Synthesis, radiolabeling, and biological evaluation of (R)- and (S)-2-amino-3-[(18)F]fluoro-2-methylpropanoic acid (FAMP) and (R)- and (S)-3-[(18)F]fluoro-2-methyl-2-N-(methylamino)propanoic acid (NMeFAMP) as potential PET radioligands for imaging brain tumors.

The non-natural amino acids (R)- and (S)-2-amino-3-fluoro-2-methylpropanoic acid 5 and (R)- and (S)-3-fluoro-2-methyl-2-N-(methylamino)propanoic acid 8 were synthesized in shorter reaction sequences than in the original report starting from enantiomerically pure (S)- and (R)-alpha-methyl-serine, respectively. The reaction sequence provided the cyclic sulfamidate precursors for radiosynthesis of (R)- and (S)-[(18)F]5 and (R)- and (S)-[(18)F]8 in fewer steps than in the original report. (R)- and (S)-[(18)F]5 and(R)- and (S)-[(18)F]8 were synthesized by no-carrier-added nucleophilic [(18)F]fluorination in 52-66% decay-corrected yields with radiochemical purity over 99%. The cell assays showed that all four compounds were substrates for amino acid transport and enter 9L rat gliosarcoma cells in vitro at least in part by system A amino acid transport. The biodistribution studies demonstrated that in vivo tumor to normal brain ratios for all compounds were high with ratios of 20:1 to115:1 in rats with intracranial 9L tumors. The (R)-enantiomers of [(18)F]5 and [(18)F]8 demonstrated higher tumor uptake in vivo compared to the (S)-enantiomers.

Synthesis, radiosynthesis, and biological evaluation of fluorine-18-labeled 2beta-carbo(fluoroalkoxy)-3beta-(3'-((Z)-2-haloethenyl)phenyl)nortropanes: candidate radioligands for in vivo imaging of the serotonin transporter with positron emission tomography.

The meta-vinylhalide fluoroalkyl ester nortropanes 1-4 were synthesized as ligands of the serotonin transporter (SERT) for use as positron emission tomography (PET) imaging agents. In vitro competition binding assays demonstrated that 1-4 have a high affinity for the SERT (K(i) values = 0.3-0.4 nM) and are selective for the SERT over the dopamine and norepinephrine transporters (DAT and NET). MicroPET imaging in anesthetized cynomolgus monkeys with [(18)F]1-[(18)F]4 demonstrated that all four tracers behave similarly with peak uptake in the SERT-rich brain regions achieved after 45-55 min, followed by a steady washout. An awake monkey study was performed with [(18)F]1, which demonstrated that the uptake of [(18)F]1 was not influenced by anesthesia. Chase studies with the SERT ligand 15 displaced [(18)F]1-[(18)F]4, but chase studies with the DAT ligand 16 did not displace [(18)F]1-[(18)F]4 thus indicating that the tracers were binding specifically to the SERT.

Synthesis and biological evaluation of trans-3-phenyl-1-indanamines as potential norepinephrine transporter imaging agents.

The development of radioligands suitable for studying the central nervous system (CNS) norepinephrine transporter (NET) in vivo will provide important new tools for examining the pathophysiology and pharmacotherapy of a variety of neuropsychiatric disorders including major depression. Towards this end, a series of trans-3-phenyl-1-indanamine derivatives were prepared and evaluated in vitro. The biological properties of the most promising compound, [(11)C]3-BrPA, were investigated in rat biodistribution and nonhuman primate PET studies. Despite high in vitro affinity for the human NET, the uptake of [(11)C]3-BrPA in the brain and the heart was not displaceable with pharmacological doses of NET antagonists.

Synthesis and biological evaluation of [11C]talopram and [11C]talsupram: candidate PET ligands for the norepinephrine transporter.

PET and SPECT ligands for the norepinephrine transporter (NET) will be important tools for studying the physiology, pathophysiology and pharmacology of the CNS noradrenergic system in vivo. A series of candidate NET ligands were synthesized and characterized in terms of their affinity for human monoamine transporters. The two most promising compounds, talopram and talsupram, were radiolabeled with carbon-11 and evaluated through biodistribution studies in rats and PET imaging studies in a rhesus monkey. Although both compounds displayed high affinity and selectivity for the human NET in vitro, these compounds did not enter the CNS in adequate amounts to be used in PET imaging studies.

Synthesis, radiosynthesis, and biological evaluation of carbon-11 and iodine-123 labeled 2beta-carbomethoxy-3beta-[4'-((Z)-2-haloethenyl)phenyl]tropanes: candidate radioligands for in vivo imaging of the serotonin transporter.

2beta-Carbomethoxy-3beta-[4'-((Z)-2-iodoethenyl)phenyl]tropane (ZIET) and 2beta-carbomethoxy-3beta-[4'-((Z)-2-bromoethenyl)phenyl]tropane (ZBrET) were synthesized as well as their nortropane congeners ZIENT and ZBrENT. Binding affinities of these compounds were determined in cells transfected to express human SERT, DAT, and NET using [3H]citalopram, [125I]RTI-55, and [3H]nisoxetine, respectively. Both ZIET and ZBrET displayed high affinity for the SERT (Ki = 0.11 and 0.08 nM, respectively). The affinities of ZIET and ZBrET for the DAT were 200 and 38-fold lower, respectively, than for the SERT. [11C]ZIET and [11C]ZBrET were prepared by alkylation of their corresponding nortropanes with [11C]methyl iodide in approximately 30% radiochemical yield (decay-corrected to end of bombardment, EOB). High specific activity [123I]ZIET was synthesized in 33% radiochemical yield (decay-corrected) by treating the 2beta-carbomethoxy-3beta-[4'-((Z)-2-trimethylstannylethenyl)phenyl]tropane (3) with no carrier-added sodium [123I]iodide and hydrogen peroxide in ethanolic HCl. Biodistribution studies in rats indicated that [123I]ZIET enters the brain readily and accumulates in SERT-rich regions. Blocking studies performed in rats demonstrated that [123I]ZIET was selective and specific for SERT-rich regions (e.g. thalamus, brainstem, and striatum). MicroPET brain imaging studies in monkeys demonstrated that [11C]ZIET and [11C]ZBrET uptakes were selectivity localized in the putamen, midbrain, caudate, thalamus, pons, and medulla. Radioactivity in the regions of high SERT density of monkey brain was displaceable with citalopram except in the putamen and caudate. Radioactivity uptake in these DAT-rich regions was significantly displaceable either by preadministration of citalopram followed by injection of RTI-113 (or vice-versa) or by administration of a mixture of DAT and SERT ligands. In conclusion, the high yield, high specific activity, one-step radiolabeling method, high selectivity and favorable kinetics, and the good results obtained with [123I]ZIET in rats support the candidacy of [11C]ZIET for in vivo visualization and quantification of brain SERT.

Synthesis and evaluation of 2-amino-4-[(18)F]fluoro-2-methylbutanoic acid (FAMB): relationship of amino acid transport to tumor imaging properties of branched fluorinated amino acids.

Radiolabeled amino acids represent a promising class of tumor imaging agents, and the determination of the optimal characteristics of these tracers remains an area of active investigation. A new (18)F-labeled branched amino acid, 2-amino-4-[(18)F]fluoro-2-methylbutanoic acid (FAMB), has been prepared in 36% decay-corrected yield using no-carrier-added [(18)F]fluoride. In vitro uptake assays with rat 9L gliosarcoma cells suggest that [(18)F]FAMB was transported primarily via the L type amino acid transport system. In vivo studies with [(18)F]FAMB demonstrated tumor to normal brain ratios of 14:1 in rats with intracranial 9L gliosarcoma tumors at 60 minutes after injection. Comparison of [(18)F]FAMB with structurally related (18)F-labeled branched amino acids demonstrated that A type transport in vitro was positively correlated with the tumor to brain ratios observed in vivo.

Radiolabeled amino acids for tumor imaging with PET: radiosynthesis and biological evaluation of 2-amino-3-[18F]fluoro-2-methylpropanoic acid and 3-[18F]fluoro-2-methyl-2-(methylamino)propanoic acid.

Novel radiopharmaceuticals, including amino acids, that target neoplasms through their altered metabolic states have shown promising results in preclinical and clinical studies. Two fluorinated analogues of alpha-aminoisobutyric acid, 2-amino-3-fluoro-2-methylpropanoic acid (FAMP) and 3-fluoro-2-methyl-2-(methylamino)propanoic acid (N-MeFAMP), have been radiolabeled with fluorine-18, characterized in amino acid uptake assays, and evaluated in vivo in normal rats and a rodent tumor model. The key steps in the syntheses of both radiotracers involved the preparation of cyclic sulfamidate precursors. Radiosyntheses of both [18F]FAMP and [18F]N-MeFAMP via no-carrier-added nucleophilic substitution provided high yields (>78% decay-corrected) in high radiochemical purity (>99%). Amino acid transport assays using 9L gliosarcoma cells demonstrated that both compounds are substrates for the A type amino acid transport system, with [18F]N-MeFAMP showing higher specificity than [18F]FAMP for A type transport. Tissue distribution studies in normal Fischer rats and Fischer rats implanted intracranially with 9L gliosarcoma tumor cells were also performed. At 60 min postinjection, the tumor vs normal brain ratio of radioactivity was 36:1 in animals receiving [18F]FAMP and 104:1 in animals receiving [18F]N-MeFAMP. On the basis of these studies, both [18F]FAMP and [18F]N-MeFAMP are promising imaging agents for the detection of intracranial neoplasms via positron emission tomography.

Synthesis of syn- and anti-1-amino-3-[18F]fluoromethyl-cyclobutane-1-carboxylic acid (FMACBC), potential PET ligands for tumor detection.

syn- and anti-1-amino-3-[18F]fluoromethyl-cyclobutane-1-carboxylic acid (FMACBC, 16 and 17), analogues of anti-1-amino-3-[18F]fluorocyclobutyl-1-carboxylic acid (FACBC), were prepared to evaluate the contributions of C-3 substitution and configuration on the uptake of these radiolabeled amino acids in a rodent model of brain tumors. Radiofluorinated targets [18F]16 and [18F]17 were prepared by no-carrier-added radiofluorination from their corresponding methanesulfonyl esters 12 and 13, respectively, with decay-corrected radiochemical yields of 30% for [18F]16 and 20% for [18F]17. In amino acid transport assays performed in vitro using 9L gliosarcoma cells, both [18F]16 and [18F]17 were substrates for L type amino acid transport, while [18F]17 but not [18F]16 was a substrate for A type transport. Biodistribution studies in normal Fischer rats with [18F]16 and [18F]17 showed high uptake of radioactivity (>2.0% dose/g) in the pancreas while other tissues studied, including liver, heart, lung, kidney, blood, muscle, and testis, showed relatively low uptake of radioactivity (<1.0% dose/g). In rats implanted intracranially with 9L gliosarcoma cells, the retention of radioactivity in tumor tissue was high at 5, 60, and 120 min after intravenous injection of [18F]16 and [18F]17 while the uptake of radioactivity in brain tissue contralateral to the tumor remained low (<0.3% dose/g). Ratios of tumor uptake to normal brain uptake for [18F]16 were 7.5:1, 7:1, and 5:1 at 5, 60, and 120 min, respectively, while for [18F]17 the ratios were 7.5:1, 9:1, and 9:1 at the same time points. This work demonstrates that like anti-[18F]FACBC, [18F]16 and [18F]17 are excellent candidates for imaging brain tumors.