Abstract
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Objectives: F-18 FDDNP has been proven to be a promising PHF-Tau protein tracer for studying AD in humans [1]. In order to improve its brain uptake and binding specificity, we have developed an automated synthesis of its analog (F-18 FEONM, 2) and evaluated it as a potential tau protein imaging agent on a tauopathy animal model using micro PET and immunohistochemistry.
Methods: The F-18 FEONM (2) was automated synthesized with commercial GE TRACERlabTM FX2 N synthesizer. Briefly, nucleophilic fluorination of the corresponding tosyl- precursor (1) in anhydrous acetonitrile with K[18F]/K2.2.2 at 100℃ for 15 min followed by purification with a semi-preparative HPLC ( Phenomenex Luna C18(2), 5 µm, 250 x 10 mm, 80 % MeOH, 2 ml/min) and solid phase extraction gave product (2) (Scheme 1). The radiochemical purity and in vitro stability of 2 were determined using an analytical HPLC ( Phenomenex Luna C18(2), 5 µm, 250 x 4.6 mm, 80 % MeOH, 0.8 ml/min). For animal study, transgenic mouse P301S of different ages (5, 9 and 12 months), with age-matched wild type serving as controls were used in this study. Dynamic 60-min micro PET imaging were performed after intravenous administration of 11.3 ± 0.5 MBq (n=3) of 2. Dynamic sinograms were produced with 12 x10 sec, 6 x 30 sec, 5 x 300 sec, 3 x 600 sec frames. Images were analyzed by co-registration to MRI atlas and scaling to olfactory bulb, presented as standardized uptake value ratio (SUVR) and further validated by immunohistochemistry stain with phospho-Tau (Ser396), phospho-Tau (Ser202, Thr205) AT8 and nissl stain.
Results: The radiochemical yield of 2 synthesized by automated method was 2.34 ± 1.29 % (EOS, N=18). The synthesis time including purification and formulation was 69.8 ± 6.4 min from EOB. In vitro radiochemical purity were > 90% within 4 hours after formulation in 5 mg/mL ascorbic acid in NaCl 0.9%. The time-activity curves of 2 in both transgenic and control mice showed rapid brain uptake and washout in 30 mins (Fig. 1). The SUVR of 2 at 15-20 min post injection revealed significantly higher uptake in hippocampus and brain stem in 9 and 12 months age of P301S mice than age-matched control (Table 1). Immunohistochemical stain of Ser396 and AT8 positive neurons match the distribution obtained from PET imaging.
Conclusions: The results of this study demonstrate that 2 may serve as a potential tau protein imaging agent. Further comparisons of 2 with other potent tau imaging agents are warrant. References: [1]. Villemagne VL, Fodero-Tavoletti MT, Masters CL, Rowe CC. Tau imaging: early progress and future directions. Lancet Neurol 2015;14:114-24. Captions: Scheme 1. Synthesis of F-18 FEONM (2). Fig.1 Brain uptake of F-18 FEONM (2) in wild type and transgenic mice P301S at 12 month age. Table 1. SUVR of F-18 FEONM (2) in wild type and transgenic mice P301S at different ages. # Indicate a statistically significant difference ( p < 0.05). ## Indicate a statistically significant difference ( p < 0.01)