Improved Synthesis and Preclinical Evaluation of [18F]FNP-59: a Radiotracer for Imaging Cholesterol Trafficking

976 Objectives: We previously reported the synthesis of FNP-59, a novel fluorinated analogue of NP-59 and cholesterol[1]. Cholesterol is normally trafficked to adrenal cortical tissue where it is esterified by ACAT1 for storage and used in the synthesis of hormones. In atherosclerotic disease foam cells also express ACAT1 for esterifying cholesterol to store and prepare it for transport and elimination. A higher yielding concise route to FNP-59 precursor and standard is required to facilitate preclinical imaging and clinical translation. A new route to FNP-59, the automated radiosynthesis, and preclinical evaluation will be reported. Methods: The synthesis of FNP-59 standard and precursor was undertaken with either acetyl, pivaloyl or benzoyl protection of the 3β-OH of cholesterol. The previous route was 11 steps long, but preparation of FNP-59 can be accomplished in 8 steps, utilizing a recently reported rearrangement observed in a steroid B ring expansion [2]. The precursor, which is generated from cholesterol in 6 steps could be utilized to prepare [18F]FNP-59 by standard substitution chemistry. Tetrabutylammonium (TBA) bicarbonate solution was used to elute fluorine-18 trapped on an anion exchange sep-pak. After an ethanolic azeotropic distillation to dry [18F]TBAF, mesylate precursor (5 mg) dissolved in DMSO (1 mL) was added to the reactor and heated at 150 °C for 20 min. The 3-pivaloyl intermediate was then treated with 1M KOH in 1:1 EtOH:H2O (1M) at 110 °C for 25 min to convert the material to [18F]FNP-59. The reaction mix was diluted with HPLC eluent and purified by semi-preparative HPLC to afford [18F]FNP-59 (117mCi, 6.5% RCY), which was then reformulated for use in ethanol, polysorbate-80, and saline. Biodistribution studies in female Sprague-Dawley rats were conducted to observe adrenal uptake and compare to previous reports using NP-59. Additionally, a study was conducted in ApoE-/- and BL6 mice to demonstrate the ability of the agent to image atherosclerosis in the aorta and carotid artery. Results: Adrenal uptake in female Sprague-Dawley rats matched the expected uptake from prior work with NP-59 [3] with increasing adrenal and ovary uptake over time. At 6 hours the adrenal uptake of 11.5% ID/g compared to 1.1% ID/g for liver will provide the required contrast for imaging adrenal uptake. Imaging in ApoE-/- and BL6 mice at 2 hours from injection showed increased aorta and carotid artery uptake in the ApoE-/-; however, based on the biodistribution data later time points should be utilized to reduce background from circulating [18F]FNP-59. Conclusions: An improved, 8 step synthesis to FNP-59 that contains the radiolabeled precursor was developed. The automated radiosynthesis of [18F]FNP-59 was successfully developed, utilizing only class 3 solvents, and a minimal amount of TBA salts. Preclinical results indicate that [18F]FNP-59 has similar properties to NP-59, indicating its potential use in imaging adrenal pathologies and atherosclerotic disease. References: [1]Winton WP, Brooks AF, Wong KK, Scott PJH, Viglianti BL. SynOpen. 2019, 03(02), 55-58. [2] (a) Tadanier, J. J. Org. Chem.1966, 31(7), 2124-2135 (b) Kranz, DP; Chiha, S; Greffen, AM; Neudorfl, JM; Schmalz, HG. Org. Lett. 2012, 14(14), 3692-3695 [3] Sarkar SD, Beierwaltes WH, et al. J Nucl Med.1975, 16(11):1038-42. Acknowledgements: This work was supported by the Center for the Discovery of New Medicine and a Michigan Memorial Phoenix Project SEED grant. We also want to thank Prof. Milton Gross for his advice and support of this project, as well as Jenelle Stauff, Janna Arteaga, and Phillip Sherman for conducting animal imaging studies