Deborah W. McCarthy

Efficient production of high specific activity 64Cu using a biomedical cyclotron.

Copper-64 (T 1/2 = 12.7 h) is an intermediate-lived positron-emitting radionuclide that is a useful radiotracer for positron emission tomography (PET) as well as a promising radiotherapy agent for the treatment for cancer. Currently, copper-64 suitable for biomedical studies is produced in the fast neutron flux trap (irradiation of zinc with fast neutrons) at the Missouri University Research Reactor. Access to the fast neutron flux trap is only possible on a weekly basis, making the availability of this tracer very limited. In order to significantly increase the availability of this intermediate-lived radiotracer, we have investigated and developed a method for the efficient production of high specific activity Cu-64 using a small biomedical cyclotron. It has been suggested that it may be possible to produce Cu-64 on a small biomedical cyclotron utilizing the 64Ni(p,n)64Cu nuclear reaction. We have irradiated both natural nickel and enriched (95% and 98%) Ni-64 plated on gold disks. Nickel has been electroplated successfully at thicknesses of approximately 20-300 mm and bombarded with proton currents of 15-45 microA. A special water-cooled target had been designed to facilitate the irradiations on a biomedical cyclotron up to 60 microA. We have shown that it is possible to separate Cu-64 from Ni-64 and other reaction byproducts rapidly and efficiently by using ion exchange chromatography. Production runs using 19-55 mg of 95% enriched Ni-64 have yielded 150-600 mCi of Cu-64 (2.3-5.0 mCi/microAh) with specific activities of 94-310 mci/microgram Cu. The cyclotron produced Cu-64 had been used to radiolabel PTSM [pyruvaldehyde bis-(N4-methylthiosemicarbazone), used to quantify myocardial, cerebral, renal, and tumor blood flow], MAb 1A3 [monoclonal antibody MAb to colon cancer], and octreotide. A recycling technique for the costly Ni-64 target material has been developed. This technique allows the nickel eluted off the column to be recovered and reused in the electroplating of new targets with an overall efficiency of greater than 90%.

Ex vivo cell labeling with 64Cu–pyruvaldehyde-bis(N4-methylthiosemicarbazone) for imaging cell trafficking in mice with positron-emission tomography

We have used copper-64-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (64Cu–PTSM) to radiolabel cells ex vivo for in vivo positron-emission tomography (PET) imaging studies of cell trafficking in mice and for eventual application in patients. 2-[18F]-Fluoro-2-deoxy-d-glucose (FDG) cell labeling also was evaluated for comparison. 64Cu–PTSM uptake by C6 rat glioma (C6) cells increased for 180 min and then stabilized. The labeling efficiency was directly proportional to 64Cu–PTSM concentration and influenced negatively by serum. Label uptake per cell was greater with 64Cu–PTSM than with FDG. However, both 64Cu–PTSM- and FDG-labeled cells showed efflux of cell activity into supernatant. The 64Cu–PTSM labeling procedure did not interfere significantly with C6 cell viability and proliferation rate. MicroPET images of living mice indicate that tail-vein-injected labeled C6 cells traffic to the lungs and liver. In addition, transient splenic accumulation of radioactivity was clearly detectable in a mouse scanned at 3.33 h postinfusion of 64Cu–PTSM-labeled lymphocytes. In contrast, the liver was the principal organ of tracer localization after tail-vein administration of 64Cu–PTSM alone. These results indicate that in vivo imaging of cell trafficking is possible with 64Cu–PTSM-labeled cells. Given the longer t1/2 of 64Cu (12.7 h) relative to 18F (110 min), longer cell-tracking periods (up to 24–36 h) should be possible now with PET.

Comparative studies of Cu-64-ATSM and C-11-acetate in an acute myocardial infarction model: ex vivo imaging of hypoxia in rats.

Copper labeled diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) is a promising agent for the imaging of hypoxic tissues. In the present study 64Cu(t1/2 = 12.8 h) labeled Cu-ATSM was used in combination with 11C (t1/2 = 20.3 min) labeled acetate as a regional perfusion marker to visualize hypoxic rat heart tissue in an acute left anterior descending (LAD) coronary artery occluded rat model using an ex vivo tissue slice imaging technique. 64Cu-ATSM was injected intravenously c.a. 10 min after occlusion and rats were sacrificed by cervical dislocation 10 min after injection. Carbon-11-acetate was injected 1 min before sacrifice to obtain a measure of blood flow. The heart was dissected, frozen, and cut into 1-mm thick slices with a gauged slicer, and 11C images were obtained with an electronic autoradiography instrument. After decay of 11C, 64Cu images were obtained in the same manner. In ischemic regions, where there was low 11C accumulation, 64Cu showed high accumulation when compared with normal regions. In rats with a large occlusion, the center of the ischemia did not show any accumulation of either 11C or 64Cu, indicating no blood supply. Cu-ATSM appears to be useful for the detection of hypoxia with contrast being observed at short times (10 min) postinjection.

Redox-active metal complexes for imaging hypoxic tissues: structure–activity relationships in copper(II) bis(thiosemicarbazone) complexes

Reduction potential and lipophilicity of the copper(II) bis(thiosemicarbazone) complexes can be independently controlled by alkylation in the diketone backbone and the N-termini of the ligand, allowing optimisation of radiopharmaceuticals strongly selective for hypoxic tissues.

Metal Radionuclides for PET Imaging

There are many possible metal-based positron-emitting radionuclides that can be utilized in positron tomography. At the present time, the most promising nuclides are copper, gallium, and yttrium. It is highly likely that radiopharmaceuticals labeled with these nuclides will be generally utilized in clinical practice over the next several years.

Non-standard isotope production and applications at Washington University

The positron emitting radionuclides, oxygen-15, nitrogen-13, carbon-11, and fluorine-18 have been produced at Washington University for many years utilizing two biomedical cyclotrons; a Cyclotron Corporation CS15 and a Japan Steel Works 16/8 cyclotron. In recent years we have become interested in the production of non-standard PET isotopes. We were initially interested in copper-64 production using the 64Ni(p,n)64Cu nuclear reaction, but now apply this technique to other positron emitting copper isotopes, copper-60 and copper-61. Copper-64 is being produced routinely and made available to other institutions. In 1999 over ten Curies of copper-64 were produced, making copper available to thirteen institutions, as well as research groups at Washington University. We are currently developing methods for the routine productions of other PET radioisotopes of interest, these include; bromine-76, bromine-77, iodine-124, gallium-66, and technetium-94m.

Accelerator production of copper-64 for pet and radiotherapy

Copper-64 is an intermediate-lived positron emitting radionuclide which is a useful radiotracer for positron emission tomography and a promising radiotherapy agent for cancer treatment. Copper-64 is currently reactor-produced, and because it can only be produced at a small number of reactor facilities, its availability is extremely limited. We have shown that high yields of 64Cu with high specific activity and radiochemical purity can be produced via the 64Ni(p,n)64Cu reaction using a 16 MeV cyclotron beam. An automated target and pneumatic transport line have been constructed and tested which allow production of over 500 mCi batches of 64Cu and delivery to a hot cell for processing. A technique for the rapid and efficient separation of 64Cu from the 64Ni target material and other reaction byproducts has been developed and the enriched target material has been recovered with an efficiency greater than 90%. These techniques can be extended to the production of other potentially useful isotopes, such as 60Cu, 61Cu and 67Cu.Copper-64 is an intermediate-lived positron emitting radionuclide which is a useful radiotracer for positron emission tomography and a promising radiotherapy agent for cancer treatment. Copper-64 is currently reactor-produced, and because it can only be produced at a small number of reactor facilities, its availability is extremely limited. We have shown that high yields of 64Cu with high specific activity and radiochemical purity can be produced via the 64Ni(p,n)64Cu reaction using a 16 MeV cyclotron beam. An automated target and pneumatic transport line have been constructed and tested which allow production of over 500 mCi batches of 64Cu and delivery to a hot cell for processing. A technique for the rapid and efficient separation of 64Cu from the 64Ni target material and other reaction byproducts has been developed and the enriched target material has been recovered with an efficiency greater than 90%. These techniques can be extended to the production of other potentially useful isotopes, such as 60C...