Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative motor neuron disorder. Genetic studies have linked mutation of the gene SOD1 to ALS pathology as well as several other pathological processes including modulation of glutamatergic function and inflammatory processes. Since therapeutic approaches for ALS are focused on glutamatergic function, we investigated modulation of glutamate transport based on its receptor function as well as excitotoxicity-induced inflammatory response. In vivo positron emission tomography (PET) imaging studies of metabotropic glutamate receptor subtype 5 (mGluR5) using [18F]FPEB ([18F]3-fluoro-5-(2-pyridylethynyl)benzonitrile) and inflammatory response using [11C]PBR28 (peripheral benzodiazepine receptor ligand 28) were done in an early and a late phase of neurodegeneration in four ALS mice expressing SOD1-G93A gene and four control base mice (C57/BL6). Accumulation of [18F]FPEB and [11C]PBR28 were quantitated in several brain areas and spinal cord to determine degeneration-induced modulation. The studies were completed with immunohistochemical analyses of mGluR5 and inflammatory response. These studies showed enhanced binding potential of [18F]FPEB in several brain areas including striatum, hippocampus, and frontal cortex. In the whole brain, the binding potential increased 49 ± 9 % from base mice to ALS-type mice and further enhanced 23 ± 4 % during disease progression. Also, in the spinal cord 6–22 %, enhanced accumulation of [18F]FPEB was observed during progression of the disease. The accumulation of [11C]PBR28 increased by 110 ± 33 % in the whole brain during progression of the disease indicating significant inflammatory process. [11C]PBR28 accumulation enhanced 89–264 % in the spinal cord and 204 % in the lungs. The end point immunohistochemical analyses verified the enhanced mGluR5 expression and inflammation. These results confirm the role of glutamate and inflammation in ALS-type pathology. These data also support the hypothesis that excessive glutamate may contribute to inflammation in the chronic neurodegenerative processes in ALS.

Four 4-phthalimide derivatives of N-(3-chlorophenyl)-2-picolinamide were synthesized as potential ligands for the PET imaging of mGlu₄ in the brain. Of these compounds, N-(3-chloro-4-(4-fluoro-1,3-dioxoisoindolin-2-yl)phenyl)-2-picolinamide (3, KALB001) exhibited improved binding affinity (IC₅₀ = 5.1 nM) compared with ML128 (1) and was subsequently labeled with ¹⁸F. When finally formulated in 0.1 M citrate buffer (pH 4) with 10% ethanol, the specific activity of [¹⁸F]3 at the end of synthesis (EOS) was 233.5 ± 177.8 GBq/μmol (n = 4). The radiochemical yield of [¹⁸F]3 was 16.4 ± 4.8% (n = 4), and the purity was over 98%. In vivo imaging studies in a monkey showed that the radiotracer quickly penetrated the brain with the highest accumulation in the brain areas known to express mGlu₄. Despite some unfavorable radiotracer properties like fast washout in rodent studies, [¹⁸F]3 is the first ¹⁸F-labeled mGlu₄ radioligand, which can be further modified to improve pharmacokinetics and brain penetrability for future human studies.

Racemic and enantiomerically pure ((S,S) and (R,R)) 2-[alpha-(2-(2-[(18)F]fluoroethoxy)phenoxy)benzyl]morpholine ([(18)F]FRB) and its tetradeuterated form [(18)F]FRB-D(4), analogs of the highly selective norepinephrine reuptake inhibitor reboxetine (2-[alpha-(2-ethoxyphenoxy)benzyl]morpholine, RB), have been synthesized for studies of norepinephrine transporter (NET) system with positron emission tomography (PET). The [(18)F]fluorinated precursor, (S,S)/(R,R)-N-tert-butyloxycarbonyl-2-[alpha-(2-hydroxyphenoxy)benzyl]morpholine ((S,S)/(R,R)-N-Boc-desethylRB), was prepared by the N-protection of (S,S)/(R,R)-2-[alpha-(2-hydroxyphenoxy)benzyl]morpholine ((S,S)/(R,R)-desethylRB) with a tert-butyloxycarbonyl (Boc) group followed by enantiomeric resolution with chiral HPLC to provide both (S,S) and (R,R) enantiomers with >99% enantiomeric purity. These compounds were then used for radiosynthesis to prepare enantiomerically pure [(18)F]FRB and [(18)F]FRB-D(4) via the following three-step procedure: (1) formation of 1-bromo-2-[(18)F]fluoroethane ([(18)F]BFE or [(18)F]BFE-D(4)) by nucleophilic displacement of 2-bromoethyl triflate (or D(4) analog) with no-carrier added [(18)F]F(-) in THF; (2) reaction of [(18)F]BFE (or [(18)F]BFE-D(4)) with N-Boc-desethylRB in DMF in the presence of excess base; and (3) deprotection with trifluoroacetic acid. The racemates, (S,S) and (R,R) enantiomers of [(18)F]FRB and [(18)F]FRB-D(4) were obtained in 11-27% (decay corrected to the end of bombardment, EOB) in 120-min synthesis time with a radiochemical purity of >98% and specific activities of 21-48 GBq/micromol (EOB). The results of the whole-body biodistribution studies with (S,S)-[(18)F]FRB-D(4) were similar to those with (S,S)-[(18)F]FRB but showed relatively faster blood clearance and no significant in vivo defluorination. Positron emission tomography studies in baboon brain also showed that (S,S)-[(18)F]FRB-D(4) may be a potentially useful ligand for imaging NET with PET.