, 2008

6. Reubi JC, Waser B, Schaer JC, et al: Somatostatin receptor sst1-sst5expression in normal and neoplastic human tissues using receptor au-toradiography with subtype-selective ligands. Eur J Nucl Med 28:836-846, 2001

7. Krenning EP, Kwekkeboom DJ, Bakker WH, et al: Somatostatin recep-tor scintigraphy with [111In-DTPA-D-Phe1] and [123I-Tyr3]-oc-treotide: the Rotterdam experience with more than 1000 patients. EurJ Nucl Med 20:716-731, 1993

8. Eisenwiener KP, Prata MI, Buschmann I, et al: NODAGATOC, a newchelator-coupled somatostatin analogue labeled with [67/68Ga] and[111In] for SPECT, PET, and targeted therapeutic applications of so-matostatin receptor (hsst2) expressing tumors. Bioconjug Chem 13:530-541, 2002

9. Ferreira CL, Lamsa E, Woods M, et al: Evaluation of bifunctional che-lates for the development of gallium-based radiopharmaceuticals. Bio-conjug Chem 21:531-536, 2010

0. Boros E, Ferreira CL, Cawthray JF, et al: Acyclic chelate with idealproperties for (68)Ga PET imaging agent elaboration. J Am Chem Soc132:15726-15733, 2010

1. Heppeler A, André JP, Buschmann I, et al: Metal-ion-dependent bio-logical properties of a chelator-derived somatostatin analogue for tu-mour targeting. Chemistry 14:3026-3034, 2008

2. Antunes P, Ginj M, Walter MA, et al: Influence of different spacers onthe biological profile of a DOTA-somatostatin analogue. BioconjugChem 18:84-92, 2007

3. Fischman AJ, Babich JW, Strauss HW: A ticket to ride: peptide radio-pharmaceuticals. J Nucl Med 34:2253-2263, 1993

4. Nayak TK, Brechbiel MW: Radioimmunoimaging with longer-livedpositron-emitting radionuclides: potentials and challenges. BioconjugChem 20:825-841, 2009

5. Mach JP, Carrel S, Forni M, et al: Tumor localization of radiolabeledantibodies against carcinoembryonic antigen in patients with carci-noma: a critical evaluation. N Engl J Med 303:5-10, 1980

6. Otsuka FL, Welch MJ, Kilbourn MR, et al: Antibody fragments labeledwith fluorine-18 and gallium-68: in vivo comparison with indium-111and iodine-125-labeled fragments. Int J Rad Appl Instrum B 18:813-816, 1991

7. Leung K: 68Ga-DOTA synthetic Her2 specific affibody. Bethesda, MD,Molecular Imaging and Contrast Agent Database (MICAD), NationalCenter for Biotechnolgy Information, 2010

8. Breeman WA, de Jong M, Kwekkeboom DJ, et al: Somatostatin recep-tor-mediated imaging and therapy: basic science, current knowledge,limitations and future perspectives. Eur J Nucl Med 28:1421-1429,2001

9. Breeman WA, Kwekkeboom DJ, Kooij PP, et al: Effect of dose andspecific activity on tissue distribution of indium-111-pentetreotide inrats. J Nucl Med 36:623-627, 1995

0. Breeman WA, De Jong M, Bernard BF, et al: Pre-clinical evaluation of[(111)In-DTPA-Pro(1), Tyr(4)]bombesin, a new radioligand for bomb-esin-receptor scintigraphy. Int J Cancer 83:657-663, 1999

1. Breeman WA, VanHagen MP, Visser-Wisselaar HA, et al: In vitro and invivo studies of substance P receptor expression in rats with the newanalog [indium-111-DTPA-Arg1]substance P. J Nucl Med 37:108-117,1996

2. Kooij PP, Kwekkeboom DJ, Breeman WAP, et al: The effects of specificactivity on tissue distribution of [In-111-DTPA-DPhe1]octreotide inhumans. J Nucl Med 35:226P, 1994

3. Breeman WA, Kwekkeboom DJ, de Blois E, et al: Radiolabelled regula-tory peptides for imaging and therapy. Anti Cancer Agents Med Chem7:345-357, 2007

4. van Essen M, Krenning EP, Kam BL, et al: Peptide-receptor radionu-clide therapy for endocrine tumors. Nat Rev Endocrinol 5:382-393,2009

5. Kwekkeboom DJ, Krenning EP, Lebtahi R, et al: ENETS consensusguidelines for the standards of care in neuroendocrine tumors: peptidereceptor radionuclide therapy with radiolabeled somatostatin analogs.

Neuroendocrinology 90:220-226, 2009

8

8

8

8

8

68Ga-labeled pharmaceuticals 321

66. Rösch F, Herzog H, Stolz B, et al: Uptake kinetics of the somatostatinreceptor ligand [86Y]DOTA-DPhe1-Tyr3-octreotide ([86Y] SMT487)using positron emission tomography in non-human primates and cal-culation of radiation doses of the 90Y-labelled analogue. Eur J NuclMed 26:358-366, 1999

67. Jonard P, Jamar F, Walrand S, et al: Effect of peptide amount on bio-distribution of [86Y]DOTA-DPhe1-Tyr3-octreotide ([86Y] SMT487).J Nucl Med 41:260P, 2000

68. Azhdarinia A, Yang DJ, Chao C, et al: Infrared-based module for thesynthesis of 68Ga-labeled radiotracers. Nucl Med Biol 34:121-127,2007

69. Gonda I, Swift DL: Losses of gallium in common laboratory ware andways to minimize them. Eur J Nucl Med 18:511-513, 1991

70. Gebhardt P, Opfermann T, Saluz HP: Computer controlled 68Ga milk-ing and concentration system. Appl Radiat Isot 68:1057-1059, 2010

71. Norenberg JP, Petry NA, Schwarz S: Operation of a radiopharmacy fora clinical trial. Semin Nucl Med 40:347-356, 2010

72. Vallabhajosula S: Molecular Imaging: Radiopharmaceuticals for PETand SPECT (ed 1). Berlin, Springer, 2009

73. Reubi JC, Waser B: Concomitant expression of several peptide recep-tors in neuroendocrine tumours: molecular basis for in vivo multire-ceptor tumour targeting. Eur J Nucl Med Mol Imaging 30:781-793,2003

74. Wild D, Wicki A, Mansi R, et al: Exendin-4-based radiopharmaceuti-cals for glucagonlike peptide-1 receptor PET/CT and SPECT/CT. J NuclMed 51:1059-1067, 2010

75. Wild D, Mäcke HR, Waser B, et al: 68Ga-DOTANOC: a first compound

for PET imaging with high affinity for somatostatin receptor subtypes 2and 5. Eur J Nucl Med Mol Imaging 32:724, 2005

76. Wild D, Mäcke H, Christ E, et al: Glucagon-like peptide 1-receptorscans to localize occult insulinomas. N Engl J Med 359:766-768, 2008

77. Ambrosini V, Campana D, Bodei L, et al: 68Ga-DOTANOC PET/CTclinical impact in patients with neuroendocrine tumors. J Nucl Med51:669-673, 2010

78. Ginj M, Schmitt JS, Chen J, et al: Design, synthesis, and biologicalevaluation of somatostatin-based radiopeptides. Chem Biol 13:1081-1090, 2006

79. Virgolini I, Ambrosini V, Bomanji JB, et al: Procedure guidelines forPET/CT tumour imaging with 68Ga-DOTA-conjugated peptides: 68Ga-DOTA-TOC, 68Ga-DOTA-NOC, 68Ga-DOTA-TATE. Eur J Nucl MedMol Imaging 37:2004-2010, 2010

0. Froidevaux S, Calame-Christe M, Schuhmacher J, et al: A gallium-labeled DOTA-alpha-melanocyte-stimulating hormone analog for PETimaging of melanoma metastases. J Nucl Med 45:116-123, 2004

1. Shetty D, Jeong JM, Ju CH, et al: Synthesis of novel 68Ga-labeled aminoacid derivatives for positron emission tomography of cancer cells. NuclMed Biol 37:893-902, 2010

2. Petrik M, Haas H, Dobrozemsky G, et al: 68Ga-siderophores for PETimaging of invasive pulmonary aspergillosis: proof of principle. J NuclMed 51:639-645, 2010

3. Knapp FF Jr, Mirzadeh S: The continuing important role of radionu-clide generator systems for nuclear medicine. Eur J Nucl Med 21:1151-1165, 1994

4. Nunn AD: Molecular imaging and personalized medicine: an uncertain

future. Cancer Biother Radiopharm 22:722-739, 2007