Note: Descriptions are shown in the official language in which they were submitted.
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?,/1-JLTICOLUINl1.J SELECTIVITY INVERSION GENE'P.ATOP. FOR
PRODUCTION OF ULTRAPURE RADIONUCLIDES
BACKGROUND ART
The use of radioactive materials in
diagnostic medicine has been readily accepted because
these procedures are safe, minimally invasive, cost
effective, and they provide unique structural and/or
functional information that is otherwise unavailable
to the clinician. The utility of nuclear medicine is
reflected by the more than 13 million diagnostic
procedures that are performed each year in the U. S.
alone, which translates to approximately one of every
four admitted hospital patients receiving a nuclear
medical procedure. [See, Adelstein et al. Eds.,
Isotopes for Medicine and the Life Sciences; National
Academy Press, Washington, DC (1995); Wagner et al.,
"Expert Panel: Forecast Future Demand for Medical
Isotopes," Department of Energy, Office of Nuclear
Energy, Science, and Technology (1999); Bond et al.,
Ind. Eng. Chem. Res. (2000) 39:3130-3134.1 More than
90 percent of these procedures are for diagnostic
imaging purposes and use technetium-99m (99mTc) as the
radionuclide. 99mTc possesses a unique combination of
convenient production and availability, coupled with
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appropriate nuclear decay mode, decay energy, and
chemical reactivity. These properties enable 99mTc to
be coupled to biolocalization agents that permit the
imaging of many diseases and virtually every part of
the human anatomy. [See, Bremer, Radiochim. Acta
(1987) 41:73-81; Steigman et al., The Chemistry of
Technetium in Medicine, National Academy Press:
Washington, D. C., (1992); Schwochau, Angew. Chem.
Int. Ed. Eng. (1994) 33:2258-2267.]
The typical life cycle of a medical
radionuclide, such as 99mTc, commencing with raw
material acquisition and proceeding through
nucleogenesis of a radiochemical and clinical
administration of the purified and sterile
radiopharmaceutical is depicted schematically in Fig.
1. Technetium-99m is used as a specific example in
this discussion because the vast majority of all
nuclear medical procedures utilize this radionuclide,
and aspects of new production technologies are
typically compared to this successful model. The
99mTc desired "daughter" is formed by (31- (or negatron)
decay of the molybdenum-99 (99Mo) "parent", which
forms as a result of the fission of uranium-235 in a
nuclear reactor. [See, Bremer, Radiochim. Acta
(1987) 41:73-81; Schwochau, Angew. Chem. Int. Ed.
Eng. (1994) 33:2258-2267; Boyd, Radiochim. Acta
(1987) 41:59-63; and Ali et al., Radiochim. Acta
(1987) 41: 65-72 . ]
Molybdenum-99 is separated from its
nucleosynthesis precursors and byproducts during
"Chemical Processing", which represents the last
stage as a"Radiochemical" according to Fig. 1. Such
"Radiochemicals" encounter far less stringent
regulation of the chemical and radionuclidic purity
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and no biological requirements (e.g., sterility and
nonpyrogenicity) are enforced. Upon completion of
"Chemical Processing", which includes generator
fabrication, the 99Mo/"mTc pair has become a
"Radiopharmaceutical" (according to Fig. 1) and is
now subject to rigorous control of the chemical
purity, radionuclidic purity, sterility, and
nonpyrogenicity.
Chemical purity is vital to a safe and
efficient medical procedure, because the radionuclide
is generally conjugated to a biolocalization agent
prior to use. This conjugation reaction relies on
the principles of coordination chemistry wherein a
radionuclide is chelated to a ligand that is
covalently attached to the biolocalization agent. In
a chemically impure sample, the presence of ionic
impurities can interfere with this conjugation
reaction. If sufficient 99mTc, for example, is not
coupled to a given biolocalization agent, poorly
defined images are obtained due to insufficient
photon density localized at the target site and/or
from an elevated in vivo background due to aspecific
distribution in the blood pool or surrounding
tissues.
Regulation of radionuclidic purity stems
from the hazards associated with the introduction of
long-lived or high energy radioactive impurities into
a patient, especially if the biolocalization and body
clearance characteristics of the radioactive
impurities are unknown. Radionuclidic impurities
pose the greatest threat to patient welfare, and such
interferents are the primary focus of clinical
quality control measures that attempt to prevent the
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administration of harmful and potentially fatal doses
of radiation to the patient.
In addition to the controls placed on the
chemical and radionuclidic purity of a
"Radiopharmaceutical", Fig. 1 also indicates that
biological requirements are instituted. The internal
administration of radiopharmaceuticals obviously
mandates that the pharmaceutical be sterile and
nonpyrogenic, and such requirements are familiar to
medical practitioners.
Complementing the favorable nuclear and
chemical characteristics of 99mTC are favorable
economics and the convenience with which this
radionuclide can be produced to meet
radiopharmaceutical specifications. Taken together,
these factors have been vital to the success of
nuclear medicine.
The chemistry underlying the separation of
99mTC from -99Mo relies on the high affinity of alumina
(A1203) for molybdate-99 (99MOO42-) and its negligible
affinity for pertechnetate-99m (99i'TCO41-) in
physiological saline solution. Fig. 2 shows a
conventional 99mTC generator or 1199mTc cow", in which
the 99MoO42- parent is immobilized on an A1203 sorbent
from which the 99mTc043*- can be conveniently separated
by ascending elution of a physiological saline
solution into a vacuum container. [See, Bremer,
Radiochim. Acta (1987) 41:73-81; Schwochau, Angew.
Chem. Int. Ed. Eng. (1994) 33:2258-2267; Boyd,
Radiochim. Acta (1982) 30:123-145; and Molinski, I.nt.
J. Appl. Radiat. isot. (1982) 33:811-819.1
The above "conventional generator" affords
99mTCO41_ of adequate chemical and radionuclidic purity
for use in patients and has the benefits of ease of
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use, compact size, and the safety of having the
principal radiologic hazard (i . e., 99M0O42-)
immobilized on a solid A1203 support. The latter
benefit eases restrictions on transport of the
generator to the nuclear pharmacy and simplifies
manual processing by the nuclear medicine technician.
Given the preeminent position of 99mTc in
nuclear medicine and the simple and effective
operation of the conventional 99mTc generator shown in
Fig. 2, the logic and design of this radionuclide
generator have become the industry standard for
nuclear medicine. This generator methodology is not,
however, universally acceptable for all
radionuclides, especially for those having low
specific activity parent sources or those
radionuclides proposed for use in therapeutic nuclear
medicine. The difficulties of using the conventional
generator technology with low specific activity
parent radionuclides; that is, the microquantities of
the parent radioisotope present as a mixture with
macroquantities of the nonradioactive parent
isotope(s), derive from the need to distribute
macroquantities of parent isotopes over a large
volume of support so as not to exceed the sorbent
capacity. Large chromatographic columns are not
practical for nuclear medical applications as the
desired daughter radionuclide is recovered in a large
volume of eluate and, as such, is not suitable for
clinical use without secondary concentration.
Radionuclides useful in therapeutic nuclear medicine
represent unique challenges to the conventional
generator technology and warrant further discussion.
The use of radiation in disease treatment
has long been practiced, with the mainstay external
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beam radiation therapy now giving way to more
targeted delivery mechanisms. By example, sealed-
source implants containing palladium-103 or
iodine-125 are used in the brachytherapeutic
treatment of prostate cancer; samarium-153 or
rhenium-188 conjugated to diphosphonate-based
biolocalization agents concentrate at metastasis in
the palliative treatment of bone cancer pain; and
radioimmunotherapy (RIT) employs radionuclide
conjugation to peptides, proteins, or antibodies that
selectively concentrate at the disease site whereby
radioactive decay imparts cytotoxic effects.
Radioimmunotherapy represents the most selective
means of delivering a cytotoxic dose of radiation to
diseased cells while sparing healthy tissue. [See,
Whitlock, Ind. Eng. Chem. Res. (2000), 39:3135-3139;
Hassfjell et al., Chem. Rev. (2001) 101:2019-2036;
Imam, J. Radiation Oncology Biol. Phys. (2001)
51:271-278; and McDevitt et al., Science (2001)
294:1537-1540.1 In addition, the recent explosion of
information about disease genesis and function
arising from the human genome project is expected to
propel RIT into a leading treatment for
micrometastatic carcinoma (e.g., lymphomas and
leukemias) and small- to medium-sized tumors.
Candidate radionuclides for RIT typically
have radioactive half-lives in the range of 30
minutes to several days, coordination chemistry that
permits attachment to biolocalization agents, and a
comparatively high linear energy transfer (LET). The
LET is defined as the energy deposited in matter per
unit pathlength of a charged particle, [see, Choppin
et al., J. Nuclear Chemistry: Theory and
Applications; Pergamon Press: Oxford, 1980] and the
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LET of a-particles is substantially greater than (3-
particles.
By example, a-particles having a mean
energy in the 5-9 MeV range typically expend their
energy within about 50-90 m in tissue, which
corresponds to several cell diameters. The lower LET
(31--particles having energies of about 0.5-2.5 MeV may
travel up to 10,000 m in tissue, and the low LET of
these 0'---emissions requires as many as 100,000 decays
at the cell surface to afford a 99.99 percent cell-
kill probability. For a single a-particle at the
cellular surface, however, the considerably higher
LET provides a 20-40% probability of inducing cell
death as the lone a-particle traverses the nucleus.
[See, Hassfjell et al., Chem. Rev. (2001) 101:2019-
2036 . ]
Unfortunately, the LET that makes a- and
(31--emitting nuclides potent cytotoxic agents for
cancer therapy also introduces many unique challenges
into the production and purification of these
radionuclides for use in medical applications.
Foremost among these challenges is the radiolytic
degradation of the support material that occurs when
the conventional generator methodology of Fig. 2 is
used with high LET radionuclides. [See, Hassfjell et
al., Chem. Rev. (2001) 101:2019-2036; Gansow et al.,
In Radionuclide Generators: New Systems for Nuclear
Medicine Applications; Knapp et al. Eds., American
Chemical Society: Washington, DC (1984) pp 215-227;
Knapp, et al. Eds., Radionuclide Generators: New
Systems for Nuclear Medicine Applications American
Chemical Society: Washington, DC (1984) Vol. 241;
Dietz et al., Appl. Radiat. Isot. (1992) 43:1093-
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1101; Mirzadeh et al., J. Radioanal. Nucl. Chem.
(1996) 203:471-488; Lambrecht et al., Radiochim. Acta
(1997) 77:103-123; and Wu et al., Radiochim. Acta
(1997) 79:141-144.]
Radiolytic degradation of the generator
support material can result in: (a) diminished
chemical purity (e.g., radiolysis products from the
support matrix can contaminate the daughter
solution); (b) compromised radionuclidic purity
(e.g., the support material can release parent
radionuclides to the eluate: termed "breakthrough");
(c) diminished yields of daughter radionuclides
(e.g., a-recoil can force the parent radionuclides
into stagnant regions of the support making their
decay products less accessible to the stripping
eluent); (d) decreases in column flow rates (e.g.,
fragmentation of the support matrix creates
particulates that increase the pressure drop across
the column); and (e) erratic performance (e.g.,
variability in product purity, nonreproducible
yields, fluctuating flow rates, etc.)..
Medical radionuclide generators typically
employ three fundamental classes of sorbents for use
in the conventional methodology depicted in Fig. 2:
(a) organic sorbents (e.g., polystyrene-
divinylbenzene copolymer-based ion-exchange resins,
polyacrylate supports for extraction chromatography,
and the like), (b) inorganic sorbents (e.g., A1203,
inorganic gels, and the like) and (c) hybrid sorbents
(e.g., inorganic frameworks containing surface-
grafted organic chelating or ion-exchange
functionalities, silica supports used in extraction
chromatography, and the like).
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A variety of organic sorbents, most notably
the conventional cation- and anion-exchange resins,
have been proposed for use in nuclear medicine
generators [see, Molinski et al., Int. J. App1.
Radiat. Isot. (1982) 33:811-819; Gansow et al., in
Radionuclide Generators: New Systems for Nuclear
Medicine Applications, Knapp et al. Eds., American
Chemical Society, Washington, DC (1984) pp 215-227;
Mirzadeh et al., J. Radioanal. Nucl. Chem. (1996)
203:471-488; and Lambrecht et al., Radiochim. Acta
(1997) 77:103-1231 due to the well documented
chemical selectivity [see, Diamond et al., In Ion
Exchange, Marinsky Ed., Marcel Dekker, New York
(1966) Vol. 1, p 277; and Massart, "Nuclear Science
Series, Radiochemical Techniques: Cation-Exchange
Techniques in Radiochemistry," NAS-NS 3113; National
Academy of Sciences (1971)] and the widespread
availability of these materials. Unfortunately,
organic-based ion-exchange resins frequently fail or
are severely limited in applications using the
conventional generator logic, and typically do so at
radiation levels far below those needed for routine
human use.
By example, polystyrene-divinylbenzene
copolymer-based cation-exchange resins are used in a
generator for the a-emitter 212Bi, but such materials
are limited to approximately two week "duty cycles"
(i.e., the useful generator lifetime accounting for
chemical and physical degradation) for 10-20 mCi
generators. Radiolytic degradation of the
chromatographic support reportedly leads to
diminished flow rates, reduced 212Bi yields, and
breakthrough of the radium-224 (224 Ra) parent. [See,
Mirzadeh et al., J. Radioanal. Nucl. Chem. (1996)
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203:471-488.] Similarly, a 213Bi generator employing
an organic cation-exchange resin was limited to a
shelf life of approximately one week at an activity
level of 2-3 mCi of the a-emitting 225Ac parent.
[See, Mirzadeh et al., LT. Radioanal. Nucl. Chem.
(1996) 203:471-488; and Lambrecht et al., Radiochim.
Acta (1997) 77:103-123.]
With the US Food and Drug Administration's
recent approval of yttrium-90 (90Y)-based RIT for
widespread human use, more efficient generator
technologies for this radionuclide continue to emerge.
Yttrium-90 forms by (3l- decay of the strontium-90
(90Sr) parent radionuclide and, thus, represents a two
component separation involving Sr(II) and Y(III)
(presuming a chemically pure 90Sr stock). Although a
variety of 90Y production methods have been proposed,
[see, Dietz et al., Appl. Radiat. Isot. (1992)
43:1093-1101; Horwitz et al., U.S. Patent No.
5,368,736 (1994); and Ehrhardt et al., U.S. Patent
No. 5,154,897 (1992)] each technology is challenged
by scale-up to Curie levels of production due to
problems arising from radiolysis of the solution
medium and the support matrix. The inadequacies of
the solvent extraction and ion exchange-based
generators for 90Y have been briefly reviewed in works
proposing macrocyclic host/guest chemistry as the
basis for the separation of 90Y from 90Sr. [See, Dietz
et al., Appl. Radiat. isot. (1992) 43:1093-1101; and
Ehrhardt et al., U.S. Patent No. 5,154,897 (1992).]
In these reports, the 90Sr was separated
from 90Y in 3 M HNO3 on a Sr(II) selective
chromatographic support containing a lipophilic crown
ether. This extraction chromatographic material
showed exceptional stability to y radiation from a
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6oCo source, although some diminution of Sr(II)
retention was noted. Unfortunately, the presence of
radiolysis-induced gas pockets adversely affects the
chromatographic performance of this conventional
generator. Consequently, the 9 Sr was stripped after
each processing run to minimize radiolytic
degradation of the support; however, it became
increasingly difficult to achieve efficient stripping
of 90Sr upon repeated use.
The use of inorganic materials in
radionuclide generators has been greatly influenced
by the A1203-based conventional 99mTc generator
technology. [See, Bremer, Radiochim. Acta (1987)
41:73-81; Schwochau, Angew. Chem. Int. Ed. Eng.
(1994) 33:2258-2267; Boyd, Radiochim. Acta (1987)
41:59-63; Boyd, Radiochim. Acta (1982) 30:123-145;
Molinski, Int. J. Appl. Radiat. Isot. (1982) 33:811-
819; Benjamins et al., U.S. Patent No. 3,785,990
(1974); Panek-Finda et al., U.S. Patent No. 3,970,583
(1976); Matthews et al., U.S. Patent No. 4,206,358
(1980); Benjamins et al., U.S. Patent No. 4,387,303
(1983); Weisner et al., U.S. Patent No. 4,472,299
(1984); Monze et al., Radiochim. Acta (1987) 41:97-
101; Forrest, U.S. Patent No. 4,783,305 (1988); Quint
et al., U.S. Patent No. 4,833,329 (1989);
Vanderheyden et al., U.S. Patent No. 4,990,787
(1991); Evers et al., U.S. Patent No. 5,109,160
(1992); Ehrhardt et al., U.S. Patent No. 5,382,388
(1995); and Knapp et al., U.S. Patent 5,729,821
(1998).] Although the inorganic sorbents represent
an improvement with respect to radiolytic stability,
such inorganic materials frequently exhibit poor ion
selectivity, slow partitioning kinetics, and poorly
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defined morphologies that inhibit good
chromatographic performance.
Using the 99mTC generator example, a two
component separation (1 . e., 99mTC041 from 99MOO42 in
physiological saline solution) is required, for which
A1203 is well suited. For more complicated parent
daughter relationships, however, several very
different chemical species can appear between the
parent and daughter in a given decay chain (e.g., a
gas, a tetravalent cation, and a divalent cation
separate 224Ra and 21aBi) and identifying a single
inorganic sorbent capable of retaining all but the
desired daughter radionuclide is difficult.
Rhenium-188 (188Re) is receiving attention
as a therapeutic nuclide for the prevention of
restenosis after angioplasty, for pain palliation of
bone cancer, and in certain RIT procedures given the
similarity of its coordination chemistry with that of
its widely studied lighter congener Tc. Rhenium-188
is formed by (31- decay of tungsten-188 (188W) , which is
produced by double neutron capture of enriched 186W in
a high flux nuclear reactor. Inefficiencies arising
in the nucleosynthesis of 188W result in a low
specific activity parent; that is, trace 1$$W is
present in macroquantities of the 1$$W isotope. Such
a mass of tungstate (W042-) requires a large column so
that the capacity of A1203 for W042' is not exceeded.
Large chromatographic columns yield the 188Re daughter
in large volumes of solution, and a variety of
secondary concentration procedures have been devised
to address this shortcoming. [See, Knapp et al. Eds.,
Radionuclide Generators: New Systems for Nuclear
Medicine Applications, American Chemical Society:
Washington, DC (1984) Vol. 241; Mirzadeh et al., J.
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Radioanal. Nucl. Chem. (1996) 203:471-488; Lambrecht,
et al., Radiochim. Acta (1997) 77:103-123; Knapp et
al., U.S. Patent No. 5,729,821 (1998); Knapp et al.,
U.S. Patent No. 5,186,913 (1993); and Knapp et al.,
U.S. Patent No. 5,275,802 (1994).]
Another seldom discussed shortcoming of the
conventional generator methodology as applied to 7"88Re
arises after the generator has concluded its duty
cycle and the isotopically enriched 186W must be
extracted from the bulk A1203 matrix. Recovery of the
isotopically enriched 186W for further neutron
irradiation is an important part of the economical
production and use of 18SRe, but the distribution of
macroquantities of isotopically enriched 186W target
materials over a large volume of A1203 inhibits cost
effective processing.
The 188Re "gel generator" attempts to
overcome some of the challenges faced by the
inorganic A1203-based 188Re generator, and is based on
the formation of a highly insoluble zirconyl
tungstate [ZrO(WO4)] gel. [See, Ehrhardt et al., U.S.
Patent No. 5,382,388 (1995) and Ehrhardt et al., U.S.
Patent No. 4,859,431 (1989).] This concept has
several advantages over A1203-based generators, but
still suffers from the fundamental drawbacks of
applying the conventional generator methodology to
therapeutic radionuclides.
Although the ZrO (W04) gel generator for 7L88Re
can permit the use of smaller column volumes than the
A1203-based generators, the recovery of valuable
isotopically enriched 186W for subsequent irradiation
is still complicated. Additional considerations
include variable chromatographic behavior and flow
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rates, as the precipitated ZrO(W04) solids are not of
well defined particle sizes or morphologies.
The inorganic materials discussed here are
not immune to radiolytic degradation, especially with
the high LET radionuclides. Several early versions
of the a-emitting 212Bi generator [see, Gansow et al.,
in Radionuclide Generators: New Systems for Nuclear
Medicine Applications; Knapp et al. Eds., American
Chemical Society: Washington, DC (1984) pp 215-227;
and Mirzadeh, S. Generator-Produced Alpha-Emitters.
Appl. Radiat. Isot. (1998) 49:345-349] used inorganic
titanates to retain the long-lived thorium-228 parent,
from which the 224Ra daughter elutes and is
subsequently sorbed onto a conventional cation-
exchange resin. Over time, the titanate column
material succumbed to radiolytic degradation,
creating fine particulates that forced separations to
be performed at elevated pressures.
The hybrid sorbents can be subdivided into
extraction chromatographic materials and engineered
inorganic ion-exchange materials. Most of the
published applications of hybrid materials have used
well-known extraction chromatographic methods [see,
Dietz et al., in Metal Ion Separation and
Preconcentration: Progress and Opportunities; Bond et
al. Eds., American Chemical Society, Washington, DC
(1999) Vol. 716, pp 234-250], whereas the preparation
and use of engineered inorganic materials is a more
recent phenomenon. Extraction chromatography
overcomes the poor ion selectivity and slow
partitioning kinetics of inorganic materials by using
solvent extraction reagents physisorbed to an inert
chromatographic substrate. [See, Dietz et al., in
Metal Ion Separation and Preconcentration: Progress
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and Opportunities; Bond et al. Eds., American
Chemical Society, Washington, DC (1999) Vol. 716, pp
234-250 . ]
The radiolytic stability of extraction
chromatographic supports is improved when the inert
substrate is an amorphous inorganic material such as
silica, with the most profound results reflected as
sustainable flow rates over the generator duty cycle.
Such "improved" radiolytic stability is deceptive,
however, as the fundamental chemical reactions
underlying the parent/daughter separation still
involve molecules constructed from an organic
framework that remains susceptible to radiolytic
degradation. Likewise, organic-based chelating
moieties have been introduced into engineered
inorganic ion-exchange materials to improve ion
selectivity, but such functionalities continue to
suffer the effects of radiolysis.
Preliminary reports using hybrid sorbents
as conventional generator supports in the production
of 213Bi have appeared. [See, Lambrecht et al.,
Radiochim. Acta (1997) 77:103-123; Wu et al.,
Radiochim. Acta (1997) 79:141-144; and Horwitz et
al., U.S. Patent No. 5,854,968 (1998).] Initial
investigations have relied on sorption of 22 5Ra by
organic cation-exchange resins, which showed
substantial degradation over a short period of time
giving reduced yields of 213Bi, poor radionuclidic
purity, and unacceptably slow column flow rates.
[See, Mirzadeh et al., J. Radioanal. Nucl. Chem.
(1996) 203:471-488; and Lambrecht, et al., Radiochim.
Acta (1997) 77:103-123.] Initial improvements
centered on sorption of the 225Ac parent of 213Bi on
Dipex Resin, an inert silica gel-based support to
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which a chelating diphosphonic acid diester is
physisorbed. [Horwitz et al., React. Funct. Polymers
(1997) 33:25-36.] The silica substrate exhibits
greater radiolytic stability than the previously
employed organic cation-exchange resins; however,
radiolytic damage (i.e., discoloration) was observed
surrounding the narrow chromatographic band in which
the 225Ac parent is loaded, ultimately leading to
breakthrough of the 225Ac parent. [See, Lambrecht et
al., Radiochim. Acta (1997) 77:103-123; and Wu et
al., Radiochim. Acta (1997) 79:141-144.]
An incremental improvement in this
generator centered on reducing the radiation density
by dispersing the 225Ac parent radioactivity over a
larger volume of the chromatographic support, which
is achieved by loading the Dipex Resin with 225Ac in
a batch mode rather than in a narrow chromatographic
band. [See, Wu et al., Radiochim. Acta (1997) 79:141-
144.] Unfortunately, this batch loading process is
awkward and the Dipex Resin still suffers from
radiolytic degradation of the chelating diphosphonic
acid diester upon which the separation efficiency
relies.
Despite industry preferences for the
conventional generator depicted in Fig. 2, the
fundamental limitations discussed above are
compounded by radiolytic degradation of the support
medium when using high levels of the high LET
radioactivity useful in therapeutic nuclear medicine.
The severity of these limitations coupled with the
ultimate liability of compromised patient safety
argue for the development of alternative generator
technologies, especially for therapeutically useful
radionuclides.
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An ideal generator technology should
provide operational simplicity and convenience as
well as reliable production of the theoretical yield
of the desired daughter radionuclide having high
chemical and radionuclidic purity. As deployed for
diagnostic radionuclides, the conventional generator
technology generally meets these criteria, although
purity and yield have been observed to fluctuate.
[See, Boyd, Radiochim. Acta (1982) 30:123-145; and
Molinski, Int. J. Appl. Radiat. Isot. (1982) 33:811-
819 . ]
The conventional generator is poorly
suited, however, to systems involving low specific
activity parents (e.g., the 188W/188Re generator
discussed above) as well as with the high LET
radionuclides useful in therapeutic nuclear medicine.
In order to safely and reliably produce
therapeutically useful radionuclides of high chemical
and radionuclidic purity, a new paradigm in
radionuclide generator technology is required. A
shift in the fundamental principles governing
generator technologies for nuclear medicine, and for
therapeutic nuclides specifically, is supported by
the fact that the inadvertent administration of the
long-lived parents of high LET therapeutic
radionuclides would compromise the patient's already
fragile health; potentially resulting in death.
Because the conventional generator strategy depicted
in Fig. 2 relies on long-term storage of the parent
radionuclide on a solid support that is constantly
subjected to high LET radiation, no assurances can be
made regarding the chemical and radionuclidic purity
of the daughter radionuclide over an approximate 14-
60 day generator duty cycle.
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Additional support for fundamental changes
in radionuclide generator technology derives from the
rapidly increasing trend towards automation of
routine tasks such as synthesis operations in
biotechnology and high throughput blood screening in
the clinical laboratory. Radionuclide generator
technologies, as practiced in the nuclear pharmacies,
presently lag behind in the automation of routine
activities. In the nuclear medicine arena,
increasing federal regulations safeguarding patient
health and business competition/profitability are
likely to drive the industry towards automation. The
introduction of computer-controlled liquid delivery
systems into the nuclear pharmacy will permit a
departure from the vacuum container-based generators
of Fig. 2. A reduction in the number of manual
operations also serves to minimize the radiation dose
to the nuclear medicine technician, while
simultaneously reducing the liabilities attributable
to human error.
The adverse effects of radiolytic
degradation described above pose enormous challenges
in the development of new therapeutic radionuclide
generators. Any damage to the support material of a
conventional generator compromises the separation
efficiency, potentially resulting in breakthrough of
the parent radionuclides and to a potentially fatal
dose of radiation if administered to the patient.
Such a catastrophic event is theoretically prevented
by the quality control measures integrated into
nuclear pharmacy operations, but any lack of safe,
predictable generator behavior represents a major
liability to the nuclear pharmacy, hospital, and
their respective shareholders. The invention
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described hereinafter provides an alternative radionuclide
generator technology that is capable of reliably producing
near theoretical yields of medically useful radionuclides of
high chemical and radionuclidic purity.
BRIEF DESCRIPTION OF THE INVENTION
In one aspect, the invention provides a method for
producing a solution of desired daughter radionuclide that
is substantially free of impurities comprising the steps of:
(a) contacting an aqueous parent-daughter solution
containing a desired daughter radionuclide with a first
separation medium having a high affinity for the desired
daughter radionuclide and a low affinity for the parent and
other daughter radionuclides, said desired daughter and
parent radionuclides having different (i) ionic charges,
(ii) charge densities or (iii) both as they are present in
said solution, and maintaining that contact for a time
period sufficient for said desired daughter radionuclide to
be bound by the first separation medium to form desired
daughter-laden separation medium and a desired daughter-
depleted parent-daughter solution; (b) removing the desired
daughter-depleted parent daughter solution from the
separation medium; (c) stripping the desired daughter
radionuclide from the desired daughter-laden separation
medium to form a solution of desired daughter radionuclide;
(d) contacting the solution of desired daughter radionuclide
with a second separation medium having a high affinity for
the parent radionuclide and a low affinity for said desired
daughter radionuclide, and maintaining that contact for a
time period sufficient for said parent radionuclide to be
bound by the second separation medium to form a solution of
substantially impurity-free desired daughter radionuclide.
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In a further aspect, the invention provides a method
for producing a solution of desired daughter radionuclide that
is substantially free of impurities comprising the steps of:
(a) providing an aqueous parent-daughter radionuclide solution
containing a desired daughter radionuclide; (b) contacting the
parent-daughter solution with a first separation medium having
a high affinity for the desired daughter radionuclide and a
low affinity for the parent and other daughter radionuclides
such that the decontamination factor of the desired daughter
radionuclide from the parent radionuclide impurities of said
first separation medium under the conditions of contact is
greater than or equal to 102, said desired daughter and parent
radionuclides having different (i) ionic charges, (ii) charge
densities or (iii) both as they are present in said solution,
and maintaining that contact for a time period sufficient for
said desired daughter radionuclide to be bound by the first
separation medium to form desired daughter-laden separation
medium and a desired daughter-depleted parent-daughter
solution; (c) removing the desired daughter-depleted parent
daughter solution from the separation medium; (d) stripping
the desired daughter radionuclide from the desired daughter-
laden separation medium to form a solution of desired daughter
radionuclide; (e) contacting the solution of desired daughter
radionuclide with a second separation medium having a high
affinity for the parent radionuclide and a low affinity for
said desired daughter radionuclide such that the
decontamination factor of the desired daughter radionuclide
from the parent radionuclide impurities of said first
separation medium under the conditions of contact is greater
than or equal to 102, and maintaining that contact for a time
period sufficient for said parent radionuclide to be bound by
the second separation medium to form a solution of
substantially impurity-free desired daughter radionuclide.
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In a still further aspect, the invention provides
a method for producing a solution of desired daughter
radionuclide that is substantially free of impurities
comprising the steps of: (a) providing an aqueous parent-
daughter radionuclide solution containing a desired daughter
radionuclide that is selected from the group consisting of
90Y , 99mTC, 103Pd' 111In , 125 1 , 188Re , 201,I,1 , 47SC, 212Bi , 213Bi ,
211At, and 223Ra; (b) contacting the parent-daughter solution
with a first separation medium having a high affinity for
the desired daughter radionuclide and a low affinity for the
parent and other daughter radionuclides such that the
decontamination factor of the desired daughter radionuclide
from the parent radionuclide impurities of said first
separation medium under the conditions of contact is greater
than or equal to 102, said desired daughter and parent
radionuclides having different ionic charges as they are
present in said solution, and maintaining that contact for a
time period sufficient for said desired daughter
radionuclide to be bound by the first separation medium to
form desired daughter-laden separation medium and a desired
daughter-depleted parent-daughter solution; (c) removing the
desired daughter-depleted parent daughter solution from the
separation medium; (d) stripping the desired daughter
radionuclide from the desired daughter-laden separation
medium to form a solution of desired daughter radionuclide;
(e) contacting the solution of desired daughter radionuclide
with a second separation medium having a high affinity for
the parent radionuclide and a low affinity for said desired
daughter radionuclide such that the decontamination factor
of the desired daughter radionuclide from the parent
radionuclide impurities of said first separation medium
under the conditions of contact is greater than or equal to
102, and maintaining that contact for a time period
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sufficient for said parent radionuclide to be bound by the
second separation medium to form a solution of substantially
impurity-free desired daughter radionuclide.
In a yet further aspect, the invention provides a
method for producing a solution of desired daughter
radionuclide that is substantially free of impurities
comprising the steps of: (a) providing an aqueous parent-
daughter radionuclide solution containing a desired daughter
radionuclide that is selected from the group consisting of
90Y, 99m,I,C, i03Pd, iZiln, 125I , issRe , 20i7,1 , 47Sc , 2i2Bi , 2i3Bi ,
211At, and 223 Ra; (b) contacting the parent-daughter solution
with a first separation medium having a high affinity for
the desired daughter radionuclide and a low affinity for the
parent and other daughter radionuclides such that the
decontamination factor of the desired daughter radionuclide
from the parent radionuclide impurities of said first
separation medium under the conditions of contact is greater
than or equal to 102, said desired daughter and parent
radionuclides having different charge densities as they are
present in said solution, and maintaining that contact for a
time period sufficient for said desired daughter
radionuclide to be bound by the first separation medium to
form desired daughter-laden separation medium and a desired
daughter-depleted parent-daughter solution; (c) removing the
desired daughter-depleted parent daughter solution from the
separation medium; (d) stripping the desired daughter
radionuclide from the desired daughter-laden separation
medium to form a solution of desired daughter radionuclide;
(e) contacting the solution of desired daughter radionuclide
with a second separation medium having a high affinity for
the parent radionuclide and a low affinity for said desired
daughter radionuclide such that the decontamination factor
of the desired daughter radionuclide from the parent
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radionuclide impurities of said first separation medium
under the conditions of contact is greater than or equal to
102, and maintaining that contact for a time period
sufficient for said parent radionuclide to be bound by the
second separation medium to form a solution of substantially
impurity-free desired daughter radionuclide.
In another aspect, the invention provides a method
for producing a solution of desired daughter radionuclide
that is substantially free of impurities comprising the
steps of: (a) providing an aqueous parent-daughter
radionuclide solution containing a desired daughter
radionuclide that is selected from the group consisting of
90Y' 99m,rC' 103pd' ZllIn' 125I , 188Re, 201,1,1' 47S,G,, 212Bi' 213Bi'
211At, and 223 Ra; (b) contacting the parent-daughter solution
with a first separation medium having a high affinity for
the desired daughter radionuclide and a low affinity for the
parent and other daughter radionuclides such that the
decontamination factor of the desired daughter radionuclide
from the parent radionuclide impurities of said first
separation medium under the conditions of contact is greater
than or equal to 102, said desired daughter and parent
radionuclides having both different ionic charges and charge
densities as they are present in said solution, and
maintaining that contact for a time period sufficient for
said desired daughter radionuclide to be bound by the first
separation medium to form desired daughter-laden separation
medium and a desired daughter-depleted parent-daughter
solution; (c) removing the desired daughter-depleted parent
daughter solution from the separation medium; (d) stripping
the desired daughter radionuclide from the desired daughter-
laden separation medium to form a solution of desired
daughter radionuclide; (e) contacting the solution of
desired daughter radionuclide with a second separation
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medium having a high affinity for the parent radionuclide
and a low affinity for said desired daughter radionuclide
such that the decontamination factor of the desired daughter
radionuclide from the parent radionuclide impurities of said
first separation medium under the conditions of contact is
greater than or equal to 102 , and maintaining that contact
for a time period sufficient for said parent radionuclide to
be bound by the second separation medium to form a solution
of substantially impurity-free desired daughter
radionuclide.
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The pi-esent invention contemplates a method
for producing a solution of a desired daughter
radionuclide that is substantially free of
impurities. That method comprises the steps of
contacting an aqueous parent-daughter radionuclide
solution containing a desired daughter radionuclide
with a first separation medium having a high affinity
for the desired daughter radionuclide and a low
affinity for the parent and other daughter
radionuclides. The parent and desired daughter
radionuclides have one or both of different ionic
charges or different charge densities or both as they
are present in that solution. That contact is
maintained for a time period sufficient for the
desired daughter radionuclide to be bound by the
first separation medium to form desired daughter-
laden separation medium and a solution having a
lessened concentration of desired daughter
radionuclide (compared to the initial parent-daughter
radionuclide solution).
The solution having a lessened
concentration of desired daughter radionuclide is
removed from the desired daughter-laden separation
medium. The desired daughter radionuclide is
stripped from the desired daughter-laden separation
medium to form a solution of desired daughter
radionuclide. The solution of desired daughter
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radionuclide is contacted with a second separation
medium having a high affinity for the parent
radionuclide and a low affinity for the desired
daughter radionuclide. In preferred embodiments, no
chemical adjustment is made to the solution before
elution on the second separation medium (guard
column). That contact is maintained for a time
period sufficient for parent radionuclide, if
present, to be bound by the second separation medium
to form a solution of substantially impurity-free
desired daughter radionuclide. The solution of
substantially impurity-free daughter radionuclide is
typically recovered, although that solution can be
used without recovery for a reaction such as binding
of the radionuclide to a medically useful agent.
The present invention has several benefits
and advantages.
In one benefit, the method does not require
the use of air or gas to separate some of the
solutions from one another, which in turn provides
better chromatographic operating performance and
better overall chemical and radionuclidic purity.
An advantage of a contemplated method is
that the separation media have longer useful
lifetimes because they tend not to be degraded by
radiation due to the relatively little time spent by
high linear energy transfer radionuclides in contact
with the media.
Another benefit of the invention is that
radionuclides having high purity can be obtained.
Another advantage of the invention is that
greater latitude in the selection of commercially
available pairs of separation media are available,
and appropriate elution solutions are easily prepared
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for the production of different radionuclides for
medical and analytical applications.
A still further benefit of the invention is
that the high separation efficiency of the separation
media permits daughter radionuclides to be recovered
in a small volume of eluate solution.
A still further advantage of the invention
is that the chemical integrity of the separation
medium is preserved, which provides a more
predictable separation performance and reduces the
likelihood of parent radionuclide contamination of
the daughter product.
Still further benefits and advantages will
be readily apparent to the skilled worker from the
disclosures that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings forming a portion of this
disclosure,
Fig. 1 is a schematic drawing modified from
Bond et al., Ind. Eng. Chem. Res. (2000) 39:3130-3134
that shows the seven primary steps in the production
of medically useful radionuclides and their
respective purity and regulatory requirements.
Fig. 2 is a schematic drawing that shows
the conventional generator methodology using an
ascending flow elution as deployed for 99mTc.
Fig. 3 is a schematic depiction of the
generic logic of the multicolumn selectivity
inversion generator described herein and in which PSC
refers to Primary Separation Column and GC refers to
Guard Column.
Fig. 4 shows the radioactive decay scheme
from 232U to aospb, highlighting the key impurities
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(radium and lead nuclides that can interfere with the
medical use of the desired radionuclide, 212Bi) in the
development of a multicolumn selectivity inversion
generator for 212Bi .
Fig. 5 is a graph that plots dry weight
distribution ratios, DW, for Ba(II) [open squares] and
Bi(III) [open circles] vs. [HCl] in molarity on a
TOPO Resin primary separation column.
Fig. 6 is a graph of counts per minute per
milliliter (cpm/mL) of eluate versus bed volumes (BV)
of eluate passed through a column at 25( 2) C during
the loading (0.75-4.75 BV), rinsing (4.75-8.75 BV),
and stripping (8.75-12.25 BV) procedures in the
separation of Ba ( I I) [open squares] from Bi ( I I I)
[open circles] by TRPO Resin using 0.20 M HC1 as the
preequilibration, load, and rinse solutions and 1.0 M
NaOAc in 0.20 M NaCl as a strip solution. The
horizontal dashed line indicates background counts.
No 133Ba(II) was observed in the range 8.75-12.25 BV
after a spilldown correction.
Fig. 7 is a graph that shows D, values for
Bi ( III ) vs.[Cll-] in molarity for a sulfonic acid
cation-exchange resin guard column in a 1.0 M sodium
acetate/sodium chloride solution at pH 6.5 [closed
squares] versus a solution of 0.0122 M HC1 at pH 1.9
[closed circles].
Fig. 8 is a graph of counts per minute per
milliliter (cpm/mL) of eluate versus bed volumes (BV)
of eluate passed through a column at 25( 2) C during
the loading (1-12 BV), rinsing (12-24.5 BV), and
stripping procedures (24.5-37 BV) in the separation
of Ba(II) [open squares] from Bi(III) [open circles]
by Dipex Resin using 1.0 M HNO3 as the
preequilibration, load, and rinse solutions and 2.0 M
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HCl as a strip solution. The horizontal dashed line
indicates background counts. No 207Bi(III) was
detected during loading. Counts from 133Ba (II)
reached background levels after passage of 30 BV.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
An answer to the problems posed by
radiolytic degradation when using high LET
radionuclides is found in the present invention that
separates parent and desired daughter radionuclides
from a solution containing both using a method that
is broadly referred to herein as multicolumn
selectivity inversion. The term "parent
radionuclide" is often used in the singular herein
for convenience with the understanding that a
contemplated solution containing parent and desired
daughter radionuclides can and usually does contain a
plurality of parent radionuclides as are well-known
from radioactive decay schemes, as well as one or
more daughter nuclides that include the desired
daughter nuclide and its daughter nuclides.
A contemplated method preferably uses a
plurality of chromatographic columns for the
separation. The separation medium packings of those
columns have different selectivities for the parent
and desired daughter radionuclides, and those
selectivities are inverted from the selectivities
that are usually used for similar separations as
practiced in the conventional generator methodology
of Fig. 2. That is, the first separation medium
contacted with an aqueous solution containing the
parent and desired daughter has a greater selectivity
for the desired daughter than for the parent or other
daughters that may be present, whereas at least one
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later-contacted separation medium has a greater
selectivity for the parent than for the desired
daughter radionuclide. It should be noted that a
plurality of second separation media can be used in
one separation, with those media being in separate or
the same guard columns as is appropriate to the
specific media employed.
Solution storage of the radioactive parent
and daughters has the profound advantage of
minimizing radiolytic degradation of the
chromatographic separation material that is
responsible for the product purity because the
majority of the radiolytic damage is relegated to the
solution matrix, for example, water, rather than to
the separation medium.
The integrity of the separation medium is
further maintained by using high chromatographic flow
rates (e.g., by an automated fluid delivery system)
to minimize the duration of contact between the
radioactive solution and the separation medium
selective for daughter radionuclides. Preserving the
chemical integrity of the separation medium equates
to more predictable separation performance and
reduces the likelihood of parent radionuclide
contamination of the daughter product. Furthermore,
by targeting extraction of the desired daughter
radionuclides as needed rather than by eluting a
conventional generator, inorganic sorbents resistant
to radiolysis are not required and a greater variety
of chromatographic separation media with greater
solute selectivity may be employed.
To further minimize the likelihood of
parent radionuclide contamination, another separation
medium selective for the parent(s) is introduced
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downstream from the desired daughter-selective
separation medium. The addition of a second
separation column adds another dimension of security
ensuring that hazardous long-lived parent
radionuclides are not administered to the patient.
An example of such a tandem column arrangement is
depicted in Fig. 3. Exemplary desired daughter
ion/parent ion groups that can be readily separated
using the subject method include Y3+/Srz+; TcO4'-/MoO4'-;
PdCl42'/Rh3+; In3t/Cd'+; I1 /Sb3+; Re041-/W042 ; Tll+/Pbz+;
Sc3+/TiO2+ or Ti4+; Bi3+/Ra2+, Pb2+; Bi3+ /Ac3+, Ra2+; At1-
/Bi3+; and Ra2+/Ac3+, Th4+
As shown at the top of Fig. 3, parent and
desired daughter radionuclides are permitted to
approach or reach radioactive steady state in an
aqueous solution matrix that receives the brunt of
the radiation dose, rather than on the separation
medium that is responsible for the efficiency of the
chemical separation. When needed, the solution
containing the parent and desired daughter
radionuclides is contacted with (loaded on) a
chromatographic column containing a first separation
medium that is selective for the daughter
radionuclide (the primary separation column), while
permitting the one or more parents and any other
"daughters" such as those of the desired daughter
radionuclide to elute. The desired daughter and one
or more parent radionuclides have one or both of
different (i) ionic charges or (ii) charge densities
as they are present in that solution.
Thus, as to ionic charges, one of the
parent and daughter radionuclides can be a +2 cation
and the other a +3 cation, or one can be a +2 cation
and the other a -1 anion, and the like, as they are
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present in the solution used to contact the first
separation medium. Typically, the parent and desired
daughter radionuclides maintain their differences in
charge throughout the complete separation process,
but need not. For example, where TcO41- is to be
separated from Mo042- or ReO41- is to be separated from
W042, those anions maintain their charges throughout
the separation. On the other hand, bismuth and
actinium both typically have +3 charges, but bismuth
is preferentially separated from actinium as a
solution complex with chloride ions such as the
Bi.Cl41- anion whereas actinium does not form such a
complex under the same conditions and remains as an
Ac3+ cation.
Although a large number of chemical
separations can be conveniently described by
acknowledging the differences in the net ionic charge
of two or more analytes as the basis for separation,
many other separations rely on more subtle
differences in the coordination chemistry and/or
solution speciation as a means of effecting
separation. As a general approximation, the
differences in coordination preferences and/or
solution speciation between two ions can be
conveniently attributed to the different charge
densities, where electrostatic interactions
predominate.
The charge density is defined as the
overall charge per unit volume occupied by a given
mono- or polyatomic ion. The concept of charge
density is a contributing factor to Hard/Soft
Acid/Base Theory. In accordance with that Theory,
ions defined as "Hard" are not very polarizable and
typically have large absolute values of charge
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density (e.g., Li+, A13+, F-, and 02-) , whereas those
ions defined as "Soft" have lower charge densities
and are more easily polarized (e.g., Hg2+, Bi3+, I1-,
Tc041-, and the like).
Explanations based solely on differences in
ionic charge do not adequately describe the many
separations of similarly-charged analytes that are
routinely segregated based on differences in the
charge densities of those analytes; for example,
separation of Ce3+ from Lu3+ or F'- from I'-. For the
Ce3 /Lu3+ separation, the cations are of identical
charge but the well-known lanthanide contraction
effects a systematic decrease in the lanthanide ionic
radius and, hence the ionic volume, which results in
a net increase in charge density across the
lanthanide series. This net increase in charge
density can effect differences in the hydration
number (primary and secondary spheres), solution
speciation, and coordination chemistry that,
individually or collectively, can serve as the basis
for a separation.
In another example, the charge density of
the halide anions decreases upon travelling down the
group, as the ionic radius (and volume) increases and
the charge becomes more diffuse. Such differences in
charge density can be exploited for separations
because the electrostatic interactions governing ion-
ligand and ion-solvent interactions are different,
which provides a convenient chemical aspect to be
exploited for a given separation.
The concept of charge density is not
limited strictly to monatomic ions, and is readily
extended to polyatomic species; for example,
NH41+/N (CH2CH3) 1+ and Tc041-/IO31- . In each example, the
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ions are of like charge but each occupies a different
volume, thereby changing the charge density and
altering the ionic interaction characteristics and
solution speciation, as reflected in the parameters
such as free energies of hydration, overall hydration
number, complex formation constants, and the like.
The eluate from the primary separation
column (desired daughter-depleted parent-daughter
solution or solution having a lessened concentration
of desired daughter radionuclide) that contains the
parent and a lessened amount of the desired daughter
radionuclide is removed (separated) from the first
separation medium that is laden with the desired
daughter. That solution can be discarded, but is
preferably collected into a vessel and permitted to
again approach radioactive steady state so that
further amounts of desired daughter can be obtained.
The primary separation column containing the daughter
radionuclide is then typically rinsed to remove any
residual impurities that might be present such as
from the interstices prior to elution of the daughter
(stripping).
In order to maximize the convenience and
effectiveness of this multicolumn generator method,
knowledge of the solution speciation of the daughter
radionuclide and its radionuclidic parents are used
to select both the strip solution and the material or
materials of the second separation medium of the
second chromatographic column (the guard column). In
ideal practice, the daughter-selective primary
separation medium-containing column is stripped with
a solution that permits the desired daughter
radionuclide to elute directly through the guard
column without the need for any chemical adjustment
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to the solution medium, while any parent or other
daughter ion interferents are retained on that second
column.
Solution storage of the radioactive source
material and use of a multicolumn selectivity
inversion method in which the desired daughter
radionuclide is first selectively extracted and then
further decontaminated of residual parent ions by a
second separation medium-containing guard column
serve to minimize radiolytic damage to the support
medium and afford reliable production of near
theoretical yields of highly pure desired daughter
radionuclides. In a typical application, a primary
separation column exhibits a high affinity for the
desired daughter and a low affinity for the parent
and any other daughter radionuclides, whereas the
guard column contains a second separation medium that
has high affinity for the parent and a low affinity
for the desired daughter radionuclide.
Such a pairing affords a combined
decontamination factor (DF) of parent from desired
daughter radionuclide of about 104 to about 1010, or
greater, under the conditions of contacting the
multiple separation media. Separately, each column
utilized provides a DF about 102 to about 105, or
greater, under the conditions of contacting. The DF
for a given step is multiplied with the DF for the
next step or, when represented using exponents, the
DF value exponents are added for each step. A DF
value of about 1010 is about the largest DF that can
be readily determined using typical radioanalytical
laboratory apparatus.
The Decontamination factor (DF) is defined
using the following equation:
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D~naly'ele8luent
DF = limpurity~~}leffluent
[Ana'ly '8]influent
IhnPUr'tYInfluent
For a system at radioactive steady state
(e.g., 224Ra and it daughters including 212Bi and its
daughters), the denominator is about 1. This means a
DF value can be approximated by examining the
stripping peak in a chromatogram and dividing the
maximum cpm/mL for the analyte (i.e., the desired
212Bi daughter radionuclide) by the activity of the
impurities (i . e . , 224Ra parents).
Alternatively, the DF can be calculated by
taking the ratio of the dry weight distribution
ratios (D,) for an analyte and impurity. Assuming the
"influent" is at radioactive steady state (making the
denominator for DF unity ), the ratio of D,õ values for
analyte/impurity are:
analyte
A f mR = (% V
solids/100)
DF = Ao -Af impurity /
Ao-Af / V
Af mR = (% solids/100)
which simplifies after cancellation to:
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Ao -Af analyte
DF = Af
impurily
Ao-Af
Af
where Ao, Af, V, mR and % solids are as defined
elsewhere. These ratios of activities are
proportional to the molar concentrations cited
elsewhere in the definition of DF.
The fundamental differences between a
contemplated multicolumn selectivity inversion
generator technology and the conventional methodology
presented in Fig. 2 are thus at least three-fold: (1)
the storage medium for the parent radionuclides is a
solution rather than a solid support, (2) the desired
daughter radionuclide is selectively extracted from
the parent radionuclide-containing solution when
needed, and (3) a second separation medium prevents
the exit of parent radionuclides from the generator
system.
In addition to minimizing radiolytic damage
to the chromatographic support, extraction of the
minute masses of daughter (i.e., the minor
constituent) by use of the multicolumn selectivity
inversion generator shown in Fig. 3 permits the use
of small chromatographic columns. Thus, the desired
daughter radionuclide can be recovered in a small
volume of solution that is conveniently diluted to
the appropriate dose for clinical use. Typically, 90
percent of the daughter radionuclide can be delivered
in less than about five bed volumes of the first
separation medium of the first column.
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A contemplated separation method is
typically carried out at ambient room temperature.
Gravity flow through the columns can be used, but it
is preferred that the separation be carried out at
more than one atmosphere of pressure as can be
provided by a hand-operated syringe or electric pump.
The use of less than one atmosphere of pressure
(e.g., vacuum assisted flow) as can be achieved by
use of a syringe is also preferred.
The time of contact between a solution and
a separation medium is typically the residence time
of passage of the solution through a column under
whatever pressure head is utilized. Thus, although
one can admix a given solution and separation medium
and maintain the contact achieved there between a
period of hours or days, sorption by the separation
medium is usually rapid enough; that is, the binding
and phase transfer reactions are sufficiently rapid,
that contact provided by flow over and through the
separation medium particles provides sufficient
contact time to effect a desired separation.
The general concept of a selectivity
inversion between the extraction of the desired
daughter radionuclide by the primary separation
column and the retention of parents and other
interferents by the guard column represents an
important aspect of this invention. A seemingly
similar concept is briefly proposed for use with the
diagnostic 64Cu radionuclide [see, Zinn, U.S. Patent
No, 5,409,677 (1995)], however, the application of
the multicolumn selectivity inversion generator to
radiotherapeutic nuclides or to high specific
activity diagnostic radionuclides has not before been
examined or appreciated, and the ionic charges of
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both the parent and daughter radionuclides in that
disclosure are the same, +2 for copper and zinc ions.
The charge densities of the Cu2+ and Zn21 ions are also
substantially the same.
Thus, the example cited for 64Cu relies
exclusively on the use of an immobilized ligand to
complex g4Cu and removes it from macroquantities of
zinc isotopes. One reference is made to secondary
removal of zinc from the 64Cu product by an
unidentified anion-exchange resin, which is made
necessary by the poor selectivity exhibited by the
complexing ligand in the initial separation.
Furthermore, large bed volumes are required and the
64Cu product is delivered in > 20 mL of strongly
acidic solution, which requires secondary
concentration and neutralization before the 64Cu can
be conjugated to a biolocalization agent for use in a
medical procedure. The proposed 64Cu. separation
system does not discuss the identity of ionic charges
of the ions to be separated, nor any application for
use with high specific activity radionuclide
generators or high LET radiation, both of which
present unique challenges to the design of
radionuclide generators.
When radiolytic degradation of the support
material is less of a concern (e.g., for diagnostic
radionuclides), the multicolumn selectivity inversion
generator shown in Fig. 3 continues to offer many
advantages. By example, target irradiation in an
accelerator or reactor frequently requires the use of
isotopically enriched target materials to maximize
the production of the desired parent radionuclides.
Such nucleosynthesis reactions can be inefficient,
producing only low specific activity parents. By
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using the multicolumn selectivity inversion generator
and extracting only the small mass of the daughter
constituent, the macroquantities of the isotopically
enriched target ions are kept in solution and can be
more easily recovered for future irradiation.
Equally important is the small volume of solution in
which the daughter radionuclide is recovered; made
possible by the use of small columns and the logic of
the multicolumn selectivity inversion generator.
The present method is typically configured
to operate substantially free from air or gas,
thereby permitting better chromatographic
performance. The presence of interstitial gas
pockets can result in the solution passing through
the channel without flowing over, through or around
the beads; rather, the solution passes through the
channel without contacting the separation medium.
Specifically, air or gas travelling through a
separation medium can cause channeling in which less
than the desired intimate contact between the
solution and the separation medium can occur. As
such, the columns used in a contemplated method are
configured as a system for transporting and
processing liquids.
Another advantage to such an air- or gas-
less system is that there is no air or gas that must
be sterilized by filtration through sterile air
filters. As such, the components used in a
contemplated method can be of a less complicated
design than those that use combinations of air and
liquid.
The benefits of this generator technology
are profound and the versatility of the fundamental
logic presented in Fig. 3 means that a wide variety
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of radionuclides can be purified using the
multicolumn selectivity inversion generator concept.
Table 1, below, provides a list of radionuclides of
interest to nuclear medicine for imaging or therapy,
along with exemplary solution conditions and
chromatographic materials for their purification
using a multicolumn selectivity inversion generator.
The list of radionuclides and separation conditions
reported in Table 1 are not to be construed as
limiting, rather as examples showing how a variety of
parent/daughter pairs having quite different solution
chemistries, ionic charges, and charge densities can
be separated and purified for use in nuclear medical
applications. As new separation media become
available and interest increases in other
radionuclides, the multicolumn selectivity inversion
generator can be readily adapted to provide a
convenient route to the reliable production of
radionuclides of high chemical and radionuclidic
purity for use in diagnostic or therapeutic nuclear
medicine.
Table 1
Nuclidea Primary method(s) Load solution Strip
of generationb Primary solution
Key separation separation Guard column
(solute/interferent) column
90Y 90sr ((3' )->9DY 0. 5 M HNO3 3 M HNO3
Y3+/Sr2+ AOPE-EXC Sr Resin-EXC
99 ''I'c 99Mo ((3-) ->99mTc 5 M NaOH Phys. Saline
Tc041-/MoO42- ABEC Solutione
A1z03
i03pd 103Tth (p, n) 103pd 0.5 M HCl pH = 4-6
PdCl42' /Rh3+ NE-EXC cix
in SO4Z-/Cl'
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111In 112Cd (p, 2n) 11'In 0.1 M HC1 1 M HC1
In3+/Cd2+ AOPE-EXC AIX
125 1 123Sb ((X, 2n) 125I Dil. HC1 pH = 4-6
I1-/Sb3+ NE-EXC CIX
188Re 186W (2n,y) 188W (R-) ->'eeRe 5 M NaOH Phys. Saline
Re041-/W04'- ABEC Solution
A1203
2 o1T1 203T1 (p, 3n) 201Pb (EC) _>201T1 Holdback 2 M HNO3
T11 /Pb2+ reagentd Sr Resin-EXC
CIX
47sc 47T1 (n, p) 47SC HNO3/HF 2 M HC1
Sc3+/TiO2+ or Ti4+ in S042- MF-NE-EXC AOPE-EXC
212 Bi 224Ra->_>212 Pb((3-)->212Bi 0.2 M HC1 1 M NaOAc
Bi3+/Ra2+, Pb2+ NE-EXC 0.2 M NaCl
CIX
213Bi a25Ac(a)->->213Bi 0.2 M HC1 1 M NaOAc
Bi3+/Ac3+, Ra2+ NE-EXC 0.2 M NaCl
CIX
Z"At 209Bi (a, 2n) 211At Dil. HC1 pH = 4-6
At1-/Bi3+ NE-EXC CIX
223Ra 227AC(Q-)_>227Th((x)-_>223Ra Holdback HNO3
Raz+/AC3+, Th4+ reagent Dipex-EXC
Weak acid
CIX
aMedically useful radionuclides as defined
by the nuclear medicine community. [Bond et al.,
Ind. Eng. Chem. Res. (2000) 39:3130-3134] .
bSeveral production routes often exist and
those cited are the generally accepted routes for
nuclear medicine.
Widely used separation methods include: AIX
= anion-exchange chromatography; CIX = cation-
exchange chromatography; EXC = extraction
chromatography; AOPE-EXC = acidic organophosphorus
extractant-EXC; NE-EXC = neutral organic extractant-
EXC, MF-NE-EXC = multifunctional neutral organic
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extractant-EXC, ABEC = Aqueous Biphasic Extraction
Chromatography.
dHoldback reagents include carboxylates,
polyaminocarboxylates, certain inorganic anions,
chelating agents, etc.
ePhys. Saline Solution = Physiological
Saline Solution.
A contemplated method and system can
utilize one or more separation media. The separation
medium or media utilized for a given separation is
governed by the radionuclides to be separated, as is
well-known. Preferred separation media are typically
bead-shaped or of consistent size and morphology
solid phase resins, although sheets, webs, or fibers
of separation medium can be used.
One preferred solid phase separation medium
is the Bio-Rad 50W-X8 cation exchange resin in the H{
form, which is commercially available from Bio-Rad
Laboratories, Inc., of Hercules, CA. Other useful
strong acid cation-exchange media include the Bio-
Rad AGMP-50 and Dowex~ 50W series of ion-exchange
resins and the Amberlite IR series of ion-exchange
resins that are available from Sigma Chemical Co.,
St. Louis, MO. Anion-exchange resins such as the
Bio-Rad AGMP-1 and Dowex 1 series of anion-exchange
resins can also serve as separation media.
Another resin that can be used in the
present process is a styrene-divinyl benzene polymer
matrix that includes sulfonic, phosphonic, and/or
gem-diphosphonic acid functional groups chemically
bonded thereto. Such a gem-diphosphonic acid resin
is commercially available from Eichrom Technologies,
Inc., located at 8205 S. Cass Avenue, Darien, IL,
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under the name Diphonix resin. In the present
process, the Diphonix resin is used in the H' form.
The characteristics and properties of Diphonix resin
are more fully described in U.S. Patent No.
5,539,003, U.S. Patent No. 5,449,462 and U.S. Patent
No. 5,281,631.
The TEVATM resin, having a quaternary
ammonium salt, specifically, a mixture of trioctyl
and tridecyl methyl ammonium chlorides, sorbed on a
water-insoluble support that is inert to the
components of the exchange composition, is highly
selective for ions having the tetravalent oxidation
state. For example, the +4 valent thorium ions are
bound to the TEVATM resin in nitric acid solution,
whereas the actinium (Ac) and radium (Ra) ions (whose
valencies are +3 and +2, respectively) are not
substantially extracted by contact with this resin
under the same conditions. The TEVATM resin is
commercially available from Eichrom Technologies,
Inc.
In a contemplated method, the second
separation medium (ion-exchange medium) contains
diphosphonic acid (DPA) ligands or groups. Several
types of DPA-containing substituted diphosphonic
acids are known in the art and can be used herein.
An exemplary diphosphonic acid ligand has the formula
CR1R2 ( P03R2 ) 21
wherein R is selected from the group
consisting of hydrogen (hydrido) , a Cl-C$ alkyl group,
a cation, and mixtures thereof;
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R' is hydrogen or a Cl-C2 alkyl group; and R2
is hydrogen or a bond to a polymeric resin.
When R2 is a bond to a polymeric resin, the
phosphorus-containing groups are present at 1.0 to
about 10 mmol/g dry weight of the copolymer and the
mmol/g values are based on the polymer where R1 is
hydrogen. Exemplary exchange media containing
diphosphonic acid ligands are discussed hereinbelow.
One such exchange medium is referred to as
Dipex resin, which is an extraction chromatographic
material containing a liquid diphosphonic acid
extractant belonging to a class of diesterified
methanediphosphonic acids, such as di-2-ethylhexyl
methanediphosphonic acid. The extractant is sorbed
on a substrate that is inert to the mobile phase such
as Amberchrom -CG71 (available from TosoHaas,
Montgomeryville, PA) or hydrophobic silica. In this
extractant, R' and R2 are H and one R is 2- (ethyl) -
hexyl and the other is H.
Dipex resin has been shown to have a high
affinity for trivalent lanthanides, various tri-,
tetra-, and hexavalent actinides, and the trivalent
cations of the preactinide 225Ac, and to have a lower
affinity for cations of radium and certain decay
products of 225Ac. These affinities have been shown
even in the presence of complexing anions such as
fluoride, oxalate, and phosphate.
The active component of a preferred Dipex
resin is a liquid diphosphonic acid of the general
formula,
0 0
RO/ IPI IPI OR
HO __'~ OH
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where R is C6-C.1$ alkyl or aryl, and
preferably an ester derived from 2-ethyl-l-hexanol.
A preferred compound is P,P'-bis-2-(ethyl)hexyl
methanediphosphonic acid.
The active component diphosphonic acid
ester can be mixed with a lower boiling organic
solvent such as methanol, ethanol, acetone, diethyl
ether, methyl ethyl ketone, hexanes, or toluene and
coated onto an inert support, such as glass beads,
polypropylene beads, polyester beads, or silica gel
as known in the art for use in a chromatographic
column. Acrylic and polyaromatic resins such as
AMBERLITE , commercially available from Rohm and Haas
Company of Philadelphia, PA, can also be used.
The properties and characteristics of
Dipex resin are more fully described in Horwitz et
al. U.S. Patent No. 5,651,883 and Horwitz et al. U.S.
Patent No. 5,851,401. Dipex resin is available from
Eichrom Technologies, Inc.
Another useful ion-exchange resin is
DiphosilTM resin. Similar to the other DPA resins,
DiphosilTM resin contains a plurality of geminally
substituted diphosphonic acid ligands such as those
provided by vinylidene diphosphonic acid. The
ligands are chemically bonded to an organic matrix
that is grafted to silica particles. DiphosilTM resin
is available from Eichrom Technologies, Inc.
Yet another useful resin has pendent
-CR1 (P03R2) 2 groups added to a preformed
water-insoluble copolymer by grafting; that is, the
pendent phosphonate groups are added after copolymer
particle formation. For these polymers, R is
hydrogen (hydrido) , a C1-C8 alkyl group, a cation or
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mixtures thereof, and R' is hydrogen or a C1-C8 alkyl
group. A contemplated pendent -CR'(PO3R2) 2 group for
this group of resins has the formula shown below.
The particles also contain zero to about 5 mmol/g dry
weight of pendent aromatic sulfonate groups.
CR'(PO3R2)2
A contemplated pendent methylene
diphosphonate as first formed typically contains two
C1-C8 dialkyl phosphonate ester groups. Exemplary
C1,-C8 alkyl groups of those esters and other C1-C8
alkyl groups noted herein include methyl, ethyl,
propyl, isopropyl, butyl, t-butyl, pentyl,
cyclopentyl, hexyl, cyclohexyl, 4-methylcyclopentyl,
heptyl, octyl, cyclooctyl, 3-ethylcyclohexyl and the
like, as are well-known. An isopropyl group is a
preferred R group. An R' Cl-C2 alkyl group is a
methyl or ethyl group, and R1 is most preferably
hydrogen.
After formation, the alkyl ester groups are
hydrolyzed so that for use, R in the above formula is
hydrogen (a proton), Ca2+ ion or an alkali metal ion
such as lithium, sodium, or potassium ions.
Preferably, the insoluble copolymer
contains at least 2 mole percent reacted vinylbenzyl
halide with that percentage more preferably being
about 10 to about 95 mole percent. One or more
reacted monoethylenically unsaturated monomers as
discussed before are present at about 2 to about 85
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mole percent, with this monomer preferably including
at least 5 mole percent of an above monoethyleni.cally
unsaturated aromatic monomer such as styrene, ethyl
styrene, vinyl toluene (methyl styrene) and vinyl
xylene.
A useful insoluble copolymer also includes
a reacted cross-linking agent (cross-linker).
Reacted cross-linking agents useful herein are also
quite varied. Exemplary useful cross-linking agents
are selected from the group consisting of
divinylbenzene, trimethylolpropane triacrylate or
trimethacrylate, erythritol tetraacrylate or
tetramethacrylate, 3,4-dihydroxy-1,5-hexadiene and
2,4-dimethyl-l,5-hexadiene. Divinylbenzene is
particularly preferred here.
The amount of reacted cross-linker is that
amount sufficient to achieve the desired
insolubility. Typically, at least 0.3 mole percent
reacted cross-linker is present. The reacted
cross-linking agent is preferably present at about 2
to about 20 mole percent.
These contemplated particles are the
multi-step reaction product of a nucleophilic agent
such as CR1(P03R2)2-, which can be obtained by known
methods, with a substrate. Thus, CHR3* (P03R2)2, where R
is preferably an alkyl group, is first reacted with
sodium or potassium metal, sodium hydride or
organolithium compounds, for example, butyllithium,
or any agent capable of generating a diphosphonate
carbanion. The resulting carbanion is then reacted
with a substrate that is a before-discussed insoluble
cross-linked copolymer of one or more of vinyl
aliphatic, acrylic, or aromatic compounds and a
polyvinyl aliphatic, acrylic, or aromatic compound,
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for example, divinylbenzene. That copolymer contains
at least 2 mole percent of a reacted halogenated
derivative of vinyl aromatic hydrocarbon such as
vinylbenzyl chloride group, preferably from 10 to 95
mole percent, about 2 to about 85 mole percent of
monovinyl aromatic hydrocarbon such as styrene and at
least 0.3 mole percent of polyvinyl aliphatic and/or
aromatic cross-linker such as divinylbenzene,
preferably 2-20 mole percent.
The copolymer containing grafted methylene
diphosphonate tetraalkyl ester groups in an amount
corresponding to about 1.0 mmol/g of dry weight,
preferably from 2 to 7 mmol/g of dry weight, is
preferably reacted with a sulfonating agent such as
chlorosulfonic acid, concentrated sulfuric acid, or
sulfur trioxide in order to introduce strongly acidic
pendent aromatic sulfonic groups into their
structure. The presence of the sulfonate pendent
groups confers the additional advantage of
hydrophilicity to the particles and leads to a
surprising enhancement in the rate of cation
complexation without adversely affecting the observed
selectivity.
The reaction of the sulfonating agent with
a grafted copolymer containing methylene
diphosphonate groups is usually carried out when the
recovered resin product in ester form is swollen by a
halohydrocarbon such as dichloromethane, ethylene
dichloride, chloroform, or 1,1,1-trichloroethane.
The sulfonation reaction can be performed using 0.5
to 20.0 weight percent of chlorosulfonic acid in one
of the mentioned halohydrocarbon solvents at
temperatures ranging from about -25 to about 50 C,
preferably at about 10 to about 30 C. The reaction
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is carried out by contacting resin preswollen for
zero (unswollen) to about two hours with the above
sulfonation solution for 0.25 to 20 hours, preferably
0.5 to two hours.
After completion of the sulfonation
reaction, the particles are separated from the liquid
reaction medium by filtration, centrifugation,
decantation, or the like. This final, second resin
product'is carefully washed with dioxane, water, 1 M
NaOH, water, 1 M HC1 and water, and then air dried.
The sulfonation reaction and work-up in
water also hydrolyzes the phosphonate C1-C8 alkyl
ester groups. Where sulfonation is not carried out,
hydrolysis of the phosphonate esters can be carried
out by reaction with an acid such as concentrated
hydrochloric acid at reflux.
These contemplated particles contain as
pendent functional groups both methylene
diphosphonate and sulfonate groups, directly attached
to carbon atoms of aromatic units or acrylate or
methacrylate units in the polymer matrix. A
contemplated resin displays high affinity towards a
wide range of divalent, trivalent, and polyvalent
cations over a wide range of pH values. At a pH
value below one, the resins are able to switch from
an ion-exchange mechanism of cation removal to a
bifunctional ion-exchange/coordination mechanism due
to the coordination ability of the phosphoryl oxygen
atoms. The sulfonic acid groups then act to make the
matrix more hydrophilic for rapid metal ion access;
the methylene diphosphonate groups are thus
responsible for the high selectivity. Further
details for the preparation of this resin can be
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found in Trochimczuk et al. U.S. Patent No.
5,618,851.
Another particularly useful separation
medium that is described in U.S. Patent No. 5,110,474
is referred to as Sr Resin and is available from
Eichrom Technologies, Inc. Briefly, the Sr Resin
comprises an inert resin substrate upon which is
dispersed a solution a crown ether extractant
dissolved in a liquid diluent.
The diluent is an organic compound that
has: (i) a high boiling point; that is, about 170 to
200 C, (ii) limited or no solubility in water, (iii)
is capable of dissolving from about 0.5 to 6.0 M
water, and (iv) is a material in which the crown
ether is soluble. These diluents include alcohols,
ketones, carboxylic acids, and esters. Suitable
alcohols include 1-octanol, which is most preferred,
although 1-heptanol and 1-decanol are also
satisfactory. The carboxylic acids include octanoic
acid, which is preferred, in addition to heptanoic
and hexanoic acids. Exemplary ketones include
2-hexanone and 4-methyl-2-pentanone, whereas esters
include butyl acetate and pentyl acetate.
The macrocyclic polyether can be any of the
dicyclohexano crown ethers such as dicyclohexano-18-
Crown-6, dicyclohexano 21-Crown-7, or dicyclohexano-
24-Crown-8. The preferred crown ethers have the
formula: 4,4' (5' ) [ (R,R' ) dicyclohexano] -18-Crown-6,
where R and R' are one or more members selected from
the group consisting of H and straight chain or
branched alkyls containing 1 to 12 carbons. Examples
include, methyl, propyl, isobutyl, t-butyl, hexyl,
and heptyl. The preferred ethers include
dicyclohexano-18-crown-6 (DCH18C6) and
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bis-methylcyclohexano-18-Crown-6 (DMeCH18C6). The
most preferred ether is bis-4,4'(5')-
[(t-butyl)cyclohexano]-18-Crown-6 (Dt-BuCH18C6).
The amount of crown ether in the diluent
can vary depending upon the particular form of the
crown ether. For example, a concentration of about
0.1 to about 0.5 M of the most preferred t-butyl form
(Dt-BuCH18C6) in the diluent is satisfactory, with
about 0.2 M being the most preferred. When the
hydrogen form is used, the concentration can vary
from about 0.25 to about 0.5 M.
The preferred Sr Resin utilizes an inert
resin substrate that is a nonionic acrylic ester
polymer bead resin such as Amberlite XAD-7 (60
percent to 70 percent by weight) having a coating
layer thereon of a crown ether such as Dt-BuCH18C6
(20 percent to 25 weight percent) dissolved in
n-octanol (5 percent to 20 weight percent), having an
extractant loading of 40 weight percent. [See,
Horwitz et al., Solvent Extr. Ion Exch., 10(2):313-16
(1992).]
It has also been observed that Pb Resin, a
related resin, also available from Eichrom
Technologies, Inc. is also useful for purifying and
accumulating 212 Pb for the production of 212Bi. Pb
Resin has similar properties to Sr Resin except that
a higher molecular weight alcohol; that is, isodecyl
alcohol, is used in the manufacture of Pb Resin.
[See, Horwitz et al., Anal. Chim. Acta, 292:263-73
(1994).] It has been observed that Pb Resin permits
subsequent stripping of the 212Bi from the resin,
whereas it has been observed that 212 Pb is strongly
retained by the Sr Resin.
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An improved Sr Resin also available from
Eichrom Technologies, Inc. is even more selective.
This separation medium is referred to as Super
Pb(Sr)' selective resin and comprises free-flowing
particles having about 5 to about 50 weight percent
of a bis-4, 4 ' (5' )[C3-C$-alkylcyclohexano] 18-Crown-6,
such as Dt-BuCH18C6, that exhibits a partition ratio
between n-octanol and 1 M nitric acid (DCrown =
[Crownorg] /[Crown] Aq) of greater than about 103, and
usually of about 103 to about 106, dispersed onto an
inert, porous support such as polymeric resin (e.g.,
Amberchrom -CG71) or silica particles. The
separation medium is free of a,diluent, and
particularly free of a diluent that is: (i) insoluble
or has limited (sparing) solubility in water and (ii)
capable of dissolving a substantial quantity of water
that is present in the Sr Resin. See, U.S.Patent No.
6,511,603 B1.
Preferred wash and strip solutions that are
used are also selected based upon the parent and
daughter radionuclides and the desired use of the
product. The reader is directed to Horwitz et al.
U.S. Patent No. 5,854,968 and Dietz et al. U.S.
Patent No. 5,863,439 for an illustrative discussion
of this separation medium.
Yet another separation medium is
particularly useful for separating chaotropic anions
in aqueous solution. This separation medium is
available from Eichrom Technologies, Inc. under the
designation ABEC , and comprises particles having a
plurality of covalently bonded -X- (CH2CH2O) n-CH2CH2R
groups wherein X is 0, S, NH or N- (CH2CH20) m-R3 where m
is a number having an average value of zero to about
225, n is a number having an average value of about
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15 to about 225, R3 is hydrogen, C1-C2 alkyl, 2-
hydroxyethyl or CH2CH2R, and R is selected from the
group consisting of -OH, C1-Clo hydrocarbyl ether
having a molecular weight up to about one-tenth that
of the -(CH2CH2O)11- portion, carboxylate, sulfonate,
phosphonate and -NR'R2 groups where each of R' and R2
is independently hydrogen, C2-C3 hydroxyalkyl or C1-C6
alkyl, or -NR'R2 together form a 5- or 6-membered
cyclic amine having zero or one oxygen atom or zero
or one additional nitrogen atom in the ring. The
separation particles have a percent CH2O/mm2 of
particle surface area of greater than about 8000 and
less than about 1,000,000.
Exemplary chaotropic anions include simple
anions such as Br1- and I1- and anion radicals such as
TcO41-, ReO41- or IO31-. The chaotropic anion can also
be a complex of a metal cation and halide or
pseudohalide anions. A particularly useful
separation that can be effected using this separation
medium is that of 99mTcO4''- from an aqueous solution
that also contains the parent radionuclide 99MOO42-
ions. Further details concerning the ABEC
separation medium and its uses can be found in U.S.
Patents No. 5,603,834, No. 5,707,525 and No.
5,888,397.
Exemplary chelating resins include that
material known as ChelexTM resin that is available
from Bio-Rad Laboratories that includes a plurality
of iminodiacetate ligands and similar ligands can be
reacted with 4 percent beaded agarose that is
available from Sigma Chemical. Co., St. Louis, MO.
In a preferred method that utilizes
separation medium beads, the support beads that
comprise the separation medium are packed into a
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column. When a solution is passed through the beads,
the solution can flow over, through and around the
beads, coming into intimate contact with the
separation medium.
Examples
All acids were of trace metal grade, and
all other chemicals were of ACS reagent grade and
used as received. The 207Bi and 133Ba radioactive
tracers were each evaporated to dryness twice in
concentrated HNO3 and dissolved in 0.50 M HNO3 prior
to use. Standard radiometric assay procedures were
employed throughout, and all count rates were
corrected for background.
The extraction chromatographic materials
were prepared using a general procedure described
previously. [See, Horwitz et al., Anal. Chem.,
63:522-525 (1991).] Briefly, a solution of 0.25 M
tri-n-octylphosphine oxide (TOPO) in n-dodecane (0.78
g) was dissolved in about 25 mL of ethanol and
combined with 50-100 m Amberchrom -CG71 resin (3.03
g) in about 25 mL of ethanol. The mixture was
rotated at room temperature on a rotary evaporator
for about 30 minutes after which the ethanol was
vacuum distilled. The resulting solid is referred to
as TOPO Resin and corresponds to 20 percent (w/w)
loading of 0.25 M TOPO in n-dodecane on Amberchrom -
CG71. The modified TRPO Resin was prepared in a
similar manner, except that this material contains no
n-dodecane diluent and the dispersing solvent was
methanol rather than ethanol. The TRPO Resin
O
contains an equimolar mixture of Cyanex -923 (a
mixture of n-alkyl phosphine oxides) and
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dipentyl(pentyl)-phosphonate loaded to 40 percent on
50-100 m Amberchrom -CG71.
The percent solids for the Bio-Rad AGMP-50
cation-exchange resin were determined by transferring
a portion of the wet resin to a tared vial and drying
in an oven at 1100 C until a constant mass was
achieved. Each gravimetric analysis was performed in
triplicate to provide a percent solids of 48.6( 0.3)
percent. All resins were stored in tightly capped
containers and were not exposed to air for any
lengthy period of time to avoid a change in percent
solids.
All dry weight distribution ratios were
determined radiometrically by batch contacts of the
resins with the desired solutions at 25( 2) C. The
dry weight distribution ratio (D,,,) is defined as:
D = Ao -Af V
W A~ mR = (% solids/100)
where Ao = the count rate in solution prior to contact
with the resin, Af = the count rate in solution after
contact with resin, V volume (mL) of solution in
contact with resin, mR = mass (g) of wet resin, and
the percent solids permits conversion to the dry mass
of resin.
The batch uptake experiments were performed
by adding L quantities of '-33Ba or 207Bi in 0.50 M HNO3
to 1.2 mL of the solution of interest, gently mixing,
and removing a 100 L aliquot for y-counti.ng (Ao) .
One mL of the remaining solution (V) was added to a
known mass of wet resin (mR) and centrifuged for 1
minute. The mixture was then stirred gently (so that
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the resin was just suspended in the solution) for 30
minutes, followed by 1 minute of centrifugation, and
another 30 minute of.stirring. After 1 minute of
centrifugation to settle the resin, the solution was
pipeted away and filtered through a 0.45 m PTFE
filter to remove any suspended resin particles. A
100 L aliquot was then taken for y-counting (Af) .
All dry weight distribution ratios are accurate to
two significant digits.
A quantity of TRPO Resin in 0.20 M HC1 was
slurry packed into a 1.2 mL capacity Bio-Spin
disposable plastic chromatography column (Bio-Rad
Laboratories, Inc.) to afford a bed volume (BV) of
0.5 mL. A porous plastic frit was placed on top of
the bed to prevent its disruption during the addition
of eluent. The column was conditioned by eluting 3.0
mL (6 BV) of 0.20 M HC1 and followed by gravity
elution of 2.0 mL (4 BV) of 0.20 M HC1 spiked with
133Ba and 207Bi . The column was subsequently rinsed
with 2.0 mL (4 BV) of 0.20 M HC1 and the 207Bi was
stripped using 2.0 mL (4 BV) of 1.0 M sodium acetate
(NaOAc) in 0.20 M NaCl. Column eluates were
collected into tared y-counting vials, and all volumes
were calculated gravimetrically using the respective
solution densities.
A portion of 20-50 m Di.pex Resin {40
percent P,P'-bis(2-ethylhexyl)methanediphosphonic
acid on Amberchrom-CG71, Eichrom Technologies, Inc.
[see, Horwitz et al., React. Funct. Pol),mers, 33:25-
36 (1997)]) in 1.0 M HNO3 was slurry packed into a
custom plastic chromatography column to afford a BV
of 0.16 mL. Porous plastic frits were used to keep
the resin in place during chromatographic operations,
which were carried out using a custom automated low
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pressure chromatography system. The column was
conditioned by eluting 4.0 mL (25 BV) of 1.0 M HNO3
and followed by elution of 2.0 mL (12.5 BV) of 1.0 M
HNO3 spiked with 133Ba and 207Bi at a flow rate of about
0.25 mL/min. The column was subsequently rinsed with
2.0 mL (12.5 BV) of 1.0 M HNO3 and the 207Bi was
stripped using 2.0 mL (12.5 BV) of 2.0 M HC1. Column
eluates were collected into tared y-counting vials,
and all volumes were calculated gravimetrically using
the respective solution densities.
As discussed above, the use of high LET a-
and (31--emitting radiation holds great promise for the
therapy of micrometastatic carcinoma and solid tumor
masses. [See, Whitlock et al., Ind. Eng. Chem. Res.
39:3135-3139 (2000); Hassfjell et al., Chem. Rev.
101:2019-2036 (2001); Imam, Int. J. Radiation
Oncology Biol. Phys. 51:271-278 (2001); and McDevitt
et al., Science 294:1537-1540 (2001).] One candidate
a-emitter proposed for use in cancer therapy is 212Bi
[see, Whitlock et al., Ind. Eng. Chem. Res. (2000)
39:3135-3139 (2000); Hassfjell et al., Chem. Rev.
101:2019-2036 (2001); and Imam, Int. J. Radiation
ncology Bi.ol. Phys. 51:271-278 (2001) ] which forms
as part of the uranium-232 (232U) decay chain shown in
Fig. 4.
Bismuth-212 is presently obtained for use
by elution from a conventional generator in which the
relatively long-lived (i.e., 3.66 d) 224Ra parent is
retained on a cation-exchange resin and the 212 Bi is
eluted with about 1-3 M HC1 or mixtures of HC1 and
HI. [See, Mirzadeh et al., J. Radioanal. Nucl. Chem.
203:471-488 (1996) and Mirzadeh, Appl. Radiat. Isot.
49:345-349 (1998).] Radiolytic degradation of the
cation-exchange resin limits the useful deployment
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lifetime of the 212Bi generator to approximately two
weeks, [see, Mirzadeh et al., J. Radioanal. Nucl.
Chem. 203:471-488 (1998)] and a multicolumn
selectivity inversion generator can provide
advantages for the purification of 212Bi. The decay
chain leading to 212Bi also presents a challenging
testing ground for the multicolumn selectivity
inversion generator concept, and the following
detailed examples target the development of a new
21ZBi generator.
Examination of the radioactive half-lives
shown in Fig. 4 indicates that a solution of 224Ra
with t1/2 = 3.66 days is well suited to serve as the
radionuclidic source material for use in the nuclear
pharmacy. The 212Bi can be extracted from this
solution using a primary separation column selective
for Bi (III) , while permitting Ra (II) , Po (IV) , and
Pb(II), to elute. In this 212Bi example, the most
hazardous radionuclidic impurity is the comparatively
long-lived bone seeking 224Ra parent, with 212Pb (tl/2 =
10.64 h) representing somewhat less of a concern.
The behavior of Ra(II) can be extrapolated
from studies using its lighter congener Ba(II), and
this chemical analogy has been employed in the
discussion below. Fig. 5 shows a plot of D,,, for
Ba ( I I) and Bi ( I I I) vs.[HCl ] on TOPO Resin, an
extraction chromatographic material containing 0.25 M
tri-n-octylphosphine oxide (TOPO) in n-dodecane at 20
percent loading on 50-100 m Amberchrom-CG71.
This plot indicates the potential of TOPO
Resin for Bi(III) separation from Ba(II) and, by
extension from their chemical similarities, Ra(II),
in the range 0.04-0.4 M HC1. Note that values of DW
less than 10 obtained from these batch contact
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studies indicate essentially no sorption of a given
analyte (i.e., Ba(II), and by extension Ra(II), would
not be substantially retained under chromatographic
elution conditions). Operating in a chromatographic
mode, rather than in the batch mode used to generate
the data in Fig. 5, DFs of greater than 103 for Ba(II)
(and Ra(II)) from Bi(III) can be achieved.
Fig. 5 also shows that DW for Bi(III)
decreases at both extremes of the HC1 concentration,
which indicates that an HC1 concentration greater
than 1 M or a pH = 3-10 buffered strip solution can
serve as effective stripping agents. Because of the
proposed in vivo use of the radionuclide and the need
for its conjugation to a biolocalization agent, near
physiological pH values are preferred as a strongly
acidic medium inhibits the conjugation reaction and
can chemically attack the biolocalization agent.
A chromatographic study was performed to
assess the effectiveness of stripping at low acid
concentrations; specifically stripping with a
solution of sodium acetate (NaOAc) at pH = 6.5. The
chromatographic separation of Ba(II) from Bi(III)
using modified TRPO Resin (closely related to the
phosphine oxide-containing TOPO Resin) is shown in
Fig. 6, and the principle of using NaOAc at near-
neutral pH to strip Bi(III) from TRPO Resin is
confirmed.
Fig. 6 shows that Ba(II) elutes with the
first free column volume of 0.20 M HC1 load solution
(as predicted for D,, less than 10 from Fig. 5), and
decreases steadily to background levels after
approximately two bed volumes of 0.20 M HC1 rinse. A
small amount of a07Bi(zII) is detected in the column
eluate during loading, but is not statistically
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WO 03/086569 PCT/US03/11278
significant at less than twice background radiation
levels in the 207Bi window. No 133Ba (II) could be
detected in the strip solution comprising 1.0 M NaOAc
in 0.20 M NaCl, which effectively removes greater
than 85 percent of the Bi(III) in approximately two
bed volumes. This study confirms that the Bi(III)
can be effectively separated from Ba(II) and stripped
from the modified TRPO and TOPO Resins by reducing
the acid concentration from pH = 0.70 (for 0.20 M
HC1) to pH = 6.5 (1.0 M NaOAc).
The chromatogram of Fig. 6 shows that the
TRPO Resin affords a DF of Ba(II) (and Ra (II) ) from
Bi(III) of about 103 , and that this resin could serve
as an effective primary separation column in a
multicolumn selectivity inversion generator. To
ensure a high purity product and to minimize the
probability of the 224Ra and 212 Pb parents from
reaching the patient; however, a guard column was
developed that permits elution of 21_2Bi(III) while
224Ra ( I I) and 212 Pb ( I I) are retained.
Fig. 7 shows the dependence of Bi(III)
uptake on a macroporous sulfonic acid cation-exchange
resin vs. [Cl'--] at two different pH values. A C11-
concentration of about 1 M affords anionic chloro
complexes of, Bi(III) (e.g., BiC141-, BiC152-, etc.)
that are not retained by cation-exchange resins. As
a result, the Dw values for Bi(III) shown in Fig. 7
are quite low, suggesting little, if any, retention
of the anionic chloro complexes of Bi(III) under
chromatographic conditions. The retention of Ra(II)
by sulfonic acid cation-exchange resins in this pH
range is reported to be quite high [see, Massart,
"Nuclear Science Series, Radiochemical Techniques:
Cation-Exchange Techniques in Radiochemistry," NAS-NS
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CA 02482294 2004-10-12
WO 03/086569 PCT/US03/11278
3113; National Academy of Sciences, (1971)], which
suggests that 224Ra(II) would not elute from a cation-
exchange resin guard column and would not contaminate
the 212Bi(III) eluate to any significant extent.
The extraction of Pb(II) from solutions of
less than 1 M HC1 by neutral organophosphorus
extractants similar to those used in the TOPO and
TRPO Resins of the primary separation column is
reported to be quite low. [See, Sekine et al.,
Solvent Extraction Chemistry, Marcel Dekker, New York
(1977).] The proposed cation-exchange resin guard
column of Fig. 7 provides additional decontamination
from 212Pb (II) based on the observation that Pb(II)
does not form anionic chloro complexes to any
appreciable extent at [C1-] less than 1 M. Supporting
this observation are experimental results reporting
that 212Bi (III) , substantially free of its immediate
212 Pb (II) parent, can be eluted from sulfonic acid
cation-exchange resins by 0.5 M HC1 (i.e., Pb(II) is
retained by the cation-exchange resin under these
conditions). [See, Hassfjell et al., Chem. Rev.
101:2019-2036 (2001); Mirzadeh et al., J. Radioanal.
Nucl. Chem. 203:471-488 (1996); and Mirzadeh,. Appl.
Radiat. Isot. 49:345-349 (1998).] The data presented
in Figs. 5-7 combined with the literature data for
Pb(II) indicate that 212Bi can be effectively
separated from i.ts 224Ra and 212Pb parents using a
multicolumn selectivity inversion generator based on
a neutral organophosphorus extractant primary
separation column.
Fig. 8 presents an alternative to the
modified TRPO Resin primary separation column (Fig.
6) for the separation of 212Bi (III) from 224Ra (II) and
21zPb(II). Dipex Resin is an extraction
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CA 02482294 2007-08-15
28778-158
C___ Jt:, JG_ p-r=-~ Pi3L"cr=8l C' =1~=5 ~ J- ~ 0 pc :f cenC
/ _'_-':C ~F 11, -ID_S !2-e=!7y! r_=:lec.-pilo=',.Jrlo: iC
~ f Gi" , _ -,
ac_~Q ;/n 20 -~0 m zinb~rc_r.rom,-CG7! . i See, horw_ Z e
al., RFacr. Funct. Polymers 33:23-35 (1997).] Fia. 2
srio%,~s that Bi (III) is strongly retained =-om 1. 0 iJ,
(D
riN03 by Dipex Resin, while Ba (II) read ly elutes . 170
statistically significant quantities of ''p7Bi(III)
were detected during the load and rinse procedures,
and the 1.0 Pr M103 rinse brought the 1'3Ba(II) levels
to background after five bed volumes. Stripping with
2.0 hI HCl removes greater than 93 percent of the
' 'Bi (III) along with a minimal amount of 133Ea (II) in
two bed volumes. Use of the chelating ion-exchange
Dipex Resin in the primary separation column affords
overall DFs of greater than 103, but would still
require the use of guard column chemistry as
described above to rninimize the potential for
contamination of the 212Bi product by 224Ra and z''Pb.
The use
of the article "a" or "an" is intended to include one
or more.
From the foregoing it will be observed that
numerous modifications and variations can be
effectuated without departing from the true spirit
and scope of the novel concepts of the invention. It
is to be understood that no limitation with respect
to the specific eRLhOdiment illustrated is -ntend.ed or
,shculd be inferred. The disclosure is intended to
r ;_
cover bY the appended claims a11 S',ich mooi-r'a.t_ors
=G11 'VJl ii7 t il e 5=~~;Je 0T _hF
- _ 7 _
