Convenient Preparation of No-Carrier-Added Technetium-99m

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Bioconjugate Chem. 1999, 10, 832−837

Convenient Preparation of No-Carrier-Added Technetium-99m Radiopharmaceuticals Using Solid-Phase Technology Robert Dunn-Dufault,†,‡ Alfred Pollak,† John R. Thornback,*,† and James R. Ballinger‡,§ Resolution Pharmaceuticals Inc., 6850 Goreway Drive, Mississauga, Ontario, Canada, L4V 1V7, Faculty of Pharmacy, University of Toronto, Ontario, Canada, Division of Experimental Therapeutics, Ontario Cancer Institute, Toronto, Ontario, Canada, and Division of Nuclear Medicine, University Medical Imaging Centre, Toronto, Ontario, Canada . Received March 17, 1999; Revised Manuscript Received June 14, 1999

A solid-phase technetium chelation chemistry was developed as a means of preparing 99mTc radiopharmaceuticals at high effective specific activity (HSA). Three peptidic N3S 99mTc ligands [mercaptoacetyl-Gly-Gly-Gly (MAG3), picolinyl-Ser-Cys-Gly-Thr-Lys-Pro-Pro-Arg (RP063), and dimethyl-Gly-Ser-Cys-Gly-Thr-Lys-Pro-Pro-Arg (RP128)] were used. The free thiol of Cys in each was attached to a series of commercially available amine-functionalized supports in a two-step process. The amine groups on the solid supports were converted to maleimide groups followed by the attachment of the 99mTc chelators through a thiol ether linkage with Cys. The optimized loading of the supports ranged 6-122 µmol/g support as determined by amino acid analysis. Each of the peptide-loaded supports (50-100 mg) was placed in either glass syringe vessels or disposable chromatography columns. Labeling with [99mTc]pertechnetate (200-800 MBq) in the presence of stannous gluconate was achieved at room temperature for 30-60 min or in a 100 °C water bath for 10 min. Up to 80% of the activity was eluted from the column with saline to give products with purity up to 99.8% as determined by HPLC. Amino acid analysis indicated as little as 100 pmol of peptide present in the 99mTc products, demonstrating that extremely high effective specific activity can be achieved without the need for purification.

INTRODUCTION

Most radiopharmaceutical kits contain a quantity of the ligand which is in vast excess of the amount of 99mTc complex formed (i.e. the effective specific activity is low), but this quantity of ligand is safe for administration to patients. For example, a kit for the preparation of a bonescanning agent might contain 10 mg of diphosphonate; thus, each dose would contain on the order of 1 mg, but this presents no hazard. However, with newer radiopharmaceuticals, such as bioactive peptides and receptorbinding agents, administration of milligram or in some cases even microgram quantities could produce pharmacological or toxic effects or could saturate the receptor sites of interest. A purification step may be required to separate the 99mTc chelate from the excess free ligand. Purification can be as simple as a syringe column or as complex as high-pressure liquid chromatography (HPLC). Alternatively, it may be possible to prepare the 99mTc chelate at sufficiently high effective specific activity (HSA) by starting with extremely large amounts of pertechnetate. Babich et al. have prepared 99mTc-labeled chemotactic peptides by both approaches (1). However, all of these procedures are unsuitable for routine use, being labor intensive, time consuming, and inefficient in yield with respect to the radiotracer, requiring special equipment, and involving higher radiation exposure. The term “specific activity” refers to the 99mTc content of a product relative to the total technetium present (i.e., * To whom correspondence should be addressed. Phone: (905) 677-0831. Fax: (905) 677-9595. E-mail: [email protected]. † Resolution Pharmaceuticals. ‡ University of Toronto. § Ontario Cancer Institute and University Medical Imaging Centre

Figure 1. Release of an N3S HSA 99mTc radiopharmaceutical from a solid phase. Unreacted ligand remains covalently bound. 99mTc plus 99Tc). Specific activity is determined by ingrowth time between elutions of the generator and decay time since elution. “Effective specific activity” refers to the 99mTc content per total mass present and is determined by the amount of ligand, 99mTc-labeled and free, in solution. We have developed a simple method of preparing 99mTc chelates at extremely high effective specific activities by making use of a solid-phase technology (Figure 1). Many of the new N2S2 and N3S chelators require that the sulfur atom(s) be protected prior to labeling. If the protecting group is attached to a solid support, only molecules which have been released by chelating 99mTc will be present in solution while the excess precursor remains covalently attached to the support. This labeling technology can be incorporated into a kit from which an

10.1021/bc990032f CCC: $18.00 © 1999 American Chemical Society Published on Web 08/03/1999

Solid-Phase Technetium-99m Labeling

Bioconjugate Chem., Vol. 10, No. 5, 1999 833

Table 1. Amine Functionalization of the Various Commercial Solid Supports type of support

abbreviation

amino groups (µmol/g)

controlled pore glass silica acrylic copolymer agarose glass argogel

CPG SIL ACR AGR GLA ARG

100 200 50 300 50 400

HSA product can be eluted simply and safely in a process amenable to preparation of a sterile product. We have evaluated this approach using a variety of solid supports and demonstrated its utility in the preparation of three 99mTc radiopharmaceuticals: the renal imaging agent MAG3 (2) and two peptides for imaging inflammation, RP063 and RP128 (3, 4). EXPERIMENTAL PROCEDURES

Materials. Affi-Gel 102 (amine-functionalized crosslinked agarose) and Affi-Prep (N-hydroxysuccinimidefunctionalized acrylic polymer), Bio-Spin polypropylene chromatography columns (sterile with filter, 1.5 mL capacity) and accessories were purchased from Bio-Rad Laboratories (Hercules, CA). 3-Aminopropyl-functionalized silica and glass were purchased from Sigma-Aldrich Canada (Oakville, ON). Amine-functionalized controlledpore glass was purchased from CPG Inc. (Fairfield, NJ). Argogel [amine-functionalized poly(ethylene glycol)grafted polystyrene] was obtained from Argonaut Technologies Inc. (San Carlos, CA). The amine functionalization of each support is reported in Table 1. Radioactivity was measured in a Capintec CRC-15R dose calibrator using the 99mTc setting. 1H/13C NMR spectra were recorded on a Bruker 300AC spectrometer. Electrospray mass spectra (ESMS) were obtained on a Sciex API#3 mass spectrometer in the positive ion mode. Amino acid analysis (AAA) was conducted on a Waters Picotag system at the Banting Institute (Toronto, ON). Analytical RP-HPLC of each of the 99mTc-labeled products was run on a Vydac reversed-phase C-18 protein and peptide column (5 µm, 0.46 cm × 25 cm) using a Beckman Instruments (Fullerton, CA) system employing both radiometric detection set on a 99mTc window (Beckman 171 radioisotope detector with a modified yttrium silicate scintillant) and UV absorbance detection (Beckman 168 UV detector set at 215 nm wavelength). The column was eluted using a linear gradient of 100% A to 90% B over 20 min at a flow rate of 1.5 mL/min where solvent A was 0.1% trifluoroacetic acid (TFA) in water and solvent B was 0.1% TFA in acetonitrile. Reagents for peptide synthesis including solvents, resins, and protected amino acids were obtained from Applied Bioresearch Inc. (Foster City, CA) and Bachem Biosciences Inc. (Philadelphia, PA). Buffers, inorganic salts, and reagent chemicals were purchased from SigmaAldrich Canada. HPLC-grade acetonitrile was purchased from BDH Inc. (Toronto, ON). Solutions of stannous chloride (90 mM, 20 mg/mL in 1 N HCl) and sodium gluconate (60 mM, 13 mg/mL in Milli-Q water) used for radiolabeling were prepared in advance and were stored frozen until use. 99mTc in the form of sodium pertechnetate solution was obtained from Mallinckrodt Medical Inc. (Mississauga, ON) or from a DuPont 99Mo/99mTc generator. Syntheses of Peptides. Three different peptidic N3S technetium chelators were prepared to explore the universality of the solid-phase labeling method outlined.

MAG3 (mercaptoacetyl-Gly-Gly-Gly-COOH) was synthesized with a benzoyl-protected sulfur whereas RP063 (picolyl-Ser-Cys-Gly-Thr-Lys-Pro-Pro-Arg-COOH) and RP128 (N,N-dimethyl-Gly-Ser-Cys-Gly-Thr-Lys-Pro-ProArg-COOH) were prepared with acetamidomethyl (Acm) protection on the Cys. Each was synthesized using standard 9-fluorenylmethoxycarbonyl (Fmoc) chemistry starting with the carboxy-terminal amino acid (Fmoc-Gly or Fmoc-Arg) preloaded on Sasrin resin (Bachem Biosciences Inc.) using an ABI 433A automated peptide synthesizer. The products were cleaved from the resin in 95% TFA (100 mg of resin/mL) at room temperature for 2 h. The resin was removed by filtration and the peptides were precipitated by dripping the supernatant into 50 mL of tert-butyl methyl ether at 5 °C. The solids were centrifuged to a pellet, the ether decanted, and the pellet dried in vacuo. The pellets were dissolved in water and lyophilized to white powders. MAG3, RP063, and RP128 were purified by reversedphase HPLC on a Beckman Instruments HPLC system (System Gold) using a Millipore-Waters radial compression module (RCM, 0.8 cm × 10 cm) C-18 column with 0.1% TFA in water as solvent A and 0.1% TFA in acetonitrile as solvent B. The column was eluted using a linear gradient of 100% A to 100% B over 25 min at a flow rate of 1 mL/min. For each peptide, the fractions containing the correct species were pooled and lyophilized to a white powder. The authenticity of each product was confirmed by mass spectral analysis. benzoyl-MAG3: MW 367.0. ESMS: 368.0 (MH+). RP063: MW 1020.1. ESMS: 1021.1 (MH+). RP128: MW 1000.2. ESMS: 1001.0 (MH+). Free-thiol MAG3 was obtained by hydrolysis of the benzoyl sulfur-protecting group. Care was taken to exclude oxygen where possible. Free-thiol RP063 and RP128 were prepared by removal of the Acm protecting group. To the Acm-protected peptide (120 mmol) in aqueous acetic acid (30%, 2 mL) purged with argon was added mercuric acetate (180 mmol). The reaction was stirred under argon for 2.5 h at room temperature. Water (10 mL) was added and hydrogen sulfide gas was bubbled through the solution where a black precipitate formed. The black suspension was centrifuged to a clear solution and a black pellet. Each solution was decanted and filtered through a 0.2 µm PVDF syringe filter. The solutions were frozen in liquid nitrogen and lyophilized to white powders. Products were analyzed by HPLC and were typically >95% pure with traces of the Acmprotected species. Correct species were also identified by mass spectral analysis. Loading of Supports. Each of the supports, Affi-Gel 102 (AGR), silica (SIL), controlled-pore glass (CPG), glass (GLA), and Argogel (ARG) were commercially available with amine functionalization. Affi-Prep 10 acrylic polymer (ACR) was obtained N-hydroxysuccinimide-functionalized and required conversion to amine functionalization by reaction with 1,4-diaminobutane in DMF. Carbethoxymaleimide was synthesized by a literature method (5). The maleimide-functionalized supports were prepared as follows. To the amine-functionalized support (1 g) in saturated sodium bicarbonate (4 mL) at 0 °C was added powdered carbethoxymaleimide (100 mg). The reaction was shaken for 15 min after which water (15 mL) was added. The reaction was continued at room temperature for 15 min. The support was filtered and washed copiously with water. The MAG3- and RP063-loaded supports were prepared by reacting the corresponding maleimide-functionalized

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Figure 2. Synthetic scheme for loading MAG3 to a solid support. R ) ethyl or methyl. Table 2. Loading of MAG3, RP063, and RP128 to Solid Supports and 99mTc-Labeling Results support type CPG AGR ACR SIL GLA ARG

N 3S

N3S loading (µmol/g)

radiochemical yield (%)

radiochemical purity (%)

MAG3 RP063 RP128 MAG3 RP063 RP128 MAG3 RP063 RP128 MAG3 RP063 RP128 RP128 RP128

9 9 12 122 100 55 21 29 6 78 7 19 14 28

56 13 37 60 41 76 33 20 40 81 52 25 69 65

93 60 87 81 86 99 97 85 75 97 45 8 99 97

support (1 g) at 0 °C with an argon-purged solution of MAG3-SH (15 mg) or RP063-SH (25 mg) in sodium bicarbonate (0.2 N, 4 mL). After 30 min of shaking the solution, the reaction was allowed to warm to room temperature. The reaction was shaken for 3-16 h, after which the support was filtered, washed copiously with water and then an organic solvent (acetone, chloroform, dichloromethane, or diethyl ether), and dried in vacuo. Loading was determined by AAA, and the results are presented in Table 2. The RP128-loaded supports were prepared by a slightly different procedure. To each amine-functionalized support (1 g) in saturated sodium bicarbonate (3 mL) at 0 °C was added powdered carbethoxymaleimide (30 mg). The reaction mixture was shaken for 30 min and then water (5 mL) was added. The shaking was continued at room temperature for 30 min. The support was then filtered and washed copiously with water and methanol and dried in vacuo. To each support (300 mg) was added an argon-purged solution of RP1280-SH (10 mg) in phosphate-buffered saline (PBS, 0.02 N, 1 mL). After 1 h at room temperature, each support was filtered, washed with copious water and then methanol, and dried in vacuo. Each was analyzed by AAA, and results are reported in Table 2. 99mTc Labeling of Supports. Typically, to each peptide-loaded support (50-100 mg) in a glass syringe barrel or a Bio-Spin disposable chromatography column were added saline (100 µL), [99mTc]pertechnetate (200-800 MBq, 100 µL), and aqueous stannous gluconate (100 µL) containing 20 µg of stannous chloride and 1.3 mg of sodium gluconate. The reaction mixture was capped,

swirled, and left to proceed for 30-60 min at room temperature or for 10 min in a 100 °C water bath. The cap was removed and the column was fitted with a luerlock stopcock and needle. Three portions of saline (300 µL, 1 mL, and 1 mL) were added sequentially, and the dose eluted from the column into three evacuated vials. The activity of each was measured in a dose calibrator. An aliquot from fraction 1 was analyzed by HPLC. The balance of fraction 1 of each labeling was submitted for AAA in order to confirm that the dose liberated from the support was free of excess unlabeled peptide. For comparison, standard solution-phase low effective specific activity (LSA) labeling was performed as described elsewhere (2-4), and the products were analyzed by HPLC. RESULTS

Each of the support types used in this study was loaded with the free-thiol peptides by the methods outlined for MAG3 in Figure 2. Each of the three peptides was conveniently synthesized by an automated solid-phase peptide synthesizer in high yields and with typical purity of >95% based on HPLC analysis of the cleaved products. As appropriate, the sulfur-protecting groups were removed in preparation for loading to the supports. The supports were first rapidly functionalized with maleimide under mild conditions in buffered aqueous solutions, typically for 1 h. The maleimide-functionalized supports were then quenched with the thiol-containing peptides, again under mild aqueous conditions. The loading of each, reported in Table 2, was determined by AAA and ranged 6-122 µmol/g. These values represent the final loading of the peptides but do not indicate at which step the variability occurred. The final evaluation of the technology is the labeling step with 99mTc. The labeling of MAG3, RP063, and RP128 from each of the solid supports was evaluated with respect to three parameters: radiochemical yield, radiochemical purity, and the level to which the eluted fractions were free of excess unlabeled peptides. Optimal labeling was observed with RP128 loaded to Argogel and is shown in Figure 3, where it can be seen that >96% of the injected radioactivity elutes in a peak at 8.5 min (solid line) and that there is no UV absorbance associated with this peak (dotted line). To confirm that the dose liberated from the support was free of excess unlabeled peptide, the first eluted fraction from each support was tested by amino acid analysis. In each case, AAA showed only background levels of the amino acids found in MAG3, RP063, or

Solid-Phase Technetium-99m Labeling

Figure 3. HPLC radiometric trace of a HSA [99mTc]RP128 labeling from Argogel support. Injection of 50 µL onto a C-18 column (5 µm, 0.46 cm × 25 cm). The column was eluted using a linear gradient of 100% A to 90% B over 20 min at a flow rate of 1.5 mL/min where solvent A was 0.1% TFA in water and solvent B was 0.1% TFA in acetonitrile. The system was run with dual detection: 99mTc (solid line), UV detection at 215 nm (dotted line).

RP128. These values ranged from 100 pmol to 1 nmol per sample and were a function of contaminants on the detection column. Accordingly, these background levels were used in estimation of effective specific activity. Since the various supports have different adsorption properties, each was washed thoroughly to minimize any noncovalent peptide binding to the support. As supporting evidence, the UV traces of the HPLC-analyzed products showed essentially no peak corresponding to unlabeled ligands. Typical UV traces of LSA and HSA formulations of [99mTc]RP128 are shown in Figure 4, where it can be seen that the major peak at 8.0 min in the LSA preparation (solid line), representing unlabeled Acm-protected RP128, is absent in the HSA product (dotted line). The radiochemical yield and radiochemical purity of the three peptides on each of the supports is shown in Table 2. For most of these measurements, n ) 1; however, in subsequent work, the yield and purity were found to be reproducible, with a typical coefficient of variation of 90% purity is required) while a yield which is only moderate may still be acceptable. The preparation of the supports involves a two-step loading process which results in a loading value for each ligand loaded to each support type tested. The loading of the ligand to the supports varied greatly, from 6 to 122 µmol/g, despite similar loading conditions. The loading differences are in part due to differences in the level of amine functionalization on the supports (Figure 2) and in the reactivity of the amines with carbethoxymaleimide under the given conditions in the first step. Discrepancies between the loading levels of the ligands and their effective labeling suggests that simple AAA indicates the amount of total ligand bound to the support but not necessarily the amount of ligand that is structurally available to coordinate 99mTc. For example, RP128loaded silica gave 19 µmol/g loading but a meager 8% radiochemical purity upon 99mTc labeling, whereas RP063 loaded to silica showed only 7 µmol/g loading with a 45% radiochemical purity of [99mTc]RP063. This indicates the interaction of the ligand with the support greatly affects the success of the labeling. In addition, the data show a large variation in the recovered yields from the supports, ranging from 13% to as high as 81% of the dose in the collected fractions. The loss of a high percentage of the dose is undesirable in that the use of higher activities would be required to generate a suitable dose for patient use. This binding to the supports was found to be partly physical trapping of colloids, partly ionic interaction of 99mTc species with the support, and potentially some weak coordination with the vast excess of ligand still covalently attached to the supports. Recoveries from agarose and Argogel were both influenced by the length of time the supports were soaked in saline before elution, indicating diffusion out of the beads played a role in dose recovery. Ultimately, it appears that the interaction of the ligand with the supports during loading and both of these with 99mTc species during coordination with 99mTc dictates the purity of the 99mTc complex and the yield recovered. Optimization of the loading of each ligand to each type of support would be required in order to make strong evaluative comparisons. However, in general, supports with the highest amine functionalization, with the least possibility for unwanted charge interaction, and which best approximate solution-like conditions, should provide the best labeling results with the highest recovered yields. For these reasons, Argogel or a similar PEGylated support shows great potential, provided that there is no release of PEG. Initially, the supports studied were chosen for their commercial availability with amine functionalization. The mechanical and surface properties of the supports vary greatly. Silica, glass, and acrylic are rigid supports. Silica and glass have charged surfaces where acrylic is neutral. Cross-linked agarose and PEG Argogel are flexible and have gel characteristics. Agarose is 95%

Dunn-Dufault et al.

water when swollen, has moderate mechanical stability, and completely dissolves in extremes of pH, whereas Argogel swells slightly and equally well in aqueous and organic solvents, with high mechanical and chemical stability, although release of trace amounts of PEG may be a problem. With the rapid pace of developments in solid-phase chemistry, the type and quality of supports are becoming broader and the tailoring of supports to specific needs is or will soon be a possibility. The ability to use supports with high mechanical and chemical stability, coupled with a diversity of commercially available linking groups, makes a solid-phase radiopharmaceutical kit design a very real possibility. ACKNOWLEDGMENT

We thank Tam Nguyen for assistance in peptide synthesis and Resolution Pharmaceuticals Inc. for supporting this work. LITERATURE CITED (1) Babich, J. W., Solomon, H., Pike, M. C., Kroon, D., Graham, W., Abrams, M. J., Tompkins, R. G., Rubin, R. H., and Fishman, A. J. (1993) Technetium-99m-labeled hydrazino nicotinamide derivatized chemotactic peptide analogs for imaging focal sites of bacterial infection. J. Nucl. Med. 34, 1964-1974. (2) Fritzberg, A. R., Kasina, S., Eshima, D., and Johnson, D. L. (1986) Synthesis and biological evaluation of technetium99m MAG3 as a hippuran replacement. J. Nucl. Med. 27, 111-116. (3) Goodbody, A., Ballinger, J., Tran, L., Sumner-Smith, M., Lau, F., Meghi, K., and Pollak, A. (1994) A new Tc-99m labelled penta-peptide inflammation imaging agent. Eur. J. Nucl. Med. 21, 870. (4) Caveliers, V., Goodbody, A., Tran, L., Bossuyt, A., and Thornback, J. (1996) Human dosimetry of TC99m-RP128, a potential inflammation imaging agent. Eur. J. Nucl. Med. 23, 1131. (5) Bodanszky, M., and Bodanszky, A. (1984) The Practice of Peptide Synthesis, Springer-Verlag, New York. (6) Lamson, M. L., Kirschner, A. S., Hotte, C. E., Lipsitz, E. L., and Ice, R. D. (1975) Generator-produced 99mTcO4-: carrier free? J. Nucl. Med. 16, 639-641. (7) Del Rosario R. B., Jung, Y. W., Baidoo, K. E., Lever, S. Z., and Wieland, D. M. (1994) Synthesis and in vivo evaluation of a 99m/99Tc-DADT-benzovesamicol: a potential marker for cholinergic neurons. Nucl. Med. Biol. 21, 197-203. (8) DiZio, J. P., Fiaschi, R., Davison, A., Jones, A. G., and Katzenellenbogen, J. A. (1991) Progestin-rhenium complexes: metal-labeled steroids with high receptor binding affinity, potential receptor-directed agents for diagnostic imaging or therapy. Bioconjugate Chem. 2, 353-66. (9) DiZio, J. P., Anderson, C. J., Davison, A., Ehrhardt, G. J., Carlson, K. E., Welch, M. J., and Katzenellenbogen, J. A. (1992) Technetium- and rhenium-labeled progestins: synthesis, receptor binding and in vivo distribution of an 11 betasubstituted progestin labeled with technetium-99 and rhenium186. J. Nucl. Med. 33, 558-569. (10) Hirai, H. (1990) Use of tumor receptors for diagnostic imaging. A review. Acta Radiol. (Suppl.) 374, 57-64. (11) Kung, H. F., Kim, H. J., Kung, M. P., Meegalla, S. K., Plossl, K., and Lee, H. K. (1996) Imaging of dopamine transporters in humans with technetium-99m TRODAT-1. Eur. J. Nucl. Med. 23, 1527-1530. (12) Kung, H. F. (1991) Overview of radiopharmaceuticals for diagnosis of central nervous disorders. Crit. Rev. Clin. Lab. Sci. 28, 269-86. (13) O’Neil, J. P., Carlson, K. E., Anderson, C. J., Welch, M. J., and Katzenellenbogen, J. A. (1994) Progestin radiopharmaceuticals labeled with technetium and rhenium: synthesis, binding affinity, and in vivo distribution of a new progestin N2S2-metal conjugate. Bioconjugate Chem. 5, 182-193.

Solid-Phase Technetium-99m Labeling (14) Pearson, D. A., Lister-James, J., McBride, W. J., Wilson, D. M., Martel, L. J., Civitello, E. R., and Dean, R. T. (1996) Thrombus imaging using technetium-99m-labeled highpotency GPIIb/IIIa receptor antagonists. Chemistry and initial biological studies. J. Med. Chem. 39, 1372-1382. (15) Lever, S. Z., Baidoo, K. E., Mahmood, A., Matsumura, K., Scheffel, U., and Wagner, H. N. (1994) Novel technetium ligands with affinity for the muscarinic cholinergic receptor. Nucl. Med. Biol. 21, 157-164. (16) Kao, C. H., Baidoo, K. E., and Lever, S. Z. (1997) Validation and application of a solid-phase chemistry method for prepa-

Bioconjugate Chem., Vol. 10, No. 5, 1999 837 ration of high effective specific activity technetium-99m radiopharmaceuticals. Nucl. Med. Biol. 24, 499-505. (17) Flanagan, R. J., Wilson, J. S., and Wiebe, L. I. (1986) A high speed, no-carrier-added radiochemical reactor for the synthesis of halogenated radiopharmaceuticals. J. Labelled Compd. Radiopharm. 23, 1242-1243. (18) Zhu, X., Gobi, J., and Hunter, D. H. (1998) A convenient preparation of no-carrier-added MIBG via solid-phase organic chemistry. J. Nucl. Med. 39, 143P.

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