Synthesis and Preclinical Evaluation of 22-[18F]Fluorodocosahexaenoic Acid as a Positron Emission Tomography Probe for Monitoring Brain Docosahexaenoic Acid Uptake Kinetics.

Docosahexaenoic acid [22:6(n-3), DHA], a polyunsaturated fatty acid, has an important role in regulating neuronal functions and in normal brain development. Dysregulated brain DHA uptake and metabolism are found in individuals carrying the APOE4 allele, which increases the genetic risk for Alzheimer's disease (AD), and are implicated in the progression of several neurodegenerative disorders. However, there are limited tools to assess brain DHA kinetics in vivo that can be translated to humans. Here, we report the synthesis of an ω-radiofluorinated PET probe of DHA, 22-[18F]fluorodocosahexaenoic acid (22-[18F]FDHA), for imaging the uptake of DHA into the brain. Using the nonradiolabeled 22-FDHA, we confirmed that fluorination of DHA at the ω-position does not significantly alter the anti-inflammatory effect of DHA in microglial cells. Through dynamic PET-MR studies using mice, we observed the accumulation of 22-[18F]FDHA in the brain over time and estimated DHA's incorporation coefficient (K*) using an image-derived input function. Finally, DHA brain K* was validated using intravenous administration of 15 mg/kg arecoline, a natural product known to increase the DHA K* in rodents. 22-[18F]FDHA is a promising PET probe that can reveal altered lipid metabolism in APOE4 carriers, AD, and other neurologic disorders. This new probe, once translated into humans, would enable noninvasive and longitudinal studies of brain DHA dynamics by guiding both pharmacological and nonpharmacological interventions for neurodegenerative diseases.

Radiosynthesis of 20-[18F]fluoroarachidonic acid for PET-MR imaging: Biological evaluation in ApoE4-TR mice.

Dysreglulated brain arachidonic acid (AA) metabolism is involved in chronic inflammation and is influenced by apolipoprotein E4 (APOE4) genotype, the strongest genetic risk factor of late-onset Alzheimer's disease (AD). Visualization of AA uptake and distribution in the brain can offer insight into neuroinflammation and AD pathogenesis. Here we present a novel synthesis and radiosynthesis of 20-[18F]fluoroarachidonic acid ([18F]-FAA) for PET imaging using a convergent route and a one-pot, single-purification radiolabeling procedure, and demonstrate its brain uptake in human ApoE4 targeted replacement (ApoE4-TR) mice. By examining p38 phosphorylation in astrocytes, we found that fluorination of AA at the ω-position did not significantly alter its biochemical role in cells. The brain incorporation coefficient (K*) of [18F]-FAA was estimated via multiple methods by using an image-derived input function from the right ventricle of the heart as a proxy of the arterial input function and brain tracer concentrations assessed by dynamic PET-MR imaging. This new synthetic approach should facilitate the practical [18F]-FAA production and allow its translation into clinical use, making investigations of dysregulation of lipid metabolism more feasible in the study of neurodegenerative diseases.

Synthesis of ortho-Formylphenylphosphonic Acids as Covalent Probes of Active Site Lysines.

During the course of an investigation of targeted inhibition of DNA polymerase beta (pol ß) lyase activity using small molecules, we observed the formation of an aldimine between (2-formyl)phenylphosphonic acid (2FPP) and butylamine under basic aqueous conditions; complete deprotonation of the phosphonate group was required to stabilize the imine product. Results of computational docking studies suggested that the reaction of Lys-72 on the lyase active site with an aldehyde group could be facilitated by a proximal phosphonate, not only because of the phosphonate's ability to mimic phosphate interacting with the DNA binding site, but also because of its ability to shield the imine against hydrolysis. Novel pol ß lyase inhibitors were thus prepared using a 2FPP analogue with an amine linker; P-C bond formation in synthesis of this intermediate was possible with an unprotected aldehyde using palladium-catalyzed, microwave-assisted Michaelis-Arbuzov chemistry. These compounds, and structurally related derivatives lacking the aldehyde or phosphonate, were evaluated in an assay for pol ß, to assess their potential for inhibition of pol ß.