Aromatic L-Amino Acid Decarboxylase (AAAD) Inhibitors as Carcinoid Tumor Imaging Agents: Synthesis of 18F labeled 6-Fluoro-a-Fluoromethyl-M-Tyrosine (6-FFMMT)

As part of our approach to develop mechanism based tumor imaging agents, we investigated radiolabeled AAAD inhibitors as potential PET carcinoid tumor imaging agents. Poster presented at the 49th Annual Meeting of the Society of Nuclear Medicine - Abstract #1464

Aromatic L-Amino Acid Decarboxylase (AAAD) Inhibitors as Carcinoid Tumor-Imaging Agents: Synthesis of 18F-labeled α-fluoromethyl-6-fluoro-m-tyrosine (FM-6-FmT)

The aromatic l-amino acid decarboxylase (AAAD) enzyme is significantly upregulated in neuroendocrine tumors and, thus, would be a good target for PET imaging agents. α-fluoromethyl-DOPA (FMDOPA) is one of the most potent irreversible AAAD inhibitor and its non-catechol derivative, α-fluoromethyl-m-tyrosine (FMmT), is a promising AAAD imaging agent. We synthesized FMmT and its direct electrophilic fluorination provided a mixture of products identified by NMR analysis after HPLC purification as 6-fluoro-, 2-fluoro- and 2,6-difluoro-derivatives of FMmT. Using rat striatal homogenates, α-fluoromethyl-6-fluoro-m-tyrosine (FM-6-FmT) was found to have AAAD inhibitory activity comparable to that of FMDOPA. Electrophilic radiofluorination of FMmT using [18F]AcOF gave 18F labeled 6-fluoro-, 2-fluoro- and 2,6-difluoro-FMmT derivatives in 22.0%, 21.9% and 8.5% radiochemical yields, respectively. Based on its proposed mechanism of inhibition, FM-6-[18F]FmT is expected to irreversibly bind to AAAD and, hence, could be used as a PET agent to image tumors of endocrine origin containing high concentrations of AAAD. Since FM-6-FmT lacks the catechol moiety, it is expected to be better than FMDOPA since it is not a substrate for catechol-O-methyltransferase

Aromatic L‐Amino Acid Decarboxylase (AAAD) Activity in Rhesus Macaque Striatum After MAO‐B Inhibition by Ro 16‐6491

The aromatic L-amino acid decarboxylase (AAAD) (EC 4.1.1.28) enzyme converts L-DOPA to dopamine and is critical in dopamine replacement therapy in Parkinson’s disease (PD). Following stimulated release, the action of synaptic dopamine is terminated by neuronal reuptake and deamination by monoamine oxidase (MAO). While MAO inhibition is known to downregulate tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis (Lamensdorf and Finberg, 1997), the effect of MAO inhibition on AAAD is unclear. There are two MAO subtypes based on substrate and inhibitor specificity, MAO-A and MAO-B (Kinemuchi et al., 1984). Dopamine is effectively deaminated by both subtypes, but MAO-B has higher activity than MAO-A in the primate striatum, suggesting that MAO-B plays a greater role than MAO-A in striatal dopamine metabolism in primates (Paterson et al., 1995). By slowing dopamine degradation and maximizing L-DOPA efficacy, MAO-B inhibitors may be useful in PD therapy, as suggested by Napolitano et al. (1995)

Snythesis of [F-18]-ZD1839 as a PET Imaging Agent for Epidermal Growth Factor Receptors

The epidermal growth factor receptor (EGFR) is an epithelial cell membrane receptor with an intra- cellular tyrosine kinase (TK) component. EGFR-TK is involved in cell signalling critical to proliferation, apoptosis, repair and angiogenesis. More than two thirds of human cancers derive from epithelial tissues and the EFGR-TK is overexpressed in the majority of these tumors. Thus in recent years numerous selective EGFR-TK inhibitors with nanomolar affinities have been developed as potential anti-cancer agents. One potent inhibitor that has progressed the furthest toward clinical registration is ZD1839 (or Iressa). ZD1839 is approved for clinical use in Japan and is close to obtaining U.S. FDA approva l. ZD1839 is a fluorine-containing anilinoquinazoline which can be isotopically labelled with 18F for use in clinical oncology as a PET imaging agent. Since extensive pre-clinical and clinical data on ZD1839 are available, the use of 18F-ZD1839 to identify patients who would benefit from Iressa treatment and monitor its efficacy would be straightforward. Abstract from the 15th International Symposium on Radiopharmaceutical Chemistry - Session 1: Oncology: Molecular Markers and Tumour Diagnosis I - Chemistry and Pharmacology

MAO-B Inhibition Downregulates Striatal Aromatic Amino Acid Decarboxylase (AAAD) Activity in Rhesus Monkeys in Vivo

The goal of this study is to determine the effect of MAO blockade on AAAD activity in non-human primate brain, in vivo, using PET imaging with fluoro-m-tyrosine (FMT), a selective PET imaging agent for assessing AAAD activity. Paper presented at the 49th Annual Meeting of the Society of Nuclear Medicine, Abstract number 385

Brief Communications Serotonin Transporter Availability in the Amygdala and Bed Nucleus of the Stria Terminalis Predicts Anxious Temperament and Brain Glucose Metabolic Activity

anxiety and affective disorders. Positron emission tomography scans using [ 11 C]DASB [ 11 C]-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile]to measure 5-HTT availability (an index of receptor density and binding) were performed in 34 rhesus monkeys in which the relationship between regional brain glucose metabolism and anxious temperament was previously established. 5-HTT availability in the amygdalohippocampal area and bed nucleus of the stria terminalis correlated positively with individual differences in a behavioral and neuroendocrine composite of anxious temperament. 5-HTT availability also correlated positively with stress-induced metabolic activity within these regions. Collectively, these findings suggest that serotonergic modulation of neuronal excitability in the neural circuitry associated with anxiety mediates the developmental risk for affect-related psychopathology

Initial in vivo PET imaging of 5-HT1A receptors with 3-[(18)F]mefway.

4-trans-[(18)F]Mefway is a PET radiotracer with high affinity for 5-HT1A receptors. Our preliminary work indicated the positional isomer, 3-[(18)F]mefway, would be suitable for PET imaging of 5-HT1A receptors. We now compare the in vivo behaviour of 3-mefway with 4-mefway to evaluate 3-[(18)F]mefway as a potential 5-HT1A PET radiotracer. Two male rhesus macaques were given bolus injections of both 3- and 4-trans-[(18)F]mefway in separate experiments. 90 minute dynamic PET scans were acquired. TACs were extracted in the mesial temporal lobe (MTL) and caudal anterior cingulate gyrus (cACg). The cerebellum (CB) was used as a reference region. In vivo behavior of the radiotracers in the CB was compared based upon the ratio of normalized PET uptake for 3- and 4-trans-[(18)F]mefway. Specific binding was compared by examining MTL/CB and cACg/CB ratios. The subject-averaged ratio of 3-[(18)F]mefway to 4-trans-[(18)F]mefway in the cerebellum was 0.96 for 60-90 minutes. MTL/CB reached plateaus of ~2.7 and ~6 by 40 minutes and 90 minutes for 3- and 4-trans-[(18)F]mefway, respectively. cACg/CB reached plateaus of ~2.5 and ~6 by 40 minutes and 70 minutes for 3- and 4-trans-[(18)F]mefway, respectively. The short pseudoequilibration times and sufficient uptake of 3-[(18)F]mefway may be useful in studies requiring short scan times. Furthermore, the similar nondisplaceable clearance in the CB to 4-trans-[(18)F]mefway suggests the lower BPND of 3-[(18)F]mefway is due to a lower affinity. The lower affinity of 3-[(18)F]mefway may make it useful for measuring changes in endogenous 5-HT levels, however, this remains to be ascertained

High-affinity dopamine D2/D3 PET radioligands 18F-fallypride and 11C-FLB457: A comparison of kinetics in extrastriatal regions using a multiple-injection protocol

18F-Fallypride and 11C-FLB457 are commonly used PET radioligands for imaging extrastriatal dopamine D2/D3 receptors, but differences in their in vivo kinetics may affect the sensitivity for measuring subtle changes in receptor binding. Focusing on regions of low binding, a direct comparison of the kinetics of 18F-fallypride and 11C-FLB457 was made using a MI protocol. Injection protocols were designed to estimate K1, k2, fNDkon, Bmax, and koff in the midbrain and cortical regions of the rhesus monkey. 11C-FLB457 cleared from the arterial plasma faster and yielded a ND space distribution volume (K1/k2) that is three times higher than 18F-fallypride, primarily due to a slower k2 (FAL:FLB; k2=0.54 min−1:0.18 min−1). The dissociation rate constant, koff, was slower for 11C-FLB457, resulting in a lower KDapp than 18F-fallypride (FAL:FLB; 0.39 nM:0.13 nM). Specific D2/D3 binding could be detected in the cerebellum for 11C-FLB457 but not 18F-fallypride. Both radioligands can be used to image extrastriatal D2/D3 receptors, with 11C-FLB457 providing greater sensitivity to subtle changes in low-receptor-density cortical regions and 18F-fallypride being more sensitive to endogenous dopamine displacement in medium-to-high-receptor-density regions. In the presence of specific D2/D3 binding in the cerebellum, reference region analysis methods will give a greater bias in BPND with 11C-FLB457 than with 18F-fallypride