Automated radiosynthesis of [18F]atorvastatin via Ru-mediated 18F-deoxyfluorination: a prospective PET imaging tool for the assessment of statin related mechanisms of action

Introduction: Cardiovascular diseases represent the leading cause of death globally, having high incidence rates regardless of each country income level.(1) Since the 1970s, when the first statin was isolated and screened as a potent 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR) inhibitor, this class of lipid-lowering agents became the spine of primary and secondary cardiovascular disease prevention. HMGCR is an enzyme involved in the production of endogenous cholesterol and the structure-based rational quest for more powerful inhibitors ended up with numerous statins being synthesized and added to medical prescription. In addition to their unambiguous cholesterol-lowering effects, the success of statins is increasingly being connected with its pleiotropic effects.(2) More and more often statins are being associated with potential protective effects on other pathologies (e.g. respiratory, carcinogenic, viral, neurodegenerative). Although these data are strongly suggestive, the exact off-target effects of statins have not been proven in vivo. On the other hand, some patients revealed to be statin-resistant or statin-intolerant.(3) Therefore, an increasingly specific knowledge of the subcellular mechanisms affected by statins and the development of a more sensitive toolbox to investigate this subject is currently challenging in medicinal chemistry. Objective: Herein, we propose the radiofluorination of atorvastatin, the most widely used statin in the prevention of cardiovascular risk factors and one of the bestselling drugs in pharmaceutical history. By taking advantage of the fluorobenzene ring present in atorvastatin, we intend to produce the radiolabeled analogue [18F]atorvastatin without changing the physicochemical characteristics of the original molecule. We believe that this positron emission tomography (PET) tracer may have the potential to unveil some of the statin-related mechanisms of action by supporting the development of highly sensitive in vitro quantitative techniques to compare cellular and subcellular localization. Off-target activity and pharmacokinetics can potentially be identified by mapping the radiotracer biodistribution over time. To evaluate these hypotheses, in vitro and in vivo experiments are currently ongoing. Methods: First, a phenol-derived [18F]atorvastatin precursor with a ketal and tert-butyl ester protected side-chain was synthesized and then the phenol substituent was complexed to a ruthenium fragment to decrease electron density and activate the arene for subsequent nucleophilic aromatic substitution.(4) This strategy unlocks the access to 18F-fluorination of aromatic rings lacking electron withdrawing groups. 18F-Deoxyfluorination and deprotection were manually optimized and then translated to automation, taking into account the particularities and constraints (i.e. limitation of volumes, fluid trajectories, arrangements) of the selected synthesis module (Synthra RNplus), to obtain [18F]atorvastatin reformulated into an injectable physiological saline solution. Results: To date, the full synthesis, purification and reformulation of [18F]atorvastatin has been consistently achieved, reinforcing the robustness of this 18F-deoxyfluorination method. After a total time of approximately 75 min., stable (at least up to 3 h when incubated in solution or human serum) [18F]atorvastatin with radiochemical purity ≥95% was obtained in moderate isolated yields of 20.4% ± 1.9% (d.c.) during the manual optimization tests, being already adequate for the planning and performance of pre-clinical assays (molar activity 112 ± 77 GBq/µmol). Despite the reduced radiochemical yields obtained to date with the chosen automated module (approx. 5%), the acquired data forecast an interesting margin of progression for the enhancement of the final characteristics and efficiency of the process. Conclusions: Here we demonstrate the suitability of the late-stage 18F-deoxyfluorination strategy to be automated in order to achieve the radiolabeling of phenol derived Ru-coordinated complexes to routinely produce [18F]atorvastatin. In vitro and in vivo studies are currently ongoing to evaluate the potential of this radiolabeled statin to identify off-target activity. Ultimately, [18F]atorvastatin might be used to distinguish between statin-resistant and non-resistant patients for personalized therapy, thereby the first in human PET studies are already being envisaged at our department. References: (1) Roth GA et al. [2017], J Am Coll Cardiol, 70(1):1; (2) Oesterle A et al. [2017], Circ Res, 120(1):229; (3) Reiner Ž [2014] Nutr Metab Cardiovasc Dis, 24(10):1057; (4) Beyzavi MH et al. [2017] ACS Cent Sci, 3(9):944.