Synthesis and Automated Labeling of [¹⁸F]Darapladib, a Lp-PLA₂ Ligand, as Potential PET Imaging Tool of Atherosclerosis

Abstract

Atherosclerosis and its associated clinical complications are major health issues in industrialized countries. Lipoprotein-associated phospholipase A 2 (Lp-PLA 2) was demonstrated to play an important role in atherogenesis and to be a potential risk prediction factor of plaque rupture. Darapladib is one of the most potent Lp-PLA 2 inhibitors with an IC 50 of 0.25 nM. Using its affinity for Lp-PLA 2 , we describe herein the total synthesis of darapladib radiolabeling precursor and the automated radiolabeling process for positron emission tomography (PET) imaging via an arylboronate moiety. The tracer thus obtained was tested in a mouse model of atherosclerosis (ApoE KO) and compared with the widely used [ 18 F]fluorodeoxyglucose ([ 18 F]FDG) PET tracer, known to label metabolically active cells. [ 18 F]Darapladib showed a significant accumulation within mice aortic atheromatous plaques dissected out ex vivo compared to [ 18 F]FDG. Incubation of the radiotracer with human carotid samples showed a strong accumulation within the atherosclerotic plaques and supports its potential for use in PET imaging.

Late-Stage Copper-Catalyzed Radiofluorination of an Arylboronic Ester Derivative of Atorvastatin

“…It is known from the literature that the atorvastatin side chain [43], the presence of the two non-functionalized mono-substituted benzene rings and a pyrrole core [44], and the basic salts in solution [16] can all influence copper oxidation states, which might explain the limited [ 18 F]F − conversion. Nevertheless, the radiofluorination yields obtained in this work are in line with what has been reported for complex heteroaromatic molecules, especially if containing several phenyl groups in its structure [33,35,38], and should still be sufficient to proceed for the development of [ 18 F]atorvastatin ( 8 ) preclinical screening assays after a fast and nearly quantitative deprotection of the side chain [45] (Figure S4).…”

“…To circumvent the downsides of azeotropic evaporation, especially when automated where manipulation and close control of the conditions are challenging, a few solid-phase extraction (SPE) drying procedures have been rising in the literature [21,27,28,30], some even able to avoid the use of bases [29]. However, being very recent, they still lack a proper multicentre evaluation and assessment into more than just simple (hetero)arenes, as some authors claim not being able to reproduce them [38] and when attempted by us for the 18 F-fluorination of the arylboronic ester derivative of atorvastatin ( 6 ), invariably led to no detectable [ 18 F]F − conversion (despite shown to be successful when tested first in some of the same simple aryl boronic acid esters used in our previous work [39]).…”

“…Therefore, a case-by-case optimization still seems to be necessary, being extremely difficult to reach a standardized procedure for every labeling precursor, which might explain the reason why the exact same methodology has been very rarely repeated in the literature. An analysis through the Cu-mediated works published, and hereby referenced [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39], shows that [Cu(OTf) 2 (py) 4 ] is still by far the most common catalyst used (against other options such as Cu(OTf) 2 , Cu(OTf) 2 (associated with diverse pyridine derivatives), or Cu(CF 3 SO 3 ) 2 ), and the typical amounts for all of them are between 5 to 30 µmol while the Bpin labeling precursor may vary from 4 to 60 µmol. The reaction temperatures are usually kept around 120 °C ± 10 °C while anhydrous DMA and dimethylformamide (DMF) are the solvents almost exclusively reported, with the first one having the propensity for better conversion efficacies [39] which can arguably be due to its higher boiling point and resistance to bases.…”

“…One of these strategies, the late-stage copper-mediated oxidative 18 F-fluorination of arylboronic ester and acid derivatives, has received great attention from radiochemistry research groups but is still not routinely applied in the production of clinical PET radiopharmaceuticals. Numerous basic-research proposals for improving this Cu-catalyzed reaction have been successfully reported and conceptualized with simple heteroaromatic groups [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33], but advanced applications to more complex molecules with potential clinical value are sparse and generally reveal very fluctuating 18 F-fluorination efficiencies [33,34,35,36,37,38]. Following previous work from our group [39], where we applied this Cu-mediated strategy to several structurally different drug-like molecules and investigated the influence of a range of temperature, solvents, catalyst, and precursor amounts, we aimed to go up in terms of complexity, applicability, and relevance.…”

Recent Progress in Nucleophilic Fluoride Mediated Fluorine-18 Labeling of Arenes and Heteroarenes

“…Representative examples of radiopharmaceuticals 1 芳香亲核氟-18 标记反应 亲核取代(S N Ar)广泛应用于芳香族分子的官能化 [6] , 使用放射性氟标记试剂进行的芳香亲核取代(S N Ar)是一 种常见的形成 C(sp 2 )- 18 LG 18 LG = N + R 3 , NO 2 , I, Br, OTs, OTf, etc. [26][27][28][29][30][31][32][33][34][35][36] . 此后该课题组又改进了几种常用放 射性药物的标记方法 [37] , 并研究了杂环对氟-18 标记放 射化学收率的影响及上百种不同杂环位点上的具体影 响 [38] .…”