Article pubs.acs.org/ac
Harvesting 67Cu from the Collection of a Secondary Beam Cocktail at the National Superconducting Cyclotron Laboratory Tara Mastren,†,§ Aranh Pen,‡ Shaun Loveless,§ Bernadette V. Marquez,§ Elizabeth Bollinger,§ Boone Marois,‡ Nicholas Hubley,‡ Kyle Brown,† David J. Morrissey,∥ Graham F. Peaslee,‡ and Suzanne E. Lapi*,†,§ †
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63139, United States Department of Chemistry, Hope College, 35 East 12th Street, Holland, Michigan 49422, United States § Department of Radiology, Washington University School of Medicine, 510 S. Kingshighway Boulevard, St. Louis, Missouri 63110, United States ∥ Department of Chemistry, Michigan State University, 578 South Shaw Lane, East Lansing, Michigan 48824, United States ‡
ABSTRACT: Isotope harvesting is a promising new method to obtain isotopes for which there is no reliable continuous supply at present. To determine the possibility of obtaining radiochemically pure radioisotopes from an aqueous beam dump at a heavy-ion fragmentation facility, preliminary experiments were performed to chemically extract a copper isotope from a large mixture of projectile fragmentation products in an aqueous medium. In this work a 93 MeV/u secondary beam cocktail was collected in an aqueous beam stop at the National Superconducting Cyclotron Laboratory (NSCL) located on the Michigan State University (MSU) campus. The beam cocktail consisted of ∼2.9% 67Cu in a large mixture of co-produced isotopes ranging in atomic number from ∼19 to 34. The chemical extraction of 67Cu was achieved via a two-step process: primary extraction using a divalent metal chelation disk followed by anion-exchange chromatography. A significant fraction (74 ± 4%) of the 67Cu collected in the aqueous beam stop was recovered with >99% radiochemical purity. To illustrate the utility of this product, the purified 67Cu material was then used to radiolabel an anti-EGFR antibody, Panitumumab, and injected into mice bearing colon cancer xenografts. The tumor uptake at 5 days postinjection was found to be 12.5 ± 0.7% which was in very good agreement with previously reported studies with this radiolabeled antibody. The present results demonstrate that harvesting isotopes from a heavy-ion fragmentation facility could be a promising new method for obtaining high-quality isotopes that are not currently available by traditional methods.
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Cu-67, a theragnostic isotope in nuclear medicine, will be available in the cooling loops of FRIB.4−7 Cu-67 has a half-life of 2.58 days that matches the biological half-life of antibodies and β decays with an average β− energy of 141 keV to 67Zn, followed by the emission of an imageable 184 keV γ ray (48.7%). The emitted γ ray can be used for single photon emission computed tomography (SPECT) imaging, and the β− particles produced from the decay can provide therapy; therefore, attaching this radioisotope to an antibody would provide a targeted theragnostic agent. Currently 67Cu is typically produced via the 68Zn(p,2p)67Cu reaction which requires highenergy proton accelerators (∼100 MeV). Several national laboratories have linear accelerators capable of producing 67Cu but cannot always dedicate beam time to making this isotope.8−10 Harvesting 67Cu from the cooling loop at FRIB
sotope harvesting from heavy-ion fragmentation facilities is a potential new pathway for obtaining isotopes not currently available by other means.1,2 With the ongoing construction of the Facility for Rare Isotopes (FRIB), there has been an increased interest in harvesting unused isotopes from the cooling loop of the beam dump at the new facility. During routine operation at FRIB, heavy ions such as uranium will be accelerated and fragmented by passage through a rotating carbon disk that acts as the fragmentation target. The isotope desired for nuclear physics research will be selected by the Advanced Rare Isotope Separator (ARIS), and the unreacted primary beam, and many other isotopes, will be deposited into a water-filled beam dump and thus into the cooling loop.3 The primary beam will react with the water in the beam dump and produce many long-lived isotopes that could be used in a wide range of applications such as medicine, astrophysics, stockpile stewardship, and others.1,2 Several designs are currently under consideration for harvesting isotopes of interest from the cooling loop as an ancillary service at the new facility. © 2015 American Chemical Society
Received: June 19, 2015 Accepted: September 25, 2015 Published: September 25, 2015 10323
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technique with a silicon-PIN diode in the A1900 focal plane and cyclotron RF signal prior to delivery of the beam to the experimental end station.12 Three separate samples were collected of the secondary beam using a modified version of the water target system described by Pen et al.2 Due to the higher intensity of the beam compared to previous experiments, a 12.7 μm tantalum foil was used in place of the 8 μm Kapton film to contain the water. Additionally, room temperature water was circulated (at 100 mL/min) through a fluorinated ethylene propylene (FEP) loop inside the water cell to remove excess heat. Separation Chemistry Development and Validation. A 100 mL solution containing Cu2+, Zn2+, Ni2+, Co2+, Fe2+, and Mn2+, each at a final concentration of 100 ppb, was made to validate the chemistry using appropriate amounts of atomic absorption standards (Sigma) and diluted to 100 mL with 1.25 M NH4OAc pH 5 and passed through a chelation disk (Empore) functionalized with iminodiacetic acid groups. At pH 5, the carboxylate functional groups on the chelation disk are negatively charged, thus attracting positively charged ions. The metal ions in the test solution existed as 2+ ions with the exception of manganese, which likely existed as both a 2+ ion and as a MnO molecule. The addition of 10 mL of 6 M HCl to the chelation disk extracts 2+ metals previously chelated by the disk without removing the 3+ metals chelated to the disk. Following passage through the chelation disk, the 6 M HCl fraction was added to an anion-exchange column containing 2.5 g of AG1-X8 resin (Bio-Rad). The elution constants of these metals with the quaternary-amine-functionalized resin vary with the concentration of hydrochloric acid.13 Various amounts of 6, 4, 2.5, and 0.5 M HCl were used to elute the different metallic ions from the anion-exchange resin. Each fraction was evaporated to dryness and reconstituted in 5 mL of 3% HNO3 (the metallic ions were presumed to exist in solution as Cu2+, Ni2+, Zn2+, Co2+, Mn2+, and Fe2+). An aliquot of each fraction was diluted by a factor of 500 and measured via ion chromatography14 to monitor the separation of the different elements. The separation of cobalt from copper was more difficult than from the other divalent metals due to similar elution constants of these two metals.13 Since the elution constant for cobalt rises at a steeper rate than that of copper between 6 and 3 M hydrochloric acid, the concentration of HCl was varied in this range while monitoring the copper−cobalt separation with the radiotracers 55Co and 64Cu. A 3.7 MBq amount of each of 55Co and 64Cu (produced at Washington University in St. Louis) in 10 mL volumes of 6 M HCl were added to an anion-exchange column with 2.5 g of AG1-X8 resin, and the eluate was collected. Three additional fractions of 10 mL of 6 M HCl were added to the resin, and the eluate was also collected. Then 10 mL fractions of 4, 2.5, and 0.5 M HCl were added to the resin, and the eluate was collected in sequence. The eluate from each fraction was analyzed by an HPGe detector (Canberra) to detect the characteristic γ rays of 64Cu (1345 keV) and 55Co (931, 477, and 1408 keV) to determine the distribution of the isotopes. The chemistry of vanadium is much more complex than that of the metals described earlier, and it is likely that vanadium ions delivered to the aqueous beam dump existed in several different oxidation states. The ion chromatography method cannot be used to measure the concentration of vanadium; therefore another method was developed to monitor the separation. Vanadium was oxidized to the +5 oxidation state by
could provide research quantities (theoretical saturation yields of ∼2 Ci) of this isotope as a complementary source. Previously, an idealized experiment was performed to separate 67Cu from a preseparated secondary beam (73% pure) that also contained some nickel, zinc, germanium, and gallium isotopes using a two-part chemical purification process after collection in water.1 However, the isotopes in the cooling loop at FRIB will not be preseparated because the cooling loop will contain the full suite of isotopes produced during fragmentation of the primary beam. In the present work, a less idealized case is studied where an unseparated secondary beam cocktail was collected containing a wide range of contaminant isotopes and only ∼2.9% 67Cu. A chemical procedure was developed for separation of 67Cu from the water samples with this collection of isotopic contaminants, most of which were first-row transition metals. The majority of the elements present in the secondary beam cocktail were present as one or more radioactive isotopes. Characteristic γ rays from each radioisotope were analyzed to monitor the separation of 67Cu from the different elements. Three separate samples were collected and shipped to Washington University in St. Louis, MO, USA, for chemical separation and γ analysis. Cu-67 was chemically separated from the beam contaminants with high radiochemical purity. To illustrate the utility of this product, radiolabeling studies were conducted with an anti-EGFR antibody (Panitumumab).11 Biodistribution studies were conducted with this radiopharmaceutical in tumor-bearing animals to demonstrate proof of concept.
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EXPERIMENTAL SECTION Beam Collection. A 25 particle-nA 130 MeV/u 76Ge beam was fragmented using a 508 mg/cm2 thick beryllium production target in the A1900 projectile-fragment separator.12 The aluminum wedge used in the previous study to preseparate the beam was removed from the center of the A1900 to obtain a broad range secondary beam. The focal plane (FP) massselection aperture was set to 10 mm, and the momentum acceptance slits were opened to 2% (full momentum acceptance). The experimental parameters for the separator and beamline are listed in Table 1. The momenta of the transmitted fragments are ±1% of the central value of the magnetic rigidity, Bρ = 3.2822 Tm. The beam intensity and purity were measured using the standard ΔE/time-of-flight Table 1. Experimental Parameters for the 67Cu Secondary Beam Production at the NSCL block
description
value
primary beam target D1 Bρ I1 slits D2 Bρ I2 slits D3 Bρ D4 Bρ D5 Bρ D6 Bρ exit window air gap water target window water target
beryllium dipole selection magnet momentum acceptance (Δp/p) dipole selection magnet momentum acceptance (Δp/p) dipole selection magnet dipole selection magnet dipole beamline magnet dipole beamline magnet zirconium air tantalum H2O
508 mg/cm2 3.2822 Tm 2.03% 3.2822 Tm 2% 3.2822 Tm 3.2822 Tm 3.2822 Tm 3.2822 Tm 75 μm 89 mm 12.7 μm 73 mm 10324
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carbonate buffer (pH 9) in a final volume of 60 μL. The reaction was incubated at 37 °C for 1 h and buffer exchanged into 0.1 M ammonium acetate buffer (pH 5.5) using a Zeba spin-desalting column, according to manufacturer instructions. The concentration of the NOTA-Bz-NCS-Panitumumab conjugate was measured using the Bradford assay.15 Evaluation of Immunoreactivity. The immunoreactivity of the conjugated NOTA-Bz-NCS-Panitumumab was measured to verify that the conjugation did not affect the antigen-binding properties of the antibody, as described in Method 3 by Konishi et al.16 The final effective specific activity of 64Cu-NOTA-BzPanitumumab was 185 kBq/μg. Radiolabeling was confirmed to be >99% by radio thin-layer chromatography. The radiolabeled Panitumumab was brought to a final concentration of 4.18 μg/ mL in 0.1 M NH4OAc at pH 5.5. Serial dilutions were performed so that solutions ranging from 0.0625 to 4.18 μg/ mL were obtained. A 50 μL aliquot of each concentration was added to 500 μL of 3 × 106 HCT-116 cells in suspension containing 0.1% bovine serum albumin in PBS, in triplicate. Cell solutions were incubated for 2 h. Cells were centrifuged at 1000g for 3 min to form a pellet, and the supernatant was removed. Cells were then washed three times with 500 μL of phosphate buffered saline, centrifuging as described earlier with removal of the supernatant between each wash. Standards containing the initial activity of each concentration along with the cell pellets were counted using a γ counter (PerkinElmer). The immunoreactive fraction was calculated using method 3 as described by Konishi et al.16 The ratio of bound vs total antibody was graphed against the total antibody (pmol) in Prism 6 (Graphpad). Animal Models. All animal procedures were performed as stated in the Guide for Care and Use for Laboratory Animals by the National Institutes of Health under a protocol approved by the Animal Studies Committee at Washington University in St. Louis. Female Athymic Nu/Nu mice (National Cancer Institute, Bethesda, MA, USA) aged 6−9 weeks were anesthetized with a ketamine/xylazine cocktail (Vedco, St. Joseph, MO, USA). A 100 μL aliquot of approximately 3 × 107 cells/mL HCT-116 colon cancer cells suspended in saline were subcutaneously injected into the shoulder. Tumors were allowed to grow for 2 weeks before biodistribution studies. Radiolabeling and Biodistribution of 67Cu-NOTA-BzPanitumumab. The 2.5 M HCl fractions from the three separations were mixed together, brought to dryness, and reconstituted in 30 μL of 0.1 M HCl. A 25 μL aliquot (451 kBq) of 67Cu was added to 25 μL of 0.5 M NH4OAc, and the final solution had a pH of 5.5. A 111 μL aliquot of 5.5 mg/mL NOTA-Bz-Panitumumab was added to the mixture and incubated at 37 °C for 30 min. Radiolabeling was confirmed using radio thin-layer chromatography. The solution was brought to 400 μL by the addition of saline, and nonspecifically bound 67Cu was challenged with EDTA (final EDTA concentration of 1 mM). A 113 kBq amount of 67Cu-NOTA-bz-Panitumumab was injected via tail vein into three mice bearing HCT-116 colon cancer xenografts. Biodistribution studies were performed at 5 days postinjection. Organs and tumors were harvested and weighed. The radioactivity associated with each organ was assayed with a γ ray detector, background and decay corrected, to determine the percentage of injected dose per gram of organ (%ID/g).
the addition of hydrogen peroxide. A UV−vis spectrophotometer set at 450 nm was used to measure the absorbance of this solution. Calibration curves for vanadium in 6, 2.5, and 0.5 M hydrochloric acid were obtained for concentrations ranging from 2 to 12 mg/L using an atomic absorbance standard of vanadium. Linear calibration curves were obtained from these data. In order to test the vanadium separation procedure, 100 mL of 1 mg/L solution of vanadium in 1.25 M NH4OAc pH 5 was passed through a chelation disk. A 10 mL aliquot of 6 M HCl was then passed through the disk and collected. A 20 μL aliquot of H2O2 was added to a 1 mL aliquot of the eluate and its absorbance measured at 450 nm. The remaining 9 mL of 6 M HCl was added to a column containing 2.5 g of AG1-8X resin. Two additional 10 mL 6 M HCl fractions followed by a 10 mL 2.5 M fraction were eluted from the column, and 20 μL of H2O2 was added to 1 mL aliquots from each fraction and their absorbance measured at 450 nm the absorbance peak for V(V) complexes. Titanium in an aqueous solution is readily oxidized to TiO2, which is insoluble in water. However, the concentration of titanium is expected to be very low in the aqueous beam stop; therefore it is likely that TiO2 will be soluble at the relevant concentration. As with vanadium, the titanium concentration cannot be measured using ion chromatography. However, in acidic medium and in the presence of hydrogen peroxide, titanium exhibits a bright yellow color representative of its +4 oxidation state. Calibration solutions were made in 6, 2.5, and 0.5 M HCl for concentrations ranging from 2 to 12 mg/L. Linear curves were obtained for the absorbance at 405 nm as a function of concentration. An atomic absorption standard of TiO2 was used to make a 100 mL of 1 mg/L solution of titanium in 1.25 M NH4OAc at pH 5. This was passed through a chelation disk followed by 10 mL of 6 M HCl. A 1 mL aliquot of the 6 M HCl eluate was collected, 20 μL of H2O2 was added, and the absorbance was measured at 405 nm. The remaining 9 mL of 6 M HCl was subjected to anion-exchange chromatography in the same manner as described for vanadium. A 20 μL aliquot of H2O2 was added to a 1 mL aliquot from each fraction, and their absorbances were measured at 405 nm the absorbance peak for Ti(IV) complexes. 67 Cu Separation. First, each 100 mL sample from the NSCL was adjusted to 1.25 M solution of NH4OAc by adding 9.6 g of trace-metals-grade ammonium acetate and pH adjusted to 5 by adding ∼3 mL of trace-metals-grade acetic acid. The ∼100 mL solution was then passed through the chelation disk as previously described.1 The filter holder and disk were transferred to a clean vacuum flask, and 10 mL of 6 M tracemetal-grade hydrochloric acid was passed through to remove the 67Cu and other 2+ metal contaminants bound to the chelation disk. The 6 M HCl elution was then transferred to an anion-exchange column with AG1-X8 resin, the 10 mL eluate was collected, and then two additional 10 mL fractions of 6 M hydrochloric acid were passed through the column and collected. Next 10 mL of 2.5 M hydrochloric acid was added to the resin, and the eluate was collected. Finally, 10 mL of 0.5 M HCl was added to the resin and the eluate collected. All eluates were analyzed for radiochemical purity by observing the characteristic γ rays using an HPGe detector. Conjugation of NOTA-Bz-NCS to Panitumumab. Briefly, Panitumumab (Genentech) was conjugated to 5 mol equivalents of NOTA-Bz-NCS (Macrocyclics) in 0.1 M sodium 10325
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184.6, 300.2, and 393.5 keV). It is important to note that 67Ga was also present in the secondary beam that decays to the same daughter nuclide. Since greater than 99% of the gallium remains bound to the chelation disk and more than 99% of the copper elutes from the disk upon the addition of 6 M HCl, during the separation procedure, the contribution to the characteristic γ rays from 67Ga could be calculated and used to correct the total 67 Cu activity. Separation Chemistry Development and Validation. The theoretical beam contaminants delivered to the water cell and their decay products were modeled using the LISE++ code with the EPAX3 cross-sections and the Nucleonica decay engine, respectively.1,2 The theoretical contaminants, both radioactive and stable, at 3 days postirradiation are listed in Table 2. Preliminary experiments determined that >99% of the titanium, vanadium, and manganese would be separated from 67 Cu with the chelation disk and the first three 6 M HCl fractions. This likely resulted from the formation of TiO2, H2VO4−, Mn2+, and MnO217−19 in aqueous solutions at pH 5. Iron, likely existing as Fe2+, was found to elute from the column in the 0.5 M HCl fraction. The best separation achieved for cobalt and copper was the use of three 6 M HCl fractions, which removed ∼80% of the cobalt. Arsenic, in aqueous solutions at pH 5, exists both as the neutral compound H3AsO3 and the negatively charged complex H2AsO4−. These arsenic species were expected to pass through the chelation disk.20 Therefore, the separation strategy developed for the separation of 67Cu from the aqueous beam dump consisted of the use of the chelation disk, followed by anion exchange using three fractions of 6 M HCl, a 2.5 M HCl fraction which contained the desired product, and a 0.5 M HCl fraction. Separation of 67Cu from the Secondary Beam Samples. Figure 1 shows the separation profile of 67Cu from all of the contaminant isotopes in the secondary beam as measured by the HPGe analysis of the characteristic γ rays. The HPGe spectra were obtained immediately after each separation. Additional HPGe spectra taken for longer time intervals were obtained after the short-lived isotopes decayed to measure the
RESULTS Beam Collection. A 76 MeV/u unanalyzed 67Cu secondary beam with a purity of 2.9% was obtained by fragmenting a 76Ge Table 2. Contaminating Elements Present in the Secondary Beam Collections at 3 days Postbombardment, the Isotopes Used To Follow the Separation of Each Element, and the Initial and Final Elemental Contaminants to 67Cu Ratios contaminating element Ge As Ga Zn Ni Fe Cu Cr K Ca V Sc Mn Se Co
identifying isotopes 69
Ge As 72 Ga 69m Zn 57 Ni 59 Fe 67 Cu 51 Cr 43 K 47 Ca 48 V 46 Sc, 47Sc, 48 Sc 52 Mn 75 Se 58 Co 74
initial elemental contaminant to 67 Cu ratio
Final Elemental Contaminant to 67 Cu ratioa
30 24 5.48 5.48 3.55 2.58 1.87 1.13 0.81 0.81 0.42 0.39
BD BD 0.1096 BD BD BD 1.87 BD BD BD BD 0.0078
0.32 0.13 0.06
BD BD 0.018
a
BD = below detection. Contaminants included both radioactive and stable isotopes of each element. The identifying isotopes are those radioactive isotopes of each element that were measured by HPGe analysis.
primary beam and delivered to the S1 vault at the NSCL where the water target station was set up to collect the incoming secondary beam. Three different samples were collected for approximately 12 hours each. An aliquot of 1 mL from each of the collections was analyzed with an HPGe detector to measure the total 67Cu activity using the characteristic γ rays (91.3, 93.3,
Figure 1. Flowchart showing the chemical separation of the isotopes present in each of the collections. Labels a−i correspond to the spectra shown in Figure 2. 10326
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concentrations of long-lived isotopes. Figure 2 shows the HPGe spectra taken at each stage of the separation. Each γ ray was assigned to an isotope using the Lund/LBNL Nuclear Data Search database.21 In the case where different isotopes had γ rays that could not be resolved, other, less abundant γ rays were used to calculate the activity in order to obtain the contribution to the peak from each radioisotope. All measured activities were corrected for decay during analysis, and the results were decay-corrected to the end of bombardment. The identifying isotopes, theoretical amount of each element with respect to 67Cu, and the final elemental contamination to 67Cu ratios are listed in Table 2. The success of the chemistry was measured by the ratio of the final contaminant concentration vs the initial contaminant concentration. There are many cases where this ratio, the decontamination factor, was so large that no residual activity could be measured. The poorest decontamination factor was observed for cobalt, where the final solution could only be decontaminated by a factor of 3. The majority (74 ± 4%) of the 67Cu was obtained in the 2.5 M fraction with a radiochemical purity of >99%. The other contaminants present in the 67Cu fraction, as measured with a HPGe detector for 12 h and decay corrected to the end of the bombardment, were 58Co (0.07%), 48Sc (0.06%), 47Sc (0.06%), and 72Ga (0.30%). No additional radioactive contamination was detected in the 2.5 M HCl 67Cu fraction. The 67Cu fraction was brought to dryness by heating followed by reconstitution in 20 μL of 0.1 M HCl. 67 Cu-NOTA-Bz-Panitumumab Labeling and Biodistribution Studies. The immunoreactivity (measurement of the retained functionality of the antibody after conjugating with the NOTA chelate) was measured to be 97 ± 2%. NOTA-BzPanitumumab was radiolabeled with 67Cu at a final specific activity of 740 Bq/μg. Biodistribution studies at 5 days postinjection for 67Cu-NOTA-Bz-Panitumumab is shown in Figure 3. The %ID/g of the tumor was found to be 12.5 ± 0.7%, which is in very good agreement with results reported by Chang et al.11
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DISCUSSION Our results show that 67Cu was separable with high radiochemical purity from an aqueous secondary fragment beam that contained many contaminants. With the exception of Ga, Sc, and Co, we were able to remove all contaminants down to the limits of detection. An anti-EGFR antibody, Panitumumab, was successfully conjugated with NOTA-Bz-NCS without affecting its antigenbinding properties. The purified 67Cu extracted from the contaminants was then used to radiolabel the Panitumumab conjugate to obtain a biodistribution profile for 67Cu-NOTABz-Panitumumab in HCT-116 colon cancer xenografted mice as a proof-of-concept that the product obtained was chemically reactive and could potentially be used for targeted therapy. The tumor uptake of 67Cu-NOTA-Bz-Panitumumab was similar to that reported previously even though the specific activity was much lower. The injected amount of panitumumab in this study was 150 μg which is lower than the 1 mg used to block the EGFR receptors.11 Additionally, the 89Zr-Dfo-Panitumumab data are different from those of this study in that different tumor sizes between both studies could affect differences in tumor uptake due to vascular permeability.22,23 In this case, tumor uptake was similar; however more investigations are
Figure 2. HPGe spectra for each part of the separation strategy. Panels a−i correspond to the separation steps shown in Figure 1. The 2.5 M HCl fraction (g) is where 74 ± 4% of the 67Cu was eluted. The detector dead time for each measurement was kept below 5% to minimize the contribution from sum peaks.
Figure 3. Biodistribution of 67Cu-NOTA-Bz-Panitumumab in HCT116 colon cancer xenografts. The %ID/g of the tumor was measured to be 12.5 ± 0.7%.
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needed to determine the biological effects of different specific activities on tumor uptake. The effective specific activity of 67Cu-NOTA-Bz-Panitumumab was low (20 nCi/μg). Given that the elements in the secondary beam had one or more isotopes that could be used as a radiotracer or interfering activity, and the radiochemical purity of 67Cu was measured to be greater than 99%, it is suspected that the lower specific activity did not result from poor separation of the secondary beam. An aliquot of the separated 67Cu was shipped to Missouri University Research Reactor (MURR) for elemental analysis using inductively coupled plasma mass spectrometry, and it was found to contain parts per million quantities of nonradioactive copper and iron which would compete strongly for incorporation into NOTA thus lowering the specific activity. It is expected that these contaminants came from an outside source as the amount of contaminants in the beam would have produced picomolar quantities not the millimolar quantities observed via ICP-MS. Potential sources of contamination were the tantalum foil used as the entrance window to the liquid water target cell in addition to the glassware, vials, and chemicals used in the separation process. The maximum specific activity that can be expected from the proposed harvesting method is ∼400 mCi/ ug which is well within the realm of reported specific activities for 67Cu produced by Brookhaven24 and is likely to be suitable for preclinical and clinical applications. One limitation of the present study compared to future possibilities at FRIB was that the mixture of isotopes present in the secondary beam primarily consisted of elements located in period 4 of the periodic table. It is estimated that ∼60% of the primary beams to be run at FRIB will be uranium. Fragmentation and fission of the uranium beam will create isotopes of essentially all of the elements lower in atomic number than uranium. The additional contamination generated from uranium fragmentation will require additional separation steps and potentially further loss of 67Cu in the process. Future research will include the separation efficiency for 67Cu and other isotopes of interest from heavy-element beams along with decontamination factors for elements between selenium and uranium in the periodic table. Another limitation of the present study was the exclusion of mixed bed resin as a first step in the separation strategy. At the FRIB facility the cooling water will be circulated through columns containing mixed bed resins in order to control the level of radioactivity in the water. Isotopes of interest for applications research may have to be eluted from these resins. Future experiments will be also performed using the mixed bed resin that will be employed at FRIB.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the help and support from the A1900 staff and NSCL Operations group for providing the 67Cu secondary beam, the MSU Environmental Health and Safety group for their help and oversight, Dave Daugherty at Hope College for help with the end station, and Jon Elson and Tom Voller at Washington University in St. Louis, MO, USA, for help with the radiation detector system and shipping procedures, respectively. The studies presented in this work were conducted in the MIR Pre-Clinical Pet-CT Facility of the Washington University School of Medicine with the support from the Siteman Cancer Center Small Animal Imaging Core. This work was supported by the DOE Office of Science Grant DE-SC0007352 and in part by the NSF under Cooperative Agreement PHY-11-02511. T.M. and S.E.L. are also supported by the Department of Energy National Nuclear Security Administration under Award No. DE-NA0000979.
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CONCLUSIONS Cu-67 was obtained with high radiochemical purity, and the decontamination factors for elements from potassium through selenium were determined. Cu-67 was acquired in the proper chemical form to be used for radiolabeling molecular probes. These studies support the hypothesis that 67Cu can be harvested from the aqueous beam stop at FRIB for use in preclinical research.
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REFERENCES
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DOI: 10.1021/acs.analchem.5b02322 Anal. Chem. 2015, 87, 10323−10329
Article
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DOI: 10.1021/acs.analchem.5b02322 Anal. Chem. 2015, 87, 10323−10329
