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.