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of 68Ge breakthrough and low metal ion impurities across the generator life span. If any
of these criteria are not met, a pre-purication step (vide infra) must be included, which
increases the complexity of the radiosynthesis. Biodistribution dosimetry studies of
68
Ge in rats (with extrapolation to human) demonstrated that at least 16 mCi (600 MBq)
can be administered before giving the patient an eective dose of 10 mSv[12]. A view
on this issue posited by Irina Velikyan, and echoed here, is: “This amount exceeds the
limit recommended by European Pharmacopoeia monographs (0.001%) by 35 000–50 000
times. These results imply that the
68
Ge(IV) limit currently recommended by European
Pharmacopoeia monograph could be increased at least 100 times without compro-
mising patient safety”[13]. Increasing the limit for
68
Ge breakthrough would not cause
a chemical problem for radiotracer synthesis because of the signicant dierence in
coordination requirements for germanium(IV) compared to gallium(III). However, non-
radioactive metal ion contamination, whilst not a realistic concern from a regulatory
perspective (the limit of 10 μg GB q
−1
for iron and zinc in the European Pharmacopeia
monograph is high compared to what is observed in routine practice), even minor levels
of contaminants can aect radiolabeling eciency.
In a direct comparison, the eluate from both the SnO
(iThemba) and TiO2 (EZAG)
2
generators contains Fe, Cu, Mn, and Al ion levels in the 1–20 ppm range, making direct
radiolabeling (without purication) more challenging; whereas the SiO
-based generator
2
produced by Isotope Technologies Garching (ITG) has metal ion levels below 0.1 ppm[14].
DOTA-RG D
when using the eluate from a SiO
required to achieve quantitative radiolabeling when using the SnO
(EZAG) generators. It should also be noted that titanium breakthrough from TiO
could be quantitatively radiolabeled at 20 μg levels after 15 minutes at 90 °C
2
-based generator (ITG), whereas pre-purication was
2
(iThemba) and TiO2
2
-based
2
columns can often be signicant and can interfere with radiolabeling, dependent on the
chelator used[15, 16]. Green etal. used both the SiO
ators to make clinical doses of [
68
Ga]Ga-DOTA-NOC over an extended period[17]. Radio-
-based (ITG) and TiO2 (EZAG) gener-
2
labeling can be carried out using the generator eluate from both generators; however,
with the TiO
(EZAG) generator, more peptide (60 vs. 30 μg) was required for quantitative
2
radiolabeling, and the elution had to be fractionated before use; the extent to which this
should be a consideration depends on the required molar activity for imaging (vide infra).
In a further study, the TiO
(EZAG) and SiO2-based (ITG) generators were compared with
2
the eluate used for radiolabeling common chelators DOTA and NOTA[18]. At 5 μg at 95 °C,
no dierence was noted between the generators, whereas when the amount of chela-
tor and/or temperature is decreased, the yield when using the TiO
decreases. Pre-purication of the eluate from the TiO
mirror the yields achieved from the SiO
the TiO
generator produced by IRE ELiT is a relatively new product, there have been no
2
-based (ITG) generator without purication. Since
2
(EZAG) generator is required to
2
(EZAG) generator
2
similar published studies to provide a relevant comparison. It could be assumed, given the
column matrix is the same, that it should be similar to the TiO
(EZAG) generator. How-
2
ever, given the low elution volume and elution mechanism, fractionation would be more
challenging, which may mean that any metal impurities (including
in the 1.1 ml eluate. From a chemistry perspective, metal-free SiO
68
Ge) are concentrated
-based generators oer
2
Chapter 9: Labeling withGallium-68 295
the best compatibility with kit radiosynthesis, allowing for high-molar-activity tracers
to be produced. However, at present, the only generators licensed for clinical use are
TiO
based.
2
III
Gallium is a group 13 p-block element with the electronic conguration [Ar]3d104s24p1. A
full d-orbital makes the oxidation chemistry relatively simple, and whilst gallium(I) and
gallium(II) exist, only gallium(III) is generally stable in aqueous solution. Gallium(III) has a
small ionic radius with a high charge density and is classied as a hard Lewis acid, preferring to bind to non-polarizable hard Lewis bases, most commonly oxygen and nitrogen.
The coordinating groups most often used for
nates[19–22]. The bonding of gallium(III) is highly ionic and labile - requiring polydentate
chelators, which relies on the chelate eect (i.e. ve- and six-membered chelate rings)
to ensure adequate stability. Gallium(III) forms complexes that are almost exclusively
six-coordinate and have an octahedral geometry. Hence, linear, tripodal, and macrocyclic
hexadentate (and higher) chelators are generally utilized.
Compared to other radioisotopes in nuclear medicine,
challenge for radiochemists in the form of hydroxide formation[23]. At low pH values
(<3), gallium(III) is present in its ionic, or hexahydrate, form [Ga(H
chelators used in
68
Ga radiochemistry will protonate and be unusable at such a low pH.
Above pH 7, soluble but unreactive [Ga(OH)
and isotope requirements, radiolabeling must be carried out in the pH range 3–7; however, in this range, other hydroxide species ([Ga(OH)]
sent; see Figure9.1[24–26]. Whilst Ga(OH)
most important because of its low solubility; as Ga(OH)
solution, the equilibrium shifts, causing further Ga(OH)
to form colloidal species. Ga(OH)
precipitation occurs under conditions that are the most
3
amenable to common chelator radiolabeling conditions at pH 4–5 and increasing with
temperature.
To make
68
Ga more available for subsequent reactions, an intermediate complex
is often formed, which should be signicantly more preferred than the formation
of Ga(OH)
but of low enough stability to allow the gallium(III) to be available for
3
further reactions. The most common and eective of these “transfer reagents” are
acetate, citrate, and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and
68
most
Ga radiosynthesis is carried out using one of them in situ as part of the buer
medium[20, 23]–although there are strong reasons to avoid HEPES, where possible,
from a regulatory perspective[27].
Citrate solutions are often used for instant thin-layer chromatography (ITLC) analysis
68
of
Ga radiochemical reactions and give a clear indication of the amount of free gallium
present; however, this technique is rarely able to distinguish between radiolabeled tracer
68
Ga are amines, carboxylates, and phospho-
68
Ga poses an additional
]3+. However, most
2O)6
]ˉ is exclusively present. To match chelator
4
2+
, [Ga(OH)2]+, and Ga(OH)3) are pre-
is never the major species, it is practically the
3
precipitates and is removed from
3
formation that also precipitates,
3
296 Handbook of Radiopharmaceuticals
0.8
(a)
% Total gallium(III)
8
pH
78
0.6
0.4
0.2
3•
Ga
[Ga(OH)]
[Ga(OH)3]
[Ga(OH)
[Ga(OH)2]
2 •
•
•
]
4
345
(b)
1. 0
[Ga(cit)2]
[Ga(cit)]
3•
0.8
0.6
0.4
0.2
345
[Ga(cit)OH]
67
•
6
[Ga(OH)4]
[Ga(OH)3]
•
Figure 9.1 (a) Speciation of a 10−9 M Ga(III) aqueous solution. Source: Jackson, G.E. and Byrne, M.J. [24]. © 1996.
SNMMI. (b) Speciation of a 10
−9
M Ga(III) aqueous solution in the presence of 4 × 10−5 M citrate. Source: Jackson,
G.E. and Byrne, M.J. © 1996. SNMMI.
and colloidal gallium. Therefore, an additional parallel iTLC method should be used to
distinguish colloidal gallium, such as using a mobile phase of 1 M ammonium acetate in
50:50 methanol:water[15, 28]. A dual-TLC method, similar to that used routinely in technetium-99m radiopharmacies, should be exclusively adopted.
To design an ideal chelator for 68Ga, a range of criteria need to be met:
Fast radiolabeling
• Given the very short half-life of
Longer radiosynthesis will limit the lifetime of the generator and the total number
of patient doses that can be produced from one unit.
Quantitative (>95%) incorporation
• Purication can be time-consuming, adds complexity to any radiosynthetic
procedure, and wastes valuable isotope, thus limiting practical utility.
Mild radiolabeling conditions
• The more ecient the radiolabeling of a chelator is at room temperature at an
injectable pH (5–7), the better. Extremes of pH and/or temperature are often
incompatible with conjugated biomolecules and lead to decomposition.
68
Ga, radiolabeling within <10 minutes is essential.
Chapter 9: Labeling withGallium-68 297
Low precursor concentration (high molar activity)
• Purication of radiolabeled tracer from excess starting material is time-con-
suming, often practically challenging, and may be incompatible with routine clinical
production. Therefore, in practice, any unlabeled precursor will still be present
on administration and will compete with the radiolabeled derivative, potentially
blocking binding to the targeted proteins. Molar activity is expressed as the
amount of radioactivity (Bq or Ci) bound per mole of biologically active molecule
(mol)[29].
Metal ion selectivity
• If the chelator does not eectively discriminate from other metal cations (espe-
cially zinc(II), aluminum(III), and titanium(IV)), more precursor, time, or temperature
will be required for radiolabeling, which can have a negative impact on one or more
of synthesis time, process complexity, and molar activity.
Versatile and robust conjugation chemistry
• Conjugation of a bifunctional derivative of the chelator needs to be suciently
robust to prevent cleavage of the
range of conjugation chemistries have been successfully carried out for both
and other radiometals[30, 31].
High in vivo stability
68
Ga complex from the biomolecule in vivo. A
68
Ga
• Upon administration, radiopharmaceuticals are in a challenging environment,
at a very low concentration compared with potential competitors, such as other
endogenous metal ions and biomolecules. Because of their chemical similarities,
68
Ga mimics iron(III) in vivo. Transferrin, the iron transport protein, has a high binding
constant (logK=20.3) for gallium(III)[23], resulting in uptake in the liver and lungs.
Therefore, chelators for
transferrin for gallium binding. Complex stability measurements are usually carried
out by competing apo-transferrin either directly, or in serum with the gallium-68
complex. Chelator stability is also often tested directly against excess competitor
metal anions to ensure selectivity. However –given that in vitro experiments are
carried out under idealized conditions using small volumes–high in vitro stability
does not necessarily translate to in vivo, which must also be investigated[22]. It is
important to consider that kinetic stability can be of high importance under these
conditions and so should be taken in to account along with thermodynamic stability.
This encourages the exploitation of hexadentate chelators and macrocycles.
The most common
68
Ga-radiopharmaceutical design is the conjugation of a bifunc-
tional chelator (BFC) to a targeting vector–most often a small peptide[30]. In recent
years, there have been a number of reviews of
ments, and a number of derivatives of each chelator have been designed to tune various
properties (charge, lipophilicity, and functional group for conjugation)[19, 21, 22, 30].
Here, we focus on the most widely used chelators, principally those that have been taken
through to clinical studies (see Figure9.2). Each chelator is described, followed by a
comparison of their relative properties.
68
Ga are often designed to compete eectively with apo-
68
Ga chelators, including recent develop-
Handbook of Radiopharmaceuticals
9.4.1 NOTA: Chelator Characteristics
1,4,7-Triazacyclononane-N,N″,N″-triacetic acid (NOTA) is a macrocyclic chelator made
from the triazamacrocycle 1,4,7-triazacyclononane (TACN ) with acetate donors attached
to the secondary amines. NOTA binds to gallium(III) with a N
where the metal ion sits above the N
plane of the macrocycle with the acetate pendant
3
arms wrapping around to form unstrained ve-membered chelate rings. This gives a
relatively high formation constant (logK
ions[32, 33]. Radiolabeling of NOTA with
) of 30.98 and high selectivity over other metal
ML
68
Ga can be carried out at room temperature to
form complexes that have high stability in vivo[34–36]. The only minor limitation for
NOTA radiolabeling is the necessity for the reaction to be at slightly acidic pH (4–5), which
may result in degradation of highly sensitive biomolecules; however, this is not usually a
concern with peptide conjugates over the short radiolabeling time required (ca. 10 minutes)[15]. NOTA has three carboxylic acid groups that can be used for conjugation to the
targeting moiety. However, sacricing a carboxylate donor may reduce complex stability
whilst modifying the overall charge, and so is not preferred. Using glutaric acid as a pen-
dant arm on the TACN, rather than acetic acid, forms the 1,4,7-triazacyclononane-1-glutaric acid-4,7-acetic acid(NODAGA) chelator, which can be conjugated to a biomolecule
without sacricing the preferred donor atom set (due to the additional functional group
on the pendant arm) and can increase the exibility of the linker[37, 38]. Replacement
of the carboxylic acid pendant arms with phosphonic and phosphinic acid groups has
been carried out to form the chelators 1,4,7-triazacyclononane-1,4,7-tri(methylene
phosphonic acid) (NOTP) and 3,3′,3”-(((1,4,7-triazonane-1,4,7-triyl)tris(methylene))
tris(hydroxyphosphoryl))tripropanoic acid (TRAP), respectively, which can increase
gallium(III) binding selectivity relative to other metal cations[39–44].
coordination sphere
3O3
68
Ga
Figure 9.2
Chemical structures of common
chelators used for
68
Ga radiolabeling.
Chapter 9: Labeling withGallium-68 299
9.4.2 DOTA: Chelator Characteristics
1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) is the most widely uti-
lized general-purpose chelator, and a wide range of metals can adequately be chelated
68
for nuclear imaging and radioisotope therapy applications (
177
Lu,
86/90
Y, 89Zr, and
225
Ac, amongst others)[21, 30, 45, 46]. Whilst not optimal for all of
Ga,
64/67
these metal ions, the ability to accommodate a metal into the central macrocyclic cavity
(smaller metal ions/6 coordinate geometry) or sitting above the N
cycle with the acetic acid pendant arms wrapping around (larger metal ions/octadentate)
oers a useful compromise for multipurpose applications. This has resulted in it being
the most widely used chelator for
it has a N
binding mode with steric strain in the macrocyclic backbone, as the central
4O2
68
Ga[28]. Crystal structures of Ga-DOTA complex show
macrocyclic cavity size is slightly too large for the small ionic radius of gallium(III)[47].
The result of this is the relatively low formation constant of 21.33; with radiochemical
reactions requiring elevated temperatures to quantitatively radiolabel at usable concentrations[48]. The labeling kinetics are slow as the metal ion needs to be located in the
central cavity to form the stable complex.
68
Ga-DOTA complexes are generally accepted
to have sucient in vivo stability for most molecular imaging applications[30, 49, 50]. The
DOTA binding mode leaves a free carboxylic acid group for conjugation to the targeting
moiety, which also conveniently allows the formation of a neutral complex (three carboxylate groups balancing the charge of the gallium(III) ion). Hence, a BFC DOTA derivative is
not required for gallium(III), simplifying the production of the conjugate. However, many
derivatives of DOTA have been investigated, to either rigidify the backbone or provide
alternate functional groups for specic conjugation reactions with biomolecules[45].
1,4,7,10-Tetraazacyclododececane,1-(glutaric acid)-4,7,10-triacetic acid (DOTAGA) deriv-
atives (analogous to the NODAGA derivatives) have become more popular to produce
bioconjugate structures that retain the 8-coordinate binding mode for larger radiomet-
als[51, 52]. Whilst this is not required for the 6-coordinate
68
Ga (although the additional
negative charge can be used to inuence biodistribution), it has started to replace DOTA
when developing agents that can be radiolabeled with either
68
Ga and
pair theranostics (vide infra)[46, 53].
44/47
Cu,
plane of the macro-
4
177
111
Sc,
In,
Lu for matched-
N,N′-Bis(2-hydroxybenzyl)ethylendiamine-N,N′-diacetic acid (HBED) is an acyclic chelator
developed from an ethylenediaminetetraacetic acid (ED TA) backbone with substituted
phenol binding groups to give a high formation constant for gallium(III) of 38.51[54]. Its
most common derivative, N,N’-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-
N,N’-diacetic acid (HBED-CC), has carboxylic acid groups that can be used for bioconju-
gation, and its acyclic nature lends itself to rapid kinetics of complex formation, even
at room temperature[55]. HBED derivatives were not the main focus of initial research
work to develop chelators for gallium-68 applications. However, its use in the design
of the successful PSMA targeted tracer PSMA-11 indicated that it is t for purpose as a
300 Handbook of Radiopharmaceuticals
component of a radiotracer[56]. HBED is an average chelator for 68Ga; it does not have
outstanding properties but has limited weaknesses. It can be radiolabeled rapidly at
moderate concentrations at room temperature and is suitably stable for in vivo use.
68
Ga
radiopharmaceuticals that use HBED as the BFC are present as a diastereomeric mixture
that does not reach equilibrium when radiolabeling at room temperature[15, 56]. It must
therefore be conrmed–for each tracer designed with this chelator–that this does not
aect target binding, complex stability, and, ultimately, diagnostic ability. Historically,
HBED was radiolabeled at elevated temperatures (>90 °C); however, there has been a shift
recently to more routine synthesis at room temperature, especially in kit-radiosynthesis
68
of [
Ga]Ga-PSMA-11. These considerations should be taken into account when utilizing
this chelator as a component in a radiopharmaceutical.
Tris(hydroxypyridinone) (THP) is an acyclic chelator based on iron(III) chelating agents
inspired by siderophores and contains three hydroxypyridinone units to provide an O
donor set. In contrast to the multipurpose DOTA, THP oers ideal characteristics for
gallium(III) coordination. THP can be used to radiolabel biomolecules at room tempera-
ture at pH 6.5 almost instantaneously at low chelator concentrations, making its use ideal
for kit radiolabeling of biomolecules, especially those that are sensitive to pH or temper-
ature[15, 57–62].
6
9.4.5 Chelator Comparison
DOTA and HBED are the most commonly used chelators for 68Ga: DOTA because of its
success with SSTR-targeted agents, and
(vide infra). Neither provides optimal coordination properties for
shown to be eective in the specic clinical situations to which they have been applied.
They both have signicant limitations, however, which may limit their utility in future
applications. DOTA is not particularly selective for
so the resultant molar activity can be low, especially if generator eluate pre-purication
is not carried out. DOTA requires heat for ecient radiolabeling, which limits its appli-
cation to biomolecule conjugates that are not temperature sensitive. While HBED does
not require heat for ecient kit radiolabeling without pre-purication, the formation of
isomers–particularly an isomeric mixture that has not reached equilibrium–may cause
problems in applications beyond PSMA-11, either through complex instability or variation
in binding prole/pharmacokinetics between the two stereoisomers.
As neither DOTA nor HBED is ideal for
research to develop optimized chelators that can be selectively radiolabeled at room
temperature without pre-purication at low chelator concentration (to increase tracer
molar activity), forming a complex that is stable in vivo and can be readily conjugated
to biomolecules without dominating their characteristics. Both NOTA and THP can be
radiolabeled with
68
Ga at room temperature at low chelator concentrations (<5 μM), with
68
Ga-HBED because of its success with PSMA-11
68
Ga, but both have been
68
Ga compared to other metal ions,
68
Ga, there has been signicant ongoing
Chapter 9: Labeling withGallium-68 301
THP having the advantage of eective radiolabeling at pH 6.5[15]. In direct competition
studies, in which equimolar amounts of two chelators are combined and reacted with
68
Ga, THP showed >95% radiochemical yield when in competition with DOTA, NOTA, and
HBED at room temperature at both pH 3.5 and 6.5.
The selection of a chelator when designing a novel bioconjugate depends on some key
considerations:
• Is the targeting vector temperature or pH-sensitive?
• What is the commercial availability (and associated intellectual property) of the
desired chelator?
• Should the synthesis ultimately be carried out in kit form? (At room temperature?
In one step?)
• Is the design of an analogous sister therapeutic agent a key goal?
• How easily modiable is the chelator or linking unit to modify the pharmacoki-
netic prole?
There is no one-size-ts-all chelator for all situations and applications.
Many other chelators have been developed or repurposed for
recent examples include the “DATA” compounds (that are carboxylate-substituted
6-amino-perhydro-1,4-diazepine (AAZTA) derivatives)[63–69], modied siderophores[70–73], and others[74–77]. The majority of these can be radiolabeled in mild
conditions suitable for kit radiosynthesis and may be viable alternatives in radiopharmaceutical design. Of particular note is the recent translation of the hybrid chelator
DATA into a clinical study as part of the room temperature single-vial kit in the form of
DATA-TOC[78].
Attaching a chelator with ideal
68
Ga radiolabeling characteristics to a biomolecule
known to bind to a target protein does not guarantee that a successful bioconjugate
will be generated. Often, a range of related derivatives must be synthesized to optimize receptor binding in vitro. Linkers are usually introduced (with varying structures) to
physically separate the binding unit from the chelator in an attempt to limit disruption
to binding caused by the chelator. Relatively speaking, the introduction of a linker, chela-
tor, and radiometal is a signicant modication to the binding molecule/vector (in terms
of molecular weight), which can cause a profound eect on binding. Therefore, once a
targeting molecule has been identied for
68
Ga radiolabeling, the whole tracer should be
designed iteratively rather than simply tagging a biomolecule, which can be eective for
18
F and 11C radiolabeling strategies.
Additionally, if a bioconjugate is optimized in vitro for binding anity, there is
no guarantee that it will behave optimally in vivo. The selection of a lead compound
based on binding anity alone does not necessarily make the best tracer. Attaching a BFC (and often a linker) can have a profound impact on the in vivo pharmacokinetics, including biodistribution, target binding, and clearance. Hober and coworkers
recently carried out a comprehensive preclinical chelator comparison study for imaging
HER2 using a small protein[79], highlighting the key feature of variable clearance
route and time when varying chelator/isotope combinations. Jain etal. compared
68
Ga radiolabeling;
302 Handbook of Radiopharmaceuticals
the biodistribution of three 68Ga-labeled fatty acid derivatives with various chelators
(diethylenetriaminepentaacetic acid (DTPA), NOTA, and NODAGA), observing that
structurally similar tracers with similar molar activities and lipophilicities can behave
very dierently in vivo, likely because of variation of overall charge[80]. Signicant
dierences were seen in uptake in target tissue (heart), blood clearance, and the level
of hepatic clearance. The importance of chelator choice and linker design was demonstrated by the formation of two deferoxamine (DFO) conjugated PSMA derivatives and
comparing them to PSMA-11 (PSMA-HBED-CC)[77]. Chelator replacement and linker
modication caused dramatic changes in tumor uptake and clearance prole in preclinical studies. Importantly, the clearance prole was shown to correlate with both plasma
protein binding and cellular dissociation constants, rather than lipophilicity. Baranski
etal. modied the structure of PSMA-11 (PSMA-HBED-CC) by introducing varying numbers of amino acids in the linker[81]. This linker extension results in a signicant modication of the biodistribution pattern when tested in preclinical studies, specically
by reducing uptake in dose-limiting organs whilst retaining tumor uptake levels. The
lead tracer had similar tumor uptake whilst reducing spleen uptake from 17.88 ± 2.87
to 2.70 ± 1.07 ID g
−1
and liver uptake from 139.44 ± 21.40 to 68.44 ± 15.24 ID g−1. Other
structural modications have also been carried out in an attempt to increase tumor-tokidney ratios of PSMA analogs[82, 83].
In general, the most important properties that inuence the in vivo prole of a radiotracer are charge and lipophilicity. The smaller the relative dierence in size of the chelator to the binding unit, the larger the eect, with small molecules and peptides the
most signicantly perturbed. However, there are also examples in which large-molecularweight biomolecules such as antibodies can be inuenced by chelator attachment[20,
84], although in some applications, chelator modication or minor structural linker modications have a more limited inuence on the pharmacokinetic prole[70, 85–87]. It is
important to note that varying only the metal center can also have an eect on the phar-
macokinetics of the radiotracer, which can be very important in theranostic applications
(vide infra) particularly if PET data is being used to calculate personalized dosimetry for
therapeutic isotope administration.
When 68Ga rst emerged into clinical use, cGMP radiosynthesis of novel unlicensed radio-
pharmaceuticals used exible and programmable synthesis units that were designed for
other radioisotopes[88]; they were not developed to handle the high reaction volumes
from elution of
generator elute were too high for administration, and other metal ions present in the
eluate, including titanium(IV), aluminum(III), and zinc(II), can compete with
tor binding and reduce radiolabeling eciency. The simplest way to reduce the amount
68
of
Ge and total volume is through fractionation, with 80% of the activity eluting in
68
Ge/68Ga generators (c. 5 ml). In addition, 68Ge breakthrough levels in the
68
Ga for chela-
Chapter 9: Labeling withGallium-68 303
the 1 ml fraction between 1.5 and 2.5 ml[89]. This method oers a workaround and has
allowed radiopharmaceuticals to be synthesized for clinical studies (with the amount
68
of
Ge breakthrough in the fraction used reduced by around 75%), but it does not solve
the problem.
Meyer etal. proposed an ionic exchange method via the formation of [
68
Ga][GaCl4]ˉ,
allowing aqueous elution in a small volume[90]. However, the method is limited by the
requirement for pre-dilution of the eluate, and the purication from other metal contaminants is not optimal. Zhernosekov etal. provided an elegant solution using a weak
cation exchange cartridge to trap
68
Ga, followed by elution in a weak acid/acetone solu-
tion[91]. This can be used directly for radiolabeling or evaporated and reconstituted in
aqueous buer[92]. Introduction of cationic pre-purication allowed the full generator
eluate to be used, the radioactivity to be concentrated, the acid concentration to be
reduced, and most competing metal ions to be removed. All of these factors contribute
to an increase in radiolabeling eciency and dealing eectively with the issue of
68
Ge
contamination.
Alternate eluent solutions were subsequently used to increase reactivity and/or bio-
compatibility. Eppard etal. replaced acetone with ethanol to elute puried
68
Ga from the
cation exchange resin, to provide a more acceptable reaction mixture for the preparation
and formulation of directly injectable
nologies have more recently been developed for ethanol-based cationic
68
Ga-radiopharmaceuticals[93, 94]. Monolith tech-
68
Ga purication,
with lower elution volumes and acid concentrations required to achieve quantitative
release[16]. These highly ecient, low-volume, small-size monolith columns allow
integration of purication as part of a miniaturized lab-on-a-chip/microuidic synthesis
system, as they decrease backpressure in comparison to particle-packed columns.
Additionally, the presence of ethanol in
68
Ga radiosynthesis seems to improve reaction
eciency, even when compared with aqueous solutions, potentially via radiolysis stabilization by scavenging of free radicals[27, 95, 96]. Avoiding organic solvents entirely is also
possible, with elution of the ion-exchange cartridge using a high-ionic-strength saline
solution becoming a more widely used approach[97]. Alternatively, cation-exchange
cartridge purication can also be combined with a subsequent anion cartridge step to
release
68
Ga in a pure aqueous media[98], but this increases complexity.
Given the similarity to gallium(III), removal of iron(III) is challenging using pre-puri-
cation techniques, with amounts generally only reduced by an order of magnitude. Their
chemical similarity also means chelators are often not particularly selective between
these cations. Therefore, the easiest way to avoid iron(III) interfering with metal ion
chelation is to ensure that its presence is limited in the rst place. This can be achieved
through generator design, quality of reagents used, and equipment used (spatulas,
needles, etc. should be metal-free). The elution yield slowly decreases across the generator lifetime as the remaining
68
Ge decreases. A generator can be eluted multiple
times daily, with the time allowed between elutions dictating subsequent yields. After
one half-life (68 minutes), 50% of the activity can be eluted; 4 hours is required for >90%
recovery. If the generator has been left without elution for a signicant period (usually >48 hours), it is common practice to discard the rst elution[27, 28]. A pre-elution is
304 Handbook of Radiopharmaceuticals