Current Medicinal Chemistry, 2012, 19, 2667-2688
`
`2667
`
`Bifunctional Chelators in the Design and Application of Radiopharmaceuticals for
`Oncological Diseases
`D. Sarko1,2, M. Eisenhut3, U. Haberkorn1 and W. Mier*,1
`
`1Department of Nuclear Medicine, University Hospital Heidelberg, INF 400, 69120 Heidelberg, Germany; 2Department of
`Pharmaceutical Chemistry, Faculty of Pharmacy, Al-Baath University, P.O. Box 77 Homs, Syria; 3Department of
`Radiopharmaceutical Chemistry, DKFZ, INF 280, 69120 Heidelberg, Germany
`Abstract: Radiopharmaceuticals constitute diagnostic and therapeutic tools for both clinical and preclinical applications. They are a
`blend of a tracer moiety that mediates a site specific accumulation and an effector: a radioisotope whose decay enables either molecular
`imaging or exhibits cytotoxic effects. Radioactive halogens and
`lanthanides are
`the most commonly used
`isotopes for
`radiopharmaceuticals. Due to their ready availability and the facile labeling metallic radionuclides offer ideal characteristics for
`applications in nuclear medicine. A stable link between the radionuclide and the carrier molecule is the primary prerequisite for in vivo
`applications. The radionuclide is selected according to its physical and chemical properties i.e. half-life, the type of decay, the energy
`emitted and its availability. Bifunctional chelating agents are used to stably link the radiometal to the carrier moiety of the
`radiopharmaceutical. The design of the bifunctional chelator has to consider the impact of the radiometal chelate on the biological
`properties of the target-specific pharmaceutical. Here, with an emphasis on oncology, we review applications of radiopharmaceuticals
`that contain bifunctional chelators, while highlighting successes and identifying the key challenges that need to be addressed for the
`successful translation of target binding molecules into tracers for molecular imaging and endoradiotherapy.
`Keywords: Bifunctional chelating agent (BFCA), DOTA, nuclear medicine, radiochemistry, radiometals, radiopharmaceuticals, targeting,
`theranostics.
`
`INTRODUCTION
`
`Radiopharmaceuticals are diagnostic and therapeutic tools for
`the early identification and treatment of pathological changes. The
`radioisotopes comprised
`in radiopharmaceuticals emit either
`gamma rays for diagnostic use or alpha or beta particles for
`therapeutic use [1]. The radiopharmaceuticals used in molecular
`imaging improve drug development since molecular and functional
`imaging enhance our understanding of disease and drug activity
`during preclinical and clinical trials [2, 3].
`The term “molecular imaging” is broadly used in combination
`with
`imaging
`modalities
`that
`provide
`primarily
`morphological/anatomical as well as molecular information [4].
`The definition of “molecular imaging” can be formulated as “the in
`vivo application of
`imaging probes
`for
`the non-invasive
`visualization, characterization, and quantification of physiological
`processes at the molecular level” [5, 6]. Molecular imaging
`provides the potential for earlier detection, characterization and
`“real time” monitoring of disease, evaluation of treatment as well as
`investigating the efficacy of drugs [7]. Nowadays, the most
`sensitive molecular imaging modalities are the radionuclide-based
`positron emission tomography (PET) and single photon emission
`computed tomography (SPECT) imaging techniques. They provide
`the sensitivity required to visualize and determine most interactions
`between physiological targets and specific biomolecules at the
`picomolar scale [8].
`Imaging modalities can generally be divided into anatomical
`and molecular imaging techniques. In contrast to computed
`tomography (CT), magnetic resonance
`imaging (MRI) and
`ultrasound, the technologies used in anatomical imaging, which are
`characterized by a high spatial resolution, the molecular imaging
`modalities (optical imaging, PET and SPECT) offer the potential to
`identify changes due to diseases at molecular and cellular levels
`before tissue structural changes (e.g. metastasis) become visible.
`They rely on the injection of radiotracers at nanomolar blood
`concentrations.
`
`*Address correspondence to these authors at the University Hospital Heidelberg,
`Department of Nuclear Medicine, Im Neuenheimer Feld 400, D-69120 Heidelberg,
`Germany; Tel: +49-6221-567720; Fax: +49-6221-5633629;
`E-mail: walter.mier@med.uni-heidelberg.de
`
`At higher doses radiopharmaceuticals exert cytotoxic effects.
`The strong cytotoxic effects obtained with beta- and alpha-emitting
`isotopes are used to treat tumors. This targeted radionuclide therapy
`is termed endoradiotherapy. It is an approach for the systemic
`treatment and represents an alternative to the chemotherapeutic
`treatment of malignant tumors.
`The selection of the radionuclide depends on its physical and
`chemical parameters. In general, the element half-life, the type of
`decay, the type of emission(s) with the related energy, and of course
`the costs and availability should be considered before selecting a
`specific radionuclide [9]. A selection of radiometal nuclides suited
`for the incorporation into biomolecules is listed in Table 1. In this
`review, we describe commonly used strategies for the radiolabeling
`of probes for imaging and therapy. Furthermore, an overview of the
`most useful radiometal-chelator systems for radiopharmaceutical
`application for molecular imaging is given with emphasis on
`oncological diagnosis and therapy.
`
`THE COMPOSITION OF RADIOPHARMACEUTICALS
`
`The majority of radiopharmaceuticals currently available for
`applications in nuclear medicine are based on radiometals. In
`general, a radiometal-based radiopharmaceutical can be divided
`into four parts: a
`targeting biomolecule, a pharmacokinetic
`modifying linker, the bifunctional chelating agent (BFCA), and the
`radiometal (e.g., 67Cu, 90Y, 99mTc, 111In, 177Lu, 186Re, 188Re, and
`213Bi) [10-12]. The targeting biomolecule serves as a “carrier” for
`the specific delivery of the radiometal isotope. The BFCA is a
`prerequisite for the radiolabeling of biomolecules with metallic
`radionuclides. In addition to the physical and chemical properties,
`the appropriate selection of the radiometal depends on factors such
`as the biodistribution of the radiometal used, the targeted tissue, and
`the clearance rate of the radiometal complex from both target and
`non-target tissues [13].
`The use of radioactive metals has led to the development of
`several BFCAs that serve as a cross-linker between the carrier and
`the radiometal. Besides the coordinating sites that chelate the
`proper metallic radionuclide, the BFCA bears a functional group
`enabling the attachment to the carrier [12].
`Different types of BFCAs with different donor atoms and
`chelator framework are required since the radiometals used in
`
`1875-533X/12 $58.00+.00
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`© 2012 Bentham Science Publishers
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`2668 Current Medicinal Chemistry, 2012 Vol. 19, No. 17
`
`Sarko et al.
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`nuclear medicine vary significantly in their coordination chemistry
`depending on their nature and oxidation state. The electronegativity
`and oxidation state play a key role in the establishment of metal-
`ligand complexes. The complexes have to be formed in high yields
`(>99.5%) with high specific activities. The high stability and the
`preservation of the receptor-binding affinity of the labeled carrier
`are the crucial requirements for the in vivo application of
`radiometal-labeled
`biomolecules
`[14]. The
`labeling
`of
`radiopharmaceuticals with a radiometal can be difficult if the metal
`ion forms insoluble hydroxides at physiological pH. It is therefore
`necessary that the BFCAs complexes and stabilizes the radiometal.
`A high thermodynamic stability and kinetic inertness, under
`physiological conditions, are required to prevent the release and
`dissociation of the radiometal from its complex and thus avoid the
`accumulation in non-targeted organs. The radionuclides provide
`either the visualization source for molecular imaging or the source
`of the cytotoxic dose in radiotherapy.
`to modify
`in order
`The
`linker region can be varied
`pharmacokinetic properties of the substance. This can, for example,
`lead to an improvement of the target to background/blood (T/B)
`ratios to reduce the accumulation in non-targeted organs, while
`maintaining the uptake rate at the site of interest. Several types of
`linkers were explored (cationic, anionic, neutral or metabolically
`cleavable) and reviewed comprehensively by Liu et al. [12, 15-17].
`A selection of the most commonly used radioisotopes is shown in
`Table 1.
`Radiopharmaceuticals can be classified upon their application
`as diagnostic and therapeutic agents. Positron- and γ -emitting
`radioisotopes are utilized for diagnostic applications (PET or
`SPECT), while β-emitters are useful for therapeutic applications.
`
`DIAGNOSTIC RADIOPHARMACEUTICALS
`
`applied
`are
`radiopharmaceuticals
`Generally, diagnostic
`in very
`low
`intravenously
`(i.v.). They can be detected
`concentrations (low picomolar range) [8]. They can be labeled with
`a metallic (i.e. 99mTc, 68Ga, and 111In) or non-metallic (i.e. 18F and
`123I) radionuclide. They rely on a gamma-emitter for SPECT or a
`positron-emitter for PET [5, 20-23]. Their ultimate goal is to enable
`an anatomical description of organs as well as to evaluate their
`
`physiological function. Additionally, they can be used to determine
`the efficiency of a therapeutic treatment. Besides 99mTc, which
`remains the most widely used SPECT isotope, 111In is useful for
`gamma and SPECT imaging, and is often used as the imaging
`surrogate for 90Y since 90Y is a pure β-emitter. For PET imaging it
`is advantageous if the radionuclide does not emit radiation other
`than the 511-keV gamma photons from positron annihilation. This
`minimizes the impairment of the spatial resolution due to the high
`β+ energy and reduces the radiation burden for the patient. The
`worldwide number of PET-based studies was assumed to reach 3.2
`million in 2010 [22].
`
`THERAPEUTIC RADIOPHARMACEUTICALS
`
`by
`accessible
`only
`is
`disease
`disseminated
`Today
`chemotherapy. The chemotherapeutic agents used in systemic
`cancer therapy exert their effect mainly on proliferating cells. As a
`consequence normal tissue which is physiologically proliferating,
`such as bone marrow, intestinal or dermal epithelia is affected.
`Consequently, the systemic side effects of chemotherapeutic agents
`are one of the major problems of cancer chemotherapy [24].
`This has led to the requirement of new therapeutic strategies.
`The aim of drug-targeting is the reduction of extra-target effects in
`order to reduce the therapeutic index. Selective delivery or the
`selective activation within the targeted tissue minimizes toxic side
`effects. The improvement of the pharmacokinetic profile should be
`accomplished by the drug targeting strategies. However, diverse
`drug-targeting concepts have been followed in order to fulfill those
`principles [24]. The majority of
`the active drug-targeting
`applications rely on receptor-based drug-targeting principles, where
`a previously modified delivery system with receptor-specific-
`ligands conveys the drug to its target (Fig. 1).
`External beam irradiation, implantable “seeds” or systemic
`administration are the three routes of site specific administration to
`convey therapeutic doses of ionizing radiation to specific disease
`sites [25]. Brachytherapy comprises the use of seeds, which are
`only beneficial for the treatment of accessible tumors [26, 27]. In
`contrast,
`the systemic administration of radiopharmaceuticals
`designed for tumor-specific localization makes it possible to treat
`disseminated
`tumors [9, 14, 28-33]. Radiolabeled molecules
`
`Table 1.
`
`
`Commonly Used Radiometals and their Physical Characteristics
`
`Element
`
`Half-Life
`
`Mode of Decay
`
`Eγ (KeV)
`
`Availability/Production
`
`IT (100%)
`
`β- (92%) EC (8%)
`
`β- (100%)
`
`β+ (19%) EC (41%) β- (40%)
`
`β- (100%)
`
`β- (100%)
`
`99mTc
`
`186Re
`
`188Re
`
`64Cu
`
`67Cu
`
`177Lu
`
`86Y
`
`90Y
`
`66Ga
`
`67Ga
`
`68Ga
`
`111In
`
`213Bi
`
`6.02 h
`
`3.7 d
`
`16.98 h
`
`12.7 h
`
`2.58 d
`
`6.71 d
`
`14.7 h
`
`2.67 d
`
`9.5 h
`
`3.26 d
`
`68 min
`
`2.8 d
`
`45.6 min
`
`β+ (33%) EC (66%)
`
`2335, 2019, 1603, 1248, 1043
`
`β- (72%)
`
`β+ (56%) EC (44%)
`
`EC (100%)
`
`β+ (90%) EC (10%)
`
`EC (100%)
`
`2288
`
`4150, 935
`
`
`
`1880, 770
`
`
`
`β- (98%) α (2%)
`
`8.38 MeV (from 213Po)
`
`Data collected from Reichert et al. and Anderson et al. and the references cited therein [18, 19].
`
`Eβ (KeV)
`
`
`
`1071, 934
`
`2116, 1965
`
`656
`
`
`
`557, 484, 395
`
`91, 93, 185
`
`141
`
`137
`
`155
`
`
`
`
`
`
`
`
`
`
`
`91, 93, 185, 296, 388
`
`
`
`245, 172
`
`440
`
`99Mo/99mTc Generator
`
`Reactor
`
`188W/188Re Generator
`
`Cyclotron
`
`Accelerator
`
`Reactor
`
`Cyclotron
`
`90Sr/90Y Generator
`
`Cyclotron
`
`Cyclotron
`
`68Ge/68Ga Generator
`
`Cyclotron
`
`225Ac/213Bi Generator
`
`Petitioner GE Healthcare – Ex. 1017, p. 2668
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`

`

`BFCAs: The Core of Radiopharmaceuticals
`
`Current Medicinal Chemistry, 2012 Vol. 19, No. 17 2669
`
`
`
`AA
`
`
`
`BB
`
`
`
`B`B`
`
`
`
`A`A`
`
`
`
`
`
`
`
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`
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`DoseDose
`
`
`Fig. (1). Idealized scheme showing the dose effect relationship of targeted drugs and conventional drugs. Curve A illustrates the dose dependency of effects
`observed with drug-carrier conjugates: the therapeutic effect is achieved at lower doses than with conventional drugs (curve B). As a consequence, the
`corresponding side effects are minimized in the case of the targeted therapy (dotted line A') in comparison to the conventional therapy (dotted line B').
`
`Effect
`Effect
`
`designed to deliver therapeutic doses of ionizing radiation are
`referred to as “therapeutic radiopharmaceuticals”. Endoradio-
`therapy offers an additional advantage over the normal targeted
`therapy, since the ionizing radiation has the ability to penetrate all
`physiological barriers, cellular internalization is not required. High
`tumor accumulation ratios and a fast blood clearance, which
`minimizes radiation damage of non-targeted tissues, are the
`characteristics of an
`ideal
`therapeutic
`radiopharmaceutical.
`Therefore, complexes formed from radiometals and BFCAs should
`have a high thermodynamic stability and kinetic inertness (Fig. 2).
`
`
`
`
`
`
`
`
`Fig. (2). The basic structure of targeted drugs. Whereas targeted prodrugs
`have to be designed to release the drug at the site of action, targeted
`radiopharmaceuticals require a stable linker that guarantees the retention at
`the site of action. This can be achieved by using bifunctional chelators.
`
`
`
`LinkerLinker
`
`
`
`DrugDrug
`
`
`
`VectorVector
`
`The choice of the β-emitter depends on the size and the location
`of the tumor. While high-energy β-emitters such as 90Y are used for
`the treatment of large tumors (long penetration range (~12 mm)),
`the medium or low-energy β-emitters such as 177Lu are preferred for
`small tumors or metastases. Radiotherapy has been started five
`decades ago with
`the
`treatment of
`thyroid malignancy by
`radioiodine. The main prerequisite for radiotherapy is the specific
`localization of the therapeutic doses in the targeted tissues. The
`development of [111In]DTPA-octreotide (OctreoScan) for
`the
`diagnosis of somatostatin receptor positive tumors has provoked the
`succession of new
`receptor-based
`target-specific
`therapeutic
`radiopharmaceuticals and motivated the introduction of new
`therapeutic radionuclides, as well as new bifunctional chelators
`(BFCAs) [14, 34-43].
`
`THE CARRIER SYSTEMS
`
`Peptide Radiopharmaceuticals
`
`Due to their favorable pharmacokinetic profiles, small synthetic
`receptor-binding peptide-based radiopharmaceuticals have been
`established for diagnostic and therapeutic applications in oncology
`[28, 39]. Usually, the application of peptide carriers requires an
`in
`vivo
`over-expressed
`receptor
`and
`sufficient
`stability.
`Additionally, the conjugated chelator should not interfere with the
`binding affinity of the peptide.
`The possibility to obtain these compounds by solid phase
`synthesis approaches, invented by the Nobel laureate Bruce
`Merrifield [44], has opened a new era in nuclear medicine [28, 29,
`39, 45-48]. With automated solid phase peptide synthesis (SPPS),
`peptides can be produced in a straightforward manner for a variety
`of diagnostic and therapeutic applications [49, 50]. Due to their low
`molecular weight,
`fast clearance,
`rapid
`tissue or
`tumor
`accumulation and their low antigenicity, peptides are preferred over
`proteins, such as antibodies, for molecular imaging and tumor-
`targeting. Various receptors are over-expressed in particular tumor
`types and many of them have been targeted either by modifying
`natural peptides or by panning phage libraries [51, 52]. Endogenous
`peptides are usually associated with a short half-life and have their
`proper physiological effects, which lead to undesired adverse
`reactions. Several peptide modifications,
`like
`the conscious
`incorporation of D-amino acids, capping, amidation or cyclization,
`have been introduced to enhance the stability and to improve the
`receptor binding characteristics of peptides [53, 54]. Attachment of
`the BFCAs may alter the pharmacokinetics of the peptide carrier
`and should therefore be performed in a rational manner to avoid any
`interference with the targeting sequence [55, 56]. To avoid receptor
`saturation, which would result in an unfavorable pharmacokinetics,
`receptor-based
`radiopharmaceuticals are used
`in very
`low
`concentrations. The somatostatin receptor binding octreotide and its
`analogs are the most commonly applied peptide radiopharma-
`ceuticals [57]. After labeling with 111In, 90Y, 64Cu or 177Lu, they
`have shown excellent results for the treatment of patients with
`neuroendocrine
`tumors. Examples
`of
`approved
`peptide
`radiopharmaceuticals are 111In-DTPA-OctreoScan® (Fig. 3) and the
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`Petitioner GE Healthcare – Ex. 1017, p. 2669
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`2670 Current Medicinal Chemistry, 2012 Vol. 19, No. 17
`
`Sarko et al.
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`
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`
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`O
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`OO
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`N
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`NN
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`N
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`NN
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`OH
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`N
`
`NN
`
`OOO
`
`OH
`
`OHOH
`
`O
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`OO
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`O
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`OO
`
`NH
`
`NHNH
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`OOO
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`OOO
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`NHNHNH
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`NHNHNH
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`NHNHNH
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`NHNHNH
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`OHOHOH
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`OOO
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`SS
`SS
`SS
`
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`HNHNHN
`
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`
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`
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`HOHOHO
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`
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`
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`
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`HOHOHO
`
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`H3CH3CH3C
`
`OH
`
`OHOH
`
`
`DOTATOCDOTATOC
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`DOTATOCDOTATOC
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`
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`
`HOHOHO
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`HNHNHN
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`
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`Octreoscan®Octreoscan®
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`Octreoscan®Octreoscan®
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`HO
`
`HOHO
`
`HO
`
`HOHO
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`
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`
`Fig. (3). The chemical structure of DTPA-OctreoScan as compared to DOTATOC. This kind of presentation illustrates the similarity of the acyclic chelator
`DTPA with the cyclic Chelator DOTA. The small change of the peptide (3Tyr versus 3Phe) and the physicochemical differences of the two different chelators
`cause significant changes of the pharmacokinetics of the two pharmaceuticals.
`
`99mTc-labeled NeoTect, an analog which is used for the detection of
`lung cancer.
`the peptide derivative
`its excellent performance
`Due
`to
`standard of peptide
`DOTATOC has become
`the gold
`radiopharmaceuticals. It has found wide application for large
`number of carcinoid tumors expressing somatostatin receptor
`subtypes SSTR2 and SSTR5. The binding to these receptors
`induces receptor mediated endocytosis of the tracer. DOTATOC
`labeled with 68Ga allows the diagnosis by PET (as shown in Fig. 4),
`90Y labeled DOTATOC has been proven to be effective for
`endoradiotherapy (Fig. 5).
`
`Monoclonal Antibodies
`
`In 1993 the US Food and Drug Administration (FDA) approved
`the first murine monoclonal antibody (MAb) for the diagnosis of
`recurrent colorectal and ovarian cancer [39]. While the preclinical
`results were promising for MAbs, clinical studies often only
`showed limited accumulation in the tumor, accompanied by slow
`blood clearance, due to the high molecular weight, resulting in only
`modest
`tumor
`to blood/background (T/B) ratios. The poor
`penetrating properties due to the large molecular weight result in
`low tumor uptake and set hurdles to the use of short physical half-
`life radioisotopes such as 99mTc. Furthermore, the first generation of
`
`Bone
`metastases
`Spleen
`Liver
`metastases
`Kidneys
`
`Bladder
`
`A
`
`B
`
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`Fig. (4). Positron emission tomography imaging of a 46 year old patient with a massive infiltration of the liver by metastases of neuroendocrine tumor (A).
`Status after intraaterial treatment with the beta emitting radiotherapeutics 90Y-DOTATOC (B). 68Ga-DOTATOC depicts the eradication of the liver metastases
`and the occurrence of multiple bone metastases.
`
`Petitioner GE Healthcare – Ex. 1017, p. 2670
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`

`

`BFCAs: The Core of Radiopharmaceuticals
`
`A
`
`B
`
`C
`
`Current Medicinal Chemistry, 2012 Vol. 19, No. 17 2671
`D
`
`
`RituximabRituximab
`
`RituximabRituximab
`
`
`
`90Y90Y90Y
`
`
`ZevalinZevalin
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`ZevalinZevalin
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`
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`Fig. (5). Planar images of different revealing the influence of the coordination environment imposed by "chelators" together with the changes of image quality
`of different SSTR binding octreotide derived radiopharmaceuticals. (A) 177Lu-DOTATOC (B) 111In-octreoscan (C) 99mTc-HYNIC-[D-Phe1,Tyr3-octreotide] and
`(D) 90Y-DOTATOC (recording of Bremsstrahlung).
`
`
`
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`
`
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`
`Fig. (6). Schematic representation of the effects of the anti-CD20 monoclonal antibody Rituximab and its 90Y-labeled derivative Zevalin. The FDA-approved
`radiopharmaceutical Zevalin has an additional therapeutic effect caused by the ionizing radiation. This is exemplified by the high efficiency of Zevalin in the
`treatment of non-Hodgkin lymphomas [58].
`
`
`therapeutic effecttherapeutic effect
`
`(apoptosis induction)(apoptosis induction)
`
`
`additionaladditional
`
`cytotoxic effectcytotoxic effect
`
`of the β-radiationof the β-radiation
`
`MAbs bore the risk of immunogenic reactions due to murine origin
`of the protein. The approval of the 90Y-labeled anti-CD20
`monoclonal antibody (Zevalin®, Bayer Schering Pharma AG,
`Germany) was a considerable advance for radioimmunotherapy in
`oncology (Fig. 6) [9, 32].
`
`Other Carrier Systems
`
`A wide variety of carrier systems have been tested for the
`systemic administration of diagnostic or therapeutic radiopharma-
`ceuticals. The spectrum of different compounds covers small
`organic or inorganic molecules [59], peptides [36], peptidomimetics
`[60, 61], proteins [37], oligonucleotides [48, 62], or macromole-
`cules, such as monoclonal antibodies [32], antibodies [31],
`microparticles [63], liposomes [64], nanoparticles [65], polymeric
`
`micelles [63], dendrimers [66], or hydrogels labeled with a
`radionuclide [65, 67].
`
`Radiometals and the Bifunctional Chelating Agent (BFCA)
`
`The stability, i.e. the release of the radiometal from a
`radiopharmaceutical in human serum is predominantly determined
`by the kinetic inertness of the chelator used. Generally, macrocyclic
`chelators such as DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-
`tetraacetic acid) form more kinetically inert complexes than their
`acyclic analogs such as DTPA (diethylenetriamine-N,N,N',N'',N''-
`pentaacetic acid) (Fig. 7). In contrast, acyclic chelators exhibit
`faster metal-binding kinetics compared with macrocyclic BFCAs.
`This is of great importance for radiometals with short half-lives.
`The chelators form coordinative bonds with central metal ions.
`
`Petitioner GE Healthcare – Ex. 1017, p. 2671
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`2672 Current Medicinal Chemistry, 2012 Vol. 19, No. 17
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`Sarko et al.
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`Fig. (7). The chemical structures of the typically used acyclic chelators DTPA and CHX-A''-DTPA, an isomer of DTPA with a cyclohexyl backbone
`substitution, as well as the macrocyclic chelators NOTA (1,4,7-triazacyclononane-1,4-7-triacetic acid) and DOTA.
`
`Depending on the size, the charge, and the electron configuration of
`the radiometals, the coordination number differs from 2-8.
`The design of radiometal-based pharmaceuticals relies on the
`organ or tissue to be targeted. The overall charge of the metal
`complex, as well as the nature of the radiometal influence the
`properties of the tracer. Usually, lipophilic complexes exhibit
`affinity to the liver, cationic complexes tend to accumulate in the
`heart, anionic complexes tend to clear through the kidneys, and
`non-charged complexes are required in order to achieve blood–
`brain barrier penetration.
`The treatment of a broad variety of diseases such as heart
`diseases, brain disorders, infections, kidney and liver abnormalities
`as well as cancer have benefited from the progression made in the
`field of nuclear medicine [19]. Developments of radiopharma-
`ceuticals benefit from the nuclear technology offered by nuclear
`reactors, accelerators, cyclotrons and generators. SPECT requires
`radiopharmaceuticals enclosing radionuclides with gamma radiation
`while PET necessitates radiopharmaceuticals labeled with positron-
`emitting radionuclides (Table 1).
`
`Metal-Based Radiopharmaceuticals
`
`The labeling with a radiometal can be difficult since metal ions
`can form insoluble hydroxides at physiological pH. This leads to
`the necessity of BFCAs, as they can complex and stabilize the
`radiometal. Therefore, the conjugation to a BFCA which exhibits
`higher affinity to the radiometal than to the hydroxyl ions (OH¯) is
`a prerequisite. In order to achieve high specific activities, the BFCA
`conjugate has to be used in very low concentrations. The formation
`of insoluble hydroxides occurs faster than the complexation with
`the ligands offered by the BFCA. Therefore, the complexation is
`performed in the presence of weak coordinating ligands such as
`acetate or citrate, since they form weak coordination complexes
`with the labeling radiometal.
`Despite the believe that the BFCAs are ‘innocent’ molecules,
`the nature of a BFCA (i.e. geometry, lipophilicity, overall charge)
`plays a key
`role
`in
`the pharmacokinetics of
`targeted
`radiopharmaceuticals [40]. This influence is decreased in larger
`biomolecules. Isomerism is an additional factor to be avoided or
`minimized since it can exert an impact on the physical and chemical
`properties of the radiopharmaceuticals, which in turn alters the
`
`pharmacokinetic properties [12]. Hydrophilic BFCAs have an
`additional advantage since they facilitate the blood clearance by
`renal tracer excretion.
`The standard procedure for the measurement of the stability of
`radiometal-labeled biomolecules is the determination of its stability
`in human serum. The monitoring techniques are based on radio-
`TLC, radio-HPLC, and LC-MS [68-70].
`Acyclic and macrocyclic chelators have been developed (Fig.
`7). The ultimate goal of the development of chelators is to obtain
`kinetically inert complexes and to reduce the rate of dissociation of
`the radiometal in vivo. For example, it has been shown that
`90Y−DTPA is not very stable in vivo, and uncomplexed 90Y
`accumulates in the bone marrow. Consequently, the macrocyclic
`chelator DOTA, which forms more kinetically inert complexes, has
`replaced DTPA. Generally, acyclic chelators have faster metal-
`binding kinetics in comparison to their macrocyclic analogs. This
`behavior is of great importance for radiometals with short half-lives
`[17, 71, 72].
`
`THE PRODUCTION OF THE RADIOISOTOPES
`Production of the Radioisotope 99mTc
`Radionuclide generators consist of a long-lived radionuclide
`(parent radionuclide) that decays to a short-lived isotope (daughter
`isotope). The design of the generator allows the separation of the
`radionuclide produced from the parent isotope by a single step
`elution from an ion exchange column. 99mTc is obtained from the
`parent radionuclide 99Mo, which itself is available at low costs (Fig.
`8). Due to their large spectrum of applications, technetium
`generators are very economical and therefore a sustainable source
`for an isotope of medical interest.
`The core of the generator is a solid column made up of
`2-)
`activated (acidified) alumina. The molybdate anion (MoO4
`polymerizes in acidic solution and forms a stable polymer that
`consists of Al[Mo6O24]9-. This polymer is adsorbed on the column
`[12]. In contrary, the pertechnetate ion, which does not polymerize,
`does not bind to the column. Therefore, 99mTc can be eluted from
`the column as pertechnetate using physiological saline. The
`- in saline explains why the
`exclusive availability of 99mTcO4
`chemistry has to be performed in aqueous solutions. Thirteen
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`BFCAs: The Core of Radiopharmaceuticals
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`Current Medicinal Chemistry, 2012 Vol. 19, No. 17 2673
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`
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`42Mo42Mo9842Mo
`
`9898
`
`stablestable
`
`
`
`92U92U23592U
`
`235235
`
`7 × 108 y7 × 108 y
`
`
`
`
`
`
`
`
`
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`
`
`
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`Fig. (8). Schematic presentation of the radioisotopes involved in the production and decay of the technetium-99m generator.
`
`n,γn,γ
`
`n,fn,f
`
`
`
`42Mo42Mo9942Mo
`
`9999
`
`66 h66 h
`
`β -, γβ -, γ
`
`
`1 31 3
`
`
`
`%%
`
`
`β-β-
`
`87%87%
`
`
`
`43Tc43Tc9943Tc
`
`9999
`
`2.1 × 105 y2.1 × 105 y
`
`γγ
`
`
`140 keV140 keV
`
`
`
`43Tc43Tc43Tc
`99m
`
`99m99m
`
`6 h6 h
`
`
`
`β-294 keVβ-294 keV
`
`
`β -β -
`
`3.7 ×3.7 ×
`
`
`
`%%
`
`
`
`1 0-31 0-3
`
`
`
`44Ru44Ru9944Ru
`
`9999
`
`stablestable
`
`percent of 99Mo decays directly to 99Tc. Consequently, this isotope
`is a by-product, which is contained in the 99mTc eluate. This isotope
`is the only by-product found in the generator eluate. The yield of
`99mTc depends on the age of the generator and the elution intervals.
`The 99mTc/99Tc ratio (with a maximum of 87%) decreases with the
`regeneration time.
`
`Production of Gallium Radioisotopes
`Radioactive gallium isotopes cannot be found in nature. 68Ga
`(half-life 68 min), 67Ga (half-life 78.3 h (3.26 d)), and 66Ga (half-
`life 9.5 h) are the three radioisotopes suited for the usage in nuclear
`medicine [73]. 68Ga is the Ga-isotope most widely used in nuclear
`medicine. It serves as an efficient surrogate for 99mTc [73]. It is the
`daughter radioisotope of germanium-68 (68Ge, half-life 270.8 days)
`(Fig. 9).
`While the short half-life of 68Ga prohibits the shipment of the
`radioisotope over long distances, the long half-life of the parent
`radioisotope 68Ge allows the production of generators (anticipated
`average life 1-2 years). This is the basis of a cost-effective
`production of 68Ga. The use of different stationary and/or mobile
`phases resulted in the development of a diversity of 68Ge/68Ga
`generators, which were discussed in a recent review [74]. The high
`positron emission energy of 68Ga (1899 keV) affects the spatial
`resolution. However, the half-life is excellent for preparation,
`purification, and imaging of tracers with rapid pharmacokinetics.
`The number of applications for 67Ga and 66Ga are relatively small in
`comparison with 68Ga. 67Ga was first produced for human use in
`1953 [75]. 67Ga decays by electron capture to form the stable
`isotope 67Zn. It is a pure γ-emitter with multiple gamma photons of
`
`different energies. The half-life of 78.3 h makes it available from
`commercial sources. 67Ga is most commonly used for inflammation
`and tumor imaging applications.
`
`Production of 111In and 113mIn
`The most widely used radioisotope of indium is 111In. It has a
`half-life of 2.83 days and is produced in cyclotrons by the
`112Cd(p,2n)/111In nuclear reaction (Fig. 10). It decays by electron
`capture with the emission of gamma photons of 173 and 247 keV
`(89% and 95% abundance, respectively).
`Due to its appropriate gamma energy, half life and its excellent
`chemical properties, 111In is widely used in gamma scintigraphy. It
`was first evaluated in vivo in 1969 [18]. The first FDA-approved
`peptide radiopharmaceutical for clinical use was the 111In-labeled
`somatostatin analog OctreoScan®. The half-life of 111In makes it
`ideal for applications in research where imaging is performed over
`longer intervals lasting up to several days [29].
`113mIn (half-life = 1.7 h) is formed using the 113Sn/113mIn
`generator where the parent isotope has a half-life of 115 days. The
`usage of 113mIn is limited by its high gamma energy of 392 keV.
`
`Production of 86Y and 90Y
`90Y (t1/2 = 64.06 h) and 86Y (t1/2 = 14.7 h) are the two
`radioisotopes of yttrium
`that have
`found applications as
`radiopharmaceuticals. 90Y is a pure β -emitter.