Improved synthesis of 20-[18F]fluoroarachidonic acid for PET-MR imaging of ApoE mice

Introduction: Levels of inflammatory arachidonic acid metabolites are elevated in Alzheimer's disease (AD) brains and cerebrospinal fluid. Imaging arachidonic acid in the brain can offer insight into neuroinflammation and AD pathogenesis. PET imaging has shown higher uptake of ¹¹C-AA in senile plaque-containing brain regions in AD patients compared to subjects without AD. As compared to ¹¹C, ¹⁸F has a more favorable half-life, as such a 20-[¹⁸F]fluoroarachidonic acid (¹⁸F-FAA) probe would be advantageous; however, previous syntheses of the ¹⁸F-FAA labeling precursor using epoxidation and Wittig reactions have disadvantages such as low selectivity and yield, making the production of ¹⁸F-FAA a great challenge.

Methods: The ¹⁸F-FAA labeling precursor, 20-tosyloxyarachidonic acid methyl ester was synthesized via a convergent synthesis route utilizing multiple copper-catalyzed C-C coupling reactions. Radiofluorination to ¹⁸F-FAA was accomplished in a two-step one-pot reaction with a single HPLC purification. The stability of ¹⁸F-FAA in mouse serum at 37 ˚C at 1 and 2 h, and in the formulation (5 μM HEPES in saline containing 50 mg/mL bovine serum albumin) at 2, 4, 6, and 8 h at room temperature was determined. PET-MR images of rhApoE3 and rhApoE4 mice (n = 5/group) were taken in a 45-min dynamic scan after bolus injection through the tail vein to determine the brain uptake and body biodistribution of ¹⁸F-FAA.

Results: The ¹⁸F-FAA labeling precursor was successfully synthesized using the new synthesis route. The chemical purity of the precursor is >95%. ¹⁸F-FAA was radiosynthesized with a radiochemical purity of ≥ 99% and high specific activity. The octanol-PBS partition coefficient (LogP) of ¹⁸F-FAA was measured by gamma counter to be 1.81 ± 0.08. Stability studies show that ¹⁸F-FAA is stable in formulation, with purity ranging from 96.20 ± 0.16 at 2 h to 92.41 ± 0.50 after 8 h incubation at room temperature, as well as >99% pure after 2 h in mouse serum at 37 ˚C. In PET images, standard uptake values (SUV_mean) were determined in rhApoE3 mice at 22.5 min after injection for brain (0.52 ± 0.04), liver (3.48 ± 0.22), kidney (2.15 ± 0.32), and muscle (0.49 ± 0.07), and at 42.5 min after injection for the brain (0.59 ± 0.05), liver (3.37 ± 0.24), kidney (2.00 ± 0.33), and muscle (0.48 ± 0.04). For rhApoE4 mice, SUV_mean values were 0.44 ± 0.05 in brain, 3.42 ± 0.25 in liver, 1.96 ± 0.28 in kidney, and 0.43 ± 0.07 in muscle at 22.5 min after injection, and 0.51 ± 0.06 in brain, 3.19 ± 0.26 in liver, 1.80 ± 0.25 in kidney, and 0.46 ± 0.09 in muscle at 42.5 min after injection.

Conclusions: The ¹⁸F-FAA labeling precursor was synthesized by an improved method and ¹⁸F-FAA was successfully prepared with excellent radiochemical purity and high specific activity. Brain uptake of ¹⁸F-FAA was observed by PET in rhApoE3 and rhApoE4 mice with concentration increasing over time. ¹⁸F-FAA is a promising PET tracer and the improved synthesis will aid in further studies.

Research Support: This work was supported by R01AG067063 and R21AG056518 from the U.S. National Institutes of Health.

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