Bioconjugate Chem. 2006, 17, 223−225
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TECHNICAL NOTES Novel and Efficient Preparation of Precursor [188Re(OH2)3(CO)3]+ for the Labeling of Biomolecules Sang Hyun Park,†,* Sepp Seifert,‡ and Hans-Jurgen Pietzsch‡ Division of Radioisotope Production and Application, Korea Atomic Energy Research Institute, Daejeon, Republic of Korea, and Forschungszentrum Rossendorf, Institute for Bioinorganic and Radiopharmaceutical Chemistry, PF 510 119, D-01314 Dresden, Germany. Received June 30, 2005; Revised Manuscript Received September 8, 2005
A novel and efficient method for preparing 188Re(I) tricarbonyl precursor [188Re(OH2)3(CO)3]+ has been developed by reacting [188Re]perrhenate with Schibli’s kit in the presence of borohydride exchange resin (BER) as a reducing agent and an anion scavanger. The precursor was produced in more than 97% yield by reacting a solution of tetrahydroborate exchange resin (BER, 3 mg), borane-ammonia (BH3‚NH3, 3 mg), and potassium boranocarbonate (K2[H3BCO2], 3 mg) in 0.9% saline with a solution of sodium perrhenate (Na188ReO4) with up to 50 MBq and concentrated phosphoric acid (85%, 7 µL) at 60 °C for 15 min. HPLC and TLC revealed 0% unreacted [188Re]perrhenate ion and 85% of the desired precursor complex, remaining perrhenate (7(3%), colloidal 188ReO2 (97% determined by means of HPLC and TLC. 188Re(I) Tricarbonyl Histidine (2). A 10 mL vial containing histidine (500 µL) was capped with a rubber stopper. A solution of 800 µL of [188Re(OH2)3(CO)3]+ with up to 50 MBq was added into the vial by a 1 mL syringe. The reaction vial was sealed and heated to 75 °C for 30 min. After 30 min, the reaction mixture was cooled to room temperature (Scheme 1). Yield: >97% determined by means of HPLC and TLC. Tetrahydroborate Exchange Resin (BER). The tetrahydroborate exchange resin (BER) as a reducing agent was prepared by the reported method. Chloride-form resin (Amberlite ionexchange resin, 12.5 g) was slurry-packed with water into a 30-mL fritted glass funnel mounted on a filter flask. Then, an aqueous sodium tetrahydroborate solution (200 mL, 0.25 M) was slowly passed through the resin over a period of 30 min. The resulting resins were washed thoroughly with distilled water
Figure 1. HPLC chromatograms of
Figure 2. HPLC chromatogram of
188
Re(I) Tricarbonyl Histidine (2)
until free of excess and finally with ethanol. The tetrahydroborate-form anion-exchange resin was then partially air-dried by removing ethanol on the surface of the BER. This resin was analyzed for its tetrahydroborate content by hydrogen evolution upon an acidification with 0.08 M HCl, and the average capacity of BER was found to be 2.5 mequiv of tetrahydroborate ion per gram.
RESULTS The HPLC chromatogram of 188Re(I) tricarbonyl precursor 1 and 188ReO4- showed that retention times of those species are 4.7 and 9.8 min, respectively. The retention time of the 188Re(I) tricarbonyl precursor was compared to that of 99mTc(I) tricarbonyl precursor and they were found to be identical. The radiolabeling yield of 188Re(I) tricarbonyl precursor in the reaction mixture was determined by the HPLC analysis and found to be 100%. The HPLC chromatogram of 188Re(I) tricarbonyl precursor in the reaction mixture is shown in Figure 1. The complex is stable (>95%) for approximately 3 h. After this time, decomposition of the complex was observed. Paper electrophoresis investigations in aqueous solution confirmed the cationic charge of 188Re(I) tricarbonyl precursor in a neutral solution. The assay for the formation of the 188Re(I) tricarbonyl precursor, reduced hydrolyzed 188Re, and [188Re]perrhenate ion
188
Re(I) tricarbonyl precursor and Na188ReO4.
Re(I) tricarbonyl histidine (2).
Bioconjugate Chem., Vol. 17, No. 1, 2006 225
Technical Notes
LITERATURE CITED
Figure 3. BER as a solid-phase reducing agent and an anion scavenger.
was achieved by investigating their positions using an instant thin-layer chromatography (ITLC). 188Re(I) tricarbonyl precursor: >95% (Rf ) 0.4); reduced hydrolyzed 188Re: less than 3% (origin); [188Re]perrhenate ion: 0% (Rf ) 0.8). The HPLC chromatogram of 188Re(I) tricarbonyl histidine showed that the retention time of the complex is 11.4 min. The radiolabeling yield of 188Re(I) tricarbonyl histidine in the reaction mixture was determined by the HPLC analysis and found to be >97%. The HPLC chromatogram of 188Re(I) tricarbonyl histidine (2) in the reaction mixture is shown in Figure 2. The retention time of the 188Re(I) tricarbonyl histidine was compared to that of 99mTc(I) tricarbonyl histidine and they were found to be identical.
DISCUSSION Tetrahydroborate exchange resin (BER) contains tetrahydroborate ion (BH4-) bound to the cation which is supported on polystyrene matrix, and the cation has a quaternary alkylammonium functionality used for adhering the tetrahydroborate ion or negatively charged species. BER is shown in Figure 3. According to the present study, employing the borohydride exchange resin (BER) as a novel reducing agent and an anion scavenger, 188Re(I) tricarbonyl precursor having high radiochemical purity and labeling efficiency can be prepared without nitrogen gas flushing and ice bath cooling compared to the conventional method (9). The BER is advantageous in terms of being stable over a wide range of pH (2-11) and applicable to biologically active molecules, as well as being easily removable through filtration when being administrated (10), thus providing the potential to economically and effectively produce 188Re(I) tricarbonyl precursor without the formation of unreacted 188ReO -, colloidal 188ReO , and negatively charged impurities 4 2 by reinforcing the conventional reducing agent requiring very stringent condition for preparation.
ACKNOWLEDGMENT This work was supported by the Nuclear R & D Program of the Korean Ministry of Science and Technology and the Technical Cooperation Program of the International Atomic Energy Agency. We would like to thank K. Landrock for her assistance in preparing HPLC, ITLC, and electrophoresis.
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