Abstract
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Objectives: It is reported that rosmarinic acid, a type of polyphenol, inhibits the deposition of amyloid β protein (Aβ) and has a preventive effect against Alzheimer's disease (AD) (1). However, its biological mechanism has not been fully elucidated. In this study, we have established a radiosynthesis method for [11C]methylrosmarinate, which is a derivative of rosmarinic acid and examined the biodistribution in normal rats including the brain accumulation.
Methods: The radiosynthesis of [11C]methylrosmarinate was performed via [11C]methyl triflate that was produced by gas-phase method. The precursor was used rosmarinic acid and radiolabeling was carried out at room temperature. We investigated reaction solvents of precursor (acetone, DMF or DMSO), and types of bases (NaOH or TBAH) and its amount (1.4 - 9.8 as a molar ratio). Then, after HPLC separation, SPE columns (tC2, C8, tC18 or HLB cartridge column) for purification and the radiolysis prevention of final product were considered. [11C]methylrosmarinate (35.4 ± 12.3 MBq) was injected into normal Wistar rats (n=6, body weight 206 ± 6.6 g) and biodistribution was imaged by micro PET-CT (Siemens Inveon) under isoflurane anesthesia (n=3). Then, radioactivity of autopsied samples and weights were measured by γ-counter system.
Results: The labelling rate of [11C]methylrosmarinate with acetone and NaOH (molar ratio; 4.2) was more than 75 % (based on the radioactivity obtained by the radiolabeling). In purification step, more than 85 % of radioactivity could be recovered by using tC2 column and 1 mL ethanol as an eluent. In optimized condition, [11C]methylrosmarinate was synthesized over 2 GBq (molar activity; approximately 30 GBq/µmol). The radiochemical purity was over 95 % after 80 min by adding 9 mL of 2 mg/mL sodium ascorbate in saline. In the pharmacokinetics of [11C]methylrosmarinate PET, enterohepatic circulation in the liver and intestines and excretion from the kidney were observed. However, there was only background level accumulation in any brain region (SUVmean=0.73 ± 0.19) nor other major organs, suggesting no specific uptake in the brain. Autopsy study revealed accumulation in the brain was 0.25 ± 0.04 % ID/g.
Conclusions: We succeeded to establish [11C]methylrosmarinate radiosynthesis method using SPE column. In addition, [11C]methylrosmarinate has low physiological accumulation in the brain of normal rats. Accumulation in the brain should be further evaluated the difference between a rosmarinic acid and methylrosmarinate in the future study. (1) Hamaguchi et al. Am J Pathol. 2009, Dec, 175(6), 2557-65.