Aspects of 6-[18F]fluoro-L-DOPA preparation. Deuterochloroform as a substitute solvent for Freon 11.

AIM: Replacement of the ecologically harmful solvent Freon 11 (CFCl(3)) by chloroform for the module-assisted preparation of 6-[(18)F]fluoro-L-DOPA based on the electrophilic radiofluorodestannylation of the precursor N-formyl-3,4-di-tert-butoxycarbonyloxy-6-(trimethylstannyl)-L-phenylalanine ethyl ester. MATERIALS, METHODS: The TRACERlab Fx FDOPA module (GE Medical Systems) was used for the preparation of 6-[(18)F]fluoro-L-DOPA. Cyclotron-produced [(18)F]F(2) gas (5 GBq) was passed through a cooled solution (5 degrees C) of the stannyl precursor (45 mg) in CDCl(3) (10 ml). After the [(18)F]fluorination step, HCl (2 ml, 6 mol/l) was added to the solution. Then the reaction mixture was heated at 80 degrees C for 5 min under vacuum to evaporate the chloroform. The hydrolysis to remove the protecting groups was completed by heating the closed reactor at 130 degrees C for 8 min. After cooling to 20 degrees C the reaction mixture was purified by HPLC with two polymer-based RP columns (PRP-1, 7 microm, 10 x 250 mm, Hamilton) using a solution of AcOH/AcONa (pH 4.7) as eluent. The 6-[(18)F]fluoro-L-DOPA fraction was collected and sterile filtrated. RESULTS: Three types of stabilised chloroform were tested for the radiofluorination of the precursor. Only by use of deuterochloroform stabilised with silver no significant losses of radioactivity were observed. Thus, 6-[(18)F]fluoro-L-DOPA purified by HPLC was obtained in decay-corrected radiochemical yields of 25+/-3%, ready for human use. CONCLUSION: CDCl(3) has proved to be a convenient solvent for the module-assisted preparation of 6-[(18)F]fluoro-L-DOPA. In this way the use of the polluting Freon 11 can be avoided.

Module-assisted synthesis of the bifunctional labelling agent N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]SFB).

The three-step radiosynthesis of N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]SFB) was adapted to a remotely controlled synthesis module. After optimization of the reaction conditions, the final [(18)F]SFB was obtained in decay-corrected radiochemical yields of 34-38% (related to [(18)F]fluoride; n=12) within a synthesis time of 68 min. The radiochemical purity was in the range of 93-96%.

Radiolabelling of isopeptide N epsilon-(gamma-glutamyl)-L-lysine by conjugation with N-succinimidyl-4-[18F]fluorobenzoate.

The isopeptide N(epsilon)-(gamma-glutamyl)-L-lysine 4 was labelled with 18F via N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB). A modified approach for the convenient synthesis of [18F]SFB was used, and [18F]SFB could be obtained in decay-corrected radiochemical yields of 44-53% (n = 20) and radiochemical purity >95% within 40 min after EOB. For labelling N(epsilon)-(gamma-glutamyl)-L-lysine with [18F]SFB the effects of isopeptide concentration, temperature, and pH were studied to determine the optimum reaction conditions. The coupling reaction was shown to be temperature and pH independent while being strongly affected by the isopeptide concentration. Using the optimized labelling conditions, in a typical experiment 1.3GBq of [18F]SFB could be converted into 447MBq (46%, decay-corrected) of [18F]fluorobenzoylated isopeptide within 45 min, including HPLC purification.

Radiochemical synthesis and tissue distribution of Tc-99m-labeled 7alpha-substituted estradiol complexes.

The diagnosis and staging of breast cancer could be improved by the development of radiopharmaceutical imaging agents that provide a noninvasive determination of the estrogen receptor (ER) status of tumor cells. Agents labeled with (99m)Tc would be especially valuable in this regard. In attempting to achieve this goal, we synthesized four (99m)Tc-labeled 7alpha-substituted estradiol complexes. One complex utilizes the "3+1" mixed ligand design to introduce the Tc metal, whereas the other three took advantage of the cyclopentadienyltricarbonylmetal (CpTM) design. The Tc moieties were attached to the 7alpha position of estradiol with a hexyl tether, a monoether tether, or a polyether tether. The corresponding rhenium compounds have binding affinities for the ER of 20-45% compared with estradiol. Radiochemical yields of the (99m)Tc-labeled compounds ranged from approximately 15% for the CpT-Tc complexes to 95% for the 3 + 1 inorganic complex. Tissue distribution studies in immature female rats showed low nonreceptor-mediated uptake in the target organs and high uptake in nontarget organs such as the liver and fat. These complexes represent the first time that estradiol has been labeled at the 7alpha position with (99m)Tc and provide a further refinement of our understanding of ligand structure-binding affinity correlations for the ER.

Synthesis of novel progestin-rhenium conjugates as potential ligands for the progesterone receptor.

To assist in the development of technetium-based radiopharmaceuticals that are useful for the diagnostic imaging of steroid receptor-positive breast tumors, we have synthesized a series of small-sized metal chelates according to 'n + 1' mixed-ligand, thioether-carbonyl and organometallic designs. In these preliminary investigations, rhenium was used as a model for the radioactive technetium. The metal chelates contain the rhenium metal in several oxidation states, being + 5, + 3, and + 1, and they were attached to 21-substituted progesterone derivatives. A competitive receptor-binding assay (rat uterine cytosol, 0 degrees C) was used to determine the binding affinity of these conjugates for the progesterone receptor. The highest affinity of 9% (RU5020 = 100%) was obtained with a '3 + 1' mixed-ligand complex, containing a NMe group as the central donor atom in the tridentate ligand part. This value reflects a relative binding affinity of 75% compared with the parent steroid progesterone.

Synthesis and binding affinities of new 17 alpha-substituted estradiol-rhenium "n + 1" mixed-ligand and thioether-carbonyl complexes.

The development of technetium and rhenium-based radiotracers for the steroid receptor system requires the use of suitable donor groups on the steroid to provide stable binding sites for the metal. Previous approaches have mainly exploited methods involving various N- and S-coordinating chelate systems or organometallic complexes. In this work, we have prepared several novel chelate systems attached to a series of 17 alpha-substituted estradiol derivatives and examined their binding to the estrogen receptor (ER). The neutral "n + 1" mixed-ligand and dithioether-carbonyl complexes that we prepared contain the metal in three oxidation states, +5, +3 or +1, attached to a 17 alpha-substituted estradiol derivative through a thiol group, an isocyanide group, or a dithioether unit, respectively. In our preliminary investigations, we used rhenium as a nonradioactive analog of the radionuclide technetium. All complexes synthesized were evaluated in a competitive radiometric receptor binding assay at 0 degree C and 25 degrees C to determine their relative binding affinities (RBA) to the ER (relative to 3,17 beta-estradiol, RBA = 100%). The complexes show binding affinities up to 23.4% at 0 degree C and 14.1% at 25 degrees C.