Translocator protein (TSPO) is a biomarker of neuroinflammation, which is a hallmark of many neurodegenerative diseases and has been exploited as a positron emission tomography (PET) target. Carbon‐11‐labelled PK11195 remains the most applied agent for imaging TSPO, despite its short‐lived isotope and low brain permeability. Second‐generation radiotracers show variance in affinity amongst subjects (low‐, mixed‐, and high‐affinity binders) caused by the genetic polymorphism (rs6971) of the TSPO gene. To overcome these limitations, a new structural scaffold was explored based on the TSPO pharmacophore, and the analogue with a low‐affinity binder/high‐affinity binder (LAB/HAB) ratio similar (1.2 vs. 1.3) to that of (R)‐[11C]PK11195 was investigated. The synthesis of the reference compound was accomplished in six steps and 9 % overall yield, and the precursor was prepared in eight steps and 8 % overall yield. The chiral separation of the reference and precursor compounds was performed using supercritical fluid chromatography with >95 % ee. The absolute configuration was determined by circular dichroism. Optimisation of reaction conditions for manual radiolabelling revealed acetonitrile as a preferred solvent at 100 °C. Automation of this radiolabelling method provided R and S enantiomers in respective 21.3±16.7 and 25.6±7.1 % decay‐corrected yields and molar activities of 55.8±35.6 and 63.5±39.5 GBq μmol−1 (n=3). Injection of the racemic analogue into a healthy rat confirmed passage through the blood–brain barrier.

In addition to its use for the study of biomolecules in living systems, bioorthogonal chemistry has emerged as ap romising strategyt oe nable protein or drug activation in as patially and temporally controlled manner.T his study demonstrates the application of ab ioorthogonal inverse electron-demand Diels-Alder (iEDDA) reaction to cleave trans-cyclooctene (TCO) and vinyl protecting groups from carboxylic acid-containing molecules. The tetrazine-mediated decaging reactionp roceeded under biocompatiblec onditions with fast reaction kinetics (< 2min). The anti-inflammatory activity of ketoprofen was successfully reinstated after decaging of the nontoxic TCOprodrug in live macrophages. Overall, this work expandst he scope of functional groups and the application of decaging reactions to an ew class of drugs.Early research in the fieldo fb ioorthogonal chemistry focused on ligation reactions such as the Staudinger reaction, [1] coppercatalysed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), [2,3] palladium-catalysed cross-couplings, [4] ruthenium-catalysed olefin metatheses, [5] strain-promoted azide-alkyne cycloaddition (SPAAC), [6] tetrazole photoinduced 1,3-dipolar cycloadditions, [7,8] and inverse electron-demandD iels-Alder (iEDDA) tetrazine ligation. [9,10] Of these, the iEDDA reactionb etween transcyclooctene (TCO) and at etrazine is one of the more selective and fastestb ioorthogonal reactions to date. [11] Since it was first introduced by Fox et al., [9] this reactionh as been used in numerous biological applicationss uch as cell and in vivo pretargeting imaging. [12][13][14] Recently,b ioorthogonal cleavage reactions have emerged as promising strategies to control the activation of caged proteins,f luorophores, and small-molecule drugs in living systems. [15] The TCO-tetrazine iEDDA ligation can be re-engineered into ac leavage reaction by placing a leaving group at the allylic positiono fT CO. After the initial cycloaddition and elimination of nitrogen, the 4,5-dihydropyridazine now contains an appropriatelyp laced substituent that eliminates upon tautomerisation. [10] Robillard'sg roup reported the first use of the TCO-tetrazine reaction for bioorthogonal decaging to release amine-containing drugs ( Figure 1A), in which they demonstrated the release of doxorubicin (Dox) from aT CO carbamate prodrug in vitro. [16] They then applied this "click-to-release" strategy to successfullytrigger the release of Dox and monomethyl auristatin E( MMAE) from an antibody-drug conjugate (ADC). [17,18] Mejia Oneto and co-workers also reported targeted in vivo activation of aD ox-TCO carbamate prodrug by injecting an alginate hydrogel modified with tetrazines near the tumour site. [19] Al imitation of the click-toreleases trategy is the need for delivery,a nd therefore optimisation of the pharmacokinetic properties, of both the prodrug and the tetrazine. [20,21] However,t he previously mentioned approaches demonstrate the potential of bioorthogonal decaging reactions for targeted drug activation in vivo.Bioorthogonal chemist...

Purpose Positron emission tomography (PET) studies with radioligands for 18-kDa translocator protein (TSPO) have been instrumental in increasing our understanding of the complex role neuroinflammation plays in disorders affecting the brain. However, (R)-[11C]PK11195, the first and most widely used TSPO radioligand has limitations, while the next-generation TSPO radioligands have suffered from high interindividual variability in binding due to a genetic polymorphism in the TSPO gene (rs6971). Herein, we present the biological evaluation of the two enantiomers of [18F]GE387, which we have previously shown to have low sensitivity to this polymorphism. Methods Dynamic PET scans were conducted in male Wistar rats and female rhesus macaques to investigate the in vivo behaviour of (S)-[18F]GE387 and (R)-[18F]GE387. The specific binding of (S)-[18F]GE387 to TSPO was investigated by pre-treatment with (R)-PK11195. (S)-[18F]GE387 was further evaluated in a rat model of lipopolysaccharide (LPS)-induced neuroinflammation. Sensitivity to polymorphism of (S)-GE387 was evaluated in genotyped human brain tissue. Results (S)-[18F]GE387 and (R)-[18F]GE387 entered the brain in both rats and rhesus macaques. (R)-PK11195 blocked the uptake of (S)-[18F]GE387 in healthy olfactory bulb and peripheral tissues constitutively expressing TSPO. A 2.7-fold higher uptake of (S)-[18F]GE387 was found in the inflamed striatum of LPS-treated rodents. In genotyped human brain tissue, (S)-GE387 was shown to bind similarly in low affinity binders (LABs) and high affinity binders (HABs) with a LAB to HAB ratio of 1.8. Conclusion We established that (S)-[18F]GE387 has favourable kinetics in healthy rats and non-human primates and that it can distinguish inflamed from normal brain regions in the LPS model of neuroinflammation. Crucially, we have reconfirmed its low sensitivity to the TSPO polymorphism on genotyped human brain tissue. Based on these factors, we conclude that (S)-[18F]GE387 warrants further evaluation with studies on human subjects to assess its suitability as a TSPO PET radioligand for assessing neuroinflammation.